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Formability of porous tantalum sheet-metalTo cite this article Paul S Nebosky et al 2009 IOP Conf Ser Mater Sci Eng 4 012018
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Formability of Porous Tantalum Sheet-Metal
Paul S Nebosky1 Steven R Schmid
1 and Timotius Pasang
2
1Dept of Aerospace amp Mechanical Eng University of Notre Dame IN USA 46556
2Dept of Mechanical amp Manufacturing Eng AUT University New Zealand
E-mail schmid2ndedu
Abstract Over the past ten years a novel cellular solid Trabecular Metal
TM has been
developed for use in the orthopaedics industry as an ingrowth scaffold Manufactured using
chemical vapour deposition (CVD) on top of a graphite foam substrate this material has a
regular matrix of interconnecting pores high strength and high porosity Manufacturing
difficulties encourage the application of bending stamping and forming technologies to
increase CVD reactor throughput and reduce material wastes In this study the bending and
forming behaviour of Trabecular MetalTM
was evaluated using a novel camera-based system for
measuring surface strains since the conventional approach of printing or etching gridded
patterns was not feasible A forming limit diagram was obtained using specially fabricated 165
mm thick sheets A springback coefficient was measured and modeled using effective
hexagonal cell arrangements
1 Introduction
Every year orthopaedic surgeons perform around 1000000 total hip replacement (THR) surgeries
worldwide A total hip replacement usually consists of a stem which is inserted into and bonds with
the femur an acetabular cup a head which attaches to the stem and a cup liner for wear resistance
Porous coatings may be included on any orthopaedic implant where tissue ingrowth is desired For
good ingrowth properties scaffolds must have open cells and high through-porosity Arguably the best
scaffold is Trabecular MetalTM
illustrated in Fig 1 which leads to rapid and strong bone ingrowth [1
2] Trabecular MetalTM
has been used as a tissue scaffold and incorporated into acetabular cups knee
spacers and shoulder implants It has a tensile elastic modulus of roughly 7-10 GPa (compared to
subchondral bone with a stiffness of 2 GPa) a yield strength of 45-50 MPa and an ultimate tensile
strength of 59-68 MPa [3]
Currently final part geometries can be obtained in two ways First bulk Trabecular MetalTM
can
be machined to its final shape Conventional machining of Trabecular MetalTM
involves electrical
discharge machining (EDM) or milling As such machining Trabecular MetalTM
is costly not only
because of material loss but also due to expensive manufacturing and cleaning operations The second
approach involves forming before CVD
1 To whom any correspondence should be addressed
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
ccopy 2009 IOP Publishing Ltd 1
Figure 1 Porous tantalum foam used for osseointegration in orthopaedic implants (a) SEM image of
Trabecular MetalTM
(b) SEM of trabecular bone (c) total hip replacement with Trabecular MetalTM
bone ingrowth scaffold Source Courtesy of Zimmer Inc
A new approach namely forming Trabecular MetalTM
using bending and forming operations has
several advantages principally cost and reliability To develop sheet forming as a viable method for
shaping Trabecular MetalTM
this research seeks to determine the forming limits bendability and
springback of Trabecular MetalTM
Knowing the forming limits will allow designers to lay out
Trabecular MetalTM
performs and plan such operations
2 Bendability and Springback Some geometries such as interior surfaces of total knee replacements can be produced solely through
bending operations A previous paper investigated the ability of Trabecular MetalTM
to survive
bending without cracking or failure [4] In that paper a manufacturing strategy involving performing
of nestable blanks for CVD followed by bending operations was described to achieve the final part
shape However this presents an additional concern in that control and prediction of elastic springback
is needed to achieve final shapes with tight tolerances Springback results from elastic recovery after
the removal of loads used to plastically deform the workpiece [5]
21 Experimental Investigation of Springback
According to the springback model derived by Gardiner [11] and later expanded by Johnson and Yu
[13] springback depends on the die radius sheet thickness yield strength and Youngs modulus For
this study only die radius R was a factor that could be reasonably varied
Additional factors were necessary to reflect the differences between Trabecular MetalTM
and sheet
metal bending Early forming tests indicated that strain rates affected cracking in Trabecular MetalTM
[4] leading to the inclusion of punch speed clearance C and stroke length
As received the net-formed specimens had dimensions of 254 mm x 1016 mm x 15 mm and
were then prepared using the appropriate embossing Experiments were performed using a wiping die
installed on an Instron 8800 hydraulic test machine (see details in [4])
22 Results
Figure 2 breaks the data into the four significant categories and plots the springback factor as a
function of RT The curve indicates the springback factor of Trabecular MetalTM
as calculated using
the approach of Gardiner [10] The results show several important trends First springback factor
increases with decreasing die radius which is consistent with previous studies Similarly springback
factor decreases with increasing clearance In addition clearance appears to have a greater effect on
springback at larger die radii
Figure 3 shows the results of the numerical simulations plotted on top of confidence regions
determined experimentally for the four combinations of R and C In general the springback factor
increases with decreasing aspect ratio
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
2
Figure 2 Springback factor as a function of
RT The only two significant factors were die
radius and clearance
Figure 3 Springback factor as a function of
aspect ratio
23 Discussion
From lot to lot the springback properties of Trabecular MetalTM
have varied greatly While some lots
of specimens such as those net-formed in a low nitrogen environment proved very ductile and
bendable others such as those shaped using EDM with nitrogen in the CVD chamber have been
more brittle Nitrogen in the CVD chamber can react with tantalum to cause nitrogen embrittlement of
the specimen Likewise the EDM process may reduce bendability due to the presence of a recast layer
and the formation of tantalum oxide on the surface of the specimen Under net-formed conditions the
surface chemistry would not be altered
The samples used for these experiments were created under the standard CVD atmosphere
subjecting them to nitrogen embrittlement However net-forming the samples prevented recast and
the creation of tantalum oxide Thus they showed reasonable bendability
During bending it can be assumed that all deformation occurred at the bend radius Outside this
region the workpiece was assumed to remain unbent Experimentally this assumption proved
accurate as curvature outside the region of the bend was undetectable at full punch extension (Fig
4a) However simulations run at the larger clearance showed significant curvature outside the area of
the bend (Fig 4b) Deformation in this region was found to be elastic as releasing the punch
straightened the workpiece away from the bend radius
Figure 4 The observed workpiece behaviour
during wiping die bending (a) The
experimentally observed workpiece behaviour
for the larger clearance at full punch extension
(b) The deformation behaviour as predicted by
the numerical models
3 Determination of the Forming Limit Diagram
31 Experimental Set Up
Forming-limit diagrams (FLDs) have been widely applied since the 1960s for evaluating the ability of
sheet metal to be plastically deformed to desired geometries The general approach to FLDs is beyond
the scope of this paper but the general theory is reviewed by [5 15] and the approach for determining
a FLD has been standardized by ASTM [16] It should be noted however that determining an FLD
for a particular sheet metal involves inscribing or printing a (usually circular) grid pattern on the sheet
deforming the sheet and measuring the resulting surface strains The FLD is a plot of major versus
minor strain at incipient failure resulting from these experiments
Since the bone ingrowth sections of orthopaedic implants can consist of thin sheets of Trabecular
MetalTM
the extension of forming-limit diagrams to this material is useful for implant designers
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
3
Because of the unique characteristics of the material several departures were necessary from the
ASTM standardized test method
The first testing change involved specimen size Traditional sheet-metal is readily available in
large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular
MetalTM
is never as large as typical sheet-metal parts as found in automobiles for example In most
cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on
a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer
perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of
the forming conditions associated with implant geometries and greatly simplified sample preparation
Manufacturability and porosity also contributed to a change in specimen size By decreasing
specimen size the effect of changing porosity within a given specimen and from sample to sample on
the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from
752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in
punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were
performed on specimens 25 mm wide In most cases four samples of each width were tested with
three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual
examination of samples was performed and samples of poor quality were not tested
For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2
mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm
With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy
sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor
These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and
prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm
An ARAMIS camera system was used to monitor the strain evolution of the surface during
forming The ARAMIS system consists of two computer integrated CCD cameras Typically the
angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During
testing the cameras record images at a specified time interval For testing images were captured once
each second Once testing is complete the computer software converts this two-dimensional image
data to three-dimensional object data
The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a
maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests
hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates
were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head
displacement continued until the formation of visible cracks
32 Results
While most specimens showed some formability others failed after little or no plastic deformation
Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer
examination each of these brittle specimens had surface defects that led to quick failure These were
in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-
serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect
of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the
forming limits for the brittle samples while the upper curve represents the forming limits of the other
more ductile specimens These curves were determined as follows First each deformed specimen
was classified as brittle or ductile The failed strain states those within the immediate vicinity of the
crack were recorded and plotted on the forming-limit diagram Critical strain states those where
failure did not occur but which directly surrounded areas of failure were also recorded Finally the
forming limits were derived empirically by drawing the curves below the failed strain states but above
the safe and critical ones
For a plane-strain condition the limiting major strain for formable Trabecular MetalTM
was found
to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
4
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
Formability of Porous Tantalum Sheet-Metal
Paul S Nebosky1 Steven R Schmid
1 and Timotius Pasang
2
1Dept of Aerospace amp Mechanical Eng University of Notre Dame IN USA 46556
2Dept of Mechanical amp Manufacturing Eng AUT University New Zealand
E-mail schmid2ndedu
Abstract Over the past ten years a novel cellular solid Trabecular Metal
TM has been
developed for use in the orthopaedics industry as an ingrowth scaffold Manufactured using
chemical vapour deposition (CVD) on top of a graphite foam substrate this material has a
regular matrix of interconnecting pores high strength and high porosity Manufacturing
difficulties encourage the application of bending stamping and forming technologies to
increase CVD reactor throughput and reduce material wastes In this study the bending and
forming behaviour of Trabecular MetalTM
was evaluated using a novel camera-based system for
measuring surface strains since the conventional approach of printing or etching gridded
patterns was not feasible A forming limit diagram was obtained using specially fabricated 165
mm thick sheets A springback coefficient was measured and modeled using effective
hexagonal cell arrangements
1 Introduction
Every year orthopaedic surgeons perform around 1000000 total hip replacement (THR) surgeries
worldwide A total hip replacement usually consists of a stem which is inserted into and bonds with
the femur an acetabular cup a head which attaches to the stem and a cup liner for wear resistance
Porous coatings may be included on any orthopaedic implant where tissue ingrowth is desired For
good ingrowth properties scaffolds must have open cells and high through-porosity Arguably the best
scaffold is Trabecular MetalTM
illustrated in Fig 1 which leads to rapid and strong bone ingrowth [1
2] Trabecular MetalTM
has been used as a tissue scaffold and incorporated into acetabular cups knee
spacers and shoulder implants It has a tensile elastic modulus of roughly 7-10 GPa (compared to
subchondral bone with a stiffness of 2 GPa) a yield strength of 45-50 MPa and an ultimate tensile
strength of 59-68 MPa [3]
Currently final part geometries can be obtained in two ways First bulk Trabecular MetalTM
can
be machined to its final shape Conventional machining of Trabecular MetalTM
involves electrical
discharge machining (EDM) or milling As such machining Trabecular MetalTM
is costly not only
because of material loss but also due to expensive manufacturing and cleaning operations The second
approach involves forming before CVD
1 To whom any correspondence should be addressed
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
ccopy 2009 IOP Publishing Ltd 1
Figure 1 Porous tantalum foam used for osseointegration in orthopaedic implants (a) SEM image of
Trabecular MetalTM
(b) SEM of trabecular bone (c) total hip replacement with Trabecular MetalTM
bone ingrowth scaffold Source Courtesy of Zimmer Inc
A new approach namely forming Trabecular MetalTM
using bending and forming operations has
several advantages principally cost and reliability To develop sheet forming as a viable method for
shaping Trabecular MetalTM
this research seeks to determine the forming limits bendability and
springback of Trabecular MetalTM
Knowing the forming limits will allow designers to lay out
Trabecular MetalTM
performs and plan such operations
2 Bendability and Springback Some geometries such as interior surfaces of total knee replacements can be produced solely through
bending operations A previous paper investigated the ability of Trabecular MetalTM
to survive
bending without cracking or failure [4] In that paper a manufacturing strategy involving performing
of nestable blanks for CVD followed by bending operations was described to achieve the final part
shape However this presents an additional concern in that control and prediction of elastic springback
is needed to achieve final shapes with tight tolerances Springback results from elastic recovery after
the removal of loads used to plastically deform the workpiece [5]
21 Experimental Investigation of Springback
According to the springback model derived by Gardiner [11] and later expanded by Johnson and Yu
[13] springback depends on the die radius sheet thickness yield strength and Youngs modulus For
this study only die radius R was a factor that could be reasonably varied
Additional factors were necessary to reflect the differences between Trabecular MetalTM
and sheet
metal bending Early forming tests indicated that strain rates affected cracking in Trabecular MetalTM
[4] leading to the inclusion of punch speed clearance C and stroke length
As received the net-formed specimens had dimensions of 254 mm x 1016 mm x 15 mm and
were then prepared using the appropriate embossing Experiments were performed using a wiping die
installed on an Instron 8800 hydraulic test machine (see details in [4])
22 Results
Figure 2 breaks the data into the four significant categories and plots the springback factor as a
function of RT The curve indicates the springback factor of Trabecular MetalTM
as calculated using
the approach of Gardiner [10] The results show several important trends First springback factor
increases with decreasing die radius which is consistent with previous studies Similarly springback
factor decreases with increasing clearance In addition clearance appears to have a greater effect on
springback at larger die radii
Figure 3 shows the results of the numerical simulations plotted on top of confidence regions
determined experimentally for the four combinations of R and C In general the springback factor
increases with decreasing aspect ratio
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
2
Figure 2 Springback factor as a function of
RT The only two significant factors were die
radius and clearance
Figure 3 Springback factor as a function of
aspect ratio
23 Discussion
From lot to lot the springback properties of Trabecular MetalTM
have varied greatly While some lots
of specimens such as those net-formed in a low nitrogen environment proved very ductile and
bendable others such as those shaped using EDM with nitrogen in the CVD chamber have been
more brittle Nitrogen in the CVD chamber can react with tantalum to cause nitrogen embrittlement of
the specimen Likewise the EDM process may reduce bendability due to the presence of a recast layer
and the formation of tantalum oxide on the surface of the specimen Under net-formed conditions the
surface chemistry would not be altered
The samples used for these experiments were created under the standard CVD atmosphere
subjecting them to nitrogen embrittlement However net-forming the samples prevented recast and
the creation of tantalum oxide Thus they showed reasonable bendability
During bending it can be assumed that all deformation occurred at the bend radius Outside this
region the workpiece was assumed to remain unbent Experimentally this assumption proved
accurate as curvature outside the region of the bend was undetectable at full punch extension (Fig
4a) However simulations run at the larger clearance showed significant curvature outside the area of
the bend (Fig 4b) Deformation in this region was found to be elastic as releasing the punch
straightened the workpiece away from the bend radius
Figure 4 The observed workpiece behaviour
during wiping die bending (a) The
experimentally observed workpiece behaviour
for the larger clearance at full punch extension
(b) The deformation behaviour as predicted by
the numerical models
3 Determination of the Forming Limit Diagram
31 Experimental Set Up
Forming-limit diagrams (FLDs) have been widely applied since the 1960s for evaluating the ability of
sheet metal to be plastically deformed to desired geometries The general approach to FLDs is beyond
the scope of this paper but the general theory is reviewed by [5 15] and the approach for determining
a FLD has been standardized by ASTM [16] It should be noted however that determining an FLD
for a particular sheet metal involves inscribing or printing a (usually circular) grid pattern on the sheet
deforming the sheet and measuring the resulting surface strains The FLD is a plot of major versus
minor strain at incipient failure resulting from these experiments
Since the bone ingrowth sections of orthopaedic implants can consist of thin sheets of Trabecular
MetalTM
the extension of forming-limit diagrams to this material is useful for implant designers
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
3
Because of the unique characteristics of the material several departures were necessary from the
ASTM standardized test method
The first testing change involved specimen size Traditional sheet-metal is readily available in
large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular
MetalTM
is never as large as typical sheet-metal parts as found in automobiles for example In most
cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on
a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer
perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of
the forming conditions associated with implant geometries and greatly simplified sample preparation
Manufacturability and porosity also contributed to a change in specimen size By decreasing
specimen size the effect of changing porosity within a given specimen and from sample to sample on
the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from
752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in
punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were
performed on specimens 25 mm wide In most cases four samples of each width were tested with
three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual
examination of samples was performed and samples of poor quality were not tested
For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2
mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm
With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy
sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor
These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and
prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm
An ARAMIS camera system was used to monitor the strain evolution of the surface during
forming The ARAMIS system consists of two computer integrated CCD cameras Typically the
angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During
testing the cameras record images at a specified time interval For testing images were captured once
each second Once testing is complete the computer software converts this two-dimensional image
data to three-dimensional object data
The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a
maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests
hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates
were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head
displacement continued until the formation of visible cracks
32 Results
While most specimens showed some formability others failed after little or no plastic deformation
Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer
examination each of these brittle specimens had surface defects that led to quick failure These were
in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-
serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect
of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the
forming limits for the brittle samples while the upper curve represents the forming limits of the other
more ductile specimens These curves were determined as follows First each deformed specimen
was classified as brittle or ductile The failed strain states those within the immediate vicinity of the
crack were recorded and plotted on the forming-limit diagram Critical strain states those where
failure did not occur but which directly surrounded areas of failure were also recorded Finally the
forming limits were derived empirically by drawing the curves below the failed strain states but above
the safe and critical ones
For a plane-strain condition the limiting major strain for formable Trabecular MetalTM
was found
to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
4
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
Figure 1 Porous tantalum foam used for osseointegration in orthopaedic implants (a) SEM image of
Trabecular MetalTM
(b) SEM of trabecular bone (c) total hip replacement with Trabecular MetalTM
bone ingrowth scaffold Source Courtesy of Zimmer Inc
A new approach namely forming Trabecular MetalTM
using bending and forming operations has
several advantages principally cost and reliability To develop sheet forming as a viable method for
shaping Trabecular MetalTM
this research seeks to determine the forming limits bendability and
springback of Trabecular MetalTM
Knowing the forming limits will allow designers to lay out
Trabecular MetalTM
performs and plan such operations
2 Bendability and Springback Some geometries such as interior surfaces of total knee replacements can be produced solely through
bending operations A previous paper investigated the ability of Trabecular MetalTM
to survive
bending without cracking or failure [4] In that paper a manufacturing strategy involving performing
of nestable blanks for CVD followed by bending operations was described to achieve the final part
shape However this presents an additional concern in that control and prediction of elastic springback
is needed to achieve final shapes with tight tolerances Springback results from elastic recovery after
the removal of loads used to plastically deform the workpiece [5]
21 Experimental Investigation of Springback
According to the springback model derived by Gardiner [11] and later expanded by Johnson and Yu
[13] springback depends on the die radius sheet thickness yield strength and Youngs modulus For
this study only die radius R was a factor that could be reasonably varied
Additional factors were necessary to reflect the differences between Trabecular MetalTM
and sheet
metal bending Early forming tests indicated that strain rates affected cracking in Trabecular MetalTM
[4] leading to the inclusion of punch speed clearance C and stroke length
As received the net-formed specimens had dimensions of 254 mm x 1016 mm x 15 mm and
were then prepared using the appropriate embossing Experiments were performed using a wiping die
installed on an Instron 8800 hydraulic test machine (see details in [4])
22 Results
Figure 2 breaks the data into the four significant categories and plots the springback factor as a
function of RT The curve indicates the springback factor of Trabecular MetalTM
as calculated using
the approach of Gardiner [10] The results show several important trends First springback factor
increases with decreasing die radius which is consistent with previous studies Similarly springback
factor decreases with increasing clearance In addition clearance appears to have a greater effect on
springback at larger die radii
Figure 3 shows the results of the numerical simulations plotted on top of confidence regions
determined experimentally for the four combinations of R and C In general the springback factor
increases with decreasing aspect ratio
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
2
Figure 2 Springback factor as a function of
RT The only two significant factors were die
radius and clearance
Figure 3 Springback factor as a function of
aspect ratio
23 Discussion
From lot to lot the springback properties of Trabecular MetalTM
have varied greatly While some lots
of specimens such as those net-formed in a low nitrogen environment proved very ductile and
bendable others such as those shaped using EDM with nitrogen in the CVD chamber have been
more brittle Nitrogen in the CVD chamber can react with tantalum to cause nitrogen embrittlement of
the specimen Likewise the EDM process may reduce bendability due to the presence of a recast layer
and the formation of tantalum oxide on the surface of the specimen Under net-formed conditions the
surface chemistry would not be altered
The samples used for these experiments were created under the standard CVD atmosphere
subjecting them to nitrogen embrittlement However net-forming the samples prevented recast and
the creation of tantalum oxide Thus they showed reasonable bendability
During bending it can be assumed that all deformation occurred at the bend radius Outside this
region the workpiece was assumed to remain unbent Experimentally this assumption proved
accurate as curvature outside the region of the bend was undetectable at full punch extension (Fig
4a) However simulations run at the larger clearance showed significant curvature outside the area of
the bend (Fig 4b) Deformation in this region was found to be elastic as releasing the punch
straightened the workpiece away from the bend radius
Figure 4 The observed workpiece behaviour
during wiping die bending (a) The
experimentally observed workpiece behaviour
for the larger clearance at full punch extension
(b) The deformation behaviour as predicted by
the numerical models
3 Determination of the Forming Limit Diagram
31 Experimental Set Up
Forming-limit diagrams (FLDs) have been widely applied since the 1960s for evaluating the ability of
sheet metal to be plastically deformed to desired geometries The general approach to FLDs is beyond
the scope of this paper but the general theory is reviewed by [5 15] and the approach for determining
a FLD has been standardized by ASTM [16] It should be noted however that determining an FLD
for a particular sheet metal involves inscribing or printing a (usually circular) grid pattern on the sheet
deforming the sheet and measuring the resulting surface strains The FLD is a plot of major versus
minor strain at incipient failure resulting from these experiments
Since the bone ingrowth sections of orthopaedic implants can consist of thin sheets of Trabecular
MetalTM
the extension of forming-limit diagrams to this material is useful for implant designers
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
3
Because of the unique characteristics of the material several departures were necessary from the
ASTM standardized test method
The first testing change involved specimen size Traditional sheet-metal is readily available in
large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular
MetalTM
is never as large as typical sheet-metal parts as found in automobiles for example In most
cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on
a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer
perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of
the forming conditions associated with implant geometries and greatly simplified sample preparation
Manufacturability and porosity also contributed to a change in specimen size By decreasing
specimen size the effect of changing porosity within a given specimen and from sample to sample on
the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from
752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in
punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were
performed on specimens 25 mm wide In most cases four samples of each width were tested with
three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual
examination of samples was performed and samples of poor quality were not tested
For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2
mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm
With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy
sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor
These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and
prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm
An ARAMIS camera system was used to monitor the strain evolution of the surface during
forming The ARAMIS system consists of two computer integrated CCD cameras Typically the
angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During
testing the cameras record images at a specified time interval For testing images were captured once
each second Once testing is complete the computer software converts this two-dimensional image
data to three-dimensional object data
The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a
maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests
hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates
were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head
displacement continued until the formation of visible cracks
32 Results
While most specimens showed some formability others failed after little or no plastic deformation
Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer
examination each of these brittle specimens had surface defects that led to quick failure These were
in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-
serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect
of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the
forming limits for the brittle samples while the upper curve represents the forming limits of the other
more ductile specimens These curves were determined as follows First each deformed specimen
was classified as brittle or ductile The failed strain states those within the immediate vicinity of the
crack were recorded and plotted on the forming-limit diagram Critical strain states those where
failure did not occur but which directly surrounded areas of failure were also recorded Finally the
forming limits were derived empirically by drawing the curves below the failed strain states but above
the safe and critical ones
For a plane-strain condition the limiting major strain for formable Trabecular MetalTM
was found
to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
4
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
Figure 2 Springback factor as a function of
RT The only two significant factors were die
radius and clearance
Figure 3 Springback factor as a function of
aspect ratio
23 Discussion
From lot to lot the springback properties of Trabecular MetalTM
have varied greatly While some lots
of specimens such as those net-formed in a low nitrogen environment proved very ductile and
bendable others such as those shaped using EDM with nitrogen in the CVD chamber have been
more brittle Nitrogen in the CVD chamber can react with tantalum to cause nitrogen embrittlement of
the specimen Likewise the EDM process may reduce bendability due to the presence of a recast layer
and the formation of tantalum oxide on the surface of the specimen Under net-formed conditions the
surface chemistry would not be altered
The samples used for these experiments were created under the standard CVD atmosphere
subjecting them to nitrogen embrittlement However net-forming the samples prevented recast and
the creation of tantalum oxide Thus they showed reasonable bendability
During bending it can be assumed that all deformation occurred at the bend radius Outside this
region the workpiece was assumed to remain unbent Experimentally this assumption proved
accurate as curvature outside the region of the bend was undetectable at full punch extension (Fig
4a) However simulations run at the larger clearance showed significant curvature outside the area of
the bend (Fig 4b) Deformation in this region was found to be elastic as releasing the punch
straightened the workpiece away from the bend radius
Figure 4 The observed workpiece behaviour
during wiping die bending (a) The
experimentally observed workpiece behaviour
for the larger clearance at full punch extension
(b) The deformation behaviour as predicted by
the numerical models
3 Determination of the Forming Limit Diagram
31 Experimental Set Up
Forming-limit diagrams (FLDs) have been widely applied since the 1960s for evaluating the ability of
sheet metal to be plastically deformed to desired geometries The general approach to FLDs is beyond
the scope of this paper but the general theory is reviewed by [5 15] and the approach for determining
a FLD has been standardized by ASTM [16] It should be noted however that determining an FLD
for a particular sheet metal involves inscribing or printing a (usually circular) grid pattern on the sheet
deforming the sheet and measuring the resulting surface strains The FLD is a plot of major versus
minor strain at incipient failure resulting from these experiments
Since the bone ingrowth sections of orthopaedic implants can consist of thin sheets of Trabecular
MetalTM
the extension of forming-limit diagrams to this material is useful for implant designers
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
3
Because of the unique characteristics of the material several departures were necessary from the
ASTM standardized test method
The first testing change involved specimen size Traditional sheet-metal is readily available in
large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular
MetalTM
is never as large as typical sheet-metal parts as found in automobiles for example In most
cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on
a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer
perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of
the forming conditions associated with implant geometries and greatly simplified sample preparation
Manufacturability and porosity also contributed to a change in specimen size By decreasing
specimen size the effect of changing porosity within a given specimen and from sample to sample on
the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from
752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in
punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were
performed on specimens 25 mm wide In most cases four samples of each width were tested with
three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual
examination of samples was performed and samples of poor quality were not tested
For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2
mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm
With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy
sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor
These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and
prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm
An ARAMIS camera system was used to monitor the strain evolution of the surface during
forming The ARAMIS system consists of two computer integrated CCD cameras Typically the
angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During
testing the cameras record images at a specified time interval For testing images were captured once
each second Once testing is complete the computer software converts this two-dimensional image
data to three-dimensional object data
The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a
maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests
hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates
were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head
displacement continued until the formation of visible cracks
32 Results
While most specimens showed some formability others failed after little or no plastic deformation
Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer
examination each of these brittle specimens had surface defects that led to quick failure These were
in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-
serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect
of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the
forming limits for the brittle samples while the upper curve represents the forming limits of the other
more ductile specimens These curves were determined as follows First each deformed specimen
was classified as brittle or ductile The failed strain states those within the immediate vicinity of the
crack were recorded and plotted on the forming-limit diagram Critical strain states those where
failure did not occur but which directly surrounded areas of failure were also recorded Finally the
forming limits were derived empirically by drawing the curves below the failed strain states but above
the safe and critical ones
For a plane-strain condition the limiting major strain for formable Trabecular MetalTM
was found
to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
4
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
Because of the unique characteristics of the material several departures were necessary from the
ASTM standardized test method
The first testing change involved specimen size Traditional sheet-metal is readily available in
large blanks and can be easily evaluated using relatively large tooling However bulk Trabecular
MetalTM
is never as large as typical sheet-metal parts as found in automobiles for example In most
cases the scaffold sections of implants are relatively small For example a proximal ingrowth pad on
a hip stem may only be 50 mm long and ingrowth layers covering entire stems are not much longer
perhaps only 150 mm in length Thus reducing specimen size provided a more realistic simulation of
the forming conditions associated with implant geometries and greatly simplified sample preparation
Manufacturability and porosity also contributed to a change in specimen size By decreasing
specimen size the effect of changing porosity within a given specimen and from sample to sample on
the forming-limit curve was reduced For the tests performed here the sheet porosities ranged from
752 to 814 For these reasons a specimen length of 100 mm was selected Specimen widths in
punch tests ranged from 10 mm to 100 mm in increments of 10 mm while tensile tests were
performed on specimens 25 mm wide In most cases four samples of each width were tested with
three samples taken at widths of 50 and 80 mm and six analyzed at a width of 40 mm A visual
examination of samples was performed and samples of poor quality were not tested
For orthopaedic implants the desired thickness of the ingrowth layer usually ranges from 15-2
mm However limitations of the CVD process restrict the smallest sheet thickness to roughly 3 mm
With thinner layers localized tantalum deposition occurs and causes warping resulting in a wavy
sheet To avoid this problem sheets 100 x 100 x 33 mm were manufactured in the CVD reactor
These were then cut in half using wire EDM to obtain a target thickness of 165 mm As received and
prior to testing sheet thickness ranged from 152 mm to 20 mm with an average of 18 mm
An ARAMIS camera system was used to monitor the strain evolution of the surface during
forming The ARAMIS system consists of two computer integrated CCD cameras Typically the
angle γ between the cameras ranges from 10-40deg with the optimal angle ranging from 25-30deg During
testing the cameras record images at a specified time interval For testing images were captured once
each second Once testing is complete the computer software converts this two-dimensional image
data to three-dimensional object data
The experimental setup for the punch tests is given in [4] Two pull-action latch clamps with a
maximum combined hold-down capacity of 6200 N fixed the specimens in place For the tensile tests
hydraulic grips were used All tests were performed on Instron 8800 test machines Deformation rates
were 635 mmmin for the punch tests and 0635 mmmin for the tensile tests Cross-head
displacement continued until the formation of visible cracks
32 Results
While most specimens showed some formability others failed after little or no plastic deformation
Of the 44 specimens tested 37 were formable while the remaining 7 performed poorly Upon closer
examination each of these brittle specimens had surface defects that led to quick failure These were
in the form of a serrated surface (Figure 5a) or an undiscovered crater Specimens with smooth non-
serrated surfaces without defects generally performed well (Figure 5b) To compensate for the effect
of brittle samples two forming-limit curves were constructed (Figure 6) The lower curve shows the
forming limits for the brittle samples while the upper curve represents the forming limits of the other
more ductile specimens These curves were determined as follows First each deformed specimen
was classified as brittle or ductile The failed strain states those within the immediate vicinity of the
crack were recorded and plotted on the forming-limit diagram Critical strain states those where
failure did not occur but which directly surrounded areas of failure were also recorded Finally the
forming limits were derived empirically by drawing the curves below the failed strain states but above
the safe and critical ones
For a plane-strain condition the limiting major strain for formable Trabecular MetalTM
was found
to be roughly 5 Likewise minor strains generated during testing ranged from -8 to 14 By
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
4
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
comparison low-carbon steel can be formed with major strains of 40 in the plane-strain condition
Minor strains generated during testing can range from -50 to 60 Clearly Trabecular MetalTM
has
much less formability than conventional sheet metals most likely attributable to its porosity and
material brittleness
Figure 5 Trabecular Metal
TM specimens
showing samples with (a) little formability -
note the visibly striated surface and (b)
good formability
Figure 6 Forming-limit curves for
Trabecular MetalTM
Brittle specimens displayed little formability To the naked eye all deformation appeared to be elastic
due to springback after punch removal However the cameras were able to capture slight plastic
strains For the seven brittle specimens the limiting major strain was less than 1 in the plane-strain
condition Minor strains measured during testing varied from about -1 to 4
33 Discussion
Not surprisingly Trabecular MetalTM
had much less formability than conventional sheet metals This
result was anticipated from prior testing and its nature as a cellular solid In essence the pores in the
material act as stress concentrations reducing formability By this reasoning formability should
increase with decreasing porosity However less porous foams would provide smaller volumes for
bone ingrowth making them less desirable for orthopaedic applications
In general formability increases with the uniformity of the strain field developed during forming
Lubrication improves formability by fostering a more uniform strain distribution One way to
incorporate lubrication into Trabecular MetalTM
forming would be to use sheets of solid lubricant such
as polyethylene between the punch and workpiece Such sheets provide lubrication while eliminating
costly cleaning procedures that would be needed to remove the remnants of lubricating oils or fluids
The result would be a more uniform strain distribution an ability to develop strain states with larger
positive minor strains and ultimately the ability to produce more complicated parts
Based on the empirical experiences with this material it is clear that commercial application will
require some acceptable failure threshold Although the sheets used for forming appear to be free of
surface defects a porous material contains a large number of stress concentrations While many
successful parts may be made using sheet forming there still will be some that fracture prematurely
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
5
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6
4 Conclusions The manufacturing burden associated with Trabecular Metal
TM can be significantly reduced by
forming performs This paper examined the behavior of Trabecular MetalTM
in bending and forming
operations Based on the results presented herein it can be concluded
1 Statistical analysis of the experimental results showed that die radius R and clearance C were
the only factors that influenced springback Although material properties such as the yield
strength σo and Youngs modulus E do influence springback they were assumed to be
constant for a given sample lot
2 Trabecular MetalTM
was shown to have non-negligible formability which introduces new
manufacturing and design strategies for orthopaedic implants Simple geometries that do not
require high forming strains can be manufactured using forming operations
3 Before commercial application sheet forming of Trabecular MetalTM
must overcome several
remaining hurdles First the sheets used for forming must be free of surface defects or
blemishes These include surface striations along with any craters or pits Thickness
variations throughout the sheet must also be avoided Finally it must be determined if
forming operations are actually cheaper than machining
5 Acknowledgments
The authors would like to thank the financial support of the Trabecular Metal Division of Zimmer
Holdings LLC The authors would also like to thank the personal support of Dr Robert Pogge Mr
Robert Cohen and Dr Michael Hawkins for this research In addition the efforts of Dr David Coe of
Trilion Quality Systems to assist with experimental setup and camera operation are gratefully
acknowledged
6 References [1] Bobyn J Stackpool G Hacking S Tanzer M and Krygier J 1999 J Bone amp Joint Surg 81-B
907-913
[2] Bobyn JD Toh K-K Hacking A Tanzer M and Krygier JJ 1999 J Arthroplasty 14 347-353
[3] Zardiackas LD Parsell LD Dillon DW Mitchel LA Nunnery R and Poggie R 2001 J
Biomedical Materials Research Part B Applied Material 58 180-187
[4] Nebosky PS and Schmid SR 2007 Trans NAMRI 35 57-64
[5] Kalpakjian S and Schmid SR 2006 Manufacturing Processes for Engineering Materials 5th ed
(Prentice-Hall)
[6] Sturm R and Fletcher B 1941 Product Engineering 12 526-528
[7] Sturm R and Fletcher B 1941 Product Engineering 12 590-594
[8] Chapman F Hazlett T and Schroeder W 1942 Product Engineering 13 382-383
[9] Schroeder W 1943 Transactions of the ASME 65 817-827
[10] Sachs G 1951 Principles and Methods of Sheet-metal Fabricating New York (Reinhold
Publishing Corporation)
[11] Gardiner F 1957 Transactions of the ASME 79 1-9
[12] Queener C and DeAngelis R 1968 Transactions of the ASM 61 757-768
[13] Sidebottom O and Gebhardt C 1979 Experimental Mechanics 19 371-377
[14] Johnson W and Yu T 1981International Journal of Mechanical Sciences 23 619-630
[15] Hecker SS 1975 Sheet Metal Industries 13 42-48
[16] ASTM E 2218-02 2006 (American Society for Testing and Materials)
[17] Kwon JW Lee DN and Kim I 1994 Scripta Metallurgica et Materialia 35 613-618
Processing Microstructure and Performance of Materials IOP PublishingIOP Conf Series Materials Science and Engineering 4 (2009) 012018 doi1010881757-899X41012018
6