machinability assessment of titanium alloy ti-6al-4v for biomedical applications
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Machinability Assessment of Titanium Alloy Ti-6Al-4V for Biomedical Applications
Ashwin Polishetty1, Guy Littlefair2 and Praveen Kumar K3 1Deakin University, Geelong, Australia
2Head, School of Engineering, Deakin University, Geelong, Australia
3Department of Mechanical Engineering, VIT University, Vellore, India
[email protected], [email protected], [email protected].
Keywords:Titanium alloy, Ti-6Al-4V, Machining, Casting, Femoral implant
Abstract: Titanium alloy (Ti-6Al-4V) has a wide range of application in various fields of
engineering. Titanium is mainly used to manufacture aerospace components like landing gear,
fuselage, wings, engines etc. and biomedical components like hip joint, knee joint, dental implants
etc. Titanium has outstanding material properties such as corrosion resistance, fatigue strength,
tensile strength and a very good biocompatibility which makes this material very alluring for
biomedical applications.
Contrary, the machinability of the material is problematic because of the phase transformations and
thus, titanium alloy is a challenge for machining operation. This research is a comparative analysis
between the implants manufactured by traditional method of casting and machining. The femoral
stem of the hip joint replacement is designed and the component is machined using a five-axis CNC
machine.The machined component was subjected to surface roughness testing, tensile testing and
bulk hardness testing. The values were compared with the values of titanium implants manufactured
by casting.
Introduction
Pure titanium and titanium alloys are a commonly used material for design applications in
aerospace and biomedical industries. They are used for knee joint, hip joint, dental, spinal cord
replacement etc. They play a vital role in biomedical application because they have excellent
specific strength, corrosion resistance, light weight, no allergic problems and have the best
biocompatibility among the metallic biomaterials [1]. Biomaterials are artificial or natural
materials, used in the making of structures or implants, to replace the lost or diseased biological
structure to restore its form and function.
Titanium has all the properties that a biomaterial should have such as excellent mechanical
properties, biocompatible, high corrosion and wear resistance and osseointegration [2]. Pure
titanium and titanium alloys are used in a wide range of applications which demands high level of
reliable performance.
Mechanical biocompatibility is an important factor since the research and development of beta type
titanium alloys are increasing. There is a development of new titanium alloy with super elasticity.
The young’s modulus of biomaterial is said to be always equal to that of bone. If the young’s
modulus of biomaterial is greater than that of bone, bone resorption occurs.
Titanium alloy Ti-6Al-4V the most widely used titanium alloy for biomedical application has low
young’s modulus than that of stainless steel and Cobalt based alloys [3]. However, its young’s
modulus is slightly greater than that of bone. Titanium alloys are classified as commercially pure
titanium alloy, alpha beta titanium alloy and metastable beta titanium alloy [5]. Of these Ti-6Al-4V
Advanced Materials Research Vols. 941-944 (2014) pp 1985-1990Online available since 2014/Jun/06 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.941-944.1985
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is the commonly used biomaterial for medical implants and is known as the “work horse” of
titanium industry. They have an enhanced biocompatibility, reduced elastic modulus, superior strain
controlled and notch fatigue resistance. The poor shear strength and wear resistance have
nevertheless limited their biomedical use [3]. Titanium alloy Ti-6Al-4V are not only used in
biomedical application, they are used in direct manufacturing of parts and prototype for racing and
aerospace industry, in marine application, gas turbines etc. The main limitation of pure titanium is
its high cost, rarely found metal and has phase transformation problem due to which its production
cost is increased.
Machinability is defined as the ease or the difficulty with which a material can be machined under a
given set of cutting conditions including cutting speed, feed and depth of cut. Machinability of a
work material is accessed in terms of four factors: tool life, cutting forces, power requirements and
surface finish [6]. Titanium machining has hindered a more wide application due to its relatively
high cost. To minimize the inherent cost problems, successful application must take advantage of
the special features and characteristics of titanium. The machinability of titanium and titanium
alloys is limited by some characteristics and is considered difficult, due to its relatively high tensile
strength, low ductile yield, 50% lower modulus of elasticity and approximately 80% lower thermal
conductivity than that of steel [7]. In the tool contact zones high pressures and temperatures occur.
At higher temperatures caused by friction the titanium becomes more chemically reactive and there
is a tendency for titanium to “weld” to tool bits during machining operation. During machining of
titanium alloys the cutting depth should be large, cutting speed should be from 12 to 80 m/min and
approximately 50% lower when high speed steel tools are used. The heat generated should be
removed by a large volume of cooling lubricants. Chlorinated cutting fluids should not be used
because titanium can be susceptible to stress corrosion failures in the presence of chlorine.
CBN/PCBN and Poly Crystalline Diamond (PCD) tools are employed at high speed machining of
titanium alloy [8].
Experimental Design
The objective of the study is to investigate the surface roughness, tensile strength and material
removal rate for the femoral stem machined in 5-axis machine using titanium alloy Ti-6Al-
4V.Machining of titanium alloy Ti-6Al-4V for femoral stem of a hip joint replacement was done
using a five axis machine (Spinner U-620) as shown in fig.1 and fig.2.
Fig. 1 Face milling operation Fig. 2 End milling operation
The implant profile was designed using solidworks software. The required G-code was given tothe
five axis machine for the necessary operation to be performed. Once the coding is done the design
was stimulated on the screen of the five axis machine. After placing the workpiece in the 5-axis
machine the probe was used to set the origin for the cutting tool before the machining operation.
The parameters that were used for machining operation are listed below in the table 1.
1986 Materials and Processes Technologies V
Table 1 Face milling and end milling operation parameters
Cutting
Tool
Spindle
Speed(rpm)
Cutting
SpeedVc(m/min)
Overall
FeedF
Feed per
ToothFz(mm/rev)
Table
Feed(mm/min)
Depth of
Cut
(Axial)mm
Depth of cut
(Radial)mm
50mm
diameter
face milling
cutter
2000 314 0.4
0.04
800 0.5 20
16mm
diameter solid
carbide
end
2000 100 0.2 0.05 400 3 6.4
8mm
diameter
solid carbide
end
3000 75.3 0.05 0.01 150 0.2 0.7
The surface roughness, tensile strength and material removal rate of the machined titanium alloy Ti-
6Al-4V sample (as shown in fig.3) are determined. The surface roughness test is carried out in
Taylor Hobson machine and the surface roughness of the femoral stem is determined using ultra
software. The tensile test is carried out in Instron machine of maximum capacity 30KN and the
UTS, yield strength and elongation (%) are determined.
Fig. 3 Machined femoral stem in a 5-axis machine
Results and discussion
The results obtained from the experiments are discussed in this section using graphs and tables.
Surface Roughness Test
The surface roughness of face milling operation and end milling operation is shown in fig.4.
Advanced Materials Research Vols. 941-944 1987
Fig. 4Surface roughness comparisons for face and end milling operations
Surface roughness of the machined surface depends upon the feed rate, cutting speed and depth of
cut. The surface finish is directly proportional to the feed rate and inversely proportional to cutting
speed and depth of cut. In casting process there are residual stress layers that are formed due to
heating of the metal during casting. Due to the presence of air gaps in casting the surface of the
casted femoral stem is rough. Therefore the surface roughness of the casted femoral is very high.
The machining operation of titanium alloy Ti-6Al-4V obtained an average surface roughness of
0.3420µm which is lower than that obtained in casting process. As the cutting speed used for the
machining operation increases from 75m/min to 314m/min, the surface roughness of the machined
titanium alloy decreases. As the surface roughness decreases the texture of the machined surface is
fine which is suitable for the implant.
Tensile test
The tensile test was performed on the titanium alloy Ti-6Al-4V. The UTS, yield strength and
elongation (%) are determined and shown in table 4.
Table 4 Tensile test values for titanium alloy Ti-6Al-4V
Properties Ti-6Al-4V
Ultimate tensile strength (MPa) 872
Yield strength (MPa) 968
Elongation (%) 5
The titanium alloy Ti-6Al-4V has higher elastic limit as it is visible from the yield strength value.
This necessarily means that it can handle higher loads before it deforms plastically. The load
bearing capacity of the alloy is higher hence there is no shape change in stress-strain curve of the
material for higher applied loads. Hence, this material is used for applications where high tensile
forces are applied. There is very little work hardening in the material. The curve droops down once
yielding starts which indicates the softening of material once plastic deformation begins. It can be
considered that the material is safe only till its elastic limit is reached. The applicability is not clear
for loads between yield strength and the UTS. Hence, it is beneficial to verify the properties after
machining or necessary heat treatments. As the loads in the human body are lesser than the elastic
limit of titanium alloy Ti-6Al-4V, it can be used effectively for making hip joint implants. Till the
elastic limit is reached there can be volume changes in the material but no shape changes since the
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Face Milling End Milling
Surf
ace
Ro
ugh
ne
ss µ
m
Trail 1
Trail 2
Trail 3
1988 Materials and Processes Technologies V
deviatoric component is zero till that limit. The loads on the material should be clearly analysed
before its application.
Material Removal Rate
The material removal rate for face milling operation was 8cm3/min and for end milling operation
was 7.7cm3/min. These material removal rates are low to machine a titanium alloy but it is effective
because the thickness of the femoral stem was 3mm. These material removal rates are good for a
component with small thickness. With the material removal rate of 8cm3/min and 7.7cm3/min, the
femoral stem was machined in 10min. In casting process to cast a femoral stem it takes 2 to 3 hrs.
After casting is done the casted femoral stem has to be machined to get a fine surface finish. The
time consumed to manufacture a femoral stem in casting is more as compared to machining process.
With the same parameters, more than 18 femoral stems can be machined in a five axis machine in 3
hrs.
Conclusion
As discussed in the above chapters the following conclusions are made,
1. The ultimate tensile strength and yield strength of the machined femoral stem are higher so that
they can withstand high tensile forces acting on them. Titanium alloy Ti-6Al-4V has high UTS
and yield strength as compared to other biomaterials like stainless steel, Co-Cr etc. Hence,
titanium alloy Ti-6Al-4V is the best choice of biomaterial for medical implants.
2. The surface roughness of the femoral stem decreases as the cutting speed is increased. The
surface roughness of the machined titanium alloy is low as compared to the casted titanium
alloy. The surface texture of the machined femoral stem is fine as that of a bone. It can
withstand high stresses from the adjacent bones.
3. The machined titanium alloy is harder than the normal titanium alloy. The hardness of the
titanium alloy Ti-6Al-4V is 29 HRC which is harder than the bone. It has the capacity to
withstand high stresses from the adjacent bones.
4. The obtained material removal rate is low for a machining process but it is effective to machine
a component with a thickness of 3mm.
5. The time taken to manufacture a femoral stem is short in machining process as compared to
casting process.
Future work
Due to the limited scope of this project and best use of the available resources, tests such as wear
resistance and corrosion resistance are planned under future work.
The values of the test will be taken and compared with the values of the femoral stem manufactured
by casting process. The other components of hip joint such as acetabular cup, the head of femoral
stem can be machined using five axis machines and the articular interface made out of plastic can
be assembled to make a hip joint.
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Materials and Processes Technologies V 10.4028/www.scientific.net/AMR.941-944 Machinability Assessment of Titanium Alloy Ti-6Al-4V for Biomedical Applications 10.4028/www.scientific.net/AMR.941-944.1985