170 surface treatment, computer methods and experimental ... · 170 surface treatment, computer...

Fatigue strength of light metals - influence of near

structure materials state

W. Zinn, B. Scholtes

Institute of Materials Technology, University Gh Kassel,

D-347W KasW, Germany

e-mail: [email protected]

Abstract

Mechanical surface treatments, such as shot peening or deep rolling, are well knownprocesses to improve the fatigue strength of metallic components. This is due to favourablemicrostructural alterations in relatively thin surface layers connected with the surfacetreatments applied as a consequence of near surface inhomogeneous plastic deformations.Typical examples are shown, demonstrating the fatigue strength increase of mechanicallysurface treated specimens. In the case of light weight materials, e. g. magnesium, aluminiumor titanium base alloys, process parameters have to be well adapted in individual cases toachieve optimum near surface materials states, taking the wide range of mechanical propertiesattainable with such materials as a result of their specific microstructures into account.

In this paper, a short survey of characteristic examples is given, demonstrating theinfluence of process parameters and microstructures on near surface materials propertiesresulting from mechanical surface treatments. Depth distributions of macro and microresidual stresses are analysed together with microstructural observations. An important pointfor the effectiveness of mechanical surface treatments is the stability of the near surfacematerials states during loading history. This aspect is treated for the case of fatigue loading.

1 Fatigue Strength of Mechanically Surface Treated Compo-

nents

There are many examples in practice, where mechanical surface treatments areused to improve the fatigue strength of components. Figs. 1 and 2 give cha-racteristic examples. Fig. 1 demonstrates, that shot peening of AlMg4.5Mn in-creases the fatigue strength by about 50% [1]. In the case of welded specimens(see Fig. 2) as a consequence of weld seam geometry and microstructure, onlysmall fatigue strength values are observed. An appropriate shot peening treat-ment however raises fatigue strength of welded specimens up to the resp. valueof the not peened base material.

Transactions on Engineering Sciences vol 17, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533

170 Surface Treatment, Computer Methods and Experimental Measurements

In all cases, the combined effects of near surface properties resulting fromthe mechanical surface treatments applied are responsable for the fatiguestrength values achieved. Maximum strength levels can only be realized, if opti-mum combinations of near surface properties are created by the surface treat-ment processes. Among others, the most important influencing parameterswhich have to be taken into account are near surface strength distributions(which may be specified by hardness distributions), the distributions of microand macro residual stresses (in most cases detected by X-ray diffractiontechniques) and also the surface topographies characteristic for the treatmentsapplied [2].

Figure 1: Comparisonbetween woehler cur-ves of shot peened(S330; 0,2mm A2;cov. 200%) anduntreated bendingfatigue specimens [1]

stress

ampl

itud

e (MPa)

-& -*

ro ro

aB

in

o i

n c

0

0 0

C

AIMg4,5Mn (5083)reversed bending R=-1d=6mm, P =50%

N

I shot peened

\v : : |; not peened

numbers of cycles

200 •

100 -

60 •

AIMg4,5Mn (5083)reversed bending Rd=6mm, P =50%

:^NUc

1\l; 1 N^

1 \ I I I \ I I I

\ I welded and shot peened ; ;

; j not welded, not peened; ;

Figure 2: Influence ofshot peening (S230;0,2mm A2; cov.200%) on the bendingfatigue strength ofwelded specimens incomparison with thebase material [1]

10 10 10 10number of cycles

Nominal or local strength concepts [3, 4] as well as the local fatigue strengthconcept [5] together with appropriate fracture mechanics concepts may be usedto predict qualitatively or quantitatively the resulting fatigue strengths or lifeti-mes resp. of the components under consideration. But this is successfully pos-sible only, if all relevant process parameters and their influence on materialsproperties in near surface layers are known.


Surface Treatment, Computer Methods and Experimental Measurements 171

2 Influence of Process Parameters on Near Surface Properties

In Fig. 3, near surface residual stress distributions of shot peened Mg-alloyAZ31 are plotted for different shot peening conditions. Amounts of compressiveresidual stress values achieved are only small and do not exceed 65% of thetensile yield strength of the material. Maximum values, which always occur wellbelow the surface, are observed for combined treatments with steel shots andglass beads. Such treatments are applied to avoid corrosion damage by steelimpurities remaining in the Mg-alloy surface. Depth distributions of interferenceline half-width values in the lower part of Fig. 3 show, that consequences ofshot peening treatments can be detected up to a surface distance of about 0.4mm. Near the surface, considerably higher values are measured than in the inte-rior.

-S170;8A;200%-S330;8A;200%- S330; 8A; 200% + GP100; 1 50%- S230: 8A; 200% + GP100; 1 50%

0,2 0,4distance from surface [mm]

Figure 3: Depthdistributions of resi-dual stresses and in-terference line half-width values in Mg-alloy AZ31 for theshot peening treat-ments indicated

•—S170; 8A; 200%•— S330;8A;200%k—S330;8A;200% +GP100: 1 5O%>—S230; 8A; 200% + GP100; 1 50%

0,2 0,4distance from surface [mm]

For deep rolled specimens, much thicker surface layers with compressiveresidual stresses can be achieved than in the case of shot peening (see Fig. 4). Inthe case presented here, a hydrostatic rolling device and cylindrical specimenswere used. Increasing the rolling pressure from 25 bar to 100 bar or 150 bar


1 72 Surface Treatment, Computer Methods and Experimental Measurements

resp. increased maximum amounts of residual stresses as well as layer thicknessaffected by the process. It is interesting to note, that for the highest rolling pres-sure applied compressive residual stresses immediately at the surface are onlysmall. Depth distributions of interference line half-width values (see lower partof Fig. 4) confirm, that with increasing rolling pressure, surface distances of upto approximately one mm revealed an increase of micro residual stresses due torolling induced plastic deformations.

Figure 4: Depth distri-butions of residualstresses and interfe-rence line half-widthvalues for cylindricalspecimens deep rolledwith the pressures indi-cated

0,4 0,8 1,2distance from surface [mm]

Also in the case of aluminium base alloys, the influence of process parameters ofmechanical surface treatments on the resulting distributions of strength and resi-dual stresses has been investigated. A typical example is shown in Fig. 5 [6].The same shot size S230, but different peening intensities and coverages wereused. As one can see, thickness of affected surface layer can be increased byincreasing peening intensity as well as coverage.



AIZn4,5Mg1—•— S230, 0,2mm A2, 200%—O—S230, 0,3mm A2, 200%—A— S230, 0,2mm A2, 400%—O—S230, 0,2mm A2, 200% + GPl 00

0.10 0,15 0,20 0,25distance from surface (mm]

Figure 5: Influence ofshot peening processparameters on resul-ting depth distributi-ons of residual stres-ses [6]

3 Influence of Material and Materials State on Near Surface

Properties

Of course, beside process and process parameters, the material treated and itsmicrostructural state is of importance for the resulting properties of near surface

0 •t

*oTQ_2 -100 .<D iat 150 •0)

2 <«> -250-

-300 -

"""0-o-o-o-t

\.A-AIMg3

b= 3=O *AIMg4,5M

» o 0—AJ99.5

-A—

^•\

-a —

~~V

S*

\

-o-t-o—

Af' —?~

shot peenedS230; 0,2mm A2; co+ GP100; 0,2mm N:

TT— P

oo v. >150%

Figure 6: Depth distribu-tions of residual stressesof strain hardenable andage hardenable Al-alloysfor the shot peeningtreatment indicated


shot peenedS230; 0,2mm A2; cov. 20O%+ GP100; 0,2mm N; cov. >150%

0,1 0,2 0,3 0,4distance from surface [mm]



ayers. For aluminium base alloys, typical examples are shown in Fig. 6. Strainhardenable as well as age hardenable alloys have been investigated.

Characteristic properties of the materials used are listed in Tab.l. In all ca-ses, the same shot peening process indicated in Fig. 6 had been applied. Cleardifferences between the measured depth distributions of residual stresses can bedetected.alloy

A199,5AlMg3AlMg4,5MnAlMgSilAlZn4,5MglAJCuMgl

RpO,2[MPal24207163291336305

Rm[MPa]100270317310389432

HV 5/30

33808193114132

A(%]

2814,520,48,513,623,5

remarks

cold rolledcold rolledrecrystallized

hot age-hardened, as del.hot age-hardened, as del.cold age-hardened, as del.

specif.

»F8«F24W28«F30F35«F40

Table 1: Mechanical properties and conditions

Except for AJ 99.5, always pronounced residual stress maxima are observedbelow the surface, but amounts of compressive residual stresses are different foreach material. A rough correlation between shot peening induced residual stres-ses and materials strenght exists, as can be seen in Fig. 7.

Figure 7: Correlations ob-served for the materialsinvestigated between hard-ness, yield strength andtensile strength (above)and residual stresses(below)

ofOB

0^"*(maxinr

(surface)

" Num) ^

to» t

/N/P..

/ny

io>

0>2in

\ N,

\\\|

\20 40 80 100 120 140

HV5/30



The correlation between hardness and yield strength or tensile strength resp.can be described by approximately parallel straight lines. As shown in the lowerpart of Fig. 7 also hardness roughly correlates with shot peening residual stres-ses at the surface and maximum values below the surface. A clear tendencyexists, that residual stress values increase with materials hardness. The fact thatvalues for AlMg4.5Mn are somewhat lower and those for AlMgSil higher thanthe general trend can be explained by the strain hardening state of both materi-als. AlMg4.5Mn is recrystallized with a small yield strength to tensile strengthratio whereas AlMgSil was investigated in a highly plastically deformed state.

Also depth distributions of interference line half-width values, which areshown in Fig. 8 strongly depend on the materials state treated. It is interestingto note, that shot peening induced changes of half-width values are e. g. in thiscase much more pronounced for AlCuMgl than for AlMgSil. An explanation isthat half-width values not only depend on density but also on distribution ofdislocations produced during the shot peening process. Characteristic examplesof near surface dislocation structures after shot peening operations are shown inFigs. 9 and 10.

Figure 8: Depth distri-butions of interferenceline half-width values ofdifferent Al-base alloysfor the shot peeningtreatment indicated

4 0 •

3 S-

3 Oi

2 5.

0

L

"***

"- .

0 0.

Ishot peenedS230; 0,2mm A2;+ GP100; 0,2mm

•~-~*_0 AIMg3

AIM g4 , 5Mn —Ai

"* £| QQ q I *

i2 0,4 0

cov. 200%N; cov. >150% "

g •

6 0,distance from surface (mm]

4 0 1

¥ '8*-o 35-

"5> 30-S53 25-

2.0-

~" \ rA

\ Z

shot peenedS230; 0,2mm A2; cov. 200%+ GP100; 0,2mm N; cov. >150%

AJMgSil-*

* AlCuMgl• A




After shot peening of AlMgl, a dislocation cell structure has developed near thesurface (see Fig. 9) whereas in the case of AJCuMgl a more or less randomdislocation structure can be seen with dislocation bundles around the precipita-tions.

Figure 9: Near surface (z = 0.12 mm) dislocation distribution in shot peenedAlMgl (SI70; 54-58 HRC; p = 0.24 bar; cov. 98%)

Figure 10: Near surface (z = 0.05 mm) dislocation distribution in shot peenedAlCuMgl (S230; 0,2mm A2; cov. 200% + GP100; 0,2mm N; cov.



To assess the consequences of residual stress distributions, introduced bymechanical surface treatments, knowledge about their stability during loadinghistory is of importance. A typical example for residual stress relaxation in alu-minium base alloys during fatigue is presented in Fig. 11 for the case of bendingfatigue of AlZn4.5Mgl. Relaxation of surface residual stresses is shown. Thespecimen side denoted ,,upper side" was loaded in tension during the first quar-ter of the first loading cycle. For the stress amplitudes indicated, which are rela-ted to the ultimate tensile strength of the materials state investigated, as ex-pected, stress relaxation is the stronger, the higher the stress amplitude appliedwas.

Figure 11: Stress re-laxation in bendingfatigue tests ofAlZn4,5Mgl

10 10 10 10 10number of cycles

Figure 12: Depth dis-tribution of residualstresses after fatiguetests of AJZn4,5Mgl

0,2 0,3 0,4 0,5distance from surface [mm]

A typical observation for aluminium base alloys is, that stress relaxation is themost pronounced during the first loading cycle. Then, stress values remain al-most constant or decrease only slightly. The relaxation behaviour of residualstresses immediately at the surface may be quite different compared to that ofsubsurface residual stresses. An example is shown in Fig. 12. As one can see,stresses near the maximum below the surface relax somewhat faster than at thesurface. To a first approximation surface residal stress relaxation can be correla-



ted with hardness or strength of the materials investigated, as shown in Fig. 13.Stress values measured before fracture or for 10^ loading cycles resp. are plot-ted as a function of hardness for different aluminium base alloys investigated,cyclically loaded with the stress amplitudes indicated. There exists a tendencythat residual stresses are the more stable, the higher materials hardness andhence strength is. For a better understanding of residual stress relaxation,knowledge about dislocation rearrangement in surface treated specimens in thecourse of fatigue tests are necessary.

Figure 13: Surfaceresidual stress relaxati-on versus hardness HV5/30

References1. Zinn, W. Untersuchungen zum Dauerschwingverhalten von Stumpf-

schweiflverbindungen aus den naturharten Aluminiwnlegierungen AlMgBundAlMg4,5Mn, DVS-Verlag, Dusseldorf, 1990.

2. Scholtes, B. Eigenspannungen in mechanisch randschichtverfestigtenWerkstoffzustanden, DGM Informationsgesellschaft Verlag, Oberursel,1990.

3. Radaj, D. Ermudungsfestigkeit, Springer-Verlag, Berlin and New-York,1995.

4. Zenner, H. Lebensdauerkonzepte, Beschreibung - Kritik - Entwicklungen,in Bauteillebensdauer, Nachweiskonzepte, DVM-Berichte 800, 1997.

5. Starker, P., Wohlfahrt H. & Macherauch, E. Surface Crack Initiationduring Fatigue as a Result of Residual Stresses, Fatigue of EngineeringMaterials and Structurs, 1979, 1, 3-9.

6. Zinn, W. & Scholtes, B. Generation and stability of shot peening residualstresses in different aluminium base alloys, Proceedings forth europeanconference on residual stresses, Cluny, 1996 (in press).

Shot peening was carried out by MIC, Unna, which is gratefully acknowledged.


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