10.1002==3527606580.ch49

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20 Effect of Test Temperature on Fatigue of Shot Peened Magnesium Alloys Jens Wendt, Andre Ketzmer and Lothar Wagner Chair of Physical Metallurgy and Materials Technology, Technical University of Brandenburg at Cottbus, Cott- bus, Germany 1 Abstract The effect of test temperature ranging from -25 to +50°C on mechanical properties of the high- strength wrought magnesium alloy AZ80 was evaluated. While yield stress and tensile strength within this temperature range continuously increased with decreasing temperature, the 10 7 cy- cles notch fatigue strength in fully reversed loading exhibited a marked minimum at T = 0 °C. Shot peening was found to improve the fatigue strength at all tested temperatures. 2 Introduction High-strength wrought magnesium alloys are considered for substituting steels and even alumi- num alloys as suspension components in future automobiles due to their high strength to weight ratio [1-3]. For this application, good HCF performance of notched components is one of the most important requirements. Since notched components respond particularly well to a shot peening treatment owing to the interaction of the notch root stress field with the process-indu- ced residual compressive stresses [4, 5], the effect of shot peening on the fatigue performance using a wide variation in Almen intensities was studied. In contrast to applications in transmis- sion gear housings and engine blocks, where mechanical properties at elevated (120-150 °C) temperatures are important, typical temperatures for suspension parts in automobiles are in the range -25 to +50 °C. 3 Experimental The wrought magnesium alloy AZ80 (nominal composition in weight percent: 8Al, 0.5Zn, 0.2 Mn, balance: Mg) was received as extrusion from Otto Fuchs Metallwerke, Mei-nerzhagen, Germany. The rectangular bar had a cross section of 110 x 70 mm (extrusion ratio ER: 9). Spe- cimens were machined with the load axis parallel to the extrusion direction (L). The microstructure of AZ80 is shown in Figure 1. The average a-grain size is about 30 ~m. A discontinuous precipitation of Mg 17Al12 is clearly seen by optical microscopy. Tensile tests were performed at various temperatures on threaded cylindrical specimens ha- ving gage lengths and diameters of 20 and 4 mm, respectively. The initial strain rate was 8.3 ·10-4 s-l. For fatigue testing, threaded circumferentially notched specimens (Fig. 2) with a geometrical notch factor of about k t = 3.4 were machined. After machining, roughly 200 ~m were removed

387 from the surface of the specimens by electrolytical polishing to ensure that any machining effect that could mask the results was absent. This electropolished condition (EP) is taken as reference to which shot peened conditions (SP) will be compared. 60 Figure 1: Microstructure of AZ80 Figure 2: Geometry of fatigue specimens Shot peening (SP) was performed with an inj ector type machine using spherically condi- tioned cut wire SCCW 14 (0.36 mm average shot size). Specimens were shot peened to full co- verage using Almen intensities from 0.18 to 0.55 mmN. During shot peening, the surface of the specimens close to the notch was masked by an adhesive tape to ensure that the notch factor was not affected by removal of material from these regions. The exact value of the geometrical notch factor was calculated for each individual specimen by measuring the net diameter (d n ), notch depth (t) and notch root radius (p) using the formula (1) for axial loading [6]: 1 k t == 1 +---;:::=================== 2.5 (1) d n 2p +0.11 (~~ +~ J(~ r 1 dn +- 0.10 2p ( :J O.55 + 1.6 fd:: ~~V-£;; After shot peening, the change in surface layer properties was determined by roughness mea- surements through profilometry, microhardness-depth profiles and measurements of residual stresses by means of the incremental hole drilling method as described in detail elsewhere [7]. Axial fatigue tests were performed in fully reserved loading (R = -1) using a resonance tester and frequencies of about 60 to 70 Hz. These tests were done in an environmental chamber at temperatures ranging from -25 to +50 °C.

388 4 Results and Discussion Tensile properties at the various temperatures are summarized in Table 1. As seen in Figure 3, yield stress 0-0.2 and tensile strength UTS continuously decrease as the temperature increases while both uniform strain cu and fracture strain El increase. Table 1: Tensile properties of AZ80 at various temperatures Temperature E 0-0.2 UTS eu EI [DC] [GPa] [MPa] [MPa] [%] [%] 100 35 220 320 12.3 17.0 50 46 240 330 7.8 9.3 25 43 245 335 8.9 10.1 0 45 255 340 8.8 9.1 -10 45 260 340 6.9 7.0 -25 49 280 360 7.2 7.3 400 20 375 350 15 ro 325 ;R 0.. 6 0 300 10 c CJ) "f§ CJ) ~ 275 U5 U5 250 5 225 200 0 -50 -25 0 25 50 75 100 125 Temperature rOC] Figure 3: Tensile properties of AZ80 vs. test temperature Shot peening markedly changes the surface topography as seen in Figure 4. With an increase in Almen intensity from 0.18 to 0.55 mmN, the surface roughness steadily increases. Owing to marked work-hardening in AZ80, the near-surface microhardness significantly increases during shot peening (Fig. 5). Increasing the Almen intensity from 0.18 to 0.55 mmN leads to greater depths of plastic deformation. Shot peening-induced residual stresses as deter- mined by the hole drilling method are illustrated in Figure 6. Shot peening to 0.18 mmN Almen intensity leads to maximum residual compressive stresses at the surface, while peening to 0.55 mmN results in a marked drop of near-surface stresses (Fig. 6). For HCF testing, specimens were shot peened only to the low Almen intensity of 0.18 mmN.

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389 EP SP 0.18 mmN SPO.33 mmN SP 0.48 mmN SP 0.55 mmN Figure 4: Surface topographies and roughness profiles after shot peening of AZ80 0 -10 co -20 a.. 6 -30 en -40 en ~ -50 (j) ro -60 ::::l "'0 -70 "Ci5 Q) -80 0:: -90 -100 0 100 200 300 400 500 600 600 500 400 300 100 200 160 ........ ------------------, 150 N o 140 1- \ I o > 130 I- · ·F · · · · · / ~ 120 I- ,,~ : ! ~ ; ~ : :..:.::.:..:.. :.: : · 1 ~ 110 1- 1, ""..c. · · · · · · · · · · 1 C "E 100 1- \"" ""'-""' · · · · · 1 ro e 90 t- ,""" "IIII"<!"' : · ·..,. · ·· 1 ~ 80 ~_..: .... ::=-======;;:E:2:::::::==::::::ii:~I:=::IIIL..-~ 70 .......... ~.~ .. !.~ ... b .. 9.rq.~.~.~.~ . 60'--'--.& ...... I-..a ....... L.. .......... ...L...L. ...&-.L~ ........ L.-.L..-1--I--.& ...... I-..a ....... L .... L.-I o Distance from surface [I-Im] Distance from surface [!J.m] Figure 5: Microhardness-depth profiles after shot peening Figure 6: Residual stress-depth profiles after shot peening The effect of test temperature on the HCF performance in fully reversed (R = -1) axial loa- ding of notched specimens of AZ80 is shown in Figure 7. For the electropolished (EP) reference condition (Fig. 7a), the S-N curves at temperatures of T = 50, 25 and -25°C are quite similar and can be characterized in the HCF regime by a com- mon scatterband. However, the RCF performance at the temperatures of T = 0 °C and T =-10 °C is clearly inferior to the other temperatures tested. For example, the 10 7 cycles fati- gue strength of notched AZ80 is only 50 MPa in terms of (Ja . k t at a test temperature of T= -10°C whereas it is 175 MPa at T= 50°C (Fig. 7a). After shot peening, the temperature dependent ranking of the HeF performance in AZ80 is quite the same (Fig. 7b). Again, the S-N curves at temperatures of T = 50°C and 25 °C are si- milar while there is a pronounced drop in HCF performance at lower temperatures of T = 0 °C and T= -10°C. From Figure 7, the 10 7 cycles fatigue strengths of the electropolished references and the shot peened conditions were taken and replotted in Figure 8 vs. test temperature. Within the range of temperatures studied, the fatigue strength improvement caused by shot peening is independent

390 of temperature and amounts to roughly 75 MPa in terms of a a . k t . This indicates that even at the highest tested temperature of T = 50°C, no significant thermal relaxation of the shot peening- induced residual compressive stresses is likely to occur. ro 400 I----f-f-+-++t+tt---ll--l-+-H+H-I--l---+--l-+++t+t--+-t-H+H EP ~ \ - ~ 350 ~-+-++++tH---+--lII'H+t1+t--+-+++-H+tt---+-+-+-++H+t---\ ~ ~. '\ ~ro 300 1----f-+-+-+H++1~~ ~~." ,Mi'. • • 0)- 250 ~-+-++++tH---.f---:rII~-1+.-"I'4--+-++-H+tt---+-+-+-++H+t----\ ~ 200 r---:'-----'----L~~+--+-1-Hi~ ""-; ~_\_l+t+_1_____H_.w.+++k__l ~ 150 ~ ~~:~ f-t-+++-I+ttt-'~_' ~-+-,,~.' "':H-!+' t++i· ......... "''"'t'. -~lIItd:-l;H1l1H1~-; co • 00C "Ii:: I;',., ~ ~ 1 00 f--t-+++-I+ttt--+-+-H-+tfTI.=-f'~twtll--;;::---I ~ ..... -10°C ' • .,.:-/" . C U5 50 T -25°C f-t-+++-I+ttt---+--t-+-+-++++t---f--l--I-++FHIt-r--j ro 400 1--- ~-+-t-++tttt----ll--t-t-t-fj SP, 0.18 mmN ~ \ 6 350 ~I--t-t-t+++-I+-l~~\ 'fII-t--m-++tH-_t_t-+++t+t+--+-+-++ttttt------t ~ ~~~ .. ro 300 ~1--t-t-t+++++__+-N-'ilI!\"'lIII+1IIH+-_t_t-l- b ~. ·~f 250 ~H-I+H+1+--+++--t-'M-~D--+-+ .a . ~, ~ ~ 200 r-'O'--'----'5---'---0'-'-'-OC-'-'------r--l-I--1--'4DI-++-f.P:!I~+J+~~H+f-1OlII:~~ ~ 150 II 25°C f--+--I-++++++t---e--+-I-' - _. ~ ~ • O°C ~ U5 100 T -10°C Cycles to failure, N F Cycles to failure, N F a) Condition EP b) Condition SP Figure 7: S-N curves in axial loading (R = -1) of notched (k t = 3.4) specimens of AZ80, effect of test temperature 250 ro 0... 6 200 ~ co b 150 cD -0 :E 100 C. E • co CJ) 50 CJ) • EP ~ U5 • SP, 0.18 mmN 0 -40 -20 0 20 40 60 Temperature [DC] Figure 8: Fatigue strengths (R = -1) of notched specimens of AZ80 vs. temperature Presumably, the marked minimum in 10 7 cycles fatigue strengths of both the electropolished and shot peened conditions at around T = 0 °C is due to environmental rather than mechanical aspects. The humidity of the lab air in the test chamber was found to be at a maximum at around T = 0 °C. Thus, corrosion fatigue may playa significant role in fatigue of AZ80 at temperatures around 0 °C. Further work is needed to understand the comparatively poor fatigue performance of magne- sium alloys at low temperatures.

391 5 Acknowledgements The authors gratefully acknowledge the support of this work by the Bundesministerium flir Wirtschaft (BMWi) under contract 138/99. Thanks are also due to Ms. G. Rodenbeck for car- rying out the residual stress measurements. 6 References [1] T. K. Aune and H. Westengen, Magnesium Alloys and their Applications (Eds.: B. L. Mordike and F. Hehmann) DGM, 1992,221. [2] H. Friedrich and S. Schumann, Magnesium 2000 (Eds.: E. Aghion and D. Eliezer) MRI, 2000,9. [3] G. L. Song and A. Atrens, Advanced Engineering Materials, Wiley-VCH (1999),11. [4] C. Gerdes and G. Llitjering, Shot Peening (Ed.: H. O. Fuchs), American Shot Peening Society, Paramus, 1984, 175. [5] L. Wagner, C. Gerdes and G. Llitjering, Titanium Science and Technology, DGM, 1985, 2147. [6] W. Beitz and K.-H. Grote, Handbook for Mechanical Engineering, Springer, 2001, E 103 (in German). [7] 1. Lindemann, D. Roth-Fagaraseanu and L. Wagner, Shot Peening (Ed.: L. Wagner), Wiley-VCH, 2002, in press.

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