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Shot peening on the high-strength wrought magnesium alloy AZ80—Effect of peening media
Journal of Materials Processing Technology 210 (2010) 445–450
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Shot peening on the high-strength wrought magnesium alloy AZ80—Effect of peening media P. Zhang ∗ , J. Lindemann, C. Leyens Lehrstuhl Metallkunde und Werkstofftechnik, BTU-Cottbus, Postfach 101344, Konrad-Wachsmann-Allee 17, 03046 Cottbus, Germany
a r t i c l e
i n f o
Article history: Received 6 April 2009 Received in revised form 6 October 2009 Accepted 11 October 2009
Keywords: Shot peening High cycle fatigue (HCF) Wrought magnesium alloy Peening media Overpeening effect
a b s t r a c t Influence of shot peening media on fatigue performance of the high-strength wrought magnesium alloy AZ80 has been investigated at Almen intensities ranging from 0.04 to 0.4 mmN. By the use of different peening media (including glass beads, Zirblast B30 and Ce-ZrO2 (ZrO2 beads stabilized by Ce)), the improvement of about 60–75% in fatigue strength was achieved at optimum conditions. Peening AZ80 with Ce-ZrO2 shots resulted in the fewest surface defects, lowest roughness, highest maximum compressive residual stress and highest improvement of fatigue strength. A pronounced overpeening effect was observed when glass beads or Zirblast B30 shots were used as peening media. In contrast, specimens shot-peened by Ce-ZrO2 shots exhibited an extremely broad process window, i.e. the overpeening effect disappeared. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Light-weight magnesium alloys exhibit great potential in automobile for weight saving. Currently, one of the most extreme challenges faced by high-strength wrought magnesium alloys is to replace aluminum alloys as suspension components in automobiles (Becker et al., 2005). However, insufficient fatigue properties, such as lower fatigue strengths compared to Al alloys and high environmental sensitivity, limited the applications of magnesium alloys. Wagner and co-workers studied the fatigue properties of wrought magnesium alloys AZ80, AZ31, and Al alloys 2024 Al (Dörr et al., 1999) and 6082 Al (Hilpert and Wagner, 2000), they reported that fatigue strengths of the magnesium alloys were much lower than those of their counterparts. Eliezer et al. (2001) investigated the corrosion fatigue behaviors of die-cast and extruded AZ91D, AM50 and AZ31 magnesium alloys, they demonstrated that fatigue life of magnesium alloys significantly decreased in corrosive environment (3.5% NaCl). Therefore, it is of particular importance to improve fatigue performance of magnesium alloys. Mechanical surface treatments such as shot peening (SP), roller burnishing (RB) and deep rolling (DR) are widely used to enhance fatigue life of structural metallic materials. In the previous work, SP with glass beads (Zhang and Lindemann, 2005a) and RB (Zhang and Lindemann, 2005b) were utilized to improve fatigue performance of the high-strength magnesium alloy AZ80. Consequently,
∗ Corresponding author. Tel.: +49 355 694383; fax: +49 355 692709. E-mail address: [email protected] (P. Zhang). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.10.005
the improvements of about 60 and 110% in fatigue strength were achieved by SP with glass beads and RB, respectively. Although RB is more effective in improving the fatigue properties than SP, it is difficult to be applied on components with irregular geometry. In contrast, SP is of particular interest due to the high productivity and low cost. However, the available data obtained in the previous work (Zhang and Lindemann, 2005a) and in literature (Wagner, 1999) showed that SP of the high-strength wrought magnesium alloy AZ80 induced a pronounced overpeening effect, resulting in a narrow process window for SP. The overpeening effect significantly restricts the application of SP in magnesium industry. In the present work, in-depth investigations on SP of the highstrength wrought magnesium alloy AZ80 were performed. SP was conducted with different SP media in order to obtain the influence of peening media on fatigue performance of AZ80. Especially, attempts were made to elucidate the overpeening effect of magnesium alloys.
2. Experimental The high-strength wrought magnesium alloy AZ80 (nominal composition in wt.%: 8Al, 0.5Zn, 0.2Mn, balance: Mg) was used in this work. After forging, the alloy exhibits a single ␣-phase structure with an average grain size of about 30 m. The microstructure and texture of the magnesium alloy were described in details in the previous paper (Zhang and Lindemann, 2005a). The mechanical properties of AZ80 were summarized in Table 1.
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Table 1 Tensile properties of AZ80. Alloy
E (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
AZ80
45
226
337
18.2
Table 2 Parameters of peening media. Peening medium
Composition (wt.%)
Diameter (m)
Hardness
Density (g cm−3 )
Zirblast B30 Ce-ZrO2 Glass beads
67% ZrO2 , 31% SiO2 , 1% Al2 O3 , <0.1% Fe2 O3 80–90% ZrO2 + 10–20% CeO2 70% SiO2 , 10% CaO, 15% Na2 O + K2 O, 5% MgO
425–600 600–800 300–400
650–800 (HV1) 1224 (HV3) 47 (HRC)
3.8 6 2.5
For fatigue testing, hour-glass shaped round specimens (6 mm gauge diameter) were used. After machining, a layer with a thickness of about 100 m was removed from the surface of specimens by electrolytical polishing (EP) in order to avoid any influence of machining on the fatigue results. Fatigue tests were performed under rotating beam loading (R = −1) at a frequency of about 100 Hz in air.
SP was performed with an injector type machine using different SP media including Zirblast B30 and Ce-ZrO2 (ZrO2 spherical particles stabilized with Ce). In the present study, steel shots were abandoned since Mueller and Robriguey (2003) reported that steel shots induced the contamination of iron and thus destroyed the corrosion resistance of magnesium alloys. Details of the peening media are listed in Table 2. To determine the best HCF response,
Fig. 1. Surface topographies of AZ80 after SP: (a) B30, 0.15 mmN, (b) B30, 0.40 mmN, (c) Ce-ZrO2 , 0.10 mmN, (d) Ce-ZrO2 , 0.40 mmN, (e) glass, 0.15 mmN and (f) glass, 0.40 mmN.
P. Zhang et al. / Journal of Materials Processing Technology 210 (2010) 445–450
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Fig. 3. Microhardness–depth profile of AZ80 after SP.
Fig. 2. Surface roughness (maximum height Ry, arithmetical mean roughness Ra) profile of AZ80 after SP: (a) Ry and (b) Ra.
specimens were shot-peened by Zirblast B30 and Ce-ZrO2 to full coverage using Almen intensities in the range of 0.04–0.4 mmN. The surface properties of specimens after SP were determined by roughness measurements through profilometry, measurements of the microhardness–depth profiles and residual stress measurements. The incremental hole drilling method was used to measure the residual stress distribution induced by the shot peening (Lu and Flavenot, 1987). This technique cannot measure the full residual stress distribution since it cannot take into account the redistribution of residual stress during the incremental drilling process. However, the information is interesting for estimating the compressive residual stress on the surface and the approximate depth of compression layer.
roughness increases significantly with increasing Almen intensity, indicating that the surface damage becomes severely when Almen intensity increases. At a given Almen intensity the Ce-ZrO2 shotpeened specimens result in the smallest increase of roughness. This result is consistent with the SEM observation of specimen surface (see Fig. 1), peening AZ80 by using Ce-ZrO2 introduces the fewest defects on specimen surface among the three peening media. Fig. 3 illustrates the microhardness–depth profiles after SP. Since SP induces plastic deformation, there is a pronounced increase in microhardness in the near-surface region. The thickness of the plastic deformation layers can roughly be estimated by the changes in microhardness. The thickness is in the range of 100–200 m. Moreover, increasing Almen intensity leads to an increase in the depth of plastic deformation. The residual stress distribution in AZ80 after SP is demonstrated in Fig. 4. It can be seen that SP induced compressive residual stresses. Peening AZ80 with Ce-ZrO2 results in the largest maximum compressive residual stress, while those induce by SP with glass or B30 are comparable at a given intensity. In addition, the maximum compressive residual stress of the specimen shotpeened by Ce-ZrO2 at 0.10 mmN was located on the specimen surface. At a high Almen intensity of 0.3 mmN, however, the maximum compressive residual stress moved towards the interior of specimens. Since surface defects such as microcracks are very few at low Almen intensities, the residual stress may not be relaxed by the surface defects. With increasing Almen intensity, the number
3. Experimental results The surface topography of the high-strength wrought magnesium alloy AZ80 after SP is shown in Fig. 1. Surface damages induced by SP increase distinctly with Almen intensity. As the intensity is higher than 0.20 mmN, peening AZ80 with Zirblast B30 results in severe surface defects such as overlaps and microcracks, consistent with the previous observation obtained in the shot-peened AZ80 with glass beads (Zhang and Lindemann, 2005a). In contrast, peening AZ80 with Ce-ZrO2 yields a high quality surface, severe surface defects are hardly observed even at the highest Almen intensity of 0.40 mmN (Fig. 1(d)). The quantity results given by surface roughness measurements (maximum height of the profile (Ry) and arithmetical mean roughness (Ra)) are shown in Fig. 2. The
Fig. 4. Residual stress–depth profile after SP.
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2 orders of magnitude at Almen intensities ranging from 0.10 to 0.4 mmN. S–N curves for AZ80 alloy after SP at the optimum conditions for different SP media are illustrated in Fig. 5(b). The fatigue strength (107 cycles) increases from 100 to 160 and 175 MPa after SP by using Zirblast B30 and Ce-ZrO2 , respectively. The corresponding improvements in fatigue strength are about 60 and 75%. With regard to the improvement in fatigue life, peening AZ80 with Ce-ZrO2 is more effective compared to glass shots (Zhang and Lindemann, 2005a) or Zirblast B30. As reported previously (Zhang and Lindemann, 2005a), all fatigue cracks in the reference specimens (EP) nucleated at the surface, while fatigue cracks for AZ80 shot-peened with glass beads under optimum condition initiated below the surface. Similar to SP with glass beads, SP of AZ80 with B30 and Ce-ZrO2 under optimum conditions also resulted in the subsurface fatigue crack nucleation (See Fig. 6(a) and (c)). With increasing Almen intensity from 0.15 to 0.40 mmN, however, the fatigue crack nucleation site in AZ80 peened with B30 shifted from subsurface to the surface (Fig. 6(b)). It can be clearly seen that the severe surface damages induced by heavier SP acted as the nucleation site of the fatigue cracks. In contrast, peening with Ce-ZrO2 always resulted in subsurface fatigue crack nucleation, independent of Almen intensity (Fig. 6(c) and (d)).
4. Discussion
Fig. 5. Fatigue response of AZ80 after SP: (a) fatigue life vs. Almen intensity and (b) S–N curves after optimum SP.
of surface defects increased significantly (see Fig. 1). Therefore, the residual stress may partially be relaxed by the surface defects, and the maximum residual stress shifted from surface to the interior of specimens. The changes in surface layer properties such as the surface roughness, thickness of deformation layer and maximum compressive residual stress after SP with different peening media were summarized in Table 3. The effect of Almen intensity on fatigue life at the stress amplitudes of 175 MPa is shown in Fig. 5(a). An evident overpeening effect was found in the specimens shot-peened by Zirblast B30. The highest life improvements of roughly 2 orders of magnitude were observed after peening with the intermediate Almen intensity of 0.15 mmN. These results are consistent with the previous observations in AZ80 shot-peened with glass beads (Zhang and Lindemann, 2005a). In contrast, peening AZ80 with Ce-ZrO2 leads to a rather wide process window, the overpeening effect disappears. Run-outs (107 cycles) are found already at intermediate Almen intensities of about 0.08–0.10 mmN. The fatigue life is improved by roughly
The present results show that the peening media have a significant influence on the surface layer properties and subsequent fatigue response of AZ80. Peening AZ80 with Ce-ZrO2 shots results in the lowest surface roughness and the highest maximum compressive residual stress compared to those peened by glass beads and Zirblast B30. Consequently, the specimens peened by Ce-ZrO2 shots achieve the highest improvement of about 75% in fatigue strength at the optimum condition. In contrast, the improvement of fatigue strength by glass and B30 shots is about 60%. In addition, specimens peened by glass and B30 shots exhibit a pronounced overpeening effect, i.e. there is a dramatic drop in fatigue life at high Almen intensities (see Fig. 5(a)). On the contrary, SP with Ce-ZrO2 leads to an additional advantage, i.e. the process window becomes rather broad, the overpeening effect disappears. The different response in the characteristics of surface layer and fatigue properties of AZ80 induced by peening media may be attributed to the different properties of the peening media. It is known that energy transfers from shots to the target material during the impact process of SP. The energy of shots is a function of shot size, density and velocity. Since intensity is a measure of strain energy induced, shots with small size and low density have to travel at significantly higher velocities than shots with larger size and higher density in order to achieve the same intensity. For a given Almen intensity, the velocities of glass beads and B30 shots are higher since these shots present a lower density and a smaller size than Ce-ZrO2 shots. The correspondent strain rates during SP with glass beads and B30 should be higher than that by using CeZrO2 . Due to the limited deformability of the magnesium alloy, the increase in strain rate would induce more severe surface damages (Fig. 1). In addition, the compressive residual stresses in specimens
Table 3 Surface layer properties after SP. Peening medium
Almen intensity (mmN)
Roughness Ry (m)
Thickness of deformation layer (m)
Maximum Compressive residual stress (MPa)
Zirblast B30
0.15
17
125–175
75
CeZrO2
0.10 0.30
5 8
125–175 200–250
100 116
Glass
0.15
10
100–150
78
P. Zhang et al. / Journal of Materials Processing Technology 210 (2010) 445–450
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Fig. 6. Fatigue crack nucleation sites of AZ80 (SEM): (a) B30, 0.15 mmN, (b) B30, 0.40 mmN, (c) Ce-ZrO2 , 0.10 mmN and (d) Ce-ZrO2 , 0.40 mmN (arrow indicates crack nucleation site).
peened by glass and B30 are also smaller than those in Ce-ZrO2 peened specimens (see Fig. 4). In fact, the crack initiation mechanism for a shot-peened material is dependent on the competition of the benefic effect of compressive residual stresses and the negative effect of surface defects (Hutmann, 2003). The compressive residual stresses are quite low for the B30 and glass-peened specimens, while the negative effect of surface defects generated by SP was more favorable by using B30 and glass shots. As the microcracks reached the critical size during heavier SP, they may act as the crack initiation source and propagate in the subsequent fatigue tests (see Fig. 6(b)). These critical microcracks defeat the beneficial effects induced by SP, eventually leading to the overpeening effect, i.e. a pronounced drop in fatigue life of AZ80 at higher Almen intensities (see Fig. 5(a)). Compared to glass and B30, peening AZ80 with Ce-ZrO2 results in the fewest surface defects, lowest surface roughness and highest maximum compressive residual stress. Hence, when AZ80 is peened by Ce-ZrO2 , the benefic effect of compressive residual stresses on the surface overwhelms the negative effect of surface defects. As a result, the overpeening effect disappeared in specimens peened by using Ce-ZrO2 . Significant improvements in fatigue life can also be achieved at high Almen intensities (see Fig. 5(a)). The process window of SP becomes much broader. The effect of SP media on fatigue life of AZ80 was also investigated by Wagner and co-workers (Dörr et al., 1999). In those studies, different SP media including SCCW14, glass beads, S330, SCCWS23 were utilized. The high-strength wrought magnesium alloy AZ80 presented a pronounced overpeening effect and an extremely narrow process window at a small optimum intensity of about 0.05 mmN, irrespective of the particular peening medium (see Fig. 5(a)). Obviously, the present results are superior with regard to the overpeening effect. The difference between the present work and the results reported by Wagner and coworkers (Hilpert and Wagner, 2000) is probably caused by the
different microstructure and ductility of target materials. The wrought magnesium alloy AZ80 used in the present work exhibits a single ␣-structure with a high elongation of 18% (Zhang and Lindemann, 2005a). While the wrought magnesium alloy used by Wagner et al. (Hilpert and Wagner, 2000) consists of an ␣- and -phase mixed structure, the ductility is much lower than that of the present alloy. Since the estimated strain rate during SP is roughly in the range of 103 to 104 s−1 (Altenberger, 2003), the brittle -phase may be broken even at a low intensity, regardless of the particular peening media. The broken -phase possibly acts as the source of fatigue crack and propagates during fatigue, leading to the overpeening effect of AZ80. Hence, SP media have little effect on fatigue performance of AZ80 in reference Wagner (1999). 5. Summary In the present work, the influence of peening media including glass bead, Zirblast B30 and Ce-ZrO2 on high cycle fatigue properties of AZ80 has been investigated. The results showed that SP media had a significant influence on the surface layer properties and subsequent fatigue response of AZ80. Peening AZ80 with CeZrO2 introduced the fewest surface defects, smallest roughness and highest compressive residual stresses. Regardless of different peening media, SP improved the fatigue strength of the high-strength magnesium alloy AZ80. Peening AZ80 with Ce-ZrO2 achieved the highest improvement of about 75% in fatigue strength at the optimum condition. Additionally, peening AZ80 with Ce-ZrO2 resulted in an extremely broad process window, i.e. the overpeening effect disappeared. The benefic effect of compressive residual stresses on the surface overwhelmed the negative effect of surface defects, leading to the absence of overpeeing effect. Such a broad process window makes SP applicable for practical magnesium applications.
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Acknowledgements This work is sponsored by BMBF and Shanghai Pujiang Program (Grant No. 06PJ14062). The authors would like to thank Otto Fuchs Metallwerke, Meinerzhagen, Germany, for providing the magnesium alloy, and OSK Kiefer GmbH, Oppurg, Germany, for shot peening. One of the authors is grateful to DAAD for granting a scholarship to him (PZ). References Altenberger, I., 2003. Alternative mechanical surface treatments: microstructures, residual stresses & fatigue behavior. In: Wagner, L. (Ed.), Shot Peening. WileyVCH, Weinheim, pp. 421–434. Becker, J., Bogon, P., Gers, H., Kiefer, A., Leyens, C., Newiak, F., Roll, K., Straube, O., Viehweger, B., 2005. Magnesium—Knetlegierungen fur den Automobilbau. ATZ – Automobiltechnische Zeitschrift 107, 922–929 (in German). Dörr, T., Hilpert, M., Beckmerhagen, P., Kiefer, A., Wagner, L., 1999. Influence of shot peening on fatigue performance of high-strength aluminum- and magnesium alloys. In: Nakonieczny, A. (Ed.), Shot Peening. IMP, Poland, Warsaw, pp. 153–160.
Eliezer, A., Gutman, E.M., Abramov, E., Unigovski, Y., 2001. Corrosion fatigue of diecast and extruded magnesium alloys. J. Light Met. 1, 179–186. Hilpert, M., Wagner, L., 2000. Response of light alloys to mechanical surface treatments: comparison of magnesium and aluminum alloys. In: Kainer, K.U. (Ed.), Magnesium Alloys and their Application. Wiley-VCH, Weinheim, p. 525. Hutmann, P., 2003. The application of mechanical surface treatment in the passenger car industry. In: Wagner, L. (Ed.), Shot Peening. Wiley-VCH, Weinheim, pp. 3–12. Lu, J., Flavenot, J.F., 1987. Application of the incremental hole drilling method for the measurement of residual stress distribution in shot-peened components. In: Wohlfahrt, H., Kopp, R., Vöhringer, O. (Eds.), Proceeding of 3rd International Conference on Shot Peening. Garmisch-Partenkirchen, Germany, pp. 279– 286. Mueller, C., Robriguey, R., 2003. Influence of shot peening on the fatigue and corrosion behavior of the die cast magnesium alloy AZ91 hp. In: Wagner, L. (Ed.), Shot Peening. Wiley-VCH, Weinheim, pp. 271–277. Wagner, L., 1999. Mechanical surface treatments on titanium, aluminum and magnesium alloys. Mater. Sci. Eng. A 263, 210–216. Zhang, P., Lindemann, J., 2005a. Influence of shot peening on high cycle fatigue properties of the high-strength wrought magnesium alloy AZ80. Scripta Mater. 52, 485–490. Zhang, P., Lindemann, J., 2005b. Effect of roller burnishing on the high cycle fatigue performance of the high-strength wrought magnesium alloy AZ80. Scripta Mater. 52, 1011–1015.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Shot peening on the high-strength wrought magnesium alloy AZ80—Effect of peening media P. Zhang ∗ , J. Lindemann, C. Leyens Lehrstuhl Metallkunde und Werkstofftechnik, BTU-Cottbus, Postfach 101344, Konrad-Wachsmann-Allee 17, 03046 Cottbus, Germany
a r t i c l e
i n f o
Article history: Received 6 April 2009 Received in revised form 6 October 2009 Accepted 11 October 2009
Keywords: Shot peening High cycle fatigue (HCF) Wrought magnesium alloy Peening media Overpeening effect
a b s t r a c t Influence of shot peening media on fatigue performance of the high-strength wrought magnesium alloy AZ80 has been investigated at Almen intensities ranging from 0.04 to 0.4 mmN. By the use of different peening media (including glass beads, Zirblast B30 and Ce-ZrO2 (ZrO2 beads stabilized by Ce)), the improvement of about 60–75% in fatigue strength was achieved at optimum conditions. Peening AZ80 with Ce-ZrO2 shots resulted in the fewest surface defects, lowest roughness, highest maximum compressive residual stress and highest improvement of fatigue strength. A pronounced overpeening effect was observed when glass beads or Zirblast B30 shots were used as peening media. In contrast, specimens shot-peened by Ce-ZrO2 shots exhibited an extremely broad process window, i.e. the overpeening effect disappeared. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Light-weight magnesium alloys exhibit great potential in automobile for weight saving. Currently, one of the most extreme challenges faced by high-strength wrought magnesium alloys is to replace aluminum alloys as suspension components in automobiles (Becker et al., 2005). However, insufficient fatigue properties, such as lower fatigue strengths compared to Al alloys and high environmental sensitivity, limited the applications of magnesium alloys. Wagner and co-workers studied the fatigue properties of wrought magnesium alloys AZ80, AZ31, and Al alloys 2024 Al (Dörr et al., 1999) and 6082 Al (Hilpert and Wagner, 2000), they reported that fatigue strengths of the magnesium alloys were much lower than those of their counterparts. Eliezer et al. (2001) investigated the corrosion fatigue behaviors of die-cast and extruded AZ91D, AM50 and AZ31 magnesium alloys, they demonstrated that fatigue life of magnesium alloys significantly decreased in corrosive environment (3.5% NaCl). Therefore, it is of particular importance to improve fatigue performance of magnesium alloys. Mechanical surface treatments such as shot peening (SP), roller burnishing (RB) and deep rolling (DR) are widely used to enhance fatigue life of structural metallic materials. In the previous work, SP with glass beads (Zhang and Lindemann, 2005a) and RB (Zhang and Lindemann, 2005b) were utilized to improve fatigue performance of the high-strength magnesium alloy AZ80. Consequently,
∗ Corresponding author. Tel.: +49 355 694383; fax: +49 355 692709. E-mail address: [email protected] (P. Zhang). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.10.005
the improvements of about 60 and 110% in fatigue strength were achieved by SP with glass beads and RB, respectively. Although RB is more effective in improving the fatigue properties than SP, it is difficult to be applied on components with irregular geometry. In contrast, SP is of particular interest due to the high productivity and low cost. However, the available data obtained in the previous work (Zhang and Lindemann, 2005a) and in literature (Wagner, 1999) showed that SP of the high-strength wrought magnesium alloy AZ80 induced a pronounced overpeening effect, resulting in a narrow process window for SP. The overpeening effect significantly restricts the application of SP in magnesium industry. In the present work, in-depth investigations on SP of the highstrength wrought magnesium alloy AZ80 were performed. SP was conducted with different SP media in order to obtain the influence of peening media on fatigue performance of AZ80. Especially, attempts were made to elucidate the overpeening effect of magnesium alloys.
2. Experimental The high-strength wrought magnesium alloy AZ80 (nominal composition in wt.%: 8Al, 0.5Zn, 0.2Mn, balance: Mg) was used in this work. After forging, the alloy exhibits a single ␣-phase structure with an average grain size of about 30 m. The microstructure and texture of the magnesium alloy were described in details in the previous paper (Zhang and Lindemann, 2005a). The mechanical properties of AZ80 were summarized in Table 1.
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Table 1 Tensile properties of AZ80. Alloy
E (GPa)
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
AZ80
45
226
337
18.2
Table 2 Parameters of peening media. Peening medium
Composition (wt.%)
Diameter (m)
Hardness
Density (g cm−3 )
Zirblast B30 Ce-ZrO2 Glass beads
67% ZrO2 , 31% SiO2 , 1% Al2 O3 , <0.1% Fe2 O3 80–90% ZrO2 + 10–20% CeO2 70% SiO2 , 10% CaO, 15% Na2 O + K2 O, 5% MgO
425–600 600–800 300–400
650–800 (HV1) 1224 (HV3) 47 (HRC)
3.8 6 2.5
For fatigue testing, hour-glass shaped round specimens (6 mm gauge diameter) were used. After machining, a layer with a thickness of about 100 m was removed from the surface of specimens by electrolytical polishing (EP) in order to avoid any influence of machining on the fatigue results. Fatigue tests were performed under rotating beam loading (R = −1) at a frequency of about 100 Hz in air.
SP was performed with an injector type machine using different SP media including Zirblast B30 and Ce-ZrO2 (ZrO2 spherical particles stabilized with Ce). In the present study, steel shots were abandoned since Mueller and Robriguey (2003) reported that steel shots induced the contamination of iron and thus destroyed the corrosion resistance of magnesium alloys. Details of the peening media are listed in Table 2. To determine the best HCF response,
Fig. 1. Surface topographies of AZ80 after SP: (a) B30, 0.15 mmN, (b) B30, 0.40 mmN, (c) Ce-ZrO2 , 0.10 mmN, (d) Ce-ZrO2 , 0.40 mmN, (e) glass, 0.15 mmN and (f) glass, 0.40 mmN.
P. Zhang et al. / Journal of Materials Processing Technology 210 (2010) 445–450
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Fig. 3. Microhardness–depth profile of AZ80 after SP.
Fig. 2. Surface roughness (maximum height Ry, arithmetical mean roughness Ra) profile of AZ80 after SP: (a) Ry and (b) Ra.
specimens were shot-peened by Zirblast B30 and Ce-ZrO2 to full coverage using Almen intensities in the range of 0.04–0.4 mmN. The surface properties of specimens after SP were determined by roughness measurements through profilometry, measurements of the microhardness–depth profiles and residual stress measurements. The incremental hole drilling method was used to measure the residual stress distribution induced by the shot peening (Lu and Flavenot, 1987). This technique cannot measure the full residual stress distribution since it cannot take into account the redistribution of residual stress during the incremental drilling process. However, the information is interesting for estimating the compressive residual stress on the surface and the approximate depth of compression layer.
roughness increases significantly with increasing Almen intensity, indicating that the surface damage becomes severely when Almen intensity increases. At a given Almen intensity the Ce-ZrO2 shotpeened specimens result in the smallest increase of roughness. This result is consistent with the SEM observation of specimen surface (see Fig. 1), peening AZ80 by using Ce-ZrO2 introduces the fewest defects on specimen surface among the three peening media. Fig. 3 illustrates the microhardness–depth profiles after SP. Since SP induces plastic deformation, there is a pronounced increase in microhardness in the near-surface region. The thickness of the plastic deformation layers can roughly be estimated by the changes in microhardness. The thickness is in the range of 100–200 m. Moreover, increasing Almen intensity leads to an increase in the depth of plastic deformation. The residual stress distribution in AZ80 after SP is demonstrated in Fig. 4. It can be seen that SP induced compressive residual stresses. Peening AZ80 with Ce-ZrO2 results in the largest maximum compressive residual stress, while those induce by SP with glass or B30 are comparable at a given intensity. In addition, the maximum compressive residual stress of the specimen shotpeened by Ce-ZrO2 at 0.10 mmN was located on the specimen surface. At a high Almen intensity of 0.3 mmN, however, the maximum compressive residual stress moved towards the interior of specimens. Since surface defects such as microcracks are very few at low Almen intensities, the residual stress may not be relaxed by the surface defects. With increasing Almen intensity, the number
3. Experimental results The surface topography of the high-strength wrought magnesium alloy AZ80 after SP is shown in Fig. 1. Surface damages induced by SP increase distinctly with Almen intensity. As the intensity is higher than 0.20 mmN, peening AZ80 with Zirblast B30 results in severe surface defects such as overlaps and microcracks, consistent with the previous observation obtained in the shot-peened AZ80 with glass beads (Zhang and Lindemann, 2005a). In contrast, peening AZ80 with Ce-ZrO2 yields a high quality surface, severe surface defects are hardly observed even at the highest Almen intensity of 0.40 mmN (Fig. 1(d)). The quantity results given by surface roughness measurements (maximum height of the profile (Ry) and arithmetical mean roughness (Ra)) are shown in Fig. 2. The
Fig. 4. Residual stress–depth profile after SP.
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2 orders of magnitude at Almen intensities ranging from 0.10 to 0.4 mmN. S–N curves for AZ80 alloy after SP at the optimum conditions for different SP media are illustrated in Fig. 5(b). The fatigue strength (107 cycles) increases from 100 to 160 and 175 MPa after SP by using Zirblast B30 and Ce-ZrO2 , respectively. The corresponding improvements in fatigue strength are about 60 and 75%. With regard to the improvement in fatigue life, peening AZ80 with Ce-ZrO2 is more effective compared to glass shots (Zhang and Lindemann, 2005a) or Zirblast B30. As reported previously (Zhang and Lindemann, 2005a), all fatigue cracks in the reference specimens (EP) nucleated at the surface, while fatigue cracks for AZ80 shot-peened with glass beads under optimum condition initiated below the surface. Similar to SP with glass beads, SP of AZ80 with B30 and Ce-ZrO2 under optimum conditions also resulted in the subsurface fatigue crack nucleation (See Fig. 6(a) and (c)). With increasing Almen intensity from 0.15 to 0.40 mmN, however, the fatigue crack nucleation site in AZ80 peened with B30 shifted from subsurface to the surface (Fig. 6(b)). It can be clearly seen that the severe surface damages induced by heavier SP acted as the nucleation site of the fatigue cracks. In contrast, peening with Ce-ZrO2 always resulted in subsurface fatigue crack nucleation, independent of Almen intensity (Fig. 6(c) and (d)).
4. Discussion
Fig. 5. Fatigue response of AZ80 after SP: (a) fatigue life vs. Almen intensity and (b) S–N curves after optimum SP.
of surface defects increased significantly (see Fig. 1). Therefore, the residual stress may partially be relaxed by the surface defects, and the maximum residual stress shifted from surface to the interior of specimens. The changes in surface layer properties such as the surface roughness, thickness of deformation layer and maximum compressive residual stress after SP with different peening media were summarized in Table 3. The effect of Almen intensity on fatigue life at the stress amplitudes of 175 MPa is shown in Fig. 5(a). An evident overpeening effect was found in the specimens shot-peened by Zirblast B30. The highest life improvements of roughly 2 orders of magnitude were observed after peening with the intermediate Almen intensity of 0.15 mmN. These results are consistent with the previous observations in AZ80 shot-peened with glass beads (Zhang and Lindemann, 2005a). In contrast, peening AZ80 with Ce-ZrO2 leads to a rather wide process window, the overpeening effect disappears. Run-outs (107 cycles) are found already at intermediate Almen intensities of about 0.08–0.10 mmN. The fatigue life is improved by roughly
The present results show that the peening media have a significant influence on the surface layer properties and subsequent fatigue response of AZ80. Peening AZ80 with Ce-ZrO2 shots results in the lowest surface roughness and the highest maximum compressive residual stress compared to those peened by glass beads and Zirblast B30. Consequently, the specimens peened by Ce-ZrO2 shots achieve the highest improvement of about 75% in fatigue strength at the optimum condition. In contrast, the improvement of fatigue strength by glass and B30 shots is about 60%. In addition, specimens peened by glass and B30 shots exhibit a pronounced overpeening effect, i.e. there is a dramatic drop in fatigue life at high Almen intensities (see Fig. 5(a)). On the contrary, SP with Ce-ZrO2 leads to an additional advantage, i.e. the process window becomes rather broad, the overpeening effect disappears. The different response in the characteristics of surface layer and fatigue properties of AZ80 induced by peening media may be attributed to the different properties of the peening media. It is known that energy transfers from shots to the target material during the impact process of SP. The energy of shots is a function of shot size, density and velocity. Since intensity is a measure of strain energy induced, shots with small size and low density have to travel at significantly higher velocities than shots with larger size and higher density in order to achieve the same intensity. For a given Almen intensity, the velocities of glass beads and B30 shots are higher since these shots present a lower density and a smaller size than Ce-ZrO2 shots. The correspondent strain rates during SP with glass beads and B30 should be higher than that by using CeZrO2 . Due to the limited deformability of the magnesium alloy, the increase in strain rate would induce more severe surface damages (Fig. 1). In addition, the compressive residual stresses in specimens
Table 3 Surface layer properties after SP. Peening medium
Almen intensity (mmN)
Roughness Ry (m)
Thickness of deformation layer (m)
Maximum Compressive residual stress (MPa)
Zirblast B30
0.15
17
125–175
75
CeZrO2
0.10 0.30
5 8
125–175 200–250
100 116
Glass
0.15
10
100–150
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Fig. 6. Fatigue crack nucleation sites of AZ80 (SEM): (a) B30, 0.15 mmN, (b) B30, 0.40 mmN, (c) Ce-ZrO2 , 0.10 mmN and (d) Ce-ZrO2 , 0.40 mmN (arrow indicates crack nucleation site).
peened by glass and B30 are also smaller than those in Ce-ZrO2 peened specimens (see Fig. 4). In fact, the crack initiation mechanism for a shot-peened material is dependent on the competition of the benefic effect of compressive residual stresses and the negative effect of surface defects (Hutmann, 2003). The compressive residual stresses are quite low for the B30 and glass-peened specimens, while the negative effect of surface defects generated by SP was more favorable by using B30 and glass shots. As the microcracks reached the critical size during heavier SP, they may act as the crack initiation source and propagate in the subsequent fatigue tests (see Fig. 6(b)). These critical microcracks defeat the beneficial effects induced by SP, eventually leading to the overpeening effect, i.e. a pronounced drop in fatigue life of AZ80 at higher Almen intensities (see Fig. 5(a)). Compared to glass and B30, peening AZ80 with Ce-ZrO2 results in the fewest surface defects, lowest surface roughness and highest maximum compressive residual stress. Hence, when AZ80 is peened by Ce-ZrO2 , the benefic effect of compressive residual stresses on the surface overwhelms the negative effect of surface defects. As a result, the overpeening effect disappeared in specimens peened by using Ce-ZrO2 . Significant improvements in fatigue life can also be achieved at high Almen intensities (see Fig. 5(a)). The process window of SP becomes much broader. The effect of SP media on fatigue life of AZ80 was also investigated by Wagner and co-workers (Dörr et al., 1999). In those studies, different SP media including SCCW14, glass beads, S330, SCCWS23 were utilized. The high-strength wrought magnesium alloy AZ80 presented a pronounced overpeening effect and an extremely narrow process window at a small optimum intensity of about 0.05 mmN, irrespective of the particular peening medium (see Fig. 5(a)). Obviously, the present results are superior with regard to the overpeening effect. The difference between the present work and the results reported by Wagner and coworkers (Hilpert and Wagner, 2000) is probably caused by the
different microstructure and ductility of target materials. The wrought magnesium alloy AZ80 used in the present work exhibits a single ␣-structure with a high elongation of 18% (Zhang and Lindemann, 2005a). While the wrought magnesium alloy used by Wagner et al. (Hilpert and Wagner, 2000) consists of an ␣- and -phase mixed structure, the ductility is much lower than that of the present alloy. Since the estimated strain rate during SP is roughly in the range of 103 to 104 s−1 (Altenberger, 2003), the brittle -phase may be broken even at a low intensity, regardless of the particular peening media. The broken -phase possibly acts as the source of fatigue crack and propagates during fatigue, leading to the overpeening effect of AZ80. Hence, SP media have little effect on fatigue performance of AZ80 in reference Wagner (1999). 5. Summary In the present work, the influence of peening media including glass bead, Zirblast B30 and Ce-ZrO2 on high cycle fatigue properties of AZ80 has been investigated. The results showed that SP media had a significant influence on the surface layer properties and subsequent fatigue response of AZ80. Peening AZ80 with CeZrO2 introduced the fewest surface defects, smallest roughness and highest compressive residual stresses. Regardless of different peening media, SP improved the fatigue strength of the high-strength magnesium alloy AZ80. Peening AZ80 with Ce-ZrO2 achieved the highest improvement of about 75% in fatigue strength at the optimum condition. Additionally, peening AZ80 with Ce-ZrO2 resulted in an extremely broad process window, i.e. the overpeening effect disappeared. The benefic effect of compressive residual stresses on the surface overwhelmed the negative effect of surface defects, leading to the absence of overpeeing effect. Such a broad process window makes SP applicable for practical magnesium applications.
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Acknowledgements This work is sponsored by BMBF and Shanghai Pujiang Program (Grant No. 06PJ14062). The authors would like to thank Otto Fuchs Metallwerke, Meinerzhagen, Germany, for providing the magnesium alloy, and OSK Kiefer GmbH, Oppurg, Germany, for shot peening. One of the authors is grateful to DAAD for granting a scholarship to him (PZ). References Altenberger, I., 2003. Alternative mechanical surface treatments: microstructures, residual stresses & fatigue behavior. In: Wagner, L. (Ed.), Shot Peening. WileyVCH, Weinheim, pp. 421–434. Becker, J., Bogon, P., Gers, H., Kiefer, A., Leyens, C., Newiak, F., Roll, K., Straube, O., Viehweger, B., 2005. Magnesium—Knetlegierungen fur den Automobilbau. ATZ – Automobiltechnische Zeitschrift 107, 922–929 (in German). Dörr, T., Hilpert, M., Beckmerhagen, P., Kiefer, A., Wagner, L., 1999. Influence of shot peening on fatigue performance of high-strength aluminum- and magnesium alloys. In: Nakonieczny, A. (Ed.), Shot Peening. IMP, Poland, Warsaw, pp. 153–160.
Eliezer, A., Gutman, E.M., Abramov, E., Unigovski, Y., 2001. Corrosion fatigue of diecast and extruded magnesium alloys. J. Light Met. 1, 179–186. Hilpert, M., Wagner, L., 2000. Response of light alloys to mechanical surface treatments: comparison of magnesium and aluminum alloys. In: Kainer, K.U. (Ed.), Magnesium Alloys and their Application. Wiley-VCH, Weinheim, p. 525. Hutmann, P., 2003. The application of mechanical surface treatment in the passenger car industry. In: Wagner, L. (Ed.), Shot Peening. Wiley-VCH, Weinheim, pp. 3–12. Lu, J., Flavenot, J.F., 1987. Application of the incremental hole drilling method for the measurement of residual stress distribution in shot-peened components. In: Wohlfahrt, H., Kopp, R., Vöhringer, O. (Eds.), Proceeding of 3rd International Conference on Shot Peening. Garmisch-Partenkirchen, Germany, pp. 279– 286. Mueller, C., Robriguey, R., 2003. Influence of shot peening on the fatigue and corrosion behavior of the die cast magnesium alloy AZ91 hp. In: Wagner, L. (Ed.), Shot Peening. Wiley-VCH, Weinheim, pp. 271–277. Wagner, L., 1999. Mechanical surface treatments on titanium, aluminum and magnesium alloys. Mater. Sci. Eng. A 263, 210–216. Zhang, P., Lindemann, J., 2005a. Influence of shot peening on high cycle fatigue properties of the high-strength wrought magnesium alloy AZ80. Scripta Mater. 52, 485–490. Zhang, P., Lindemann, J., 2005b. Effect of roller burnishing on the high cycle fatigue performance of the high-strength wrought magnesium alloy AZ80. Scripta Mater. 52, 1011–1015.