Influence of shot peening on notched fatigue strength of the high-strength wrought magnesium alloy AZ80
Introduction
Light-weight magnesium alloys are considered as ideal candidates for weight saving in automobile. However, the fatigue properties of magnesium alloys cannot reach the requirement as load bearing parts in automobile, which significantly restrict the application of magnesium alloys [1], [2]. It is reported that the fatigue properties can be improved by element addition, heat-treatments or by mechanical forming processes (such as extrusion, rolling and forging) [3], [4], [5]. Moreover, the fatigue properties can further be enhanced by mechanical surface treatments [6], [7], [8], [9].
Mechanical surface treatments such as shot peening (SP), roller burnishing (RB) and deep rolling (DR) have been widely used to improve the fatigue performance of metallic materials [10], [11], [12], [13]. The principle of mechanical surface treatments is based on introducing compressive residual stresses and work-hardening in near-surface region, which retard the crack nucleation and/or propagation [14]. It has been demonstrated that SP can improve the fatigue properties of magnesium alloys [6], [8], [15]. However, the beneficial effect of SP can only be achieved by carefully selecting the peening parameters to avoid the strong overpeening effect [6], [8]. More recent results showed that the peening media had a significant influence on the fatigue properties of magnesium alloy AZ80 [15]. By using Ce–ZrO2 shots, the process window of SP became broad, and the overpeening effect in the high-strength wrought AZ80 disappeared [15].
Virtually most engineering components contain geometrical discontinuities (so-called notches). During service there is a stress concentration in notches, resulting in a significant reduction in the fatigue life of components [16], [17]. Therefore, it is of particular importance to investigate the effect of SP on the notched fatigue strength of magnesium alloys. So far, the most investigations on SP of magnesium alloys were performed by using smooth specimens [6], [8], [15], the available information on the influence of SP on the notched fatigue strength of magnesium alloys is very limited [18]. It has been reported that SP can effectively improve the notched fatigue strength of AZ80 [18]. However, steel shots (SCCW14) were utilized for SP in that study [18], which severely deteriorated the corrosion resistance of the high-strength magnesium alloy AZ80 due to the iron contamination [19]. Moreover, necessary information on the influence of SP process parameters including SP media on the notched fatigue strength of magnesium alloys is yet unavailable.
In the present investigation, shot peening has been performed on the notched specimens of the high-strength wrought magnesium alloy AZ80. Various process parameters including SP media (glass, Zirblast B30 and Ce–ZrO2 shots) and Almen intensity have been used in order to obtain the optimum SP condition.
Section snippets
Experimental
The material used in this investigation was a high-strength wrought magnesium alloy AZ80 (nominal composition in wt.%: 8Al, 0.5Zn, 0.2Mn, balance: Mg). It was supplied by Otto Fuchs Metallwerke, Meinerzhagen, Germany. After casting, the alloy was extruded and then forged to a rectangular bar. The final microstructure of the alloy consisted of equiaxed grains of α-phase with an average grain size of about 30 μm. The details for the preparation of the alloy were described elsewhere [8]. The
Influence of notch on S–N curves at EP condition
S–N curves of AZ80 at the reference condition (EP) are shown in Fig. 3. It can be seen that fatigue strength is considerably reduced by geometrical stress concentrations. The fatigue strength (the stress amplitude at 107 cycles) decreases from 100 to 45 MPa as the notch factor increases from 1.0 to 2.3. In order to determine the influence of the geometrical notch factor on fatigue strength, the data in Fig. 3a are re-plotted in terms of maximum notch root stresses (σa·Kt). The notched fatigue
Conclusions
In the present work, the influence of SP on the notched fatigue strength of the high-strength wrought magnesium alloy AZ80 was investigated by using different peening media (including glass, Zirblast B30 and Ce–ZrO2 shots) and various Almen intensities ranging from 0.04 to 0.60 mm N. The conclusions are summarized as follows:
Notch reduces the fatigue strength of AZ80 remarkably, the fatigue strength (at 107 cycles) decreases from 100 to 45 MPa as the notch factor increases from 1.0 to 2.3. The
Acknowledgements
Project is sponsored by BMBF. 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.
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