Study On Surface Characteristics And Fatigue Behavior Of Warm Shot Peened GW92K Magnesium Alloy

Magnesium alloys at higher temperatures are more plastic than at room temperature.This paper proposes warm shot peening (WSP) to improve their fatigue strength bystrengthening the surface layer, thus promoting the use of magnesium alloy. The high cyclefatigue behavior of as-extruded and extruded-aged GW92K Mg alloys were firstlyinvestigated. The WSP system with peening medium for magnesium alloys was built.Further study was put on effect of WSP on surface characteristics such as surfacetopography, microstructure, microhardness, compressive residual stress of as-extruded andextruded-aged GW92K. High cycle fatigue properties under optimum shot peening (SP)and WSP were tested. And finally attempted to explain the WSP strengthening mechanismof magnesium alloy.The average grain size of GW92K Mg alloy is21μm, with yield strength208MPaand elongation22%; After aging treatment, the yield strength is improved to291MPa andelongation reduced to10%, with the grain size almost the same. The fatigue strength at107cycles of the as-extruded and extruded-aged GW92K alloy are125MPa and145MPa.Fatigue crack of as-extruded and extruded-aged specimen were initiated at sample surface.At the same Almen intensity, WSP induces larger maximum compressive residualstress, higher subsurface hardening and higher surface roughness than SP does. WhenAlmen intensity is0.55mmN (WSP), mircocrack occurs at the specimen. Ra(arithmeticalmean roughness) and Ry(maximum height) of SPed specimen increased from1.06μm and10.6μm to3.3μm and19.6μm with Almen intensity increased from0.10mmN to0.55mmN. While Raand Ryof WSPed specimen increased from1.1μm and11.3μm to3.6μm and22.6μm. Extension twin (1012)is observed in WSPed specimens’ deformationlayer. When Almen intensity is0.30mmN, the compressive residual stress of SPed andWSPed specimen is104MPa and109MPa at the distance of88μm from surface. Theoptimum Almen intensities of SP and WSP for as-extruded GW92K are0.10and0.15mmN. The fatigue strength of optimum SPed and WSPed GW92K are175MPa and185MPa.The surface characteristics of WSPed extruded-aged GW92K alloys exhibits the samephenomenon as the as-extruded alloys. Kt(stress concentration coefficient) of SPed andWSPed specimen increases from1.14and1.19to1.39and1.44respectively, with Almenintensity increased from0.10mmN to0.55mmN. The optimum Almen intensities of SP andWSP for extruded-aged GW92K are0.10and0.20mmN. The fatigue strength of optimumSPed and WSPed GW92K are195MPa and210MPa. SPed and WSPed specimens’ fatiguecrack initiate at region located in the sub-surface.SP and WSP have the similar mechanism in surface strength and fatigue resistance.compressive residual stress, structure refinement and increased microhardness help toimprove its high-cycle fatigue property, while surface roughness increases does detrimentalto the high-cycle fatigue properties. Compressive residual stress transfers fatigue crackinitiation source into sub-surface and surface hardening material improves the the criticalstress, thereby enhancing the anti-fatigue properties of magnesium alloys.The effect of WSP on fatigue property is better than that of SP because: WSP induceshigher subsurface hardening and larger maximum compressive residual stress, meanwhile,the surface of WSPed specimen is more plastically deformed but less damaged. Becauseextruded-aged GW92K possesses higher strength, WSP induces low surface roughness intoextruded-aged GW92K than as-extruded GW92K, so the negative effect of surfaceroughness are weakened, leading to a better fatigue property for extruded-aged GW92K.