Investigation On Shot Peening Treatment Of Mg-8Gd-3Y Alloy And Its Characterization

GW83 rare-earth Magnesium alloy is widely used in aerospace,automotive and biomedical fields due to its high specific strength,good stiffness and biocompatibility.Shot peening（SP）is one of the important methods for surface deformation strengthening.By optimizing the surface layer microstructure and introducing the compressive residual stress（CRS）field,SP can significantly improve the surface mechanical properties of the material.Therefore,SP technique is widely applied in industrial production.In order to improve the surface mechanical properties of GW83 alloy,conventional SP treatment and other advanced SP techniques,such as multiple SP,stress SP and warm SP etc.were carried out in this paper.Meanwhile,the residual stress field,microstructure and mechanical properties of the treated deformation layer were characterized.Research on the residual stress distribution showed that conventional SP introduced a relatively high level of CRS on the surface of the material,and the maximum CRS appeared in the subsurface.As the SP intensity increased,the maximum CRS increased.When the intensity reached 0.54 mm N,the surface CRS and maximum CRS values were-143 and-184 MPa,respectively.The multiple SP process could effectively increase the CRS value and the influenced depth in the deformed layer.At the intensity of 0.54+0.13 mm N of multiple SP,the surface CRS and maximum CRS of GW83 alloy were-153 and-200 MPa,respectively.Under the same SP intensity,stress SP and warm SP significantly increased the value of CRS and the depth of affected zone.Among them,the improvement of residual stress by stress SP was mainly manifested in the loading direction,but not obvious in the perpendicular one to the loading direction.The relaxation behavior of the residual stress induced by SP under high temperature and cyclic loading was studied,and it was found that the thermal relaxation of the residual stress,e.g.,in a range of 150～300℃,mainly occurred in the initial stage of annealing.The higher annealing temperature resulted in the faster the relaxation of CRS.Zener-Wert-Avrami function could be used to evaluate this relaxation behavior of CRS.By this function,the residual stress relaxation activation enthalpy of GW83 alloy was calculated to be 60 k J/mol,which was less than the self-diffusion activation energy of magnesium alloy.The relaxation of CRS of GW83alloy under tension-tension cyclic loading mainly occurred in the initial stage.With the increase of the cycle times,the relaxation rate of the residual stress slowed down and gradually stabilized.The greater the applied load,the faster the residual compressive stress relaxes and the smaller the stress stability value.The dynamic simulation on random impact of multiple projectiles on GW83 alloy was performed with ANSYS finite element software.The simulation results showed that the maximum CRS appeared on the subsurface layer instead of on the collision surface.With the increase of SP coverage,the surface CRS and the maximum CRS both increased,and the depth corresponding to the maximum CRS also gradually increased.As the SP speed increased,both the surface CRS and the maximum CRS increased,and the influenced depth of CRS also increased,and the depth corresponding to the maximum CRS migrated into the matrix.XRD line analysis method was used to study the microstructure of the deformed layer,the results indicated that the domain size was significantly refined,the microstrain and the dislocation density were significantly increased.The microstructure of the peened layer presented a gradient variation,the surface domain size was the smallest,the microstrain and the dislocation density were the largest.When the SP intensity was 0.54+013 mm N,the surface domain size was 24 nm,and the dislocation density reached 2.8×10¹⁵ m^-2,which was far higher than that of the original material.Under the same SP intensity,both stress SP and warm SP could further improve the microstructure of the deformed layer.The grain refinement mechanism of shot peened GW83 alloy was studied by TEM observation.The grain refinement process mainly included:i)the introduction of deformation twins and a large number of dislocations in the initial large grains;ii)subdivision of large grains into substructures by twin-twin interaction and dislocation cells interacting twins;iii)evolution of sub-grains into ultrafine and nanocrystallines through dislocation activity and crystal grain rotation.The microstructure evolution of the peened layer at high temperature showed that during the isothermal annealing process,a large amount of Mg₂₄Y and Mg₅（Gd,Y）phases precipitated in the deformed layer,indicating that SP could accelerate the precipitation of the second phase.After isothermal annealing,the refined domains grew up,the microstrain and dislocation density were greatly decreased.Through linear regression analysis,the growth activation energy of the peened GW83 alloy was 125 k J/mol,and the microstrain relaxation activation energy was 108 k J/mol.The study of the hardness of the deformed layer showed that the hardness change showed a gradient trend,indicating that the hardness was maximum at the top surface and gradually dropped with the increase of the layer depth.The surface hardness of the multiple SP sample was the highest,reaching 156 HV,which was nearly two times higher than that of the original sample.Under the same SP intensity,both stress SP and warm SP could effectively improve the hardness of the deformed layer.After isothermal annealing,due to the precipitation of a large amount of nanophase,the hardness of the surface layer increased.When the temperature was kept at 250℃for 32 min,the hardness reached a maximum of 175 HV,which was 33%higher than that of the peened samples.SP significantly improved the hardness and yield strength of the surface layer in GW83alloy.The surface yield strength of GW83 alloy after SP was tested by in-situ tensile X-ray stress.The results indicated that the surface yield strength after SP was 248 MPa,which was24%higher than that of the original material.The CRS induced by SP and the optimization of the microstructure,including the refinement of domain size,the increase of the density of defects such as dislocations,were important reasons for the increase in the hardness and surface yield strength.