Effect Of Inter-pass Heat Treatment On The Microstructure And Mechanical Properties Of Rare Earth Magnesium Alloy During Large Height-to-diameter Ratio Upsetting-extrusion

Mg-Gd-Y-Zn-Zr alloys containing long period stacking-ordered(LPSO)phases are used in aerospace and defense industries because of their low density and excellent room temperature and high temperature mechanical properties.Due to process limitations,large Mg-Gd-Y-Zn-Zr alloy components often require large high-diameter-ratio cast bars made by reciprocal upsetting and extrusion(RUE)process,however,multiple thermal deformation not only increases the cost,but also causes grain growth due to repeated heating.In order to solve the above problems,this paper proposes to add annealing and annealing-aging heat treatment between passes based on a large height-to-diameter ratio upsetting-extrusion process in order to obtain a short process billet making method,and systematic experiments have been conducted at different extrusion temperatures.The results show that inter-pass heat treatment can significantly increase the recrystallization rate of the alloy after a single upsetting-extrusion deformation.Annealing after upsetting induced static recrystallization(SRX),which initially increased the recrystallization rate of the alloy,during which residual dislocations in the deformed grains were consumed and the microstructure was refined by the transition from low-angle grain boundaries(LAGBs)to highangle grain boundaries(HAGBs)and by recrystallized grain growth nibbling.With grain boundary migration,solute elements precipitated out to form particle β phase,which promoted dynamic recrystallization(DRX)by particle stimulated nucleation(PSN)mechanism during the subsequent extrusion process.The annealed samples were subjected to 66 h peak aging treatment,and the precipitated β’ phase was accelerated to a large number of diffusely distributed granular β phases during the preheating process before extrusion,which not only improved the contribution of PSN mechanism to DRX,but also inhibited the grain growth through the pegging effect,so the organization was further refined.The final 460°C extruded inter-pass annealed-aged sample obtained the highest recrystallization rate and the smallest grain size of 88.1% and 7.03 μm,respectively,as a result of the combined effect of continuous dynamic recrystallization(CDRX),discontinuous dynamic recrystallization(DDRX),SRX,and PSN mechanisms.The inter-pass heat treatment enhanced the fine-grain strengthening,but weakened the dislocation strengthening and weave strengthening,and the strength of the inter-pass annealed samples decreased to different degrees,while the microstructure of the inter-pass annealedaging samples was further refined and fine-grain strengthening dominated,and the strength of some samples increased.However,the inter-pass heat treatment resulted in a significant increase in recrystallization rate,which facilitated fine-grain coordinated deformation and improved elongation(EL),while the more uniform microstructure reduced inter-sample property differences and residual stresses,which was beneficial for subsequent alloy component forming.Therefore,comprehensive mechanical properties and microstructure,in this experimental condition 460°C upsetting-450°C annealing 5h-200°C aging 66h-460°C extrusion is the best experimental parameters,the yield strength(YS),ultimate tensile strength(UTS),EL of this sample are 246 MPa,329MPa,13.4%,where fine-grain strengthening is the main strengthening mechanism.The grain refinement and high Schmid factor resulted in a good elongation.