Research On Microstructure,Properties And Surface Strengthening Of Mg-Y-Nd-Zr Rare Earth Magnesium Alloy By Equal Channel Angular Pressing

Equal channel angular pressing(ECAP)is a common severe plastic deformation process in the refinement of magnesium alloy grain.However,due to the poor plastic deformation ability of magnesium alloys at room temperature,the ECAP of most magnesium alloys was usually carried out at high temperatures.Extrusion at high temperature is easy to cause grain growth,which will weaken the effect of grain refinement.In this paper,the forging and ECAP extrusion deformation methods are used to deform the Mg-2Y-0.6Nd-0.5Zr alloy.On the one hand,forging can refine the grain size,reduce casting defects,and improve the plastic deformation ability of the alloy,so that the deformation temperature of ECAP is reduced;on the other hand,the forging and ECAP deformation processes can be superimposed to obtain a larger amount of deformation,breaking through the grain refinement limitation of single ECAP and obtaining excellent mechanical properties.In addition,in order to further enhance the surface strength and fatigue performance of extruded Mg-2Y-0.6Nd-0.5Zr alloy,the alloy was also subjected to subsequent ultrasonic surface rolling processing(USRP)treatment.The results show that:After forging deformation at 350 ?,the grain size of casted Mg-2Y-0.6Nd-0.5Zr alloy was significantly refined,from 140?m refined to 25?m.Under the combined effect of fine grain strengthening and work hardening,the tensile strength of the alloy increased from 120 MPa and 244 MPa,but the elongation did not change much,from 10.5% to 12.3%.Forging also changed the preferred orientation of the as-casted alloy during solidification,and the pyramidal orientation weakened,the orientation of the basal and cylinder is enhanced.The forged Mg-2Y-0.6Nd-0.5Zr alloy has the best ECAP deformation effect at 360 ?.After one pas of ECAP at this temperature,the grain size is further refined from 25?m to 14.8?m.However,due to the weakening of work hardening and the transformation of texture after one pass of ECAP,the strength of the alloy has been reduced,and the tensile strength has dropped from 244 MPa to 233 MPa,the elongation increased from 12.3% to 17.4%.With the increasing of ECAP passes,the grain size of the alloy decreased gradually,and the uniformity of the structure increased,but the grain refinement effect gradually weakened.The refinement effect during the first two passes of ECAP was significant,from the 25?m of as-casted refined to 14.8?m and 4.5?m respectively.After four passes of ECAP,the refinement effect was weakened,and the grain size after four and six passes of ECAP were 3?m and 1.9?m,respectively.The tensile strength of the alloy increased first and then decreased with the increasing of ECAP passes,and reached a peak value with ?262 MPa after four passes of ECAP.After six passes of ECAP,some inclined texture was formed in the alloy,which resulted in the increase of Schmidt factor,coupled with a high degree of recrystallization to soften the alloy,so its strength was slightly reduced compared with that after four passes of ECAP.The elongation of the alloy increased continuously with the increasing of ECAP passes,and the elongation reached 25.2% after six passes of ECAP.The surface roughness of the extruded Mg-2Y-0.6Nd-0.5Zr alloy was significantly reduced after USRP treatment.The surface roughness increased with the increasing of the spindle speed,and decreased first and then increased with the increasing of static pressure.The surface roughness after rolling at a static pressure of 0.08 MPa and spindle speed of 500 r/min was the lowest,from the initial 1.08?m to 0.09?m.Under the combined effect of fine grain strengthening and work hardening,the microhardness was enhanced,and the hardness gradually decreased from the surface to the core,and finally when close to the base hardness value it tends to be stable.After rolling at a static pressure of 0.08 MPa and spindle speed of 500 r/min,the hardness of the outermost layer reached 92.5 HV.After USRP,the residual stress on the surface of the specimen was caused by the inconsistent degree of tissue deformation at different depths of the specimen.The residual stress increased with the increasing of deformation degree of the alloy,and the surface residual stress reached-226 MPa after rolling at a static pressure of 0.08 MPa and spindle speed of 500 r/min.