Study On High-temperature Deformation Behaviour,Thermal Spinning Microstructure And Property Of Casted Mg-4Gd-3Y-0.5Zn-0.3ZrZn Alloy

Mg-RE alloys have a higher strength, excellent thermo-stability and good creepresistance. The specificity of RE can change the behavior of phase formation andtransformation during solidification and subsequent heat treatments. However it is stillnot clarified how the strengthening mechanism comes from. There are also somedrawbacks such as poor hot workability, low plasticity at room temperature for Mg-REalloys, so it is necessary that deep study on the strengthening effects and deformationbehavior under high temperature should be made in order to supply some guidance forthermal deformation like subsequent power spinning. Spinning is the preferredtechnology used for making tube workpieces applied in aerospace areas. There is hugedifficulities for the spinning of magnesium alloys which needs urgent study.The solution treatment was firstly carried out in the present study to investigate themicrostructure evolution, mechanical properties and strengthening mechanism of theMg-4Gd-3Y-0.5Zn-0.3Zr under different conditions. Through thermal compressionsimulation experiment, high temperature deformation behavior, micro-structuralevolution and deformation mechanism were made clear. With reasonable formingprocess, tube work-pieces were used for hot power spinning experiment. Afterwards,microstructure evolution among different passes in both axial and tangential directionswas observed in detail, Analysis on the reasons and corresponding improved methods asfor the defects and flaws during the spinning process are put forward..The grain size is relatively small for the casted Mg-4Gd-3Y-0.5Zn-0.3Zr alloybecause of the rare earth rich inside grains. The β’ and Mg12(Gd,Y)Zn phases,distributed randomly along grain boundaries, together with the LPSO phase insidegrains offer the high strength. At550℃,10h is the optimal parameter of T4treatmentprocess. Under this condition, there are much more LPSO phases transformed fromMg12(Gd,Y)Zn phases inside grains and almost all the particle phases get dissolved backinto grains again which is the source of solution strengthening effect.The results of the thermal simulation experiment show that the flow-stress of thecasted Mg-4Gd-3Y-0.5Zn-0.3Zr alloy rise as the strain rate increases or the temperaturedecreases.The calculated average theraml activation energy Q is354.1415KJ/mol andthere exists obvious dynamic re-crystallization. The deformation mechanism is basedmainly on the dislocation slip. After systematic observation on the microstructureevolution under different temperatures and strain rates, the optimal paratmeters forthermoplastic deformation is425℃～450℃，0.001s-1～0.1s-1.Based on sound design steps, hot power spinning of the casted tube work-pieces was carried out, before which is the homogenization treatment at355℃,12h. Afterbeing spun, microstructures along axial and tangential directions are strengthened with ahigher yield strength. This strengthening mechanism is very helpful for revolved parts.Technical defects such as toroidal cracks, inner micro-cracks and cracks at the terminalsappeared during the spinning and so did the over-burning phenomenon because of muchtoo higher temperature.