Thermal Deformation Behavior And Extrusion Process Of Mg-6Zn-5.5Cu-1Y-0.6Zr Alloy

Mg-Zn alloy is one of the most widely and commonly used magnesium alloys.The maximum solid solubility of Zn in magnesium is 6.2%and it decreases with the increase of temperature.In order to improve the performance of Mg-Zn alloy,Cu element is added to Mg-Zn alloys,so as to improve the solution temperature.The more elements dissolved into the magnesium matrix could achieve solid solution strengthening and aging strengthening effect of magnesium alloys.The Y element adds to the magsium alloys could form the Mg-Y compound and the Mg-Y phases can improve the thermal stability of the magnesium alloys.Mg-6Zn-5.5Cu-lY-0.6Zr alloy is selected as the research object in order to obtain the lowcost,highstrength and goodplastic magnesium alloy.Microstructures and mechanical properties of the extruded,annealed and hot compressed alloys are investigated,in order to obtain the optimal hot working schedule of the experinental alloy.The main conclusions are summarized as follows:The as-cast Mg-6Zn-5.5Cu-lY-0.6Zr alloy is mainly consists of the net-like MgZnCu phase and the skeleton-like I-Mg3YZn6 phase.After homogenized at 430℃x24h,the I-Mg3YZn6 phase is decreased,and a large number of W-Mg3Y2Zn3 phase appeares at the grain boundaries.At the same time,a large number of nanoscale rod-like MgZn2 phase precipitates in the alloy.The stress-strain curves of Mg-6Zn-5.5Cu-1Y-0.6Zr alloy exhibit a typical dynamic recrystallization characteristion under the deformation temperature of 523K～673K and strain rate of 0.001s-1～1s-1.The activation energy for hot deformation of the experimental alloy is Q=222.31kJ/mol.The constitutive equations is shown as follows:σ=69.44{(17.27×10-17×Z)0.16+[(17.27×10-17×Z)0.32+1]0.5}Z=(?)exp(222286.5/RT)The thermal processing map of Mg-6Zn-5.5Cu-lY-0.6Zr alloy under different true strain is drawn,and the rheological instability area is analyzed.So the reasonable processing parameters of the alloy are obtained.That is,when the deformation rate is at a low degree,the alloy is suitable for hot deformation in the temperatures range from 573K to 673K.After extrusion of Mg-6Zn-5.5Cu-lY-0.6Zr alloy at different temperatures and speeds,the recrystallized grain size increases with the increase of extrusion temperature and extrusion speed.It can be seen that the extrusion temperature and extrusion speed have a significant effect on the mechanical properties of the alloy.With the increase of extrusion temperature and extrusion speed,the strength of the alloy decreases and the plasticity increases.After extrusion at 250℃ with 2mm/s,the alloy exhibits the best mechanical properties,which the tensile strength is 384 MPa,the tensile yield strength is 373 MPa,and the elongation is 11.4%,respectively.Through the T5 and T6 heat treatments,the hardness of the experimental alloy reaches the peak-hardness of 103 HV and 84 HV at 10 h.The strength of the specimen increases under the aging treatment of 250℃ and 2mm/s.The optimal ultimate tensile strength and tensile yield strength of Mg-6Zn-5.5Cu-1Y-0.6Zr alloy are 392 MPa and 385 MPa,respectively.At the same time,the elongation of the alloy is 7.5%.