近液相线铸造与等通道角挤压制备ZK60半固态等温晶粒粗化

Lsothermal Grain Coarsening of Near-liquidus Casting and Equal Channel Angular Extrusion Formed ZK60 in the Semi-solid State

分别采用近液相线铸造法和等通道角挤压制备了镁合金ZK60半固态坯,采用光学显微镜和图像分析仪研究了这两种坯料在等温热处理过程中晶粒的粗化和晶粒内液相产生的机理。研究表明,当固相率在0.65~0.85这个区间时,在温度一定的条件下,两种不同状态半固态坯料晶粒的粗化速率随着保温时间的延长而降低;铸态坯料的粗化系数K由固相率为0.65时的570μm3/s,下降到固相率为0.8时的319μm3/s,然后上升到固相率为0.85时的859μm3/s。而等通道角挤压制备的坯料其粗化系数由固相率为0.65时的146μm3/s,下降到固相率为0.85时的98.8μm3/s。铸态坯料晶粒间的液滴来源于非平衡凝固产生的晶内共晶相和晶粒以合并形式长大时所夹杂的液相。

Grain sizes of alloys in the semi-solid state are very important during isothermal heat treatment.In this paper,magnesium ZK60 semi-solid billets were prepared by near-liquids casting and equal channel angular extrusion,respectively.Isothermal grain coarsening of as-cast and extruded billets has been investigated by Optical micrograph and image dissector camera system.The formation mechanics of intragranular liquid droplets has also been studied.The results show that grain coarsening rates of both as-cast and extruded billets are decreased with increase in holding time at the same temperature,when the width of the semi-solid interval ranges from 0.68 to 0.85.The coarsening rate constant K of as-cast billet decreases from 570 μm3/s(solid fraction equals to 0.65) to 319 μm3/s(solid fraction equals to 0.8),and then it increases to 859 μm3/s(solid fraction equals to 0.85) during isothermal heat treatment.However,the coarsening rate constant K of extruded billets decreases from 146 μm3/s(solid fraction is 0.65) to 98.8 μm3/s(solid fraction equals to 0.85) during isothermal heat treatment。The formation of intragranular liquid droplets of as-cast billets originates from eutectic of intragranular,which is produced by non-equilibrium freezing,and entrapped liquid,which is created by coalescence of solid particles.