Mg-Al-Zn alloy thin strips were prepared by vertical twin-roll casting.Effects of Al and Ca on the microstructure and the surface defects of the thin strips were investigated.The results show that with the increase of Al content,the quantity of surface cracks of the thin strips increases rapidly and their locations are moved to the center of the thin strips.The alloy melts with more than 0.10%Ca(mass fraction)stick on the roller and a poor surface quality of the thin strips appears.With the increase of Al content,the microstructure of the thin strips evolves from singleα-Mg phase,characterized by the uniform and equiaxed recrystallization grains, to the combination of dendriticα-Mg with Al supersaturatedα-Mg.The minor addition of Ca can obviously refine the grains of Mg-3Al-1Zn thin strip because the formation of Al2Ca and the finest grains are obtained by adding 0.08%Ca.
The development status of twin-roll strip casting for magnesium alloys in China was summarized as well as the new progress when several kinds of twin-roll strip casting technologies were developed and used.Horizontal twin-roll casting (HTRC) of magnesium alloys has attracted much attention and has been industrialized in China.Vertical twin roll casting(VTRC) of the magnesium alloys can reach a speed of higher than 30 m/min and its research and development are just beginning and exhibit exciting potential.By comparing the process characteristics of the two technologies,the process stability of HTRC for the magnesium alloys is better,and the casting speed and the cooling rate of VTRC for the magnesium alloys are higher.The quality of the products by the two technologies needs to be improved and further investigated.
The AZ31 sheets were prepared by extrusion.The effects of the extrusion processing parameters including the temperature extrusion ratio,and structure of the extrusion die on the microstructure and mechanical properties of the as-extruded AZ31 sheets were investigated.The results show that the partial grains grow abnormally.and the mechanical and anisotropic properties of the as-extruded AZ31 sheets have little change at the extrusion temperatures of 380-400℃and the extrusion ratio of 13.3.With the increase of the extrusion ratio,the microstructure of the as-extruded AZ31 sheets by conventional die becomes finer and more uniform,and the elongation rate increases,but the strength decreases and its anisotropy becomes worse.Under the porthole die,finer and more uniform microstructure,higher mechanical properties and better anisotropy can be brought for the as-extruded AZ31 sheets The extruded AZ31 sheets by the porthole die have better anneal process of 300℃and 1 h.
