Grain Refinement, Partial Remelting And Thixoforming Of Magnesium Alloys

Magnesium alloys,as the lightest structural metallic materials,have large application potential.Semisolid forming has been consinder as the most promising forming technology for magnesium alloys.Thixoforming,one of semisolid processing methods,is the main method of semisolid forming because it is more convenient for billet heating, and transportation and easier for the realization of automatic control.However, the microstrctures of cast magnesium alloys are relatively coarse,which seriously affects the resulted semisolid microstructures and also impairs their mechanical properties.Therefore, in order to obtain semi-solid microstructures with uniform anf fine primary particles,magnesium alloys should be grain refined.In this thesis,AZ91 and AM60 magnesium alloys were refined by Al-Ti-B or MgCO3.Effects of them on the microstructure and mechanical properties of these alloys and corresponding refining mechanisms have been investigated using optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscope(SEM) with energy dispersive spectrometer (EDS)and electron probe micro-analyzer (EPMA).In addition,the microstructural evolution during partial remelting and subsequent thixoforming of the grain-refined magnesium alloys were studied.The results indicated that:Addition amount of the refiners,addition temperature, holding time, cooling rate and casting ingot size had different effects on the microstructure.The grain size of AZ91D alloy could be decreased from 422μm to 76μm when 0.3%Al-Ti-B was added at 750℃and then helded for 30 min.The grain size of AM60B alloy could be decreased from 328μm to 73μm when 0.3% Al-Ti-B was added at 780℃and then helded for 30 min.Addition of 1.2% MgCO3 into AM60M alloy at 790℃and subsequent holding for 10min,its grain size was decreased to 69μm.The refining mechanism of formation Al-Ti-B is:part of the formed TiB2 particles was acted as the nucleus substrates and the other played a pinning role for crystal growth.For MgCO3,the formed Al4C3 particles were the nucleus substrates.The analysis on liquid fraction,phase transition and temperature variation of partially melted specimens showed that the AZ91D Mg alloy specimen withΦ15mm×15mm refined by Al-Ti-B would not reach solid-liquid equilibrium state until it was heated for 20 min.During partial melting,the microstructure evolution of the refined magnesium alloys could be divided into four steps,the initial coarsening due to the dissolution of interdendritic eutectic,structure separation resulted from the melting of residual eutectic, spheroidization due to partial melting of primary particles and final coarsening attributed to mergence and Ostwald ripening. The primary particles of the refined alloys were smaller and more spheroidal than those of the not refined alloys.The finer the as-cast microstruture, the smaller and more spheroidal the primary particles in the semisolid microstruture.The effects of the three parameters,such as injection speed,holding time of ingots and holding temperature on the microstructure and mechanical performance of thixoformed AM60 alloy (refined by MgCO3)was investigated.The results indicated that the castings showed an optimum microstructure and mechanical performance when they formed under the conditions of heating for 130min at 610℃and injection speed of 3m/s.The tensile strength of the thixoformed alloy was up to 176MPa.