Study On Microstructure And Properties Of Thixo-Diecasted AZ91D Magnesium Alloy

Semisolid forming or semisolid processing(SSF/SSP) of metals is an advanced metal processing technology.Thixoforming,one of SSP methods,is convenient during the billet heating,transportation of SSP.And automation can be achieved easily. Therefore,Thixoforming is a main method of SSP.The magnesium alloy AZ91D,as an experimental material was used.The effects of ingot size and SiC addition on microstructure of mutative-size AZ91D alloy initial billet during semisolid isothermal heat treatment are studied.and the effects of the parameters of thixo-Diecasting on microstructures and mechanical properties of magnesium alloy AZ91D without/with SiC were also investigated in the thesis.Exists of different microstructures from center to edge of AZ91D magnesium alloy billet after isothermal heat treatment,which the size of solid particles gradually changes from large to small,and the particles tend to gradually spheroid and more liquid.with increasing the billet diameter,it is found that the difference of microstructures further increases from center to edge of the billet,there is an obvious "dimensional effect"of microstructure in semi-solid AZ91D alloy without SiC introduced.By contraries,the magnesium alloy AZ91D with SiC addition.the average size of solid particles becomes finer than that without SiC addition after isothermal heat treatment.And the solid particles size of the billet with SiC from edge and center area is almost identical dimension,spheroid and homogeneous distribution.From the billet with diameterφ15mm,φ45mm,φ70mm with SiC addition after semisolid isothermal heat treatment,the difference of microstructures from center to edge of the billet increased, but the tendency will be weaken.According to the self-designed die chamber combination of the die-casting machines in the liquid,a set of semi-solid die-casting trail experiments is carried out.The thixo-Diecasted experiments which changed respectively injection speed,holding temperature,holding time of the billets,the initial results were obtained.When they are respectively 590℃and 130min,good mechanical properties and microstructure are gained.The low injection speed is more suitable for thixo-Diecasted.Easier to suffer casting defects,such as gas porosity,from high injection speed.Based on the defects in die-casting,the results were analyzed,and proposed that the mold gate is too small, semisolid particles size are too big.Therefore,the semisolid die-casting mold was redesigned,and SiC was introduced into AZ91D alloy to be modified.And then,to change the injection speed,holding temperature,holding time of the billets during semisolid isothermal heat treatment for thixo-diecasting.Finally,a set of the optimized parameters was obtained as following the holding temperature of 590℃,the holding time of 110min,and injection speed of 2m/s.the UTS and hardness of thixo-diecasted AZ91D alloy are 182MPa and HV67.87 respectively.Meanwhile,the primary solid phase particles are distributed uniformly in the microstructure with an average diameter of 87μm and solid fraction of 58%.