Semisolid Microstructural Evolution And Influence Factors For The Mg-Al-Si Alloy

Magnesium alloys are the lightest structural materials widely commercially available andhave great potential for applications in automotive, aerospace industries and others in recentyears. However, the challenge of Mg-Al-Si alloy is that the Mg₂Si phase is prone to formingundesirable, coarse Chinese script shape under low solidification rate resulting from theeutectic reaction, which would greatly deteriorate the mechanical properties of the magnesiumalloys and become a principal obstacle for the application of Si-containing magnesium alloys.SSM processing is regarded as an advanced forming technology for magnesium alloy, but theresearch of semisolid process for Mg-Al-Si alloy is less. In this thesis, the Mg-9Al-1Si alloysemisolid billets were fabricated by strain-induced melt activation （SIMA） process andsemisolid isothermal heat-treatment process, and the effects of different heat-treatmentprocessing parameters （compressive ratio, holding temperature and holding time） and thealloy content （Al and Si content） on the semisolid microstructures of Mg-Al-Si alloy werestudied. Moreover, the semisolid microstructural evolution and influence factors for theMg-Al-Si alloy were also investigated. Through the research not only can determine theisothermal processing parameters, but also can enrich semisolid theoretical basis, theinfluence of theoretical significance is obviously.Experimental results indicated that a non-dendritic microstructure of Mg-9Al-1Si alloycould be obtained by the strain-induced melt activation （SIMA） process and semisolidisothermal heat treatment process, respectively. The morphology of α-Mg grains transformedfrom dendritic shape to spherical shape and the eutectic Mg₂Si phase transformed fromChinese script shape to globular shape during the SIMA process. Compared with the SIMAprocess, without the compressive processing, although the morphology of α-Mg grainstransformed from dendritic shape to spherical shape but the eutectic Mg₂Si phase transformedfrom Chinese script shape to polygon shape during the semisolid isothermal heat treatment.With the compressive ratio varying from0%to30%, the volume fraction of liquidgradually increased, average size of α-Mg grains increased and the globular tendency becamemore obviously. With the compressive ratio increased, the average size Mg₂Si particle wasnot significant change but its shape factor increased. With the holding temperature varyingfrom560℃to575℃, the average size of α-Mg grain and its globular tendency increased andthe volume fraction of liquid increased. The average size and shape factor of Mg₂Si particleincreased. With the holding time varying from5min to30min, the volume fraction of liquid gradually increased, average size of α-Mg grains increased and the globular tendency becamemore obviously, the average size and shape factor of Mg₂Si particle increased. The resultsshowed that the optimized SIMA processing parameters were suggested at570℃for20minand compressive ratio was chosen30%for the Mg-9Al-1Si alloys.With the holding temperature varying from560℃to575℃, the volume fraction ofliquid gradually increased, average size of α-Mg grains increased and the globular tendencybecame more obviously, the average size of Mg₂Si particle increased but its shape factordecreased. With the holding time varying from5min to30min, the average size of α-Mg grainand its globular tendency increased, the volume fraction of liquid increased, the average sizeof Mg₂Si particle increased but its shape factor decreased obviously. The results showed thatthe optimized semisolid isothermal heat treatment parameters were suggested at570℃for20min for the Mg-9Al-1Si alloys.The effects of the alloy content （Al and Si content） on the as-cast and semisolidmicrostructures of Mg-Al-Si alloy were investigated. With the Al content increased, thehypoeutectic composition transformed into hypereutectic composition in the as-castmicrostructure of Mg-Al-Si alloy; the liquid phase increased, the morphology of α-Mg grainbecame more globular and its size decreased in the semisolid microstructure of Mg-Al-Sialloy. With the Si content increased, abundant polygonal primary Mg₂Si phase existed in thesemisolid microstructure of Mg-Al-Si alloy, the primary Mg₂Si particle distributed in theα-Mg grain boundary and hampered its growth resulted in α-Mg grain size decreased. Afterdifferent semisolid process, all the dentritic Mg₂Si changed into nearly spherical particles.The semisolid microstructural evolution mechanisms for Mg-Al-Si alloys were studied,including the spheroidization mechanism of α-Mg grain, the spheroidization mechanism ofeutectic Mg₂Si phase and primary Mg₂Si phase at grain boundary. The results indicated thatthe spheroidization mechanism of α-Mg grain produced by isothermal heat-treatment processwas the dendrites melting mechanism. However, the penetration of liquid phase along thesub-grain boundary was the main spheroidization mechanism of α-Mg grain produced bySIMA process. The spheroidization mechanism of eutectic Mg₂Si should be attributed to theresolution precipitation. The spheroidization mechanism of primary Mg₂Si was associatedwith the dendrites melting and “Rayleigh shape instability”.