Investigation On Integration Of Non-equilibrium Solidification And Solid State Transformation Of A356 Aluminum Alloy

Solidification and heat treatment are two closely related processes in the fabrication of Al-Si-Mg based casting alloys.The solidification process mainly controls the size of the grains in the alloy and minimizes the casting defects such as shrinkage porosity,shrinkage cavity,and macrosegregation,etc.The heat treatment process is mainly expected to obtain fine strengthening phases that are uniformly distributed,and to modify the morphology,size and number density of hard or brittle phases to improve the comprehensive mechanical properties of the alloys.Therefore,solidification and heat treatment are combined to affect the ultimate mechanical properties of the alloys.With the rapid development of advanced solidification techniques,the non-equilibrium of materials is significantly enhanced,and thus remarkably affects the solid-state transformation occurring in the subsequent heat treatment process.How to understand the effect of non-equilibrium solidification on subsequent solid-state transformation,and effectively tailor the microstructure of alloys via the combination of non-equilibrium solidification and solid-state transformation,is of great importance to modify the mechanical properties of Al-Si-Mg casting alloys.In this thesis,a commercial Al-Si-Mg?A356?casting aluminum alloy was solidified under different non-equilibrium solidification conditions.The effects of non-equilibrium solidification on the subsequent solid state transformations occurring upon solid solution and artificial aging treatments,the microstructural evolution,and the mechanical properties were investigated systematically.By uniting the non-equilibrium solidification,solid state transformations,and cold deformation,the mechanical properties of the alloy was improved significantly.The main conclusions are summarized as follows:?1?Upon melting process of the A356 alloy,the alloy melt was refined with C₂Cl₆.When the refining temperature is 993 K,the needle-like cavity defects in the as-solidified sample are reduced pronouncedly.Whereas the refining temperature is beyond or below 993 K,the refining effect is weak.By applying overheating treatment and high purity argon gas purification in the alloy melt in the temperature range of 993 to 953 K,the development of dendrites is effectively inhibited,which effectively reduces the shrinkage defects.?2?The non-equilibrium solidification of A356 casting alloy with wide range of cooling rates?0.12-96 K/s?was achieved by sand mould and step-like metal mould casting methods.The results indicate that?1?with increasing the cooling rate upon solidification,the secondary dendrite arm spacing and the size of eutectic Si can be continuously reduced,?2?the solubility of Mg and Si in Al matrix can be gradually increased,and?3?the formation of Fe-rich intermetallics can be suppressed to some extent.When the cooling rate is larger than 1.2 K/s,a number of nano-sized Si particles form in the Al matrix,and their number density increases with further increasing the cooling rate.When the cooling rate is higher than 20 K/s,a number of nano-sized Al?about 10 nm in diameter?particles are observed in the eutectic Si.The formation of these hierarchical microstructures has led to simultaneous increase in strength and ductility of the alloy with increasing the cooling rate upon solidification.?3?Subjected to solid solution treatment,the increase in the cooling rate upon solidification may facilitate the spheroidization of the eutectic Si and dissolution of the Fe-rich phase??-Fe?Al9FeMg3Si5??.The nano-sized Si and Al particles within the Al matrix and eutectic Si exhibit high thermal stability,and are influenced by the solid solution heat treatment.The solid solution treatment leads to a substantial improvement of strength and ductility of the alloys,especially the ductility,whereas does not change the trend of simultaneous increase in strength and ductility with increasing the cooling rate upon solidification.?4?The artificial aging treatments on the peak solution treated alloys lead to precipitation of a high density short rod-like metastable?'phase.Nano-sized Si twins are observed in the Al matrix of the sample solidified at 96 K/s.The aging treatments do not change the main features of the microstructures of peak solution treated alloys,such as the secondary dendrite arm spacing of primary?-Al phases,and the size and amount of the nano-sized Si and Al particles within Al matrix and eutectic Si.The increase in the cooling rate upon solidification causes the increase in the density of?'precipitates and the decrease of their size in the peak aged alloys.The aging treatments lead to a significant increase in strength of the alloys and a certain reduction in their ductility,whereas do not change the trend of simultaneous increase in strength and ductility with increasing the cooling rate upon solidification.?5?A route by the combination of a multi-passes cold rolling and artificial aging was developed to strength the A356 alloy solidified at a cooling rate of 96 K/s.After multi-passes rolling,the primary?-Al dendrites are stretched along the rolling direction,and the eutectic Si phase is remarkably refined and spheroidized.Subjected to six passes rolling,the ultimate tensile strength?UTS?and elongation to fracture?ETF?of the alloy reach 280 MPa and 35%,respectively.Further artificial aging the cold-rolled sample at 453 K for 4 h leads to a remarkable increase in UTS up to 375 MPa and a decrease in ETF to 26%.When the aged sample is cold rolled by three passes,the UTS and ETF of the sample reaches up to 480 MPa and 12%,respectively,which has been comparable to some high strength extruded aluminum alloys.?6?The results of this study indicate that by properly uniting the non-equilibrium solidification,solid state transformation,and cold deformation,the mechanical properties of Al-Si-Mg based casting alloys can be comprehensively improved.The processing route developed in this work may easily be applied in industrial scale production.