The Microstructure And Formation Of The Directionally Solidified Al-Si-Mg Ternary Near Eutectic Alloy

Currently, the directional solidification technology is widely used to obtain excellentorganization and properties of the materials with special orientation. The application ofdirectional solidification technique, can get even single crystal orientation structure,significantly improve the performance of material needed. This study will select Al-Si-Mgternary eutectic alloy as the research object of directional solidification experiments, todiscuss changes of organizational morphology, growth mechanism and mechanicalproperties.The downward directional solidification of the alloy with chemical composition ofAl-12.7Si-4.97Mg(wt%) was carried out at the pulling-down rates of1,3,5,10,15and25mm/min under the constant temperature gradient (5K/mm). By means of metallographicmicroscope and scanning electron microscope, to observe the microstructure morphology,and analyse its growth mechanism.Hardness and tensile strength are measured bymicrohardness instrument and Electronic universal tensile testing machine.The experimental results show that, in the microstructure of the Al-Si-Mg ternarynear eutectic alloy, organizational morphology of-Al phase is not too big change underdifferent speeds, but its size, a dendrite arm spacing and the secondary dendrite armspacing is tapering and smaller with the increase of the speed of the pulling-down; Underdifferent speed of pulling-down, Mg2Si phase morphology changes greatly in ternaryeutectic (-Al+Mg2Si+Si)E, appears such as the shape of the letter Xâ†'Yâ†'Vâ†'I changetendency; Under the same speed of pulling-down, degree of directional solidification isdifferent in different parts of the organization for the same sample, directionalcharacteristic is more obvious in steady-state district.With the speed of pulling-down increasing, material microhardness is tendency torising in result of grain refining, by linear fitting, the equation is calculated betweenhardness and the pulling-down speed: HB17.23exp(R/15.58)77.45. The tensilestrength and yield strength of samples after directional solidification are slightly higherthan the original as-cast specimens through tensile test. The main fracture mode of theoriginal as-cast specimens and directional solidification speciments are analysed by the tensile fracture surface analysis.