Effect of La addition on microstructure and mechanical properties of hypoeutectic Al-7Si aluminum alloy

摘要

The effect of La addition(0,0.1,0.2,0.4,wt.%)on the microstructure,tensile properties and fracture behavior of Al-7Si alloy was investigated systematically.It is found that the La appears in the Al-7Si alloy in the form of Al4La and Al2Si2La phases.La addition can refine the secondary dendrite arm spacing(SDAS)and eutectic Si particles,which are decreased by 7.9%and 7%,respectively,with the optimal La content of 0.1wt.%.Because when 0.1wt.%La is added,a relatively higher nucleation undercooling of 37.47℃ is observed.Higher undercooling degree suggests that nucleation is accelerated and subsequent growth is restrained.After T6 heat treatment,compared with the without La,the ultimate tensile strength of the alloy with 0.1wt.%La is enhanced by 5.2%from 333 MPa to 350.2 MPa and the elongation increases by 73%from 7.37%to 12.75%,correspondingly.The fracture mode evolves from the ductile-brittle mixed fracture to ductile fracture mode.However,when La element content reaches a certain value of 0.4wt.%,serious segregation takes place during the solidification process.The formed brittle phases deteriorate the tensile properties of the alloy and the fracture mode of Al-7Si-0.2/0.4 La changes to mixed ductile-brittle fracture mode.