Study On Modification Of La/P In High Silicon Aluminum Alloy

The microstructure of hypereutectic Al-Si alloys, synthesized by conventionalcasting routines, normally consist of very coarse primary Si and eutectic Si crystalswith sharp sides and edges, which greatly aggravate the extrudability, machinability,strength and ductility of Al-Si alloys. Therefore, structural modification must be donein order to achieve fine Si phases with beneficial shapes and even distributions. In thispaper, pure La and Al-La-P composite alloy were used to modify the hypereutecticAl-20%Si alloy. The morphology, composition and mechanical property before andafter modification were analyzed by SEM, EDS and XRD. The main results ang theconclusions are following:Pure Nd can effectively refine the primary and eutectic silicon in alloy. Theoptimal modification parameters of La were content of0.3%and modification time of60min. The morphology of primary silicon after modification transforms frompentalpha and irregular shape to ellipsoid, and the grain size of primary silicon isrefined from78.42μm to35.52μm. A majority of eutectic silicon changes fromlamellar morphology to small short rod. The strength of the alloy modified by Laincreases by about from157MPa to179MPa，and its hardness increases by about from67.33HB to74.7HB.Basing on the modification by La, Al-La-P composite alloy was prepared by themix-melting method in this Paper. Al-La-P composite alloy is a new modifier to Al-Sialloy, and La, P are major alloy elements. Al-La-P composite alloy was used tomodify Al-20%Si alloy, the experimental results indicate that Al-La-P composite alloyhas a good modification effete to Al-20%Si alloy. Al-La-P composite alloy can changethe morphology eutectic Si and primary Si from their original coarse plateletstructures to less harmful firer fibrous structures. The average size of primary Sireduced to19.74μm, mechanical properties of tensile strength and hardness reach to186MPa and79.63HB. So, The metamorphic ability of Al-La-P is greater than La.After solution treated and age treated, the size of primary Si changed a little, thecorner gets smooth.The morphology of eutectic silicon particles granulate after heattreatment.P salt and liquid Al reaction can generate AlP, it served as a heterogeneous core,makes primary silicon size fine; The modification of La addition on Si takes effectsthrough the preferential absorption of La atoms onto the growth edges of Si and segregation of La atoms in the vicinity of Si/Al interfaces to suppress the growth of Sicrystals.