Study On The Microstructures And Mechanical Properties Of Al-Si-Li Alloys

The addition of Li to Al-Si alloys can not only change the morphology of eutectic Si,but also improve the mechanical properties after heat treatment.Up to now,the researches on Al-Si-Li alloys is very little.Thus,the investigations on the Al-Si-Li alloys will benefit for the development of new aluminum alloys and thus extend the applications of the aluminum alloys.In this work,the microstructure of a series of Al-Si-Li with varying compositions were investigated by using optical microscope,scanning electron microscope,transmission electron microscope and the mechanical properties including hardness and tensile properties were examined.Also,the kinetics of the precipitation during aging of the Al-Si-Li alloy were investigated by using the differential scanning calorimetry technique.Based on the TEM examination and the kinetics analysis,the precipitation sequence in Al-Si-Li alloy was proposed.The microstructure of the as-cast Al-Si-Li alloys with varying compositions are composed of primary a-Al,binary Al+Si eutectic phase and ternary Al+AlLiSi+Si eutectic phase.The volume fraction of the eutectic phases increase with increasing Si and Li content.For the as-cast alloys,with increasing the content of the alloying elements,the hardness increases but the electrical conductivity decrease.After homogenization,the hardness of the alloys decreases and the electrical conductivity of the alloys increase compared to those of the as-cast alloys.After extrusion,the alloys were heat treated under varying solution and aging heat treatment.Based on the variations of hardness and electrical conductivity,the optimum heat treatment was determined as 550℃×1h for solution heat treatment and 200℃×6h aging heat treatment,respectively.After aging,all the alloys showed precipitation hardening.Compared to the alloys as-solutioned,the yield strength and tensile strength of the alloys after aging were increased but the elongation were decreased.Both the yield strength and tensile reached a peak value for the S2 alloy.The yield strength and the tensile strength of the S2 alloy after aging are 217 MPa and 223 MPa,which are increased by 197%and 92%,respectively,compared to those,73 MPa and 116 MPa after solution.After aging,stringer-like precipitations coherent with the matrix were observed in the matrix and particle-like precipitations were observed in the grain boundary around which there exists distinct precipitation free zone.The kinetics of the precipitation during aging were determined by using DSC technique.Based on the TEM observation and kinetics analysis,the precipitation sequence in the Al-Si-Li was propose.That is:Super saturated solutions→atomic cluster→GP zone→L"→L’→AlLiSi.