Study On Optimization Of Heat Treatment And Mechanical Properties Of 15%Mg₂Si/A356-1.5Fe Composites

Aluminum alloys have been widely used in various fields due to their excellent properties,such as low density and high specific strength.More and more aluminum scraps make the recycling become very important to the sustainable development of society.The high content of impurity Fe in the aluminum scrap may form the brittle phases which deteriorate the recycling aluminum alloys' properties.To date,the effective technique of Fe removal has not been developed.However,it should be noted that Fe-rich phases have the advantages of high hardness and high melting point.Based on this consideration,the present group proposed that the recycling aluminum alloys with high Fe content were chosen as the matrix to prepare the aluminum matrix composites reinforced by Mg₂ Si.The reinforcement of Mg₂ Si was formed in the Al melt by reaction between Si and Mg.The composite system was 15%Mg₂Si/A356-1.5Fe optimized in our previous study.In the present study,mechanical vibration and combining modification by Mn/Cr were applied to improve the mechanical properties of this composite.The heat treatment parameters were optimized by orthogonal experiment for this composite.The high temperature tensile property was tested.Some conclusions are obtained as follows:Under the vibration condition in the solidification process,the eutectic silicon and needle like ?-Fe phase could be refined.With the increasing of the vibration frequency,the tensile strength of the composite was improved.The tensile strength of the composite without vibration condition was only 69 MPa.The tensile strength was improved by about 50% to 104 MPa when the vibration frequency was 200 Hz.The needle-like ?-Fe phase was effectively modified by combination of Mn and Cr to nodular and rod-like ?-Fe phase.When Mn and Cr contents were both 0.5%,the tensile strength of the 15%Mg₂Si/A356-1.5Fe composites was 123 MPa and improved by 18% compared to the composite without modification.In all factors of heat-treatment,the solution temperature and solution time have significant influence on the properties of hardness and tensile strength of the 15%Mg₂Si/A356-1.5Fe composites.After being heat-treated,Mg₂ Si and Si phases became globular and distributed more uniformly.The friction and were properties were improved.Compared to the as-cast composites,the frictional coefficient decreased from 0.7 to 0.4.The worn surface was smoother.The single-stage solution at 550? was better than double-stage solution treatment.Base on the result of orthogonal test,the optimized heat treatment was solution at 550? for 4h,cooled by water,aging at 170? for 4h.The tensile strength of the 15%Mg₂Si/A356-1.5Fe by the optimal heat-treatment was 154 MPa and improved by 25% compared to that of the as-cast sample.High temperature tensile strength of the heat-treated 15%Mg₂Si/A356-1.5Fe composites and A356-1.5Fe recycled alloy were both tested at 200? and 300?.With increasing in the temperature,the tensile strength of A356-1.5Fe recycled alloy decreased sharply.As for the 15%Mg₂Si/A356-1.5Fe composites,its tensile strength decreased slowly.Compared to room temperature,its tensile strength decreased by 28% and 57% at 200? and 300?,respectively.However,these ratios were only 12 and 23% for the composites.The tensile strength of the composites was more than twice than that of the A356-1.5Fe recycled alloys at 300?.The composites show the excellent high temperature mechanical properties.