Effect Of Heat Treatment On Microstructures And Properties Of Semi-Solid Formed Hypereutectic Al-Si-Cu-Mg Alloy

Hypereutectic Al-Si alloys are widely used in the automotive industry,especially in the production of pistons and engines,because of the advantages of good thermal stability,excellent wear resistance and small coefficient of thermal expansion.However,there are generally coarse and sharp edges of primary Si phases which cause irreversible damage to the alloy matrix and weaken the properties of the hypereutectic Al-Si alloy.Therefore,the emphasis of industrial production of hypereutectic Al-Si alloy is to improve the morphology of Si phase.In this paper,we studied the effects of solution treatment and aging treatment on the microstructures and properties of semi-solid formed hypereutectic Al-Si-Cu-Mg alloy which was prepared by semi-solid thixotropic forming process.The semi-solid slurry of hypereutectic Al-Si-Cu-Mg alloy was prepared by electromagnetic stirring,and the effect of electromagnetic stirring on the microstructures and mechanical properties of the alloy was studied.The results showed that the microstructure of the alloy was improved after electromagnetic stirring.When the electromagnetic stirring time was 60s,the tensile strength,elongation and hardness of the alloy were 192MPa,2.33%and98.3HB respectively.The block-like sharp edges became dull and the size of primary Si phase was reduced.Three different pressure ratios of 400MPa,600MPa and 800MPa were selected during semi-solid extrusion,respectively.As the pressure ratio increased,the primary Si phases became finer and more round and smooth in the alloy.When the pressure ratio of the semi-solid extrusion was 600MPa,the alloy exhibited excellent mechanical properties,tensile strength was 237MPa,elongation was 2.59%,and hardness was 105.8HB.In order to study the effect of heat treatment on the microstructures and mechanical properties of semi-solid formed hypereutectic Al-Si-Cu-Mg alloy,the solution treatment time and aging treatment time were changed.With the increase of solid solution time,the strip-like eutectic Si phase was necked,melted and granulated,the bulk-like primary Si phase was passivated,the size of Al₂Cu strengthening phases decreased,the properties of the alloy were improved.The circularity and the average diameter of Si phases became optimized in the alloy per unit area after solution treatment for 10h.The mechanical properties of the alloy reachd the peak value,the tensile strength was 311MPa,the yield strength was 291MPa,and the elongation was 2.20%,the hardness was 132.0HB.At the same time,the peeling pit on the surface of the alloy was small,and the furrow was small and not obvious.The wear weight and average friction coefficient of the alloy were low,and the values were 2.4mg and 0.3913,respectively,which proved that the alloy had a good wear resistance.The eutectic Si phase coarsened when the solid solution time over 10h.The mechanical properties of the alloy were improved after aging treatment.When the aging was 1h,the Q''phase of the punctate precipitates appeared in the alloy;when the aging was 8h,the??phases of the rod-like precipitates appeared in the alloy,and the number of punctate precipitates decreased;the number of rod-like and punctate precipitate phases decreased in the alloy after ageing for 12h.