Influences Of Pulse Electric Current Treatment On The Solidification Microstructures And The Mechanical Properties Of Al-Si Piston Alloys

As an important part of the engines, pistons play a significant role in bearing and transporting the load. Among piston materials Al-Si alloys are becoming dominant gradually due to their better specific strength and shape stability. It is said that every time the properties of pistons are improved, the"speed up revolution"in engines will take place.The pulse electric field grain refining technique that originates abroad takes much more progress domesticly. It is applied to many kinds of materials, and is investigated more deeply. It is expected to be used extensively in industrial fields. However, in order to improve pistons'wear resistance and high temperature resistance, we should increase the Si content in the Al-Si alloys. But this leads to uneven distributions in the morphology of the primary and eutectic Si phases. When we add chemical substance to modify them, it will also bring us problems of pollution, over-modification, and errors in composition controlling.In light of the issue that implies above, three different compositions of Al-Si piston alloys were prepared, and were subjected to pulse electric current treatment (PECT) at different temperatures. In virtue of optical microscopy (OM), SEM and EDS, we observed and analyzed the solidification microstructures including the morphology of the matrix and the distribution and configuration of the secondary phases. Accordingly, the mechanical properties of the specimen prepared previously were tested.The results indicate that PECT can refine the grains of Al-Si alloys, which refer to decreasing the grain size of the alloys with the Si content less than 12.6wt%. We also found the relations between the PECT temperature and the PDAS (primary dendrite arm spacing) of the alloys with Si content more than 12.6wt%. The morphology and distribution of the primary Si, eutectic Si, and the secondary phases varied obviously. Finally, we derived the best process parameter for PECT on Al-Si piston alloys from the testing of the tensile strength, elongation and microhardness of alloys prepared.In order to elucidate the effect and mechanism of PECT, we thought the melt of Al-Si piston alloy as melt of Al-Si binary alloy approximately, in terms of the thermodynamic regular solution model. On the basis of equilibrium transformation of the solid-liquid phase of Al-Si binary alloy, we demonstrated that the PECT's affect on the melt could arise from the variations of the interaction energy, which was based on an"electro-pulse modification"idea, and these variations resulted in the changes of Gibbs free energy in solid-liquid phase transformation. On the other hand, according to that PECT can accelerate the nucleation of Al, the growth of the primary Si and eutectic Si are surpressed, then changing their morphology and distributions.