Effect Of Silicon Particle Morphology On Properties Of A356 Alloy Made From Electrolytic Low Content Titanium Aluminum Alloy

A new technique of adding titanium to aluminum was depicted: a way of adding titanium by means of electrolysis method. The variation of silicon morphology and mechanical properties of the A356 alloys made from the electrolytic low-titanium aluminum alloy which being produced in this way were investigated.The change of silicon phase versus solution time during heat treatment at 535 ℃ was studied. The geometry parameters of silicon granules were measured and their variation during the heat treatment as well as probability distribution was analyzed. The results shows that after being heated for about 1 hour the long-radius average and aspect ratio average decrease drastically to about the minimum, subsequently, the variation becomes gradually. With the prolongation of the solution treatment time, the short-radius of the silicon particle increases. But the diameter average decreases at first, approaches to the minimum after treated 2 hours, then increases with the solution time. The investigation indicates that the probability distribution of long-radius, diameter average and aspect ratio enhance during the solution treatment and, the change of the long-radius and the aspect ratio are evident particularly, of which the high distribution concentration is approximately in the range of 2-4 hours. The results reveal that after being solution treated for about 2-4 hours, the desirable spheroidization is obtained and the distribution is more evenly at the same time.The rule of tensile properties versus heat preservation time at 535 ℃ was also studied. The aging treatment is 165℃ × 8h. It reveals that the tensile toughness of the samples being treated for 3-4 hours were almost as good as the ones being treated for 6-8 hours, which was consistent with the variation of the silicon phase. But the alteration of yield toughness was slight. The variation of mechanics properties during the aging course was also investigated. The strength and hardness increase while the plasticity decreases at the course of aging. The heat treatment technique about thiskind of A356 alloy was preliminarily obtained. The trend of the tensile properties during age course suggests that 2-4 hours is the optimal age time at 165 ℃ after solution treated at 535 ℃ for 3 hours.The influence of the silicon phase morphology on the fatigue and fracture toughness was carefully studied. We find that the fatigue crack initiation life is not monotonous during the solution stage. The results indicate that the fatigue crack initiation life is longer for the samples solution treated for about 4 hours. And the influence of the silicon granule morphology on fatigue crack growth rate as well as fatigue crack path was also investigated. The change of silicon granule has little effect on the fatigue crack propagation at Paris region while it is not true at the low fatigue crack propagation rate region. Near the threshold area, the fatigue crack resistance of the specimen being treated 3 hours is the best. The fatigue crack locus does not preferential go into preliminary a - Al, but is more prone to the eutectic region. The fatigue crack path went round silicon particle in most case, while only a few silicon particle was cut into halves. The fatigue crack growth resistance enhances as soon as the silicon particle becomes round and moderate big, which contribute to the low propagation rate. At last, the rule of 4-6 solution hours treatment for the optimal fracture toughness was found. All in all, 4 hours or even longer heat treatment is more preferable in order to obtain the desirable fatigue and fracture properties.