Effects of Mg on the impact strength of Al-Si-Cu alloys

Abstract

Effects of addition of Mg (0.11.2wt%) to Al-Si-Cu alloys on the impact strength and structures were examined by means of Charpy impact test and metallographic method.The following four types of specimens were used for the experiments.(a) As cast.(b) Solution treated at 500°C for 240 min.(c) Age hardened at 160°C for 300 min. after solution treatment.(d) Age hardened at 200°C for 300 min. after casting.The results obtained were as follows:(1) The impact strength of Al-Si-Cu alloys containing Mg was much increased by solution treatment at 500°C for 240 min. as observed in Al-Si and Al-Si-Cu alloys. The behavior of impact rupture was characterized by a great increase in time for formation and propagation of cracks with a small increase in maximum load, which resulted in a great increase in impulse. The above behavior would be attributed to the dissolution of Cu and Si into matrix and the changes in shape and amount of crystallites.(2) The impact strength of as cast and solution treated alloys were likely to be decreased with the increase in Mg content. The behavior of impact rupture was characterized by a decrease in time for formation and propagation of cracks with a constant maximum load. The above behavior would be attributed to the increase in remaining undissolved crystallites due to the addition of Mg.(3) The impact strength of the alloys age hardened at 160°C for 300 min. after solution treatment and those age hardened at 200°C For 300 min. after casting were likely to be decreased with the increase in Mg content.The above behavior of impact rupture was characterized by the decrease in time for formation and propagation of cracks with the increase in maximum load, which led to the decrease in impulse. The behavior of the former alloys would be attributed to the composite effect of changes in both of shape and amount of crystallites (as described in (1)) and age hardening. Therefore, the increase in maximum load would be attributed to the effect of age hardening.(4) It was found that the hardness of Al-Si-Cu-Mg alloys after ageing was not different between the specimen age hardened at 200°C for 300 min. after casting and that age hardened at 160°C for 300 min, after solution treatment, but the impact strength was much different between the both. It was observed by fractography that the former specimen showed brittle fracture and the latter showed ductile fracture, though they had the same composition and hardness.