Impact strength of Al-Si alloys solidified and held in mould kept at 500°C

Observation of impact strength of Al-Si alloys (5th report)

Abstract

After Al-10%Si alloys were cast in a mould heated at 500°C, they were treated in two ways. Some of them were held in a mould at 500°C for 0.513hrs. Others were quenched in water immediately after solidification and reheated to 500°C and held at that lemperature for 113hrs. The relation between the impact strength and the structures of alloys thus treated was studied by means of Charpy impact tests and metollographic observations.
The following results were obtained:
(1) The size and distribution of eutectic silicon crystals were markedly affected by heating process after casting. After Al-10%Si alloys were cast in a mould heated at 500°C constant and were held there at the same temperature for a long time, there occurred little change in configuration of eutectic silicon crystals during holding, which were distributed in acicular or thin tabular form. As the results of Charpy impact tests, these alloys had little impact strength, which was hardly changed during the further holding.
However, when the alloys were quenched in water immediately after solidification and reheated to 500°C, the configuration in eutectic silicon crystals was gradually changed from acicular to spheroidal form durinng the holding at 500°C.
(2) When the alloys had previously been quenched to room temperature after casting, and then held at 500°C, the shape of eutectic silicon cry stals was converted into spheroidal form and the impact strength was greatly increased.
The difference in impact strength of these alloys would be attributed to the change in configuration of eutectic silicon crystals which may depend on their previous heat history, not on their high temperature heat treatment.
(3) In Al-Si alloys modified by the addition of metallic sodium, the configuration in eutectic silicon crystals showed almost the same behavior as that of unmodified alloys.