Effect of Mn Addition on Microstructure, Mechanical Properties and Die Soldering of Rheocasting Al-7Si-0.3Mg Alloys

Abstract

The effect of Mn additions (up to 0.8 wt.%) into Al-7Si-0.3Mg alloy on the microstructure, mechanical properties and die soldering was firstly investigated in gravity die casting conditions, with the aim to obtain an optimized Mn addition level. Especially, the stirring testing on H13 die material was used to elucidate the effect of Mn additions on the Fe concentration in the Al melting as a function of melting temperature (690 °C) and holding time (up to 32 h), which can provide an indirect sign of die soldering. On this basis, the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to be beneficial for microstructure, strength and die soldering. Secondly, rheocasting of Al-7Si-0.3Mg alloy without (Mn-free alloy) and with 0.6 wt.% Mn (Mn-containing alloy) was performed. The addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy was found to have following effects: (i) solidification temperature of α-Al is increased and a higher (about 20 °C) rheocasting temperature is therefore needed, but no significant die soldering problem was observed even with increasing casting temperature, (ii) Mn-containing phase forms along grain boundaries and within Al matrix, which increases tensile and yield strength but decreases elongation in as-cast, T5 and T6 conditions, (iii) number density and volume fraction of pores increases, and (iv) die soldering slightly improves. This paper demonstrates that the addition of 0.6wt.%Mn into Al-7Si-0.3Mg alloy is beneficial for microstructure, strength and die soldering in both gravity die casting and rheocasting conditions.