Microstructural Modification Hardness and Surface Roughness of Hypereutectic Al-Si Alloys by a Combination of Bismuth and Phosphorus

Abstract

This research aimed to investigate the effects of bismuth (Bi) and phosphorus (P) addition on the microstructure and roughness of a B390 hypereutectic Al-Si alloy. Microstructural variation of as-cast alloys showed that P is an effective refiner of primary Si and its distribution, while Bi has the opposite effect. The liquidus temperature increased with greater amounts of P, resulting in a wider solidification range. The solidus temperature increased with Bi content, resulting in a shortened solidification range. The benefit of Bi on eutectic silicon was through changing its morphology from fibrous to lamellar structures. It was observed that both primary Si and eutectic Si were modified when both P and Bi were added to the alloys. For effective refinement, the size of primary Si can be reduced by 50.6% and, the P content should not be less than 0.1 wt% with a Bi content of 0.5–1.0 wt%. Furthermore, an intermetallic phase size reduction also occurs. P addition results in a significant (5.85%) increase in hardness. Adding Bi (up to 1.0 wt%) together with P (up to 0.1 wt%) did not reduce the effect of P on the hardness of the modified alloys. The most effective reduction in roughness achieved during cutting at both low and high speeds was 26.5% and 57.7%, respectively, from modifying the B390 alloy using 0.1 wt%P with 1.0 wt%Bi.

Keywords

• phosphorus
• bismuth
• hypereutectic Al-Si alloys
• primary silicon
• modification