Relationship between machinability, microstructure, and mechanical properties of Al-7Si alloy

In this work, commercially available Al-7Si alloy was used, and the microstructure was modified by the additions of AlSr15, AlTi5B1, and Al3B. Cylindrical samples were cast into a sand mold and the machinability of the alloys was characterized by means of surface roughness, chip morphology, and buildup edge of the chips. Tool life and surface finish can be improved by decreased chip size by lowering the energy required for machining. The surface finish is an indicator of the quality of the material during the machining process. Therefore, in addition, melt quality was measured by means of reduced pressure test and the calculation of the bifilm index was used to compare the melt quality with machinability. It was found that there is a good correlation between bifilm index and surface roughness of the machined parts. Thus, for a better surface finish, melt quality, which can be quantified by bifilm index, has to be high. AlSr15 modified alloy exhibits the lowest built-up edge with the lowest surface roughness, because strontium modification causes coarse silicon crystals to transform to fine fibrous structure, which enhances machinability.