Microstructure, Hardness, Tensile Strength, and Sliding Wear of Hypoeutectic Al–Si Cast Alloys with Small Cr Additions and Fe‐Impurity Content

Abstract

Herein, the influence of Cr additions on the solidification parameters, as‐cast microstructures, and mechanical properties of Al–Si alloys with Fe‐impurity is investigated. Al–Si–Cr alloys with 3% and 7% Si and 0.25% and 0.40% Cr are developed. Solidification experiments are performed using an instrumented cooling casting apparatus permitting the obtainment of vertical upward‐solidified ingots. The morphological and phase structures are characterized by optical and scanning electron microscopy and X‐ray diffractometry. Hardness, tensile, and dry sliding wear tests are conducted to determine the mechanical properties. The Cr addition to the Al–Si alloys leads to the formation of α‐Al(Si, Fe)Cr interdendritic precipitates, suppressing the formation of undesired needle‐like β‐intermetallic compounds (Al5FeSi). It is noted that both higher alloy Cr content and cooling rate induce refinement in the dendrite arm spacing, and improvement in both hardness and ultimate tensile strength when considering the alloys with identical Si content. The coefficient of friction slightly increases with increasing alloy Cr content and with coarsening of the microstructure. Correlations between the wear rate and secondary dendrite arm spacing are proposed, in which the wear rate decreases with decrease in the secondary dendrite arm spacing and increase in the alloy Cr content and hardness.