The effects of Cr addition on microstructure, hardness and tensile properties of as-cast Al–3.8wt.%Cu–(Cr) alloys

Abstract

An investigation is made on the effects of Cr addition (0.25 and 0.50wt%) on the solidification evolution, microstructure formation, hardness and tensile properties of Al–3.8Cu–(Cr) alloys. Solidification experiments were carried out in a vertical furnace using a metallic mold cooled from the bottom, thus permitting solidified ingots under transient heat flow conditions to be obtained. The solidification system was instrumented with thermocouples at positions along the length of the ingot to permit the solidification cooling rate to be determined from bottom to top of the ingots. The ingots were transversally and longitudinally sectioned to extract specimens for metallographic analyze, hardness and tensile tests. The macrostructures are shown to be entirely columnar along the length of the Al–3.8Cu and Al–3.8Cu–0.25Cr alloys ingots, whereas the Al–3.8Cu–0.50Cr alloy is shown to have a columnar-to-equiaxed transition close to the top. For any alloy ingot examined, the Al-rich matrix is characterized by a cellular morphology for high cooling rates followed by cellular/dendritic transitions with the decrease in cooling rate. The addition of Cr to the Al–3.8Cu alloy promoted the formation of Al23CuFe and Al7CuCrFe phases precipitated in the matrix and eutectic mixture, preventing formation of isolated Al–Fe intermetallic compounds. Hardness and tensile strength increased with increasing alloy Cr content, with highest values of both properties associated with the Al–3.8Cu–0.50Cr alloy ingot. Experimental equations correlating hardness/tensile strength and cellular spacing are proposed. These equations were coupled to the theoretical Hunt–Lu model to estimate hardness and ultimate tensile strength as a function of the cellular spacing.