Preparation of Al-Al₂Cu Composites In-situ Processed Via Reactive Infiltration

Al-Al₂Cu composites due to their superior properties such as low density, high thermal stability, low thermal expansion coefficient, high hardness and good wear properties are appropriating candidates for automotive and aerospace industries. One advantage of the in-situ methods for processing composites is that the second phase materials are thermodynamically stable and therefore they can have a strong bond with the matrix alloy. In this study, Al-Al₂Cu composites were processed by molten aluminum reaction with copper wires placed in a preheated steel mold. The solidified samples were then heat-treated to supplement the reaction between Al and Cu and convert the remained Cu to intermetallic. The diameter of the copper wires, the temperature at which the molten metal is poured, the time and temperature of heat treatment, as well as other material and processing parameters, were optimized for this purpose in order to transform the copper wires into rod-like intermetallic while also preventing the uniform distribution of the resulting intermetallic particles in the sample. Therefore, a preform was made by using 500 µm diameter copper wires with 99% purity. This preform was placed inside a steel mold pre-heated at 520 °C and subsequently molten pure aluminum (99.6% purity) at 720 °C was poured into the mold. As a result of the reaction between Cu and Al, aluminum- copper intermetallic compounds had been shaped within the form of continuous fibers. In order to complete the reaction the samples were heat-treated in a tube furnace at 570 °C for 20 min. Reference samples were prepared from pure aluminum as well as Al-Cu alloy with the same copper content as composites to be compared with the heat-treated and as-cast composites. The findings show that the acid pickling action improves solid-state inter-diffusion and homogenous infiltration into the Cu wires. The Cu core in the composite benefits from a uniform inter-diffusion in the solid-state thanks to a lengthy heat treatment period (40 min). Al₄Cu₉ composites made under and above eutectic heat treatment, respectively, have Al₂Cu rosette-shaped in the matrix and around the Cu wires. The heat-treated composite produced the better tensile strength (81 MPa) and toughness (14 KJm^–3) as compared to the composite Not-heat treated above the eutectic temperature (78 MPa and 12 KJm^–3). The specimens were characterized by optical microscopy, SEM, EDS, microhardness measurement and tensile tests. The results showed that by using the abovementioned procedure the Al-Al₂Cu composites with rod-shaped intermetallic can be produced successfully.