Unidirectional flipped multiple-pass friction stir process: an innovative step in the fabrication of in situ Al-Cu composites

The in situ Al-Cu composites were manufactured by powder metallurgy with state-of-the-art microwave sintering tailed by friction stir process in two different ways: unidirectional overlapped two-pass and unidirectional flipped two-pass. Novelty is claimed in the flipped process. An attempt was made to investigate the addition of copper beyond the solubility limit and the critical composition of an Al-Cu alloy (4.6 wt.%). The overlapped process enforced higher temperature, cumulative strain, and strain rate. Since Al and Cu are high-stacking fault energy metals, both dynamic recrystallization and dynamic recovery occurred and resulted in grain refinement and higher fractions of Al₂Cu. The self-hard and brittle nature of Cu and Al₂Cu improved strength (Al-3wt.%Cu, 231.23 MPa), hardness (Al-6wt.%Cu, 82.5 HV), and deteriorated ductility (Al-7wt.%Cu, 5.2%). The formed Al₂Cu at the interface were surrounded by Al particles and formed passive films Al₂O₃ and Cu₂O enhanced corrosion resistance (Al-5wt.%Cu, 0.00717191 mpy). The process densely compacted the material, minimized porosity, decreased dislocation density, and increased strain aided in better electrical conductivity (Al-5wt.%Cu, 145.92%IACS). The flipped process circumvented excessive heating and embrittlement of the material thereby improving strength without loss of ductility (Al-7wt.%Cu, 235.85 MPa and 25.53%). Al-3wt.%Cu with minimum corrosion current (5.681 µA/cm²) exhibited maximum resistance to corrosion (0.169852 mpy). The highest electrical conductivity was noticed for (Al-5wt.%Cu, 104.17%IACS).