Effects of Heat Treatment and Extrusion on Microstructure and Properties of A390 Alloy Hollow Billet Fabricated via DC Casting

An A390 alloy hollow billet was fabricated via a DC casting process prior to heat treatment and extrusion. The microstructural evolution and properties of the A390 alloy were studied by means of optical microscopy, scanning electron microscopy (SEM), electrical conductivity, tensile and hardness tests. The results show that primary Si particles experienced a slight change in that the sharp edges and corners became smooth. The eutectic Si particles were partially modified, the morphology changed from needle-like to rod-like and granular and the particle size increased during homogenization and T6 heat treatment. Some coarse primary Si particles fractured with cracks inside or separated into smaller ones, while fine eutectic Si particles being uniformly distributed in the matrix with a more rounded morphology after the extrusion process. Intermetallic phases were still distributed at the grain boundary after homogenization and T6 heat treatment, while the extrusion process was successful in changing the distribution of intermetallic phases to be uniform in the matrix. The Al8FeMg3Si6 (pi) phase disappeared and it was substituted by the Al5Cu2Mg8Si6 (Q) and theta phase during homogenization, and Fe was dissolved into the Q phase. The electrical conductivity increased after the heat treatment and extrusion process. The ultimate tensile strength (UTS) and hardness significantly increased after the T6 heat treatment as a result of precipitate hardening. The tensile strength and hardness value were reduced after the extrusion process and that can be ascribed to a small portion of primary Si particles being fractured with crack inside during the extrusion process. However, the elongation increased significantly after the extrusion process and it can be attributed to the microstructural evolution, especially the changes of eutectic Si improved the continuity of the matrix.