Investigation of the thermal cycle and mechanical properties of Al 6061 produced by binder jetting

Binder Jetting (BJ) of Al alloys can open new scenarios in the Additive Manufacturing (AM) field for many of the alloys that cannot be processed by Laser Powder Bed Fusion (LPBF) technologies because of their susceptibility to hot cracking (e.g., 2xxx, 6xxx and 7xxx series Al alloys). This study aims to investigate the de-binding, sintering and age-hardening cycles of binder-jetted specimens manufactured using a pre-alloyed Al 6061 powder feedstock, by microstructure characterization and mechanical testing. Dilatometry test results indicated that only a pure N2 atmosphere was effective in sintering the alloy. The optimization of the heat-treatment revealed that an optimal thermal cycle must include an initial solution step at 530 degrees C with water quenching and aging at 175 degrees C for 8 h. Mechanical tests, including tensile and fracture toughness tests, demonstrated that 6061-T6 exhibits isotropic mechanical behaviour. Tensile properties at room temperature were comparable to conventionally processed alloys but with lower elongation. Fracture toughness in air at room temperature resulted in substantially lower than extruded Al 6061-T6 specimens, and unstable crack growth was observed. The analysis of the fracture surfaces indicated that specific discontinuities such as oxide layers and other defects, such as pores, could have acted as stress-concentration points and preferential path for crack growth, favouring the development of brittle fracture mechanisms.