A stepwise sintering process for magnesium alloys fabricated by binder jetting additive manufacturing

The natural oxide layer on magnesium alloy particles hinders mass transfer and diffusion, making rapid and effective densification during sintering difficult, which challenges the progress and application of binder jetting in magnesium alloy forming. To address this, this study introduces a stepwise sintering process which is a refined sintering strategy based on the established two-step sintering method. The stepwise sintering process involves controlled heating rates and intermediate temperature holds, which minimize residual stresses, enhance densification, and optimize grain growth. A thorough investigation was conducted on the appearance, microstructure, densification, phase composition, and mechanical properties of AZ91D magnesium alloy for each stepwise sintering strategy from TH1 to TH5. The results show that under the TH4 sintering process, the sample has a uniform microstructure with minimal porosity or defects, leading to optimal performance with a density of 91.77 %, compressive strength of 202.33 MPa, and tensile strength of 89.45 MPa. Compared to the TH1 two-step sintering process, these properties have improved by 14.7 %, 41.9 %, and 106 %, respectively. Moreover, the sample maintains high shape fidelity. As the sintering process changes, the density and mechanical properties of the sample initially increase and then decrease, but the phase composition remains consistent as the alpha-Mg matrix with (3-Mg17Al12 phase. Notably, in the TH4 process, the (3-Mg17Al12 phase content peaks at 7.3 %. In conclusion, this study demonstrates that the stepwise sintering process can substantially enhance sintering density compared to the traditional two-step sintering method, which is crucial for optimizing the sintering process in binder jetting forming of magnesium alloys. It offers an innovative approach for densification improvement, sintering efficiency, and shape retention, thus providing insights for advancements in this field.