Numerical investigation on the reuse of recycled powders in powder bed fusion additive manufacturing

The reuse of recycled powder in laser powder bed fusion (LPBF) has significant economic and practical value. While numerous investigations have proposed the negative effects of recycled powder in LPBF, further research is still needed to develop effective schemes for powder reuse, which plays a critical role in sustainable development of additive manufacturing. In this article, the spreading and subsequent selective melting of actual Ti-6Al-4 V powder mixture with different mixing proportions of recycled powder (χ_r, weight percentage) were reproduced by using discrete element method (DEM) and computational fluid dynamics (CFD) simulations, where the influences of recycled powder content on the stability of LPBF process were systematically investigated/evaluated and corresponding mechanisms were explored. The results showed that for current cases, χ_r ≤ 60% can guarantee a stable LPBF process and desired powder bed/printing region quality. However, if χ_r > 60%, the LPBF process becomes unstable, which can result in not only the degraded powder bed but also the rough surface and internal defects in the printed region. The underlying mechanisms can be ascribed to: (1) during powder spreading, the excessive χ_r would exacerbate the interaction between particles to cause the unexpected particle scattered movement during deposition, thereby degrading the powder bed quality; (2) during laser melting, the formed loose powder bed would impede the heat absorption and dissipation, resulting in unstable liquid flow in the molten pool, thus deteriorating the quality of the printing region. Therefore, it can be confirmed that maintaining the powder bed quality in LPBF is crucial for achieving recycled powder reuse, and χ_r = 60% could be regarded as the threshold of current research cases.