Premature failure of an additively manufactured material

Additively manufactured metallic materials exhibit excellent mechanical strength. However, they often fail prematurely owing to external defects (pores and unmelted particles) that act as sites for crack initiation. Cracks then propagate through grain boundaries and/or cellular boundaries that contain continuous brittle second phases. In this work, the premature failure mechanisms in selective laser melted (SLM) materials were studied. A submicron structure was introduced in a SLM Ag–Cu–Ge alloy that showed semicoherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. This structure led to a remarkable strength of 410 ± 3 MPa with 16 ± 0.5% uniform elongation, well surpassing the strength-ductility combination of their cast and annealed counterparts. The hierarchical SLM microstructure with a periodic arrangement of precipitates and a high density of internal defects led to a high strain hardening rate and strong strengthening, as evidenced by the fact that the precipitates were twinned and encircled by a high density of internal defects, such as dislocations, stacking faults and twins. However, the samples fractured before necking owing to the crack acceleration along the external defects. This work provides an approach for additively manufacturing materials with an ultrahigh strength combined with a high ductility provided that premature failure is alleviated. An analysis of metallic alloys fabricated through layer-by-layer deposition processes has revealed critical factors in preventing these materials from unexpectedly breaking. In selective laser melting (SLM) technology, thin layers of metal powders are assembled into three-dimensional objects using rapid heating and cooling steps. Zhi Wang from the South China University of Technology in Guangzhou and colleagues now show that the microstructure of a silver-copper-germanium alloy formed through SLM can affect the material’s strength. Using optical and X-ray microscopy, the team found that regularly spaced precipitates formed inside the 3D-printed alloy prevented abrupt atomic sliding movements, giving the material a higher natural strength and good ductility. Fractures that occurred at lower than expected stress levels were identified as arising from errors in the printing process, such as pores and unmelted powder particles. A submicron structure strategy was introduced in a selective laser melted (SLM) Ag-Cu-Ge alloy, showing semi-coherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. It leads to a remarkable strength of ~410 MPa with ~16% ductility, well surpassing the strength-ductility combination of their cast counterparts. The hierarchal SLM microstructure and high density of internal defects leading to a high strain hardening rate and strong strengthening. Premature failure occurred due to the external defects, such as pores and unmelted particles. This work paves a way for additively manufacturing materials towards high strength–ductility synergy.