Metal additive manufacturing products often exhibit better mechanical properties. However, these products often have early failures during service due to defects (such as pores and unmelted particles), thus becoming the source of cracks. Cracks will propagate along the grain boundaries or where there is a continuous brittle secondary phase. The latest research from South China University of Technology reported the early failure mechanism of laser melting products in selected areas. The secondary microstructure of the Ag-Cu-Ge alloy produced by SLM shows that the semi-coherent precipitated phases are discontinuously distributed, but are arranged periodically along the cell crystal boundary. The strength of the printed Ag-Cu-Ge alloy can reach 410 ± 3MPa, and the elongation can reach 16 ± 0.5%. This value far exceeds the casting alloy and the annealed alloy of the same composition. SLM periodic structure and hierarchical distribution of precipitated phases and high-density internal defects will lead to higher strain hardening rate and stronger hardening strength, which can be proved by the twinning of precipitated phases and the enrichment of internal defects. These defects mainly include dislocations, stacking faults and twins. However, the fracture of the sample before necking is due to accelerated fracture at the defect. This work provides a possible way for additive manufacturing of ultra-high strength and ultra-high plasticity materials and preventing early failure.
Additive manufacturing technology, especially SLM technology, has attracted widespread attention and achieved tremendous development in recent years. The products it prints out often have higher strength. However, the products manufactured by additive are often anisotropic, the microstructure is also inconsistent in different areas, and the morphology is also different at different scales. For example, the typical characteristics of SLM, such as the melt channel will be different when viewed at the macro and micro angles, and the composition phase and distribution at the micro angle and nano scale are also different. The microstructure is basically typical anisotropy in the length direction, especially the change along the length direction is more obvious in the presence of heating of the sample. The anisotropy of this microstructure in the length direction makes the mechanical properties of SLM products difficult to predict. In addition, the SLM sample also has a non-equilibrium phase due to rapid cooling and rapid heating. Although the strength of SLM products will be higher than equivalent materials, they often fail before reaching the corresponding strength, that is, early failure. This phenomenon is especially obvious when manufacturing Al-12Si and AlMgSi. The work hardening curve is consistent with different stress-strain curves, indicating that the material fails before necking. The high density of internal defects and the introduction of new defects play a very important role in the failure of the material before the expected strength is reached. At this time, SLM will have the same tensile strength and breaking strength. This behavior is called early failure, and most SLM materials have the phenomenon of failure before necking, even in the theory of materials with greater strength. The reason for the early failure is due to the existence of defects such as pores and unmelted particles as the source of crack initiation, and then the crack expands along the grain boundary or cell boundary. The materials manufactured by SLM present a typical unique unstable cell structure, such as SLM Al-12Si, AlSi10Mg, CoCrMo and 316L. The cell boundary is called a layer-by-layer connection, which accelerates the crack propagation. In order to give full play to the advantages of SLM technology in high strength, it is very necessary to study the causes of early failure and obtain measures to reduce early failure, which is critical to give full play to the advantages of SLM and make SLM quickly become a competitive manufacturing process in the future.
Here, we report the mechanism of early failure in additive manufacturing. Choose Ag-Cu-Ge for the experiment. This is because the Ag-Cu alloy does not form a complex intermetallic compound phase due to the low Cur content, and Cu-Ge has a high probability of forming Cu3Ge and / or Cu5Ge phases, which precipitates in the Ag matrix. Therefore, the precipitation of complex intermetallic compounds and the harm they bring can be avoided. The internal mechanism of early failure is studied and a road map is designed to avoid early failure of products in SLM. Adding 1 ± 0.2wt% Ge to Ag-7.3wtCu can promote the co-separation of Cu / Ge in the primary Ag. The formation of eutectic α + β phase will lead to the formation of macro secondary structure, which is similar to the phase and morphology obtained in the Cu-10Sn alloy SLM. The eutectic structure formed from the molten pool is mainly the periodic arrangement of the primary enriched Ag-α phase and the secondary Cu-rich phase (Cu3Ge and Cu5Ge) in the matrix. The generation of fine tissue is caused by the SLM process being too cold.