Additive manufacturing metal materials are becoming more and more popular in the fields of automobiles, biomaterials and aviation. They usually contain high-density dislocations, stacking faults and other structural defects and fine microstructures such as nano cell structures, resulting in materials with high strength and poor plasticity.
Recently, Professor Wang Zhi and his collaborators of South China University of Technology used laser selective melting (SLM) to control the microstructure of silver alloys through alloy design, change the interface of nano-cell structure to continuously precipitate brittle phases, and obtain regularly arranged discontinuous precipitated phases. An ultra-high-strength silver alloy with a tensile strength of 410 MPa is obtained, which is much stronger than the as-cast and annealed silver alloys, and has good plasticity (16%). At the same time, it is clearly pointed out that high-density structural defects and defects such as holes, unmelted particles and micro-cracks are the root causes of premature failure of additive manufacturing metals.
Related achievements were published in NPG Asia Materials (3-year average IF = 8.139) under the title of “Premature failure of an additively manufactured material”, and the journal also published editorial reviews titled: Additive manufacturing: 3D-printed alloys find their hidden strength.
Thesis link: https://www.nature.com/articles/s41427-020-0212-0
In this study, by adding a small amount of germanium to the silver-copper alloy, a second phase of discontinuous Cu- and Ge-rich precipitated periodically in the subgrain boundary of the cell structure was obtained, and the precipitated phase had a semi-coherent relationship with the Ag matrix. These semi-coherent precipitated phases can effectively hinder the movement of dislocations and increase the strength. At the same time, these precipitated phases can twin during the deformation process to release local stress, avoid crack initiation and improve plasticity.
The TEM microstructure of the SLM Ag-Cu-Ge alloy after tensile deformation shows that there are a large number of dislocations, stacking faults (SFs) and twins in the Ag matrix, and there are also twins inside the precipitated phase. Periodic misfit dislocations are clearly visible at the interface between the precipitated phase and the Ag matrix. Tensile fractures indicate that cracks initiate at external defects (holes and unmelted powder particles) and propagate along internal defects.
In summary, this study obtained a discontinuous precipitation strengthening phase of cell structure subgrain boundary through alloy design, prepared an ultra-high strength silver alloy with tensile strength of 410 MPa and plastic deformation of 16%, revealing cracks in the holes The initiation of unmelted particles and the propagation of internal structural defects is the cause of premature failure of additive manufacturing metals. These findings provide a new theory for additive manufacturing of high-strength and tough metals.