Metal additive manufacturing (MAM) is revolutionizing the production methods in many industries, especially in the aerospace, automotive and biomedical fields. However, the further widespread adoption of MAM has many technical problems, and one of the main obstacles is the control of the grain structure. Poorly controlled grain structure will affect its thermal cracking properties and lead to anisotropic mechanical properties, especially in high-performance alloys. The alloys currently used in industry are initially designed for conventional production processes, and are not optimized for MAM processes. New alloys with high strength and optimal solidification properties are needed to maximize the absorption of MAM as a competitive manufacturing route for high-performance components.
For decades, it has been recognized that fine and equiaxed grains can reduce the tendency of thermal cracking and improve its performance, for example, by strengthening the Hall-Petch relationship. However, in MAM, due to the extremely high cooling rate and unbalanced solidification of the thermal gradient, the main feature of the grains is the columnar and textured microstructure. Therefore, the formation of equiaxed grains in MAM is a huge challenge. Although progress has been made in obtaining fine equiaxed grains by adding grain refiners to the MAM of aluminum alloys, there are still no commercial refiners that can effectively refine the microstructure of titanium grains.
Under the leadership of Professor Mark Easton of the RMIT Additive Manufacturing Center and Professor Hamish L. Fraser of Ohio State University (co-corresponding author), Dr. Dong Qiu and Dr. Duyao Zhang and the Commonwealth Scientific and Industrial Research Organization (CSIRO) ), The University of Queensland and the University of Nevada cooperate to design tunable microstructures for MAM components (especially light alloys).
The theory behind the project is based on the interdependence theory (Acta Mater.2011, 59, 4907) proposed by Professor David StJohn and others. This titanium-copper alloy has a higher ability to supercool the structure, which is due to the distribution of alloying elements during the solidification process, which can overcome the negative effects of high thermal gradients in the laser. The printing process does not require any special process control or other treatment. The printed titanium-copper alloy sample has a completely equiaxed fine grain structure. Compared with conventional alloys under similar processing conditions, they also show excellent mechanical properties, such as high yield strength and uniform elongation, which is attributed to the use of high cooling rate and multiple thermal cycles to form super Fine eutectoid microstructure.
On December 5, 2019, related research results were recently published in "Natrue" magazine, entitled "Additive manufacturing of ultrafine-grained high-strength titanium alloys" (additive manufacturing of ultra-fine grain high-strength titanium alloys).
The Ti-Cu alloy manufactured by the MAM process has completely fine equiaxed primary grains and eutectoid flakes, and has excellent mechanical properties. Experiments have shown that tunable microstructures can be realized on multiple microstructure length scales through MAM. The proposed new alloy design strategy focuses on synergistically controlling the thermodynamics of alloying elements and the solidification conditions of MAM. The authors also hope that their alloy design concepts can be applied to other alloy systems and develop more high-performance engineering alloys for MAM in the future.