FU Zicong, YANG Shiting, TIAN Xianhui, LANG Fengchao, LI Jijun, ZHANG Weiguang
Surface Technology. 2024, 53(5): 85-95.
DOI: 10.16490/j.cnki.issn.1001-3660.2024.05.009
Due to the large temperature gradient, it is easy to accumulate residual stress inside the material, and residual stress will damage the normal function and structural integrity of the component during the forming process of selective laser melting technology. A suitable heat treatment process can release the residual stress generated during processing, which can improve the mechanical properties of the material. In order to accurately measure the mechanical properties of materials at the micro-nano scale, nanoindentation is often used. However, in the test of indentation hardness and modulus of elasticity, there are indentation size effects, which can affect the measurement results. Therefore, in order to study the effects of different preparation methods and annealing temperatures on the micro-nano mechanical properties and size effects of materials, the TC4 titanium alloy prepared by SLM technology by powder feeding was annealed at different temperatures in this paper. At the same time, the nanoindentation method was used to systematically analyze the micro-nano mechanical properties and size effects of materials in different states to provide a more comprehensive experimental basis for the study of nanoindentation size effects of additive manufacturing materials. Firstly, the original alloys were annealed at 600 ℃, 700 ℃, 800 ℃ and 900 ℃, respectively. After that, the surface of the alloy was physically polished and chemically etched and the microstructures of the original and four kinds of annealed alloys were observed with a scanning electron microscope. When the surface of the specimen was polished into a mirror, nanoindentation technique was used to measure the nano-hardness and elastic modulus of the original and four annealed alloys. During the test, the maximum load was set as 0.75 mN, 3 mN, 25 mN, 100 mN, 200 mN, 300 mN and 400 mN, respectively. In addition, when unloading to 10% of the maximum control load, it was held for 60 seconds to reduce the influence of thermal drift on the experimental results. Finally, the nano-hardness were fitted functionally by the proportional sample resistance model, Nix-Gao model and Meyer law. The results showed that the hardness and elastic modulus of the five kinds of Ti-6Al-4V alloy all decreased with the increase of the pressing depth, which showed the typical indentation size effect. The nano-hardness of the original and four annealed alloys measured by experiment was 3.66 GPa, 4.36 GPa, 3.96 GPa, 3.88 GPa and 4.77 GPa, respectively. The elastic modulus of the original and four annealed alloys was 113.1 GPa, 125.2 GPa, 102.1 GPa, 100.3 GPa and 108.7 GPa, respectively. The nano-hardness calculated based on the proportional specimen resistance model was 3.53 GPa, 4.34 GPa, 3.92 GPa, 3.52 GPa and 4.04 GPa, respectively. The nano-hardness calculated based on the Nix-Gao model was 3.68 GPa, 3.94 GPa, 4.07 GPa, 3.85 GPa and 4.47 GPa, respectively. The Meyer index fitted based on the Meyer‘s law was 1.75, 1.86, 1.82, 1.80 and 1.81, respectively, all of which was less than 2, it was positive indentation size effect. The three models can describe the indentation size effect of the original and annealed alloys well. The Nix-Gao model can establish the relationship between nanohardness and indentation depth directly, therefore the fitting results are closer to the experimental results, and the calculated hardness values are also the most accurate.
