Effect of quenching and tempering on microstructure and properties of Super Invar alloy fabricated by laser powder bed fusion for semiconductor equipment
摘要
Although laser powder bed fusion (LPBF) enables the near-net-shape fabrication of complicated-shaped Super Invar alloy components, tailored heat treatment upon the as-built raw blanks is absolutely necessary and extremely significant for sophisticated-dimensional control and high-stiffness scenarios in semiconductor equipment and other precision devices. This paper thoroughly investigates the effect of quenching at different solution-treatment temperatures (830, 930, 1030, and 1130 °C, respectively) and tempering on the microstructures, mechanical, magnetic, and thermal expansion properties of vertically and horizontally built Super Invar alloy samples. The vertically built sample possesses comparable grain sizes but higher dislocation densities, proportions of high-angle grain boundaries, hardnesses, and residual stresses than the horizontally built one, despite their same building volume. These microstructure differences, combined with the LPBF intrinsic grain morphology, further cause very distinct effects of heat treatment on the microstructures, mechanical, magnetic, and thermal expansion properties of Super Invar alloy samples built along different directions. Overall, quenching and tempering do not lead to apparent grain refinement but cause an enhancement in dislocation density and the consumption of high-angle grain boundaries. The dislocation density and hardness deteriorate as the solution-treatment temperature increases within the range of 830–1130 °C. The columnar-grain and equiaxed-grain regions within the as-fabricated Super Invar alloy predominantly exhibit dominating < 100 > and secondary < 111 > orientations. The vertically built samples solution-treated at 930 °C or even higher temperatures exhibit more notable texture composition changes than those treated at 830 °C or built horizontally. The thermal expansion properties of all investigated quenched and tempered Super Invar alloy samples fabricated via LPBF conform to the ASTM F1684 Standard. Both vertical and horizontal coefficients of thermal expansion (CTEs) above room temperature are generally positively correlated with the solution-treatment temperature within the range of 830–1130 °C. An apparent discrepancy in Curie temperature values determined from magnetization curves along different directions and thermal gravimetric analysis is observed. The saturation magnetization measured along the vertical direction is mainly affected by the texture distribution within equiaxed-grain zones on top surfaces, while the horizontally measured saturation magnetization shows a close correlation with columnar-grain zones within side surfaces. Compared to as-built samples, very slight ultimate tensile strength reduction combined with a notable yield strength decline (~ 90 MPa for both vertically and horizontally built bar samples) can be observed in all heat-treated samples. Vertically built samples solution-treated at 930 °C or higher temperatures display more pronounced texture composition changes than those treated at 830 °C or built horizontally. Although heat treatment causes significant elastic property degradation for most heat-treated specimens due to residual stress release combined with the generation of interior tensile stress, rigidity enhancement can be observed in LPBF-built Super Invar alloy samples solution-treated at 1030 and 1130 °C. This is mainly due to the preferred < 111 > and < 110 > orientations of equiaxed and columnar grains generated during solid-solution treatment at these high temperatures. The optimum heat treatment practice for LPBF-built Super Invar alloy to obtain superior rigidity (vertical elastic modulus: 117.2 GPa, horizontal elastic modulus: 160.2 GPa), acceptable strength performance, and inapparent thermal expansion anisotropy with relatively low average CTE values (approximately 0.5 × 10–6 ℃−1, 30–100 °C) is identified as water quenching after solid-solution treatment at 1030 °C for 0.5 h, furnace cooling after subsequent tempering at 315 °C for 1 h, and final aging at 95 °C for 48 h. The effectiveness of this heat treatment scheme on stiffness improvement was further proved via application verification on a raw blank of the lens barrel used in the lithography machine.