Effect of Solid-Solution Treatment and Cooling Method on Texture Evolution and Properties of Super Invar Alloy Fabricated by Laser Powder Bed Fusion

Abstract

Tailored heat treatment is crucial for achieving the desired low coefficient of thermal expansion (CTE) and high elastic modulus (E) of laser powder bed fusion (LPBF) processed Super Invar alloy (Fe-32Ni-4Co) in semiconductor equipment and other precision devices. This paper thoroughly investigates the effect of solid-solution treatment (at 830 ℃ and 1030 ℃) and cooling method (Furnace cooling, air cooling, oil cooling and water quenching) on the microstructures, mechanical properties, magnetic properties and thermal expansion behaviors of LPBF manufactured Super Invar alloy samples. Faster cooling rate is favorable for the development of stronger textures of Copper components, while WQ and particularly FC can lead to Goss component enhancement. The as-built Super Invar alloy samples contain relatively high residual compressive stresses. Solid-solution treatment at higher temperature and faster cooling rates can more effectively relieve these compressive residual stresses and induce higher internal tensile stresses in the interior. Solid-solution treatment at 1030 ℃ results in lower strength, hardness and elastic modulus than at 830 ℃. The hardness, strength, elastic modulus and average CTE within 30~100 ℃ are generally negatively correlated with cooling rate despite the abnormal enhancement in water-quenched samples. Attributed to water-quenching-induced <111> and <110> texture generation, the water-quenched samples, particularly solution-treated at 1030 ℃, exhibit notable rigidity recovery. Strong magnetostriction effect, which may be attributed to the poor elastic property, is observed within the oil-cooled samples showing the lowest thermal expansions. A negative correlation between Curie temperature and cooling rate can be clearly identified. The vertically-measured saturation magnetization variation generally follows the intensity variation of dominant Goss textures of the columnar grains within side surfaces, while the horizontally-measured saturation magnetization value is positively correlated with the intensity of notable Copper textures within the equiaxed-grain zone. The furnace-cooled samples solution-treated at 830 ℃ exhibit the highest rigidity (Vertical E: 155.7 GPa. Horizontal E: 130.5 GPa) and strength but even higher CTEs (above 1.0×10^-6 °C^-1, 30~100 °C) than the standard suggests. The oil-cooled treated samples solution-treated at 830 ℃ possess the lowest CTEs (0.20×10^-6~0.40×10^-6 °C^-1, 30~100 °C) at the sacrifice of relatively small E decline (Vertical E: 136.6 GPa. Horizontal E: 123.5 GPa). The water-quenched ones solution-treated at 1030 °C demonstrate comprehensively acceptable CTEs (0.40×10^-6~0.80×10^-6 °C^-1, 30~100 °C) and superior stiffness performance (Vertical E: 152.4 GPa. Horizontal E: 119.3 GPa). Targeted selection towards detailed requirement of static and dynamic dimensional stability under specific circumstances among these recommended heat treatment practices would be advisable during engineering application of LPBF processed Super Invar alloy components.