Continuous Heating Dissolution and Continuous Cooling Precipitation Diagrams of a Nickel-Titanium Shape Memory Alloy

Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet been used to analyse precipitation processes during continuous temperature changes. We have enabled DSC to analyse dissolution and precipitation processes in situ during heating as well as during cooling from the solution annealing temperature. Results are presented as CHD and CCP diagrams, including information from microstructure analysis and the associated changes in the austenite/martensite transformation temperatures.

Modified Solution Annealing Heat Treatments of Powder Bed Fused Inconel 718

Inconel 718 (IN718) is a superalloy commonly used in aerospace, turbomachinery, and applications with extreme conditions due to its creep and corrosion resistance and high strength and hardness at a wide range of temperatures. The recent development of metal additive manufacturing offers a new approach to fabricate complex IN718 parts with minimal machining. However, additively manufactured IN718 parts suffer from anisotropic mechanical properties and are usually inferior to conventionally produced parts. This is especially noticeable under dynamic loading conditions, where they suffer from lower fatigue strength and life in addition to lower reliability. This study focuses on post-processing heat treatments that aim to homogenize the microstructure of the additively manufactured IN718 and reduce the defects produced during fabrication. In this work, we developed modified solution annealing treatments for IN718 samples, followed by tensile testing. X-ray diffraction and scanning electron microscopy imaging were used to evaluate the microstructure. The results show that by increasing the initial heat treatment temperature, better isotropy, Young’s modulus, and ductility were produced at the cost of a slight reduction in the tensile strength. The further rise in the heat treatment temperature increased the grain size significantly and produced unfavourable precipitate morphology, which caused a brittle behaviour.

HEAT TREATMENT EFFECT ON MARAGING STEEL MANUFACTURED BY LASER POWDER BED FUSION TECHNOLOGY: MICROSTRUCTURE AND MECHANICAL PROPERTIES

Laser Powder Bed Fusion (L-PBF) is a widespread additive manufacturing technology in industrial applications, for metal components manufacturing. Maraging steel is a special class of Fe-Ni alloys, typically used in the aerospace and tooling sectors due to their good combination of mechanical strength and toughness. This work analyses the heat treatment effect on the microstructure and hardness value of 300-grade maraging steel manufactured by the L-PBF process. The considered heat treatment consists of a solution annealing treatment followed by quenching and ageing hardening treatment. The effect of ageing temperature is reported, in a wide temperature range. Results show that solution annealing treatment fully dissolves the solidification structure caused by the L-PBF process. Moreover, the ageing hardening treatment has a significant impact on the hardness, hence on strength, of L-PBF maraging steel. The optimal ageing conditions for the L-PBF maraging steel are identified and reported: in particular, results show that the hardness of 583 HV is achieved following ageing treatment at 490 °C for 6 hours. A higher treatment temperature leads to over-ageing resulting in a decrease of hardness. Conversely, an excessive ageing time does not seem to affect the hardness value, for the ageing temperature of 490 °C.

Microstructure and Hardness Evolution of Solution Annealed Inconel 625/TiC Composite Processed by Laser Powder Bed Fusion

This study deals with the Inconel 625 (IN625) alloy reinforced with micro-TiC particles processed by laser powder bed fusion. The microstructure and hardness in the as-built and solution-annealed states were investigated. The microstructures of the as-built IN625 and IN625/TiC states were primarily made up of columnar grains along the building direction. After the solution annealing at 1150 °C for 2 h, the IN625 alloy consisted of equiaxed grains due to recrystallization and grain growth. On the contrary, the solution-annealed IN625/TiC composite still presented columnar grains. Therefore, the TiC particles hinder the recrystallization, indicating higher microstructure stability for the composite. For the IN625/TiC composite, both the reduced alteration of the grains and the more intensive formation of carbides prevent a remarkable hardness reduction in the solution-annealed state.