Weldability of Cast Superalloys - Effect of homogenization heat treatments on hot cracking susceptibility of cast Alloy 718, ATI® 718Plus®, and Haynes® 282®

Doktorsavhandling, 2020

Ni- and Ni–Fe-based superalloys are used in high-temperature sections in the rear end of aero engines owing to their superior mechanical properties compared to those of the other alloys. Traditionally, structural components have been manufactured as single piece castings. However, the recent trend for the fabrication of hot structural components has changed, instead of the traditional single piece castings, to welding of wrought parts in sections where high strength is required, and cast parts, where complex geometrical shapes are needed. This can be challenging, as superalloys are prone to weld-cracking phenomenon known as “hot cracking”. Especially, the cast materials are known to be more prone to cracking owing to the higher extent of segregating phases that remain from the casting process. Traditionally, Alloy 718 has been used for manufacturing hot structural components of aero engines. New alloys, such as ATI® 718Plus® and Haynes® 282® have been introduced over the past decade in view of the need for materials that could surpass the maximum operative temperature of Alloy 718 thereby improving the overall aero engine efficiency. The cast versions of ATI® 718Plus® and Haynes® 282® have been developed during the recent years; however, very little is known about their weldability performance. Prior to welding, hot isostatic pressing treatments are performed to homogenize the material and eliminate any porosity that remained from the casting process. Although the standards for HIP treatment of cast Alloy 718 have matured in the aerospace industry, no such standard heat treatments are available for the cast versions of ATI® 718Plus® and Haynes® 282®. In the current study, different heat treatments were performed to investigate the homogenization effect on the segregating phases. Moreover, the effect of homogenization heat treatments on the heat-affected zone liquation cracking was evaluated by means of Varestraint and Gleeble weldability tests. The results contribute to better understand the relationship between microstructural changes and the susceptibility towards cracking. This knowledge can be used in the production to select appropriate homogenization heat treatment and ultimately avoid the cracking problems during welding.