Alloy modification for additive manufactured Ni alloy components-part I: effect on microstructure and hardness of Invar alloy

Alloy 36 (1.3912), also known as "Invar, " is an alloy with 36% nickel. The alloy has a remarkably low thermal expansion coefficient in certain temperature ranges. This peculiarity is called the invar effect, which was discovered in 1896 by the Swiss physicist Charles edouard Guillaume Sahoo and Medicherla Mater today Proc 43:2242-2244, (2021). Therefore, it is used in applications in which dimensional stability is critical, such as molding tools for composite materials in aerospace, automotive applications, or liquified natural gas (LNG) cargo tanks. Moreover, increasingly complex structures and the optimization of resource efficiency also require additive manufacturing steps for the production or repair of components Frazier J Mater Eng Perform 23:1917-1928, (2014); Treutler and Westling, (2021). Additively manufactured components have a heterogeneous microstructure and anisotropic mechanical properties Guevenoux et al. (2020). In addition, the manufactured components require subsequent machining surface finishing, like finish milling, to achieve their final contour. Nickel iron alloys are difficult to machine Zheng et al. Adv Mater Res 988:296-299, (2014). Additionally, inhomogeneous microstructure may lead to unstable cutting forces and conditions. In part I of this investigation, the initial alloy 36 is modified with the elements Ti, Zr, and Hf up to a maximum of 0.33 wt.-%. The influence of the modification elements on the microstructure as well as on the hardness of the AM components is examined. Furthermore, one modification is applied to metal arc welding process and investigated. Part II focuses on the effect of the alloy modifications on machinability as well as on the surface integrity of plasma-transferred-arc-welded (PTA) and finish milled invar components.