Vacuum induction melting and vacuum arc remelting of Co-Al-W-X gamma-prime superalloys

Co-Al-W alloys strengthened with the L1 2 gamma-prime phase have promise as next generation high temperature materials due to the ability to engineer a high gamma-prime content alloy with a higher gamma-prime solvus and higher melting point than many Ni-base gamma-prime strengthened alloys. Furthermore, these Co-Al-W gamma-prime alloys are interesting as potential cast-and-wrought alloys because they have a relatively narrow range of solidification temperature and large range of temperature between the gamma-prime solvus and the solidus, suggesting than manufacturing via an ingot metallurgy route would be feasible. However, since J. Sato et al discovered gamma-prime in the Co-Al-W alloy system in 2006, the focus in the literature has been on characterizing the structure and properties of these alloys and measuring and assessing the thermodynamics of the alloy system primarily for application as castings for turbine blade applications. To date the author is not aware of any publications describing the microstructure of vacuum induction melted, vacuum arc remelted ingots of a size more than about 2kg. Most work has been performed using small, laboratory-scale, cast-and-hot-rolled samples or samples cast as single crystals. This paper presents ATI's experience in assessing the feasibility of manufacturing a cast-and-wrought billet product in the Co-Al-W-X alloy system. Three 22 kg heats were produced to examine a small range of alloy compositions of potential commercial interest: Co-9Al-9W, Co-9Al-10W-2Ti, and Co-9Al-10W-2Ti-0.02B, respectively. Each heat was vacuum-induction-melted and vacuum-arc-remelted then open-die forged. The ingot microstructure has been characterized. Hot workability during billetizing will be described and microstructure and hardness of hot worked and heat treated product will be presented.