Other high temperature superalloy compositions have been based on the Ni group alloys known as Inconel, with the two most frequent variants being 625 and 718. The choice of which alloy to use will depend on the maximum operating temperature and the part strength requirements. However, in the AM sector there has been some initial preference for the 625 alloy because the post-build heat treatment cycle is relatively straight forward, and it was the material of choice within the motorsport sector from where a lot of early AM work was derived.
These Ni based alloys have a high alloy content, which enables them to withstand a variety of severe corrosive environments. However, even in more severe corrosive environments the combination of nickel and chromium resists oxidizing chemicals, and the molybdenum content resists non-oxidizing environments. The presence of molybdenum also makes these alloys very resistant to pitting and crevice corrosion. Furthermore, the niobium acts to stabilize the alloy against sensitization during welding, preventing subsequent intergranular stress corrosion cracking.
As rough rules of thumb it’s possible to select which alloy to use according to the following criteria.
–Choose the 625 alloy for its high strength and resistance to creep, rupture, and corrosion at high temperatures, even in the range of 980ï‚°C to 1140ï‚°C. 625 has nominal room temperature tensile strength between 827-1034MPa.
–If higher oxidation resistance or strength is required then the 718 can be used at a slightly lower maximum operating temperature, which is usually in the range of 700ï‚°C to 760ï‚°C, even though it is known that some 718 AM parts have also been used at temperatures in excess of 1000ï‚°C. The higher strength is achieved because this alloy can be age hardened, which is usually achieved in a two-stage process after an initial solution treatment followed by rapid cooling. The 718 alloy has nominal room temperature tensile strength between 1170-1275MPa in the fully aged condition.
The values given above for tensile strength are only indicative of the performance of these alloys. It has been reported within the AM sector that higher values have been achieved, and for this reason it is always advisable to have your own verified test results following whichever heat treatment cycle has been selected, or specified. Note, both alloys can also be used down to the extremely low cryogenic temperatures.
Following on from the success of these two Ni based superalloys there has also been considerable interest in other high temperature alloys that offer even greater performance. Today proprietary laser powder bed fusion process parameters exist for other Inconel group alloys, as well as others like Hastelloy X, C276, and MAR-M247.
This information is excerpted from a Renishaw paper, The status quo of metal alloys for additive manufacturing.
