We're welding 0.040" thick 718 to a slightly thicker (0.043") mating part. The original concept for the tool used copper for the backing with a small groove on the backside of the weld. However, due to time and money contraints, they want to make this backing material out of something else. Both stainless steel and aluminum were brought up as options. I'd prefer to use copper so we can suck enough heat away from the weld area. With using stainless, I'm concerned that if the groove isn't big enough, we may risk welding the 718 to the stainless tool. Aluminum might be ok but I'm not positive on whether or not it's going to conduct enough heat away. Again, this material is 0.040" and we're TIG welding it. Any thoughts?
By Lawrence 
Date 02-20-2009 19:52
Edited 02-21-2009 21:14
Are we talking about an argon backup fixture for welding inco 718? or an argon backup fixture made of inco 718 or both?
There are some prohibitions in print about using copper to fabricate backpurge tools and heat sinks for precipitation hardenable superalloys because the hot inco has an affinity for the copper and cracking has been noted. This being said you can look at manufacturers of jet engine parts that regularly use copper heat sinks and copper argon backups.
One solution is to build your backups out of copper and have a thin nickel plate applied. This way you get the formability and heat sink properties of copper (not to mention the ease in drilling argon flow holes in custom backups)
Aluminum would not be advisable for fixturing if it is close to the HAZ for a number of reasons.
Say a little more about your project.
Along with everything Lawrence mentioned, remember that the reason the molten metal doesn't stick to the copper parts is because it conducts heat so rapidly. Stainless is not nearly as good a conductor of heat. I tink Your concern about stainless is wawanted.
