Plug welding in copper

[kenphil]

We are having trouble with plug welds cracking around the outside perimeter of the weld. It is roughly .75 deep. Is there a special procedure for welding a plug in copper which prevents cracking. These plug welds have to be pressure tight when completed.

 

[metengr]

What welding process are you using and filler metal?

 

[kenphil]

We are using a mig procedure using cu110 copper wire. We are preheating to 700 to 900F. We have to do this for the conductivity. This is a casting which has water flow thru it for cooling.

 

[metengr]

Review the information below and, if necessary, come back with any questions.

http://www.brazing.com/techguide/procedures/copper_welding.asp

 

[kenphil]

My experience is mainly in steel welding. At a shipyard where I worked, you were never allowed to weld in a circle. Their theory was that the stresses were all focused inward and therefore would crack on the outside of the circular weld. Whenever a circular weld was needed, welds were placed parallel to the axis at the outside of the circle on either side. My question is in the design of this area not the method of welding itself. The welding itself is approved, the question is joint design.

 

[metengr]

Their theory was that the stresses were all focused inward and therefore would crack on the outside of the circular weld. Whenever a circular weld was needed, welds were placed parallel to the axis at the outside of the circle on either side. My question is in the design of this area not the method of welding itself.

Nonsense. We have plug welded materials with no problems.

 

[kenphil]

ok I can understand your response. But we are having serious problems with plugs cracking. We use the same procedure for all of our other copper welding with no problems. I am just trying to understand where the problem is coming from. The casting is roughly a ton of copper. Is there a way I can send pictures of the joint and cracks to you.

 

[metengr]

Post them on engineering.com (for new posts you can use the attachment feature in step 4) or use the following

http://flypicture.com/

 

[MintJulep]

What are you using for shielding gas?

 

[kenphil]

argon helium mix

 

[btrueblood]

110 Copper is "electrolytic tough pitch", and from the article that metengr posted,

The high oxygen content in tough pitch copper can lead to emritterment in the heat affected zone and weld metal porosity. Phosphorus deoxidized copper is more weldable, with porosity being avoided by using filler wires containing deoxidants (Al, Mn, Si, P and Ti).

...assuming "embritterment" should have been "embrittlement", the problem may be with your electrodes. Pure, oxygen-free copper, or fillers with deoxidants as noted may give better results. Having tried what you are doing in a previous life, the above "may"'s are almost "will"'s, but I bow to others with more knowledge of metallurgy.

I have also seen/heard about Cu welds with high porosity that were ultimately traced to high oxygen content in the argon purge/shield gas mix. You might check on that, though welding grade Argon is usually pretty low in O2.