Controlling Thermal Distortion - Tube Assemblies

[afortier]

We are encountering large variability across rigidly fixtured automotive exhaust parts that we are assembling when we remove them from the fixtures. Our customer has reported as much as a 1/2" variation between flanged end locations. The parts are made from 3", .065 aluminized mild steel. They have 3 weld joints (butt) and two filet welds at the end flanges. Welding process is tig, miller dynasty welders with experienced operators. The parts are pre-bent, then production trimmed in fixtures ensuring repeatability of fitment, and are usually welded with a zero gap. We are looking for any suggestions in terms of post weld treatments or any tricks that some of you may know to minimize this type of distortion. We are not allowed to use flex sections for this part.

[Rocky D]

You are noticing that the part will twist in the direction of the weld pass. There are ways to correct it. First tack the tubes 1/4" apart, then, do one of the following...1) you have three joints, so weld the 2 outside ones CW and the inner one CCW...2) weld each joint starting and 6 o'clock and run to 12 o'clock. Now if you are still getting distortion, carefully notice how much the tube moves when you weld in a certain direction...then combat that by welding the same amount in the opposite direction. 3) make 1 inch welds opposite each other like tightening the lug nuts on a wheel. Weld the flanges in the same way, not continuously in the same direction....that causes the flange to rotate. As long as you split up the weld direction, you will reduce the rotation.
To qualify myself, I welded pneumatic ducting in aircraft for 30+ years.

[afortier]

Thank you rocky. I will post back with results.