Distortion is not caused by welding itself; it is caused by heating metal that is restrained. A weld pool expands against cooler material around it, and when everything cools again the contraction has to go somewhere. Thick sections can absorb that, thin-wall tube cannot.
A tube that was straight finishes bowed, a flange face that was flat pulls out of true, and a run that measured correctly on the bench no longer lines up under the car. The movement is not random, which is what makes it controllable: it goes where the heat was put and where the restraint was.
Most of the outcome is decided before the first full pass. Tacks placed evenly around the joint, alternating from side to side, give the part a chance to equalise as it is built. Tacking one side thoroughly before touching the other loads the joint in one direction and the full weld then works against it.
Squareness should be checked after tacking and before welding, not afterwards. A joint that has already pulled at the tack stage will not come back, and the later passes simply lock the error in and add their own movement on top.
The single most effective technique is to stop welding in one continuous run. Splitting the joint into segments and working them on opposite sides keeps the heat distributed, and back-stepping, where each run finishes into the previous one rather than starting from it, pulls the contraction back on itself.
Shorter runs with a pause between them also let the part cool evenly instead of building a hot spot that pulls everything towards it. On thin-wall exhaust tube the difference between one continuous pass all the way round and a balanced sequence is often the difference between a part that fits and one that has to be cut and reset.
Fixtures hold alignment while the joint is made, which is essential for flange faces, bolt patterns and any part that has to mate to something else. Pins through bolt holes maintain spacing, and a straight bar clamped along a tube run holds the line while the seam is welded.
Clamping harder is not the same as clamping better. Over-restrained parts store stress that is released the moment the clamps come off, and the distortion simply arrives later. Fixtures that control direction while still allowing some movement usually give a truer part than rigid ones.
Cooling should be air, not water. Quenching a hot stainless or titanium weld adds residual stress and can change the structure of the metal in a way that no amount of straightening will fix. Clamps left in place while the part cools to room temperature are doing useful work; clamps removed while it is still hot are not.
Heat sinks and dampening clamps placed either side of the weld draw heat out and reduce the hot zone, which is why they are common on thin-wall work. The aim is not to cool the weld faster but to keep the hot area smaller so the contraction has less metal to pull against.
No exhaust moves nothing. A long run of tube is a chain of separate parts joined by slip joints, V-band clamps, flex sections and rubber hangers, all of which exist partly to absorb the movement that welding creates. Small deviation along a pipe is normal and harmless.
Distortion only becomes a problem where it changes how something fits: a flange that will not seal, a tip that does not sit straight in the bumper, a mounting point that no longer lines up. That is the standard we work to on TIG stainless steel welding for tube assemblies, and it is why each length of exhaust pipe is checked against its position in the run rather than on its own.
