Welding distortion is an all too familiar annoyance. Sheet metal has the tendency to warp which can make a structure more prone to buckling or make a piece visibly undesirable or make a table not function properly. While its uncommon to weld high tolerance components, any welding distortion can easily ruin the part and cause it to no longer fit within tolerances. For all of these reasons, all welders should have strong understanding of how to prevent and/or control welding distortion.
Why Welding Distortion Occurs
An electric arc from welding can hit temperatures of 3,000 to 20,000 °C. This puts massive local thermal loads onto a piece of metal which causes the distortion welders are far too familiar with. As metal heats up it begins to expand, when this expansion is local it causes the part to begin to bend. For this reason heat isn't the only issue, if you heated a piece of metal uniformly it would all expand at the same rate and there wouldn't be any distortion or warping. However, welding only heats up a small area radiating from the point of contact with the welding arc which causes only the heated area around the arc to expand leading to distortion.
Controlling Welding Distortion
There are multiple ways to control welding distortion, and in practice its best to use as many of them as you can. Heating the part evenly in an oven or through alternative means can help reduce distortion. Strong work holding solutions that hold parts firmly in place can also assist with distortion. Increasing the amount of a material that the heat can flow into around a weld can also dramatically help reduce distortion. Stitch welding also helps prevent distortion by reducing the maximum heat in a given area.
Heating The Part Evenly
Using an oven or other heating mechanism which can uniformly heat up the entire part can help reduce welding distortion. This works by reducing the thermal difference between the local welded area and the surroundings, causing uniform expansion.
To do this properly it's also important to ensure that the part also cools evenly. Hanging the component on a rack vertically while ensuring even air flow on both sides of the component can help ensure even cooling.
In practice this method isn't as ideal as the heat generated in welding is far to hot where if you heated up your part enough it would be simply too hot for a human to work on. Heating a part does however have the bonus of increasing ductility which can prevent thermal cracking and therefore increase weldability. Thermal cracking, however, is separate from welding distortion. Thermal cracking is caused when brittle material experiences local thermal expansion and has a material structure that is too rigid to allow for expansion. By heating up metal the ductility is increased with allows the material to expand without experiencing the high stresses that lead to cracking.
Strong work holding can help prevent the part from distorting by simply keeping it snug in place. In order for a part to distort, thermal expansion causes internal pressure or stress within a part which leads to bending. By reacting this pressure with work holding solutions you can prevent distortion. One downside to this is that the material will want to distort to relieve stress and if the work holding isn't uniform around the weld it is unlikely to completely prevent distortion. Work holding measures also don't address the issue of residual stress caused during the welding process.
Give The Heat Somewhere To Go
The most effective measure that is also easier to do in practice is to simply give the heat somewhere to go. This is why thicker members do not have as many issues with welding distortion, there is simply enough meat in the part for the heat to be adequately absorbed without distorting. However, just because you are working on a thin part doesn't mean that you can't add some meat to it. Fortunately, conductivity between metals is quite good which allows you to clamp on a chunk of metal to give it similar thermal characteristics as a thicker member.
To further increase the effectiveness you can use a member that has similar properties to a heat sink. Select a member that allows for ample cooling by having a high surface area to volume ratio which increases convection cooling. Using such members allows it to quickly cool and therefore draw even more heat away from the weld area further reducing the risk of welding distortion.
This is why heat sinks have so many thin fins, the large surface area to volume ratio where airflow can travel causes them to cool rapidly. Keeping the heat sink cool in comparison to the heat emitting body makes the heatsink act as a heat pump because heat always wants to travel from warm to cool areas. This can be achieved by using a heavier C-Channel member, though if you really need some serious welding distortion prevention there is more that you can do.
A copper or aluminum or other highly conductive material will draw far more heat away from the weld than a standard steel member. You can also increase the pressure of the heat sink on the part by using a strong clamp which increases the real surface area in contact at the microscopic level and therefore increases the amount of area for which heat can travel through.
Stitch welding works by reducing the amount of time a local area is being heated. Instead of doing one long pass, parts are welded together with multiple much shorter passes that are spaced out. This allows the heat to be better controlled. Once a pass is given time to cool you can then weld another pass adjacent to it. Stitch welding prevents the max heat of the component form climbing too high which leads to reduced distortion and helps to prevent burning through.
Stitch welding combined with adding a heat sink locally to your weld adds adequate welding distortion control for most projects without any undesired effects.
DIMIDE is a tool manufacturer based out of Milford, CT with the current goal of perfecting the common C-Clamp. Our clamps are impact compatible to easily deliver high clamping force. We are also hard at work on making our clamps modular so that they can fit any job.