There are three main elements to distortion: Weld configuration, material selection, and heat input. Weld configuration can be explained as the weld type, fillet weld or groove weld, and the distance of the weld from the neutral axis of the part. Different materials have different reactions to heat, some materials distort more and some less. The last element, heat input, is the amount of heat generated by the welding process directly absorbed by the weldment.
When it comes to welding configurations, the easiest model to use is the double v-groove weld.
From this model you can see that there would be an equal amount of weld on each side of the neutral axis. This will result in little to no distortion, in this plane, if welded in the correct sequence. When looking at a single v-groove weld this is no longer the case.
Here you can see there will be substantially more weld on the upper half of the weldment causing the edges of the plate to extend upwards. The distortion is upwards, or away from the neutral axis due to the majority of the weld material being on the top half. However there are a few ways to still result in a straight part from this groove weld. The first option would be to pre-bow the plate to allow for the movement, and the second option would be to adhere or clamp another member to restrain the part. Keep in mind if you restrain the part internal stresses will increase and cracking will be more prominent.
Material selection can be one of the most important things to keep in mind when considering distortion. Materials that do not absorb heat will tend to cause the most distortion. The most commonly known material for distorting is austenitic stainless steel. To control distortion of stainless steel joint configurations and heat input are key. For other materials such as aluminum or low carbon steels, there is no substantial decrease in distortion by changing grades of materials. For example the difference between an AISI 1008 and AISI 1030 steel would show minimal to no difference. However from using an aluminum alloy to an austenitic stainless alloy will show a substantial change.
Heat input is an often overlooked variable. The amount of heat input varies greatly on welding process. Heat input can be calculated as follows for most processes:
You may be wondering what this has to do with distortion. When determining distortion the least amount of heat input results in the least amount of distortion. To control distortion the main factor in this equation is travel speed. As you can see below even a slight increase in travel speed can result in a significant decrease in heat input.
When welding stainless steel alloys it is a common misconception that lower travel speeds result in better weld quality. In most cases the gas cup or gas flow rate is not adequate for the optimal travel speeds to be reached. For example when GTAW thin sections of stainless steel a #6 gas cup is commonly used, when you could actually use a #12 and get substantially higher travel speeds at higher amperages with the same weld quality.
When welding vertical down with GMAW you often see less distortion. Heat input is the main reason for this outcome. When traveling vertical down optimal travel speeds are often reached to stay ahead of the weld puddle.
Keep all these variables in mind the next time you weld. These tools can be very useful on every weldment from heavy structural beams to microchip applications. A little extra work ahead of time can save hours or days in the end.