Distortion can negatively impact any weld. It is critical to predict and minimize weld distortion to maintain predetermined dimensions. Manufacturers that fully understand weld distortion are better prepared to anticipate it and work to mitigate its negative effects.
What is Weld Distortion?
Weld distortion occurs when weld metal, or the adjoining base metal, contracts and expands. Several variables can result in weld distortion, including temperature variations that cause a change in the physical properties of the metal, or when the welding process is applied unevenly to one side of the weld or the other.
Every part has a strict dimensional requirement that could be put in jeopardy if the material distorts during welding. Manufacturers must minimize weld distortion to achieve the predetermined dimensions required for the design to function correctly.
There are three types of weld distortion:
- Transverse: This type of distortion occurs across the weld joint.
- Longitudinal: Distortion that occurs along the length of the joint.
- Angular: This distortion creates an angle on a surface that was supposed to be flat, for instance.
All three forms of weld distortion occur to some degree or another at the same time in every real-world application. Manufacturers use different techniques to limit each one individually and to control the others.
Planning to Minimize Weld Distortion
Before the first arc is struck, thorough planning is required to predict and mitigate weld distortion. Some weld distortion can be minimized during the weld itself. Other weld distortion is unavoidable and must be corrected after the weld is complete.
The first step of planning is to classify each segment of the welded assembly (or weldment) into one of three different categories of tolerances:
- Tight tolerance
- Medium tolerance
- Loose tolerance
The tight tolerance sections are dealt with first. Since some degree of weld distortion is inevitable on tight tolerance welds, the team must plan for correcting this unavoidable distortion post weld. This requires them to add the appropriate amount of extra material eto machine away after the weld is complete, making the finished surfaces meet their pre-planned dimensions.
Next, the team must plan for medium and low tolerance welds.
The decision of whether to mitigate distortion during the weld or to rectify distortion afterward is made on a case-by-case basis. If they decide to work to limit distortion during the weld, the team must:
- Choose fixture designs that limits distortion.
- Hold and clamp parts rigidly during welding.
- Choose weld joint designs that produce less distortion — smaller welds create less distortion than bigger welds.
- Design a sequence of assembly that can reduce distortion. For example, they may build some rigidity into the design of parts. They may also put the critical framework of a welded assembly in place early so it has its own structure. Then, they can add more components later to avoid unnecessarily large distortion.
Post-Planning: Minimizing Distortion During Welding
The last line of defence against distortion is the people doing the work. The manufacturer must hire, train and rely on welders to be experts in welding distortion. They must offer their teams support, such as prevention training, dimensional measuring equipment and inspection personnel with high-tech tools that can detect and limit distortion throughout the welding process.
It is critical to stop and assess at pre-planned breaks throughout the process.
The engineering and welding staffs should work together during these planned pauses to monitor distortion as they go, re-evaluate and adjust to limit future distortion. If the first set of welds is bending to the right, for example, they will recalculate and compensate to the left on the second set of welds.
These periods of stopping and assessing cannot be arbitrary — they must be built into the planning phase.
Dealing with Unplanned Distortion
If distortion occurs that wasn't predicted, teams generally use mechanical means to correct the problem. Straightening is a process that forces the part back into the correct shape to limit remaining distortion. In other cases, the team may apply significant mechanical force by putting the part in a massive press.
In other cases, teams literally fight fire with fire.
Sometimes the best remedy is to attack the distortion using the same mechanism that caused the problem in the first place — heat. A process called flame straightening requires teams to use a torch or add some weld in a different location in the assembly to intentionally produce a distortion in a beneficial direction.
They apply the same shrinkage stresses responsible for the original distortion, but in a way that helps overall assembly. Although this is a low-tech technique, it requires a heightened understanding of the forces of distortion and thermal pressure.
Distortion is a part of welding. It cannot be avoided, but by understanding the forces involved, teams can plan to minimize weld distortion. Whether they attack the distortion during the weld or after the weld depends on the situation, but either way, manufacturers must be intimately familiar with the physics and techniques involved in order to mitigate this potentially harmful phenomenon.