Sheet metal fabrication projects that require welding will typically experience some degree of welding distortion—resulting in the expansion and contraction of the weld and base metal. While common, welding distortions can threaten the structural integrity or aesthetic of your project. Depending on the type, size and material of the distortion, different corrective techniques are available to fix it.
Types of Welding Distortion
The amount of residual stress and the degree of restraint used during the welding process impacts the type of distortion you may experience. The root cause of welding distortion is high heat application.
Several types of welding distortion are possible, and your project can experience more than one type simultaneously. These types include:
- Transverse shrinkage: Shrinkage results from contractions that run perpendicular to the weld. This type of distortion travels vertically through the cross-section and occurs parallel to the weld
- Longitudinal shrinkage: This distortion results from forces parallel to the weld. This type of shrinkage occurs perpendicular to the weld
- Angular distortion: This distortion is a form of transverse shrinkage and tends to be the most frequently occurring type. Angular distortion occurs when stronger contractive forces on one side of the neutral axis cause the material to bend in that direction
- Bowing and dishing: When the weld is not balanced along the neutral axis of the cross-section, longitudinal shrinkage in the welds bend the section into a curved shape
- Buckling: Frequently occurring in thin metal structures, buckling causes sheet metal to curve along the width. This is often time-consuming and difficult to correct
How to Correct Distortion
Preventative measures should be set in place to avoid welding distortion. However, if it does unfortunately occur, there are several well-established techniques to rework and correct the sheet metal.
Correcting welding distortion can be achieved mechanically or thermally:
1. Mechanical Corrective Techniques
Hammering and pressing are the primary mechanical corrections for weld distortion. Mechanical straightening involves using force with a hammer, wedge, pneumatic ram, machine press or similar instrument to push the distorted area flat.
2. Thermal Corrective Techniques
With thermal correction, heat creates local stress that is sufficiently high enough that the component is pulled back into shape upon cooling. This technique, also known as flame straightening, is applied with the following methods:
- Spot heating: This technique removes buckling in thin sheet structures. Distortion is corrected by applying heat to spots on the convex side
- Line heating: Heating the component in a straight line along the welded joint on the opposite side to the weld often corrects angular distortion
- Wedge-shaped heating: Wedge shapes are heated to correct distortion in larger complex fabrications
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