Defects in welding are most commonly caused by improper welding procedure. While undesirable, welding defects are common and easy to correct once the cause is determined. 

In sheet metal fabrication, the four most common welding defects are porosity, overlap, distortion and lack of fusion. These defects are generally easy to detect and preventable in future applications. 

Here’s a closer look at these common welding defects and the best ways to prevent them.

1. Porosity

Simply put, porosity translates to gas bubbles in the welded surface. This occurs when shielding gas becomes trapped within the weld and releases after the weld hardens. The release of gas leaves tiny holes in the weld that are especially destructive in Metal Inert Gas (MIG) or stick welds. 

Weld porosity can result in internal and external cracks, as well as weakened weld joints. To prevent this defect, sheet metal fabricators should: 

  • Be sure to specify similar metal types if using multiple forms
  • A certified welding inspector should use x-rays and ultrasound technology to check the welded metal for holes

2. Overlap

Overlap is defined as an excess of weld metal beyond the weld root. This occurs when molten metal flows over the surface of the base material and cools without fusing. Weld overlap is common in fillet or butt welds, which are frequent weld types in sheet metal fabrication. 

Weld overlap may occur due to insufficient heat during the welding process, little to no fusion of the metal or concentrated stress under load. To prevent this defects, welders should: 

  • Use a metal type and grade that is  more agreeable with high-heat conditions
  • Ensure welders are skilled and experienced

3. Distortion

Distortion in welding is caused by excessive heat warping the base metal. This usually occurs in thinner gauge sheet metals because they lack the surface area to dissipate heat. Distortion can also occur when creating longer welds, as metal is exposed to heat for an extended period of time. 

Distortion is problematic because warped metal disrupts the aesthetics of the metal and is not structurally sound. To prevent weld distortion, welders should: 

  • Avoid stainless steel, since it’s especially prone to shifting position during a weld
  • Use a more weldable metal type and grade to reduce the amount of passes 

4. Lack of Fusion

Lack of fusion occurs when the base metal and the weld metal do not completely adhere together. This is commonly caused by incorrect welding angle, excessive speed or insufficient arc length. 

Lack of fusion is consequential because the weld connection will not remain strong or last long. To prevent this weld defect, welders should: 

  • Use a thinner piece of metal
  • Ensure welders are properly trained

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