It is important for welders and welding inspectors to understand the various types of flaws that can be present in a weld. Knowing the flaws and their causes can help prevent these flaws from occurring or help accurately repair the weld. Below is an explanation of three types of weld flaws- porosity, slag inclusion, and incomplete fusion- and their common causes.
Porosity is small cavities of gas or air present in a weld. Porosity occurs when dissolved gases and gases released during the welding process do not have enough time to escape before solidification. Surface porosity occurs on the surface of the weld. It can be seen, and it looks like small holes in the material. Subsurface porosity occurs within the weld itself. It can only be detected with technology such as phased array or industrial radiography. Elongated porosity is when the flaw is stretched. The most common causes of weld porosity includes impurities on the surface of the metal, excess shielding gas, damp electrodes, and backing bar material that doesn't match the metal being welded.
Slag refers to foreign materials trapped inside the weld. Slag is the residue of the flux coating in MMA welding, which is a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. A common cause of slag is the presence of a coating on the component being welded. For example, aluminum is often coated with aluminum oxide. During the welding process, the aluminum oxide can become trapped inside the weld. In order to prevent slag, cleaning of the component before beginning the welding process is required.
Incomplete fusion occurs when the weld material and the parent piece are not completely fused together. Welds with incomplete fusion are weak and unlikely to pass code inspection. Incomplete fusion is commonly caused when not enough weld material is deposited to fill the weld joint, there are gaps between weld beads at the root, and the surface of the component is dirty.
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