Undercut
Undercut is a sharp groove at the edge of the weld bead, caused by excessive heat input, which acts as a notch. It leads to porosity and reduced mechanical properties.
Physical-chemical mechanism
Undercut occurs when the welding heat (welding current) is too high. This causes the metal at the edge of the bead to melt faster than the center. The sharp temperature difference between the edges and the core creates an uneven stress state, leading to a groove or notch.
Metallurgical relevance & welding risks
Undercut can cause porosity at the edge of the weld, which weakens the mechanical properties like strength and toughness. Additionally, it can promote cracking and hot cracking, especially when the metal is under mechanical stress.
Workshop practice & actions to take
To avoid undercut, control the welding heat input. This can be done by optimizing welding parameters like current and travel speed. Using shielding gas or backing strips can also help minimize the temperature difference between the edges and the core.