Fusion Line

The fusion line is the boundary where the base material has just melted during welding and mixed with the weld metal. When you look at the weld, you can see whether the bond was clean or whether you just 'laid it on top'. A clean fusion line means the base material was properly engaged and the weld will hold. When you make microsections, you look right there for lack of fusion, cracks, or inclusions. Adjust the heat and travel speed so the fusion line runs evenly, then the weld will be proper.