Copyright Goodheart-Willcox Co., Inc. 148 Gas Tungsten Arc Welding Handbook Melt-through. Occurs at a gap in the weld joint or place where metal is thin. How to correct: • Decrease current. • Increase arc length. • Increase travel speed. • Decrease root opening. Root underfi ll. The root of the weld penetration does not extend below the lower edges of the joint. How to correct: • Decrease travel speed. • Decrease amperage. • Decrease arc length. • Increase fi ller wire deposition. Excessive root reinforcement. The penetration beyond the joint root is uneven and too tall. How to correct: • Move the torch throughout the weld length in a consistent pattern. • Add fi ller metal as required to maintain an even penetration.