Dilution

Every weld pool mixes two sources of metal: the filler being added and the base metal the arc melts as it advances. Dilution is the share that comes from the base metal. For a structural butt or fillet weld this is rarely discussed, because filler and base metal are usually similar alloys and the mix barely changes the outcome. For overlay and cladding welding it is the central variable, because the entire point of the deposit is to put a different alloy — a corrosion-resistant or wear-resistant layer — on top of a cheaper structural base metal, and dilution is what determines how much of that base metal ends up inside the layer meant to protect it.

Higher dilution pulls the deposit’s chemistry toward the base metal and away from the filler’s intended composition. On a corrosion-resistant overlay this can move the alloy below the composition its corrosion performance depends on; on a hardfacing deposit it can soften the working surface. Both failures are invisible on the surface and only show up in service or on a chemical analysis of the deposit — a much later and more expensive point to discover them.

Dilution is controlled through the same variables that shape penetration: heat input, travel speed, and the process itself. Single-pass, high-energy processes drive molten base metal deep into the weld pool and dilute heavily; multi-layer strip cladding, where later layers barely touch the original base metal, dilutes far less. Because the mechanism is the same energy-and-travel-speed relationship that governs penetration, real-time thermal monitoring of an overlay pass reads as a consistency check on dilution even though it cannot measure deposit chemistry directly: a heat input excursion on an overlay pass is a dilution excursion by the same physics that makes it a penetration excursion on a structural weld.