base metal
The metal to be welded (rather than that used to weld).
base metal: the workpiece being joined, not the filler
In welding and metal joining, the base metal is the principal material that forms the structure being assembled. It is the part you are joining to something else, as opposed to the filler metal (the rod, wire, or powder added during the process). The base metal may be steel, aluminum, copper, titanium, or any alloy suitable for the application; its composition, thickness, and condition determine the welding procedure, heat input, and cooling rate required for a sound joint.
The term distinguishes clearly between the two materials involved in fusion joining. When a welder runs a bead of mild steel filler wire into a joint between two carbon steel plates, those plates are the base metal. If the filler metal were to pool and cool without proper fusion into the base, you would have adhesion failure rather than a true weld. The base metal must reach its melting point or at least enter the plastic range for genuine metallurgical bonding to occur.
Properties that matter
The composition and thickness of the base metal govern the heat input you can safely apply. Heavy sections of low-carbon steel tolerate higher amperage and travel speed; thin sheet requires careful attention to avoid burn-through. Alloy steels, stainless grades, and non-ferrous metals demand specific filler metals and preheat or interpass temperatures. A base metal with high carbon content or high restraint (thick sections with limited access) is prone to hydrogen cracking if cooled too quickly; the welding engineer must specify preheat, slower cooling, or stress-relief heat treatment to manage this risk.
Defects in the base metal before welding, such as mill scale, rust, oil, or laminations, can prevent proper fusion or trap contaminants in the weld pool. Surface preparation, therefore, is not optional; it is part of controlling the base metal environment. Similarly, the grain structure of the base metal (whether hot-rolled, cold-worked, or cast) affects how it will behave in the heat-affected zone adjacent to the weld.
In repair and fabrication shops, the term base metal is also a commercial reality: it represents the bulk of material cost and weight in a welded assembly. Filler metal consumption is typically only 5 to 15 percent of the base metal mass. This is why material selection, inventory, and traceability of the base metal are critical to both quality and cost control in structural and pressure-vessel work.