Weld Metal

Weld metal refers to the metal part formed by the melting and solidification of filler materials (such as welding rods, welding wires) and/or base materials (the workpiece being welded) during the welding process. It is the core area of the weld joint, and its composition, structure and performance directly affect the welding quality. The following is a detailed analysis:

1. The formation process of weld metal

Melting and mixing

During welding, a high-temperature heat source (arc, laser, etc.) melts the filler material (such as welding wire, flux) and part of the base material to form a molten pool.

The filler metal and base material in the molten pool are mixed in proportion (depending on the welding process) and finally solidified into weld metal.

Example: In manual arc welding, the gas and slag generated by the decomposition of the electrode coating protect the molten pool, and the core metal fuses with the base material.

Solidification and crystallization

When the molten pool cools, the metal changes from liquid to solid, forming columnar crystals or equiaxed crystal structures.

The solidification rate, temperature gradient and alloy composition jointly determine the grain size and orientation.

2. Composition characteristics of weld metal

Chemical composition

Different from the parent material: Due to the addition of filler materials, the chemical composition of weld metal may be different from that of the parent material (such as when low-carbon steel electrodes weld high-carbon steel).

Alloying design: Add alloying elements (such as Mn, Si, Ni) through welding wire or coating to improve strength, toughness or corrosion resistance.

Typical organization

Casting organization: Rapid cooling leads to non-equilibrium organization (such as martensite, bainite), which needs to be adjusted by post-weld heat treatment.

Segregation phenomenon: Solute elements are enriched at grain boundaries, which may cause hot cracks (such as segregation of sulfur and phosphorus in steel).

3. Key performance of weld metal

Mechanical properties

Strength and toughness: Affected by alloy composition and cooling rate, it is necessary to match the requirements of the parent material (such as low-temperature impact toughness of high-strength steel welds).

Hardness: High carbon equivalent welds are prone to form hard and brittle organizations, and heat input and interlayer temperature need to be controlled.

Defect sensitivity

Porosity and slag inclusions: caused by insufficient protection of the molten pool or incomplete cleaning.

Cracking: Hydrogen-induced cracking (cold cracking) or solidification cracking (hot cracking) are common problems.