Metal in a fusion weld consisting of that portion of the base metal and filler metal melted during welding. See MIXED ZONE and UNMIXED ZONE.
Weld metal is an admixture of melted base metal and deposited filler metal, if filler is used. Typical weld metals are rapidly solidified and have a fine- grain dendritic microstructure. In most arc welding processes filler metal is added. Some welds are com- posed of only remelted base metal; for example, electron beam and resistance welds are made without filler metal.
Microstructure-The microstructure of weld metal is considerably different from that of the base metal of similar composition. The difference in microstructure is not related to chemical compositions, but to different thermal and mechanical histories of the base metal and the weld metal.
The structure of the base metal is a result of a hot rolling operation and multiple recrystallization of the hot-worked metal. In contrast, the weld metal has a solidified or cast structure and has not been mechanically deformed. This structure and its attendant mechanical properties are the direct result of the sequence of events that occur as the weld metal solidifies. These events include reaction of the weld metal with the gases in the vicinity of the weld and with non-metallic liquid phases (slag or flux) during welding, and also reactions that took place in the weld after solidification.
Solidification- The unmelted portions of grains in the heat-affected zone at the solid-liquid interface serve as nucleation sites for weld metal solidification. Metals grow more rapidly in certain crystallographic directions. Therefore, favorably oriented grains grow for substantial distances, while the growth in others that are less favorably oriented is blocked by faster growing grains. As a result, weld metal often exhibits a macrostructure, described as columnar, in which the grains are relatively long and parallel to the direction of heat flow. This structure is the natural result of the influence of favorable crystal orientation on the competitive nature of solidification grain growth. Weld metal solidification of most commercial metals involves microsegregation of alloying and residual elements. This action is associated with, and in large measure, responsible for the formation of dendrites. A dendrite is a structural feature which reflects the complex shape taken by the liquid-solid interface during solidification.
Strengthening Mechanisms- Practical methods for strengthening weld metal are fewer than for base metal. For example, weld metal is not usually cold- worked. However, there are four mechanisms for strengthening weld metal, and where applicable, mechanisms are additive:
(1) Solidification grain structure
(2) Solid solution strengthening
(3) Transformation hardening
The first mechanism is common to all welds, and the second is applicable to any alloy type, but the third and fourth apply to only specific groups of alloys. See MIXED ZONE and UNMIXED ZONE.