Basic Metallurgy of Fusion Welding

Basic Metallurgy of Fusion Welding

A typical fusion welded joint varies in metallurgical structure from the fusion zone to the base material with consequential variations in mechanical properties. This is because of the fact that fusion welding processes result in melting and solidification with very high temperature gradient within a small zone with the peak temperature at the center of the fusion zone. In general, a weld can be divided in four different zones as shown schematically in Fig. 1.
.


Figure.1. Schematic presentations of several zones in a fusion welded joint

The fusion zone (referred to as FZ) can be characterized as a mixture of completely molten base metal (and filler metal if consumable electrodes are in use) with high degree of homogeneity where the mixing is primarily motivated by convection in the molten weld pool. The main driving forces for convective transport of heat and resulting mixing of molten metal in weld pool are: (1) buoyancy force, (2) surface tension gradient force, (3) electromagnetic force, (4) friction force. Similar to a casting process, the microstructure in the weld fusion zone is expected to change significantly due to remelting and solidification of metal at the temperature beyond the effective liquidus temperature.
The weld interface, which is also referred to as mushy zone, is a narrow zone consisting of partially melted base material which has not got an opportunity for mixing. This zone separates the fusion zone and heat affected zone.
The heat affected zone (HAZ) is the region that experiences a peak temperature that is well below the solidus temperature while high enough that can change the microstructure of the material. The amount of change in microstructure in HAZ depends on the amount of heat input, peak temp reached, time at the elevated temp, and the rate of cooling.