With a suitable example explain how the weld metal solidifies and crystallizes.
Answers
During the solidification of the liquid metal fast moving disordered atoms get
arranged in a definite order to form crystal lattices such as BCC, FCC, and HCP etc. As
soon as heat source moves away, gradually heat is lost to the base metal. Rate of heat
loss depends on temperature of weld metal, weld pool size, thickness and thermal
properties base metal etc.
On cooling, liquid metal losses energy in the form of latent heat and so the
temperature is lowered which in turn decreases the average inter-atomic distance
between mobile & disordered atoms. On further cooling, attractive forces between
atoms prevent them moving away from one another and eventually completely liquid to
solid transformation takes place. Solidification of pure metals begins with temperature
arrest and then continues to take place at a constant temperature as shown in Fig. 33.1
(a) while alloys solidify over a range of temperature. The arrest in temperature is
attributed to evolution of the latent heat of solidification on transformation from the liquid
metal to solid state.
The microstructure development in the FZ depends on the solidification behavior of the weld pool. The principles of solidification control the size and shape of the grains, segregation, and the distribution of inclusions and porosity. Solidification is also critical to the hot-cracking behavior of alloys. Sometimes, it is convenient to consider the FZ as a minicasting. Therefore, parameters important in determining microstructures in casting, such as growth rate (R), temperature gradient (G), undercooling (DT), and alloy composition determine the development of microstructures in welds as well. Comprehensive reviews of weld pool solidification based on these parameters are available in the literature.1,2