COMPUTATIONAL WELDING MECHANICS
Microstructure Evolution
The composition of the base metal must be specified. We prefer the format ((Fe,*), (C, 0.12), (Mn, 1.3),...). Here (Fe,*) means the balance of the fraction is Fe. We have to specify whether composition is in weight % or atomic %. Also the initial microstructure of the base metal must be specified. We prefer a format such as:
(ferrite, wt% 80, gASTM 2, comp ((Fe,*), (C, 0.12), (Mn, 1.3),..)) (pearlite wt%20, gMicrons 50), etc.
The composition of the weld metal is a function of the mixing of filler metal with base metal.
For each phase, we suggest the name of the phase and for each attribute of the phase the name of the attribute, the value of the attribute and its dimensions. For example, grain size could be specified by either ASTM number (gASTM) or diameter in microns {gMicrons). Note, strictly speaking pearlite is not a phase as Gibbs would define a phase, but we suspect that most metallurgists prefer to call pearlite a phase. The software should understand that pearlite is actually a eutectic microstructure consisting of lamellae of iron carbide phase and ferrite phase.
For microstructure analysis, we must specify an initial microstructure. If no microstructure is specified, we could assume an equilibrium microstructure with some estimated or default grain size. As a material point moves past the weld or as the weld moves past a material point, the temperature of the material point changes and the microstructure evolves. The latent heats of transformation in the solid state have a small effect on the solution of the thermal problem. The different phases can have different values of thermal conductivity, specific heat and specific enthalpy. For the thermal stress solver, the microstructure determines the specific volume or density as a function of temperature and hence the strain due to thermal expansion and phase changes. It also determines the macroscopic and microscopic elasticity tensor (Young’s modulus and Poisson’s ratio), yield stress, hardening modulus and viscosity.
