The determination of temperature distribution and heat flow helps to predict the microstructure and mechanical properties at different welding zones. The primary objective of welding heat flow modeling is to provide a mathematical tool for thermal data analysis, design iterations, or the systematic investigation of the thermal characteristics of any welding parameters. In fusion welding processes heat is supplied by the arc or by suitable heat and this heat is utilized to melt the electrode and a filler material and some portion of the parts to be welded.
Welding Thermal Process:
- The welding heat source moves at a constant speed along a straight path. The end result, after either initiating or terminating the heat source, is the formation of a transient thermal state in the weldment.
- At some point after heat-source initiation but before termination, the temperature distribution is stationary, or in thermal equilibrium, with respect to the moving coordinates.
- The intense welding heat melts the metal and forms a molten pool. Some of the heat is conducted into the base metal and some is lost from either the arc column or the metal surface to the environment surrounding the plate.
- Three metallurgical zones are formed in the plate upon completion of the thermal cycle: the weld-metal (WM) zone, the heated-affected zone (HAZ), and the base-metal (BM) zone.
- The peak temperature and the subsequent cooling rates determine the HAZ structures, whereas the thermal gradients, the solidification rates, and the cooling rates at the liquid-solid pool boundary determine the solidification structure of the WM zone.
- The size and flow direction of the pool determines the amount of dilution and weld penetration. The material response in the temperature range near melting temperatures is primarily responsible for the metallurgical changes.
Relation to Welding Engineering Problems:
To model and analyze the thermal process, an understanding of thermally induced welding problems is important. A simplified modeling scheme, with adequate assumptions for specific problems, is possible for practical applications without using complex mathematical manipulations
Welding Metallurgy: Metallurgical structures in the HAZ and cracking in the WM usually occur under the transient thermal condition. Therefore, a transient thermal model is needed to analyze cracking and embrittlement problems.
Welding Distortion The temperature history and distortion caused by the welding thermal process creates nonlinear thermal strains in the weldment.
Thermally related welding problems can be categorized as:
· SOLIDIFICATION RATES IN THE WELD POOL
· COOLING RATES IN THE HAZ AND ITS VICINITY
· THERMAL STRAINS IN THE GENERAL DOMAIN OF THE WELDMENT
The HAZ is always bounded on one side by the liquid-solid interface during welding. This inner-boundary condition is the solidus temperature of the material. The liquid weld pool might be eliminated from thermal modeling if the interface could be identified. A conduction heat-transfer model would be sufficient for the analysis of the HAZ.
The thermal strains caused by welding thermal cycles are caused by the nonlinear temperature distribution in the general domain of the weldment. Because the temperature in the material near the welding heat source is high, very little stress can be accumulated from the thermal strains.