Computer modeling of heat flow in welds
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I.
INTRODUCTION
FUSION welding is the method of choice for assembling most large metal structures such as ships, bridges, nuclear reactors, pipelines, trains, and cars. In these structures safety and economy are important issues. In simple terms, a fusion weld is produced by moving a localized intense heat source along the joint. The chemical composition of the weld metal, energy, and position of the arc must be carefully controlled to achieve the desired weld quality. Often filler metal is added. This complex process that mimics the entire steelmaking process in a volume of roughly one cubic cm in a time span of less than one minute has largely been developed by experiment, i.e., trial and error. The process is so complex that to date mathematical modeling has contributed little to modem welding technology. This presentation argues that recent developments in computational weld mechanics now enable the heat transfer in real welding situations to be analyzed or simulated accurately, perhaps more accurately than the data can be measured. The critical first step in an accurate analysis of the physical behavior of welds is to compute the transient temperature field, T(x, y, z, t), for any point of interest, (x, y, z), and for any instant of time, t. For a given material and joint design, this temperature field largely determines the size of the fusion zone and heat-affected zone, the microstructure, residual stress, distortion, hydrogen content, and it is fundamental to understanding and analyzing weld defects. For a range of materials and joint designs, the temperature field, together with the chemical composition and transformation kinetics, is the basis for predicting the microstructure in the FZ and HAZ. In determining the residual strain and stress, the temperature dependent stress-strain relationship also plays an important role. In short, any computer simulation of these topics depends on and is sensitive to the accuracy of the computed temperature field. In particular, the heat source generates a transient temperature field that has important consequences. It modifies J. GOLDAK and M. BIBBY, Professors, and J. MOORE, R. HOUSE, and B. PATEL, Graduate Students, are with the Department of Mechanical and Aeronautical Engineering, Carleton University, Ottawa, ON, Canada, K1S 5B6. Manuscript submitted May 24, 1985.
METALLURGICALTRANSACTIONS B
the microstructure by solidification, recrystallization, grain growth, and phase transformations such as ferrite to austenite to martensite. The microstructure controls the thermal and mechanical properties of the weldment. The transient temperature field causes thermal expansion, stress, and strain that usually plastically deforms in the weld neighborhood and results in residual stress and strain; i.e., when the weldment cools a stress remains and the structure is distorted from its original shape. Distortion is the bane of fabricators because it increases their costs and delays production. It is managed by a variety of techniques including balanced welds, rigid fixturing,
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