Solidification phenomenas of weld metals (1st report) characteristic solidification morphologies, microstructures and solidification theory

摘要

Recent years have seen key problem areas of fusion welding being successfully researched. Previous research studies have led not only to the development of high-performance/high-quality welding machines, automation of welding and less labour-intensive welding processes, but also to clarification of welding phenomena, evaluation and improvement of joint performance, clarification of welding defect occurrence mechanisms, the development and implementation of welding defect prevention methods, etc. The results of materials-science investigations have further shown that the mechanical properties of weld metals as well as the occurrence of solidification cracking and porosity are strongly affected by welding solidification phenomena, most notably features such as crystal growth morphologies, microstructures and microsegregation behaviours during solidification. To determine the solidification phenomena affecting fusion weld metals, it is therefore initially important to have an awareness of solidification morphologies and microsegregation mechanisms. The constitutional supercooling theory is normally employed to explain solidification morphologies and microstructures considering solute discharge to the planar solidification interface front (when the equilibrium partition coefficient at the solid-liquid interface k{sub}0 1) and solute absorption from this front (when k{sub}0 > 1) as well as identifying their specific features through consideration of the relationships between cooling rates during solidification, growth rates at the crystal interface, temperature gradients, solute concentrations (contents of alloying elements), partition coefficients, etc. The understanding and modelling of solidification phenomena have seen major progress achieved through the development of perturbation theory, considering interfacial tension alongside important recent advances in the development of rapid solidification theory, Fundamental aspects of general solidification theory, rapid solidification theory and welding solidification phenomena are documented in other recent technical papers and publications, most notably: 'Fundamentals of solidification', 'Noteworthy fundamental aspects of the solidification of metals', 'Relationship between solidification conditions and microstructures', 'Modelling of welding metallurgical phenomena', 'Mathematical modelling of welding phenomena', etc. On the other hand, evaluation of the hot cracking (solidification) susceptibility during welding solidification and the occurrence and propagation mechanisms of solidification cracking have been long and extensively researched, and the resulting techniques have been developed in conjunction with the elaboration of cracking experimental methods and an understanding of microstructures and microsegregation behaviours. At present, progress is also being slowly made in clarification of the application circumstances of strain at individual grains and grain boundaries during solidification. This series of reports profiles recent developments in the study of welding solidification phenomena (also including rapid solidification phenomena during techniques such as surface melting treatment). The 1st report focuses on solidification phenomena affecting the fusion weld metals of general-purpose alloys, outlining solidification microstructures, morphologies and constitutional supercooling theory. The 2nd report discusses solidification theory, the redistribution of solutes, microsegregation behaviours and the formation of segregation products. The 3rd report examines solidification cracking mechanisms and the relationship between microsegregations and solidification cracking susceptibilities, and combines the basic data obtained to provide an understanding of weld metal phase diagrams. Various other factors, such as the temperature history and temperature distribution changes during welding, the molten pool morphology, the molten metal flow inside the molten pool, etc. also stro