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Parameters that control the solidification of castings also control the solidification and microstructure of welds. However, various physical processes that occur due to the interaction of the heat source with the metal during welding add a new dimension to the understanding of the weld pool solidification. Conventional theories of solidification over a broad range of conditions can be extended to understand weld pool solidification. In certain cases, because of rapid cooling rate effects, it is not unusual to observe nonequilibrium microstructures. Recent developments in the application of computational thermodynamics and kinetic models, studies on single-crystal welds, and advanced in-situ characterization techniques have led to a better understanding of weld solidification and microstructures.

By: S.A. David, S.S. Babu, and J.M. Vitek

Parameters that control the solidification of castings also control the solidification and microstructure of welds. However, various physical processes that occur due to the interaction of the heat source with the metal during welding add a new dimension to the understanding of the weld pool solidification. Conventional theories of solidification over a broad range of conditions can be extended to understand weld pool solidification. In certain cases, because of rapid cooling rate effects, it is not unusual to observe nonequilibrium microstructures. Recent developments in the application of computational thermodynamics and kinetic models, studies on single-crystal welds, and advanced in-situ characterization techniques have led to a better understanding of weld solidification and microstructures.

INTRODUCTION

In welding, as the heat source interacts with the material, the severity of thermal excursions experienced by the material varies from region to region, resulting in three distinct regions in the weldment (Figure 1). These are the fusion zone (FZ), also known as the weld metal, the heat-affected zone (HAZ), and the unaffected base metal (BM). The FZ experiences melting and solidification, and its microstructural characteristics are the focus of this article. 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 (∆T), 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

Most knowledge of weld pool solidification is derived from the extrapolation of the knowledge of freezing of castings, ingots, and single crystals at lower thermal gradients and slower growth rates.1–6 In addition, rapid solidification theories have been extended to welds solidified at very high cooling rates.7–14However, microstructure development in the FZ is more complicated15,16because of physical processes that occur due to the interaction of the heat source with the metal during welding, including re-melting, heat and fluid flow, vaporization, dissolution of gasses, solidification, subsequent solid-state transformation, stresses, and distortion. These processes and their interactions profoundly affect weld pool solidification and microstructure. In recent years, phenomenological modeling of welding processes has provided unprecedented insight into understanding both the welding process and the welded materials. A variety of sophisticated models that employ analytical and numerical approaches are capable of describing many physical processes that occur during welding.15–25.

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