A Three-Stage Mechanistic Model for Solidification Cracking During Welding of Steel
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UCTION
MULTIPLE mechanisms for solidification cracking during metal manufacturing processes such as casting, welding, and more recently additive manufacturing have been proposed. Sigworth[1] and Eskin et al.[2] reviewed the research field in 1996 and 2004. Some of the first studies of importance include Pellini’s ‘‘strain theory of hot tears’’ proposed in 1952,[3] Humphrey and Jennings ‘‘shrinkage-brittleness theory’’ in 1948,[4] and Borland’s ‘‘generalized theory’’ in 1960.[5] The generalized theory combined and modified the ‘‘strain’’ and ‘‘shrinkage-brittleness’’ theories. All these early solidification cracking theories are in the agreement that solidification cracking occurs with thermally mechanically induced strain at the late stages of solidification when the fraction of liquid is less than 0.1. In 1976, Feurer[6] studied liquid presence between grains and argued that a solidification crack will L. AUCOTT is with AWE Plc, Aldermaston, Reading, RG7 4PR, UK. D. HUANG and A.C.F. COCKS are with the Department of Engineering, University of Oxford, Oxford, OX1 3PJ, UK. H.B. DONG and S.W. WEN are with the Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK. Contact e-mail: [email protected] J. MARSDEN is with the Tata Steel, Swinden Technology Centre, Rotherham, S60 3AR, UK. A. RACK is with the European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France. Manuscript submitted September 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleate as a pore if the liquid is no longer able to fill the inter-granular openings. However, this study only considered the contribution of solidification shrinkage. In 1988, Guven and Hunt[7] emphasized the role of tensile stresses in the formation of solidification cracks. In 1999, Rappaz et al.[8] proposed a RDG model by extending Feurer approach[6] to include feeding associated with tensile deformation of the solidified material. The RDG model was the first hot tearing model with a physically sound basis. However, phase changes and the grain boundary, where cracking occurs, were not taken into account. In 2003, Campbell[9] emphasized that solidification cracking criteria generally neglect the importance of thermo-mechanical aspects and simply consider the alloy’s solidification temperature range: the larger the freezing range, the more susceptible the alloy will be to solidification cracking. In 2015, Kou[10] developed a model focusing on events occurring at the grain boundary, such as: separation of grains from each other, lateral growth of grains toward each other, and liquid feeding between grains. To date, the potential driving forces for solidification cracking are well-established: solid contraction in a thermal gradient, solidification shrinkage, and a high sensitivity to solute segregation. However, nucleation and propagation mechanisms are inconclusive and hindered by a lack of direct experimental observation to validate the theories proposed. Only recently have experimental observations on solidification cracking started to emerge: first for transpar
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