A Multi-scale Thermomechanical-Solidification Model to Simulate the Transient Force Field Deforming an Aluminum 6061 Sem
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FUSION welding is a joining process that uses solidification of the base metal and sometimes a filler metal to make the weld. In aluminum alloy welding, a major defect that can arise is hot cracking[1] in which an intergranular crack forms during the terminal stages of solidification. A hot crack forms within the two-phase mushy zone behind the weld pool, rupturing liquid films that are present at grain boundaries. Hot cracking occurs due to the combination of limited intergranular liquid flow in response to solidification shrinkage, and semisolid deformation caused by thermal contraction and mechanical constraints.[2,3] To date, various mechanisms have been proposed to predict the formation of this defect.[3,4] As shown in early work by Pellini[5] and later Magnin,[6] hot cracking in castings can be related to the total strain accumulated in the semisolid. Other researchers[7,8] have shown that high strain rates seem to hinder the accommodation of deformation within the semisolid material, and consequently fracture occurs. Based on these findings, several strain and strain rate-based criteria have been developed to qualitatively predict hot crack formation[2,7–14] in both casting and welding. Although the actual strain and strain rate fields within a semisolid weld are position and time dependent, existing hot cracking criteria use an averaged value across the semisolid weld to assess crack susceptibility.[2,14] Such a simplifying assumption limits the
H.R. ZAREIE RAJANI, Ph.D. Researcher, and A.B. PHILLION, Assistant Professor, are with the School of Engineering, The University of British Columbia, Kelowna, BC, Canada. Contact e-mail: [email protected] Manuscript submitted May 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
application of hot cracking criteria, as they are not able to predict the position and size of this defect[3] for a given set of welding parameters. This assumption is nonetheless used due to the challenge of obtaining the non-uniform strain and strain rate fields within the semisolid during welding. Experimentally, it is quite difficult to obtain the transient strain field due to the short lifetime of the weld pool and the high temperature conditions. Numerical models, in contrast, are able to predict this behavior as they are not limited by time and temperature, but are complex due to the two-phase nature of semisolids and the interaction between the semisolid weld and the base metal. In order to simulate the transient strain fields during welding,[15] a model must first take into account the shape of the weld domain and then couple the resulting mesh with the force field ~ ftotal that causes deformation. Considering the semisolid weld as the simulation domain, the first step requires a mesh matching the microstructure of the weld fusion zone, while the second step requires detailed knowledge of the force field that deforms the semisolid weld. This total force field consists of the external forces that act on the boundaries of the semisolid weld, ~ f@Xfusionjsemisolid weld , and the internal for
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