Investigation of Hot Cracking Behavior in Transverse Mechanically Arc Oscillated Autogenous AA2014 T6 TIG Welds
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HOT cracking during welding was studied experimentally.[1–6] Different manifestations of hot cracking during fusion welding are as follows: (1) solidification cracking in the fusion zone (FZ), (2) liquation cracking in the partially melted zone/heat-affected zone (PMZ/ HAZ), and (3) a combination of the two. Of these various manifestations, aluminum alloys commonly experience both solidification cracking in the FZ and liquation cracking in the PMZ, which are mainly intergranular. Generally, solidification cracking occurs during the terminal stage of solidification, when the tensile stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal.[7,8] Liquation cracking is produced due to the combination of thermally induced strains arising during the welding process and formation of low melting liquid film along the grain boundary and to some extent at the grain interior.[9,10] Solidification cracking has been known to be favored by the factors that decrease the solid-solid contact area during the terminal stages of solidification. Two of the most important factors are the extent of formation of low-melting eutectics and grain size. Lowmelting eutectics at the grain boundaries may exist as a liquid film to a temperature well below the equilibrium solidus and reduce the grain boundary contact area to a minimum.[3] Also, the coarser the grain structure, the less the grain boundary contact areas for a given amount of nonliquid. Hence, coarse-grained FZ structures are N.S. BIRADAR, Research Scholar, and R. RAMAN, Professor, are with the Welding and Equipment Design Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. Contact e-mail: [email protected] Manuscript submitted October 13, 2011. Article published online May 26, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
generally more prone to solidification cracking than is fine-grained material. Similarly, liquation cracking has been known to be favored by the following factors: (1) the extent of liquation, (2) the grain structure, (3) the hot ductility, and (4) weld metal contraction and the degree of external restraint.[9] Significant effort was devoted to characterize the relative weldability of different alloys, using a variety of weldability tests.[10–16] One test that is used currently to qualify the weldability of aluminum alloys is the circular patch test (CPT),[10,12,17] which is a representative test that attempts to reproduce the actual welding conditions in small test samples. Hence, the formation of fine-grained structure in the weld metal and reduced liquation in the PMZ is important in controlling hot cracking. Many methods were reported in the literature to control the grain structure in the weld metal. Grain refinement techniques such as inoculation with heterogeneous nucleants,[18,19] arc pulsation by pulsing welding current,[20] doublesided arc welding,[21] weld pool stirring,[22] longitudinal oscillation of weld table
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