Process-Controlled Microstructure and Cast Morphology of Dendrite in Pulsed-Current Gas-Metal Arc Weld Deposits of Alumi
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THE mechanical properties of a weld joint often become very sensitive to the microstructure of the weld deposit, largely because of the welding parameters. Earlier observation[1] shows that the presence of coarse dendrites in a weld metal can be considered as one of the primary causes of the weakening of the weld metal. Therefore, to achieve the desired properties in the weld joint, control over the coarsening of the microstructure of the weld metal may be considered as of great importance. Various techniques can be used for this purpose; these are primarily known as vibrating the solidifying weld pool,[2] using an alloying addition as a grain refiner in the filler metal,[3,4] lowering the current and heat input,[5] keeping down the heat built up in the weld pool, and interrupting the solidification of the weld pool by introducing pulsed current.[6] The refining of the microstructure of the weld metal through minor alloying additions in the filler wire of the gas-metal arc welding (GMAW) has been found to be effective to a significant degree[3,4] in improving the mechanical properties of the V.K. GOYAL, Workshop Superintendent, is with I.G. Engineering College, Sagar, (M.P.)-470 004, India. Contact e-mail: prakgfmt@ iitr.ernet.in P.K. GHOSH, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, India. J.S. SAINI, Emeritus Professor, is with the Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee. Manuscript submitted November 20, 2006. Article published online July 11, 2007. 1794—VOLUME 38A, AUGUST 2007
weld joint. However, the use of pulsed-current GMAW (P-GMAW) process has been found[7] to provide more improvement in the mechanical properties of a weld metal containing a grain refiner by further refining its microstructure, largely by interrupting the metal deposition under the pulsed current. The interruption in the metal deposition affecting the solidification of the weld pool, in an attempt to refine its microstructure, largely depends upon the pulse parameters affecting the heat input and the heat built up in it. The heat input varies as a function of the energy input, whereas the heat built up in the weld pool of the P-GMAW process primarily depends upon the heat consumed in the remelting of the solid fraction grown in it during the interruption in the energy input at the pulse-off period. Thus, control over the pulse parameters may have a significant influence on the microstructure of the weld deposit in the P-GMAW process. The selection of the welding parameters in the P-GMAW process, however, is comparatively more complicated than it is in the conventional GMAW process, due to the involvement of a large number of simultaneously interacting pulse parameters in the P-GMAW process. The primary parameters of the P-GMAW process are considered to be the peak current (Ip), base current (Ib), pulse frequency (f), peak current duration (tp) and base current duration (tb). The simultaneous influence of the pulse p
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