Alternating grain orientation and weld solidification cracking
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INTRODUCTION
A L T H O U G H magnetic arc oscillation had been employed in numerous applications with varying degrees of success, the first in-depth study on the effect of magnetic arc oscillation on the microstructure and properties of autogenous GTA welds, to the best of our knowledge, was that carried out by Tseng and Savage ~ in 1971. Magnetic arc oscillation was used in conjunction with the Varestraint test 2 to evaluate the effect of magnetic arc oscillation on the microstructure and hot-cracking susceptibility of HY-80 steel welds. The subgrain size and hot-cracking susceptibility of the welds were reduced by magnetic arc oscillation. The effect of arc oscillation on the grain structure, however, was not reported. Garland 3 investigated the effect of torch vibration on the grain structure and solidification-cracking susceptibility of autogenous GTA welds of an A1/I.7 to 2.8 pct Mg alloy. The fish-bone t e s t a w a s used for evaluating the solidification-cracking susceptibility of the welds. Fine equiaxed grains were produced by torch vibration and solidification cracking was reduced. Sharir et al. s obtained fine equiaxed grains in autogenous GTA welds of tantalum by using magnetic arc oscillation. David and L i u , 6'7 and subsequently Scarbrough and Burgan, 8 reduced the size of the columnar grains in the autogenous GTA welds of iridium alloys by magnetic arc oscillation. The latter investigators reported reduction in the rejection rate of the welded parts due to hot cracking. It seems clear from the previous studies mentioned above that the solidification-cracking susceptibility of a weld can be significantly affected by its grain structure, and the grain structure in turn can be significantly affected by magnetic arc oscillation during welding. It was observed by Savage e t al. 9 that welds made with a tear-shaped weld pool are more susceptible to solidification cracking than welds made with an elliptically shaped pool. Davies and Garland 1~pointed out that this is because in a tear-shaped weld pool low melting point segregates tend to be trapped at the grain boundaries along the weld centerline when the essentially straight columnar grain ~ growing from opposite sides of the weld pool impinge against each other. S. KOU and Y. LE are, respectively, Professor and Graduate Student at the Department of Metallurgical and Mineral Engineering, University of Wisconsin, Madison,'WI 53706. Manuscript submitted October 26, 1984. METALLURGICAL TRANSACTIONS A
In an elliptically-shaped weld pool, on the other hand, curved columnar grains H growing perpendicular to the trailing edge of the pool do not impinge against each other and, therefore, do not trap low melting point segregates along the weld centerline. It has been observed in casting that fine-grained materials are less susceptible to solidification cracking than coarsegrained materials. ~2-15 This is due to the fact that in finegrained materials low melting point segregates tend to be distributed over a larger grain boundary area and, therefore, becom
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