Hot Cracking in Directed-Energy Surface Processing
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MRS BULLETIN/MAY 1988
rapid solidification processing. This article will discuss the current understanding of hot cracking in surface melts in terms of present modeling capabilities and through the examination of some case histories. Sources of Hot Cracking The susceptibility of a material to hot cracking is complex. Many factors contribute to crack formation. These factors can be categorized as mechanical or metallurgical. Mechanical factors include geometrical influences and the stresses and strains induced during processing. The underlying driving force for cracking is the tensile stresses produced by thermal contraction during melt solidification and cooling. A qualitative description of the stress distribution transverse to a melt track is given in Figure 1. The relatively low compressive stresses produced during heating give way to the tensile stresses within the melt zone during cooling. Metallurgical determinants of hot cracking are also significant. They include melt and substrate chemistries, solidification structure, and microstructural coarseness. While the tensile stresses in the process zone continue to increase as cooling proceeds, the temperature at which hot cracks initiate strongly depends on metallurgically based crack sensitivity factors. Cracking Observations The importance of material properties and processing conditions to crack sensitivities will be introduced using case histories from the surface processing literature. The modifications implemented to control cracking serve to illustrate the underlying factors controlling cracking susceptibilities. The large effort within the welding commu-
nity to further the understanding and ability to control hot cracking in weldmen ts is also illustrative. Parallels between surface-melt processing and fusion welding are obvious. In fact, both the heat source and typical power densities used in processing can be equivalent. A major difference, however, is the short interaction times (high scan rates) and resultant high thermal gradients and rapid solidification rates typically obtained in surface-melt processing. In fusion welding, hot cracking is principally attributed to the presence of low-melting-point liquid films either during final solidification or upon reheating. In a generalized theory of hot cracking by Borland,3 it was recognized that liquid present in the form of a film allows high stresses to build up at the bridges joining adjacent grains. It is now widely recognized that hot cracking is promoted by high impurity contents (which segregate to form lowmelting films), low-melting phases, or wide-freezing ranges.4 Hot cracking or microfissuring of fully austenitic stainless steels during welding is a widespread, well-documented problem. In general, cracking is controllable in these materials by adjustments in melt chemistry which ensure high temperature delta-ferrite phase formation during initial solidification.5 The benefits of this solidification pattern are attributed to the partitioning of impurities such as silicon and phosphorus to t
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