Introduction Many alloys are susceptible to cracking during solidification, which is called solidification cracking in weld- ing (Ref. 1) and hot tearing in casting (Refs. 2–4). Figure 1 shows an exam- ple of solidification cracking. A 1.6- mm-thick 6061 Al sheet was bead-on- plate welded by using the gas tungsten arc welding (GTAW) process without a filler metal. The mushy zone (a semisolid) be- tween the weld pool and the complete- ly solidified weld metal is weak and susceptible to cracking. As the weld pool moves forward, both the solidi- fied weld metal and the portion of the workpiece behind the pool cool and contract. The higher density of the sol- id than the liquid causes the weld met- al to contract during solidification. So- lidification shrinkage can be as much as 6.6% in the case of Al (Ref. 2). The thermal expansion coefficient causes both the solidified weld metal and the workpiece to contract during cooling. Thus, both solidification shrinkage and thermal contraction induce tensile strain in the mushy zone. The more tightly the workpiece is clamped down or connected to a rigid body, the more tensile strain is induced by its thermal contraction. The semisolid material in the mushy zone has little strength because it consists of dendritic grains that are still separated by liquid. The problem is that the semisolid also has little duc- tility during the terminal stage of so- lidification when there is insufficient liquid between grains for them to move and rearrange themselves to ac- commodate tensile strains without cracking. Numerous theories or models have been proposed for hot tearing (Ref. 5). Some of them assumed the mushy zone will crack when the tensile stress, strain, or strain rate exceeds a critical value. Apblett and Pellini’s strain theo- ry in 1954 (Ref. 6) assumed that grains are separated by thin liquid films near the end of solidification and that cracking occurs when the highly localized strains in the liquid films fi- nally exceed the critical limit they can withstand. Prokhorov’s theory in 1962 (Ref. 7) focused mainly on the thermo- mechanical factor of cracking, assum- ing that cracking can occur if the rate of strain accumulation with tempera- ture drop dε/dT exceeds a critical val- ue. The model of Feurer in 1977 (Ref. 8), on the other hand, focused mainly WELDING RESEARCH A Simple Index for Predicting the Susceptibility to Solidification Cracking The crack susceptibility of an Al alloy can be reduced if the filler metal reduces the maximum steepness of its curve of temperature T vs. square root of fraction solid (f S ) 1/2 BY S. KOU ABSTRACT The present study proposed to use the maximum │dT/d(f S ) 1/2 │ of an alloy as a simple index for its susceptibility to solidification cracking. The index was based on a recent criterion for cracking that considered the phase diagram, solidification shrink- age, strain rate, cooling rate, and liquid feeding. However, other factors may also af- fect the susceptibility, e.g., the dihedral angle and secondary phases. Curves of T vs. (f S ) 1/2 of Al alloys were plotted to find the maximum │dT/d(f S ) 1/2 │ using commercial thermodynamic software package Pandat and database PanAluminum based on the Scheil solidification model of no solid-state diffusion. Several critical predictions were made based on the index, including the crack susceptibility reduction by Al filler met- als, the relative crack susceptibility of wrought Al alloys 2014, 2024, 2219, 6061, and 7075, and the most crack susceptible compositions of binary Al-Si, Al-Cu, and Al-Mg alloys. These predictions were verified by reported crack susceptibility tests and pub- lished filler metal guides. Although the predicted crack susceptibility of Al-Mg alloys was too high, it was not caused by the index but by the Scheil model because it doesn’t consider diffusion. It was explained that, because of the very high (17.5 wt-%) Mg solubility in solid Al, significant Mg diffusion from the interdendritic liquid into Al- rich dendrites can occur and reduce the maximum │dT/d(f S ) 1/2 │ significantly, consis- tent with the good weldability of Al-Mg alloys. KEYWORDS • Solidification Cracking • Aluminum Alloys • Filler Metals • Crack Susceptibility Index • Fraction Solid • 2014 Al • 2024 Al • 2219 Al • 7075 Al • 6061 Al • 4043 Al Filler Metal • 4145 Al Filler Metal • Al-Si Alloys • Al-Cu Alloys • Al-Mg Alloys S. KOU ([email protected]) is professor in the Department of Materials Science and Engineering, University of Wisconsin, Madison, Wis. WELDING JOURNAL / DECEMBER 2015, VOL. 94 374-s
A Simple Index for Predicting the Susceptibility to Solidification Cracking
May 17, 2023
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