This study investigates the effects of welding velocity on weldability of dissimilar cast/rolled high-entropy alloys and the applicability of cryogenic temperatures. Cast HEA side of dissimilar weld metal (WM) indicates a larger dendrite packet and dendrite arm spacing (DAS) than rolled WM. Size difference is associated with epitaxial growth from each base metal (BM). As the welding velocity increases (6-10 m min −1 ), shrinkage voids and DASs decreases. Dissimilar welds show tensile properties comparable to cast BM, where there are tensile fractures of dissimilar welds. Cryogenic properties of dissimilar welds are superior to those of room-temperature welds, because deformation twins and dislocation densities are significantly formed at 77 K. Therefore, dissimilar welds can be applied to the production of cryogenic products.
In this study, the weldability of the as-cast CM247LC superalloy for turbine blade applications was metallurgically evaluated in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone (HAZ) of gas tungsten arc welds, and the cracks were characterized as liquation cracks, since a cobble or dropletshaped crack surface consistent with a liquid film was clearly confirmed. Microstructural analysis of the cracking surface and thermodynamic calculations helped elucidate the metallurgical mechanisms of the liquation cracking. In other words, the cracking was attributed to liquation of the γ-γ’ eutectic colony and the constitutional liquation of the MC-type carbides: these phases existed in the as-cast microstructure. In particular, it was calculated that liquation of the γ-γ’ eutectic colony during welding occurs at least at 1488 K and that constitutional liquation of MC-type carbides begins at 1411 K, while the equilibrium solidus temperature of the CM247LC alloy is 1530 K. Finally, the liquation cracking susceptibility was quantitatively evaluated through a spot-Varestraint test, and it was confirmed for the first time that the higher susceptibility of as-cast samples can be suppressed by employing a pre-weld heat treatment such as solution treatment.
In order to quantitatively evaluate the solidification cracking susceptibility in laser welds of type 310S stainless steel, a transverse-Varestraint testing system using a laser beam welding apparatus was newly constructed. The timing-synchronization between the laser oscillator, welding robot and hydraulic pressure devices was established by employing high-speed camera observations together with electrical signal control among the three components. Moreover, the yoke-drop time measured by the camera was used to prevent underestimation of the crack length. The laser beam melt-run welding used a variable welding speed from 10.0 to 40.0 mm/s, while the gas tungsten arc welding varied the welding speed from 1.67 to 5.00 mm/s. As the welding speed increased from 1.67 to 40.0 mm/s, the solidification brittle temperature range of type 310S stainless steel welds was reduced from 146 to 120 K. It follows that employing the laser beam welding process mitigates the solidification cracking susceptibility for type 310S stainless steel welds.
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