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Although hot cracks frequently occur during metal additive manufacturing (AM), there are very few fundamental studies on the cracking behaviour and susceptibility. In this study, a horizontal tensile-type hot cracking test was investigated for evaluating the solidification cracking susceptibility of alloy 718 quantitatively during AM, particularly in laser powder bed fusion. The factors influencing the cracking susceptibility were also examined. The solidification cracking was reproduced by the melting of the AMed specimen with a tensile load under conditions of heat source simulating the AM process. The critical initial tensile stress for solidification crack initiation could apply as an evaluation index for cracking susceptibility. The critical initial stress decreased with increasing the laser power. A similar tendency was observed during the melting of the powders set on tiny groove for simulating AM process. The critical strain rate for solidification cracking (CST) was derived by a thermal elastic–plastic analysis using the critical initial stress measured experimentally. The CST at the laser power of 1125 W was higher than that of 1000 W for no crack condition. Therefore, high CST corresponding to penetration shape with high laser power induce to deteriorate the solidification cracking susceptibility.

The formation conditions of lacy ferrite in austenitic stainless steels solidified with primary ferrite was clarified through crystallography in order to increase the amount of lacy ferrite which contributed to the improvement of properties. Lacy ferrite was formed when the misorientation between ferrite and austenite from the Kurdjumov–Sachs (K-S) relationship was ≤ 7° on the close-packed plane and ≤ 9° in the close-packed direction during solidification. In contrast, vermicular ferrite was formed if the misorientation exceeded these values. The temperature gradient during solidification was controlled unidirectionally by two-pass laser welding. Following the performance of two-pass welding with a highly unidirectional laser, it was confirmed that the formation ratio of lacy ferrite to total ferrite significantly increased in the second-pass weld metal by remelting the first-pass weld metal, in which the heat flow direction and preferential growth direction of ferrite and austenite were controlled to be almost parallel. The growth ratio while maintaining the K-S relationship established between ferrite and austenite in the first-pass weld metal, and/or the ferrite formation ratio that satisfies the K-S relationship, increased in the second-pass weld metal because the growth of both the primary ferrite and secondary austenite was crystallographically independent of each other during solidification.

In the welding of austenitic stainless steels, the formation of approximately 5% δ-ferrite phase is often used to decrease the susceptibility to solidification cracking. The partition coefficients of impurity elements such as phosphorous and sulfur of the ferrite phase are high, which expand the temperature range of solidification. At the same time, it is well known that the solidification cracking susceptibility increases with increasing δ-ferrite content above 20%. Thus, high δ-ferrite content increases the solidification cracking susceptibility, even though the δ-ferrite phase has high solubility of the impurity elements. The aim of this work was to investigate the effects of the δ-ferrite content and the concentration of carbon and silicon on the solidification crack susceptibility of Fe–18%Cr stainless steel with various nickel concentrations. The ferrite content was controlled by adding various amounts of nickel. The brittle temperature range (BTR) hardly changed with increasing δ-ferrite content, regardless of the type and concentrations of carbon and silicon. Therefore, the effect of the ferrite content on solidification cracking susceptibility must be small. In addition, the BTR increased with the concentrations of carbon and silicon. The specimens containing higher concentrations of carbon exhibited higher BTRs than those containing silicon. It is considered that the solidification segregation corresponding to the partition coefficient of carbon to the δ-ferrite phase induces an increase in cracking susceptibility. In addition, the formation of austenite during solidification reduces cracking susceptibility owing to the higher solubility of carbon.

Hot cracking susceptibility of Haynes® 282® with varying amount of C (0.05–0.09 wt%), Mn (0.03–0.12 wt%), Si (0.03–0.16), B (0.005–0.006 wt%) and Zr (0–0.01) are investigated. Synergistic role of C and B is found on solidification and heat affected zone (HAZ) liquation cracking susceptibility. High amount of C and B promote formation of eutectic constituents during final stages of solidification and promote crack healing by backfilling effect. When C and B are added in low amount the crack healing does not occur due to the absence of eutectic consituents therefore cracking extent increases. Thermodynamics simulations indicate C and B tie up to MC carbides and M3B2 borides during solidification. Scanning Electron Microscopy and Nanoscale secondary ion mass spectrometry analysis reveal C and B to be present both in solid solution and in form of precipitates to Ti-Mo rich carbides and Mo rich borides, respectively. In HAZ, these phases promote liquation cracking where cracking extent correlates to the amount carbides and borides. Lower C and B is found to reduce the liquation cracking in the HAZ. Furthermore, a high temperature homogenization heat treatment at 1190 °C excarbates the cracking by dissolving the borides and releasing B to the grain boundaries.