This study identified a cracking that occurred continuously in the built direction of an additive manufactured part of alloy 718 by the laser metal deposition method and clarified the cracking behavior. The solidification microstructure of alloy 718 was evaluated by EDS, EPMA, and extraction residue, and it was found that Laves phase, Ti carbides, and Al oxides were formed between the dendrite microstructures. A fracture surface analysis of the cracks revealed that the cracking in the additive manufacturing was hot cracking associated with the liquid film and that the cracking occurred along the grain boundaries. The fracture surface at the beginning of the cracking showed a solidification cracking fracture surface. The experimental investigation for crack propagation during additive manufacturing and the analytical evaluation of the mechanical state around the cracking using thermal elastic-plastic analysis elucidated the cracking behavior during additive manufacturing. If the cracking exists at the fusion boundary, it is suggested that the cracking initiates from the solid-liquid interface towards the solidification direction. Moreover, tensile strain occurred around the cracks during additive manufacturing, particularly the strain was biggest at the crack edge on the fusion boundary. Therefore, the continuous cracking that occurred beyond each layer in the additive manufacturing of alloy 718 in this study can be considered solidification cracking, and its behavior can be further explained as a notch extension cracking that starts from the solidification cracking and continuously occurs in the solid-liquid coexistence temperature region.

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The chemical composition of alloy 718 powder was optimized to suppress a solidification cracking in the additive manufactured part by the laser metal deposition method. The relationship between the condition of additive manufacturing and defects was evaluated. A lack of fusion was observed on the low heat input (low energy density), and the solidification cracking occurred on the high heat input (high energy density). The powder composition of alloy 718 was attempted to be optimized from a theoretical approach of the solidification cracking susceptibility. It was found that Si and B had a significant influence on the solidification cracking, therefore a preventive powder with reduced Si and B was developed. The effects of Si and B on solidification cracking were compared by reproducing the solidification conditions during additive manufacturing and evaluating the solidification brittle temperature range (BTR) using the Varestraint test. It was found that the BTR of the preventive material was smaller, and the solidification cracking susceptibility was improved. In addition, the solidification cracking did not occur in additive manufacturing using the preventive powder. In other words, it is suggested that the countermeasure for weld solidification cracking leads to the prevention of solidification cracking in additive manufacturing directly, and the effectiveness of the optimization of powder composition was verified.

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Friction stir welding of the dissimilar metals steel and aluminum involves slight scraping of the joint surface on the steel side using a tool to be metallurgically joined. However, excessive contact between the tool and workpiece during the welding generates excessive heat, which significantly reduces the joint strength. Therefore, we focused on a joint method, which uses the anchor effect of the protrusions formed by an additive manufacturing method, to establish a new dissimilar metal friction stir welding technique without involving cutting with a tool. In this study, we used a laser additive manufacturing system to form a 1-mm high inverted truncated conical protrusion on the steel side joint surface of maraging steel and investigated the effect of the optimal protrusion shape for maximizing the joint strength. For the root diameter of the protrusion of at least 1.1 mm, the protrusion did not break owing to plastic flow during friction stir welding. Numerical analysis showed that the joint strength increased as the gap between protrusions decreased; however, when the gap was 0.29 mm or less, the deformation behavior under tension changed, and the joint strength decreased as the gap decreased. Numerical analysis also showed that the joint strength increased with increasing tilt angle of the inverted truncated cone, but decreased with increasing tilt angle of ≥70°. Additionally, the joint strength obtained by the experiment was approximately 40% lower than the joint strength obtained by numerical analysis owing to the insufficient plastic flow during friction stir welding, and inability of sufficiently filling the gaps between the protrusions with aluminum, which resulted in defects. However, the correlation between tensile strength and protrusion shape predicted by numerical analysis showed the same trend as the experimental results, suggesting usefulness of the numerical analysis in optimizing the anchor shape.

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Weld residual stresses are affected by phase transformation of low-alloy steels. When phase transformation occurs, transformation plasticity occurs in addition to transformation expansion. Conventional predictions of welding residual stress using the finite element method often consider only transformation expansion, and there are few cases in which transformation plasticity is taken into factor. In this study, welding residual stress analysis that factors in transformation plasticity was performed to verify the effect of transformation plasticity on welding residual stress. The results show that the prediction accuracy of welding residual stress due to transformation expansion of low-alloy steel is improved by factors in transformation plasticity.

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The purpose of this study is to clarify the effect of Ti addition on the solidification cracking susceptibility of carbon steel. Using two-phase peritectic solidification model considering MnS and TiC crystallization, the effect of Ti addition on the BTR of carbon steel was theoretically investigated. The results suggest that Ti addition improves the BTR of carbon steel. On the other hand, when Ti was added in excess, the BTR of carbon steel deteriorated due to an increase in the amount of Ti segregation, thus suggesting the existence of an appropriate amount of Ti depending on the amount of C. The appropriate amount of Ti could be uniformly arranged by the Ti/C ratio regardless of the amount of C. The Ti/C = 5 to 8 was shown to be the appropriate range for Ti addition. In order to experimentally confirm the effect of Ti addition shown by the theoretical study, Ti-variable materials were prepared for 0.05 and 0.15%C steels. EPMA analysis of the weld metal microstructure showed the formation of TiC in the Ti-added material. The BTR of the Ti-variant material was evaluated by the Trans-Varestraint tests, and the BTR improvement effect of Ti addition was confirmed for the 0.15%C steel, and the appropriate Ti addition range was Ti/C=6.3. In this study, the effect of BTR improvement by Ti addition and the appropriate range of Ti addition were proposed based on the theoretical model for BTR of carbon steel. In addition, the effect of Ti addition was experimentally confirmed by evaluating the BTR.

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