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The selective laser melting (SLM) process involves irradiation from a high-power laser to melt and sinter powdered material. The high temperature gradient near the laser spot generates high residual stress that can lead to failure of fabricated parts. Estimation of the residual stress for real transient SLM processes by simulation involves considerable computation effort which makes its application in real industrial SLM processes rather difficult. To tackle this challenge, a two-stage coupled transient thermomechanical model with a quasi-transient heat source was developed. In the first stage, a transient thermal analysis was performed to acquire temperature distribution of the full SLM process. The temperature distribution serves as thermal loading input for subsequent transient thermal stress analysis. This quasi-transient thermomechanical model can provide residual stress data consistent with that of a conventional full transient model in 68% less computation time. The data provided by the new model also includes the dynamic stress release due to re-heating and re-melting in overlapped laser scanned regions which was in close confirmation with experimental results. This efficient quasi-transient model is useful for rapid analysis and optimization of SLM processing parameters. [ABSTRACT FROM AUTHOR]

Fusion-based additive manufacturing (AM) has significantly grown to fabricate Nickel-based superalloys with design freedom across multiple length scales. Several phenomena such as feedstock/energy source/melt pool interactions, solidification and phase transformations occur during fusion-based AM processes of Nickel-based superalloys, which determine the ultimate microstructure and mechanical performance of the built parts. In this review, we elaborate a comprehensive discussion on AM Nickel-based superalloys and influential factors including feedstock characteristics (powder morphology, chemistry, contamination, flowability, recycling) and AM processing (parameters, and powder spreading/wall/balling/spattering effects) on their microstructure (micro-segregation, phases formations and grain structures), defect generation (sub-surface/internal defects, microcracks, surface roughness, and residual stress). Furthermore, the mechanical properties of AM Nickel-based superalloys such as tensile, creep and fatigue at room/elevated temperatures are analyzed in accordance with the initial, and post processing effects. Additionally, the commonly utilized modeling approaches in literature to predict the microstructure and mechanical behavior of these alloys are highlighted. Finally, the current challenges and mitigation approaches for future research are identified considering the gaps in the AM Nickel-based superalloys. [ABSTRACT FROM AUTHOR]

Martensitic phase transformation is an important strengthening mechanism for 17-4 PH stainless steel; this study has investigated the martensite fractions of 17-4 PH stainless steel processed by selective laser melting (SLM) with systematic variation in energy densities through adjustments in laser power, scanning speed, and hatch distance. Interestingly, the fractions of martensite have shown a positive correlation with the energy densities for specific hatch distance. Finite element analysis has suggested that an increase in energy density could promote higher residual stress to induce higher fractions of martensite due to greater thermal contraction during SLM process. Furthermore, this study has shown that a decrease in the hatch distance could induce higher fractions of reverted austenite due to more laser scanning overlaps, leading to a decrease in martensite fractions. Prediction equations based on SLM parameters for martensite fractions in 17-4 PH stainless steel have been established; this work demonstrates that martensite fractions of 17-4 PH stainless steel in as-built state can be designed by SLM process. • 17-4 PH stainless steel has been investigated by different SLM parameters. • Martensite fractions can be controlled by both energy density and hatch distance. • Higher energy density can increase martensite fractions for a given hatch distance. • Finite element analysis successfully correlates with experiments. • An equation to predict martensite fractions has been established. [ABSTRACT FROM AUTHOR]