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The effect of varying amounts of CoAl2O4 inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl2O4 resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl2O4 content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al2O3 particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl2O4 as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) is a popular additive manufacturing (AM) technique for the fabrication of precise components. The success of SLM relies on accurate control of the melt pool, where the metal powder melts and solidifies. Fluid flow inside the melt pool is driven by forces like Marangoni force and recoil pressure. SLM process begins with the fabrication of support, which involves the process of joining dissimilar metals. A strong support-base substrate connection ensures the success of subsequent additive manufacturing. In this study a comprehensive model was developed to predict the dimensions and characteristics of the melt pool with a focus on the first layer of the SLM process. An initial numerical simulation was done to obtain transient melt pool temperature and velocity distribution during the SLM process. The results were then coupled with a theoretical analysis development of the proposed model. The effectiveness of the model was validated by experimental results and melt pool characteristics from previous publications. The model shows Root Mean Square Error (RMSE) of 0.53 and 0.49 for height-to-width ratio and normalized area, respectively. The Mean Absolute Percentage Error (MAPE) of normalized area is 15.7 % for the majority of the data, which demonstrates a good accuracy of the model. In addition, the transition from conduction mode to keyhole mode can be identified with the model. This model can be used to rapidly and effectively predict the melt pool characteristics for SLM and other similar AM processes with a wide range of processing parameters. [ABSTRACT FROM AUTHOR]

Abstract This article presents a study on the influence of selective laser melting (SLM) process on microstructure and property of a cemented carbide system containing high entropy alloy. Analysis along the building direction indicated variation of chemical composition and microstructure, and this was influenced by two effects, firstly the dilution effect due to elemental diffusion from the baseplate and secondly the elements evaporation caused by high-power laser. At the lower half of the specimen, high fraction of η-carbide formed near the level of baseplate, and there were chemical gradients of major binder elements along the building direction. At the upper half of the specimen, there were relatively less variation in chemical composition and more homogeneously distributed phases including WC, W 2 C, η-carbide and FCC metal binder. The hardness of the lower half specimen varied from 711.7 HV1 (bottom of the specimen) to 1178.6 HV1 at 1 mm height. For the upper half of the specimen, hardness values could range from 1306.8 HV1 to 1413.4 HV1 and fracture toughness varied from 9.74 MPa m1/2 to 13.29 MPa m1/2. Highlights • A complex cemented carbide system was subjected to selective laser melting process. • The cemented carbide system contains a high entropy alloy. • Composition and microstructure of the cemented carbide were heterogeneous. • Dilution effect, evaporation, and laser scanning tracks could affect the built. • Hardness and toughness varied due to variation in composition and microstructure. [ABSTRACT FROM AUTHOR]

This article presents detailed microstructure characterization of cemented carbide fabricated by selective laser melting process. Nickel-coated tungsten carbide powders were used for this study; the selective laser melting process induced complex microstructure consisting of partially-melted carbide, carbide precipitates and dendritic microstructure. Four phases were identified, i.e. MC, M 2 C, FCC and η in the as-built specimen. CALPHAD phase diagram was utilized to describe possible phase formation sequence during selective laser melting process, which involved melting, solidification and repeated heating. Although the composition of initial powder was very simple (spherical W 2 C-WC and Ni-coating), selective laser melting allowed the decomposition of initial carbides, subsequent formation of dendritic structure and precipitation of additional carbides. To the best of authors' knowledge, for the first time, detailed microstructure characterization of cemented carbide fabricated by selective laser melting is presented. [ABSTRACT FROM AUTHOR]

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]

• Agglomeration of CoAl 2 O 4 inoculants follows the scan direction. • Modifying the scan speeds and energy densities can inhibit the agglomeration. • CoAl 2 O 4 was reduced within the melt pool to from Co-rich particles. • The extent of melt pool remelts the Co-rich particles present in deposited layers. • Insufficient energy input may lead to a slight fraction of unreacted CoAl 2 O 4. [Display omitted] The investigation aims to clarify the influence of scan parameters on the morphology and phase constitution of CoAl 2 O 4 inoculants in Inconel 718 (IN718) superalloy processed by selective laser melting (SLM). Gas atomized IN718 powder feedstock was uniformly blended with 0.2 wt% of CoAl 2 O 4 flakes and processed with a range of laser scan parameters to understand the effect of the inoculant additions on microstructure. Similar to the reactions observed in investment cast Ni-superalloys, the CoAl 2 O 4 particles were found to chemically react with elemental Al, Cr, and Ti present in the melt pool of IN718 and become reduced into Al 2 O 3 , Cr 2 O 3 , and TiO 2 oxides after the SLM process. Analyses of single melt pool beads revealed the existence of Co-rich metallic particles distributed along the surface of the oxides. However, since SLM processing involves remelting of the previously deposited layers to minimize the occurrence of porosity, this results in the dissolution of the Co-rich metallic particles that form when the CoAl 2 O 4 particles are reduced in the melt pool. As such, the ability of the metallic Co particles to contribute to grain refinement becomes limited during SLM processing. Furthermore, the inoculant particles were found to agglomerate along the scan direction. The underlying mechanisms can be attributed to the Marangoni convection following the moving center of laser beam. It was found that decreasing energy density and increasing scan speed can effectively reduce the size and number density of the agglomerated oxide particles. Faster laser scan velocities lead to reduced convective flow within the melt pool and reduce the tendency for agglomeration. However, the reduced magnitude of energy input limited the time over which the CoAl 2 O 4 particles could react with the melt pool. This led to the formation of lack of fusion defects and incomplete chemical reaction between CoAl 2 O 4 and IN718 after the SLM process. The effectiveness of CoAl 2 O 4 inoculant particles on grain refinement in as-built SLM IN718 microstructures is strongly dependent on the melt pool physics and laser scan conditions. [ABSTRACT FROM AUTHOR]

An effective strategy to strengthen a superalloy processed by selective laser melting (SLM) is proposed. The aim is to increase the yield strength of Inconel 718 fabricated by SLM to beyond 1400 MPa, which has never been achieved before. In this study, various NbC additions (0.0%, 0.5%, 1.0%, and 5.0% by weight) were added in the powder bed of Inconel 718, and two types of post-SLM heat treatments were investigated, i.e., solution heat treated plus aging (STA) and direct aging (DA). With NbC addition, smaller depth of melt pool and finer dendritic cells were obtained. Both STA and DA promoted the precipitations of γ′ and γ″. STA eliminated the cellular dendrites and induced grain growth while DA retained the as-built cellular dendrites, grain size, and nano-carbide from NbC addition, rendering a significant 326.2 MPa increase in yield strength. In this work, 0.5% NbC addition exhibited a record-high yield strength of 1461 MPa and ultimate tensile strength of 1575 MPa for Inconel 718 processed by laser manufacturing process according to literature data to-date. [ABSTRACT FROM AUTHOR]

The selective laser melting (SLM) process is an additive manufacturing technique that can fabricate three-dimensional workpiece by laser scanning and sintering of designated material in powder form on a preset scanning route along a bed of powders. This process involves a moving laser heating source which causes local transient mass and heat transfer with phase change, i.e., melting and solidifying, in a pool of melted powders. To fully investigate such a complicated transport process with reasonable computation cost, a novel quasi-transient numerical model (hopping model) was proposed and validated experimentally in this study. This approach includes a volumetric heat source defined as an elliptical Gaussian laser spot which hops along the scan path. Inconel 718 (IN718) superalloy powder is selected as the material for demonstration. Thermal analysis was carried out using a number of laser power and scan speeds in the range from 80 W to 120 W and from 80 mm/s to 140 mm/s, respectively. The comparison of molding appearance of the work pieces from the developed hopping model, full transient model, effective-transient model, and experiments illustrates the accuracy and the efficiency of the proposed hopping model. Molding appearance obtained from the quasi-transient model was consistent with experimental results with a relative error of less than 1.71% on the width of the IN718 stripes fabricated by SLM. For the cases analyzed in the current study, the quasi-transient model showed a 99% and a 60% reduction in both the computation time and the amount of computer memory needed compared to the full transient model and the effective-transient model, respectively. This novel quasi-transient simulation technique can be used to rapidly and effectively optimize process parameters for intelligent additive manufacturing. [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]