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This investigation rationalizes the impact of oxide particles on the melt pool and subsequent formation of microstructure in superalloy Inconel 718 (Alloy 718) processed by laser powder bed fusion (L-PBF). A mixture of Alloy 718 powder with 0.2 wt pct of CoAl2O4 was processed without scan rotation. It was found that the physical presence of oxides appeared to increase the degree of melt pool penetration by inhibiting the heat transfer. Without favorable thermal conditions to trigger heterogeneous nucleation, a more noticeable tendency to form columnar grains was characterized in samples with CoAl2O4. It is believed that the presence of oxide particles in the melt pool contributed to decreasing the cooling rates during solidification and retarded the columnar-to-equiaxed transition. A comparatively deeper degree of melt pool penetration also enhanced the relative density of samples with CoAl2O4. Despite the occurrence of the balling phenomenon, the reduced Marangoni convection also contributed to less micro-humping and better continuity of the melt tracks that decreased the surface roughness by at least 16.7 pct. The addition of CoAl2O4 to the powder feedstock significantly impacted the processing window of L-PBF processed Alloy 718 and may potentially be used as a means to engineer the microstructure. [ABSTRACT FROM AUTHOR]

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]

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]

• 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]

High-power disk laser welding a steel plate using the Inconel 82 (IN82) filler wire with a 1.2 and 3.0 m/min feeding rate has been investigated in the experiment. The rapid thermal cycle combined with convection induced by the keyhole mechanism in laser welding results in the rapid solidification of the fusion zone (FZ). However, the microstructure of the FZ is not homogeneous at the macroscopic scale. The dilution of the FZ is important in determining the final microstructure of the weldment. For the specimen with a 1.2 m/min wire feeding rate, a lower amount of Ni-based IN82 filler is introduced into the weld pool, and the dilution of the FZ is between 65% and 100%. The BCC structure with high density boundaries dominates the entire FZ. For the specimen with a 3.0 m/min wire feeding rate, part of the filler melt is trapped on the top of the weld pool, and solidified into austenite alloyed with a Ni concentration above 15 at%. The range of dilution in the FZ with a 3.0 m/min wire feeding rate is decreased to 50–90%. There are hot cracks initiated/propagated along interdendritic austenite and in the austenite free of boundaries. Boundaries, especially for high-angle ones, in the BCC structure retard hot crack propagation in the FZ. The application of quantitative chemical analyses of Fe or Ni concentrations in the weldment provides a good approximation in evaluating the dilution of the FZ in laser welding. The methodology proposed in this study shows potential to obtain the dilution of any specific location in the FZ for industrial application in the future. [ABSTRACT FROM AUTHOR]