Search

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]

• 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]

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]