Microstructural evolution, nanoprecipitation behavior and mechanical properties of selective laser melted high-performance grade 300 maraging steel

Selective laser melting (SLM) additive manufacturing (AM) technology has become an important option for the precise manufacturing of complex-shaped metallic parts with high performance. The SLM AM process involves complicated physicochemical phenomena, thermodynamic behavior, and phase transformation as a high-energy laser beam melts loose powder particles. This paper provides multiscale modeling and coordinated control for the SLM of metallic materials including an aluminum (Al)-based alloy (AlSi10Mg), a nickel (Ni)-based super-alloy (Inconel 718), and ceramic particle-reinforced Al-based and Ni-based composites. The migration and distribution mechanisms of aluminium nitride (AlN) particles in SLM-processed Al-based nanocomposites and the in situ formation of a gradient interface between the reinforcement and the matrix in SLM-processed tungsten carbide (WC)/Inconel 718 composites were studied in the microscale. The laser absorption and melting/densification behaviors of AlSi10Mg and Inconel 718 alloy powder were disclosed in the mesoscale. Finally, the stress development during line-by-line localized laser scanning and the parameter-dependent control methods for the deformation of SLM-processed composites were proposed in the macroscale. Multiscale numerical simulation and experimental verification methods are beneficial in monitoring the complicated powder-laser interaction, heat and mass transfer behavior, and microstructural and mechanical properties development during the SLM AM process.

Additive layer manufacturing is a blanket term for a wide range of processes operating on the same underlying principle. 3D geometry is created by depositing material, layer by layer to create a final 3D geometry. Selective laser melting (SLM) is a branch of additive layer manufacturing, using a laser to fuse a powder bed of metal into each layer. This thesis investigates the SLM process and its application to nickel based superalloy materials, IN625 and IN718. IN625 and IN718 are similar nickel-based superalloys developed for use in aerospace gas turbine engines. In their conventionally manufactured form, these materials have excellent high temperature mechanical properties which make them idea for use in the hot section of gas turbine engines. The aim of this thesis was to investigate how these materials interact with the SLM process and how the material produced can be optimised to improve the range of applications it can be used for. A gap in knowledge regarding a detailed understanding of how the powders morphological and rheological properties influence its ability to be processed by SLM was identified and investigated. A wide range of characterisation methods were implemented with certain important properties being identified to assess a powders processability, namely the particle size distribution and how a significant content of fine particles below 10 μm in size can be detrimental to processability. There is also a lack of a standard powder characterisation methodology specifically for SLM applications. This is addressed with certain methods and measurements being suggested as most promising for wider SLM application. Avalanche flow testing is found to be most applicable to the critical recoating process in SLM and most able to differentiate suitable and unsuitable SLM powders. Following characterisation of the raw material feedstock powder, this thesis also investigates the influence of processing parameters on the microstructure of the material produced by the SLM process. Significant microstructural changes were observed as a result of process parameter changes. This was identified to potentially enable for in-situ modification of material microstructure to suit a manufactured material to its end application. Of the process parameters investigated, laser scan speed was most interesting, suggesting that a faster laser scan speed was able to create a similar microstructure to a much slower one. This was attributed to the reheating effect of the laser beam returning quickly to the adjacent scan line. The validity of this explanation was investigated using a simple, computational thermal model. The result is a new understanding of laser scan speed SLM and its nonlinear relationship with material temperature and microstructure evolution. Finally, post process heat treatments of SLM manufactured IN718 material were investigated. This investigation was in response to a gap in current knowledge regarding heat treatments designed specifically for SLM material. SLM IN718 has been found to have reduced high temperature mechanical properties, specifically stress rupture, which limits its application in demanding environments. In this thesis a range of post process homogenisation heat treatments were investigated, with treatments between 1030 °C and 1060 °C being found to produce material with characteristics consistent with material with excellent stress rupture properties. This novel heat treatment route could provide a method for SLM IM718, and the increased design and geometric freedoms, to be applied in more demanding applications. An evolution of the grain structure in the material was also observed and measured during high temperature homogenisation treatments. This was investigated in the final chapter, and a novel mechanism is suggested for the process of grain coarsening observed. Previously published literature explains similar evolutions as recrystallisation however this did not fit the observations during this thesis. The evolution of grain structure was observed using a process of quasi in-situ electron back scatter diffraction, and a mechanism of grain boundary length reduction, followed by grain growth, is suggested to better fit the observations. It was determined that grains are preferentially selected for growth based on their proximity to a ‘path of least resistance’ of lower angle grain boundaries. The results of this work should benefit industrial users of SLM in the fabrication of Nickel-Based Superalloy material for aerospace applications. The conclusions on powder characterisation offer an insight into available methods to better control and characterise powder feedstock materials for consistent production. Aerospace users especially may find the work regarding post process heat treatments designed specifically for SLM material, to recover lost stress rupture performance, useful in enabling the use of SLM materials, and the design freedom that brings with it, in mor demanding environments than are currently possible.

In this study, the effects of laser volumetric energy density (η) on the metallurgical defect behavior and microstructure evolution of H13 die steel fabricated by selective laser melting (SLM) additive manufacturing are systematically studied, and underlying thermal mechanisms are revealed. The results indicate that the metallurgical defect behavior is significantly affected by the applied η, which is controlled by laser power P and scanning speed v. With increasing P or decreasing v, η increases, the metallurgical defects such as pores and poor fusion initially decrease and then increase, and the density initially increases and then decreases. The typical microstructures induced by SLM are columnar dendrites and equiaxed dendrites. Their growth direction, distribution, and size at different positions in the molten pool are quite different. The size of the columnar crystals with directional full growth is highly correlated with the applied η. As the applied η increases, the length and diameter of the columnar crystals increase, but grains with nonuniform distribution are obtained under a higher η of 122.22 J mm−3. Under the optimized η of 111.11 J mm−3 (P = 200 W, v = 1000 mm/s), the H13 die steel samples fabricated by SLM are near-fully dense and have almost no metallurgical defects (the density reaches 99.13%), and the dense columnar crystals with uniform distribution are obtained. This study may provide a theoretical and experimental basis for the design and optimization of SLM processing parameters and the reliable fabrication of SLM-processed parts with controlled defects and microstructures.

Selective laser melting (SLM) additive manufacturing (AM) of hard-to-process W-based parts with the addition of 2.5 wt.% TiC was performed using a new metallurgical processing mechanism with the complete melting of the high-melting-point powder. The influence of SLM processing parameters, especially laser scan speed and attendant laser fluence (LF), on densification behavior, microstructural development, and hardness/wear performance of SLM-processed W-based alloy parts was disclosed. The densification response of SLM-processed W-based parts decreased both at a low LF of 10.7 J/mm2, caused by the limited SLM working temperature and wetting characteristics of the melt, and at an excessively high LF of 64 J/mm2, caused by the significant melt instability and resultant balling effect and microcracks formation. The laser-induced complete melting/solidification mechanism contributed to the solid solution alloying of Ti and C in W matrix and the development of unique microstructures of SLM-processed W-based alloy parts. As the applied LF increased by lowering laser scan speed, the morphologies of W-based crystals in SLM-processed alloy parts experienced a successive change from the cellular crystal to the cellular dendritic crystal and, finally, to the equiaxed dendritic crystal, due to an elevated constitutional undercooling and a decreased thermal undercooling. The optimally prepared W-based alloy parts by SLM had a nearly full densification rate of 97.8% theoretical density (TD), a considerably high microhardness of 809.9 HV0.3, and a superior wear/tribological performance with a decreased coefficient of friction (COF) of 0.41 and a low wear rate of 5.73 × 10−7 m3/(N m), due to the combined effects of the sufficiently high densification and novel crystal microstructures of SLM-processed W-based alloy parts.

The fabrication of TiB2/AlSi10Mg composites by selective laser melting (SLM) additive manufacturing has been conducted. The influence of laser processing parameters on the densification behavior, microstructure evolution, and tensile properties of the SLM-processed composites is addressed. With the increase in laser volume energy density, the densification rate increased and reached 99% at a laser speed of 1000 mm/s. Meanwhile, the TiB2 particles underwent a partial melting behavior with the formation of an irregular pattern in the solidified part and complete melting with the smooth surface of the reinforcing particles as the laser volume energy increased. The cellular-dendritic microstructure and the width of the eutectic phase of the as-fabricated composites were significantly refined due to the high cooling rate and complete melting of the reinforcing particles as the nucleation sites using the laser energy density of 117 J/mm3. Therefore, the microhardness, ultimate tensile strength, yielding strength, and elongation of the as-fabricated TiB2/AlSi10Mg composites obtained in this process condition were 131.3 HV0.2, 375 MPa, 260 MPa, and 3.1%, respectively, which were significantly higher than those of the unreinforced AlSi10Mg alloy.

Selective Laser Melting Additive Manufacturing of TiC/AlSi10Mg Bulk-form Nanocomposites with Tailored Microstructures and Properties

ABSTRACTA nearly fully dense grade 300 maraging steel was fabricated by selective laser melting (SLM) additive manufacturing with optimum laser parameters. Different heat treatments were elaborately applied based on the detected phase transformation temperatures. Microstructures, precipitation characteristics, residual stress and properties of the as-fabricated and heat-treated SLM parts were systematically characterized and analyzed. The observed submicron grain size (0.31 μm on average) suggests an extremely high cooling rate up to 107 K/s. Massive needle-shaped nanoprecipitates Ni3X (X = Ti, Al, Mo) are clearly present in the martensitic matrix, which accounts for the age hardening. The interfacial relations between the precipitate and matrix are revealed by electron microscopy and illustrated in detail. Strengthening mechanism is explained by Orowan bowing mechanism and coherency strain hardening. Building orientation-based mechanical anisotropy, caused by ‘layer-wise effect’, is also investigated in as-fabricated and heat-treated specimens. The findings reveal that heat treatments not only induce strengthening, but also significantly relieve the residual stress and slightly eliminate the mechanical anisotropy. In addition, comprehensive performance in terms of Charpy impact test, tribological performance, as well as corrosion resistance of the as-fabricated and heat-treated parts are characterized and systematically investigated in comparison with traditionally produced maraging steels as guidance for industry applications.

Parametric analysis of thermal behavior during selective laser melting additive manufacturing of aluminum alloy powder

HighlightsOptimized L-PBF parameters enabled high-density fabrication of In625 with stable melt-pool morphology.Solution heat treatment promoted grain equiaxation and reduced lattice distortion in L-PBF In625.Porosity exhibited a non-monotonic evolution, decreasing at 1090 °C and increasing at 1150 °C.Heat treatment at 1150 °C provided the best overall balance among strength, ductility, and anisotropy reduction.The results clarify the microstructure–defect–property relationship in heat-treated L-PBF In625.What are the main findings?Optimized L-PBF parameters enabled the fabrication of high-density In625 components.Heat treatment temperature strongly affects porosity and grain evolution.Grain equiaxation occurs below 1090 °C in L-PBF-fabricated In625.What are the implications of the main findings?Heat treatment enables simultaneous optimization of strength and ductility.Defect evolution explains the reduction in anisotropy after heat treatment.The results guide engineering applications of L-PBF In625.Heat treatment is essential for In625 fabricated by laser powder bed fusion (L-PBF), as it significantly influences microstructural evolution, defect behavior, and mechanical performance. In this study, the effects of different solution heat treatments on L-PBF-fabricated In625 were systematically investigated. Industrial computed tomography was employed to characterize internal defects before and after heat treatment, while optical microscopy, EBSD, TEM, and EDS were used to analyze microstructural evolution. Room-temperature tensile tests evaluated mechanical properties. The results show that heat treatment at 1090 °C reduces porosity from 0.33% to 0.25%, whereas increasing the temperature to 1150 °C results in a further increase in porosity to 0.45%. This non-monotonic behavior is interpreted as the result of competing mechanisms, including partial closure of small pores at 1090 °C and pore coarsening/enlargement at higher temperatures, with the latter possibly involving the growth of sub-resolution pores into the CT-detectable range. Complete grain equiaxiality occurs after heat treatment at 1090 °C or higher, with average grain sizes below 100 μm, although grain coarsening becomes pronounced at higher temperatures. Samples heat-treated at 1150 °C exhibit reduced mechanical anisotropy, achieving tensile strength above 919 MPa and elongation up to 60%. These results clarify the mechanisms by which heat treatment governs microstructure–defect–property relationships in L-PBF In625, guiding its engineering application.

Microstructural evolution and mechanical properties of Ti55531–0.5Nb fabricated by selective laser melting under different heat treatments

Influence of heat treatment parameters on microstructure and mechanical performance of titanium alloy in LPBF: A brief review

Microstructure evolution and mechanical properties of selective laser melted bulk-form titanium matrix nanocomposites with minor B4C additions

The effect of hydrogen treatment on microstructures evolution and mechanical properties of titanium alloy fabricated by selective laser melting

Effect of heat treatment on microstructural heterogeneity and mechanical properties of 1%C-CoCrFeMnNi alloy fabricated by selective laser melting

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