Laser Powder Bed Fusion Technique Research Articles

Nanometer-scale porosity evolution and mechanical properties of additively manufactured 17-4PH stainless steel

Understanding of the microstructure and mechanical properties of additively manufactured (AM) 17-4PH stainless steel has matured significantly, but combined wrought-level strength and ductility has yet to be achieved. Identifying the microstructural obstacle(s) impeding this milestone is critical for many future applications and industries interested in this AM alloy. In this work, microstructure, mechanical properties, and fracture behavior is evaluated for AM 17-4PH built via the laser powder bed fusion technique (LPBF) and heat treated via a broad range of techniques to achieve wrought-level 17-4PH strength. The highest strength/ductility was achieved with a two-stage hot isostatic press (HIP) and solution anneal (SA) treatment followed by an aging heat treatment, but wrought-level ductility could not be matched. Persistent nano-porosity (0.06–0.23 μm) was identified and correlated to fracture surface dimpling (R2 = 0.73–0.85) indicating a possible role in this ductility limitation. Contextualization of this study's mechanical properties dataset within the literature suggests that this observation is endemic to the current state-of-the-art processing techniques for AM 17-4PH.

Blistering and retention behavior of laser powder bed fused tungsten alloys under hydrogen plasma irradiation

In this study, pure tungsten (W) fabricated by powder metallurgy and laser powder bed fusion (LPBF) formed W-Ta and W-ZrC alloys were exposed to hydrogen plasma. Severe blistering was found in LPBFed W-Ta specimens due to a strong [111] crystallographic texture and the large number of particle-matrix interfaces in the LPBFed W-ZrC inhibits the blister growth. The retention of LPBFed W alloys is lower than that of pure W due to the presence of large columnar grains and cracks. The results highlight the capability of tailoring the microstructures of W alloys by the LPBF technique to achieve improved plasma-compatible performance.

Modelling and characterization of novel honeycomb structures with mass gradient produced by additive manufacturing

The dissemination of additive manufacturing methods has facilitated the design and production of complex structures which have a high strength-to-weight ratio. Cellular materials such as honeycombs have low-weight and high capacity to absorb energy which makes them desirable for the aerospace and automotive industries. The present work covers the study and comparison of metal-based regular honeycombs and functionally graded honeycombs. The latter encompass radial and linear/longitudinal gradients. Three repeating unit cells were studied: regular hexagons, Plateau and lotus. The structures were produced in aluminium using the laser powder bed fusion technique. Selected samples were submitted to a stress-relieving heat treatment. Numerical and experimental methods were used to assess the in-plane compressive properties. Finite element analysis was used to obtain the simulated force–displacement curves of each structure, allowing for the calculation of specific stiffness, absorbed energy and yield strength. The experimental method consisted of the compression of three specimens of three types of regular structures with and without stress-relieving heat treatment. The heat treatment reduced the yield strength and stiffness whilst increasing the ductility of the samples. The mechanical behaviour of the structures was found to depend upon a combined effect of the type of gradient, relative density, and unit cell structure. The results showed that an increase in the relative density would enhance the specific mechanical properties. The lotus configuration displayed the highest specific mechanical properties, as its geometry reduces the stress concentrations. The numerical results showed a reasonable match with the experimental results.

Strengthening aluminum matrix composite with additively manufactured 316L stainless steel lattice reinforcement: Processing methodology, mechanical performance and deformation mechanism

This study investigates various processing approaches, interface characteristics, and mechanical properties of an aluminum (Al) matrix composite reinforced with additively manufactured (AM) 316L stainless-steel (SS) lattice. The AM-316L-SS lattice, boasting a 30 % infill density, was fabricated using the laser powder bed fusion technique. The Al-matrix was integrated with the AM-316L-SS reinforcement through experimentation involving pre-stirring of the molten Al under pressurized and unpressurized die-casting conditions. A well-bonded interface, with cohesive regions, was obtained in the pre-stirred structures. In contrast, regions of decohesion were displayed in cast structures without pre-stirring, leading to the presence of pores and the manifestation of imperfect bond interfaces. Compression tests conducted on the sound composite demonstrated enhanced mechanical properties, with a compressive strength of approximately 163 MPa, significantly higher than that of the pure Al-matrix (approximately 80 MPa). Localized interface cracking was initiated by stress gradients and plastic deformation, leading to microcrack propagation into the Al-matrix.

Tradeoff of structural layouts of a compact heat exchanger additively manufactured for space exploration

The innovative thermal management system named Two-Phase Mechanically Pumped Loop (2PMPL) has been investigated to design probes for Venus exploration. This study aims integrating the 2PMPL evaporator inside the probe shell, using the Laser Powder Bed Fusion technique, to reduce the thermal system size and to increase the payload. In this paper, four evaporator designs have been developed and compared to the latest available reference solution. The overall assessment of those designs has been performed by comparing the results of basic Computational Fluid Dynamics analysis, structural behavior, mass and Three-Phase Contact Line (TPCL) length, highlighting specific strengths of each solution.

Influence of laser printing mode on thermal behaviors, forming characteristics, and microstructure evolutions of additive manufactured Ti6Al4V alloy

Ti6Al4V parts fabricated by laser powder bed fusion (LPBF) technology have been widely used in such fields as aerospace, automotive and medical implants. This study investigates the effect of laser scanning modes on thermal behaviors, forming characteristics, and microstructural evolutions of LPBF-fabricated Ti6Al4V parts. The numerical simulations on the temperature field provide a theoretical explanation of various surface morphologies, surface roughness values and relative densities of samples. The rotation between adjacent layers diminishes the large directional thermal stress generated by X scanning or Y scanning, the Ti6Al4V samples using XY scanning and Island scanning present smooth surface and higher relative density (> 99.0%). The observed staggered martensite within the columnar β grains is due to the epitaxial solidification across the deposited layers with 90° or 37° rotation. The martensite growth of LPBF-processed Ti6Al4V components using X scanning has a similar inclination along the building direction and presents anisotropic characteristics. These findings provide new inspirations for achieving high-performance titanium alloy components with specific microstructure by LPBF technique using a proper laser scanning mode.

Preparation of ultrahigh-strength and ductile nano-lamellar eutectic high-entropy alloy via laser powder bed fusion

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) has become one of the hottest research topics owing to its comprehensive mechanical properties. In this study, the AlCoCrFeNi2.1 EHEA was fabricated via the laser powder bed fusion (L-PBF) technique, and the grain morphology, eutectic structure, mechanical properties, and deformation mechanism were investigated. The results indicate that the samples have superfine grains and nano-lamellar eutectic structures, attributable to the high cooling rate of the L-PBF process. Furthermore, the phenomenon of solute trapping was observed in the boundary of the FCC and B2 semi-cohesive phase in the nano-lamellar eutectic structure. Based on the special eutectic structure of the AlCoCrFeNi2.1 EHEA prepared using the L-PBF technique, the samples exhibited an exceptional tensile strength of up to 1.6 GPa, accompanied by tensile ductility of over 9.5 %. The improvement of mechanical properties may be attributable to the Hall–Petch strengthening and back stress effect caused by refining the eutectic structure, whereas the growth of the nano-lamellar structure may be closely related to the solute-trapping effect.

Additive manufacturing of Mn-Al permanent magnets via laser powder bed fusion

Permanent magnet materials based on Mn demonstrate high potential to fill the gap between lower-performance bonded ferrites and high-performance magnets based on rare-earth elements. Mn-Al permanent magnets were processed into bulk via the laser powder bed fusion (LPBF) technique. Gas-atomized Mn54Al46 precursor particulates were fused into cube-shaped magnets of the high-temperature ε phase with ∼93 % bulk density. A 450 °C anneal for 30 min transformed the ε phase into largely the ferromagnetic τ phase. The magnetic properties were compared to an as-cast reference bulk Mn54Al46 sample prepared from ε phase via the same heat treatment. The LPBF sample exhibited a finer grain size, a ∼45 % gain in coercivity, and higher remanence due to grain elongation and a weak <001>τ texture parallel to the printing direction. Despite the loss of density, the LPBF sample had a ∼70 % improvement in (BH)max compared to the as-cast bulk reference sample. The success of this process demonstrates that the LPBF technique is a viable method to densify the metastable Mn-Al τ phase from particulate into bulk while retaining a fine grain size, developing magnetic anisotropy, and preventing phase decomposition. Optimal properties of the LPBF magnet were Mr = 41 Am2/kg, Hci = 129 kA/m, and (BH)max = 7.2 kJ/m3 along the print direction.

Tribological Behaviour of Low Temperature Plasma Assisted Carburized 316 L Steel Produced by L-PBF Technique

Austenitic stainless steels produced by Laser Powder Bed Fusion (L-PBF) are interesting materials because of their excellent corrosion resistance. Due to their relatively low hardness, the tribological response of these materials is poor, which limits their use in applications where control of wear degradation is important. Nevertheless, low-temperature plasma-assisted carburisation is an interesting process for improving the wear resistance of austenitic stainless steels, as has been observed for wrought materials. In fact, the increase in hardness is guaranteed by a surface layer of expanded austenite (S-phase) with a thin top layer of amorphous carbon. In this work, AISI 316 L, produced by the L-PBF technique, was carburised using 5 different plasma gas mixtures (by varying the CH4/H2 ratio) at 475°C for 7 hours. The samples obtained were then subjected to a detailed microstructural characterisation in order to obtain information on surface modification. The morphological features of the surface were examined by SEM observations in top view and in cross-section. The tribological performance was evaluated by pin-on-flat tests (alumina sphere as counter-material) with 2 different applied loads and a stroke length of 5 mm. Friction coefficient, wear rate (stylus profilometer) and wear mechanisms (SEM) were also evaluated. Preliminary results show an increase in wear resistance of all plasma treated materials compared to the untreated material. The improved tribological performance was discussed in relation to the abrupt increase in surface hardness.

Analysis of physical, chemical, mechanical, and microbiological properties of Ti–35Nb–7Zr–5Ta and Ti–6Al–4V discs obtained by machining and additive manufacturing

The objective was to compare the physical, chemical, mechanical, and microbiological properties between discs (n = 10) of Ti–6Al–4V and Ti–35Nb–7Zr–5Ta (TNZT) obtained by Machining (M) and additive manufacturing (AM) by Laser Powder Bed Fusion technique to identify the influence of chemical composition and processing technique on material properties. Using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), wettability, surface free energy, roughness by confocal laser microscopy and atomic force microscopy (AFM), Vickers microhardness (VM), and colony forming units (CFU) against S. aureus. Two-way ANOVA (p < 0.05) was applied. Higher roughness and irregularity were observed in the AM discs. The chemical composition of the EDX alloys was compatible with the concentrations expected and available in the literature. For TNZT the manufacturing technique interfered in the present phases (α and β) due to different cooling rates. The wettability and surface free energy of TNZT was higher than that of Ti–6Al–4V, and there was no significant difference for the manufacturing techniques. Ti–6Al–4V showed greater hardness than TNZT and the M technique greater than AM. There was no difference in S. aureus CFU between the groups. It was concluded that TNZT alloy showed higher hydrophilicity, surface free energy, roughness, lower hardness, manufacturing techniques that interfered in its phases, and no differences for CFU compared to Ti–6Al–4V. The AM technique generated more irregular and rough surfaces, and lower hardness, without significant changes concerning M in terms of chemical composition, wettability, surface free energy, and bacterial formation.

Rate controlling deformation mechanisms in SS316L stainless steel manufactured using laser powder bed fusion technique

We conducted uniaxial tensile strain rate jump tests to account for the strain rate controlling mechanisms in the additively manufactured SS316L stainless steel. Special emphasis is placed on the role of high dislocation density and chemical segregation on dictating the rate sensitive behavior. Lowest strain rate sensitivity is found, despite the highest dislocation density in the as build state, which signified the role of solute segregation at cell walls in increasing the activation volume and forms the strong basis for rejecting dislocation forest as the strain rate controlling obstacles. Consistent increase in the strain rate sensitivity with annealing is found consistent with the chemical homogenisation of the solutes causing smaller inter-obstacle spacing causing decreasing activation volume. Experimental findings are supported by CALPHAD simulations and mechanical threshold stress (MTS) modelling framework. Significant difference between the instantaneous and steady state rate sensitivity is understood using the dynamic pileup dislocation density readjustment mechanism causing considerable flow transient while adjusting the mobile dislocation density on strain rate change in the presence of solute decorated dislocation cells. Present investigation helps in gaining newer insights into the deformation mechanisms of the additively manufactured alloys featuring cellular microstructures.

Thermal-structural hybrid Lagrangian solver and numerical simulation-based correction of shape deformation of stainless-steel parts produced by laser powder bed fusion

An efficient thermal-structural numerical solver for Additive Manufacturing has been developed based on a modified Lagrangian approach to solve the energy conservation equations in differential form. The heat transfer is modeled using the finite difference method applied to a deforming Lagrangian mesh. The structural solver has been enhanced with the proposed effective quasi-elastic differential approach for modeling the elastoplastic behavior of materials. The algorithm is relatively simple to implement yet is highly effective. The solver can predict shape deformations of metal parts printed using the laser powder bed fusion technique. The second key capability of the solver is the auto-compensation of distortions of 3D-printed parts by proposing a corrected geometry of a surface to be printed, in order to ensure minimal deviation of the actual printed part from the desired one, even under non-optimal operating conditions or for complex shapes. All the simulation results have been verified in real-life experiments for 3D parts of sizes ranging from 10 to 15 mm up to 40 mm.

Enhanced structural integrity of Laser Powder Bed Fusion based AlSi10Mg parts by attaining defect free melt pool formations

This research aims to fabricate an AlSi10Mg parts using Laser Powder Bed Fusion technique with enhanced structural integrity. The prime novelty of this research work is eliminating the balling and sparring effects, keyhole and cavity formation by attaining effective melt pool formation. Modelling of the Laser Powder Bed Fusion process parameters such as Laser power, scanning speed, layer thickness and hatch spacing is carried out through Complex Proportional Assessment technique to optimize the parts' surface attributes and to overcome the defects based on the output responses such as surface roughness on scanning and building side, hardness and porosity. The laser power of 350 W, layer thickness of 30 µm, scan speed of 1133 mm/s, and hatch spacing of 0.1 mm produces significantly desirable results to achieve maximum hardness and minimum surface roughness and holding the porosity of < 1%. The obtained optimal setting from this research improves the structural integrity of the printed AlSi10Mg parts.

Advancements in Laser Powder Bed Fusion of Carbon Nanotubes-Reinforced AlSi10Mg Alloy: A Comprehensive Analysis of Microstructure Evolution, Properties, and Future Prospects

Laser powder bed fusion (L-PBF) stands out as a promising approach within the realm of additive manufacturing, particularly for the synthesis of CNT-AlSi10Mg nanocomposites. This review delves into a thorough exploration of the transformation in microstructure, the impact of processing variables, and the physico-mechanical characteristics of CNT-AlSi10Mg nanocomposites crafted via the L-PBF technique. Moreover, it consolidates a substantial corpus of recent research, proffering invaluable insights into optimizing L-PBF parameters to attain the desired microstructures and enhanced properties. The review centers its attention on pivotal facets, including the dispersion and distribution of CNTs, the formation of porosity, and their subsequent influence on wear resistance, electrical and thermal conductivity, tensile strength, thermal expansion, and hardness. In line with a logical progression, this review paper endeavors to illuminate the chemical composition, traits, and phase configuration of AlSi10Mg-based parts fabricated via L-PBF, juxtaposing them with their conventionally manufactured counterparts. Emphasis has been placed on elucidating the connection between the microstructural evolution of these nanocomposites and the resultant physico-mechanical properties. Quantitative data culled from the literature indicate that L-PBF-produced parts exhibit a microhardness of 151 HV, a relative density of 99.7%, an ultimate tensile strength of 70×103 mm3N.m, and a tensile strength of 756 MPa.

Surface Texture and Microstructural Characterization of Thin-Walled Ti6Al4V Part Processed Using Laser Powder Bed Fusion Technique: Effect of Build Direction

Abstract Objective: The current research investigates the surface texture and microstructural characterization of thin-walled Ti6Al4V along the build direction processed using laser powder bed fusion (LPBF) technology using an intra-comparison approach. Methodology: The two-dimensional and three-dimensional surface morphology and multi-scale surface roughness analysis of all Ti6Al4V samples were performed using an opto-digital microscope (with extended focus imaging coupled with high dynamic range imaging). Moreover, the scanning electron microscope, microhardness tester, and X-ray diffraction techniques were used to analyze the microstructural and microhardness values. Findings: (1) The lath thickness was relatively thicker in the LPBF-processed Ti6Al4V sample’s microstructure at central locations than in the top and bottom locations. (2) The areal surface roughness (Sa), Rk, and Sk values were relatively lower for the middle region than for the bottom and top regions of the thin-walled part, implying nonuniform surface topography along the build direction. (3) The middle region had a higher surface texture and texture amplitude symmetry periodicity than the top and bottom regions along the build direction. Value: Overall, the established methodology employed on the thin-walled Ti6Al4V part processed using LPBF technology enables the selection criteria of a suitable surface finishing process to achieve isotropic finish for practical industrial applications.

Laser powder bed fusion of full martensite Cu-Al-Mn-Ti alloy with good superelasticity and shape memory effect

A copper-based shape memory alloy Cu-11.85Al-3.2Mn-0.1Ti with desirable superelasticity and shape memory effect was printed by laser powder bed fusion (LPBF), which demonstrated the feasibility of engineering applications through the deformation and recovery of complex corrugated structural components. The microstructure of LPBF-printed Cu-11.85Al-3.2Mn-0.1Ti was composed of full β1' martensite with a high density of stacking faults and twin crystals. The abnormal phase constitution of full martensite in the printed Cu-11.85Al-3.2Mn-0.1Ti is correlated to substrate preheating and thermal cycling during LPBF. The start and finish transformation temperatures of martensite (Ms and Mf) and austenite (As and Af) were 172.5 °C, 144.4 °C, 180.2 °C and 209.2 °C, respectively. The printed samples exhibited compressive strength of 1319 MPa with a compressive fracture strain of 16.1%. The printed alloy displayed remarkable superelasticity and shape memory effect. The superelastic recovery rate reached a maximum value of 87% at 6% pre-strain, corresponding to a superelastic recovery strain of 5.22%. The mechanism of superelasticity is attributed to the rubber-like deformation behavior. Moreover, the pre-strained samples demonstrated a desirable shape memory effect under the temperature stimulation, where the shape memory recovery rate of up to 100% at a pre-strain of 6%, corresponding to a shape memory recovery strain of 0.78%. The reasons for the favorable shape memory effect were: (i) the thermoelastic inversion of martensitic transformation and (ii) the precipitation of γ2(Cu9Al4) phase along the grain boundaries. As a result, these findings suggest that the LPBF technique can produce Cu-Al-Mn-Ti parts with superelasticity and good shape memory properties, which provided valuable references for printed copper-based shape memory alloys with full martensitic structures.

Effects of shell scanning and build orientation on dynamic properties of laser powder bed fusion AlSi10Mg alloy

Additively manufactured (AM) products use two scanning strategies to create the central core region and the inner region near the free-surface circumference i.e. the shell. Manufacturers usually machine the shell to meet standard mechanical property testing. However, it's essential to determine how the shell affects an AM product's mechanical properties.This study focused on the dynamic tension and compression mechanical properties of an AlSi10Mg alloy made using the laser powder bed fusion technique. Machined and non-machined samples with and without shells of different build sizes and orientations underwent testing using split Hopkinson bar systems.Machined products displayed anisotropic dynamic behavior, favoring vertically built samples for maximum dynamic stress and horizontally built samples for elongation. However, samples with a shell showed apparent isotropic dynamic behavior due to pore distribution in the shell regime. The high porosity in the shell region resulted from the high intensity of heating energy input during scanning. Differences in shell shape between Z (load parallel to the direction of the building) and XY (load tangential to the direction of the building) samples and a slower, repeated laser scanning process in the shell region contributed to differences in porosity distribution between orientations. High porosity negatively impacted mechanical behavior and reduced dynamic mechanical properties.

Design of a low Young’s modulus Ti-Zr-Nb-Sn biocompatible alloy by in situ laser powder bed fusion additive manufacturing process

A new metastable β Ti-22Zr-11 Nb-2Sn (at%) biomedical titanium alloy was elaborated from elemental powders by in situ laser powder bed fusion technique (L-PBF). Iterative design of experiments was used to identify optimized manufacturing parameters to obtain dense and homogeneous parts (>99.3%) with low unmolten niobium fraction (<0.05%). The microstructural and mechanical properties of this alloy were investigated in its as-fabricated state and after two heat-treatments at 400 °C and 700 °C, respectively. The alloy, showing a β-type microstructure, possesses a very low Young’s modulus (∼50 GPa) combined with a high tensile strength reaching about 1100 MPa after heat-treatment. These properties are very suitable for medical devices in osseous site such as hip prostheses or dental implants and the results were compared to the Ti-6Al-4 V ELI (wt%) medical grade fabricated from pre-alloyed powders. Thus, the present work demonstrates the significant potential of the in situ L-PBF technique to elaborate highly biocompatible titanium alloys with tailored chemical compositions.

Assessment of Additive Manufactured IN 625's Tensile Strength Based on Nonstandard Specimens.

The study aimed to evaluate the tensile strength of additively manufactured (AMed) IN 625 using sub-sized test pieces and compare them to standard specimens. Cylindrical round coupons of varying diameters were manufactured along the Z-axis using the laser powder bed fusion technique and subjected to heat treatment. The simulation of the alloy solidification predicted the formation of several intermetallics and carbides under equilibrium conditions (slow cooling), apart from the γ phase (FCC). Sub-sized tensile specimens with different gauge diameters were machined from the coupons and tensile tested at ambient temperature. The results showed that sub-sized specimens exhibited lower tensile and yield strengths compared to standard specimens, but still higher than the minimum requirements of the relevant ASTM standard for AMed IN 625. The lower strength was attributed to the "size effect" of the test specimens. Fracture surfaces of the sub-sized test specimens exhibit a mixed character, combining cleavage and microvoid coalescence, with improved ductility compared to standard test pieces. The study highlights the importance of adapting characterization methods to the particularities of manufactured parts, including reduced thicknesses that make sampling standard-size specimens impractical. It concludes that sub-sized specimens are valuable for quality control and verifying compliance with requirements of AMed IN 625 tensile properties.

A new grain refinement route for duplex stainless steels: Micro-duplex stainless steel matrix composites processed by laser powder bed fusion

The employment of grain refinement approaches is regarded as a favorable strategy to increase mechanical properties of duplex stainless steels (DSSs). In this work, a new grain refinement route was successfully developed via laser powder bed fusion (LPBF) of micro-duplex stainless steel (SS) matrix composites along with a subsequent quenching process. For as-built composites, TiCxNy nanoparticles were first formed in-situ via the introduction of submicron/micron-sized TiC particles with low cost into 2205 DSSs, and fine equiaxed high-temperature ferrite (δ-ferrite) grains were formed due to the ultra-high cooling rate of LPBF and heterogeneous nucleation sites provided by in-situ formed TiCxNy nanoparticles. The formation of TiCxNy nanoparticles inhibited the epitaxial growth of δ-ferrite grains, and no obvious textures were found in as-built composites. Subsequently, fine austenite grains were produced at grain boundaries of fine unrecrystallized δ-ferrite during quenching treatment of as-built composites, which enabled heat-treated composites to achieve an ensemble of outstanding ultimate tensile strength and uniform elongation. The δ-ferrite recrystallization was mainly inhibited by the existence of TiCxNy nanoparticles. This study can provide a groundwork for designing grain refinement methods to produce high-performance DSS parts with complex geometry based on LPBF technique.