Laser Powder Bed Fusion Technique Research Articles

3D printing of ductile equiatomic Fe-Co alloy for soft magnetic applications

The equiatomic Fe-Co alloy is an ideal soft magnetic material due to the combination of high saturation magnetization, permeability and low coercivity it offers. The ordered B2 phase in it, however, makes it highly brittle and hence unsuitable for industrial-scale manufacturing of bulk soft magnets. In this work, we employ the laser powder bed fusion (LPBF) technique—a widely used one for additive manufacturing of metallic parts—to fabricate bulk equiatomic Fe-Co blocks with a microstructure that almost-exclusively contains the disordered body centered cubic (BCC) phase. It plus the high dislocation density (an inherent attribute of the LPBF technique) impart considerable ductility to the as printed alloy, which will make it highly amenable for post-processing mechanical operations such as machining. Subsequent annealing heat treatment (HT) will facilitate disordered BCC to ordered B2 phase transformation and reduction in dislocation density and residual stresses, all of which will maximize the soft magnetic properties. This study demonstrates a viable method to manufacture equiatomic Fe-Co parts with desired geometry and magnetic properties.

Effects of notch-load interactions on the mechanical performance of 3D printed tool steel 18Ni300

The capability of additive manufacturing to produce parts with complicated geometrical features is one of the unique advantages of this technology over traditional production methods. However, the intricate interaction between concentrated stress fields imposed by various geometrical profiles, inherent defects, and external loads must be comprehensively recognized to leverage the aforementioned ability. Consequently, this study investigated the simultaneous effects of macroscopic notches and microscale defects on the performance of tool steel 18Ni300 under quasi-static and cyclic loading regimes. The specimens were manufactured using the laser-powder bed fusion technique, a common method for industrial applications that rely on metals. Eleven (horizontally built) sample designs, consisting of external or internal notches with different characteristics, were considered in this study to ensure a comprehensive comparison. The results showed that the surface quality was slightly higher along the notch roots than in the other areas. All the notches, regardless of their types, reduced ductility. Notch strengthening was generally observed in the notched samples under quasi-static loads. However, the degree of strengthening was directly related to the stress concentration factor, unlike for typical conventional metals. This unexpected behavior was attributed to the inherent defects in the manufacturing process. The surface defects at the notch roots played a dominant role in governing fatigue failures. A modification of the Murakami approach was proposed to estimate the fatigue life of notched components fabricated via additive manufacturing. Finally, the applicability of the Solberg–Berto diagram to predict the failure locations in horizontally manufactured samples was evaluated. The diagram largely agreed with the experimental data obtained from this study.

Microstructure, mechanical, and wear properties of thin-walled Ti6Al4V parts produced using laser powder bed fusion technique

The present work aims to study the variation of microstructure, microhardness and wear properties of the laser powder bed fusion (LPBF) processed Ti6Al4V thin-walled sample along the build direction. The results revealed that a harder α′ and Ti3Al phases in the Ti6Al4V’s microstructure. The lath thickness was thinner at the top and bottom than the middle region. The microhardness value variation was found to be around ∼ 5 % along the build direction. A linear relationship between microhardness and wear loss was observed. The presence of the Ti3Al phase resulted in inferior sliding wear of the thin-walled part along the build direction. However, thecoefficient of friction values was influenced by variable lath thickness along the build direction. The outcome of the present study is technically captivating and crucial for the design and development of advanced industry-specific thin-walled Ti6Al4V end-use parts.

Microstructural consistency in the additive manufactured metallic materials: A study on the laser powder bed fusion of AlSi10Mg

In the current study, the microstructural consistency in a laser powder bed fusion (L-PBF) AlSi10Mg material is investigated. A rectangular sample with a height of 40 mm is fabricated through the L-PBF technique. The main aim of the study is to analyze the effect of the repeated thermal cycles on the hierarchical characteristics of the microstructure in solid state. While the top of the sample represents the material experienced the minimum numbers of repeated thermal cycles, by moving towards the bottom of the sample more repeated thermal cycles experienced by the material. The microstructure of the material is investigated in terms of the grain and cell structures, and the precipitates characteristics. It is observed that the grain and cell structures are consistent throughout the height since they develop during the solidification process and are not affected by the repeated thermal cycles. On the other hand, these repeated thermal cycles affect the characteristics of the precipitates and the chemistry of the matrix. Although the precipitates’ characteristics change under the effect of the repeated thermal cycles, these changes do not affect the strength of the material significantly.

Investigation of laser powder bed fusion manufacturing and post-processing for surface quality of as-built 17-4PH stainless steel

Laser powder bed fusion (LPBF) technique enables to manufacture complex parts and difficult-to-cut materials with saving the material costs and manufacturing time. However, there still has been technical challenges and qualities issues for LPBF products to be used in engineering applications. In order to obtain the high-quality surface products including minimal defects and voids, this study systematically investigated and determined LPBF manufacturing process and post-processing using a 17-4PH stainless steel. It was experimentally found that design of LPBF process conditions could only attain the densification of as-built products, while surface qualities were not as good as commercial products. Therefore, the post-processing was indispensably explored by conducting mechanical polishing, physical vapor deposition coating, and hot isostatic pressing for the high surface quality and densification of as-built specimens. The surface qualities of the specimens, fabricated with optimum volumetric energy density of 97.66 J/mm3, were carefully characterized for their surface roughness, gloss, surface energy and frictional behaviors. After the post-processing, the surface roughness and gloss were dramatically improved by more than 800% over the as-built parts, and the porosity was reduced by up to 57.8%. This study demonstrates that optimum LPBF process and post-processing could enhance the qualities of LPBF products up to the same or beyond level of commercial products, comparing the defects, roughness, gloss, and touch sensation. The LPBF technique coupled with post-processing, if optimized, can show the great potential for the use in a wide variety of real-world engineering applications including mobile phones, jewelry, electronic appliances, artificial teeth and bones.

Texture evolution during high strain-rate compressive loading of maraging steels produced by laser powder bed fusion

Dynamic mechanical behavior and texture evolution in maraging steels, manufactured using the laser powder bed fusion (LPBF) technique, was investigated at strain rates of 2000 s −1 , 2300 s −1 , 3200 s −1 for the as-built samples, and 650 s −1 , 680 s −1 , 950 s −1 for the heat-treated samples. Split-Hopkinson Pressure Bar (SHPB) was used for high strain rate compression tests. The texture of the deformed samples was measured using the electron backscattering diffraction (EBSD) technique. A strong strain rate dependence on the crystallographic texture of as-built LPBF-maraging steel samples was observed. However, strain rate does not have any significant effect on texture evolution in the heat treated maraging steel samples. • Maraging steels rod samples were produced using laser powder bed fusion technique (LBBF) and subjected to heat treatment at 490 °C for 6 h. • High strain rate compressive behavior of LPBF-maraging steel samples in as-built and heat treat conditions were investigated by Split Hopkinson pressure bar apparatus. • Texture of deformed as-built and heat-treated samples were studied by electron backscattering diffraction technique (EBSD) where significant texture development in as-built samples was observed and no texture change for heat-treated samples was seen.

Compressive Behaviour of Additively Manufactured Periodical Re-Entrant Tetrakaidecahedral Lattices at Low and High Strain-Rates

Compressive deformation behaviour of additively manufactured lattice structures based on re-entrant tetrakaidecahedral unit-cell geometry were experimentally investigated under quasi-static and dynamic loading conditions. Specimens of four different structures formed by three-dimensional periodical assembly of selected unit-cells were produced by a laser powder bed fusion technique from a powdered austenitic stainless steel SS316L. Quasi-static compression as well as dynamic tests using split Hopkinson pressure bar (SHPB) apparatus at two strain-rates were conducted to evaluate the expected strain-rate sensitivity of the fundamental mechanical response of the structures. To evaluate the experiments, particularly the displacement fields of the deforming lattices, optical observation of the specimens using a high-resolution camera (quasi-static loading) and two synchronised high-speed cameras (SHPB experiments) was employed. An in-house digital image correlation algorithm was used in order to evaluate the anticipated auxetic nature of the investigated lattices. It was found that neither of the investigated structures exhibited auxetic behaviour although strain-rate sensitivity of the stress–strain characteristics was clearly identified for the majority of structures.

A mesoscopic digital twin that bridges length and time scales for control of additively manufactured metal microstructures

We present our recent development of an integrated mesoscale digital twin (DT) framework for relating processing conditions, microstructures, and mechanical responses of additively manufactured (AM) metals. In particular, focusing on the laser powder bed fusion technique, we describe how individual modeling and simulation capabilities are coupled to investigate and control AM microstructural features at multiple length and time scales. We review our prior case studies that demonstrate the integrated modeling schemes, in which high-fidelity melt pool dynamics simulations provide accurate local thermal profiles and histories to subsequent AM microstructure simulations. We also report our new mechanical response modeling results for predicted AM microstructures. In addition, we illustrate how our DT framework has been validated through modeling–experiment integration, as well as how it has been practically utilized to guide and analyze AM experiments. Finally, we share our perspectives on future directions of further development of the DT framework for more efficient, accurate predictions and wider ranges of applications.

Numerical Study on Effect of the Building Height and Material Properties on Thermal Distortion of Large Cylinders Manufactured through Laser Powder Bed Fusion

The laser powder bed fusion technique involves depositing layers by selectively melting a material. This leads to rapid melting and solidification, followed by repeated reheating and remelting. The complex thermal hysteresis and resultant thermal gradient inevitably cause thermal stress and distortion in additively manufactured products. The thermal stress and distortion lead to dimensional inaccuracies, defects such as delamination and cracking, and finally, adverse effects on the product life. In this study, a thermomechanical analysis of a large cylinder (hollow shape, with a height of 750 mm, an outer diameter of 500 mm, and a thickness of 20 mm) was performed using the finite element method to investigate the effects of building height and material properties on thermal distortion. Specifically, the evolution of thermal distortion was systematically interpreted with the thermal hysteresis. Depending on the building height of the cylinder, the thermal hysteresis was influenced by the bed plate and the thermal effect of the predeposited and postdeposited layers. In addition, the thermal distortion tendencies of Ti-6Al-4V and Inconel 718 metal powders were examined based on the material properties, particularly thermal diffusivity. The maximum thermal distortion occurred at approximately 100 mm below the top surface of the cylinder for both materials. Moreover, the thermal distortion of the Ti-6Al-4V was more significant than that of Inconel 718 because of the lower thermal diffusivity of the former. Key words: Powder bed fusion, Thermo-mechanical analysis, Thermal distortion, Thermal hysteresis, Material properties, Metal powders

Significance of grain refinement on micro-mechanical properties and structures of additively-manufactured CoCrFeNi high-entropy alloy

Grain refinement is an effective approach to improve mechanical properties of conventionally-manufactured high entropy alloys (HEAs). Additive manufacturing of HEAs is a new materials challenge and increasing reports are available for exploring the optimal processing parameters and post-manufacturing treatments to advance the physical and mechanical properties of additively-manufactured (AM) HEAs. At the current stage of the development of AM HEAs, it is necessary to investigate the significance of grain refinement on their mechanical properties and structures. In the present study, a CoCrFeNi HEA is manufactured by a laser powder-bed fusion technique using pre-alloyed HEA powders on which grain refinement was conducted by high-pressure torsion for up to 8 turns under 6 GPa at room temperature. The results from nanoindentation and Vickers microhardness testing demonstrate high strain hardening capability and increased plasticity, thus potentially high ductility, in the nanostructured AM CoCrFeNi HEA. X-ray diffraction analysis demonstrates the structural evolution with decreasing crystallite size, increasing microstrain and expanding lattice parameter with grain refinement in the HEA. The structural changes justify the estimation by nanoindentation of the rate-controlling mechanism of the grain boundary-mediated dislocation activity for the nanostructured AM HEA. This study provides advantages of nanostructuring for current developments in the AM technology of HEAs.

Effects of printing volumetric energy densities and post-processing treatments on the microstructural properties, phase transformation temperatures and hardness of near-equiatomic NiTinol parts fabricated by a laser powder bed fusion technique

Equiatomic and near-equiatomic NiTinol alloys, mainly due to the exhibition of adjustable mechanical and thermal shape memory properties, are widely exploited to produce different smart and functional devices. Laser powder bed fusion technique was used in this research to process dense NiTinol parts from a prealloyed near-equiatomic NiTi powder, using three different volumetric energy densities. In the next step, samples, processed using a specific set of printing parameters, were annealed using two different procedures. Effects of different used processing volumetric energy densities and post-processing heat treatments on the microstructural properties, phase composition, crystallographic information, phase transformation behavior and hardness of the processed samples were investigated. As-printed Parts with Ni contents ranging from 55.12 to 55.27 wt%, density higher than at least 96% of theoretical and a similar phase structure as that of used powder, dominated by austenitic NiTi, were obtained. Applying a hot isostatic processing treatment at 1100 °C for 4 h or annealing the samples in a pressureless sintering furnace, working under a vacuum environment at the same temperature and holding time, had significant effects on the properties of the printed parts. These treatments, in addition to increasing the density of the printed samples, their average grain size and reducing the residual stresses, shifted the phase transformations to lower temperatures. These results suggest that post-printing heat treatments affect and, as similar to NiTinol alloys having Ni contents above 55.7 wt%, can be used as a strategy to alter the phase transformation temperatures of printed less Ni-rich near-equiatomic NiTinol alloys and consequently obtain NiTinol parts suitable for different applications.

Experimental validation of laser powder bed fusion simulation

In many industrial sectors, laser powder bed fusion (L-PBF) is the main additive manufacturing technology for producing end-usable metal parts. Although L-PBF technique has been developed in the last twenty years, ensuring process feasibility and achieving maximum product quality at the first building session is still a difficult goal to pursue. Simulation software packages are available in the market for the prediction of induced stresses and deformation in L-PBF products to help the user getting the part right at the first time. In this paper, Amphyon software by Additive Works is tested and experimentally validated for the production of Ti6Al4V parts in an EOSINT M270 Dual Mode machine. First, the sensitivity of the software is evaluated by changing the main process parameters by +/- 20% with respect to Ti64 reference values. After calibration, the software is validated by comparison of the predicted deformed shape of a reference part with the real geometry using 3D scanning. Experimental results show that Amphyon software is able to predict the deformed shape for L-PBF parts correctly. The deviations from the real geometry depend on a simplified simulation model that considers a limited set of parameters for the L-PBF process.

On structure and mechanics of biomimetic meta-biomaterials fabricated via metal additive manufacturing

Additively manufactured (AM) metamaterials with unparallel mechanical and biological properties have been of interest for many biomedical, aerospace, and defense applications. In this study, new findings on the structure-property relationships of a specific class of AM meta-biomaterials based on minimal surfaces which have not been well understood previously (i.e., Primitive and Schoen-IWP (IWP)) are presented. Biocompatible Ti-6Al-4V meta-biomaterials based on Primitive and IWP topologies, with three different morphological properties for each, were designed and fabricated via laser powder bed fusion technique. The microstructure, morphological, quasi-static compressive, flexural properties, and fracture mechanisms of the designed meta-biomaterials are thoroughly evaluated. Studied meta-biomaterials exhibited distinct fracture mechanisms under flexural loading depending on their topology; crack growth path was either zigzag (i.e., deflected) or straight. Additionally, Primitive meta-biomaterials exhibited ductile material behavior, under both compressive and flexural loading conditions, with pore size, porosity, and elastic gradient similar to the trabecular bone (850–1020 μm, 40–64%, and 3200–6000 MPa). However, IWP meta-biomaterials showed brittle behavior with porosity and elastic gradient closer to cortical bone (20–42% and 7000–13,000 MPa). These outcomes indicate that both mechanical behavior and fracture mechanisms of meta-biomaterials can be controlled based on topology and morphology.

Concurrent improvement of strength and ductility in heat-treated C300 maraging steels produced by laser powder bed fusion technique

Abstract In this work, C300 maraging steel powder feedstocks at different titanium contents (0.72 and 1.17 wt%) were additively manufactured using the laser powder bed fusion (LPBF) technique to systematically study the mechanical behavior of the material in both as-built and heat-treated conditions. X-ray diffraction techniques along with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to investigate the microstructural characteristics and phase formation in the as-built and heat-treated samples. After heat-treating at 490 °C for 6 h, the HighTi maraging steel showed higher strength and ductility (2057.74 MPa and 4.05%). The microstructural characterization proved that this alloy contains a higher amount of reverted austenite (17.89 wt%), which resulted at higher strength as results of the transformation induced plasticity (TRIP) effect. In terms of ductility improvement, the fiber/copper texture developed in the HighTi sample provided a higher driving force to transform the reverted austenite to martensite. As the TEM analyses revealed, needle-shaped Ni3Ti and spherical Ni3Mo precipitates were found in both LowTi and HighTi alloys, where the HighTi one showed higher volume fraction of Ni3Ti precipitates, and consequently, higher values of tensile strength, which can be explained, based on the Orowan mechanism.

Laser Powder Bed Fusion of Potential Superalloys: A Review

The laser powder bed fusion (LPBF) is an additive manufacturing technology involving a gradual build-on of layers to form a complete component according to a computer-aided design. The LPBF process boasts of manufacturing value-added parts with higher accuracy and complex geometries for the transport, aviation, energy, and biomedical industries. TiAl-based alloys and high-entropy alloys (HEAs) are two materials envisaged as potential replacements of nickel-based superalloys for high temperature structural applications. The success of these materials hinge on optimization and implementation of tailored microstructures through controlled processing and appropriate alloy manipulations that can promote and stabilize new microstructures. Therefore, it is important to understand the LPBF technique, and its associated microstructure-mechanical property relationships. This paper discusses the metallurgical sintering processes of LPBF, the effects of process parameters on densification, microstructures, and mechanical properties of LPBFed TiAl-based alloys and HEAs. This paper also, presents updates and future studies recommendations on the LPBFed TiAl-based alloys and HEAs.

Role of powder particle size on laser powder bed fusion processability of AlSi10mg alloy

Abstract Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing (AM) methods which provides an exceptional opportunity to improve the existing designs and move toward fabricating fine features and complex geometries with higher efficiencies. Considering the layer-wise nature of this technique, the possibility of fabricating fine features is tied to the ability to deposit thin powder layers in this process. Since the powder layer thickness is directly dictated by the powder particle size, finer powders are required to further enhance the ability of the L-PBF technique in manufacturing fine features and intricate geometries. Accordingly, this study aims at investigating the processability of fine AlSi10Mg powder (D50 = 9 µm) by using the L-PBF process. The densification level, surface quality and dimensional accuracy of the final parts are investigated in a wide range of process parameters and are compared to those manufactured by the commonly used AlSi10Mg powder (referred to as coarse powder with D50 = 40 µm). The underlying reasons behind the different processability of fine and coarse powders are explored from the density, surface quality, microhardness and dimensional accuracy viewpoints through analyzing the flowability, bed packing density and optical absorption of powders. Moreover, the process-microstructure-microhardness relationship is assessed in detail for both fine and coarse powders. This study reinforces the idea that the utilization of fine powders in the range used in this study for L-PBF processing is rather challenging.

An investigation into the possibility to eliminate the microstructural defects of parts printed using a Ni-rich Ni-Ti elemental powder mixture

Ni-rich Ni-Ti alloys, such as 60NiTi, show a higher hardness and dimensional stability than equiatomic or near-equiatomic NiTinol ones. These make them suitable to be employed in structural applications. Laser powder bed fusion technique (LPBF) was used in this research to print parts from a Ni-rich Ni-Ti powder mixture consisting of about 63 wt.% Ni and 37 wt.% Ti. The microstructure of the obtained parts, processed using different parameters, showed inhomogeneity, contained undesirable Ni-rich and Ti-rich regions and a significant amount of cracks. To eliminate these defects, homogenize the microstructure and to obtain the required phases, we applied hot isostatic pressing (HIP) to the printed samples with a selected set of parameters. After HIP at 180 MPa and 1050 °C for 4 h, we observed the formation of a homogeneous microstructure containing mainly NiTi and Ni3Ti phases in the printed samples. However, cracks still persisted in the microstructure of these HIP treated samples. Applying another round of HIP treatment at a temperature just slightly above the melting start temperature of the samples, treated by the elementary HIP procedure, could successfully eliminate the cracks in the microstructure of samples and also increase the crystallinity of the existing phases.

Build position-based dimensional deviations of laser powder-bed fusion of stainless steel 316L

This study investigates the effect of position on build-plate on the dimensional deviations for stainless steel 316 L samples made by laser powder-bed fusion. To understand the effect of sample position on the build-plate (substrate) with respect to shrinkage and dimensional deviation, 36 samples in a 6 × 6 array were printed with three repetitions. The value of the diameter was measured at 10 points along the vertical axis in the perpendicular and parallel directions to the flow of the assisted gas. The results of the experiment show that there is shrinkage in both directions with respect to the gas flow. However, the extent of deviation in the perpendicular direction to the gas flow is greater compared to the parallel diameters for the samples. This can be related to the pressure of assisted gas and the difference in cooling rate corresponding to the position of the samples on the building substrate. The hypothesis is proved by conducting further experiments regulating the amount of gas flow by adjusting the individual nozzle for the gas flow to the build chamber. The reason for these deviations is speculated to be related to the rheology of the melt pool. This research could lay a solid foundation for the future development of a compensation strategy to nullify the effect of shrinkage and dimensional deviations on parts made using the laser powder-bed fusion technique. The results of shrinkage of the columns appear to suggest that there is an effect on the circularity from the assisting gas.

Mechanical Properties of AlSi12 Alloy Manufactured by Laser Powder Bed Fusion Technique

Mechanical properties of AlSi12 alloy, which was manufactured by laser powder bed fusion (LPBF) technique, were investigated. Some basic properties of fabricated objects such as density and tensile strength were clarified. The suitable laser irradiation condition of fabricated objects that had own high relative density was showed. The microstructure of fabricated objects with as-manufactured condition was also evaluated. The fabricated objects exhibited the similar ultimate tensile strength and the Young’s modulus according to the building directions, although other mechanical properties slightly differed. Mechanical properties of fabricated objects made by reused powders also exhibited the similar values of those manufactured by unused powders. In contrast, the mechanical properties excluding the Young’s modulus of the fabricated objects, which were annealed, differed due to annealing treatment. Furthermore, the mechanical properties excluding the Young’s modulus of the fabricated objects for the designed shapes manufactured to the tensile test specimen were smaller than those of the fabricated objects machined from LPBFed rectangular block. Therefore, the attention is necessary for the mechanical design of fabricated object through LPBF technique and that with annealing treatment, because of the difference in the mechanical properties between the object built as the designed shape and the object made from LPBFed bulk product.

Mechanical behavior of polymeric selective laser sintered ligament and sheet based lattices of triply periodic minimal surface architectures

Advances in additive manufacturing triggered a paradigm shift in the design of functional components allowing for complex topology-driven cellular lattices to be incorporated for the aim of reducing weight, enhancing multi-functionality, and facilitating manufacturability. In this paper, the compressive mechanical behavior of different polymeric lattices based on triply periodic minimal surfaces (TPMS) are investigated both experimentally and computationally. The behavior of two classes of TPMS lattices are investigated; sheet- and ligament-based lattices. Samples are fabricated using the laser powder bed fusion technique, selective laser sintering, and characterized using micro-Computed Tomography (micro-CT) and Scanning Electron Microscopy (SEM). A finite-deformation hyperelastic-viscoplastic-damage constitutive model is calibrated and employed to capture the full compressive behavior of lattices. The computational results are compared to and validated against corresponding experimental results. Results show that sheet-based polymeric TPMS lattices exhibit a stretching-dominated mode of deformation and prove to have superior stiffness and strength as compared to TPMS ligament-based lattices. The numerical simulations are in good agreement with experimental results for ligament-based lattices while significant deviation from experimental results is observed for the sheet-based lattices which is attributed to uncertainty in measuring the actual relative density and relatively higher manufacturing defects.