Selective Laser Melting Additive Manufacturing Research Articles

Experimental investigation on selective laser melting additive manufacturing of non-ferromagnetic heterogeneous material based on TiNi alloy with Ta coating

In order to manufacture non-ferromagnetic heterogeneous material by Selective Laser Melting (SLM), it is necessary to solve the separation problem of non-ferromagnetic mixed powders after SLM forming. This research presents how to resolve this problem using TiNi/Ta mixed powders by ultrasonic vibration screening method and the analysis of SLM manufacturing experiment for non-ferromagnetic heterogeneous material. The purity of TiNi and Ta powders after separation could reach 99.7087%wt% and 98.8501wt% respectively, which both reached a relatively high purity. A TiNi alloy sample with Ta coating and TiNi/Ta gradient transition was manufactured successfully by SLM. The material interface of the sample achieves metallurgical bonding, and the color of the sample profile shows a gradient transition. The EDS analysis shows that the material composition changes from Ta ->Ta/TiNi gradient ->TiNi from surface to inside of the sample. The Ta coating contains over 92.5wt% Ta, and the TiNi matrix contains over 98.69wt% TiNi. Along the powder laying direction, it is difficult to clean the small powder near the solid-powder or solid-solid interfaces made of two different materials, which causes micro polluted areas near the interfaces. This study also provides a new method for integrated manufacturing of TiNi alloy part with Ta coating.

Vibration characteristics of additive manufactured IWP-type TPMS lattice structures

Several research has been done on the conventional strut-based lattice structures, which have the problem of stress concentration in the application. In recent years, a new lattice structure, the triply periodic minimal surface (TPMS) lattice structure is increasingly gaining attention, but its vibrational properties have been less studied. In this study, the IWP-type TPMS lattice structures and ordinary body-centered cubic (BCC) counterparts with similar topology were prepared by selective laser melting (SLM) additive manufacturing (AM) technique. The compression behavior of the structures and the energy absorption capacity were determined by uniaxial compression tests. The frequency response and the damping ratio of the structure were calculated by dynamic vibration transfer rate tests. The results show that the stiffness and inherent frequency of the lattice structure are proportional to the volume fraction and inversely proportional to the cell size. Decreasing the volume fraction and increasing the cell size can be more beneficial to achieve low-frequency vibration isolation. Moreover, the IWP-type triply periodic minimal surface lattice structures have better mechanical properties than BCC structures and have good vibration isolation properties. Insights from this paper provide a reference for improving the load-carrying and vibration isolation performance of lightweight lattice structures.

A multi-physics material point method for thermo-fluid-solid coupling problems in metal additive manufacturing processes

The metal additive manufacturing (AM) process involves complex multi-physics coupling phenomena associated with the heat transfer, fluid flow, and solid mechanics that bring challenge to the current numerical methods. In this study, a multi-physics material point method is proposed for solving thermo-fluid–solid coupling problems in metal additive manufacturing processes. In this method, the material domain is discretized by a structured grid and a set of particles with corresponding variants for the heat transfer, fluid flow, and thermal stress evolution, under both Eulerian and Lagrangian descriptions. The interaction between these fields is naturally handled by the same structured grid and particles. Moreover, a semi-implicit local iteration technique is proposed to efficiently solve heat transfer with solid–liquid phase change, an improved Chorin’s projection method is introduced to handle Darcy’s damping, and a staggered derivation scheme with a sub-cell occupation technique is proposed to solve surface tension and Marangoni forces. A set of numerical examples, including the benchmark problems and the selective laser melting AM problems, is presented to validate the proposed method, where good agreements have been achieved with the analytical, numerical, and experimental data available in the literature. It is demonstrated that the proposed method is a powerful tool for thermo-fluid–solid coupling problems in metal additive manufacturing and other multi-physics problems.

Bio-inspired, metal additive manufacturing interlocked structures: Geometrically design and fracture performance analysis

Suture-inspired interlocked interfaces in nature exhibit promising industrial applications potentials in advanced mechanical structures, devices and equipment. In this paper, the parametric design of interlocked interfaces is performed, and selective laser melting (SLM) additive manufacturing tensile specimens and compact tension (CT) fracture specimens with sutured interfaces are harvested, where AlSi10Mg raw materials are used as constituent materials. Afterwards, compact tension tensile fracture experiments and corresponding finite element simulations were performed and compared, where force-displacement curves for different interface structural geometrical parameters were harvested. The mechanical properties and failure modes of the interlocked interface structures were analyzed through experiments and simulations verification. Crack-tip opening displacement (CTOD) and fracture energy criteria were used to evaluate the fracture toughness of the interlocked specimen interfaces. In addition, parametric analysis was carried out to investigate the influence of different interlocked structural parameters on the interface mechanical performances and failure features. Accordingly, it can be concluded that the interface strength, toughness, and failure mechanisms can be rationally tuned through given geometrical parameters of the re-entrant interface microstructures.

PRODUCTION OF OXIDE DISPERSION STRENGTHENED INCONEL 718 ALLOYS USING CONVENTIONAL POWDER METALLURGY AND ADDITIVE MANUFACTURING METHODS

Oxide dispersion strengthened (ODS) Ni-based alloys having a high density of nano-oxides (NOs) (<10 nm) are considered to be good candidates for extreme environments, such as high temperature, radiation, and corrosion. In this study, ODS IN718 alloys have been produced using conventional powder metallurgy (PM) and novel selective laser melting (SLM) additive manufacturing. The effect of processing routes on the microstructure, in particular on the nano-oxide formation and structure has been investigated. It has been found that the powder metallurgy method that consists of compressing followed by sintering at 1250 and 1500 °C results in a nano-granular structure with homogenously distributed fine nano-oxides having a high number density. Similarly, SLM results in a high number density of fine nano-oxides; however, the particles exist in groups with the grains/cells. The nano-oxides are determined to be Y2Ti2O7, Y2TiO5 or YTiO3 and Y-Al-O. The deviation in the lattice parameters of Y2Ti2O7 infers the existence of some Al in the structure. This study sheds light on producing ODS IN718 alloys with high-density nano-oxides using powder metallurgy and additive manufacturing methods.

Selective Laser Melting Additive Manufactured Tantalum: Effect of Microstructure and Impurities on the Strengthening-Toughing Mechanism.

The balance between the strength and the toughness of pure tantalum (Ta) fabricated with selective laser melting (SLM) additive manufacturing is a major challenge due to the defect generation and affinity for oxygen and nitrogen. This study investigated the effects of energy density and post-vacuum annealing on the relative density and microstructure of SLMed tantalum. The influences of microstructure and impurities on strength and toughness were mainly analyzed. The results indicated that the toughness of SLMed tantalum significantly increased due to a reduction in pore defects and oxygen-nitrogen impurities, with energy density decreasing from 342 J/mm3 to 190 J/mm3. The oxygen impurities mainly stemmed from the gas inclusions of tantalum powders, while nitrogen impurities were mainly from the chemical reaction between the molten liquid tantalum and nitrogen in the atmosphere. The proportion of <110> texture decreased after vacuum-annealing at 1200 °C, while that of the <100> texture increased. Concurrently, the density of dislocations and small-angle grain boundaries significantly decreased while the resistance of the deformation dislocation slip was significantly reduced, enhancing the fractured elongation up to 28% at the expense of 14% tensile strength.

Tensile behavior of large size Zr−based bulk metallic glass composites prepared by selective laser melting

A Zr55Cu30Al10Ni5 (Zr55) bulk metallic glass composite (BMGC) with a size of 37 × 37 × 10 mm3 was fabricated by using selective laser melting (SLM) additive manufacturing technology with argon gas atomized (GAed) powder. The microstructure and tensile deformation behavior of SLMed Zr55 BMGCs are investigated. The microstructure evolution perpendicular to the laser scan direction exhibits a periodic microstructure with three different zones (amorphous, nanocrystal + amorphous and dendritic eutectic + nanocrystal zones). The stepwise tensile fracture behavior, mixing ductility with a sharp stress drop after the failure of amorphous layers is shown for the Zr55 BMGC. The nanocrystal + amorphous crystalline zone has a lower hardness and elastic modulus than the amorphous zone via the nanoindentation test, therefore, the stress drops obviously when the hard amorphous layers fail and then stress recovery follows as the soft crystalline layers pick up the load, resulting in a final failure strain of 9%. The results further verified the feasibility of SLM technology for the development of high−performance BMGCs for industrial applications.

Scanning strategy optimization for the selective laser melting additive manufacturing of Ti6Al4V

Minimizing the residual stress and deformation of the manufactured product consequent to the selective laser melting (SLM) process is always a primary focus in the realm of additive manufacturing. This paper studied the influence of different scanning strategies on the thermal-mecha nical characteristics of SLM of Ti6Al4V. Numerical simulation and experimental tests were employed concerning three scanning strategies of the SLM process: Z-shaped, outer spiral, and inner spiral. Allowing for the latent heat of phase change, thermal radiation, and convection effect, the temperature history, and distribution of the specimen were analyzed, based on which the indirect thermal-mechanical coupling method was engaged for the residual stress and deformation analysis. The result shows good consistency in deformation characteristics between the numerical prediction and experimental observation. A reasonable scanning strategy is beneficial to mitigating the non-uniform distribution of the residual stress, and compared with the outer spiral and inner spiral strategies, the z-shaped scanning strategy may have a better effect in forming accuracy and mechanical properties.

Microstructures and Hardening Mechanisms of a 316L Stainless Steel/Inconel 718 Interface Additively Manufactured by Multi-Material Selective Laser Melting

For the first time, the interfacial microstructures and hardening mechanisms of a multi-material (MM) 316L stainless steel/Inconel 718 (316L SS/IN 718) interface fabricated by a novel multi-material selective laser melting (MM SLM) additive manufacturing (AM) system have been investigated in this study. MM 316L SS/IN 718 parts were successfully built with high densification levels (&gt;99%) and low porosity content (average: ~0.81%). Microscopy analysis indicates that the interfacial microstructures are characterised by dense dislocation tangling networks, NbC and TiC, and very small amounts of Laves phase (&lt;2 wt. %). In addition, equiaxed grains (average: 45 ± 3 μm) are attained in the interfacial region, whereas both individual IN 718 and 316L SS regions exhibit show columnar grains with average sizes of 55 ± 5 μm and 85 ± 3 μm, respectively. Vickers microhardness (HV) and nanoindentation measurements exhibit that the hardness values of the interfacial region are between those of the individual material regions. A strengthening model is built to assess the contribution of intrinsic strength, solid solution, precipitations, dislocations, and grain boundaries to the overall interfacial hardness of the as-built MM alloy.

Influence of Structural Parameters on Mechanical Properties of Triply Periodic Minimal Surface Structure

Titanium alloy is widely used in aerospace and other fields due to its low density, high specific strength, corrosion resistance, and other advantages. With the development of selective laser melting additive manufacturing technology, it is possible to manufacture triply periodic minimal surface porous structures. The effects of structural parameters on the mechanical properties of a Ti-6Al-4V Diamond (D)-type triply periodic minimal surface structure are studied. According to the characteristics of porous structures, the Gibson Ashby fitting formulas of D structures are modified using the concept of equivalent cross-sectional areas. The influence of cell size and surface thickness on the compressive modulus and strength of a D structure is discussed. Prediction formulas of the mechanical properties based on cell size and surface thickness are established by combining the relative density with the structural parameters. On this basis, the density optimization method is applied to the lightweight design of a rocket-related assembly based on D structure filling. The design results verify the feasibility of a lightweight design based on triply periodic minimal surface structure filling.

Strategies to reduce pores and cracks of molybdenum fabricated by selective laser melting

Selective laser melting (SLM) additive manufacturing process has been attractive to the fabrications of Mo and Mo alloys due to the advantage of fast near net shaping of complex shaped components. In order to reduce pores and cracks, by using high purity Mo powder, process parameter optimization, as well as La2O3 particle addition and post hot isostatic pressing (HIP) were applied. The results show that when the applied linear energy density is larger than 0.48 J/mm, the SLM samples can have continuous molten tracks. When the applied volumetric energy density is in the range of 250–280 J/mm3, small cubic SLM samples can have high relative densities larger than 99.7%. For large bulk SLM samples prepared with a volumetric energy density of 259 J/mm3, the addition of 0.9 wt% La2O3 can reduce the porosity from 0.76% to 0.63%. After HIP treatment, the highest relative density of 99.6% can be obtained for large bulk sample with La2O3 addition. Cracks are observed only at the surfaces of all samples, while no cracks can be seen in the inner parts. Moreover, after adding La2O3, the cracks at the surfaces are narrowed, and the crack number density is significantly reduced by >2 times.

Selective Laser Melting (SLM) Additively Manufactured CoCrFeNiMn High-Entropy Alloy: Process Optimization, Microscale Mechanical Mechanism, and High-Cycle Fatigue Behavior.

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) possesses excellent properties including exceptional strength-ductility synergy, high corrosion resistance, and good thermal stability. Selective laser melting (SLM) additive manufacturing facilitates the convenient fabrication of the CoCrFeNiMn HEA parts with complex geometries. Here, the SLM process optimization was conducted to achieve a high relative density of as-built CoCrFeNiMn HEA bulks. The mechanisms of process-induced defects and process control were elucidated. The microscale mechanical behaviors were analyzed through in situ scanning electron microscopy observation during the compression tests on micro-pillars of the as-built HEA. The stress-strain characteristics by repeated slip and mechanism of "dislocation avalanche" during the compression of micro-pillars were discussed. The high-cycle fatigue tests of the as-built HEA were performed. It was found that a large number of nano-twins were induced by the fatigue, causing a non-negligible cycle softening phenomenon. The effects of promoted ductility due to the fatigue-induced nano-twins were illustrated. This work has some significance for the engineering application of the SLM additively manufactured CoCrFeNiMn HEA parts.

Energy Absorption Characteristics of Polygonal Bio-Inspired Honeycomb Column Thin-Walled Structure under Quasi-Static Uniaxial Compression Loading.

In this paper, we investigated the internal structure of the beetle elytra, i.e., two different structural forms I and II of the bio-inspired honeycomb column thin-walled structures (BHTS) that give the honeycomb sandwich structure frequently used in construction projects better mechanical properties and lightweight performance. BHTS specimens were fabricated by additive manufacturing selective laser melting (AM-SLM) using AlSi10Mg Al-Si alloy. In order to understand the effect of section angle number on BHTS during loading, quasi-static uniaxial compression tests were carried out and verified by numerical simulation. The experimental results showed that in the quasi-static uniaxial compression tests, the number of section angles greatly improved the energy absorption (EA) index of each BHTS: the average value of initial peak crushing force (PCF) of hexagonal BHTS increased by 108.82% and 43.44%, respectively, compared to triangular and rectangular BHTS. The average value of the mean crushing force (MCF) increased by 74.87% and 45.48%, respectively. The average value of EA increased by 89.02% and 46.64%, respectively. The results indicate that the number of section angles can be used as an effective way to enhance the EA of BHTS. This work can provide a reference for the design of high-efficiency energy absorbers and will be widely used in EA scenarios such as construction, transportation, etc.

Macro-Micro Coupled Simulation of SLM Process with Inconel 718 Superalloy

As an increasingly mature additive manufacturing technology for metal materials, Selective Laser Melting (SLM) technology has become a hot topic in many application fields. However, due to the fast-moving velocity and small scale of the laser beam in the SLM process, it is very difficult to directly observe the microstructural changes in the SLM additive manufacturing process. In this study, a macro-micro coupled simulation model of Inconel 718 SLM process was established to study the solidification behavior of the molten pool. The macro temperature field is obtained by the finite difference method based on the birth and death grid algorithm. The local temperature intercepted from the macro temperature field is employed as the input condition for phase field microstructure calculation. Dendrite morphology, intercellular spacing, and microsegregation are simulated under the coupled model. The result shows that the solidification structure of IN718 alloy in the micro molten pool formed by SLM grows in the form of a non-flat interface. The primary dendrite spacing predicted by the simulation is in good agreement with Hunt model at the initial stage of solidification. The solute trapping caused by non-equilibrium solidification makes dendrites dissolve more Nb, resulting in microsegregation.

Micro-laminated CoCrFeMnNi−TiNp/CoCrFeMnNi high-entropy alloy matrix composite with bimodal grain structure via multi-material selective laser melting (MM-SLM) additive manufacturing

Deviating from most studies on selective laser melting (SLM) additively manufactured single metals or homogeneous metal matrix composites (MMCs), this work engaged in a multi-material SLM (MM-SLM) route for micro-laminated MMCs with bimodal grain structures. Structurally hierarchical micro-laminate of CoCrFeMnNi high-entropy alloy (HEA)-based HEA−TiNp/HEA MMC was built by the MM-SLM to overcome the strength-ductility trade-off and improve the anti-impact performance under high strain-rate compression deformation. The dynamic impacts were performed on the SLM-built HEA and MM-SLM-built HEA−TiNp/HEA using a split Hopkinson pressure bar (SHPB) system, with the strain rates of 1000 s−1, 2000 s−1, and 3000 s−1. The structurally hierarchical HEA−TiNp/HEA exhibited higher strength and deformability than the HEA. The strain-rate-dependent work hardening capacity and macroscopic plasticity of the HEA−TiNp/HEA indicated a better resistance to dynamic shocks. The promising results pave a road for achieving high-performance HEA-based MMCs of the MM-SLM-built hierarchical structures with a relatively low proportion of nano-ceramic reinforcements and bimodal-grained HEA matrix.

Acoustic emission for in situ process monitoring of selective laser melting additive manufacturing based on machine learning and improved variational modal decomposition

Acoustic emission for in situ process monitoring of selective laser melting additive manufacturing based on machine learning and improved variational modal decomposition

Impact of the additive manufacturing process on the high-temperature corrosion of 316L steel in the presence of NaCl–KCl–ZnCl2 molten solar salt

A comparative study of the high-temperature oxidation and corrosion resistance of wrought and additive manufacturing selective laser melting (SLM) 316L stainless steel was performed in presence of molten NaCl–KCl–ZnCl2 salt used for thermal energy storage in the new generation of concentrated solar plants. Isothermal tests at 650 and 700 °C in a dry air atmosphere were conducted to evaluate (i) the effect of the manufacturing process and (ii) the effect of molten salts on the corrosion behavior of the steel by dimensional metrology analysis, scanning electron microscopy and X-ray diffraction characterization. The manufacturing process had a significant influence on the oxidation resistance. The wrought samples had a mass gain after 48 h that was four times greater than that of SLM samples at 650 °C, and 50% greater at 700 °C. The presence of the salt primarily increased the corrosion rate of all the samples and, again, the wrought pieces suffered 13% and 6% more corrosion in terms of metal loss than the SLM ones at 650 and 700 °C, respectively. The presence of more pathways for Cr diffusion associated with a higher number of defects in the microstructure of the SLM sample, as dislocations and grain boundaries, facilitated the formation of Cr2O3 scales in the SLM sample, avoiding the intergranular attack shown in the wrought one.

A dimensionally augmented and physics-informed machine learning for quality prediction of additively manufactured high-entropy alloy

Selective laser melting (SLM) additive manufacturing (AM) is widely used due to its significant advantages in designing and manufacturing special-shaped complex components. The process parameters of SLM determine the quality of as-built parts, but it is difficult to establish an accurate and reliable mathematical model to connect process parameters with the quality of as-built parts. However, data-driven machine learning can effectively solve the analysis and prediction problem of complex process. Therefore, a machine learning (ML) prediction method based on dimensionality augmentation and physical information is proposed, which connects the process parameters (laser power, hatching space, scanning speed, and layer thickness) of SLM with the quality characteristics (top layer surface roughness and relative density) of as-built parts. The four process parameter features (4-dimensional features) are expanded to high-dimensional features through feature engineering to characterize the quality of as-built parts. In addition, the physical information of powder melting forming in SLM process is fused with ML algorithm, the theory-guided ML is used to improve the prediction accuracy of the model. In this paper, the CoCrFeNiMn high-entropy alloy as-built samples dataset is used for network training of four ML algorithms, and three assessment indexes are used to evaluate the prediction model. The results show that dimensionally augmented and physics-informed ML model has better prediction accuracy and generalization ability. The proposed method can also provide guidance for optimizing process parameters.

Ultrafast Laser Ablation of Inconel 718 for Surface Improvement

Inconel 718 is considered difficult to machine because of its ability to maintain its properties at high temperatures. The low thermal conductivity of the alloy causes accelerated tool deterioration when machining. Selective laser melting (SLM) additive manufacturing introduces a possibility of eliminating these difficulties, and producing complex shapes with this difficult-to-machine material. However, high surface roughness and porosity usually occur at the surface of components produced through additive manufacturing. In this study, the surfaces of Inconel 718 samples produced through selective laser melting were treated using laser ablation. The process parameters for the laser ablation process were analyzed in order to achieve a minimum surface roughness. The surfaces of the samples were observed after printing and after the laser ablation process using a microscope and the roughness average was measured using a profilometer. Optimized process parameters were achieved, and have the capability to reduce the surface roughness to a minimum Ra of 3.024 µm.

Fabrication and Actuation Performance of Selective Laser Melting Additive-Manufactured Active Shape-Memory Alloy Honeycomb Arrays

Shape-memory alloy (SMA) honeycomb arrays have drawn worldwide attention for their potential active applications in smart morphing wings. However, the manufacturing of complex active SMA honeycomb arrays via conventional processes is a difficult task, and the actuation performance of the honeycomb arrays has not yet been well–investigated. In this work, the active SMA honeycomb arrays were fabricated by selective laser melting (SLM) additive manufacturing, and their actuation performance was investigated. The results show that the SLM–fabricated active SMA honeycomb arrays can generate obvious actuation performance during the transformation and exhibit a higher maximum actuation stress of 2.53 MPa at a R/t ratio of 4 and a tensile pre–strain of 35%. This research will contribute to the design and further improvement of active SMA honeycomb arrays based on SLM additive manufacturing, promoting the engineering applications for smart morphing wings.