Selective Laser Melting Process Parameters Research Articles

Influence of the Energy Density for Selective Laser Melting on the Microstructure and Mechanical Properties of Stainless Steel

We have investigated the impact of the process parameters for the selective laser melting (SLM) of the stainless steel AISI 316L on its microstructure and mechanical properties. Properly selected SLM process parameters produce tailored material properties, by varying the laser’s power, scanning speed and beam diameter. We produced and systematically studied a matrix of samples with different porosities, microstructures, textures and mechanical properties. We identified a combination of process parameters that resulted in materials with tensile strengths up to 711 MPa, yield strengths up to 604 MPa and an elongation up to 31%, while the highest achieved hardness was 227 HV10. The correlation between the average single-cell diameter in the hierarchical structure and the laser’s input energy is systematically studied, discussed and explained. The same energy density with different SLM process parameters result in different material properties. The higher energy density of the SLM produces larger cellular structures and crystal grains. A different energy density produces different textures with only one predominant texture component, which was revealed by electron-backscatter diffraction. Furthermore, three possible explanations for the origin of the dislocations are proposed.

Numerical Simulation of Moving Heat Flux during Selective Laser Melting of AlSi25 Alloy Powder

A single-layer three-dimensional model was created to simulate multi-channel scanning of AlSi25 powder in selective laser melting (SLM) by the finite element method. Thermal behaviors of laser power and scanning speed in the procedure of SLM AlSi25 powder were studied. With the increase of laser power, the maximum temperature, size and cooling rate of the molten pool increase, while the scanning speed decreases. For an expected SLM process, a perfect molten pool can be generated using process parameters of laser power of 180 W and a scanning speed of 200 mm/s. The pool is greater than the width of the scanning interval, the depth of the molten pool is close to scan powder layer thickness, the temperature of the molten pool is higher than the melting point temperature of the powder and the parameters of the width and depth are the highest. To confirm the accuracy of the simulation results of forecasting excellent process parameters, the SLM experiment of forming AlSi25 powder was carried out. The surface morphology of the printed sample is intact without holes and defects, and a satisfactory metallurgical bond between adjacent scanning channels and adjacent scanning layers was achieved. Therefore, the development of numerical simulation in this paper provides an effective method to obtain the best process parameters, which can be used as a choice to further improve SLM process parameters. In the future, metallographic technology can also be implemented to obtain the width-to-depth ratio of the SLM sample molten pool, enhancing the connection between experiment and theory.

A Finite Element Model of Melt Pool for the Evaluation of Selective Laser Melting Process Parameters

A Finite Element Model of Melt Pool for the Evaluation of Selective Laser Melting Process Parameters

Effect of processing parameters on the densification of an additively manufactured 2024 Al alloy

Understanding the densification behaviours and formation mechanisms of defects are essential to fabricate high quality and high strength aluminium components using selective laser melting (SLM) technology. In this work, the effects of laser power and scanning speed on the densification, defects evolution and their formation mechanisms in a SLMed 2024 aluminium (Al) alloy were investigated in consideration of the corresponding laser energy input, melting mode transition and microstructural evolution. The results showed that optimizing the processing parameters effectively reduced the porosity level below 1% by avoiding the lack of fusion and keyhole melting mode, and minimizing the gas pores. However, optimization of the processing parameters could not eliminate the columnar structure associated with the SLMed 2024 Al alloy, which contributed to the hot-tearing cracks in the SLMed parts. It was found that the dependence of porosity formation on SLM processing parameters was contrary to the crack density. Hence, to further improve the SLM-processability of the 2024 Al alloy it is necessary to develop SLM methods in order avoid the hot-cracking within the optimized processing parameter window associated with the minimum porosity formation.

Experimental and empirical model analysis of microsurface texturing on 316 L press-fit joints fabricated by selective laser melting

In this study, selective laser melting (SLM) was investigated for the manufacturing of 316L stainless steel press-fit joints. The accuracy of selective laser melting technique in fabrication of texture profile in shape, pitch and height of microsurface texturing was examined. The resulting insertion and removal forces achieved from the produced textured pins for cold-formed high-end fixation applications were studied. The experimental results showed that the shape, pitch and height of the texture, as well as the resultant bonding strength of the joints, can be effectively set via control of the SLM processing parameters. While trapezoidal and triangular shapes of the texture lead to stronger bonding compared with oval-shaped texture profiles, the texture height was found to have a predominant effect on bond strength. To a much lower extent, larger pitch distances also resulted in higher bond strengths. A combination of abrasive and adhesive wear mechanisms was detected via examination of the inner surface of the hub into which the press fit was inserted. Along with a process map of design of the microsurface texture geometry of metal interference fit joints, this paper also presents the underlying mechanics for their bonding. The SLM process is shown to present a useful one-step method for the manufacturing of knurl metallic interference fit pins of customisable and definable texture and ensuing bond strength.

Analysis of Microstructure and Properties of a Ti–AlN Composite Produced by Selective Laser Melting

Selective Laser Melting (SLM) is a manufacturing technique that is currently used for the production of functional parts that are difficult to form by the traditional methods such as casting or CNC (Computer Numerical Control) cutting from a wide range of metallic materials. In our study, a mixture of commercially pure titanium (Ti) and 15% at. aluminum nitride (AlN) was Selective Laser Melted to form three-dimensional objects. The obtained 4 mm edge cubes with an energy density that varied from 70 to 140 J/mm3 were examined in terms of their microstructure, chemical and phase composition, porosity, and Vickers microhardness. Scanning Electron Microscopy (SEM) observations of the etched samples showed inhomogeneities in the form of pores and unmelted and partly melted AlN particles in the fine-grained dendritic matrix, which is typical for titanium nitrides and titanium aluminum nitrides. The AlN particles remained unmelted in samples, but no porosity was observed in the interface area between them and the dendritic matrix. Additionally, samples fabricated with the presintering step had zones with different sizes of dendrites, suggesting a differing chemical composition of the matrix and the possibility of the formation of the phases forming an Ti–Al–N ternary system. The chemical composition in the microareas of the samples was determined using Energy Dispersive X-Ray Spectroscopy (EDS) and revealed differences in the homogeneity of the samples depending on the SLM process parameters and the additional presintering step. The phase composition, examined using X-ray Diffraction analysis (XRD), showed that samples were formed from Ti, TiN, and AlN phases. Porosity tests carried out using a computer microtomography revealed porosities in a range from 7% to 17.5%. The formed material was characterized by a relatively high hardness exceeding 700 HV0.2 over the entire cross-section, which depended on the manufacturing conditions.

Self-organized giant magnetic structures via additive manufacturing in NdFeB permanent magnets

Selective Laser Melting (SLM) was used to produce 3D-printed net shape NdFeB (Neodymium Iron Boron) permanent magnets that exhibit relatively large internal permanent magnetization structures, without exposure to any external magnetizing field. The macroscopic magnetization, M→(r→), does not create a significant magnetic stray field outside the as-produced sample, and is pole avoiding and solenoidal. The permanent magnetization can be detected via the stray field that appears after cutting the sample into pieces. From the field measurements it can be concluded that the magnetization is mainly in the 3D-printing planes which are parallel to M→(r→). Maximum magnetic flux densities of almost 80 mT are recorded 1 mm above the cut surfaces in the air. Dependencies of the effect on SLM process parameters, as well as on the sample size and shape are discussed. Although a deep understanding is still missing, a possible mechanism that may partly explain the formation of these self-organized macroscopic magnetization structures is proposed. The discovered effect may offer new routes for producing magnetized rare earth-transition metal (RE-TM) permanent magnets without using a magnetizer, and it shows that the SLM 3D-printing process can lead to new material behavior.

Stable formation of powder bed laser fused 99.9% silver

Additive manufacture (AM) of metals and alloys using powder-bed fusion (PBF) often employs a 400 W (1060–1100 nm wavelength) fibre laser as the primary energy source for Selective Laser Melting (SLM). Highly reflectie and thermally conductive materials such as pure silver (Ag) offer significant challenges for SLM due to insufficient laser energy absorption at the powder bed. Accordingly, this work pioneers the processing, analysis, and fabrication of 99.9% (pure) atomised Ag using PBF AM featuring a 400 W fibre laser system. The atomised pure silver powder is characterised for its morphology, size, shape, distribution and compared to current AM sterling silver. Laser-powder interaction is then investigated through single track fabrication to assess the feasibility of laser melting pure Ag. Varied process parameter single laser pass and single-track fabrication on both copper and steel build substrates are conducted and analysed with optical and scanning electron microscopy (SEM) techniques. The resulting SLM process parameters are then used to create pure Ag 3D structures and the effects of laser power, scan speed, hatch distance and layer thickness on material density is evaluated. Furthermore, SEM analysis of the 3D structures was conducted to identify optimum laser power, scan speed, hatch distance and layer thickness required to create dense pure Ag structures. The results of this study show that SLM processing of pure Ag utilising PBF AM is feasible. The optimum process parameters required for the generation of controlled track formation and 3D fabrication of pure Ag at a 97% density is reported.

Large tunable elastocaloric effect in additively manufactured Ni–Ti shape memory alloys

High-efficiency elastocaloric refrigeration requires high-performance elastocaloric materials with both large surface areas to promote heat exchange rate and large elastocaloric effects to increase the amount of heat transfer. Ni–Ti shape memory alloys (SMAs) are the most promising elastocaloric materials but they are difficult to process by conventional methods due to their poor manufacturability. Here, we successfully developed Ni–Ti SMAs with large elastocaloric effects by additive manufacturing which has the capability to fabricate complex geometries with large surface areas. The phase transformation temperatures of these additively manufactured Ni–Ti SMAs, fabricated by selective laser melting (SLM), can be tuned by varying the SLM processing parameters and/or post heat treatments and thus tunable large elastocaloric effects were achieved at different temperatures, which can be used for different applications. Owing to its large transformation entropy change and high yield strength as a result of precipitation hardening, the aged SLM fabricated alloy exhibits a remarkably large elastocaloric effect with an adiabatic temperature change as high as 23.2 K, which is among the highest values reported for all Ni–Ti SMAs fabricated by both conventional methods and additive manufacturing. Furthermore, by virtue of the high yield strength and low stress hysteresis of the aged alloy, this large elastocaloric effect shows good stability during cycling. The achievement of such large elastocaloric effects in alloys fabricated by near-net-shape additive manufacturing may accelerate the implementation of high-efficiency elastocaloric refrigeration. This study is instructive for the development of advanced high-performance solid-state refrigeration materials by additive manufacturing.

Challenges and Opportunities in the Selective Laser Melting of Biodegradable Metals for Load-Bearing Bone Scaffold Applications

The aim of this paper is to assess the current status of processing biodegradable metals (BDM) via selective laser melting (SLM), with particular emphasis on bone scaffold applications, and provide a meta-analysis on the effect of processing parameters on relative density to better direct recommendations for the future of this growing field. Synthetic bone scaffolds are becoming a popular alternative for the treatment of critical bone defects that cannot heal without surgical intervention. These scaffolds act as a bridge allowing bone to grow across the gap. Selective laser melting can achieve bone scaffolds with complex hierarchical architecture tailored specifically to the patient. SLM-manufactured titanium scaffolds have already been clinically tested with some success. Permanent titanium alloys have a higher chance of implant rejection from the innate immune reaction, coupled with complications linked to the high mismatch in stiffness between the implant and the bone. Biodegradable metals can overcome these problems by maintaining sufficient mechanical properties for load-bearing applications during healing and eventually degrade away completely. Currently, however, the use of SLM for the manufacturing of BDM scaffolds is still in its infancy as only a few peer-reviewed studies are published, with the majority of these published in the last couple of years. Literature was systematically reviewed to critically analyze and synthesize the data in the form of a meta-analysis. Only studies that included the processing parameters used for volumetric energy density (namely the laser power, scan speed, hatch spacing, and energy density) and provided as built relative densities were used. SLM of biodegradable metals is an exciting research area that requires further exploration. Apart from overcoming the problems unique to each major biodegradable metal family, the meta-analysis showed that the vast majority of studies regard the optimization of SLM processing parameters. However, these studies are specific to the powder and machine used. Rather, broader guidelines need to be developed for modern SLM machines to allow for quicker optimization for future SLM-manufactured BDM.

Optimization of selective laser melting process parameters for Ti-6Al-4V alloy manufacturing using deep learning

Ti-6Al-4V products manufactured by selective laser melting (SLM) have been extensively used in biomedical, aerospace, and many other industries. However, SLM’s many process parameters and their vast range are barriers to efficiently printing desirable products. Because SLM requires so much time, expenditure, and expert knowledge about the process and materials, SLM printing of Ti-6Al-4 V products needs a pre-processing optimization system. In this paper, an SLM optimization system is developed based on a supervised deep neural network by applying the Python programming language and the TensorFlow library. The outputs of this optimization method are the optimal SLM process parameters, which can be used to produce a product that satisfies a user requirement. With this optimization system, an SLM operator does not need much experience or to spend much time on experimentation to print a desirable product. This system is a significant, important module for pre-processing SLM printing.

Effects of process parameters on the microstructure and mechanical properties of 24CrNiMo steel fabricated by selective laser melting

In this paper, the effects of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of 24CrNiMo high-strength low-alloy steel were investigated. The optimal parameter combinations and corresponding action rules were obtained by relative density index method which directly affects the microstructure and mechanical properties of materials. As a result, the relative densities of as-received 24CrNiMo steel increased rapidly with the growth of energy densities in the range of 38–50 J/mm3, then increased slowly between 50 and 100 J/mm3 and ultimately stabilized at maximum 99.6%. The hardness and tensile strength showed an initial upward trend and afterward a fall with increasing the energy densities. In addition, high energy densities led to an increase in the sub-micron cellular and columnar grain size produced by the rapid solidification during SLM, while the grain evolved from columnar grains into cellular grains as the ratio of temperature gradient to solidification rate decreased from the bottom to the top of the molten pool. The granular bainite and the meta bainite including carbon-rich retained austenite films were obtained in as-fabricated samples. The quantity of retained austenite phase decreased due to austenite decomposition with increasing energy densities. Thus, 24CrNiMo steel exhibited a high microhardness of 374.4 HV10 and a high tensile strength of 1249.5 MPa respectively at the most optimized energy density of 85 J/mm3 because of low porosity, fine grain size, and an appropriate fraction of stable retained austenite.

Metallurgical defect behavior, microstructure evolution, and underlying thermal mechanisms of metallic parts fabricated by selective laser melting additive manufacturing

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.

Printing (Mn,Fe)2(P,Si) magnetocaloric alloys for magnetic refrigeration applications

The fabrication of brittle magnetocaloric materials into heat exchangers is currently challenging for conventional manufacturing techniques. In the present study, we demonstrated that selective laser melting (SLM), an emerging 3D printing technique, offers a new and promising solution to fabricate the (Mn,Fe)2(P,Si) magnetocaloric alloys for magnetic refrigeration applications. The hexagonal (Mn,Fe)2(P,Si) phase, which shows a first-order magnetic transition, is successfully formed in the SLM-printed samples with a very small amount of impurity phase (less than 2% in volume fraction). The SLM processing parameters (e.g., the laser power and scanning speed) have a strong influence on the microstructure, magnetic phase transition and magnetocaloric properties of the printed samples. The microscopic mechanism underlying the changes in the microstructure and phase transition temperature has also been discussed. Besides that, a large isothermal entropy change, more than three times higher than that of the benchmark Gd material in a field change of 1 and 2 T, has been obtained in the SLM-printed samples.

Effect of SLM Processing Parameters on Microstructures and Mechanical Properties of Al0.5CoCrFeNi High Entropy Alloys

Selective laser melting (SLM) to fabricate Al0.5CoCrFeNi high entropy alloys with pre-mixed powders was studied in this paper. The influences of process parameters including laser power, scanning speed, and hatch spacing on the relative density of high-entropy alloys (HEAs) were investigated. A relative density of 99.92% can be achieved by optimizing the SLM process parameters with laser power 320 W, scanning speed 800 mm/s, and hatch spacing of 60 μm, respectively. Moreover, the microstructure of the HEAs was also studied using scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was found that the microstructure of the HEAs was only composed of face-centered cubic and body-centered cubic phases, without complex intermetallic compounds. The mechanical properties of the HEAs were also characterized. At ambient temperature, the alloys had a high yield strength of about 609 MPa, tensile strength about 878 MPa, and hardness about 270 HV. Through a comparison with the corresponding alloys fabricated by vacuum induction melting, it can be concluded that the high entropy alloys fabricated by SLM had fine microstructures and improved mechanical properties.

The effects of hot isostatic pressing (HIP) and solubilization heat treatment on the density, mechanical properties, and microstructure of austenitic stainless steel parts produced by selective laser melting (SLM)

Despite ongoing optimization, selective laser melting (SLM) technology still cannot guarantee the production of completely defect-free components. This work investigates hot isostatic pressing (HIP) in a nitrogen environment and solubilization heat treatment as methods for improving the quality of 316L stainless steel components produced by SLM. The characteristics of HIP-treated specimens are firstly correlated with the initial density of samples obtained with different SLM process parameters, showing that HIP is effective at eliminating pores in components with high initial density (above 99%) but not in those with low initial density (approximately 94%). Subsequently, the mechanical properties and microstructure of 316L stainless steel specimens produced by SLM are examined in the as-built state and after various post-process conditions including solubilization heat treatment and HIP at pressures from 50 to 2000 bar. The observed effects of post-processing on the porosity and microstructure of each specimen are consistent with hardness and tensile test results, with the benefits and limitations of HIP clarified for future implementation.

Selective Laser Melting and Remelting of Pure Tungsten

The processing of pure tungsten encounters a substantial challenge due to its high melting point and intrinsic brittleness. Selective laser melting (SLM) technique is gaining popularity and offers an excellent processing approach for refractory metals. Herein, dense pure tungsten specimens are produced by optimizing SLM processing parameters. The mechanical property of the SLM‐produced tungsten with an ultimate compressive strength of about 1200 MPa, which is obviously superior to that reported in other literature, is achieved. The increased laser energy input is instrumental in raising density and surface roughness of tungsten specimens. Interestingly, additional remelting of processed layers during SLM improves the surface quality and the microstructure and achieves the highest relative density (98.4% ± 0.5%). After laser remelting, the surface roughness is reduced by 28% and a large number of fine grains are obtained. The flow of fluids caused by remelting plays a decisive role in the formation of fine grains and the defect level. Therefore, these findings offer a new insight into SLM of pure tungsten.

A study of the relationship between thermal expansion and residual stresses in selective laser melting of Ti-6Al-4V

Selective laser melting of Ti-6Al-4V is associated with residual stress generation, thermal expansion, and metallurgical changes due to high heating and cooling rates. These manufacturing flaws affect the microstructure and mechanical properties of the parts produced. In this work, process mapping is presented for producing Ti-6Al-4V using the selective laser melting process. Combinations of process parameters that lead to stable melting are experimentally identified. These process maps represent the density, coefficient of thermal expansion, and deflection of parts produced in terms of laser process parameters. As a result, an optimum process window to minimize manufacturing flaws and provide stable melting is presented. The effect of selective laser melting process parameters on the coefficient of thermal expansion of the as-deposited Ti-6Al-4V parts is investigated. A critical laser energy density of 86.8 J/mm3, EC, is introduced based on the stability of the melting process. Stable melting of Ti-6Al-4V occurs at any combination of process parameters that generates laser energy density at EC. This stable melting provides homogeneous microstructure, high density of 99.9 %, and a thermal expansion coefficient in line with that of the wrought material. The induced residual stresses and part deflections at the optimal conditions are studied. Above EC, high tensile residual stresses are observed due to variations in the coefficient of thermal expansion, temperature differential, and material composition. Lack of fusion and void formation occur below EC.

Investigation of Ti-6Al-4V Alloy <i>In Situ</i> Manufactured Using Selective Laser Melting from Elemental Powder Mixture

This paper presents the results of the study of Selective Laser Melting (SLM) process for the in-situ synthesis of Ti-6Al-4V alloy from elemental powder mixture. Elemental spherical powders of Ti, Al and V were used to prepare a powder mixture, and then bulk specimens were produced by SLM using different process parameters. The effects of SLM process parameters on samples’ relative density, their chemical composition, the formed microstructure and microhardness before and after heat treatment have been studied. It was shown that volume energy density during the SLM process significantly effects the microstructure and microhardness of Ti-6Al-4V obtained from elemental powders. The difference in microstructure morphology and microhardness remains after heat treatment.

Effects of Micron/Submicron TiC on Additively Manufactured AlSi10Mg: A Comprehensive Study from Computer Simulation to Mechanical and Microstructural Analysis

The Al-based metal matrix composites (AMMCs) reinforced by micron- and submicron-scale TiC ceramic particles were manufactured by selective laser melting (SLM) additive manufacturing. The influences of SLM processing parameters such as volumetric energy density (ɛ) and laser absorptivity of powders on the densification, microstructural evolution, mechanical behavior and corrosion behavior of the AMMCs were systematically studied, based on which the strengthening mechanisms of the SLM-processed AMMC components were clarified. Finite element simulation was run to assist in revealing the underlying mechanism. Fundamental processes such as the Marangoni flow were discussed. Through these efforts, the as-printed AlSi10Mg/1 μm TiC was SLMed into crack-free, high-density parts (= 99.2%) when ɛ was > 65 J/mm3. For the as-printed AlSi10Mg/1 μm TiC samples prepared by the best SLM parameters, high tensile strength (470 ± 14 MPa) was achieved. Average hardness of ~ 169 HV0.1 and a considerably low friction coefficient with a mean value of ~ 0.3 were also obtained. The homogeneously distributed submicron TiC particles reduced the wear rate to 1.68 × 10−11 mm3/N/m. The as-printed AlSi10Mg/1 μm TiC parts showed better performance on corrosion resistance as well due to the high relative density achieved along with continuous and compatible bonding interfaces with the Al alloy matrix. Use of large-sized TiC (e.g., 30 μm) particles should be avoided, however, since they led to reduced mechanical properties and a poorer corrosion resistance.