Selective Laser Melting Material Research Articles

Microstructure, wear and residual stresses of selective laser melting AlSi10Mg solid cylinder

Advanced industrial processing technique selective laser melting (SLM) can handle various materials. Although titanium alloys are the main material used in SLM, aluminium alloys may be employed in the future. However, producing aluminium alloys is more complicated. This work uses SLM to make an AlSi10Mg solid cylinder. The aim is to study the mechanical properties and microstructure of products. Layer thickness increases defects; thus, research advises avoiding it. The optical microscope study proved the conduction melting process's stability and hole-freeness. EDX mapping and SEM were used to compare the chemical makeup of as-cast and SLM materials. An unusual microstructure showed consistent alloying component distribution. Investigations examine wear, hardness and residual stresses. Extreme hardness was found. The component has evenly distributed compressive residual stresses within material yield limits.

Comparative analysis of microstructure, mechanical, and corrosion properties of biodegradable Mg-3Y alloy prepared by selective laser melting and spark plasma sintering

This work explored possibilities of biodegradable magnesium alloy Mg-3Y preparation by two modern powder metallurgy techniques – spark plasma sintering (SPS) and selective laser melting (SLM). The powder material was consolidated by both methods utilising optimised parameters, which led to very low porosity (∼0.3%) in the SLM material and unmeasurably low porosity in the SPS material. The main aim of the study was the thorough microstructure characterisation and interrelation between the microstructure and the functional properties, such as mechanical strength, deformability, and corrosion resistance. Both materials showed comparable strength of ∼110 MPa in tension and compression and relatively good deformability of ∼9% and ∼21% for the SLM and SPS materials, respectively. The corrosion resistance of the SPS material in 0.1 M NaCl solution was superior to the SLM one and comparable to the conventional extruded material. The digital image correlation during loading and the cross-section analysis of the corrosion layers revealed that the residual porosity and large strained grains have the dominant negative effect on the functional properties of the SLM material. On the other hand, one of the primary outcomes of this study is that the SPS consolidation method is very effective in the preparation of the W3 biodegradable alloy, resulting in material with convenient mechanical and degradation properties that might find practical applications.

Fabrication of Ti-6Al-4v cellular lattice structure using selective laser melting for orthopedic use: A review

Additive Manufacturing (AM) plays a vital role in the field of biomedical applications, particularly in the production of implants for bone repair. These implants possess an internal cellular structure that promotes bone ingrowth and is crucial for their successful integration in the human body. To meet the requirements of biomedical implants, the material used should be biodegradable, non-toxic, possess similar biomechanical properties to natural bone, and exhibit porosity. consequently, the precise design and production of implants hold great significance.This review focuses on the fabrication of Ti-6Al-4 V cellular lattice structures using selective laser melting (SLM) for orthopedic applications. It discusses the structure of bone, its characteristics, and the mechanism of bone healing, emphasizing the importance of understanding bone biology for effective implant design. The suitability of the bio-compatible Ti-6Al-4 V alloy for orthopedic use is explored, highlighting its biocompatibility, mechanical properties, and corrosion resistance and also delves into the design considerations for cellular lattice structures, including pore size, shape, and interconnectivity. These design features aim to promote bone ingrowth, enhance mechanical properties, and facilitate tissue regeneration.The review emphasizes the significance of these structures in orthopedic applications, with their combination of biocompatibility, mechanical strength, and tailored design features for bone repair and tissue engineering. It also anticipates further advancements in SLM technology and material research to enhance clinical outcomes of orthopedic implants.

Microstructure and Mechanical Properties of Nanoparticulate Y2O3 Modified AlSi10Mg Alloys Manufactured by Selective Laser Melting.

AlSi10Mg has a good forming ability and has been widely accepted as an optimal material for selective laser melting (SLM). However, the strength and elongation of unmodified AlSi10Mg are insufficient, which limits its application in the space industry. In this paper, yttrium oxide (Y2O3) nanoparticles modified AlSi10Mg composites that were manufactured using SLM. The effects of Y2O3 nanoparticles (0~2 wt.% addition) on the microstructure and mechanical properties of AlSi10Mg alloys were investigated. An ultimate tensile strength of 500.3 MPa, a yield strength of 322.3 MPa, an elongation of 9.7%, a good friction coefficient of 0.43, and a wear rate of (3.40 ± 0.09) ×10-4 mm3·N-1·m-1 were obtained with the addition of 0.5 wt.% Y2O3 nanoparticles, and all these parameters were higher than those of the SLMed AlSi10Mg alloy. The microhardness of the composite with 1.0 wt.% Y2O3 reached 145.6 HV0.1, which is an increase of approximately 22% compared to the unreinforced AlSi10Mg. The improvement of tensile properties can mainly be attributed to Orowan strengthening, fine grain strengthening, and load-bearing strengthening. The results show that adding an appropriate amount of Y2O3 nanoparticles can significantly improve the properties of the SLMed AlSi10Mg alloy.

Effect of Hydrogen on the Structure and Mechanical Properties of 316L Steel and Inconel 718 Alloy Processed by Selective Laser Melting.

The interaction of hydrogen with specimens of 316L steel and Inconel 718 alloy processed by selective laser melting (SLM) was studied. The effect of hydrogen on the mechanical properties of SLM materials, hydrogen permeability, and microstructure was investigated; besides, these values were compared with the properties of conventionally produced materials. It was shown that SLM can be successfully used to produce parts for operation in hydrogen environments at high pressure at room temperature.

Multiple recycling of nickel alloy powder for selective laser melting process: influence on properties of the powder and printed material

PurposeThe purpose of this study is the 16-fold recycling process effect of VZH159 nickel alloy powder on its features and characteristics of the printed material obtained by selective laser melting (SLM). Chemical composition, content of gas impurities, powder grading, pore volume fraction and surface morphology of powder particles, structure and properties of SLM material, surface roughness and deviations from specified geometry of the test samples were investigated.Design/methodology/approachThe experiment’s method procedure presumes the use of only recycled powder without adding any virgin powder at each build cycle. To avoid powder sloughing because of incomplete filling of the build space, a print area delimiter was used. For all manufactured samples, hot isostatic pressing was carried out in an ASEA Quintus-16 facility. Heat treatment was carried out in air furnaces. Structure investigations were carried out on a Leica DMIRM metallographic complex. Microstructure studies were carried out on a Verios 460 scanning electron microscope with X-ray microanalysis.FindingsWith the number of recycling stages, an increase in oxygen content is observed in the powder, which leads to an increment for oxides in the printed material. The 16-fold recycling does not have a significant effect on the features of the powder itself and the printed material if the build space is filled with manufacturing parts by no more than 20%.Originality/valueThe creep rupture strength of the SLM material, which appears to be a sensitive characteristic to the quality of the applied powder, does not change in the printed material after all stages of powder recycling as well.

Selective Laser Melting of 304L and 316L Stainless Steels: A Comparative Study of Microstructures and Mechanical Properties

Among various types of stainless steel, 304L and 316L have the most engineering applications. Herein, 304L and 316L powders with similar size distributions and morphologies are used as raw materials for selective laser melting (SLM) using the same parameters. It is found that SLM 304L has two phases, γ‐austenite (≈95%) and δ‐ferrite (≈5%) phases, while SLM 316L contains only γ‐austenite phase. The average grain size is found to be 4.9 μm (average grain size of γ‐austenite phase: 5.0 μm; δ‐ferrite: 3.4 μm) for SLM 304L and 16.7 μm for SLM 316L. The small grain size of 304L is attributed to the peritectic reaction during solidification. δ‐ferrite acts as a heterogeneous nucleation site for γ‐austenite. SLM 316L has higher hardness and yield strength than 304L. Results obtained from the comparative study indicate that adding ferrite stabilizer elements (e.g., Cr, Mo, Si, Nb, or Ti) into 316L can lead to peritectic reaction or austenite to ferrite solid phase transformation, which is a possible approach for grain refinement.

Hydrogen embrittlement of additively manufactured austenitic stainless steel 316 L

Additive manufacturing (AM) is a promising means of production of austenitic stainless steel (SS) parts for hydrogen service. The hydrogen embrittlement resistance of SS 316 L parts manufactured by powder-bed-fed selective laser melting (SLM) and directed energy deposition (DED) was examined using slow strain rate tensile testing. The influence of the hierarchical AM microstructures on mechanical response, microstructural evolution, and void formation were analyzed using multiscale electron microscopy. The presence of hydrogen reduced ductility in as-built DED materials, but did not significantly influence the response in as-built SLM material or heat-treated materials. Microstructural features driving these different responses are discussed.

Effects of Laser Spot Size on the Mechanical Properties of AISI 420 Stainless Steel Fabricated by Selective Laser Melting.

The purpose of this study is to investigate the effects of laser spot size on the mechanical properties of AISI 420 stainless steel, fabricated by selective laser melting (SLM), process. Tensile specimens were built directly via the SLM process, using various laser spot diameters, namely 0.1, 0.2, 0.3, and 0.4 mm. The corresponding volumetric energy density (EV) is 80, 40, 26.7, and 20 J/mm3, respectively. Experimental results indicate that laser spot size is an important process parameter and has significant effects on the surface roughness, hardness, density, tensile strength, and microstructure of the SLM AISI 420 builds. A large laser spot with low volumetric energy density results in balling, un-overlapped defects, a large re-heated zone, and a large sub-grain size. As a result, SLM specimens fabricated by the largest laser spot diameter of 0.4 mm exhibit the roughest surface, lowest densification, and lowest ultimate tensile strength. To ensure complete melting of the powder and melt pool stability, EV of 80 J/mm3 proves to be a suitable laser energy density value for the given SLM processing and material system.

Studying the Microstructural Effect of Selective Laser Melting and Electropolishing on the Performance of Maraging Steel

Selective laser melting is one of the additive manufacturing technologies that have been known for building various and complicated shapes. Despite numerous advantages of additive manufacturing technologies, they strongly influence the microstructure and typically show a relatively high surface roughness. In this study, maraging steel was produced by selective laser melting (SLM), and its microstructure, hardness and corrosion behavior before and after heat treatment were studied and compared to traditionally manufactured ones (wrought, forged samples). In addition, the effect of electropolishing on the surface roughness was evaluated. The microstructural study was carried out by scanning electron microscopy equipped with electron backscattered diffraction in three different sections: parallel to the top surface (xy), transverse cross section (xz) and longitudinal cross section (yz). The same characterization was applied to heat-treated samples, austenitized and quenched as well as the aged ones. The results showed that selective laser melting produced a fine grain martensitic structure (in the as-printed condition) with a surface roughness (Ra) of about 10 µm. There was no sign of preferred texture or anisotropy in the microstructure of as-print SLM materials. The SLM microstructure was similar in all 3 sections (xy, xz and yz). Despite finer microstructure, nano-hardness and corrosion behavior of SLM and conventional wrought maraging steel in heat-treated conditions were similar. Aging resulted in the maximum nano-hardness and the minimum corrosion potential values. Precipitation has the main role in both hardness and corrosion behavior. Electropolishing was optimized and reduced the surface roughness (Ra) by 65%.

Effects of embedded spherical pore on the tensile properties of a selective laser melted Ti6Al4V alloy

Selective laser melting (SLM) is one of the most promising additive manufacturing (AM) technologies in recent years. However, some sub-millimeter defects often appear accidently in SLM specimens, resulting in deterioration of mechanical properties. Therefore, the effects of these defects on the mechanical properties of the SLM materials are significantly important. In this study, the effect of a single spherical defect on the tensile properties was investigated by embedding a spherical pore inside the SLM Ti6Al4V specimens. Different combinations of pore diameters (Ф100 μm, Ф300 μm, Ф500 μm, Ф1000 μm) and locations (center, sub-surface, near-surface) were designed to mimic the sub-millimeter printing defects. The results demonstrated that these embedded pores had negligible influence on the elastic modulus and tensile strength but had a noticeable influence on the tensile elongation. With the increase of pore size, the elongation to failure of the specimens tended to be stochastic, stably high and stably low in sequence, which could be related to different fracture modes. The tensile specimens fractured at the embedded pore when the embedded pore size was increased to a critical diameter (approximately Ф1000 μm). Concurrently, the location of the embedded pore started to have an obvious influence on ductility, namely, the elongation to failure decreased as the embedded pore became closer to free surface.

Predicting magnetic characteristics of additive manufactured soft magnetic composites by machine learning

Selective laser melting (SLM) is one of the widely used metal additive manufacturing techniques. While SLM is able to produce high-quality products, the parameter selection process can be very complicated, especially for magnetic materials in that the iron loss and permeability properties must also be considered, which renders the parameter selecting process more complicated. This research explores the parameter selection process of magnetic material for SLM, which integrates machine and evolutionary algorithms to accurately predict magnetic characteristics, such as iron loss and permeability, and generates suggestions for the process parameters according to practical demands.

Dental implant-abutment fracture resistance and wear induced by single-unit screw-retained CAD components fabricated by four CAM methods after mechanical cycling

Statement of problemComputer-aided design and computer-aided manufacturing (CAD-CAM) methodologies allow the fabrication of custom dental implant abutments with a variety of materials and techniques. Studies on the mechanical strength of such components and the wear induced at their coupling interface during mechanical cycling are sparse. PurposeThe purpose of this in vitro study was to measure the wear patterns at the hexagonal platform of dental implants induced by the installation and mechanical cycling of custom abutments fabricated by using 4 different CAD-CAM methods and to determine the compressive static resistance of the implant-abutment combinations. Material and methodsA CAD software program was used to design a custom abutment for a single-unit screw-retained external hexagon dental implant crown. The same design file was used to manufacture with 4 CAM methods (N=40): milling and sintering of zirconium dioxide (ZO), cobalt-chromium (Co-Cr) sintered by selective laser melting (SLM), fully sintered machined Co-Cr alloy (MM), and machined and sintered agglutinated Co-Cr alloy powder (AM). Prefabricated titanium abutments were used as a control (TI). Each abutment was installed onto a dental implant (4.1×11 mm), and the specimens were mechanically aged (1 million cycles, 2 Hz, 100N, 37 °C). After mechanical cycling, the hexagonal connection of the dental implants was examined with a scanning electron microscope (SEM), and unused dental implants (NI) were examined as a control (n=10). The images were analyzed with a software program to quantify the areas that showed wear. The implant-abutment combinations were reassembled and submitted to a compression test (1mm/min) with a universal testing machine. The data obtained were submitted to 1-way ANOVA (α=.05). ResultsThe mean ±standard deviation fracture load (N) of the specimens of each group were 1005 ±187 (ZO), 1074 ±123 (SLM), 1033 ±109 (MM), 1019 ±149 (AM), and 923 ±129 (TI). These values were statistically similar (P=.213). The mean ±standard deviation wear of the implants in squared-pixels were 1.1 ±0.38×105 (ZO), 2.0 ±0.29×105 (SLM), 1.0 ±0.38×105 (MM), 1.1 ±0.27×105 (AM), 1.1 ±0.33×105 (TI), and 0.51 ±0.29×105 (NI). The results indicated that, although significantly higher than those in in the control group (NI), the wear values found in the groups TI, ZO, MM, and AM were significantly lower than in the SLM group (P<.001). ConclusionsThe CAD-CAM abutments presented the same mechanical fracture load and wear measurements as the TI group, except for the SLM material, which showed increased wear. The failure mode from the load bearing test was the fracture of the abutments for the ZO group. The implants permanently deformed or fractured for the metal abutment groups.

Low-Cycle-Fatigue Properties of a 17-4 PH Stainless Steel Manufactured via Selective Laser Melting

In this paper, the static and low-cycle-fatigue (LCF) behavior of wrought samples of 17-4 PH stainless steel (SS) manufactured via Selective Laser Melting (SLM) are presented. On the one hand, several scholars have studied SLM materials and literature reports a huge amount of data as for the high-cycle-fatigue (HCF) behavior. On the other hand, few are the data available on the LCF behavior of those materials. The aim of the present research is to provide reliable data for an as-build 17-4 PH steel manufactured via SLM techniques. Only with quantitative data, indeed, it is possible to exploit all the advantages that this technology can offer. In this regard, both quasi-static (QS) and low-cycle-fatigue tests were performed on Additive Manufacturing (AM) cylindrical samples. Through QS tests, the constitutive low has been defined. Strain-controlled fatigue tests on an electromechanical machine were performed on 12 samples designed according to the ASTM standard. Tests were continued also after the stabilization was reached (needed for the cyclic curve described with the Ramberg-Osgood equation) to obtain also the fatigue (ε-N) curve. Results show that the material has a softening behavior. The Basquin-Coffin-Manson (BCM) parameters were tuned on the basis of the ε-N combinations after rupture.

Impact of Selective Laser Melting Additive Manufacturing on the High Temperature Behavior of AISI 316L Austenitic Stainless Steel

Additive manufacturing allows production of complex geometries or customized designs that are difficult or impossible to fabricate by conventional means. However, these components have hardly ever been tested in severe conditions corresponding to real functioning at high temperature. The high temperature oxidation of AISI 316L stainless steel additively manufactured by selective laser melting (SLM) has been studied for 100 h at temperatures between 700 and 1000 °C in dry air and compared to that of wrought samples. Thermogravimetric analyses showed slower kinetics for SLM samples than for conventional coupons. In addition, SLM samples exhibit parabolic kinetics for all the studied temperatures, while conventional coupons present complete laws above 800 °C. Parabolic constant rate determined for 900 °C oxidation is one order of magnitude lower for SLM samples (1.73·10−13 g2 cm−4 s−1) than for wrought coupons (1.54·10−12 g2 cm−4 s−1). The resulting activation energy values confirm the better behavior of SLM alloys, in agreement with the formation at their surface of protective chromia Cr2O3. In contrast, additional formation of non-protective iron oxides was observed above 800 °C for the wrought samples. The different behavior could be explained by Cr depletion at the surface of conventional alloy, whereas Cr supply was still insured in the case of SLM material.

The influence of additive manufacturing on the micromilling machinability of Ti6Al4V: A comparison of SLM and commercial workpieces

Abstract Ti6Al4V produced by selective laser melting (SLM) is replacing the use of casting workpieces as it is a net-near shape process that can maintain its mechanical and biocompatibility properties and it can produce scaffolds geometry reducing the workpiece weight. Typically, the microstructure produced by SLM differs from casted workpieces, the surface roughness of SLM is also different is also higher compared to casted pieces. If it is needed precision and smooth surfaces, it is necessary to add machining after SLM, especially for the production of channels smaller than 1 mm, which is the case of micromilling. For the definition of micromilling, it is not recommended to use the ones indicated for meso scale and there are few studies on the micromilling of Ti6AL4V implants produced by SLM. This study compares the machinability of the standard commercial Ti6Al4V with produced by SLM during micromilling process using different feed per tooth configurations (from 0.5 to 4.0 μ m). The analysis of machinability considered cutting forces, surface roughness, burr formation analysis and microchips morphology. Despite presenting higher strength and hardness, SLM material presented higher machinability with lower forces, lower surface roughness and less burs, explained by the SLM microstructural of fined acicular α ’ martensite due to the rapid cooling of the material.

Correlation between selective laser melting parameters, pore defects and tensile properties of 99.9 % silver

In this study the relationship between Selective Laser Melting (SLM) process parameters, associated pore defects and resulting tensile properties of additively manufactured (AM) pure (99.9 %) silver structures are investigated using X-ray Computed Tomography (XCT) and 3D visualisation analysis. Yield strength, Young’s Modulus, failure strain and ultimate tensile strength of AM pure silver are reported. Furthermore, fracture surfaces of the samples are analysed using optical microscopy and Scanning Electron Microscopy (SEM) to investigate fracture surface porosity content and surface roughness is analysed using digital microscopy. Pore defect distribution, morphology and resultant pore surface area are reported, and average pore size calculated in relation to SLM process parameters variations. It was found that sample yield and ultimate tensile strengths of pure silver have a direct correlation with energy density at the powder bed related to SLM process parameters selected. However, Youngs Modulus values were found to be dependent on the average porosity pore size rather than sample density or number of pores. SLM pure silver bulk density was evaluated using XCT comparative analysis while varying SLM parameters and the resultant pore defect morphology and distribution discussed. The results reported in this work serve as a basis for further material development and mechanical property predictions utilising XCT analysis for pure silver and other nonstandard SLM materials.

A comparison study of the dwell-fatigue behaviours of additive and conventional IN718: The role of dislocation substructure on the cracking behaviour

The dwell-fatigue responses of high temperature materials, such as IN718, manufactured via additive manufacturing processes with different microstructures is of practical interest in terms of time-dependent cracking resistance at elevated temperature. In the present study, the dwell-fatigue cracking behaviours of IN718 manufactured via selective laser melting (SLM) with different heat treatments, and via electron beam melting (EBM) with different scanning strategies were compared at 550 °C and with a long 2160 s dwell-holding period. Comparison has also been made with a conventional forged counterpart. Detailed microstructure characterizations have been done to correlate the role of dislocation substructures on the dwell-fatigue damage mechanisms and cracking resistances. A mechanism regarding the susceptibility of the dislocation cell substructure in SLM materials to creep damage is proposed. In addition, the effects of other microstructure features on the dwell cracking resistance are also discussed.

Adhesion of HVOF-Sprayed WC-Co Coatings on 316L Substrates Processed by SLM

Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect.

Cost-affordable, high-performance Ti–TiB composite for selective laser melting additive manufacturing

Abstract