As-built Selective Laser Melting Research Articles

Influence of stress-relieving heat treatments on the efficacy of Co-Cr-Mo-W alloy copings fabricated using selective laser melting.

Purpose This study aimed to evaluate the influence of stress-relieving heat treatments on the metal-ceramic bond strength and fitness accuracy of selective laser melting (SLM)-fabricated Co-Cr alloy copings.Methods SLM-manufactured Co-Cr samples were stress-relieved at 750 (Ht-750) and 1150 °C (Ht-1150). The microstructure, surface roughness, metal-ceramic bond strength, marginal and internal fit, Vickers hardness, and residual stress were then compared with those of the non-heat-treated group (As-built). The results were analyzed using one-way ANOVA and post-hoc tests (Tukey's or Student's t test) (P = 0.05).Results The microstructure of the Ht-1150 samples had a brittle oxide layer and lower surface roughness, resulting in significantly lower bond strength values than those of the other groups. The As-built group exhibited significantly lower marginal gap values than the Ht-750 and Ht-1150 groups. Therefore, the post-heat treatments degraded the marginal fitness. The surface residual stress in all sample groups were compressive because of the sandblasting effect. The compressive stresses were larger in Ht-1150 than in As-built and Ht-750 owing to their low hardness values.Conclusions Stress-relief annealing porcelain-fused-to-metal single crowns does not improve bond strength and degrades fitness accuracy because additional post-heat treatments induce thermal distortion. These findings are expected to facilitate the direct application of As-built SLM single crowns in dentistry to minimize post-manufacturing costs and time.

Selective laser melting of in-situ CoCrFeMnNi high entropy alloy: Effect of remelting

In-situ multicomponent alloying in additive layer manufacturing shows inhomogeneous elemental distribution. This study aimed to minimize this inhomogeneity in equiatomic CoCrFeMnNi high entropy alloy (HEA) produced by selective laser melting (SLM) from a mixture of elemental powder. A detailed study was undertaken regarding the effects of remelting scan strategy on microstructural and mechanical properties, melt pool geometry, and elemental distribution of CoCrFeMnNi HEA manufactured using the SLM process. The SLM sample with remelting scan strategy showed a homogeneous elemental distribution. The remelted SLM sample showed a homogeneous hardness distribution. The X-ray diffraction of the as-built and remelted SLM showed a single-phase FCC structure, typical for a HEA. Prediction and observation of melt pool depth exhibited a lower depth in the remelted SLM samples. The microstructure of the melt pools of both as-built and remelted SLM showed a fine grain morphology, typical of rapid solidification. The boundary of melt pools in the as-built SLM and the inter-space of melt pools in the remelted SLM consisted of both coarse and equiaxed/columnar dendritic grains. The present results confirmed the possibility of fabricating a homogenized CoCrFeMnNi HEA from mixed elemental powders using SLM with remelting scan strategy.

Influence of heat treatment on the tool life while machining SLM Inconel 718 with reference to C&W Inconel 718

In this study tool life experiments were performed on Inconel 718 (IN718) alloy. The Inconel 718 samples were produced by conventional Cast and Wrought (C&W) and by Laser Powder Bed Fusion (L-PBF) techniques like Selective Laser Melting (SLM). The as-built SLM samples which were subjected to only Hot Isostatic Pressing (HIP) are labeled as SLM HIP and both Hot Isostatic Pressing and Aeronautic Heat Treatment are labeled as SLM HIP-AHT. The tool life machining tests were performed using PVD coated carbide insert under emulsion conditions. The surface integrity parameters such as surface roughness, residual stresses, and microhardness were evaluated. The wear mechanisms on the rake and flank faces of the PVD insert were characterized. It's deduced that notch and flank wear near the depth of the cutting zone were the predominant failure mode affecting the tool life. The age-hardened microstructural features of C&W and HIP heat-treated SLM samples have a negative impact on the tool life. Whereas the microstructure obtained by the SLM HIP-AHT sample indeed played an important role by exhibiting 26 % more tool life than SLM HIP samples and 79 % more tool life than the C&W sample.

Effect of phase transformation on the mechanical properties of the Co-Cr-Mo alloy fabricated by selective laser melting

In this study, we examined the effect of stress relief annealing (SRA) temperatures on the microstructure and mechanical behavior of a Co-Cr-Mo alloy fabricated by selective laser melting (SLM). For the SRA, the as-built SLM samples were annealed at 750 °C and 1050 °C. In the as built SLM sample, the γ phase was mainly formed, but some ε phases were present together. The percentage of the ε phase increased significantly after SRA at 750 °C, and a microstructure with a 100% γ phase formed after SRA at 1050 °C. Changes in mechanical properties were characterized by an increase in both hardness and yield strength, while the elongation decreased after SRA at 750 °C. This was caused by the formation of the ε phase, which possesses high strength and hardness but brittleness. After SRA at 1050 °C, the hardness and yield strength decreased, and the elongation increased compared to the as-built SLM sample because of the decrease in the residual stress. In the γ phase of the Co-Cr-Mo alloy, strain-induced γ to ε martensitic transformation occurring during deformation, and it was more marked in the as-built SLM sample with high residual stress. Therefore, when the as-built sample was deformed, the brittle ε phase formed more rapidly than in the sample after SRA at 1050 °C, resulting in low elongation. • The effect of SRA on the microstructure and mechanical properties of CCM alloy fabricated by SLM was investigated. • The phase transformation behavior was changed according to the SRA temperature. • The mechanical properties were affected by the microstructure change after SRA. • After SRA, the residual stress was reduced by the heat treatment effect.

In situ observation of anisotropic tribological contact evolution in 316L steel formed by selective laser melting

A consensus on the tribological performance of components by additive-versus conventional manufacturing has not been achieved; mainly because the tribological test set-ups thus far were not suited for investigating the underlying microstructure's influence on the tribological properties. As a result, utilization of additive manufacturing techniques, such as selective laser melting (SLM), for tribological applications remains questionable. Here, we investigate the anisotropic tribological response of SLM 316L stainless steel via in situ SEM reciprocating micro-scratch testing to highlight the microstructure's role. As-built 316L SLM specimens were compared against annealed wire-drawn 316L. We found that: (i) microgeometric conformity was the main driver for achieving steady-state friction, (ii) the anisotropic friction of the additively manufactured components is limited to the break-in and is caused by the lack of conformity, (iii) the cohesive bonds, whose strength is proportional to frictional forces, are stronger in the additively manufactured specimens likely due to the dislocation-dense, cellular structures, (iv) low Taylor-factor grains with large dimension stimulate microcutting in the form of long, thin sheets with serrated edges. These findings uncover some microstructurally driven tribological complexities when comparing additive to conventional manufacturing.

Microstructural Tailoring and Enhancement in Compressive Properties of Additive Manufactured Ti-6Al-4V Alloy through Heat Treatment.

Among laser additive manufacturing, selective laser melting (SLM) is one of the most popular methods to produce 3D printing products. The SLM process creates a product by selectively dissolving a layer of powder. However, due to the layerwise printing of metal powders, the initial microstructure is fully acicular α′-martensitic, and mechanical properties of the resultant product are often compromised. In this study, Ti-6Al-4V alloy was prepared using SLM method. The effect of heat treatment was carried out on as-built SLM Ti-6Al-4V alloy from 650–1000 °C to study respective changes in the morphology of α/α′-martensite and mechanical properties. The phase transition temperature was also analyzed through differential thermal analysis (DTA), and the microstructural studies were undertaken by optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties were assessed by microhardness and compressive tests before and after heat treatment. The results showed that heat treated samples resulted in a reduction in interior defects and pores and turned the morphology of the α′-martensite into a lamellar (α + β) structure. The strength was significantly reduced after heat treatment, but the elongation was improved due to the reduction in columnar α′-martensite phase. An optimum set of strength and elongation was found at 900 °C.

Effect of internal pores on tensile properties of Co-Cr-Mo alloy fabricated by selective laser melting

PurposeThe purpose of this paper is to examine the effect of internal pores on the tensile properties of a Co–Cr–Mo alloy fabricated by selective laser melting (SLM).Design/methodology/approachThe size and volume fraction of pores were controlled through high temperature annealing (HTA) and hot isostatic pressing (HIP).FindingsAfter HTA, the size and fraction of pores decreased compared with the as-built SLM sample, and no pores were observed after HIP. Tensile tests of the HTA and HIP samples showed nearly similar tensile deformation behavior. From the results, the authors found that the size of the internal pores formed in the SLM process had little effect on the tensile properties. The as-built SLM sample had less elongation than the HTA and HIP samples, which would not the effect of porosity, but rather the effect of the residual stress and the retained ε phase after the SLM process.Originality/valueAlthough pores are a main factor that influence the mechanical properties, the effect of pores on the tensile properties of Co–Cr–Mo alloys fabricated by SLM has not been studied. Therefore, in this study, the effect of pores on the tensile properties of a Co–Cr–Mo alloy fabricated by SLM was studied.

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.

Insight to agglomeration and chemical reactions of CoAl2O4 inoculants in IN718 processed by selective laser melting

The investigation aims to clarify the influence of scan parameters on the morphology and phase constitution of CoAl2O4 inoculants in Inconel 718 (IN718) superalloy processed by selective laser melting (SLM). Gas atomized IN718 powder feedstock was uniformly blended with 0.2 wt% of CoAl2O4 flakes and processed with a range of laser scan parameters to understand the effect of the inoculant additions on microstructure. Similar to the reactions observed in investment cast Ni-superalloys, the CoAl2O4 particles were found to chemically react with elemental Al, Cr, and Ti present in the melt pool of IN718 and become reduced into Al2O3, Cr2O3, and TiO2 oxides after the SLM process. Analyses of single melt pool beads revealed the existence of Co-rich metallic particles distributed along the surface of the oxides. However, since SLM processing involves remelting of the previously deposited layers to minimize the occurrence of porosity, this results in the dissolution of the Co-rich metallic particles that form when the CoAl2O4 particles are reduced in the melt pool. As such, the ability of the metallic Co particles to contribute to grain refinement becomes limited during SLM processing. Furthermore, the inoculant particles were found to agglomerate along the scan direction. The underlying mechanisms can be attributed to the Marangoni convection following the moving center of laser beam. It was found that decreasing energy density and increasing scan speed can effectively reduce the size and number density of the agglomerated oxide particles. Faster laser scan velocities lead to reduced convective flow within the melt pool and reduce the tendency for agglomeration. However, the reduced magnitude of energy input limited the time over which the CoAl2O4 particles could react with the melt pool. This led to the formation of lack of fusion defects and incomplete chemical reaction between CoAl2O4 and IN718 after the SLM process. The effectiveness of CoAl2O4 inoculant particles on grain refinement in as-built SLM IN718 microstructures is strongly dependent on the melt pool physics and laser scan conditions.

Influence of heat treatment on the fatigue resistance of Inconel 718 fabricated by selective laser melting (SLM)

In this research work, the influence of heat treatment on the fatigue resistance of Inconel 718 (IN718) fabricated by selective laser melting (SLM) and cast and wrought (C&W) process route was investigated. The optical image of the as-built SLM sample shows melt pool layers, within the melt pool layers both cellular and columnar dendritic structures are visible. The hardness value of the SLM sample under as-built condition is around 34% less compared to the reference (C&W) IN718. The as-built Inconel 718 samples were heat treated under Hot Isostatic Pressing (HIP) and Aeronautic Heat Treatment (AHT) conditions. During heat treatment the tensile residual stresses accumulated in the as-built sample was reduced close to zero. The hardness value has increased nearly equal to the hardness value of the reference sample which is due to the precipitate formation and reduced porosity during heat treatment. To evaluate the fatigue limit, the four point bending fatigue tests were carried out. Results showed fatigue limit of the C&W samples is high compared to the SLM samples due to the differences in their microstructure. Within the SLM samples, the HIP + AHT SLM has survived more fatigue cycles at high stress levels compared to the SLM HIP samples. This is due to the presence of more porosity which acted as the crack initiation site and the presence of δ phase in the SLM HIP samples.

Selective laser melting of Ti6Al4V: Effect of laser re-melting

Abstract Microstructural inhomogeneity in additively manufactured materials affects the material properties. The present study aims in minimizing such microstructural inhomogeneity in Ti6Al4V alloy fabricated using selective laser melting (SLM) from the gas atomized powder. A detailed and systematic study of the effect of remelting on the microstructure and mechanical properties of Ti6Al4V was undertaken. Acicular α′ martensite was present in all the samples (both in the as-built SLM and remelted) and the dimensions of the α′ phase change with the number of melting steps. The hardness of the as-built SLM sample increased and the material got homogenized with an increasing number of meltings. The ultimate tensile strength was higher in the double- and triple-melted samples while the ductility was lower than the single-melted sample. The present results clearly prove that the number of remeltings play a significant role in determining the microstructure (homogenization of the microstructure) and the mechanical properties of the SLM-built materials.

Laser ultrasonic imaging for defect detection on metal additive manufacturing components with rough surfaces.

Defects or discontinuities are inevitable during the melting and consolidation process of metal additive manufacturing. Online inspection of microdefects during the processing of layer-by-layer fusion is urgently needed for quality control. In this study, the laser ultrasonic C-scan imaging system is established to detect the surface defects of selective laser melting (SLM) samples that have a different surface roughness. An autosizing method based on the maximum correlation coefficient and lag time is proposed to accurately measure the defect length. The influences of the surface roughness on the laser ultrasound signal-to-noise ratio distribution and defect sizing accuracy are also studied. The results indicate that the proposed system can detect notches with a depth of 50 µm and holes with a diameter of 50 µm, comparable in size to raw powder particles. The average error for the length measurement can reach 1.5% if the notch is larger than 2 mm. Meanwhile, the sizing error of a 1 mm length notch is about 9%. In addition, there is no need to remove the rough surface of the as-built SLM samples during the detection process. Hence, we propose that the laser ultrasonic imaging system is a potential method for online inspection of metal additive manufacturing.

Effect of Build Orientation and Annealing on Corrosion Resistance of Additively Manufactured Duplex Stainless Steel in 3.5% NaCl

Grade 2205 Duplex stainless steel (DSS) components were built via selective laser melting (SLM) with gas atomized powder (D90 < 45μm) using a 250 W laser in a nitrogen environment. The relative density was 99.1 ± 0.3%, and the ferrite/austenite phase ratio after annealing was 53.0/47.0 ± 4.8%. Two build orientations (parallel and perpendicular to build direction), as well as as-built SLM (no heat treatment) and annealed conditions, were studied. Corrosion properties were characterized in a 3.5% NaCl electrolyte and compared to 2205 wrought. The charge transfer resistance was > 500 kohm.cm2, and the corrosion rate was < 100 nA cm−2 for all alloys, which suggest passivity. No stable pit formation occurred during cyclic polarization tests although the as-built perpendicular and annealed SLM conditions showed metastable pitting, likely due to surface porosity and/or chemical inhomogeneity. The as-built parallel condition did not show metastable pitting and had the highest average charge transfer resistance, which is evidence of possible improvements from laser melted material, but not conclusive. Ultimately, all SLM DSS 2205 was similar to 2205 wrought, implying that the build orientation and annealing were not as significant of factors as the chemical composition in predicting the passivity and pitting resistance.

Near-Surface Structure and Fatigue Crack Initiation Mechanisms of As-Built SLM Inconel 718

Challenging structural applications such as customized jet engine parts are increasingly fabricated by Selective Laser Melting (SLM) of Inconel 718 powder. The as-built surface quality of SLM parts is however inferior of the machined version and the fatigue behavior is negatively affected. The as-built fatigue response of SLM Inconel 718 was quantified here using three sets of directional specimens. Since the surface quality is influenced by powder characteristics, process parameters and layer-wise fabrication, fatigue results showed a directional contribution that was interpreted using metallography and fractography.

Effects of build- and scan-directions on magnetic field-assisted finishing of 316L stainless steel disks produced with selective laser melting

Selective laser melting (SLM) is an additive manufacturing process, which allows for producing parts of very complex geometry. While SLM may produce the desired net-shape geometry, the surface conditions of as-built SLM parts are often insufficient. Also, the as-built state may yield undesired tensile stresses at the surface. Post processes such as magnetic field-assisted finishing (MAF) processes can yield the desired surface roughness and improve the residual stress state. However, in a part made by SLM, these processes will act on surfaces with various different orientations to the built direction, and, in particular, to the laser scanning direction. This paper analyzes the influence of laser scanning direction with respect to the surface to be finished by MAF using 316L steel disks produced with different orientations and scan strategies. The build plane of an as-printed disk has greater surface roughness and hardness but lower tensile residual stresses than the plane perpendicular to the build plane. These characteristics create difficulties in material removal and smoothing but do not impede MAF’s ability to increase hardness and impart compressive residual stresses to the surface scanned by the laser. The results indicate that material removal rate and hence optimal finishing time depend on the orientation of the surface with respect to the scanning direction, which should be accounted for in process planning.

Tensile property enhancement by oxygen solutes in selectively laser melted titanium materials fabricated from pre-mixed pure Ti and TiO2 powder

Ti-based materials with high oxygen solute contents were fabricated from mixtures of Ti powder and TiO2 particles by selective laser melting (SLM). Uniformly dissolved oxygen (O) from the TiO2 particles caused c lattice expansion in α-Ti crystals, which effectively increased the strengths of as-built SLM Ti–O materials. The as-built SLM Ti–O material using 1.5 wt% TiO2 showed a yield stress of 962 MPa and 15.3% elongation. The yield stress increases calculated by the Hall–Petch equation and Labusch model were equivalent to those observed experimentally, and O solid-solution strengthening was dominant in increasing yield stress.

Processing a biocompatible Ti–35Nb–7Zr–5Ta alloy by selective laser melting

Abstract

Creep and high temperature fatigue performance of as build selective laser melted Ti-based 6Al-4V titanium alloy

The present work focuses on the room and high temperature low-cycle fatigue and creep testing of Ti-6Al-4V manufactured by selective laser melting (SLM) or laser-based powder bed fusion process (LBPF). The fatigue specimens were tested in a strain control mode with the as-built and machined surfaces, evaluating the influence of the surface roughness on the fatigue performance. This helps in understanding the potential negative influence of a surface defect/roughness on the fatigue performance of the complex components produced by SLM (in as-built state), where surface machining may not be possible due to geometrical constraints. The fatigue fractures at room and high temperature were investigated with the help of a scanning electron microscope. The creep tests were performed at three different temperatures with as-built SLM samples, demonstrating an equivalent or better performance when compared to their counterparts produced by hot-forging.

Microstructural and mechanical anisotropy of selective laser melted IN718 superalloy at room and high temperatures using small punch test

The nature of melting and solidification phenomena during selective laser melting (SLM) process leads to a strong crystallographic texture in the as-built SLM components which causes mechanical anisotropy. This anisotropy is usually reduced by either optimization of the laser scanning strategy during the SLM process or high-temperature heat treatment. Microstructure and mechanical anisotropy of SLMed Inconel 718 (IN718) superalloy were investigated in this study by means of XRD, SEM, EBSD and small punch testing (SPT) method at room and high temperatures. The SPT data were validated against uniaxial tensile testing and corresponding equations were proposed to determine yield and ultimate tensile strengths of the fabricated parts. Significant differences in mechanical behavior of the side and top views were mainly achieved due to the contribution of (100) columnar grains texture. It was found that the IN718 SLM parts possess 54% greater maximum force in the side view compared to the top one at room temperature. However, the side view maximum force lowered about 37% at 650 °C, while no significant decrease was observed for the top view sample. Fractography of small punched specimens showed ductile fracture surface for the side view compared to transgranular brittle surface for the top view at room temperature. Both views exhibited more brittle fracture at elevated temperature with different fracture surface morphologies, intergranular in the side view and cleavage along with Laves phases in the top view. The present work clearly demonstrates that SPT can successfully predict mechanical anisotropy in the as-built SLM IN718 superalloy.

The Effect of Stress Relief on the Mechanical and Fatigue Properties of Additively Manufactured AlSi10Mg Parts

The heating and cooling profiles experienced during laser additive manufacturing results in residual stresses build up in the component. Therefore, it is necessary to perform post build stress relieving towards the retention and improvement of the mechanical properties. However the thermal treatments for conventional manufacturing do not seem to completely accommodate these rapid heating and cooling cycles of laser processing techniques such as powder bed fusion. Characterizations such as density measurements on the samples were performed employing the Archimedes principle; hardness testing was performed on the Zwick micro/macro (Hv) hardness tester, SEM and Electron backscatter diffraction (EBSD). Fracture toughness and crack growth was conducted on a fatigue crack machine. All characterization was done after stress relieving of Selective Laser Melting (SLM) produced samples at 300 °C for 2 hrs was performed in a furnace. The mechanical properties appear to be rather compromised instead of being enhanced desirably. As-built SLM produced tensile specimens built in different directions exhibited significantly favorable mechanical properties. However, post stress relieve thermal treatment technique deteriorated the strength while increasing the ductility significantly. Nonetheless, fatigue crack growth and fracture toughness illustrated positive outcome in terms of fatigue life on SLM produced AlSi10Mg components in application.