Selective Laser Melting Material Research Articles

On the Potential of Using Selective Laser Melting for the Fast Development of Forging Alloys at the Example of Waspaloy

Forged high-performance components form the basis of technical innovations in the energy and transport sector. Great efforts are undertaken to develop new materials for forging, aiming at optimizing both the material performance and the effort needed for materials processing. The development of new forging alloys is accomplished using computational methods in combination with experimental techniques. The experimental characterization is time-consuming and requires a significant material effort. In high-alloy grades, inevitable segregations occur during casting which require substantial efforts for homogenizing the alloys by annealing and forging. In this work, gas atomization of powder and Selective Laser Melting (SLM) is explored as a new means to provide small, homogeneous samples of the desired alloys at reduced processing times. In this regard, gas atomization is used to produce raw material for SLM, which is used to prepare the specimens for the analysis of forging process parameters. The Waspaloy is considered in this work. SLM and conventional specimens are compared by means of hot isostatic compression tests in a dilatometer to determine the flow stress and microstructure evolution. The work shows that thermomechanical treatment of the as-printed state allows to convert the additive manufactured (AM) microstructures into structures that are useful inferring on the forging properties of the considered alloy.

Development of the titanium meshes by selective laser melting and chemical etching for using as medical implants

The development of additive manufacturing technology has allowed to begin the implementation of the concept of personalized implants in medicine. Medical implants should have not only mechanical properties close to bone tissue, but also have good biocompatibility, antibacterial and osseointegrating properties. These characteristics are determined by the relief, morphology, and composition of surface and can be modified by various methods such as etching, coating, electrochemical and chemical modification etc. In this research we produced the mesh-type implants by selective laser melting (SLM) and modified their relief and morphology of surface by chemical etching. Titanium VT1-0 powder was used as a raw material for SLM. For chemical etching we used three type of solutions: mixtures of H2SO4/HCl, H2SO4/H2O2 and NH4OH/H2O2. The surface morphology of etched implants was studied by scanning electron microscopy (SEM). Results showed that NH4OH/H2O2 solution is the most promising for formation of the surface with developed relief.

A comprehensive study of mechanical and acoustic properties of selective laser melting material

In this research work, four groups of selective laser melted specimens were built from AlSi10Mg-0403 powder. Each group represents the direction with respect to the bed in which the specimens are built (X, Y, Z and 45° orientation). The mechanical properties of the specimens are characterized in terms of yield strength, ultimate tensile strength, Young’s modulus and elongation at break. In addition to that, the acoustic emission (AE) during the testing was monitored using wide-band high-accuracy piezoelectric sensors. The AE results were related to the mechanical characteristics of the specimens in terms of the acoustic parameter-based data, the peak amplitude, cumulative energy and count rate. The mechanical results show that the specimens built along the z direction have relatively lower strength and it can be attributed to the borderline porosity formed during the SLM process. The acoustic results can identify the critical points of failure under loading. The AE technique proves to be a powerful tool in characterizing the mechanical property and can unveil the concealed information which cannot be identified directly from the mechanical results.

Microstructure and microhardness studies of Inconel 718, manufactured by selective laser melting and subjected to severe plastic deformation and annealing

Microstructure investigations and microhardness measurements have been comparatively performed for the nickel-based superalloy Inconel 718 produced by selective laser melting (SLM) and conventional technology. The superalloy produced by SLM had a finer γ grain size and a developed dislocation substructure in contrast to a conventional wrought material. High pressure torsion of the superalloy led to a refinement in the microstructure and an increase in the microhardness in both superalloys. Annealing at 600 °C for 2 hours of the deformed superalloys caused an additional increase in microhardness. The microhardness of the SLM material was slightly higher than that of the conventional superalloy.

Laser Weldability of AlSi10Mg Alloy Produced by Selective Laser Melting: Microstructure and Mechanical Behavior

Selective laser melting (SLM) allows 3D complex parts to be produced with integrated functionalities but does not permit to realize large dimension parts, as the building chamber volume of commercial SLM systems is limited. Therefore, the possibility of joining SLM-built parts by welding appears as extremely valuable. In this work, laser weldability of SLMed AlSi10Mg parts has been investigated. The cooling rates induced by the welding process are lower than the ones of SLM; therefore, typical eutectic silicon network of SLM-built parts appears broken in both the melted zone and part of the heat-affected zone, as confirmed by calorimetric measurement. This microstructural evolution caused a local decrease of hardness from 132 HV to 73 HV and a reduction of ductility caused by porosity formation in the weld bead. Moreover, the thermal treatment, performed at 320°C, can limit the gap of tensile response among welded bead and base SLM material.

Relating fracture toughness to micro-pillar compression response for a laser powder bed additive manufactured bulk metallic glass

A Zr-based bulk metallic glass produced using selective laser melting (SLM) was compared to the same alloy fabricated using traditional suction-casting. Analysis of the fracture toughness through single edge notched beam bending experiments showed a significantly reduced damage tolerance for the laser-processed material (KQ ~ 138.0 ± 13.1 → 28.7 ± 3.7 MPa √m), even though X-ray diffraction and microhardness responses were identical. Using uniaxial quasistatic micro-pillar compression, it was found that as-cast samples more readily underwent shear transformations (evidenced through discrete load drops) below the nominal 0.2% yield stress, which was connected to the higher macroscopic toughness. Differential scanning calorimetry demonstrated that the increased barrier to shear transformation for the SLM material could not be explained by the relative relaxation states. Rather, it was attributed to the greater dissolved oxygen concentration in the laser-processed material, which is postulated to decrease atomic mobility in the structure and thereby increase the activation energy required to initiate shear transformations.

Evaluation of Low Cycle Fatigue Performance of Selective Laser Melted Titanium Alloy Ti–6Al–4V

The material of Ti–6Al–4V has been widely applied in various industries, such as automobile, aerospace, and medical due to its high specific strength, superior thermal stability and strong corrosion resistance. In the recent decades, selective laser melting (SLM) has become an attractive method to fabricate Ti–6Al–4V parts, thanks to its significant advantages in low material consumption, the high degree of freedom in design, low carbon footprint, etc. Predictability of SLM material fatigue properties is especially important for the safety-critical structures under dynamic load cases. The present research is aimed at evaluating the low cycle fatigue (LCF) performance of SLM Ti–6Al–4V under high loading states. LCF tests were performed for as-built and annealed SLM Ti–6Al–4V. Comparison between LCF properties of SLM Ti–6Al–4V and the wrought Ti–6Al–4V was also made. It was found that as-built SLM Ti–6Al–4V demonstrated a comparable LCF performance with the wrought material. The LCF life of as-built SLM Ti–6Al–4V was longer than that of wrought Ti–6Al–4V at lower strain amplitudes. However, the wrought Ti–6Al–4V had better LCF performance at higher strain amplitudes. The results revealed that the porosity in the as-built SLM material exerted much more impact on the degradation of the material at high strain amplitudes. Annealing deteriorated the LCF performance of SLM Ti–6Al–4V material due to the formation of coarser grains. The cyclic Ramberg–Osgood and the Basquin–Coffin–Manson models were fitted to depict the cyclic stress–strain and the strain–life curves for the SLM Ti–6Al–4V, based on which the LCF performance parameters were determined. In addition, the fatigue fracture surfaces were observed by using scanning electron microscopy (SEM), and the results indicated that fatigue cracks originated from the surface or subsurface defects.

A damage evolution model based on micro-structural characteristics for an additive manufactured superalloy under monotonic and cyclic loading conditions

In the present work the superalloy Inconel 718 manufactured by selective laser melting (SLM) was investigated with focus on micro-structural damage evolution under both monotonic and cyclic loading conditions. Material testing and characterization reveal significantly accelerated damage evolution of the SLM material, in comparing with the forged Inconel 718. An accumulated plastic strain based damage model was proposed to predict the damage evolution in the alloys under both monotonic and cyclic loading. The damage mechanism related to detrimental precipitate particles and porous defects in the material was discussed and considered in the model, which can predict both monotonic and cyclic material degradation processes with agreement.

Investigation of Localized Corrosion Phenomena in Selective Laser Melted 316L Stainless Steel

Selective laser melting (SLM) refers to the additive manufacturing of bulk material from powder on a layer by layer basis through melting via laser and rapid solidification. In recent years, additively manufactured metallic alloys produced by SLM have garnered attention due to opportunities in waste reduction, time and cost efficiency, and fabrication of geometrically complex structures. However, limited work has been performed to investigate the corrosion properties of these alloys. Understanding these mechanisms is crucial in assessing the attendant risks associated with the use of AM alloys in engineering design. Stainless steels such as 316L produced by SLM have presented advantages due to the corrosion resistant properties of these alloys. However, the non-equilibrium microstructure in these SLM materials, significantly different from that of the wrought counterpart, result in heterogeneities that may lead to localized corrosion such as melt pool boundary attack. A point of concern in stainless steels is intergranular corrosion due to the formation of secondary phases such as chromium carbides that deplete chromium from grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. While low-carbon steels such as 316L have great resistance to deleterious chromium carbide precipitation, this phenomenon has not been investigated in great detail in the case of selective laser melted 316L. However, it has been shown that precipitation of secondary phases is accelerated in SLM alloy systems such as Inconel 625 with respect to the wrought counterpart. Intergranular corrosion can ultimately lead to intergranular stress corrosion cracking, a form of brittle failure. This study investigates localized corrosion phenomena such as melt-pool boundary attack and intergranular corrosion and the associated microstructure-based mechanisms in the corrosion of these SLM 316L alloys. Galvanostatic etching, potentiodynamic scans through the double-loop potentiokinetic reactivation method, and mass-loss testing of wrought and SLM 316L are utilized to reveal the electrochemical behavior of these alloys. Additionally, focused ion beam and transmission electron microscopy are utilized to understand the microstructural origins of the observed corrosion phenomena.

Suitability of electrical discharge machining debris particles for usage as a powder for selective laser melting: an explorative study

In this study, the properties of debris particles originating from Electrical Discharge Machining (EDM) processes are investigated. The question is addressed whether these particles are suitable to serve as a raw material for Selective Laser Melting (SLM). To this end, different EDM parameter sets were used to manufacture particles with different size distributions. After de-oiling, the resulting powders were examined by dynamic image analysis. Results show that EDM can produce particles with high sphericity. Moreover, particle size distribution can be controlled by EDM parameters. In summary, EDM-manufactured particles seem to be a promising source of powders for the SLM process.

Effect of Fe Addition on Heat-Resistant Aluminum Alloys Produced by Selective Laser Melting

The effect of Fe addition on the high-temperature mechanical properties of heat-resistant aluminum alloys produced by selective laser melting (SLM) was investigated in relation to the alloy microstructures. Fe is generally detrimental to the properties of cast aluminum alloys; however, we found that Fe-containing alloys produced by SLM had improved high-temperature strength and good ductility. Microstructural observations revealed that the increase in the high-temperature strength of the alloys was due to the dispersion of fine rod-shaped Fe-Si-Ni particles unique to the SLM material instead of the cell-like structure of eutectic Si.

Texture and Microstructural Features at Different Length Scales in Inconel 718 Produced by Selective Laser Melting.

Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their <100> axes oriented in the direction of the local thermal gradient. Consequently, prominent <100> crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90° at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected.

In situ alloying of elemental Al-Cu12 feedstock using selective laser melting

ABSTRACTThis investigation developed selective laser melting (SLM) processing parameters for the in situ fabrication of an Al-Cu12 alloy from pure elemental blends of aluminium and copper powders. Use of elevated pre-heat temperatures (400°C) created a coarser dendritic cell microstructure consisting of supersaturated Al-rich with a uniform Al2Cu phase granular microstructure compared to non-pre-heated samples. Al-Cu12 in situ samples achieved maximum tensile strength values comparable to that of sand cast pre-alloyed Al-Cu12. Processing at elevated pre-heat temperatures created components with higher ultimate tensile strength and ductility compared to standard room temperature-built samples due to it assisting a more complete melting of Al and Cu particles. Additionally pre-heating enabled an artificial age hardening, producing an equilibrium α + θ microstructure. The creation of an alloy in situ through the use of elemental powder blends represents a low-cost and flexible methodology for exploration of new SLM material compositions and potential candidate materials for semi-solid processing using SLM.

Microstructural influence on fatigue crack propagation during high cycle fatigue testing of additively manufactured Alloy 718

A study of the microstructure of additively manufactured Alloy 718 was performed in order to better understand the parameters that have an influence on the fatigue properties of the material. The specimens were manufactured using two powder bed fusion techniques – Electron Beam Melting (EBM) and Selective Laser Melting (SLM). Four point bending fatigue tests were performed at room temperature with a stress ratio of R = 0.1 and 20 Hz frequency, on material that was either in hot isostatically pressed (HIP) and solution treated and aged (STA) condition or in STA condition without a prior HIP treatment. The grains in the SLM material in the HIP + STA condition have grown considerably both in the hatch and the contour regions; EBM material, in contrast, shows grain growth only in the contour region. Fractographic analysis of the specimens in HIP + STA condition showed a faceted appearance while the specimens in STA condition showed a more planar crack appearance. The crack propagation occurred in a transgranular mode and it was found that precipitates such as NbC, TiN or δ-phase, when present, did not affect the crack path. The areas with larger grains corresponded to the faceted appearance of the fracture surface. This could be attributed to the plastic zone ahead of the crack tip being confined within one grain, in case of the larger grains, which promotes single shear crack growth mode.

Characterisation of the attenuation properties of 3D-printed tungsten for use in gamma camera collimation

BackgroundThe aim of this work was to characterise the attenuation properties of 3D-printed tungsten and to assess the feasibility for its use in gamma camera collimator manufacture.Method3D-printed tungsten disks were produced using selective laser melting (SLM). Measurements of attenuation were made through increasing numbers of disks for a Tc-99m (140 keV) and I-131 (364 keV) source. The technique was validated by repeating the measurements with lead samples. Resolution measurements were also made with a SLM tungsten collimator and compared to Monte Carlo simulations of the experimental setup. Different collimator parameters were simulated and compared against the physical measurements to investigate the effect on image quality.ResultsThe measured disk thicknesses were on average 20% above the specified disk thicknesses. The measured attenuation for the tungsten samples were lower than the theoretical value determined from the National Institute of Standards and Technology (NIST) cross-sectional database (Berger and Hubbell, XCOM: photon cross-sections on a personal computer, 1987). The laser scan strategy had a significant influence on material attenuation (up to 40% difference). Results of these attenuation measurements indicate that the density of the SLM material is lower than the raw tungsten density. However, an improved performance compared to a lead collimator was observed. The SLM tungsten collimator was adequately simulated as 80% density and 110% septal thickness. Scatter and septal penetration were 17% less than a similar lead collimator and 33% greater than tungsten at 100% density.ConclusionsSLM manufacture of tungsten collimators is feasible. Attenuation properties of SLM tungsten are superior to the lead alternative and the opportunity for bespoke collimator design is appealing.

Selective laser melting of typical metallic materials: An effective process prediction model developed by energy absorption and consumption analysis

Selective laser melting (SLM) is a laser-based additive manufacturing technique that can fabricate parts with complex geometries and sufficient mechanical properties. However, the optimal SLM process windows of metallic materials are difficult to predict, especially when exploring new metallic materials. In this paper, a universal and simplified model has been proposed to predict the energy density suitable for SLM of a variety of metallic materials including Ti and Ti alloys, Al alloy, Ni-based superalloy and steel, on the basis of the relationship between energy absorption and consumption during SLM. Several important but easily overlooked factors, including the surface structure of metallic powder, porosity of powder bed, vaporization and heat loss, were considered to improve the accuracy of the model. Results show that, to achieve near-full density parts, the energy absorption (Qa) by the local powder bed should be approximately 3–8 times greater than the energy consumption (Qc), and this finding applies to all materials investigated. The value of Qa/Qc highly depends on material properties, particularly laser absorptivity, latent heat of melting and specific heat capacity. Experiments on high-entropy alloy (CrMnFeCoNi) and Hastelloy X alloy, new metallic materials for SLM, have been further conducted to verify the model. Results confirm that the model can predict suitable laser energy densities needed for processing the various metallic materials without tedious trial and error experiments. Indications and uncertainty of the model have also been analyzed.

Comparative investigation on grindability of Inconel 718 made by selective laser melting (SLM) and casting

Inconel 718 is increasingly manufactured by selective laser melting (SLM) for its excellent capability to shape complex parts. However, due to the limitation of current SLM process accuracy, for the parts with high accuracy and surface integrity requirements, subsequent process such as grinding is still necessary. SLM Inconel 718 possesses an anisotropic metallurgy microstructure that makes its grindability different from the conventionally made Inconel 718. Therefore, the grinding wheel wear and surface integrity of SLM Inconel 718 after grinding should be investigated for in-depth understanding of the correlation of SLM material property with its grindability. In this paper, inhomogeneous material properties and mechanical properties are characterized for SLM Inconel 718. Then, grinding experiments are conducted on both SLM and casting Inconel 718 for comparison. In order to investigate the effect of layer structure on the grindability and its variation trend in the whole grinding process, a step-traverse grinding mode is proposed to reserve the process status. Finally, the grinding wheel wear, power consumption, and surface integrity are measured for both samples. Furthermore, the correlation of the material properties with the grindability is analyzed, which will shed light on the process optimization for high-quality SLM Inconel 718 grinding for its reliable service in practical applications.

Developing laser beam welding parameters for the assembly of steel SLM parts for the automotive industry

PurposeThis paper aims to identify issues with joining selective laser melting (SLM) steels with conventional cold rolled steels through remote laser beam welding.Design/methodology/approachA novel approach for substituting conventional cold rolled metal sheets with SLM metal sheets, made of 316L and 18-Ni 300, is presented. The characteristics of the interaction of wrought and SLM materials are described, and joining benchmark parameters are presented and compared to known existing joining results. Finally, the joints are assessed in line with automotive specifications. This research also addresses the importance of joining technologies for the implementation of SLM as a full-fledged manufacturing technology for the automotive industry.FindingsNew parameter ranges for laser beam welding of SLM steels are defined.Research limitations/implicationsThis research is limited to the examined steels and the used machines, parameters and equipment.Practical implicationsThe presented benchmark parameters are expected to be useful for designers, product developers and machine operators.Originality/valueLittle knowledge is available about the behavior of SLM materials and their suitability for assembly processes. Novel information about SLM steels and their interaction with conventionally produced steel sheets is presented.

Classification of materials for selective laser melting by laser-induced breakdown spectroscopy

In the present work, we introduce a possibility to improve the rapid prototyping process of selective laser melting (SLM) using laser-induced breakdown spectroscopy (LIBS) which provides a material analysis. SLM uses many disparate materials for manufacturing of parts. The elemental composition of raw materials and constructed parts is obtained from a characteristic spectrum, which is a result of LIBS measurement. We compared a high-end LIBS instrumentation with a low-cost one; the latter could be easily implemented to a SLM device. The measured data were processed using multivariate data analysis algorithms. First, the principal component analysis was employed for a visualization and dimensionality reduction. Second, the reduced data set was classified using support vector machines. Moreover, we have suggested a procedure for an automatized classification of materials and parts during the SLM process without any supervision of a spectroscopy-specialist.

Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718

Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life.