Selective Laser Melting Additive Manufacturing Research Articles

An ultra-high strength martensitic steel fabricated using selective laser melting additive manufacturing: Densification, microstructure, and mechanical properties

Martensitic steels have gained renewed interest recently for their use in automotive, aerospace, and defense applications due to their ultra-high yield strengths and reasonable ductility. A recently discovered low alloy martensitic steel, AF9628, has been shown to exhibit strengths greater than 1.5 GPa with more than 10% tensile ductility, due to the formation of ε-carbide phase. In an effort to produce high strength parts with a high degree of control over geometry, the work herein presents the effects of selective laser melting (SLM) parameters on the microstructure and mechanical properties of this new steel. An optimization framework to determine the process parameters for building porosity-free parts is introduced. This framework utilizes the computationally inexpensive Eagar-Tsai model, calibrated with single track experiments, to predict the melt pool geometry. A geometric criterion for determining maximum allowable hatch spacing is also developed in order to avoid lack of fusion induced porosity in the as-printed parts. Using this framework, fully dense samples were successfully fabricated over a wide range of process parameters, allowing the construction of an SLM processing map for AF9628. The as-printed samples displayed tensile strengths of up to 1.4 GPa, the highest reported to date for any 3D printed alloy, with up to 11% elongation. The demonstrated flexibility in process parameter selection, while maintaining full density, opens up the possibility of local microstructural refinement and parameter optimization for improved mechanical properties in as-printed parts. The process optimization framework introduced here is expected to allow successful printing of new materials in an accelerated fashion.

Heat treatment temperature-induced microstructure, microhardness and wear resistance of Inconel 718 produced by selective laser melting additive manufacturing

Additive manufacturing (AM) has been becoming more widespread and its usage area has been increasing in various industries. However, the mechanical and microstructural properties of the metal parts fabricated by additive manufacturing requires further investigation. In this study, various heat treatment temperature were applied to Inconel 718 specimens fabricated by selective laser melting (SLM) to examine their effects on microstructure, microhardness and wear behavior of the specimens. Moreover, the microhardness and wear response of specimens fabricated by SLM were compared with the wrought Inconel 718. The results provide strong evidence that heat treatment temperature is capable of altering microstructural aspects of as-built specimens. This study also shows microhardness and wear resistance is remarkably induced from the final state of microstructure. Comparative study also provides evidence that Inconel 718 specimen fabricated by SLM can have much higher wear resistance than conventionally produced Inconel 718 if it is heat-treated at certain temperature.

Microstructure evolution and mechanical properties of selective laser melted bulk-form titanium matrix nanocomposites with minor B4C additions

Crack-free and almost fully dense (≥99%) bulk-form titanium matrix nanocomposites in-situ synthesized from a ball-milled mixture of Ti-6Al-4V and B4C powders were achieved through an optimized selective laser melting (SLM) process. The effects of minor B4C addition on microstructure evolution, hardness, compressive properties and fracture mechanisms of the composites were systematically investigated. The in-situ synthesized nanoscale TiBw or TiCp ceramic reinforcements, exhibiting a quasi-continuous (TMC1) or full-continuous structure (TMC2), could play a prominent role in determining the microstructure refinement and mechanical properties of the composites. Results indicate that the SLM-processed nanocomposites exhibit a maximum increase of 45% of Vickers microhardness and 26% of ultimate compressive strength over the Ti-64 alloy. The enhancement of mechanical properties is mainly attributed to the second-phase strengthening, grain refinement strengthening as well as solid solution strengthening induced by the interstitial carbon in Ti matrix. This study provides a novel solution for microstructure tailoring with minor B4C addition and producing high-performance titanium matrix composites via SLM additive manufacturing.

Post-processing effects on the surface characteristics of Inconel 718 alloy fabricated by selective laser melting additive manufacturing

This experimental study presents the effects of various post-processes including finish machining (FM), drag finish (DF) and vibratory surface finish (VSF) on surface characteristics and wear performance of Inconel 718 specimens fabricated by selective laser melting additive manufacturing. Concentrated characteristics are surface roughness, surface topography, microstructure, microhardness, XRD pattern and wear resistance. Analysis shows that partially melted powders on the surface and just beneath the surface create an undesired layer and its thickness varies from 80 to 130 µm that leads to poor surface topography and very high surface roughness. FM and DF post-processes are capable of completely removing this layer and in the meantime, up to 96 and 88% lower surface roughness. Post-processes particularly FM leads to occur strain hardening and resulted in up to 21% increase microhardness as compared to as-built material. XRD pattern of as-built and wrought alloys shows substantial difference when considering (111) and (200) planes. This study demonstrates that post-processes enhance the wear performance of SLM manufactured Inconel 718. Coefficient of friction and the wear rate of the as-built specimen decreased more than 10% after FM process.

Melt pool geometry and microstructure of Ti6Al4V with B additions processed by selective laser melting additive manufacturing

This paper documents an investigation into the microstructures and melt pool geometry features of Ti6Al4V + (0, 1, 2, 5, 10, wt%) B alloys processed by selective laser melting (SLM). Single laser-deposited tracks were made on powder-free surface of arc-melted Ti6Al4V-xB buttons. The applicability of powder-free results was supported by evaluation of Ti6Al4V melt pool geometry deposited with and without powder. For each Ti6Al4V-xB composition, melt pools were produced over wide ranges of laser beam power (P) and scan speed (V), with melt pool geometry and microstructure information gathered into mapping in P-V space to develop P-V process window. By varying wt% B and P-V parameter, a variety of microstructures were produced. A promising microstructure consisted of a TiB network with submicron spacing. Melt pool microhardness was characterized, showing evident enhancement from arc-melted baseline for all Ti6Al4V-xB composition. This work identified Ti6Al4V-xB with 2–5 wt% B as a promising composition range for SLM processing and showed the powder-free methodology can provide melt pool scale information for trial alloy composition evaluation.

Surface quality improvement by parameters analysis, optimization and heat treatment of AlSi10Mg parts manufactured by SLM additive manufacturing

The quality and reliability of selective laser melting (SLM) built components is very important. Surface quality is key quality characteristic, and it is an emergent topic for SLM products. In this paper, the influence of processing parameters such as laser power, scanning speed, overlap rate, and hatch distance of an SLM system during the fabrication of AlSi10Mg alloy parts are investigated for surface quality. The purpose of the present paper is to analyze surface quality and optimize the process parameters for quality improvement of AlSi10Mg components developed by SLM process. The effect of laser power, scanning speed, overlap rate, and hatch distance on the front face and side face surface roughness examined in as-built (AB), solution heat treatment (SHT), and artificial aging (AA). The effect of laser power on surface quality explored empirically, and ANOVA is applied to determine the best level of laser power for minimum surface roughness, standard deviation, and fluctuations. The regression analysis is performed to develop a mathematical model and used it for the optimization of processing parameters; scan speed, overlap rate, and hatch distance for achieving minimum surface roughness. Conclusively, the process parameters 0.32 kW laser power, 0.60 m/s scan speed, 35% overlap rate and 88.7 μm hatch distance determined as optimized parameters for best surface quality in as-built condition. Furthermore, the results revealed that the average surface roughness decreased by 17% after solution heat treatment.

Compressive creep of AlSi10Mg parts produced by selective laser melting additive manufacturing technology

Compressive creep properties of AlSi10Mg parts produced by additive manufacturing selective laser melting (AM-SLM) were studied using a spark plasma sintering (SPS) apparatus, capable of performing uniaxial compressive creep tests. Stress relief-treated specimens were tested under an applied stress of 100–130 MPa in the 175–225 °C temperature range. Utilizing two different configurations, the creep tests were conducted either with or without a low-density electric current (∼2.63–3.26 A/mm2) flowing through the test specimens. The results revealed that the creep rate increased under the influence of an applied electric current. The creep parameters (i.e., stress exponent n and apparent activation energy Q), were empirically determined. The stress exponent values were found to be 19.6 ± 1.2 and 16.2 ± 1.4 with and without current, respectively, while apparent activation energy was found to be 142 ± 9 kJ/mol and 150 ± 13 kJ/mol with and without current, respectively. The experimental results, together with microstructural examination of specimens, indicate that plastic deformation was controlled by dislocation activity. Furthermore, it is suggested that the annihilation process of dislocations during creep was enhanced by the electric current.

Selective laser melting additive manufacturing of pure tungsten: Role of volumetric energy density on densification, microstructure and mechanical properties

Due to the intrinsic properties of tungsten, such as high melting point and high thermal conductivity, selective laser melting of pure W parts experiences many challenges. In this study, the effects of volumetric energy density on the densification behavior, microstructure evolution and mechanical performances of SLM-processed pure tungsten parts were investigated. A maximum density of 19.0 g/cm3 (98.4% of the theoretical density) was obtained at the optimal energy density of 1000 J/mm3 and its microstructure was free of pores and balling phenomenon. The formation mechanism of pores and cracks was systematically investigated. The microhardness and compressive strength of SLM-processed pure W parts reached 474 HV and 902 MPa, respectively, which were comparable to the samples produced by conventional manufacturing methods. The morphology of fracture demonstrated that the fracture mechanism of SLM-processed pure W parts was brittle fracture and intergranular fracture was the main fracture mode. Dry sliding wear tests showed that the wear mechanism changed with the energy density. For pure W parts processed by SLM at the optimal parameters, the adhesion of hardened tribolayers was formed. In this case, the reduced coefficient of friction (COF) of 0.45 and a low wear rate of 1.3 × 10−5 mm3·N−1·m−1 were obtained.

Warping in SLM additive manufacturing processes: estimation through thermo-mechanical analysis

In additive manufacturing (AM), uneven cooling and thermal deformations deteriorate the quality of manufactured parts. However, the complexity of the non-linear systems, which are required for the modelling of the plasticity phenomena that take place, requires long simulations, a fact that renders them impractical, especially for machine control. In this study, a proof of concept for a numerical-empirical modelling approach of the thermo-mechanical phenomena of the AM process of selective laser melting (SLM) is presented. The dynamic elastic equation is solved in a two-dimensional (2D) space utilizing finite differences (FD); the solution of the coupled dynamic thermal equation is used as input. Movement of the laser heating and melting phase change are also considered. The effects of plasticity in the x-axis of the part are taken into account, utilizing a re-meshing strategy while an empirical bottom boundary condition is used for the encapsulation of the plasticity effects of the z-axis of the part. In this way, the effects of plasticity are taken into account in the proposed thermo-mechanical model, without the need to solve complex non-linear systems. The results of this study have been experimentally validated.

Influence of Particle Size on Laser Absorption and Scanning Track Formation Mechanisms of Pure Tungsten Powder During Selective Laser Melting

Abstract A three-dimensional laser absorption model based on ray tracing was established to describe the coupled interaction of a laser beam with particles in the powder layers of pure tungsten (W) material processed by selective laser melting (SLM). The influence of particle size on the powder-to-laser absorptivity and underlying absorption behavior was investigated. An intrinsic relationship between the absorption, distribution of absorbed irradiance within the powder layers, and surface morphology and geometric characteristics (e.g., contact angle, width and height of tracks, and remelted depth) of the laser scanning tracks is presented here. Simulation conclusions indicate that the absorptivity of the powder layers considerably exceeds the single powder particle value or the dense solid material value. With an increase in particle size, the powder layer absorbs less laser energy. The maximum absorptivity of the W powder layers reached 0.6030 at the particle size of 5 μm. The distribution of laser irradiance on the particle surface was sensitive to particle size, azimuthal angle, and the position of the powder particles on the substrate. The maximum irradiance in the powder layers decreased from 1.117 × 10–3 to 0.85 × 10–3 W·μm−2 and the contour of the irradiance distribution in the center of the irradiated area gradually contracted when the particle size increased from 5 to 45 μm. An experimental study on the surface morphologies and cross-sectional geometric characteristics of SLM-fabricated W material was performed, and the experimental results validated the mechanisms of the powder-to-laser-absorption behavior that were obtained in simulations. This work provides a scientific basis for the application of the ray-tracing model to predict the wetting and spreading ability of melted tracks during SLM additive manufacturing in order to yield a sound laser processability.

Laser absorption behavior of randomly packed powder-bed during selective laser melting of SiC and TiB2 reinforced Al matrix composites

In this paper, a randomly packed powder-bed model by sequential addition method was established mathematically and physically to simulate the condition of the interaction between the laser beam and powder particles. The ray-tracing calculation was used to analyze laser absorptivity and irradiance distribution occurring on powder particle surface as well as the influence of reinforcements on the laser energy absorption during selective laser melting additive manufacturing of pure AlSi10Mg, SiC/AlSi10Mg and TiB2/AlSi10Mg materials. The simulation results reveal that addition of reinforcements enhanced the interactions of laser beam with powder particles, which were illustrated by an increase of laser ray track spots on the particles surface. A respective absorptivity fluctuated from 0.37 to 0.59 and from 0.34 to 0.49 for the randomly packed SiC/AlSi10Mg and TiB2/AlSi10Mg, both of which were significantly enhanced with respective to that of AlSi10Mg powder. The addition of ceramic reinforcement particles can significantly improve the absorption irradiance of laser for powder-bed of pure AlSi10Mg, while the SiC reinforcement was more effective in enhancing this effect than TiB2. The influence of different absorption irradiance on the melt pool dimensions and surface morphologies were studied experimentally to verify the reliability of this simulation.

Selective laser melting of Mg-Zn binary alloys: Effects of Zn content on densification behavior, microstructure, and mechanical property

Mg-Zn binary alloys with seven distinctive compositions (Zn (wt.%) = 1, 2, 4, 6, 8, 10 and 12) were prepared using a selective laser melting (SLM) additive manufacturing technology. Densification behaviors and microstructure of the SLM-processed alloys were analyzed using quantitative metallography analysis, XRD, SEM, and EDS. The mechanical property characterizations were carried out through microhardness and uniaxial tensile tests. The results showed that the increase of Zn content had a significantly deteriorating effect on the densification response of the SLM-processed Mg-Zn alloys. At 1 wt% Zn content, near full-dense products could be obtained. At 2–10 wt% Zn contents, SLM-processed alloys suffered from solidification cracking and the crack density showed an approximate “Λ-shaped” variation trend with the increase of Zn content. At 12 wt% Zn content, solidification cracks were eliminated but lots of micro-pores formed within the SLM sample. All the SLM samples exhibited a duplex microstructure composed of α-Mg matrix and Mg7Zn3 eutectic phase. With the increase of Zn content, the quantity of Mg7Zn3 increased continuously whilst its morphology gradually transformed from granular shape to nearly reticular structure. The hardness and tensile tests showed that only the Mg-1Zn sample has comparable mechanical properties with the as-cast counterpart. At higher Zn contents, mechanical properties of the SLM-processed alloys became significantly degraded, mainly because of the deterioration of the densification degree.

Implant Prosthodontic Discrepancy of Complete-Arch Co-Cr Implant Frameworks Manufactured Through Selective Laser Melting Additive Manufacturing Technology Using a Coordinate Measuring Machine.

The objective was to measure the implant prosthodontic discrepancy of complete-arch implant-supported frameworks made of cobalt-chromium (Co-Cr) fabricated using selective laser melting (SLM) additive manufacturing technologies. A completely edentulous maxillary cast with seven implant replicas was obtained. Co-Cr SLM frameworks (n = 9) from three different providers (SLM-1, SLM-2, SLM-3) were manufactured. A coordinate measuring machine was selected to measure the implant prosthodontic discrepancy (μm) on the x-, y-, and z-axis and the 3D gap (3D = √x2+ y2+z2) where implants were considered as the statistical unit (n = 7). One-way analysis of variance (ANOVA), Student-Newman-Keuls, and Tukey tests were used to analyze the data (α = .05). The mean 3D implant prosthodontic discrepancy (μm) was higher for SLM-1 (73.77 ± 27.94) than for SLM-2 (47.54 ± 22.63) and SLM-3 (47.26 ± 22.57). At the x-axis, SLM-2 showed a significantly smaller gap (16.21 ± 9.6) than SLM-3 (32.92 ± 27.77) and SLM-1 (34.77 ± 21.85). At the y-axis, however, SLM-3 presented a significantly smaller gap (27.97 ± 9.49) than SLM-2 (38.84 ± 27.82) and SLM-1 (54.35 ± 29.89). Similarly, at the z-axis, SLM-3 (4.01 ± 2.29) showed the least gap, followed by SLM-2 (9.09 ± 7.63), being significantly smaller than that of SLM-1 (16.14 ± 21.09). The three SLM additive manufacturing technologies tested showed implant prosthodontic discrepancies ranging from 4.01 to 54.35 μm, which could be considered in the clinically acceptable range. Distortion at the z-axis was significantly less compared to x- and y-axes in all the groups tested.

Rapid fabrication of TiN/AISI 420 stainless steel composite by selective laser melting additive manufacturing

Although metal matrix composites (MMCs) possess high mechanical properties for wide potential applications, there are limited kinds of ex-situ MMCs fabricated via selective laser melting (SLM) until now. In this study, TiN/AISI 420 stainless steel (SS) MMCs samples were fabricated by SLM. Densification behavior, microstructure, hardness and wear performance were investigated with different TiN contents and laser powers. It was found that the addition of TiN has a strong influence on density, which was mainly attributed to its impact on laser absorptivity of powder and wettability of liquid metal. With a higher laser power, the diffusion behavior of TiN became stronger and the densification of composites was improved with a maximum relative density of 98.2%. Both of the distribution and solid-solution of TiN particles were investigated by SEM, XRD and EDS results. The optimized composites achieved a high hardness of HRC 56.7, which is 11.8% higher than that of SLMed pure AISI 420 SS. The wear performance was enhanced due to the improved hardness and TiN reinforcement particle.

Ultra-low cost Ti powder for selective laser melting additive manufacturing and superior mechanical properties associated

One of the bottleneck issues for commercial scale-up of Ti additive manufacturing lies in high cost of raw material, i.e. the spherical Ti powder that is often made by gas atomization. In this study, we address this significant issue by way of powder modification & ball milling processing, which shows that it is possible to produce printable Ti powders based on ultra-low cost, originally unprintable hydrogenation-dehydrogenation (HDH) Ti powder. It is also presented that the as-printed Ti using the modified powder exhibits outstanding mechanical properties, showing a combination of excellent fracture strength (~895 MPa) and high ductility (~19.0% elongation).

Compositional Optimization of In718 Superalloy Powder for Additive Manufacturing

In this research, a composition optimized In718 superalloy powder suitable for additive manufacturing has been developed by using the vacuum induction melting gas atomization (VIGA) and the powder sieving process. VIGA which combines the vacuum induction melting (VIM) and gas atomization (GA) processes uses high pressure inert gas to atomize the metal melt formed by VIM to form metal droplets. These metal droplets are solidified to form metal powders during the falling process in the atomized chamber. After the sieving process, the mean particle size D50 of the powder is less than 35 μm and the particle size distribution (PSD) ranges from 10 to 55 μm (D10~D90). Besides, the produced powder has high flowability (ICarr ≦15), which is suitable for selective laser melting (SLM) additive manufacturing (AM). After the SLM process, the tensile tests are conducted at room temperature and high temperature of 650°C. The results show that the high temperature properties of the optimized In718 superalloy are superior to the commercial In718 superalloy.

Effect of hot isostatic pressing on microstructure and mechanical properties of CLAM steel produced by selective laser melting

Blanket is the key energy conversion component of a fusion reactor, it is designed with a large number of complex embedded cooling channels to remove the high nuclear heat deposition. Due to the complex inner-channeled structure, the fabrication of the blanket components has been one of the key issues of the blanket technical development. In this study, the selective laser melting (SLM) additive manufacturing (AM) processing was employed to fabricate small scale test blanket module with China low activation martensitic (CLAM) steel, and the hot isostatic pressing (HIP) was applied to improve the microstructure and mechanical properties. The microstructure and mechanical properties of the SLM built CLAM at the as-built, standard heat treatment (SHT) and HIP&SHT status were investigated and evaluated. The results showed that the relative density at the as-built status compared to the hot-rolled CLAM steel reached 99.7%. The tensile strength of the as-built status was 966.2 MPa and 829.1 MPa in the directions vertical and parallel to the building direction, respectively. After the HIP&SHT, the orientation of the microstructure and the difference in strength in different directions decreased. The strength values in the directions vertical and parallel to the building direction were similar, which were equivalent to that of the hot-rolled CLAM steel with SHT. Moreover, the impact absorbed energy in the directions vertical and parallel to the building direction was increased from 21.02 J to 9.18 J at the as-built status to 54.48 J and 18.83 J at the HIP&SHT status, respectively. Meanwhile, micro-defects were drastically decreased with the help of HIP. It could be concluded that the dislocation migration and recrystallization in HIP process is effective to improve the microstructure and mechanical properties of the SLM built CLAM steel.

Selective laser melting additive manufacturing of cancer pagurus’s claw inspired bionic structures with high strength and toughness

Microstructural feature and compressive property of Cancer Pagurus’s Claw were investigated. The spirally distributed pore canal tubules perpendicular to the exoskeleton surface and the typical helicoidal architecture were observed. Selective laser melting (SLM) technology, inspired by its flexible building strategy as well as track-by-track and layer-by-layer processing mode, was applied to fabricate bionic crab claw structure by using NiTi shape memory alloy powder. The influence of rotation increments and pore distribution patterns on compressive property was analyzed. The results indicated that relatively larger rotation increment tended to result in an increase of torsional rigidity and tangential force in the interfaces between adjacent tracks, thus contributing to the greater accumulated deformation. Three different pore distribution patterns were designed, all of which significantly enhanced the toughness of SLM-fabricated NiTi parts and showed the similar energy absorption capacity. However, in consideration of the reversible martensite transformation of NiTi alloys, the helically distributed pores could induce the more accumulated deformation energy to be released during the unloading process. Besides, this kind of helical distribution pattern assisted in an enhancement of in-plane isotropy, thus resulting in the more homogenous stress distribution during the loading process.

Fragmentation and refinement behavior and underlying thermodynamic mechanism of WC reinforcement during selective laser melting of Ni-based composites

The WC-reinforced Inconel 718 composites were successfully fabricated through selective laser melting (SLM) additive manufacturing technology. Influences of the applied laser energy densities on the thermodynamics within the molten pool, fragmentation behavior of WC particles, and underlying fragmentation mechanism were investigated through experiments and simulations. The results revealed that the fragmentation behavior of WC particles was greatly dependent on the applied laser energy input. As a relatively low laser energy density was applied, the alloyed reaction layer formed around WC particles, and then experienced a fragmentation into a certain number of carbides in the vicinity of WC particles, due to the temperature gradient and resultant thermal tensile stress exerted at the interface of WC particles and molten Inconel 718 alloy. This fragmentation of WC particle was defined as the dissolution-diffusion-fragmentation mechanism. For an elevated laser energy density of 330 J/m was used, an increased temperature gradient and attendant thermal stress formed, and the initially incorporated WC particles experienced a severe heat damage, thereby directly fragmenting the WC particles into the refined pieces. Meanwhile, the molten liquid with more intense thermal convections favored the homogeneous distribution of the broken WC particles and the formation of the alloyed reaction layer between WC particles and molten Inconel 718 alloy liquid, which was predominated by the fragmentation-dissolution-diffusion mechanism.

On the role of powder flow behavior in fluid thermodynamics and laser processability of Ni-based composites by selective laser melting

Abstract A deep understanding of the underlying relation of powder paving and subsequent melting/solidification behavior is critical in predicting and tailoring the processing quality of components fabricated by selective laser melting (SLM) additive manufacturing technology based on the mesoscopic scale. Combining discrete element method (DEM) and finite volume method (FVM), the present work proposed a newly developed three-dimensional DEM physical model coupling with mesoscopic FVM model to give a unique insight into the influence of powder particle size on the flow behavior of powder and resultant melting/solidification characteristics of SLM-processed WC/Inconel 718 composites. The contacting force among powder particles, transition of solid/liquid, and surface tension of melt were considered in the model. By characterizing the packing profiles, velocity vectors and packed state of particles, the simulation results and corresponding experimental validations generally revealed that the powder possessed an elevated flow-ability as increasing the average diameter of powder below 25 μm, due to the alleviation of the domination of adhesive force among neighboring particles. The considerably high-quality SLM-processed surface without apparent laser-induced defects was obtained with a significantly low surface roughness of 2.2 μm, attributing to the formation of a denser and more homogeneous powder-bed during SLM. However, when the average diameter of powder particles was elevated over 25 μm, the combined influence of the elevated gravity of large-scaled powder and the considerable space between neighboring powder particles worsened the flow-ability of powder. The discontinuous and loose powder-bed with a low packing density of particles had a high tendency to produce the top surface with a higher surface roughness, resulting in the formation of a number of defects in SLM-processed parts such as cave-like porosities, balling and discontinuous laser-melted tracks.