Selective Melting Research Articles

Selectively modified nanostructured fluorcanasite glass-ceramic with enhanced micromechanical properties

We report for the first time, synthesis of mesoporous nano-fluorcanasite (n-FC) glass-ceramic from selective glass melting composition using controlled heat treatment and subsequent ball milling. Particle size analysis along with FE-SEM studies confirmed successful synthesis of n-FC particulates with interpenetrating lath like microstructure. XRD study established presence of fluorcanasite, frankamenite, fluorapatite, and xonotlite phases. Pore volume analysis and characteristic adsorption/desorption isotherms of n-FC particulates revealed mesoporous nature with pore diameters in the range of 3 to 15 nm. Interestingly, TEM studies also indicated polycrystalline and mesoporous nature of n-FC particulates and the corresponding ‘d’-spacing values were in good agreement with XRD result. FT-IR spectroscopy further indicated presence of characteristic bands at defined peak positions and crystalline nature of n-FC. Finally, nanoindentation studies indicated enhanced mechanical performance of synthesised n-FC. Overall, our study has established a successful route to synthesise mesoporous nano-fluorcanasite, with favouring microstructure for potential use as bioactive reinforcement.

Study of burning rate characteristics of propellants containing Al–Mg alloy nanopowder

Nanopowder of Al–Mg alloy was synthesized using the method of electrical explosion of thin wires of hypo-eutectic Al–Mg alloy. Thin wires of hypo-eutectic Al–Mg alloy were cut by electrical discharge machining and exploded electrically. The nanopowder obtained is examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for studying the morphology of the particles. TEM and SEM images substantiate the spherical structure of the nanoparticles and the diameter of particles ranges from 20 nm to 95 nm. Solid propellants were prepared by adding the alloy nanopowder as an energetic additive element with bimodal ammonium perchlorate (AP) and Hydroxyl Terminated Polybutadiene (HTPB) propellant formulations. Burning rates of the propellant with nano Al–Mg alloy powder are compared with micro aluminised propellants and propellants with micro Al–Mg powder with the same composition. Mean burn rate of 22.03 mm s−1 was obtained at a pressure level of 9 MPa with nano Al–Mg powder and 20.4 mm s−1 with micro Al–Mg powder compared to mean burning rate of 10.03 mm s−1 at 8 MPa pressure for aluminised propellants. The improved mean burn rate of propellants containing Al–Mg alloy nanoparticles could be accounted for the reduction in the size of the alloy particle and selective melting of eutectic phases along with magnesium. In the process, it gives heat feedback to the high-temperature reaction zone, which results in augmenting further reaction of metal particles.

Effects of carbon content on microstructure and mechanical properties of SiC ceramics fabricated by SLS/RMI composite process

Selective Laser Sinering (SLS) and Reactive Melting Infiltration (RMI) were combined to fabricate complicated SiC ceramics with high performance in the research. The influence of carbon content on green parts’ mechanical properties was studied. Then, the effect of carbon density on the microstructure of SiC was studied. The results showed that the density of sintered body increased first and then decreased with the increase of carbon black content. Due to the increase of carbon density, the siliconizing property of carbonated preforms is also improved by impregnating asphalt. When the carbon density was 0.6612 g/cm3, sintered density reached 2.899 g/cm3. But the flexural strength reached peak value of 190.19 ± 1.89 MPa as the carbon density decreased to 0.5542 g/cm3. Therefore, this research lays a foundation for laser sintering to fabricate high performance and complex structure SiC ceramics by regulating carbon content.

Modeling precipitation process of Al-Cu alloy in electron beam selective melting with a 3D cellular automaton model

In additive manufacturing (AM), the microstructure evolution is rather complex and plays a critical role in the mechanical performance. To predict the microstructure, many numerical models have been proposed, but solid-state phase transformations are usually not incorporated. In this work, we develop a Cellular Automaton (CA) model to simulate the solid-state phase transformation with alloy element partition. In this model, a mixed-mode phase transformation model is used, and both the growth and decomposition of precipitates are included. With the temperature profile from the validation experiment, the model well reproduces the precipitation process in the solidified Al 2024 alloy during Electron Beam Selective Melting (EBSM). The simulation results are in good agreement with the experimental results and illustrate the causes of some critical morphological features. The impact of temperature profile on the precipitation process is studied and the results indicate: (1) the rising of the powder-bed temperature during the fabrication process might cause precipitate decomposition; (2) the precipitates are evenly distributed and their sizes are uniform when the powder-bed temperature is low.

Selective electron beam melting of high strength Al2024 alloy; microstructural characterization and mechanical properties

Abstract To date, the most commonly used aluminum alloys for additive manufacturing are eutectic/near-eutectic alloys. However, fabricating high strength aluminum parts has not yet been fully investigated due to solidification cracking and porosity formation, which make these alloys very hard to be processed. Electron beam selective melting (EBSM) of aluminum alloys has some advantages over selective laser melting. The process is not affected by reflectivity of aluminum powders, and parts exhibit less induced thermal stresses. In addition, oxidation is reduced due to vacuum condition. Therefore, the process provides high potential for the fabrication of aluminum alloys, specifically, Al2024 alloy which is widely used in different industries such as automotive and airline industries. Hence, these industries are highly benefitted from being able to build defect-free AM parts. This study is significant because Al2024 alloy is highly susceptible to solidification cracking and porosity formation, and crack-free samples with high relative densities have not been built by EBM process previously. The baseplate preheating temperature was set to 350 °C, and samples were fabricated by applying a proper powder bed preheating configuration for each layer. Different sets of processing parameters were used to investigate their effect on the performance of the parts. Archimedes technique was used to measure the relative density of samples, ranging from 95% to full-dense. The results showed that higher relative densities could be achieved by increasing the input energy to an optimum value; however, by further increasing the energy, the relative density decreases, which can be attributed to porosity formation. SEM and EBSD results showed equiaxed grains in a crack-free microstructure. Grain structure consisted of high angle grain boundaries having different crystallographic orientations with a few sub-grains. Additionally, AlCuMnFe, AlCuMnFeSi, and eutectic Al2Cu phases were uniformly distributed in the α phase matrix. Microhardness results showed an almost uniform change of hardness values in both horizontal and vertical sections, ranging from 100 HV to 110 HV for as-built samples. Moreover, tensile and yield strengths of as-built samples reached 314 MPa and 191 MPa, respectively, which can be further increased by a proper post-processing treatment.

Properties of Laser Additive Deposited Metallic Powder of Inconel 625

Abstract Paper presents results of laser additive manufacturing. Deposition of nickel based super alloy Inconel 625 was performed. Laser metal deposition is advanced manufacturing process dedicated for prototyping and low scale series production. Inconel 625 is nickel based super alloy, with high heat resistance properties. Therefore due material properties and chemical composition is characterized as a difficult to machining [1, 2]. Additive manufacturing process using focused photons beam for selective deposition of metallic powder in laser engineered net shaping (LENS) method can be used as alternative technology. High energy density of controllable laser beam combining with coaxial delivery system allow to precise deposited metallic powder. Manufacturing process are based on selective melting of additional material using laser radiation and crystallization process. An additional material in form of filler wire as well as metallic powder can be used. Advantages of using metallic powder are higher level of process control, nevertheless adequate selection of process parameters are required. High energy density of laser beam and rapid crystallization process affect on metallographic structure of deposited material. Thermal energy absorbed in material affect on phase transformation.Molten powder mixing with base material changing metallographic structure. Chemical composition of obtained overlay weld are combination of base and additive material. Therefore to achieve stable crystallization process chemical composition of additive material wassimilar to base material. Additional alloying elements could affect on mechanical properties. Deposition process using TruLaserCell 1005 laser machine was performed. To determine properties of manufactured material metallographic analysis and destructive tests were performed.

Development of Domestic Equipment for Laser Additive Technologies by Melting Metallic Powders

The modern domestic installations used for the additive laser technologies based on layer-by-layer melting of metal powders are considered. An installation was developed and manufactured for the selective melting of metal powders by laser radiation scanned along a given contour. The installation is equipped with an automatic control system and makes it possible to grow 110 × 110 × 110 mm parts with an accuracy of 20 μm and a growth rate up to 10 cm3/h. A KLP-400 installation was created for growing large-sized parts. The principle of operation of the installation is based on layer-by-layer laser facing with metal powders supplied coaxially with laser radiation. The installation can be used to produce parts 400 × 400 × 400 mm in size and about 500 kg in weight. The accuracy of the manufactured parts is 0.1–0.5 mm.

Modeling and Monitoring of the Effect of Scan Strategy on Microstructure in Additive Manufacturing

Laser powder bed fusion is an additive manufacturing process that uses a selective melting technique to build three-dimensional metal structures. This fabrication technique is useful for creating complex parts with fine resolution. However, the repeated scanning method used throughout the process results in complex thermal behavior, which introduces microstructural variations and defects in the final part. The semi-analytical model used in this work calculates the temperature distributions and thermal conditions in laser powder bed fusion to predict the microstructural variations. Additionally, an in situ process monitoring approach is used to verify the predicted influence of the processing parameters and scan strategy on the microstructure of the printed parts.

Further Studies into Crack Growth in Additively Manufactured Materials.

Understanding and characterizing crack growth is central to meeting the damage tolerance and durability requirements delineated in USAF Structures Bulletin EZ-SB-19-01 for the utilization of additive manufacturing (AM) in the sustainment of aging aircraft. In this context, the present paper discusses the effect of different AM processes, different build directions, and the variability in the crack growth rates related to AM Ti-6Al-4V, AM Inconel 625, and AM 17-4 PH stainless steel. This study reveals that crack growth in these three AM materials can be captured using the Hartman–Schijve crack growth equation and that the variability in the various da/dN versus ΔK curves can be modeled by allowing the terms ΔKthr and A to vary. It is also shown that for the AM Ti-6AL-4V processes considered, the variability in the cyclic fracture toughness appears to be greatest for specimens manufactured using selective layer melting (SLM).

Spattering effects during selective laser melting

Additive manufacturing using preplaced powder became a widely used processing method during the last few years to build complex and precise structures. The laser beam is often used as a heat source to selectively melt the powder and create the structure layer by layer. However, during the processing, imperfections can occur. The spattering, accumulation of powder particles, and denudation effects lead to a redistribution of the powder in the powder bed, a loss of powder, and agglomerations. These effects can alter the processing of the subsequent tracks and layers. On the one hand, the track dimensions can vary by varied amounts of available powder. On the other hand, the process can become unstable due to different powder surface geometries, agglomerates, and the resulting variation of energy absorption. Therefore, this work aims to explain spattering effects during powder-bed processing, which can be used for modification of the processing parameters for spatter prevention. High-speed-imaging was used to record the powder and spatter behavior during the laser powder-bed processing from different angles. It was observed that different kinds of spattering can occur. Melt pool spattering occurs due to melt instabilities, while powder particles seem to be accelerated by vapor induced gas movement, pressure waves, and the gas flow induced by the Bernoulli-effect.

Multiphysics Simulation of Nucleation and Grain Growth in Selective Laser Melting of Alloys

Abstract Selective laser melting (SLM) builds parts by selectively melting metallic powders layer by layer with a high-energy laser beam. It has a variety of applications in aerospace, medical device, and other low-volume manufacturing. Nevertheless, the lack of fundamental understanding of the process-structure-property relationship for better quality control inhibits wider applications of SLM. Recently, a mesoscale simulation approach, called phase field and thermal lattice Boltzmann method (PF-TLBM), was developed to simulate microstructure evolution of alloys in SLM melt pool with simultaneous consideration of solute transport, heat transfer, phase transition, and latent heat effect. In this paper, a nucleation model is introduced in the PF-TLBM framework to simulate heterogeneous nucleation at the boundary of the melt pool in SLM. A new method is also developed to estimate the thermal flux out of the SLM melt pool model given a constant cooling rate. The effects of latent heat and cooling rate on dendritic morphology and solute distribution are studied. The simulation results of AlSi10Mg alloy suggest that the inclusion of latent heat is necessary because it reveals the details of the formation of secondary arms, reduces the overestimation of microsegregation, and provides more accurate kinetics of dendritic growth.

Computer-aided electron-beam deflection control system

The electron beam processes – 3D selective melting (3D printing), welding, etc. are complex processes with many processing variables, which require integration of interfaces for control of the processes on different levels through hierarchical equipment models, functional data and operation control models. We developed a user interface software application for controlling the electron-beam motion along several types of trajectories and a free complex shape trajectory by controlling the deflection system parameters. Several types of motion patterns with different velocities are realized, together with the possibility of choosing a free trajectory by defining the electron-beam path by a graphical image.

Characterization of interfacial transition zone of functionally graded materials with graded composition from a single material in electron beam powder bed fusion

Abstract Electron beam selective melting (EBSM), is powder bed fusion additive manufacturing (AM) technique by adding materials layer upon layer. The capacity of EBSM to fabricate complex and multi-functional parts has made it a realm of great interest for researchers in the domain of science. However, during the fabrication of functionally graded materials (FGMs) with graded composition by EBSM using at least two different materials, it is difficult to separate and reuse these materials after the fabrication. Additionally, it is difficult to vary the powder composition within a layer to fabricate a horizontal graded structure. In this research, an element evaporation concept based on electron beam powder bed fusion, is presented for fabricating one-dimensional (1D) FGMs of (α2 + γ) TiAl alloy/(α + β) titanium alloy from a single material powder. By irradiating different sub-regions of the powder bed with electron beam energy input of different magnitude to greatly reduce the relative atomic ratio of aluminum from 47% to less than 10%, the 1D FGMs structures with (α2 + α + β) phases in the interfacial transition zone (ITZ) were successfully produced. Since the microstructure of the ITZ has a significant effect on mechanical properties for the 1D FGMs, three different overlap scan distances (0.5, 1.0 and 2.0 mm), which result in different ITZs, were utilized for evaluating the microstructure of the ITZ and resulting mechanical properties. The results showed that no defects were found in the specimens of any overlap distance. Moreover, the width of the ITZ increases as the overlap scan distance, and the tensile strength decreases with the overlap scan distance.

Additive manufacturing of composite materials and functionally graded structures using selective heat melting technique

The feasibility of using selective heat melting (SHM) to fabricate composite materials and functionally graded structures was investigated. We report, for the first time, the successful 3D printing of copper (Cu)-polyethylene (PE) composite, iron (Fe)-polyethylene (PE) composite and functionally graded CuO foams using the SHM technique. It was found that a low feed rate, high airflow rate and high airflow temperature were required for efficient delivery of heat from the emitted hot air to the powder bed, so that the PE binder particles can melt and form dense composites with smooth surfaces. The best mechanical properties were exhibited by composites with 80 vol.% PE, as lower PE concentrations led to deficient binding of the metal particles, while higher PE concentrations meant that very few metal particles were available to strengthen the composite. The strength exhibited by Cu-PE composites was comparable to engineering plastics such as polycarbonate, with the added advantage of being electrically conductive. The average conductivity of the samples, 0.152 ± 0.28 S/m, was on par with physically cross-linked graphene assemblies. By subjecting a Cu-PE composite, with Cu concentration graded from 10 vol.% to 30 vol.%, to a high temperature debinding and sintering treatment in air, CuO foam with graded porosity can be obtained. This CuO foam was observed to fail in a layer-by-layer manner under mechanical compression, which is a characteristic of functionally graded materials. Our study shows that, compared to existing 3D printing techniques, SHM can be cheaper, have wider material compatibility, occupy a smaller footprint and potentially induce less residual stresses in the fabricated parts. Therefore, it could be a valuable complement to current additive manufacturing techniques for fabricating mechanically strong composite materials and functionally graded structures.

Microstructural evolution and EBSD analysis of AlSi10Mg alloy fabricated by selective laser remelting

This study characterizes microstructural evolution and texture orientation of AlSi10Mg components through selective laser melting (SLM) by adopting different strategies during remelting. SLM selectively melts the powders and solidified layer, generating a moving melt pool and resulting in unique microstructure. The microstructural evolution, grain sizes and the texture orientation of the fabricated samples were investigated using different scanning strategies by optical microscopy, scanning electron microscopy, and electron back-scattered diffraction. Experimental results show that the inter-connected net shape of the AlSi eutectic structure tends to be widen as a result of the increase of the Si precipitation phase. The grain sizes become smaller and produce more subgrains effected by high thermal conductivity of the solidified layer. The average hardness and Young's modulus increased to 2.32 GPa and 93.26 GPa, respectively. When the remelting scanning direction is rotated to 90°, the cellular microstructure comes into domination. Due to the remelting processing, the preferential grain growth orientation is 〈100〉 both along the scanning and building direction, and the crystallographic texture is enhanced. Results show distinct epitaxial growth along the heat flow with <100> orientation can be obtained and hence isotropy is strengthened.

Assessment of trapped powder removal and inspection strategies for powder bed fusion techniques

The issue of trapped powder within a part made using powder bed fusion additive manufacturing (AM) is one of the ‘dirty secrets’ of AM, yet it has not received significant attention by the research community. Trapped powders limit the application of AM for complex geometries, including heat exchangers and dies with conformal cooling channels. Being able to detect and remove trapped powder from the build is a necessary step to avoid downstream processing and performance challenges. In this work, ‘powder challenge geometries’ with complex internal features were fabricated via laser powder bed fusion (L-PBF) and electron beam selective melting (EBSM) and were used to assess the effectiveness of several powder removal and inspection methods. Hand-held ultrasonic polishing was explored as a powder removal technique and was shown to effectively clear extremely elongated channels that grit-blasting (the current industry standard) cannot clear. X-ray computed tomography (XCT) and weighing were used to inspect and quantitatively assess the effectiveness of powder removal techniques on the challenge geometries. Using the lesser known ‘vacuum boiling’ powder removal process and the more common ultrasonic bathing process, trapped L-PBF powder was easily removed from the deep channels. Conversely, trapped EBSM powder was difficult to remove using ultrasonic polishing as the powder was sintered inside the channels. It was shown that the powder recovered by the ultrasonic polishing process had size distributions, surface chemistry, morphology and porosity similar to the virgin powder. It is suggested, on these bases, that the recovered powder could likely be recycled without detrimental effects on the process operation.

The effect of short glass fibers on the process behavior of polyamide 12 during selective laser beam melting

In additive manufacturing, polymer composites are used for setting tailored properties. Short glass fibers can be used as fillers for polyamide 12 for enhancing stiffness or tensile strength as well as for reducing shrinkage. In this paper, the effects of short glass fibers on polyamide 12 concerning powder properties, process behavior and part properties in laser beam melting of polymers (SLS) are investigated. It could be shown that by increasing the short glass fiber content powder properties as well as part properties are immensely affected. By adding glass fibers, powder properties, like flowability and diffuse reflection decrease. The isothermal crystallization changes resulting in a narrower processing window. Concerning mechanical properties, short glass fibers allow for a higher stiffness until a critical limit of filler concentration within this study is reached, after which the tensile strength decreases. The elongation of break decreases by rising the filler content.

The Influence of Selective Laser Melting Process Parameters on the Structure and Properties of Products Made from Metal Powders of Domestic Manufacture

Rational selective laser melting modes have been established for the manufacture of parts from domestic powder of steel grade 20H13. The properties of PR20H13 powder have been studied using the methods of granulometric analysis. The physical and mechanical properties of samples manufactured using rational modes of selective melting have been determined.

A comparison of material models for the simulation of selective beam melting processes

The focus of this contribution lies on the comparison of different material models for the thermo-mechanical simulation of selective laser melting (SLM) processes. A Finite Element (FE) framework for the analysis of residual stresses and distortions in laser beam melting processes is introduced. The FE solver for the thermo-mechanical problem uses a staggered algorithm, is based on the open source finite element library deal.ii and makes use of heavy parallelization. The most important aspects to be investigated here are the mechanical material behavior and stress relaxation in the range of the melting temperature. Material models, which reset certain variables as stress and plastic strain to zero at a critical temperature are compared to a thermo-viscoplastic constitutive law. Simulations, related to the SLM process of Ti-6Al-4V, are performed to compare the residual stresses and plastic strains predicted by different models after melting and cooling. Furthermore, the validity of geometrically linearized material model is analyzed.

Development of additive manufacturing method by selective melting using micro arc plasma

Metal additive manufacturing has been developed rapidly for its capability to produce metallic objects with complex structure directly. In metal additive manufacturing, the heat source is an important factor determining forming properties, and laser, electron beam and arc plasma are used as main heat sources currently. However, some problems should be noted. When laser is used as heat source, the forming of some powder materials, such as copper alloys and aluminum alloys, is difficult due to their high reflectivity. For the case of electron beam, vacuum environment is necessary resulting in high cost. As for current arc additive manufacturing, it is difficult to achieve high accuracy as a result of high energy and wire material used. This paper developed a new method for metal additive manufacturing. In this method, voltage is applied between the tool electrode and metal powder bed to cause the breakdown of the dielectric forming the micro arc plasma, which is adequate to melt the metal powder, and parts can be formed layer by layer. With this method, the better accuracy can be obtained by means of smaller current and powder bed. In addition, the plasma can be generated easily with less environmental requirements making lower cost. In this paper, the forming principle of this method was studied, and the prototype device was set up. The single point and single pass discharge melting experiments were carried out, and the influence of processing parameters on forming quality was investigated. The feasibility of this method can be verified.