Laser-assisted Additive Manufacturing Research Articles

Effects of build orientation and heat treatment on microstructure and properties of Cu–Cr–Zr alloy manufactured by laser powder bed fusion

Laser powder bed fusion was utilized to manufacture Cu–1Cr-0.19Zr alloys with varied build orientations (0°and 90°). The build orientation affected the grain orientation and texture of the studied alloy. Cu–1Cr-0.19Zr alloy manufactured in the 0° orientation exhibited an intensive <110> texture. Aging the as-fabricated alloy reduced dislocations and dislocation cells and led to the formation of recrystallized grains and face-centered-cubic structured Cr precipitates (2–7 nm). The mechanical properties and conductivity of the Cu–1Cr-0.19Zr alloy increased with the aging time. The conductivity enhancement was ascribed to the decrease of defect, and impurity scattering. Meanwhile, texture led to the anisotropy of the mechanical property in the vertical and horizontal planes. The optimal aging temperature and holding time were 500 °C and 30 min, the microhardness, electrical conductivity, and yield strength (compression) reached 220.6 HV, 62.1% IACS (International Annealed Copper Standard), and 540 MPa. The softening temperature of the peak-aged Cu–1Cr-0.19Zr alloy was 580 °C, which was enhanced by dispersed Cr precipitates. These findings guide to fabrication of Cu–Cr–Zr alloys by laser-assisted additive manufacturing.

Pulsed laser-assisted additive manufacturing of Ti-6Al-4V for in-situ grain refinement

Metal additive manufacturing (AM) enables rapid customization of complex parts. However, it leads to forming of columnar grain structures which give the AM parts anisotropic properties. In this study, we propose a pulsed laser-assisted AM (PLAAM) technique for in-situ grain refinement of Ti-6Al-4V parts. A nanosecond pulsed laser was focused onto a melt pool to generate a favorable environment for the promotion of fine equiaxed grains. The PLAAM technique provided an average prior-β grain size of 549.6 μm, compared to that of 1297 μm provided by the conventional AM technique. Moreover, the maximum value of multiples of uniform distribution of the β phase decreased from 16 to 7.7 when using the PLAAM technique, which indicates a weakened crystallographic texture. These changes confirm that the proposed PLAAM technique promotes finer and more equiaxed prior-β grains. Furthermore, because the proposed technique is a non-contact technique, it can be applied to existing processes without adjusting tool paths.

Interfacial thermal conductance of 7075 aluminum alloy microdroplets in contact with a solid at fast melting and crystallization

Ultrafast nanocalorimetry, in combination with high-speed IR thermography, is used to measure the interfacial thermal conductance (ITC) of the thermal contact of metal microdroplets with a solid during fast melting (including laser heating). IR thermography and membrane nanocalorimetry were used to measure the temperature difference at the membrane/sample interface during the melting and crystallization of aluminium alloy (AA7075) microdroplets (20 μm in diameter) over a wide range of heating and cooling rates (up to 105 K s−1). This is the first time ITC has been measured at such high heating and cooling rates with this new method. We found that the interfacial temperature difference reaches about 80 K during the solidification of microdroplets during laser heating. This result is significant for understanding various industrial laser-assisted processes. It has been established that ITC measured for AA7075 microdroplets gradually increases by an order of magnitude during melting in the range from the solidus temperature to the liquidus temperature of the alloy. This unusual behavior of ITC during melting can be important for understanding and optimizing laser-assisted additive manufacturing processes.

Literature survey to the materials used in laser-assisted additive manufacturing processes for the production of nuclear materials

Literature survey to the materials used in laser-assisted additive manufacturing processes for the production of nuclear materials

Selective laser sintering in reactive atmospheres: Towards in-situ synthesis of net-shaped carbide and nitride ceramics

Selective laser reaction sintering techniques (SLRS) were investigated to produce near net-shape non-oxide ceramic materials that were previously incompatible with powder-bed fusion additive manufacturing (AM) methodologies. Specifically, reaction-bonded layers of carbide and nitride materials (Cr3C2 and CrN) were fabricated using SLRS from a model system of Cr and Cr2O3 precursor materials. Varied particle sizes (<5–45 µm) were simultaneously chemically-converted and reaction bonded into layers during selective laser processing in 100 vol% CH4 or NH3 gas. For each precursor material, factors such as particle size, chemistry, optical properties, and microstructure were investigated using quantitative phase characterization and microstructural imaging to understand important aspects of carbide and nitride phase formation within an AM context. SLRS processing of metal or metal oxide precursor systems produced yields up to 100 wt% total for carbide formation and 100 wt% total for nitrides during reactive processing. However, when either metal or metal oxide precursor was processed by itself, the simultaneous synthesis and reaction bonding approach resulted in volumetric changes during in-situ gas-solid conversion that adversely affected layer microstructure. To circumvent the internal stresses that accompanied volumetric changes induced from conversion of the metal (which expands during conversion) and the metal oxide (which contracts), we demonstrate an alternative method for the production of near-net shape carbide and nitride materials for AM. By combining optimized ratios of metal/metal oxide precursors for processing in CH4 or NH3 reactant gas, the novel precursor formulations produced undistorted, crack-free refractory ceramic layers with sub-millimeter spatial resolution and viable layer thickness for powder bed fusion-AM. The results not only demonstrate the formation of near net-shape carbides and nitrides using AM-compatible techniques but suggest that transition metal carbide and nitrides may be compatible with AM processing even though they cannot be processed using more traditional laser-assisted AM approaches.

Variations of interfacial thermal conductance at melting and crystallization of an indium micro-particle in contact with a solid

The heat transfer between a micrometer-sized particle, as used in laser-assisted additive manufacturing, and a substrate strongly depends on the variations in interfacial thermal conductance (ITC), among other thermophysical parameters of the sample and substrate. Variations of ITC were measured on a millisecond time scale during fast melting and crystallization of an indium micro-sized sample with a mass of about 1 ng. Fast scanning nanocalorimetry was simultaneously applied with high-speed infrared thermography. The ITC between the sample and the sensor membrane was obtained. Therefore, the evolution of the temperature difference at the sample/membrane interface during fast melting and crystallization of the micro-sized indium droplet was measured. An unusual effect of gradual variation of ITC in the melting experiment was found. The method can be applied for ITC measurements during fast processes with 0.26 ms resolution. The quantification of ITC allows, for example, the optimization of melting during additive manufacturing.

Simulations of Laser Assisted Additive Manufacturing by Smoothed Particle Hydrodynamics

In this paper, we present a 3D Smoothed Particle Hydrodynamics (SPH) model with powder resolved to simulate complex Laser Assisted Additive Manufacturing (LAAM) processes. To the best knowledge of the authors, this is the most comprehensive and complete SPH model for LAAM simulations to date. Major and important underlying physical processes are considered in the model. These include the transient powder–laser interaction, heat transfer, formation and dynamics of the melt-pool, powder-melt-pool interaction, and the phase change. The model is validated for many individual processes. These validation results show that the model is stable, accurate and promising for simulations of complete LAAM processes. The SPH model is demonstrated for simulations of two common LAAM processes: Selective Laser Melting (SLM) and Direct Metal Deposition (DMD).

Development of metal matrix composites by laser-assisted additive manufacturing technologies: a review

Metal matrix composites (MMCs) generally possess superior properties than the monotonic matrix alloys, and thus, they have become excellent candidate materials in various applications. Also, the ability of property tailoring at an affordable cost is of particular importance to industries. Among the many manufacturing techniques for MMCs, laser-assisted additive manufacturing (AM) techniques have emerged and drawn increasing attention in the past decade. In the literature, a wealth of studies have been carried out on the synthesis of MMCs via laser-assisted AM techniques, as well as the property evaluation of the obtained MMCs. In this paper, we review and analyze the relevant literature and summarize the material preparation, optimization of process parameters, resultant improvements, and corresponding strengthening mechanisms for each major category of MMCs. Moreover, the limitations and challenges related to MMC synthesis using the laser-assisted AM techniques are discussed, and the future research directions are suggested to address those issues.

Texture control of Inconel 718 superalloy in laser additive manufacturing by an external magnetic field

Texture control is a major challenge in laser-assisted additive manufacturing of metal part due to the focused heat source and rapid cooling. In this study, a novel magnetic-field-assisted laser additive manufacturing method is proposed to obtain highly oriented texture. Inconel 718 samples are deposited under a transverse magnetic field of various intensities and different levels of laser power. The elemental distribution, grain morphology, and texture of as-built material are carefully analyzed using various material characterization techniques, including, SEM, EDS, and EBSD. The results indicate that the magnetic field changes the interdendritic microsegregation pattern from a continuous chain to discrete morphology. The direction of deposited material tends to align with the transverse magnetic field. The orientation of stray crystals is more strictly constrained by both vertical heat flux and transverse magnetic field, resulting in a highly oriented texture on horizontal cross-section and texture on the vertical cross-section. It is also observed that by applying the transverse magnetic field, the average grain boundary misorientation angle significantly decreases from 32° to 19°, and the maximum multiple of uniform density increases from 3.0 to 5.3. The effects of laser power as well as magnetic field strength are investigated, and the underlying orientation effect is discussed.

Laser-Assisted Additive Manufacturing of mm-Wave Lumped Passive Elements

The performance of additively manufactured microwave passive elements and interconnects continues to improve as better materials, and process techniques are developed. In this paper, laser-enhanced direct print additive manufacturing (LE-DPAM) is used to fabricate capacitors and inductors for coplanar waveguide (CPW) circuits. Acrylonitrile butadiene styrene is used as the dielectric that is printed using fused deposition modeling, and DuPont CB028 conductive paste is deposited using microdispensing. These two techniques are combined with picosecond-pulsed laser machining to achieve $\simeq 12$ - $\mu \text{m}$ slots on printed conductors, producing aspect ratios greater than 2:1. The same laser is used to fabricate vertical interconnections that allow for the fabrication of multilayer inductors. Inductances in the range of 0.4–3 nH are achieved, with a maximum quality factor of 21, selfresonance frequencies up to 88 GHz, and an inductance per unit of area of 5.3 nH/mm2. Interdigital capacitors in the range of 0.05–0.5 pF are fabricated, having a maximum quality factor of 1000, a capacitance per unit area of 1 pF/mm2, and selfresonances up to 120 GHz. All the components are made on the center line of a CPW that is 836- $\mu \text{m}$ wide. The results show that LE-DPAM enables the fabrication of compact passive circuits that can be easily interconnected with MMIC dies, which at the same time can be manufactured as part of a larger component. This enables the fabrication of structural electronics that are functional into the millimeter-wave frequency range. Bounds on the inductances and capacitances per unit area are related to material and geometry limitations.

Damage reconstruction from tri-dexel data for laser-aided repairing of metallic components

The laser-assisted additive manufacturing process for component repair requires the repair volume to generate the tool path for building up specific material in the worn area. This paper introduces a damage reconstruction algorithm benefits from tri-dexel modeling. At first, nominal and damaged models were acquired through robot-aided 3D scanning process. Then, damaged models were aligned with nominal models by aligning the associated features using the least-squares method. The area covering the defective region was manually selected, and the minimum bounding box of the area was defined and subsequently sliced into a number of grids according to a user-defined grid interval. After that, rays were projected from each grid node in three orthogonal directions to intersect the selected region of nominal and damaged models. Point set in the damaged zone was extracted by comparing intersections of rays with nominal and damaged models. STereoLithography model of damage was reconstructed based on extracted point cloud using Screened Poisson Surface Reconstruction algorithm. Reconstructed damage was compared with real damage to test the accuracy of the damage reconstruction process. Regained damage was sliced into layers to generate the tool path for material deposition. Repair experiments were conducted to deposit materials in the defective area. Illustrating examples were implemented at last to test the functionality and reliability of the proposed methodology.

Investigation of Porosity and Mechanical Properties of Graphene Nanoplatelets-Reinforced AlSi10 Mg by Selective Laser Melting

Graphene possesses many outstanding properties, such as high strength and light weight, making it an ideal reinforcement for metal matrix composite (MMCs). Meanwhile, fabricating MMCs through laser-assisted additive manufacturing (LAAM) has attracted much attention in recent years due to the advantages of low waste, high precision, short production lead time, and high flexibility. In this study, graphene-reinforced aluminum alloy AlSi10 Mg is fabricated using selective laser melting (SLM), a typical LAAM technique. Composite powders are prepared using high-energy ball milling. Room temperature tensile tests are conducted to evaluate the mechanical properties. Scanning electron microscopy observations are conducted to investigate the microstructure and fracture surface of obtain composite. It is found that adding graphene nanoplatelets (GNPs) significantly increases porosity, which offsets the enhancement of tensile performance as a result of GNPs addition. Decoupling effort is then made to separate the potential beneficial effects from GNPs addition and the detrimental effect from porosity increase. For this purpose, the quantitative relationship between porosity and material strength is obtained. Taking into consideration the strength reduction caused by the increased porosity, the strengthening effect of GNPs turns out to be significant, which reaches 60.2 MPa.

Application of cellular automata and Lattice Boltzmann methods for modelling of additive layer manufacturing

PurposeThe holistic numerical model based on cellular automata (CA) and lattice Boltzmann method (LBM) are being developed as part of an integrated modelling approach applied to study the interaction of different physical mechanisms in laser-assisted additive layer manufacturing (ALM) of orthopaedic implants. Several physical events occurring in sequence or simultaneously are considered in the holistic model. They include a powder bed deposition, laser energy absorption and heating of the powder bed by the moving laser beam, leading to powder melting or sintering, fluid flow in the melted pool and flow through partly or not melted material, and solidification. The purpose of this study is to develop a structure of the holistic numerical model based on CA and LBM applicable for studying the interaction of the different physical mechanisms in ALM of orthopaedic implants. The model supposed to be compatible with the earlier developed CA-based model for the generation of the powder bed.Design/methodology/approachThe mentioned physical events are accompanied by heat transfer in solid and liquid phases including interface heat transfer at the boundaries. The sintering/melting model is being developed using LBM as an independent numerical method for hydrodynamic simulations originated from lattice gas cellular automata. It is going to be coupled with the CA-based model of powder bed generation.FindingsThe entire laser-assisted ALM process has been analysed and divided on several stages considering the relevant physical phenomena. The entire holistic model consisting of four interrelated submodels has currently been developed to a different extent. The submodels include the CA-based model of powder bed generation, the LBM-CA-based model of heat exchange and transfer, the thermal solid-liquid interface model and the mechanical solid-liquid interface model for continuous liquid flow.Practical implicationsThe results obtained can be used to explain the interaction of the different physical mechanisms in ALM, which is an intensively developing field of advanced manufacturing of metal, non-metal and composite structural parts, for instance, in bio-engineering. The proposed holistic model is considered to be a part of the integrated modelling approach being developed as a numerical tool for investigation of the co-operative relationships between multiphysical phenomena occurring in sequence or simultaneously during heating of the powder bed by the moving high energy heat source, leading to selective powder sintering or melting, fluid flow in the melted pool and through partly (or not) melted material, as well as solidification. The model is compatible with the earlier developed CA-based model for the generation of the powder bed, allowing for decrease in the numerical noise.Originality/valueThe present results are original and new for the study of the complex relationships between multiphysical phenomena occurring during ALM process based on selective laser sintering or melting, including fluid flow and heat transfer, identified as crucial for obtaining the desirable properties.

Selective laser melting of copper using ultrashort laser pulses

Within the field of laser-assisted additive manufacturing, the application of ultrashort pulse lasers for selective laser melting came into focus recently. In contrast to conventional lasers, these systems provide extremely high peak power at ultrashort interaction times and offer the potential to control the thermal impact at the vicinity of the processed region by tailoring the pulse repetition rate. Consequently, materials with extremely high melting points such as tungsten or special composites such as AlSi40 can be processed. In this paper, we present the selective laser melting of copper using 500 fs laser pulses at MHz repetition rates emitted at a center wavelength of about 1030 nm. To identify an appropriate processing window, a detailed parameter study was performed. We demonstrate the fabrication of bulk copper parts as well as the realization of thin-wall structures featuring thicknesses below 100 $${\upmu }$$ m. With respect to the extraordinary high thermal conductivity of copper which in general prevents the additive manufacturing of elements with micrometer resolution, this work demonstrates the potential for sophisticated copper products that can be applied in a wide field of applications extending from microelectronics functionality to complex cooling structures.

Surface-active element transport and its effect on liquid metal flow in laser-assisted additive manufacturing

During laser-assisted additive manufacturing, the transport phenomena, solidification behavior, and melt pool geometry are affected by base-metal sulfur content and oxygen present in the atmosphere. The role of these surface-active elements during metal-based additive manufacturing is not well understood. In this study, the mass transport of sulfur and its effect on liquid metal convection (Marangoni flow) are examined by using an improved 3D transient heat transfer and fluid flow numerical model. An improved surface tension model is proposed to take into account the influence of sulfur content and temperature at the surface of melt pool. The sulfur mass transport equation is combined with the other conservation equations. The results show that the sulfur redistribution leads to transitional Marangoni flow. The powder addition into the melt pool results in the dilution of sulfur in the melt pool. Increase in mass flow rate and energy input results in decreased temperature coefficient of surface tension. When temperature coefficient of surface tension changes sign from positive to negative, the flow motion transition from inward flow to outward flow occurs. Two predominant types of flow pattern lead to two types of fusion boundaries of deposited track.

Selective Laser Melting of Graphene-Reinforced Inconel 718 Superalloy: Evaluation of Microstructure and Tensile Performance

Graphene nanoplatelets (GNPs) have many outstanding properties, such as high mechanical strengths, light weight, and high electric conductivity. These unique properties make it an ideal reinforcement used for metal matrix composites (MMCs). In the past few years, many studies have been performed to incorporate GNPs into metal matrix and investigate the properties of obtained metal matrix composites. Meanwhile, fabrication of MMCs through laser-assisted additive manufacturing (LAAM) has attracted much attention in recent years due to the advantages of low waste, high precision, short production lead time, and high workpiece complexity capability. In this study, the two attractive features are combined to produce GNPs reinforced MMC using selective laser melting (SLM) process, one of the LAAM processes. The target metal matrix material is Inconel 718, a nickel-based Ni–Cr–Fe austenitic superalloy that possesses excellent workability and mechanical performance, and has wide applications in industries. In the experiment, pure Inconel 718 and GNPs reinforced Inconel 718 composites with two levels of GNPs content (i.e., 0.25 and 1 wt. %) are obtained by SLM. Note that before the SLM process, a novel powder mixture procedure is employed to ensure the even dispersion of GNPs in the Inconel 718 powders. Room temperature tensile tests are conducted to evaluate the tensile properties. Scanning electron microscopy (SEM) observations are conducted to analyze the fracture surface of materials and to understand the reinforcing mechanism. It is found that fabrication of GNPs reinforced MMC using SLM is a viable approach. The obtained composite possesses dense microstructure and significantly enhanced tensile strength. The ultimate tensile strengths (UTSs) are 997.8, 1296.3, and 1511.6 MPa, and the Young's moduli are 475, 536, and 675 GPa, for 0 wt. % (pure Inconel 718), 0.25 wt. %, and 1 wt. % GNP additions, respectively. The bonding between GNPs and matrix material appears to be strong, and GNPs could be retained during the SLM process. The strengthening effect and mechanisms involved in the composites are discussed. Load transfer, thermal expansion coefficient mismatch, and dislocation hindering are believed to be the three main reinforcing mechanisms involved. It should be noted that more work needs to be conducted in the future to obtain more comprehensive information regarding other static and dynamic properties and the high-temperature performances of the GNP-reinforced MMCs produced by SLM. Process parameter optimization should also be investigated.