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Influence of the laser strategy on bi-metallic interfaces printed via multi-material laser-based powder bed fusion
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This work aims to contribute to the progress in the fabrication of multi-material structures using the Laser-Powder Bed Fusion technology. To manufacture an intralayer multi-material sample, an ad-hoc partitioning system was developed for the powder chamber. The manufacturability of a Stainless-Steel/Nickel-superalloy multi-material component, having a strong bond at the interface, which appeared cracks- and porosity free, was demonstrated. To characterize the sample in-situ and real-time, an optical monitoring system was implemented. The latter can allow for detection, in real-time and in a layer-by-layer manner, of any malfunctions during the process and contaminations between the two different materials.
Additive Manufacturing (AM) allows the manufacturing of functionally graded materials (FGM). This includes compositional grading, which enables the allocation of desired materials corresponding to local product requirements. An upcoming AM process for the creation of metal-based FGMs is laser-based powder bed fusion (PBF-LB/M) utilized for multi-material manufacturing (MM). Three-dimensional multi-material approaches for PBF-LB/M are stated to have a manufacturing readiness level (MRL) of 4 to 5. In this paper, an advancement of multi-material technology is presented by realizing an industry-relevant complex part as a prototype made by PBF-LB/M. Hence, a multi-material injection nozzle consisting of tool steel and a copper alloy was manufactured in a continuous PBF-LB/M process. Single material regions showed qualities similar to the ones resulting from mono-material processes. A geometrically defined transition zone between the two materials was achieved that showed slightly higher porosity than mono-material regions. Nevertheless, defects such as porosity, cracks, and material cross-contamination were detected and must be overcome in further MM technology development.
In this review paper, the authors investigate the state of technology for hybrid- and multi-material (MM) manufacturing of metals utilizing additive manufacturing, in particular powder bed fusion processes. The study consists of three parts, covering the material combinations, the MM deposition devices, and the implications in the process chain. The material analysis is clustered into 2D- and 3D-MM approaches. Based on the reviewed literature, the most utilized material combination is steel-copper, followed by fusing dissimilar steels. Second, the MM deposition devices are categorized into holohedral, nozzle-based as well as masked deposition concepts, and compared in terms of powder deposition rate, resolution, and manufacturing readiness level (MRL). As a third aspect, the implications in the process chain are investigated. Therefore, the design of MM parts and the data preparation for the production process are analyzed. Moreover, aspects for the reuse of powder and finalization of MM parts are discussed. Considering the design of MM parts, there are theoretical approaches, but specific parameter studies or use cases are not present in the literature. Principles for powder separation are identified for exemplary material combinations, but results for further finalization steps of MM parts have not been found. In conclusion, 3D-MM manufacturing has a MRL of 4–5, which indicates that the technology can be produced in a laboratory environment. According to this maturity, several aspects for serial MM parts need to be developed, but the potential of the technology has been demonstrated. Thus, the next important step is to identify lead applications, which benefit from MM manufacturing and hence foster the industrialization of these processes.
Inconel 718 samples have been fabricated by selective laser melting (SLM) and subjected to a series of heat treatments to study the isothermal precipitation behaviors of δ, γ′, and γ″ phases. A time–temperature–transformation (TTT) diagram for as‐built Inconel 718 is finally plotted. The morphology, size, and distribution of δ phase are studied, and the corresponding transformation kinetics is analyzed by Avrami equation. The results demonstrate that the transformation rate at 900 °C is the highest, though the nose of the TTT curve occurs at 860 °C after an incubation period of 10 min. As the aging temperature increases (620–820 °C), the precipitation rate of γ′/γ″ increases and the time to reach peak hardness decreases. The nose of the TTT curve for γ′/γ″ precipitation occurs at 820 °C after only 32 s. Nevertheless, when aged above 720 °C, the peak hardness decreases. In addition, the age hardening can be described by Avrami equation when δ phase is absent (620–770 °C). In summary, SLMed Inconel 718 exhibits a faster precipitation kinetics compared with wrought material, whereas their phase transition temperatures are similar.
The thermal diffusivity (a), linear thermal expansion coefficient (α), isobaric heat capacity (Cp) and phase transition enthalpy (ΔH) of Inconel 718 superalloy were determined by laser flash method, dilatometric method and differential scanning calorimetry in the temperature range of 298–1375…1473 К. The thermal conductivity (λ) has been calculated from the measurement results. The estimated errors of the obtained data were 2–5%, 3–5%, 2–3% and (1.5–2)×10⁻⁷ K⁻¹ for a , λ , C p and α, respectively. The approximation equations and a table of reference values for the studied properties have been obtained.
Selective Laser Melting (SLM) was investigated as new processing route for strongly hypereutectic AlSi alloys for thermal management applications in space industry. Processing conditions, microstructure and thermal expansion behavior were analyzed for AlSi10Mg+Si alloys with 25 wt% and 50 wt% Si fabricated by in-situ SLM of powder mixtures. For both Si compositions parts with densities >= 99% could be achieved using laser power >= 275 W and scan speeds
>= 1500 mm/s for the alloy containing 25 wt% Si and laser power of 400 W and scan speeds >= 1500 mm/s for the alloy containing
50 wt% Si. Considerable refinement of primary and eutectic Si was achieved for both Si compositions due to the high cooling rates of SLM. The mean particle size for the coarse primary Si of the 50 wt% Si containing alloy was below 10 µm. Additionally, unmolten Si powder particles were observed. Measurements of the coefficient of thermal
expansion (CTE) showed the tailorability of CTE with adjustment of Si content. A decrease in CTE of 43% compared to pure Al was achieved at a total Si content of 50 wt%. Experimental data was close to model calculations based on the rule of mixture and the Turner model depending on the different microstructures of the two alloy
compositions.
This paper reports the application of additive manufacturing technology to fabricate bi-dimensional lightweight composite meshes capable of demonstrating auxetic properties (negative Poisson’s ratio (NPR)) in combination with negative thermal expansion (NTE) behaviour, using as constituent materials polymers that do not exhibit NTE behaviour. To describe the combination of NPR and NTE characteristics, the designation of ‘anepectic’ is being proposed. Each mesh, obtained from varying either the material combination or the design parameters, was tested on a heated silicone bath to study the effects of the different combinations on the coefficient of thermal expansion (CTE). It was found that all meshes studied demonstrated a successful combination of NPR and NTE behaviours, and it was revealed that there is a possibility to tailor the meshes to activate the NTE behaviour within a chosen range of temperatures. For an extreme case, a Poisson’s ratio of −0.056, along with a CTE of −1568 × 10⁻⁶ K⁻¹ has been achieved.
Metallic structural materials continue to open new avenues in achieving exotic mechanical properties that are naturally unavailable. They hold great potential in developing novel products in diverse industries such as the automotive, aerospace, biomedical, oil and gas, and defense. Currently, the use of metallic structural materials in industry is still limited because of difficulties in their manufacturing. This article studied the feasibility of printing metallic structural materials with robotized laser-based metal additive manufacturing (RLMAM). In this study, two metallic structural materials characterized by an enlarged positive Poisson’s ratio and a negative Poisson’s ratio were designed and simulated, respectively. An RLMAM system developed at the Research Center for Advanced Manufacturing of Southern Methodist University was used to print them. The results of the tensile tests indicated that the printed samples successfully achieved the corresponding mechanical properties.
In this paper, the use of electrostatic polymer powder transfer methods for the preparation of multi-material layers is discussed with respect to the application in Simultaneous Laser Beam Melting (SLBM). Therefore, the basic principles of the single process steps as well as the challenges in combination with SLBM are considered verifying the critical process steps. On that base, process concepts are developed which might enable the fabrication of high quality multi-material parts in the future. Moreover, since the polymer powders typically used with Laser
Beam Melting differ strongly from common toners for e. g. electrophotographic printing, an experimental setup was built to study the powder transfer with an electrically chargeable transfer plate using polyamide 12 powder. The results of this study show that transfer of powders usable for Laser Beam Melting can be achieved, but depends on the electric field strength which is a function of the gap between transfer and substrate plate and the intermediate electric potential.
In this report, the dry delivery of polyamide 12 powders by vibrating capillary steel nozzles is investigated and discussed regarding its potential for powder layer preparation in Laser Beam Melting. Therefore, a setup including a steel nozzle assembled on a piezoelectric actuator is presented, which enables the precise control over very small powder quantities by vibration excitation. An analysis reveals that the mass flow through the nozzle can be adjusted by the vibration modes in a certain range depending on the nozzle's specifications, whereas the vibration modes themselves show a complicated behaviour. Using a positioning system in combination with the vibrating nozzle, single-layer patterns consisting of polyamide 12 are produced and characterized regarding surface homogeneity and selectivity using a laser stripe sensor.
During the development of a processing route for the Selective Laser Melting (SLM) powder-bed fabrication of the nickel superalloy CM247LC it has been observed that the ‘island’ scan-strategy used as standard by the Concept Laser M2 SLM powder-bed system strongly influences the grain structure of the material.
Optical and SEM micrographs are presented to show the observed grain structure in the SLM fabricated and Hot Isostatically Pressed (HIPped) material. The repeating pattern shown in the grain structure has been linked to the overlapping of the ‘island’ pattern used as standard in the Concept Laser M2 powder-bed facility. It is suggested that the formation of this bi-modal grain structure can be linked to the heat transfer away from the solidifying melt pool. The concept of a ‘band’ heating effect across each ‘island’ rather than ‘moving point’ heating has been suggested and has been supported by Electron Back Scattered Diffraction (EBSD) evidence. For comparison an EBSD map from a sample formed using a simple ‘back-and-forth’ strategy has also been presented and reveals a dramatically different grain structure and crystallographic orientation.
MicroCT evidence, supported by SEM microscopy, shows that in the as-fabricated material the bimodal structure caused by the ‘island’ scan-strategy translates directly into the macroscopic pattern for the regions of extensive weld cracking associated with the SLM fabrication of γ′ hardenable materials. Similar microCT data has shown that HIPping can effectively close the internal cracks to provide a retro-fix solution.
Interest in multifunctional structures made automatically from multiple materials poses a challenge for today's additive manufacturing (AM) technologies; however the ability to process multiple materials is a fundamental advantage to some AM technologies. The capability to fabricate multiple material parts can improve AM technologies by either optimising the mechanical properties of the parts or providing additional functions to the final parts. The objective of this paper is to give an overview on the current state of the art of multiple material AM technologies and their practical applications. In this paper, multiple material AM processes have been classified and the principles of the key processes have been reviewed comprehensively. The advantages and disadvantages of each process, recent progress, challenging technological obstacles, the possible strategies to overcome these barriers, and future trends are also discussed.
Miniaturization is one of the main imperatives in high-tech development and therefore a persistent challenge for mechanical engineering. Recently a freeform technique – Laser Micro Sintering – has been developed by which micro parts with an overall resolution of 30 μm can be produced from powder materials. The technique is a generative freeform fabrication method based on selective laser sintering; it can produce hollows and undercuts and does not afford shape-specific tools. Functional micro bodies can be generated from metals and ceramics.
Material microstructures are presented with a coefficient of thermal expansion larger in magnitude than that of either constituent. Thermal expansion can be large positive, zero, or large negative. Three-dimensional lattices with void space exceed two-phase bounds but obey three-phase bounds; lattices and normal materials have a trend of expansion decreasing with modulus. Two-phase composites with a negative stiffness phase exceed bounds that assume positive strain energy density. The author determined Young’s modulus and its relation to thermal expansion. Behavior of these composites is compared with that of homogeneous solids in expansion-modulus maps.
Additive manufacturing offers great potential and versatility for manufacturing high-quality and geometrically complex components. Multi-material laser-powder bed fusion is an emerging additive manufacturing approach where multiple materials are combined in order to manufacture multi-material components with new possibilities in product design and spatially tailored properties. Several multi-material delivery systems have been developed and a broad spectrum of applications have been demonstrated using multi-material laser-powder bed fusion. This work provides an overview in terms of architecture, construction and applications of all existing multi-material delivery systems developed for multi-material laser-powder bed fusion. Numerous challenges related to the deposition and processing of multi-materials which have been reported are discussed and potentials, which emerged through the use of multi-material laser-powder bed fusion are discussed together with the future perspectives.
While significant progress has been made in understanding laser powder bed fusion (L-PBF) as well as the fabrication of various materials using this technology, there is still limited adoption in the industry. One of the key obstacles identified is the lack of materials that can truly manufacture functional parts directly with L-PBF. This paper covers the emerging research on in-situ alloying and multi-metal processing. A comprehensive overview of the underlying scientific topics behind them is presented. The current state of research and progress from different perspectives (the materials and L-PBF processing parameters) are reviewed in order to provide a basis for follow-up research and development of these approaches. Defects, especially those associated with these two material processing routes, are also elucidated by discussing the mechanisms of their formation, including the main influencing factors, and the tendency for them to occur. Future research trends and potential topics are illustrated. The final part of this paper summarizes findings from this review and outlines the possibility of in-situ alloying and multi-metal processing using L-PBF.
A range of heat treatments have been developed for wrought Inconel 718 to obtain desired properties. For additive manufactured Inconel 718, the recently developed standard ASTM F3301 provides guidance for heat treatment of powder bed fusion specimens. Although this standard is based on standards developed for wrought Inconel 718, it does not include direct aging. Since direct aging reduces the number of processing steps, it can result in a post processing cost reduction if the desired properties are obtained.
In this study, we characterized the microstructure and tensile behavior of Inconel 718 specimens produced by a laser powder bed fusion process. The specimens were heat treated according to two different routines after stress relieving: a full heat treatment versus a one-step direct aging process. Differences in the resulting texture and grain morphology were observed. The ex-situ stress-strain behavior was broadly similar. However, a slight increase in yield strength was observed for the direct aged specimen. In order to understand this behavior, investigations with in-situ synchrotron energy dispersive X-ray diffraction tensile testing revealed differences in the load partitioning among different crystal directions. Importantly, the elastic anisotropy expressed by the magnitude of the diffraction elastic constants showed a dependency on the microstructures.
The Direct Laser Deposition (DLD) process has shown significant results in manufacturing due to its relevant flexibility to refurbish high-performance components (e.g. turbine blades or disks) or fabricating complex shaped parts. The solidification microstructure during the DLD process, is known to be controllable using different process parameters that induce changes in the grain structure, micro-segregation, and phase transformations. This work focuses on the effect the frequency of the pulsed laser has on the metallurgical characteristics of deposited thin walls. More specifically, the effect of three different pulsing rates (10 Hz, 100 Hz, 1000 Hz) during the deposition of the Nickel-based superalloys Inconel 718 has been studied and the results compared with parts produced by continuous wave laser mode. This work highlights how the pulsing rate significantly affected the thermal history, melt pool shape, grain size and its morphology, segregation region (Nb-enriched), and hardness. Finally, the microhardness was also evaluated and a correlation between the metallurgical characteristics and the pulsing rate was established.
Specimens made from pre-alloyed Invar (Fe-36Ni) powder were fabricated by selective laser melting and stress-relief heat treated afterwards. A relative density of the fabricated parts of 99.6 % was determined by computed tomography. The microstructure and mechanical behavior under monotonic and particularly cyclic loading at ambient temperature were investigated. Results reveal a bimodal microstructure containing columnar and equiaxed grains with an average grain size of 75 μm and pronounced texture 〈001〉 || BD. The selective laser melted Invar is characterized by a homogeneous hardness distribution revealing no gradient with respect to the height of the fabricated parts. The mechanical response under monotonic tensile loading is characterized by a fairly ductile behavior. Total strain controlled low-cycle fatigue tests with varying strain amplitudes were performed. The cyclic deformation response is characterized by highly reproducible behavior in terms of stress amplitudes and number of cycles to failure. The corresponding half-life hysteresis loops reveal perfect Masing behavior. Post-mortem fractography revealed crack initiation in direct vicinity of the surfaces. The damage tolerance of Invar is found to be promoted by its high ductility. Finally, a low coefficient of thermal expansion, very similar to conventionally processed Invar, was shown by dilatometer tests in the temperature range of 0 °C–180 °C.
Manufacturing multi-material part is one of the native advantages of selective laser melting (SLM) due to its layer-by-layer manufacturing method, which is attracting more and more attention in recent years. In an effort to reveal the dual interfacial characterization of dissimilar materials manufactured by SLM with different printing sequences, this paper presents the morphology, microstructure, element distribution, phase composition and microhardness of multi-material interfaces between SLMed 316L steel and C52400 copper alloy. Both interfaces display isolated alloy islands with various shapes and different morphologies. The melt pool at 316L/C52400 interface is deeper and narrower than that at C52400/316L interface. Small 316L and C52400 spheres with a size of 1–5 μm are formed under the surface tension, Marangoni convection in the melt pool and rapid cooling condition, in which many smaller particles with a size of <1 μm appear due to the material supersaturation and convection. Interdiffusion of elements and very fine grains with a size of <6 μm result in excellent metallurgical bonding performance. Cracks tend to originate from the interface and extend to the stainless steel side for both interfaces. No separation of the two materials caused by cracks is found in their contact area. Healing of the cracks by C52400 copper alloy at 316L/C52400 interface is easier to complete. The interface thickness depends on the building sequence of the two materials, which features a much thicker transition when building C52400 on 316L. No intermetallic compound but a trace of CuNi alloy is formed at the interface. The microhardness varies in the transition area due to the existence of isolated 316L/C52400 islands and decreases from 316L to C52400 with the highest value of 283.33 ± 5.51 HV to the lowest value of 181.33 ± 17.62 HV. This research advances the understanding of the different interfacial characterizations of dissimilar materials manufactured by SLM and provides guidance and reference for manufacturing multi-material components with complex interfaces using SLM.
This work explores the feasibility of additively manufacturing tailored microstructures through varying process parameters, to eventually control the mechanical properties and performance. The investigation focuses on controlling the heat input and thermal history during laser-powder bed fusion of IN718 through process parameter manipulation; notably the heat input parameters (power, scan speed, and hatch spacing) and island scanning parameters (island size, shift, and island overlap). The changes in preferred orientation, morphology and grain size were characterised in both the transverse and build cross-sections using scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD), while the texture development was comparatively characterised using X-ray diffraction (XRD). The solidification cell size was quantified to estimate the influence of the process parameters on the cooling rates. This was also rationalised using a thermal model resolving the scan characteristics to provide the transient temperature distribution to a numerical grain growth model. Based on the obtained microstructures, graded microstructures were generated using the island strategy and identical laser parameters throughout but changing subtle features such as the island size and shift. A suitable post-process heat treatment was applied to retain the tailored microstructures, while obtaining the required hardness.
Multi-material additive manufacturing (MMAM) enables rapid design and direct fabrication of three-dimensional (3D) objects consisting of multiple materials without the need for complex manufacturing process and expensive tooling. Through concurrent integration of multiple materials having their own unique properties, MMAM allows for facile production of multi-functional devices and systems. Various MMAM techniques have recently been developed and a wide range of new applications have been demonstrated using MMAM. This paper reviews recent advances in MMAM methods and key applications enabled by MMAM in three areas; biomedical engineering, soft robotics, and electronics.
Adopting selective laser melting (SLM), a typical technology of additive
manufacturing (AM), to form multi-material metallic composites is a challenging and
promising field. In this study, SLM 316L/CuSn10 multi-material composites was an
innovative attempt to develop functional and structural materials with excellent
properties of steel and copper alloys. Dense 316L/CuSn10 specimens with no
interfacial macrocracks were successfully fabricated. Results showed that the Vickers
microhardness gradually decreased from 329.5±12.5 HV in 316L region to
172.8HV±7.4 in CuSn10 region. The ultimate tensile strength and flexural strength of
316L/CuSn10 specimen were between 316L and CuSn10. The shear stress of
316L/CuSn10 sample was 210 MPa, which was higher than the steel/copper alloys
fabricated by other methods. It indicated an ideal interfacial bonding condition of
316L/CuSn10 multi-material, which was benefited from sufficient agitation of the
molten pools and elements diffusion in the term of continuous distribution of elements
and the enrichment of the heterogeneous alloy phases. Also, the grain refinement by re
melting and recrystallization upgraded the bonding performance at the interface. Finally,
the 316L/CuSn10 lattice structure was formed by SLM, hinting at the prospects for
industrial applications of steel/copper multi-material by SLM in future.
In this study, elemental powder mixtures of Ti25Ta, an alloy with promise for orthopaedic applications, were processed using Selective Laser Melting (SLM), an emerging manufacturing method for bespoke implants. Material density and homogeneity was investigated as a function of laser scan speed and scanning strategy. Dense (>99.99%), pore free material was obtained at optimised processing parameters and a ‘remelt’ scan strategy improved melting of the Ta powders, avoiding keyhole formation.
Tensile and ultrasonic modulus testing of the SLM Ti25Ta revealed that the processed material had a similar yield strength to SLM commercially pure Ti, namely 426 ± 15 MPa, with a significant reduction of elastic modulus to 65 ± 5 GPa. The remelt scan strategy increased the yield strength to 545 ± 9 MPa, without altering the elastic modulus, however reduced the elongation from 25 ± 1 to 11 ± 4%. TEM analysis revealed the microstructure consisted of predominantly hexagonal α′ martensite with a limited amount of orthorhombic α′′ martensite formed in the Ta-rich regions near partially melted Ta particles, specifically facilitated by enhanced diffusion occurring during the remelt scan. The composition range for the α′′ phase was observed to be approximately 40–50 wt% Ta. Electron back-scattered imaging (BSI) and back scattered diffraction (EBSD) revealed the formation of the prior β grains with close to equiaxed morphology and a slight texture in the α′ martensite. The application of the remelt scan disrupted the prior β grain structure and resulted in randomly oriented α′ laths.
Inconel 718 is one of the most commonly employed alloys for metal additive manufacturing (MAM) and has a wide range of applications in aircraft, gas turbines, turbocharger rotors, and a variety of other corrosive and structural applications involving temperatures of up to ∼700 °C. Numerous studies have investigated different aspects of the mechanical behaviour of additively manufactured (AM) Inconel 718. This study analyses the observations from more than 170 publications to provide an unbiased engineering overview for the mechanical response of AM Inconel 718 (and its variations and spread among different reports). First, a brief review of the microstructural features of AM Inconel 718 is presented. This is followed by a comprehensive summary of tensile strength, hardness, fatigue strength, and high-temperature creep behaviour of AM Inconel 718 for different types of MAM techniques and for different process and post-process conditions.
The multi-material selective laser melting (SLM) technology has been widely studied in recent years. In this paper, 316L stainless steel (316SS) and Inconel 718 (IN718) multi-material parts were manufactured by SLM and their interfacial characteristics and mechanical properties were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, electronic universal testing machine and microhardness tester. The interface metallograph and higher comprehensive mechanical performance with elasticity modulus (103 ± 3 MPa), elongation (28.1 ± 2%) and ultimate tensile strength (596 ± 10 MPa) proved good metallurgical bonding at the interfaces (∼100 μm) between 316SS and IN718. Nevertheless, some cracks and holes, which were found at or near the interfaces, suggested special SLM processing parameters (such as laser power, laser scanning speed, layer thickness, hatch spacing, remelting, fabrication sequence and layers of interface) should be applied at the interfaces.
Selective laser melting (SLM) is a metal additive manufacturing (AM) technique that can fabricate complex parts of any shape. In this paper, the interfacial microstructure and mechanical properties of 316L/CuSn10 bimetallic structure were studied, and the ability of self-developed multi-material SLM equipment to form multi-material bimetallic structures was described. The investigations of 316L/CuSn10 bimetallic structure involved microscopic features, phase analysis, microhardness, tensile properties and bending properties. Moreover, the mechanical properties of multi-material specimens were compared with that of single material samples. In addition, scanning electron micrograph shows that the width of the bimetallic fusion zone is about 550 μm, and dendritic crack sources was found on the boundary between the bimetallic fusion zone and the steel region. In the direction perpendicular to the interface, the Vickers microhardness value gradually changed from 233.1 ± 8.1 HV in the steel zone to 154.7 ± 6.0 HV in the bronze zone. The non-standard tensile samples were printed and tested for evaluating tensile properties, the ultimate strength of 316L/CuSn10 joint was 423.3 ± 30.2 MPa comparing with the 316L stainless steel of 673.1 ± 4.2 MPa and the CuSn10 Tin-bronze of 578.7 ± 30.6 MPa. Tensile stress-strain curves and fracture characteristics show that the fusion zone of steel and bronze exhibits brittle fracture mechanism. Furthermore, three-point bending test was used to evaluate the interfacial bonding strength of the bimetallic structure, and results show that the maximum flexural strength of 316L/CuSn10 bimetallic structure isn't in middle of but below that of 316L stainless steel and CuSn10 Tin-bronze. The research founding that SLM can obtain 316L/CuSn10 bimetallic structure with good joint strength by adopting island scanning strategy and inter-layer stagger scanning strategy in the interfacial layers.
The wide usage of Inconel 718 alloy is based on its fusion weldability and its availability in many different forms including cast, wrought and powder. Thus with the emergence of additive manufacturing (AM) techniques for metals, Inconel 718 is a prime candidate for materials to be considered. Powders that have been developed for powder metallurgy are readily available for use in various AM processes such as selected laser melting (SLM) powder bed. While much research has focused on optimizing the deposition parameters to achieve fully densified specimens, subsequent heat treatments and their effect on the microstructure also need to be understood. This study evaluated the microstructure of SLM specimens of Inconel 718 after various heat treatments and compared the resulting effect on the quasi-static mechanical properties.
Inconel 718 alloy samples were fabricated by selective laser melting (SLM). Microstructure and precipitation in solution-heat-treated- and double-aging-SLM-made Inconel 718 were studied by scanning and transmission electron microscopy. Electron microscope observations showed that disc-shaped and cuboidal γ″ and circular γ′ precipitates with an average size of 10–50 nm developed within cellular γ austenite matrix. The simulated, experimentally observed electron diffraction patterns, and dark-field imaging further revealed that the precipitation of three variants of γ″ in the γ matrix occurred. The coarser acicular γ″ and globular as well as plate-like δ phases precipitated at grain boundaries and also within the interior of austenite matrix. The morphology, distribution and crystallography of these precipitates and their formation mechanisms were analyzed and discussed.
Four metallic metamaterials with tailorable mechanical properties are designed using bi-material star-shaped re-entrant planar lattice structures, which do not involve pins, adhesive, welding or pressure-fit joints and can be fabricated through laser-based additive manufacturing. Three length parameters, one angle parameter and three material combinations are used as adjustable design parameters to explore structure-property relations. For each of the four designed metamaterials, the effects of the design parameters on the Poisson’s ratio (PR), coefficient of thermal expansion (CTE), Young’s modulus and relative density are systematically investigated using unit cell-based finite element simulations that incorporate periodic boundary conditions. It is found that the bi-material lattice structures can be tailored to obtain 3-D printable metallic metamaterials with positive, near-zero or negative PR and CTE together with an uncompromised Young’s modulus. In particular, it is shown that metamaterial # 1 can exhibit both a negative PR and a non-positive CTE simultaneously. These metallic metamaterials can find applications in structures or devices such as antennas and precision instruments to reduce thermomechanical stresses and extend service lives.
Several additive manufacturing technologies are applied in industrial manufacturing today. For instance, laser-based powder bed fusion of metals (also known as e.g. laser beam melting, selective laser melting) is utilized for serial production in aerospace industry [1]. The main advantage of additive manufacturing technologies is the freedom of design. Hence, functionally optimized parts in terms of lightweight, fluid dynamics etc., can be produced without having an exponential growth in manufacturing costs when increasing geometrical complexity, like it is known from conventional machining. However, predominantly mono-material processing is commercially available today. This is especially true with regards to laser-based powder bed fusion of metals.
Understanding laser interaction with metal powder beds is critical in predicting optimum processing regimes in laser powder bed fusion additive manufacturing of metals. In this work, we study the denudation of metal powders that is observed near the laser scan path as a function of laser parameters and ambient gas pressure. We show that the observed depletion of metal powder particles in the zone immediately surrounding the solidified track is due to a competition between outward metal vapor flux directed away from the laser spot and entrainment of powder particles in a shear flow of gas driven by a metal vapor jet at the melt track. Between atmospheric pressure and ∼10 Torr of Ar gas, the denuded zone width increases with decreasing ambient gas pressure and is dominated by entrainment from inward gas flow. The denuded zone then decreases from 10 to 2.2 Torr reaching a minimum before increasing again from 2.2 to 0.5 Torr where metal vapor flux and expansion from the melt pool dominates. The dynamics of the denudation process were captured using high-speed imaging, revealing that the particle movement is a complex interplay among melt pool geometry, metal vapor flow, and ambient gas pressure. The experimental results are rationalized through finite element simulations of the melt track formation and resulting vapor flow patterns. The results presented here represent new insights to denudation and melt track formation that can be important for the prediction and minimization of void defects and surface roughness in additively manufactured metal components.
Multi material processing in selective laser melting using a novel approach, by the separation of two different materials within a single dispensing coating system was investigated. 316 L stainless steel and UNS C18400 Cu alloy multi material samples were produced using selective laser melting and their interfacial characteristics were analysed using focused ion beam, scanning electron microscopy, energy dispersive spectroscopy and electron back scattered diffraction techniques. A substantial amount of Fe and Cu element diffusion was observed at the bond interface suggesting good metallurgical bonding. Quantitative evidence of good bonding at the interface was also obtained from the tensile tests where the fracture initiated at the copper region. Nevertheless, the tensile strength of steel/Cu SLM parts was evaluated to be 310 ± 18 MPa and the variation in microhardness values was found to be gradual along the bonding interface from the steel region (256 ± 7 HV0.1) to the copper region (72 ± 3 HV0.1).
The review of the available literature on Dissimilar Metal Welds and Transition Joints is an outgrowth of a Metal Properties Council, Inc. /EPRI sponsored effort to detail the references pertaining to the subject in an annotated bibliographic form. The emphasis is on carbon migration, the stress/strain state of welds, and transition joint failure mechanisms.
Purpose
– Selective laser melting (SLM) is a powder metallurgical (PM) additive manufacturing process whereby a three‐dimensional part is built in a layer‐wise manner. During the process, a high intensity laser beam selectively scans a powder bed according to the computer‐aided design data of the part to be produced and the powder metal particles are completely molten. The process is capable of producing near full density (∼98‐99 per cent relative density) and functional metallic parts with a high geometrical freedom. However, insufficient surface quality of produced parts is one of the important limitations of the process. The purpose of this study is to apply laser re‐melting using a continuous wave laser during SLM production of 316L stainless steel and Ti6Al4V parts to overcome this limitation.
Design/methodology/approach
– After each layer is fully molten, the same slice data are used to re‐expose the layer for laser re‐melting. In this manner, laser re‐melting does not only improve the surface quality on the top surfaces, but also has the potential to change the microstructure and to improve the obtained density. The influence of laser re‐melting on the surface quality, density and microstructure is studied varying the operating parameters for re‐melting such as scan speed, laser power and scan spacing.
Findings
– It is concluded that laser re‐melting is a promising method to enhance the density and surface quality of SLM parts at a cost of longer production times. Laser re‐melting improves the density to almost 100 per cent whereas 90 per cent enhancement is achieved in the surface quality of SLM parts after laser re‐melting. The microhardness is improved in the laser re‐molten zone if sufficiently high‐energy densities are provided, probably due to a fine‐cell size encountered in the microstructure.
Originality/value
– There has been extensive research in the field of laser surface modification techniques, e.g. laser polishing, laser hardening and laser surface melting, applied to bulk materials produced by conventional manufacturing processes. However, those studies only relate to laser enhancement of surface or sub‐surface properties of parts produced using bulk material. They do not aim at enhancement of core material properties, nor surface enhancement of (rough) surfaces produced in a PM way by SLM. This study is carried out to cover the gap and analyze the advantages of laser re‐melting in the field of additive manufacturing.