Ink-based laser powder bed fusion of barium titanate

The additive manufacturing process selective laser melting (SLM) uses a powder bed fusion approach to fully melt layers of powdered metal and create 3D components. Current SLM systems are equipped with either single or multiple (up to four) high-power galvo-scanning infrared fibre laser sources operating at a fixed wavelength of 1064 nm. At this wavelength, a limited laser energy absorption takes place for most metals (e.g. alloys of aluminium have less than 10% absorption and titanium 50-60% absorption). The lower absorption of 1064-nm laser sources requires higher laser powers to compensate for the loss of energy due to reflectivity and fully melt the feedstock material. This makes the use of 1064-nm lasers within current powder bed fusion SLM systems energy inefficient. Further to this, there is limited potential for scale-up of these laser sources within an SLM system architecture due to physical space requirements and high economic cost, placing further limitations on current state-of-the-art SLM productivity. This research investigates the use of low power, highly scalable fibre coupled diode laser sources and the influence of shorter laser wavelengths (450–808 nm) on material absorption and processing efficiency using a diode area melting (DAM) approach. It was found that when processing Ti6Al4V, absorption was 11% higher using 450-nm lasers when compared to using 808-nm lasers and 14% higher than 1064-nm lasers. The maximum powder bed temperature for irradiation at 450 nm and 808 nm was 1920 0C and 1760 0C respectively when using only 3.5 W of laser power. Due to the speed at which the DAM process scans the powder bed, the melt pool cooling rate was much slower (750–1400 0C/s) than traditional SLM (105–106 0C/s). This encouraged the development of β phases within the formed Ti6Al4V component. The low power, low cost, highly compact short wavelength diode laser is viable energy source for future powder bed fusion additive manufacturing systems, with potential for productivity scale-up using a DAM methodology.

PurposeThe increased use cases for laser powder bed fusion (LPBF) in the research and commercial domains necessitate a better understanding of the inputs and the processing parameters. Porosity in parts manufactured by LPBF could lead to premature failure and increased cost. The powder bed, which is selectively laser melted, must be as densely packed as possible to ensure high-density parts. This paper aims to identify and qualify the variables that affect the packing density of the powder bed.Design/methodology/approachSix different independent variables that affect the packing density of the powder were identified and quantified. The chemical composition, true powder density, powder size distribution, powder circularity and convexity and powder morphology were studied. A powder bed density capsule was printed in place to determine the actual powder bed density in the LPBF unit.FindingsParticle size destitution is the most critical aspect of the packing density in the LPBF unit. Powder with better circularity, convexity and higher powder density has proven to pack less densely than powder with many smaller particles. A more significant number of fine particles will ensure the voids between larger particles are filled, and a denser item, with less porosity, can be manufactured.Originality/valueThe independent variables quantified in this study to determine their effect on the packing densities are discussed. Adherence to the ASTM standard applicable to this industry is discussed, and the quantification method is evaluated. This work’s original contribution is identifying the effect of the ratio of D90 to D10 values based on particle diameter and its interaction within the LPBF unit to result in the highest possible packing density.

Additive Manufacturing(AM) is an advanced direct-manufacturing technology, based on the discrete-stacking principle. Laser Powder Bed Fusion (L-PBF) is one of the most promising technologies in the field of metal AM, with the characteristics of fabricating parts with complex shapes directly. For L-PBF equipment , the core device is lasers, and almost all L-PBF printers are currently equipped with infrared laser. However, due to too low absorption rate of the pure copper surface to infrared laser and high thermal conductivity between pure copper, it is extremely challenging to fabricate pure copper by traditional infrared-laser powder bed fusion(IR L-PFB). In this work, green-laser was applied to replace traditional infrared laser during L-PBF process, molten pool structure and temperature flow behavior of Green-Laser powder bed additive manufacturing pure copper was studied by mesoscopic simulation. Here we show that green-laser greatly improved the absorption rate of the pure copper surface, and the result showed that with lower cost laser process parameters (lower laser power 300W and larger hatching space 0.08 mm), pure copper parts with smoother surface, no-remelting and no obvious defects could be fabricated successfully.

Metal additive manufacturing (MAM) processes have revolutionized manufacturing and design, offering unprecedented freedom to create intricate and complex parts. Research has demonstrated that in laser powder bed fusion (LPBF) of metal additive manufactured parts, the microstructure and surface can be influenced by various process parameters. However, the influence of laser pulse parameters in LPBF remains relatively unexplored. Laser pulse parameters significantly affect the microstructure and melt pool evolution in metal powder bed additive manufacturing processes. Control over these variations is crucial for achieving desired material properties and part quality. Adjustments in pulse parameters, such as power, width, and interval, can alter grain size, orientation, and subcellular structure, thus impacting mechanical properties. Moreover, optimizing laser energy density by controlling pulse parameters can mitigate defect formation, enhancing density and mechanical properties. Hence, exploring precise control over pulse width and interval during manufacturing contributes significantly to achieving high-quality components. In this study, a meso-scale numerical model was employed to investigate the influence of pulse parameters, such as pulse width and interval on the thermal history and melt pool evolution in LPBF. The physics-based model incorporates key phenomena such as heat transfer via radiation & convection, phase change, recoil pressure, and density-driven melt pool flow. These physical phenomena play a crucial role in the surface finish and microstructure of fabricated parts, affecting the formation of defects such as balling, keyhole, and spattering. A discrete element model (DEM) was employed to construct the powder bed, while the finite volume method (FVM) simulated the thermal-fluid behavior using an initial condition derived from an STL file. Validation of the numerical model against existing literature has confirmed its capacity to accurately predict melt pool behavior, including its influence on surface roughness, as well as temperature distribution and cooling rates across different laser source pulse width and interval settings. Additionally, it can also pave the way for future research directions, including the exploration of in situ hybrid processes involving multiple lasers for surface processing and enhancement. The evolution of computational models promises to facilitate more sophisticated control strategies, ultimately enhancing outcomes and efficiency in metal additive manufacturing processes, paving the way for tailored and optimized LPBF MAM parts.

Implications for accurate surface temperature monitoring in powder bed fusion: Using multi-wavelength pyrometry to characterize spectral emissivity during processing

Powder bed fusion process: A brief review

Powder bed fusion (PBF) is recognized as one of the most common additive manufacturing technologies because of its attractive capability of fabricating complex geometries using many possible materials. However, the quality and reliability of parts produced by this technology are observed to be crucial aspects. In addition, the challenges of PBF-produced parts are hot issues among stakeholders because parts are still insufficient to meet the strict requirements of high-tech industries. This paper discusses the present state of the art in PBF and technological challenges, with a focus on selective laser melting (SLM). The review work focuses mainly on articles that emphasize the status and challenges of PBF metal-based AM, and the study is primarily limited to open-access sources, with special attention given to the process parameters and flaws as a determining factor for printed part quality and reliability. Moreover, the common defects due to an unstrained process parameter of SLM and those needed to monitor and sustain the quality and reliability of components are encompassed. From this review work, it has been observed that there are several factors, such as laser parameters, powder characteristics, material properties of powder and the printing chamber environments, that affect the SLM printing process and the mechanical properties of printed parts. It is also concluded that the SLM process is not only expensive and slow compared with conventional manufacturing processes, but it also suffers from key drawbacks, such as its reliability and quality in terms of dimensional accuracy, mechanical strength and surface roughness.

The intelligent recoater: A new solution for in-situ monitoring of geometric and surface defects in powder bed fusion

Purposedding dopants to a powder bed could be a cost-effective method for spatially varying the material properties in laser powder bed fusion (LPBF) or for evaluating new materials and processing relationships. However, these additions may impact the selection of processing parameters. Furthermore, these impacts may be different when depositing nanoparticles into the powder bed than when the same composition is incorporated into the powder particles as by ball milling of powders or mixing similarly sized powders. This study aims to measure the changes in the single bead characteristics with laser power, laser scan speed, laser spot size and quantity of zirconia nanoparticle dopant added to SS 316 L powder.Design/methodology/approachA zirconia slurry was inkjet-printed into a single layer of 316 SS powder and dried. Single bead experiments were conducted on the composite powder. The line type (continuous vs balling) and the melt pool geometry were compared at various levels of zirconia doping.FindingsThe balling regime expands dramatically with the zirconia dopant to both higher and lower energy density values indicating the presence of multiple physical mechanisms that influence the resulting melt track morphology. However, the energy density required for continuous tracks was not impacted as significantly by zirconia addition. These results suggest that the addition of dopants may alter the process parameter ranges suitable for the fabrication of high-quality parts.Originality/valueThis work provides new insight into the potential impact of material doping on the ranges of energy density values that form continuous lines in single bead tests. It also illustrates a potential method for spatially varying material composition for process development or even part optimization in powder bed fusion without producing a mixed powder that cannot be recycled.

Selective laser sintering (SLS) and Selective Laser Melting (SLM) are parent layer manufacturing processes that allow generating complex 3D parts by consolidating layers of powder material on top of each other. Consolidation is obtained by processing the selected areas using the thermal energy supplied by a focused laser beam. In SLS partial fusion of powder particles takes place, followed by a solidification of the created liquid. SLM is essentially the same process as SLS, with the difference that the particles are completely molten under the laser beam. This development is driven by the need to produce near full dense objects, with mechanical properties comparable to those of bulk materials and by the desire to avoid lengthy post processing cycles. Identification of the optimal process conditions (so-called process window) is a crucial task for industrial application of SLS/SLM processes. Operating parameters of the process are adjusted in correspondence with optical and thermal properties of the processed material. Nowadays in SLS/SLM there is a tendency to increase the speed of the fabrication as a consequence of the available higher laser powers. It leads to increase of laser scanning speeds. In these circumstances, to rely only on experimental investigations in order to adjust process and material parameters is time-consuming and ineffective. Simulation tools are strongly needed for the visualization and analysis of SLS/SLM processes. In SLM the powder grains under the laser are completely molten and form a liquid domain called melt pool. Evolution of the melt pool during the process, its interaction with the laser, the substrate and the surrounding non-molten powder strongly affect the quality of the final part. The goal of this work is to study the melt pool dynamics by means of the finite-element simulation software, built specially for SLS/SLM. The numerical model is based on the homogeneous medium hypothesis. It considers the interaction between the laser and the powder material, the phase transformations and the evolution of the material properties during the process. We also study the influence of the phase change on the process efficiency. The macroscopic model is completed by the sub-models, which allow to study at microscopic level the processes taking place in the powder bed during its laser heating and melting. Melting of separate powder particles during laser irradiation is studied by means of the improved Single Grain Model. The capillary phenomena taking place in the powder bed during SLS/SLM are also studied. The interconnection of powder grains during their melting is approached by the mechanism of liquid drops coalescence. According to the obtained results, the depth-dependent sintering threshold for powder materials is proposed.

Additive manufacturing (AM) enables the rapid fabrication of parts with complex geometries that cannot be easily manufactured with traditional methods. While originally limited to rapid prototyping, recent advances in AM technology also enable direct fabrication of functional end-use parts in, e.g., aerospace, medical devices, and military applications. However, the transition from rapid prototyping to fabricating end-use parts has also revealed technology barriers, including surface quality, accuracy, part variability, and uncertainty about the process–structure–property relationship, to name a few. Crucially, fundamental questions about friction, wear, and lubrication of AM parts have led to substantial research interest in the tribology community. This Special Issue provides significant value to the tribology community by highlighting recent advances of tribology research related to AM, defining the state-of-the-art of tribology knowledge, and framing the challenges and opportunities for future tribology research in this exciting field. It is a collection of 17 research/review papers covering a wide range of state-of-the-art topics in the tribology of additive manufacturing. All the papers have undergone a rigorous and anonymous peer-review process.Additive manufacturing technology is rapidly progressing and the future may bring many new printing methodologies. However, at present, the AM technology can be broadly grouped into seven categories: binder jetting (BJ), direct energy deposition (DED), material extrusion (ME), material jetting (MJ), powder bed fusion (PBF), sheet lamination, and vat polymerization (VP). Of particular interest is the understanding of the Process–Microstructure–Tribology (PMT) "research hotspot." Table 1 summarizes the topics covered in this Special Issue, in addition to the AM technology and the materials. Furthermore, we categorize the papers into PMT, tribology design, and surface characterization, based on the main topic of the paper. To set the stage, we summarize the contents of the papers per Table 1.Renner et al. presented a review paper focusing on the corrosion and wear properties of AM-fabricated alloys including steel, titanium, and aluminum. The paper points out that AM-fabricated alloys have better corrosion and wear properties than the casted parts, while the influence of process parameters on the microstructures does not hold true across different additive manufacturing processes and materials. Many other challenges—e.g., anisotropic behaviors, effects of heat treatments, the role of nano-particles, and failure analysis—are recommended for future studies in the field. Also noteworthy is the AM-fabricated metal parts (including PBF and DED) often end up with unique microstructures due to the rapid and repeated heating/cooling cycles and extremely large thermal gradient. Indeed, melting and solidification are highly time dependent and complex processes, making it difficult to simulate and predict. These are areas where more research is needed.Sharma et al. presented a literature review on hybrid surface metal matrix composites produced by friction stir processing and provided insight into the PMT relationship. Kang et al. studied the microstructure on the surface, sub-surface, and inner region of a commercial pure Ti part fabricated using laser PBF (LPBF). They indicated that the friction and wear behavior of the three regions are distinct. This is thought to be the consequence of the intrinsic heat treatment induced by the LPBF process. The remelting/heating and recrystallization cause microstructure coarsening and refinement between the three regions.Thasleem et al. studied the influence of various post-processing methods such as heat treatment and electric discharge alloying (EDA) on ambient and elevated temperature wear behavior of LPBF AlSi10Mg alloy and compared with the cast parts. Their results indicated that an EDA-treated part has the least wear-rate and coefficient of friction at both ambient and elevated temperatures due to its higher hardness than other samples. Thus, EDA-treating can be considered as a potential post-processing technique.Microstructure reinforcement is also an efficient way to increase wear resistance. Wang et al. studied the effect of TiB2 content on the microstructure and wear behavior of nano-TiB2p/2024Al composites fabricated by laser DED. Their results revealed that the wear-rate of an 8 wt% TiB2p/2024Al matrix composite with full equiaxed grains is almost 20 times lower than that of the unreinforced alloy due to the grain morphology-induced wear mechanism. Li et al. fabricated a dense Al–Fe–Cr quasicrystal reinforced Al matrix composite using DED. The reinforcement phases contributed to the mechanical mixing layer formation that significantly reduced the coefficient of friction and improved the wear resistance. Luo et al. fabricated short carbon fiber-reinforced nylon using ME and reported that the tribological performance improved.Rolling contact fatigue (RCF) is another critical performance for many tribological applications. Xie et al. and Fasihi et al. used laser cladding to enhance the railway rail materials. They showed that by carefully selecting cladding materials, both wear and RCF performance can be improved. However, micro-cracks may initiate from the interface between clad and unclad regions. Jalalahmadi et al. presented a predictive platform for fatigue prediction and AM-fabricated metallic parts qualification. They reported developing an integrated computational materials engineering tool that includes models of crack initiation and damage progression, exploring the design space across geometries and materials.Additive manufacturing can also be used to design and process unique functional structures, which may create some breakthroughs in tribological design. Suh highlighted the importance of design in improving the performance of all tribological systems. AM was mentioned as an innovative way to produce a part that is very difficult or even impossible to manufacture using conventional manufacturing while at the same time improve the design quality.There is a large body of tribology literature on the use of surface texturing to reduce the friction and wear characteristics of conventional materials. Surface texturing appears to offer viable flexibility for improving the tribological behavior of AM parts as well. Luo et al. reported that by designing specific surface textures—such as convex squares and triangles, processed via ME—they were able to improve the tribological performance. Hoskins and Zou designed and fabricated a micro-texture inspired by Ocellated Skink using two-photon polymerization (TPP), a VP technique producing nanostructures. They reported that wear was substantially reduced due to the texture through the controlled formation of microcracking. Maddox et al. also used TPP to design and fabricate surfaces inspired by frog toes and applied in the piston ring and liner interface. These designs reduce surface friction by an average of 18% and up to 39%, compared to a flat control. Zhang et al. used VP technology to produce various polygonal three-dimensional patterns inspired by dragonfly wings to identify how the polygonal patterns of the samples with bionic wing veins affected the skin friction. Their study provides insight into the mechanism of flow separation of the dragonfly wing and further improves the structure design. Murashima et al. used VP to design and produce a novel morphing surface that selectively performs as a low-friction or break-like surface. By applying air pressure, the surface switches between a convex and a concave shape, giving a different coefficient of friction. It is worth noting that AM of a part often tends to change a "continuous surface" into many discrete layer boundaries, inducing staircase effects due to the layer-on-layer nature. Narasimharaju et al. systematically investigated the impact of varying surface inclination angles on the build direction on the resultant surface textures. The areal surface texture characterization and particle analysis indicated that the resulted surface topographies are strongly correlated with the surface inclination angles.Modeling and simulations of the hydrodynamic effects associated with AM processes are also worth investigating. Wagner and Higgs studied the capillary and hydrodynamic effects of the interfacial flow responsible for primitive formation when the binder spreads into the powder bed and forms a bound network of wetted particles in the BJ process.The collection of articles in this Special Issue represents the active and diverse research efforts in the tribology of additive manufacturing. However, there is still a long way to go in this journey. Fundamental material research, application-oriented research, and novel tribological design are extremely worthwhile and exciting to pursue. We hope this Special Issue on the latest advancements in tribology of additive manufacturing provides insights and stimulates the generation of novel ideas with industrial applications on system diagnosis and machine design for years to come.Finally, we wish to take this opportunity to sincerely thank all the authors for their scientific contributions. Special thanks also to reviewers for their constructive and insightful comments on all the papers published in this issue.

Bed process such as laser powder bed fusion (LPBF), electron beam powder bed fusion (EPBF), non-beam powder bed processes, photopolymer bed process and slurry bed process are described. Their fabrication speed is low, which is one of the drawbacks of AM, methods to increase LPBF speed is given. Besides, application of LPBF in repair is given. In EPBF, in order to comprehend beam–powder bed interactions and the role of parameters (scan speed, beam power), it is essential to know how a beam is generated and manipulated, which is described while the roles of electric current and voltage are clarified. Processes such as high speed sintering, selective heat sintering, binder jet three dimensional printing and other emerging processes (micro heater array powder sintering, localized microwave heating based AM, multi jet fusion) are energy-efficient and cost-effective, which are described. Three types of scanning, i.e. pointwise, linewise, and areawise, are explained.KeywordsMeltingMagnetic fieldHigh speedBinder jettingMicrowaveHeaterAreawise scanning

Ti-6Al-4V alloy fabricated by laser powder bed fusion (L-PBF) and electron beam powder bed fusion (EB-PBF) techniques have been studied for applications ranging from medicine to aviation. The fabrication technique is often selected based on the part size and fabrication speed, while less attention is paid to the differences in the physicochemical properties. Especially, the relationship between the evolution of α, α’, and β phases in as-grown parts and the fabrication techniques is unclear. This work systematically and quantitatively investigates how L-PBF and EB-PBF and their process parameters affect the phase evolution of Ti-6Al-4V and residual stresses in the final parts. This is the first report demonstrating the correlations among measured parameters, indicating the lattice strain reduces, and c/a increases, shifting from an α’ to α+β or α structure as the crystallite size of the α or α’ phase increases. The experimental results combined with heat-transfer simulation indicate the cooling rate near the β transus temperature dictates the resulting phase characteristics, whereas the residual stress depends on the cooling rate immediately below the solidification temperature. This study provides new insights into the previously unknown differences in the α, α’, and β phase evolution between L-PBF and EB-PBF and their process parameters.

Powder bed fusion (PBF) is a category of additive manufacturing (AM) that is particularly suitable for the production of 3D metallic components. In PBF, only material in the current build layer is at the required melt temperature, with the previously melted and solidified layers reducing in temperature, thus generating a significant thermal gradient within the metallic component, particularly for laser based PBF components. The internal thermal stresses are subsequently relieved in a post-processing heat-treatment step. Failure to adequately remove these stresses can result in cracking and component failure. A prototype hip stem was manufactured from Ti6Al4V via laser PBF but was found to have fractured during over-seas shipping. This study examines the evolution of thermal stresses during the laser PBF manufacturing and heat treatment processes of the hip stem in a 2D finite element analysis (FEA) and compares it to an electron beam PBF process. A custom written script for the automatic conversion of a gross geometry finite element model into a thin layer- by-layer finite element model was developed. The build process, heat treatment (for laser PBF) and the subsequent cooling were simulated at the component level. The results demonstrate the effectiveness of the heat treatment in reducing PBF induced thermal stresses, and the concentration of stresses in the region that fractured.

The use of 3D printing (additive manufacturing) with metal has grown significantly in demand recently, greatly reducing the time and expense required to produce complex interconnected metal components. This method minimizes material wastage, facilitates material recycling, and eliminates the need for support materials. Among the various Metal Additive Manufacturing techniques, Powder Bed Fusion (PBF) processes stands out as the most prevalent for manufacturing parts. Within the realm of PBF, electron beam melting technique, selective laser sintering technique, and selective laser melting technique are the primary methods employed. Selective laser melting and selective laser sintering operate without the need for any special conditions, unlike EBM, which necessitates a vacuum environment. Regarding the choice of materials, laser melting/sintering processes are suitable for almost all types of metals except those which surpasses beam melting capabilities. While electron beam melting is constrained to a few materials such as titanium alloys, cobalt and chromium alloys, and nickel alloys, whereas selective laser melting and sintering allows for a broad range of materials, including iron and steel alloys. However, electron beam melting exhibit the ability to process brittle materials that would typically be challenging for melting and sintering through laser. Nevertheless, the ductility, yield testing, and ultimate testing of materials created through EBM are inferior to those processed by laser methods. Although all PBF techniques excel at creating complex structures, finishing products to have a smooth surface directly over a rough surface remains a subject of ongoing research. To attain suitable mechanical properties such as hardness, tensile strength, and endurance, critical process factors include power of laser or beam, speed for scanning, density for powder bed, thickness of laser or beam, and material characteristics. Inadequate material selection coupled with incorrect process settings can lead to issues such as porosity, slag formation, and other flaws.