Laser-based Powder Bed Fusion Research Articles

Improved Corrosion and Cavitation Erosion Resistance of Laser-Based Powder Bed Fusion Produced Ti-6al-4v Alloy by Pulsed Magnetic Field Treatment

Improved Corrosion and Cavitation Erosion Resistance of Laser-Based Powder Bed Fusion Produced Ti-6al-4v Alloy by Pulsed Magnetic Field Treatment

Additive manufacturing enabled designs for dental applications: A conceptual study

Additive manufacturing (AM), or commonly also known as three-dimensional (3D) printing, has fundamentally changed the way products are designed and manufactured due to its freeform fabrication capabilities. As AM becomes more mainstream and accessible, it is therefore important to adopt new designs, or re-designing approaches, for existing products that would allow parts to be manufactured by these techniques. This study conceptualizes and derives new suitable designs for dental abutment connector used in dental implants and proof of concept is achieved using laser-based powder bed fusion of metals (PBF-L/M). The functionality of the design is tested using simulations by running static and cyclic loading to ensure that the new design can withstand a normal human bite force. Design optimization is also conducted by varying design parameters to achieve minimal Von Mises stresses throughout the abutment. Finally, the designs are compared using various design considerations and it is concluded that snap fit design is the most suitable for dental abutment due to ease of use and manufacturability.

Improved Corrosion and Cavitation Erosion Resistance of Laser-Based Powder Bed Fusion Produced Ti-6al-4v Alloy by Pulsed Magnetic Field Treatment

Improved Corrosion and Cavitation Erosion Resistance of Laser-Based Powder Bed Fusion Produced Ti-6al-4v Alloy by Pulsed Magnetic Field Treatment

Capability of Multi-Material Laser-Based Powder Bed Fusion—Development and Analysis of a Prototype Large Bore Engine Component

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.

The microstructure and fatigue performance of Inconel 718 produced by laser-based powder bed fusion and post heat treatment

Microstructure, defects and their effects on the fatigue properties of Inconel 718 alloy after laser-based power bed fusion and subsequent heat treatment were extensively investigated. A large number of γ′ and γ″ phases were produced in the heat-treated samples while little changes in grain morphology and texture occurred. The hardness, tensile strength, and fatigue limit increased by 46%, 41%, and 27%, respectively, after heat treatment. The fatigue limit could be predicted using defects based on optical microscopy measurement of the bulk samples and examination of SEM fatigue crack origins via a combination of Murakami model and extreme value statistics.

Simulation of phase evolution in a Zr-based glass forming alloy during multiple laser remelting

Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.

Spatter Formation and Splashing Induced Defects in Laser-Based Powder Bed Fusion of AlSi10Mg Alloy: A Novel Hydrodynamics Modelling with Empirical Testing

Powder spattering and splashing in the melt pool are common phenomena during Laser-based Powder Bed Fusion (LPBF) of metallic materials having high fluidity. For this purpose, analytical and computational fluid dynamics (CFD) models have been deduced for the LPBF of AlSi10Mg alloy. The single printed layer’s dimensions were estimated using primary operating conditions for the analytical model. In CFD modelling, the volume of fluid and discrete element modelling techniques were applied to illustrate the splashing and spatter phenomena, providing a novel hydrodynamics CFD model for LPBF of AlSi10Mg alloy. The computational results were compared with the experimental analyses. A trial-and-error method was used to propose an optimized set of parameters for the LPBF of AlSi10Mg alloy. Laser scanning speed, laser spot diameter and laser power were changed. On the other hand, the powder layer thickness and hatch distance were kept constant. Following on, 20 samples were fabricated using the LPBF process. The printed samples’ microstructures were used to select optimized parameters for achieving defect-free parts. It was found that the recoil pressure, vaporization, high-speed vapor cloud, Marangoni flow, hydraulic pressure and buoyancy are all controlled by the laser-material interaction time. As the laser-AlSi10Mg material interaction period progresses, the forces presented above become dominant. Splashing occurs due to a combination of increased recoil pressure, laser-material interaction time, higher material’s fluidity, vaporization, dominancy of Marangoni flow, high-speed vapor cloud, hydraulic pressure, buoyancy, and transformation of keyhole from J-shape to reverse triangle-shape that is a tongue-like protrusion in the keyhole. In the LPBF of AlSi10Mg alloy, only the conduction mode melt flow has been determined. For multi-layers printing of AlSi10Mg alloy, the optimum operating conditions are laser power = 140 W, laser spot diameter = 180 µm, laser scanning speed = 0.6 m/s, powder layer thickness = 50 µm and hatch distance = 112 µm. These conditions have been identified using sample microstructures.

Structural and Tribological Assessment of Biomedical 316 Stainless Steel Subjected to Pulsed-Plasma Surface Modification: Comparison of LPBF 3D Printing and Conventional Fabrication.

The structural features and nanoindentation/tribological properties of 316 stainless steel fabricated by conventional rolling and laser-based powder bed fusion (LPBF) were comparatively investigated regarding the effect of surface-pulsed plasma treatment (PPT). PPT was performed using an electrothermal axial plasma accelerator under a discharge voltage of 4.5 kV and a pulse duration of 1 ms. Optical microscopy, scanning electron microscopy, X-ray diffraction, nanoindentation measurements and tribological tests were applied to characterize the alloys. The LPBF steel presented almost the same modulus of elasticity and double the hardness of rolled steel. However, the LPBF steel manifested lower dry-sliding wear resistance compared with its wrought counterpart due to its porous structure and non-metallic inclusions. Conversely, LPBF steel showed three times higher wear resistance under sliding in simulated body fluid (SBF), as compared with wrought steel. PPT led to steel modification through surface melting to a depth of 22–26 μm, which resulted in a fine cellular structure. PPT moderately improved the dry-sliding wear resistance of LPBF steel by fusion of pores on its surface. On the other hand, PPT had almost no effect on the SBF-sliding wear response of the steel. The modification features were analyzed using a computer simulation of plasma-induced heating.

Influence of Ring-Shaped Beam Profiles on Process Stability and Productivity in Laser-Based Powder Bed Fusion of AISI 316L

A major factor slowing down the establishment of additive manufacturing processes as production processes is insufficient reproducibility and productivity. Therefore, this work investigates the influence of ring-shaped beam profiles on process stability and productivity in laser-based powder bed fusion of AISI 316L. For this purpose, the weld track geometries of single tracks and multi-track segments with varying laser power, scan speed, hatch distance, and beam profile (Gaussian profile and three different ring-shaped profiles) are analyzed. To evaluate the process robustness, process windows are identified by classifying the generated single tracks into different process categories. The influence of the beam profiles on productivity is studied by analyzing the molten cross-sectional areas and volumes per time. When using ring-shaped beam profiles, the process windows are significantly larger (up to a laser power of 1050 W and a scanning speed of 1700 mm/s) than those of Gaussian beams (laser power up to 450 W and scanning speed up to 1100 mm/s), which suggests a higher process robustness and stability. With ring-shaped beam profiles, larger volumes can be stably melted per track and time. The weld tracks created with ring-shaped profiles are significantly wider than those generated with Gaussian profiles (up to factor 2 within the process window), allowing enlargement of the hatch distances. Due to the higher scanning speeds and the enlarged hatch distances for ring-shaped beam profiles, the process can be accelerated by a factor of approximately 2 in the parameter range investigated.

Crack-healing, a novel approach for a laser-based powder bed fusion of high-performance ceramic oxides

• Ceramic Powder-bed fusion sample with mechanical ultimate strength of up to 200 MPa. • Self-healing materials enable high strength in powder bed fusion of ceramics. • The healing of cracks is effective for volumes up to 10 × 10 × 10 mm. Laser-based powder bed fusion (PBF-LB) of ceramic materials have not been developed sufficiently. The process-induced crack formation is the key challenge of the associated additive manufacturing method, which prevents its successful industrial implementation. Therefore, we have adopted a strategy to accept crack formation as an inherent characteristic of the manufacturing process while enhancing the mechanical properties of the parts by repairing the cracks using a thermal post-processing. Crack healing was successfully demonstrated in the PBF-LB-manufactured alumina titanium carbide (TiC) composite parts after thermal treatment in the air at 900 °C. Crack healing is achieved through the phase transformation from TiC to TiO 2 in the form of rutile. The built parts exhibited a mechanical strength up to two times higher than values demonstrated in the previous studies on alumina PBF-LB without preheating, achieving an average flexural strength value of 181 MPa and a maximum value of 232 MPa.

Integration of piezoelectric stacks in components using powder bed fusion

Laser-based powder bed fusion of metals (PBF-LB/M) is the most commonly used additive manufacturing process for fabricating complex metal parts by selective, layer-wise melting of metallic powder using a laser beam. This manufacturing technique can easily fabricate parts with complex geometries that cannot be fabricated using conventional manufacturing processes. These parts with complex geometries are generally used by aerospace and space industries, and advancement in functionalization of additive manufactured parts is highly beneficial to these industries. However, the parts constructed using additive manufacturing are monolithic, stiff, and lightweight and hence, they are vulnerable to high amplitude resonant vibrations. This is due to the low damping factor of the materials used and the absence of interfaces and connections that contribute to structural damping in conventional structures. The integration of piezoelectric materials within these structures would enable the control of vibration characteristics. The techniques presented in this study will enable a high level of freedom in the placement of piezoelectric materials and investigate the potential of merging parts constructed using additive manufacturing with piezoelectric materials. Furthermore, a technique to track the stress state during the integration process, which is crucial for the pre-stress evaluation of integrated piezoelectric stacks, is presented and shows characteristics similar to a force cell. Pre-stress is successfully tracked during integration and in some concepts tensile stress onto the piezoelectric material is occurring. Finally, to verify the functionality for potential piezoelectric damping, power conversion was reported with laser vibrometer measurements and FE validation.

Heat treatment of Ti-6Al-4V alloy manufactured by laser-based powder-bed fusion: Process, microstructures, and mechanical properties correlations

The present study comprehensively a investigates the correlations between the heat treatment process parameters (temperature, time, and cooling rate) with the microstructure and tensile properties of Ti-6Al-4V samples manufactured by laser-based powder-bed fusion (L-PBF) additive manufacturing process. Understanding these correlations results in better engineering of the microstructure to obtain a desirable combination of ductility and strength for different applications. The primary, secondary, and tertiary α' with β particles are seen in the microstructure of the as-built sample. The heat treatment at below β transus temperature (Tβ) shows a similar microstructure to the as-built samples with a considerable increment in elongation. Increasing temperature in the range of α + β region illustrates the formation of α + β lamellar structure (primary, and secondary α) with increasing elongation and decreasing tensile strength. While increasing temperature above Tβ deteriorates the mechanical properties due to increasing the thickness of α lath, increasing the time of annealing leads to increasing and then decreasing elongation. Increasing the cooling rate from furnace cooling to air cooling and finally to water quenching results in increasing yield strength and ultimate tensile strength and decreasing the elongation due to the microstructural features such as α', α, and β. Finally, the fractography of brittle and ductile samples is presented and correlated with the obtained microstructural and mechanical properties.

Numerical study of laminar forced convection heat transfer in a rough mini-channel fabricated by Additive Manufacturing

This study investigates laminar convective heat transfer of water flowing in a mini-channel with a rough surface fabricated by Laser-based Powder Bed Fusion (L-PBF) technology. A Gaussian model was used for generating random roughness, and then the three-dimensional numerical simulation was performed in ANSYS-Fluent 19.1. The numerical results indicated a more than double increase in the Nusselt number of rough channels than that of smooth ones with a marginal pressure drop penalty compared to smooth channels, showing the potential benefits of using rough channels fabricated by L-PBF for heat transfer applications.

Hybrid additive manufacturing of hot working tool steel H13 with dissimilar base bodies using Laser-based Powder Bed Fusion

Hybrid additive manufacturing (HAM) describes the combination of additively built structures onto a conventionally manufactured base body. The advantages of both manufacturing processes are combined in one process chain. As a result, new applications can be achieved with higher cost-effectiveness. With the Additive Manufacturing (AM) process a bonding zone is created that is comparable to a welded joint. In order to evaluate the quality and mechanical properties of the bonding zone, two steels (42CrMo4 and 25CrMo4) are investigated as base body materials with the hot working tool steel X40CrMoV5-1 (AISI H13) for the AM structure. Process parameters for Laser-based Powder Bed Fusion of X40CrMo4V5-1 are developed to achieve a crack and defect free structure as well as an optimized bonding zone in dependency of the base body material. Furthermore, the chemical and mechanical properties are examined in the as-built and heat-treated state. It is observed that a crack-free material bonding is possible and samples with relative densities above 99.5% are obtained. The size of the bonding zone depends on the material of the base body as well as post-process heat treatment. An average hardness of 600 HV1 can be achieved in the “as-built” state.

Design and fabrication of personalized knee prostheses by laser-based powder bed fusion: Influence of manufacturing process on geometric accuracy

The manufacture of metal prostheses by a laser-based powder bed fusion process allows the definition of more effective customized prostheses, increasing the functionality of the operated joint, reducing the risk of implant failure, and increasing the time before revision. For this personalization to be successful, a high accuracy is required when fabricating the prosthesis surfaces. Aiming to understand the performance of this manufacturing process, this article reports the results in terms of geometrical accuracy of a cobalt-chrome alloy knee prosthesis at different process and postprocess conditions. The prosthesis was designed based on experimental data of a real subject. Starting from medical images, a personalized mathematical model of the knee featuring ligament and contact constraints was defined and used to determine the shape of the implant. The process and post-treatment parameters were defined to limit the residual stress of the component after the support removal, in order to minimize geometrical deformations. The optimal process parameters were obtained by experimental tests in combination with a simulation software for the prediction of thermomechanical deformation. The overall manufacturing procedure was validated by comparing the designed and obtained geometry, measured through an optoelectronic system and a laser scanner. Also, the properties of the components in terms of density, hardness, and roughness were verified. The results show that the proposed design procedure is feasible and accurate, reaching an average deviation between the theoretical and obtained surface of −0.02 ± 0.18 mm. The overall procedure also increased the hardness of the prosthesis.

Post-processing of additively manufactured metal parts by ultrashort laser pulses for high-quality net shape geometries and advanced functionality

Additive manufacturing by means of laser-based powder bed fusion (LPBF) offers high flexibility with respect to the generation of individualized and light-weight metal parts. However, the produced parts are typically attached to support structures and deviate a few tens of micrometers from the targeted final component in geometrical net shape and surface roughness due to the melt-based fusion process. Therefore, different post-processing techniques were examined in the past to resolve the mentioned quality drawbacks. In our work, we investigated the potential of post-processing of LPBF-generated Ti6Al4V parts with ultrashort pulse laser ablation. As a result, the support structures were effectively removed, the surface roughness was reduced by 81% and complex geometries with high shape accuracy were fabricated. Furthermore, the LBPF-generated parts were laser surface structured to investigate the potential of post-processing with ultrashort laser pulses for advanced functionality, such as water-repellent surfaces. The generation of surface structures on the LPBF-generated Ti6Al4V part changed the wetting behaviour from hydrophilic to hydrophobic with an increased contact angle from 73° up to 130°.

Additive manufacturability of superalloys: Process-induced porosity, cooling rate and metal vapour

Digital technology such as metal additive manufacturing (AM) provides flexible process design freedom to fabricate intricate three-dimensional structures layer-by-layer. However, its manufacturability relies on the fundamental understanding of melt pool physics and fluid (metal) dynamics. The effect of metal vapour and porosity induced during the laser-materials interaction can influence the additive manufacturability. In this work, composition-process relationship of laser-based powder-bed fusion (L-PBF) AM is studied via computational fluid dynamics modelling to rationalise solid-liquid-vapour transformation where empirical-based approach is used to generate thermo-physical property of about 100 nickel-based superalloys at the liquid state. It is found that with larger vapor mass loss, the porosity tends to be higher. However, the higher vapour mass loss means faster cooling rate. This is indicated that the thermal-fluid flow process, which is also governed by the thermo-physical property, strongly affects the additive manufacturability. Additive manufacturability map based upon porosity, cooling rate from liquid to solid, volatile mass loss criteria has been established to link the composition in nickel-based superalloys with their thermo-physical property. This offers a thermal-fluid science based tool in designing compositions of novel superalloys for AM applications.

On the influence of process interruptions during additive manufacturing on the fatigue resistance of AlSi12

The process of laser-based powder bed fusion of metals (PBF-LB/M) is one of the most promising technologies in the field of additive manufacturing. The high freedom of design enables the integration of functionality and production of customized parts. Main drawbacks of this technology are related to relatively low build rates and reliability. The reliability of parts can be derogated due to various process instabilities leading to process-induced defects and process errors, where the latter can even lead to an interruption of the process. Due to limited build rates, a continuation of the process would be highly desirable, however, data reporting on the structural integrity of such interruptedly built parts are hardly available in literature so far. To tackle this research gap, the current study investigates the influence of process interruptions on the properties of AlSi12 processed by PBF-LB/M. The influence of the interruption on microstructure evolution is examined by electron backscatter diffraction. Mechanical properties of these specimens under monotonic and cyclic loading ranging from low-cycle fatigue up to very high-cycle fatigue regimes are investigated and compared to data obtained from counterparts built without interruption.

Surface roughness variance on different levels of surface inclination of powder bed fused tool steel 1.2709

Evolution of additive manufacturing has allowed increased flexibility and complexity of designs over conventional manufacturing e.g. formative and subtractive manufacturing. Restricting factor of laser powder bed fusion of metals (PBF-LB/M) additive manufacturing is the as-built surface quality. To promote an understanding of the surface roughness and suitable surface measuring technologies octagon shaped tool steel 1.2709 samples was developed and manufactured. Different surface measuring technologies was also literary reviewed. Studied samples were manufactured with commercially available laser-based powder bed fusion system using standard parameter set provided by the system manufacturer. Surface roughness was measured from top and down skins from multiple different building angles in a way that process specific effects, such as direction of movement of the powder re-coater, was considered. Based on these measuring results of the samples the effect surface inclination are discussed. The results show that building angle strongly affects to surface roughness of laser-based powder bed fused parts. Surface roughness was measured to be more than five times worse in unsupported angle manufactured down facing surfaces when compared with vertical walls.

Numerical analysis of the effect of processing parameters on the microstructure of stainless steel 316L manufactured by laser-based powder bed fusion

Additive manufacturing is a highly projected technology that continues to evolve and improve. In this context, laser-based powder bed fusion (L-PBF) offers a series of advantages for manufacturing parts of high geometric complexity, impossible or too costly to process through conventional processes. However, anticipating the effect of processing parameters on the resulting microstructure remains challenging due to the complexity of the involved processes. In this sense, numerical simulation, also substantially improved in the recent decade, in particular finite element analysis, offers considerable precision, comparable to results obtained by experimentation. Therefore, in this work, the applicability of numerical simulation is evaluated to predict the effect of laser power, scanning speed, and hatch spacing on the dimensions of the molten pool, degree of porosity, and the resulting microstructure. Ansys Additive package was employed to simulate a single bed deposit, evaluate the porosity, and determine the microstructure of the 316L stainless steel processed through L-PBF. The results indicate that the lowest degree of porosity is obtained with high laser power, low scanning speed, and the lowest hatch spacing with a recommendable volumetric energy density greater than 60 J/mm3. Through the simulation of the microstructure, it was possible to obtain information on the thermal gradient, solidification rate, and melt pool characteristics. Ansys Additive proves a powerful tool to understand microstructure evolution and furthering customization of mechanical properties.