L-PBF Process Research Articles

Computational design of a crack-free aluminum alloy for additive manufacturing

The design of new alloys adapted to LPBF and combining suitable mechanical strength together with a low cracking susceptibility is a promising way to produce defect-free parts made of aluminum. The current work proposes a design procedure relying on the decrease of the brittle temperature range to mitigate the hot cracking issue and additional optimization criteria concerning the phases fractions and the solid solution strengthening to preserve the mechanical strength and some ductility. The optimization functions are included in an aggregated genetic algorithm for a rapid and efficient identification of the optimal composition. The algorithm determined a promising alloy, with a brittle temperature range of only 9 °C, and mostly constituted of FCC phase (around 90%), with some Mg 2 Si and Al 9 FeNi precipitates. The alloy is found suitable for the LPBF process with no cracks observed. Hence, the criterion for the mitigation of hot cracking is validated. Mechanical results shown a high yield stress and ultimate trensile stress but an elongation at break quite low compared to conventional manufactured aluminum alloys.

Laser-particle interaction-based heat source model of laser powder bed fusion additive manufacturing

The use of powder is one of the special features in laser powder bed fusion additive manufacturing (L-PBF AM). How the powder affects the L-PBF process is the basic mechanism determining the accurate prediction of the melt pool and the temperatures in finite element heat transfer simulations. The laser is treated as electromagnetic wave essentially and the powder particles can be heated by the formation of the inducted current on the particle surfaces. The laser energy density absorbed by the powder particles was calculated based on the laser-particle interaction. Based on the statistical analysis of particles in spatial distribution, the new volumetric heat source model was obtained and then applied to finite element heat transfer simulations. Results indicate that the temperature rise of the individual particle is not uniform and relates to the distribution of electromagnetic energy density on the surface of particles. The distribution of electromagnetic energy density on the surface of the individual particle at different positions is different. The energy absorbed by the upper particles is mainly determined by the direct irradiation of the laser. The energy absorbed by the bottom particles is mainly caused by the laser reflections. Due to the change of the mechanism for energy absorption, the maximum energy absorption occurs at the location which is near the average diameter away from the top surface of the powder bed. When the powder bed thickness is small, the laser energy density along build direction is in Gaussian distribution. With the increase of the powder bed thickness, the laser energy density distribution along build direction can be fit for 4-order polynomial function.

Trabecular-like Ti–6Al–4V scaffold for bone repair: A diversified mechanical stimulation environment for bone regeneration

A bionic scaffold with ideal osteoconductive and osteoinductive characteristics is in urgent demand. The trabecular-like scaffold and corresponding regular scaffold with cubic cells were designed through Voronoi diagram and fabricated using L-PBF process. High-cycle fatigue test, fluid simulation, bone regeneration model prediction, and in vivo experiments were conducted. The research results revealed that the irregular structure significantly weakened the fatigue property of the trabecular-like scaffold, embodied by fast ratcheting strain accumulation and premature appearance of damage strain. The trabecular-like scaffold showed similar fluid behaviors to the CT-reconstructed cancellous bone, including permeation rate, pressure, velocity, and shear rate. It could be roughly concluded that there was a diversified mechanical stimulation environment formed in irregular structure, involving micro deformation, mass transportation, fluid shear stress and pressure, biological factor diffusion, better matching the demand for osteogenic mechanical stimulation, consequently making the implanted trabecular-like scaffold show better osteogenic ability than the regular scaffold.

Investigation of build strategies for microfeatures fabrication using laser powder bed fusion

Modifying the surface topography by fabricating the microfeatures on-top of interacting surfaces has the potential to enhance the tribological behavior of mechanical components having sliding motion. In the additive manufacturing process, Laser powder bed fusion process (LPBF) is chosen to fabricate microfeatures for metals. In LPBF, microfeature characteristics are based on the energy supplied and strategies used for fabrication. In this work, a single layer single track (SLST) experiment is carried out for a different combination of the laser power and scanning speed for fabricating continuous and uniform track width with good shape controllability. Further, the interaction between the lateral and vertical deposition to a single track is also analyzed by using single layer double track (SLDT) and double layer single track (DLST) strategies. It has been found that the LPBF process parameters have a significant influence on the track's morphology and its geometry irrespective of the build strategies. The continuous and uniform tracks are formed when the LED is greater than 0.3 J/mm for investigated build strategies. The microfeatures of width 150 μm to 300 μm and height 40 μm is recommended to fabricate by SLST strategy whereas for 60 μm height of microfeature DLST strategy is recommended. If the microfeatures width is greater than 300 μm then SLDT strategy is recommended. SLST and DLST show good control on the microfeatures characteristics as compared to SLDT strategy.

Electrochemical behavior of alloy 22 processed by laser powder bed fusion (L-PBF) in simulated seawater and acidic aqueous environments

Corrosion resistance of additively manufactured (AM) alloy 22 using Laser Powder Bed Fusion (L-PBF) was studied and compared with their wrought counterparts in 5 wt.% NaCl (pH ∼ 7.0) and 5 wt.% NaCl + 1.0 M HCl (pH ∼ -0.5) solutions. The l-PBF process parameters were selected from two extremes of a volumetric energy density (VED) process window. The effect of post-manufacturing processes on corrosion was also studied; these processes involved hot isostatic pressing (HIP) and solution treatment (ST). In 5 wt% NaCl, AM alloy 22 samples corrosion characteristics compared closely to wrought alloy 22 except when process parameters resulted in large pores, which acted as initiation sites for localized corrosion. In the acidic solutions, it was observed that the porosity and grain structure significantly affected corrosion performance. In general, the l-PBF-produced samples at higher VED (750 J/mm3) had lower porosity than samples produced at lower VED (188 J/mm3), which contributed to better corrosion performance. However, the lower ∑3 grain boundaries correlated to lower corrosion performance for all samples. HIP processing significantly lowered the porosity of the samples produced at lower VED and allowed for recrystallization of grain boundaries to increase the proportion of ∑3 grain boundaries, enhancing corrosion performance to near-wrought alloy 22 even in acidic solutions and at elevated temperatures.

Surface Roughness and Design for Additive Manufacturing: A Design Artefact Investigation

AbstractLaser Powder Bed Fusion (LPBF) brings the possibility to manufacture innovative near-net-shape part designs. But unfortunately, some designed surfaces suffer from rough surface finish due to characteristics of the LPBF process. This paper explores trends in managing surface roughness and through a space industry case study, a proposed process that uses Additive Manufacturing Design Artefacts (AMDAs) is used to investigate the relationship between design, surface roughness and fatigue. The process enables the identification of design uncertainties, however, iterations of AMDA's can be required.

Numerical Analysis of Residual Stresses in Additively Manufactured Materials

A finite element modeling approach was used to study the transient temperature field and mechanical response during LPBF processing of Ti6Al4V powder. The using of the Newton-Raphson method in structural analysis is described in detail. Significant effect of process parameters on residual stress and microstructure was found.

Residual Stresses Measurements in Laser Powder Bed Fusion Using Barkhausen Noise Analysis.

In recent years, the advancement of technology brought the laser powder bed fusion process to its industrialisation step. Despite all the advancements in process repeatability and general quality control, many challenges remain unsolved due to the intrinsic difficulties of the process, notably the residual stresses issue. This work aimed to assess the usability of Barkhausen noise analysis (BNA) for the residual stress in situ monitoring of laser powder bed fusion on Maraging steel 300 (18Ni-300/1.2709). After measuring the evolution of grain size distribution over process parameter changes, two series of experiments were designed. First, a setup with an external force allows to validate the working principle of BNA on the chosen material processed using LPBF. The second experiment uses on-plates samples with different residual stress states. The results show a good stability in microstructure, a prerequisite for BNA. In addition, the external load setup acknowledges that signal variation correlates with the induced stress state. Finally, the on-plate measurement shows a similar signal variation to what has been observed in the literature for residual stress variation. It is shown that BNA is a suitable method for qualitative residual stresses variation monitoring developed during the LPBF process and underlines that BNA is a promising candidate as an in situ measurement method.

Comparison of LPBF processing of AlSi40 alloy using blended and pre-alloyed powder

Elemental powders or pre-alloyed powders can be blended to form a powder stock with new compositions. In-situ alloying of blended powder mixtures during LPBF processing speeds up the alloy development process as it allows composition adjustment. Pre-alloyed powder is typically used in industrial settings to ensure consistency of the built material, whereas blended powders are used to experiment with new alloy compositions. In this research, we compare a range of binary alloy compositions built from blended elemental powder and pre-alloyed powder to evaluate how the feedstock powder affects the built material. The alloy is a lightweight alloy with a low coefficient of thermal expansion, comprised of Al and Si. The following compositions were formed by mixing pure Al and Si powder: AlSi38, AlSi40 and AlSi42, and pre-alloyed AlSi40 powder was also used. Parametric studies were performed for both the blended and pre-alloyed powders. It was found that the optimum process parameters for the blended and pre-alloyed powder were different. The line energy for pre-alloyed powder was selected to be 0.150 J/mm (300 W, and 2000 mm/s) which yielded a density of 99.5% of the theoretical density. The optimal density for the blended powders was 99.07-99.16% which was also achieved with 0.150 J/mm (375 W, 2500 mm/s). The alloys are brittle, with Young's modulus 86-99 GPa. Coefficient of thermal expansion (CTE) reduced with increasing Si content, material built from pre-alloyed powder had 5% higher CTE than material built from blended powder.

Computational framework for the simulation of multi material laser powder bed fusion

With the capability to build components with freeform geometries, laser powder bed fusion (L-PBF) is considered as one of the most promising techniques for industrial applications. Compared to conventional manufacturing technologies, an additional advantage of l-PBF process is the capability to fabricate parts with multiple materials, such as in-situ alloying and functionally graded alloys. In this regard, here we present a computational model to simulate the l-PBF of multiple materials. Based on volume of fluid (VOF) methods and computational fluid dynamics (CFD), the multi-physics multi-material model was successfully implemented with various physics of l-PBF process, including surface tension, Marangoni shear force, recoil pressure, compositional diffusion, etc. In addition, we demonstrated the numerical algorithm of ray tracing heat source for multiple materials. With reasonable assumptions, conduction mode or keyhole mode laser melting of miscible materials can be simulated via the proposed computational framework. Furthermore, such multi-physics model is capable of simulating l-PBF of an arbitrary number of materials. The computational model can help achieve insightful understanding of the l-PBF of multiple materials.

A testbed for investigation of laser powder bed fusion at elevated atmospheric pressure

Metal additive manufacturing (AM) by laser powder bed fusion ( L -PBF) builds upon fundamentals established in the field of laser welding which include the influence of gas and plume dynamics on weld depth and quality. L -PBF demands a thorough investigation of the complex thermophysical phenomena that occur where the laser interacts with the metal powder bed. In particular, melt pool turbulence and evaporation are influenced by the ambient gas chemistry and pressure, yet typical L-PBF equipment lacks flexibility to alter the ambient pressure within the process chamber. This paper presents the design and validation of high pressure laser melting (HPLM) testbed. This system accommodates bare metal plate samples as well as manually-coated single powder layers, and operates at up to 300 psig. The open architecture of this testbed allows for full control of all relevant laser parameters in addition to ambient gas pressure and gas flow over the build area. Representative melt tracks and rasters on bare plate and powder are examined in order to validate system performance, and preliminary analysis concludes that pressure has a significant impact on melt pool aspect ratio. The HPLM system thus enables careful study of the effects of pressure on the outcomes of the L-PBF process, and can be applied in the future to materials that are challenging to process by L-PBF under ambient pressure, such as those with high vapor pressures.

Effects of Build Angle on Additively Manufactured Aluminum Alloy Surface Roughness and Wettability

AbstractLaser powder bed fusion (LPBF) was utilized to create a series of aluminum alloy (i.e., AlSi10Mg) 5 mm-diameter support pillars with a fixed height of 5 mm containing varying filet angles and build orientations (i.e., 0 deg, 10 deg, 20 deg, 30 deg, 40 deg, 50 deg, and 60 deg from the normal surface) to determine surface roughness and water wettability effects. From experiments, anisotropic wetting was observed due in part to the surface heterogeneity created by the LPBF process. The powder-sourced AlSi10Mg alloy, typically hydrophobic, exhibited primarily hydrophilic behavior for build angles of 0 deg and 60 deg, a mix of hydrophobic and hydrophilic behavior at build angles of 10 deg and 20 deg, and hydrophobic behavior at 30 deg, 40 deg, and 50 deg build angles. Measured surface roughness, Ra, ranged from 5 to 36 µm and varied based on location. 3D-topography maps were generated, and arithmetic mean heights, Sa, of 15.52–21.71 µm were observed; the anisotropy of roughness altered the wetting behavior, thereby prompting some hydrophilic behavior. Build angles of 30 deg and 40 deg provided for the smoothest surfaces. A significantly rougher surface was found for the 50 deg build angle. This abnormally high roughness is attributed to the melt pool contact angle having maximal capillarity with the surrounding powder bed. In this study, the critical melt pool contact angle was near equal to the build angle, suggesting that a critical build angle exists, which gives rise to pronounced melt pool wetting behavior and increased surface roughness due to enhanced wicking followed by solidification.

Digital twin-driven multi-variable process control of thermal manufacturing processes

Thermal manufacturing processes are complicated to the extent that they require sophisticated performance optimization. However, their implemented form often lacks advanced control strategies. This research proposes digital twins (DTs) that generalize the concept of process control towards multivariable performance optimization. To achieve this, data-driven models are used to describe the mathematical link between the process parameters and the performance indicators (KPIs), while they are all defined based on the spatiotemporal profile of the temperature field. The control strategy is then designed based on time and quality criteria, while the control enforcement is regarded in real-time manner, so that the DT concept is validated. The performance of the process models is examined through aggregation of modelling details from physics-based models from an LPBF process, including delays and dynamics. The current proposed framework seems to close the gap between the theoretical and data driven models with respect to process modelling and control efficiency.

Effect of DED coating and DED + Laser scanning on surface performance of L-PBF stainless steel parts

The advantages of the Laser Powder Bed Fusion process have led to growing interest from an industrial and academic point of view. However, the diffusion of this process has been slowed by technical challenges that have yet to be resolved, including non-optimal surface quality and low repeatability. To fix these issues and recover surface quality, several treatments are available. Our study focuses on one of them: the Directed Energy Deposition (DED) treatment, which in addition can be exploited for the repair of damaged components. In fact, the proposed study has been conducted by using AISI 316 L alloy both for producing parts throughout the L-PBF process and for coating them. Furthermore, a final Laser Scanning (LS) treatment has been carried out with the aim of maximizing the DED coating performances, in terms of surface roughness , microhardness and microstructural properties. To detect the anomalies that can emerge during the previous surface treatments, an optical monitoring system was implemented in the visible range by using a coaxial CCD camera, to monitor the geometry and morphology of the melt pool related to the DED and to the subsequent LS treatment. The results revealed many improvements in terms of surface roughness for the DED coating and even more for the LS treatment. Moreover, the latter was fundamental to improve the microhardness and the microstructural characteristics of the final coating. The data gathered and analyzed by the implemented monitoring system will allow making the treatment more stable and performing for future applications on complex geometries. • Coatings using DED + LS treatment on a L-PBF substrate were successfully achieved. • Laser scanning significantly improves DED coating performances. • Microstructure after the laser scanning treatment is an equiaxial fine grain. • The produced coating is uniform due to the stability of the melt pool. • Coatings quality was monitored in-process through optical and thermal systems.

Comparison of Lpbf Processing of Alsi40 Alloy Using Blended and Pre-Alloyed Powder

Comparison of Lpbf Processing of Alsi40 Alloy Using Blended and Pre-Alloyed Powder

Monitoring of the L-PBF process to analyze the effect of powder bed temperature on parts roughness

One of the limitations of the L-PBF technology is the high surface roughness of the parts. The resulting surface roughness morphology is a combination of the layer-by-layer building strategy and a stochastic pattern of spherical particles partially adhered to the surface. This stochastic pattern is composed by partially melted powder particles due to the heat transfer between the melt-pool and the powder particles at the part boundary. In this study, an experimental setup was designed to analyze the influence of the heat transfer of the L-PBF process on the adhered particles on the surface. Thus, the test parts consist on vertical walls distributed at different distances, from 0.2 mm to 5 mm, in order to promote different thermal gradients between surfaces. The manufacturing process was monitored using an IR camera, in order to find the relationship between the heat transfer and the mechanism of adhesion of particles. Moreover, the morphology of the resulting surface roughness of the parts was also analyzed. The results show a correlation between the temperature gradients and the number of adhered particles on the surfaces.

Effect of Density in Alloy 400 Developed by LPBF Process Exposed at High Temperatures on Oxidation Resistance in Steam Atmosphere

Effect of Density in Alloy 400 Developed by LPBF Process Exposed at High Temperatures on Oxidation Resistance in Steam Atmosphere

Design, Simulation and Optimization of an Additive Laser-Based Manufacturing Process for Gearbox Housing with Reduced Weight Made from AlSi10Mg Alloy

The gas turbine engine’s (GTE) development aims for the increasing the efficiency, strength, reliability and safety of its components. To create competitive engines, housing parts and components with high functionality and reduced weight are needed. Especially difficult in the design and production are the gearboxes for aviation GTE. Traditional technologies based on precision casting or material forming operations have significant limitations due to the complexity of fulfilling multiple different requirements. Nowadays, one of the progressive production techniques is additive manufacturing. The article presents the results of computational and experimental studies that substantiate the applicability of laser additive technology to reduce the mass of body parts by up to 15% while ensuring their strength properties. The physical and mechanical characteristics of aluminum alloys acceptable for the manufacturing of housing parts were analyzed. The necessary characteristics of the powder alloy of the Al-Si system and the technological parameters of the L-PBF of the modified housing of the gear reducer are established. Using the finite element method (FEM) the L-PBF process was numerically simulated and the technological modes for synthesis of the AlSi10Mg alloy powder were optimized. With the help of a serial 3D printer ProX320DMP, the prototype of a gear housing was manufactured.

Thermal flow characteristics and the evolution of laser absorption in laser powder bed fusion of Cu-Cr-Zr alloy

The ultra-high transience and complexity of the LPBF process brings critical challenges to understanding the underlying physics behind the defect formation, and mathematical modeling of the melt pool dynamics in LPBF is being a good method to overcome this issue. Since the laser heating is central to the LPBF process, it is vital to pay more attention to how to define the laser heat source in simulation. This study focuses on the “top free surface” heat source model that is widely used in existing literatures, of which two aspects of deficiencies are modified. On the one hand, the loading pattern of heat source at the interface is optimized by a proposed scattered heat source (SHS) model. The results show that the SHS model has an apparent advantage of less generation of artificial high temperature and better energy convergence. On the other hand, a calorimetric measurement is adopted to obtain the practical effective laser absorptivity under different process parameters. With implantation of measured absorptivity, the simulation fidelity compared to experiments is significantly enhanced in terms the track width and depth, and the absorptivity uncertainty derived from existing literatures on modeling copper alloy is well solved. Next, the physics behind the variation of effective absorptivity with scan speed is studied. Then, the characteristics of Cu-Cr-Zr alloy in LPBF induced by the huge thermal conductivity is identified by comparing with the case of commonly LPBF-used material (stainless steel 316 L). Finally, the influence of scan speed on the resultant track morphology and keyhole-induced porosity evolution is discussed.

Mechanical properties of Inconel 718 additively manufactured by laser powder bed fusion after industrial high-temperature heat treatment

Heat treatment of laser powder bed fused (LPBF-ed) Inconel 718 is crucial to achieve desired mechanical properties. Residual stresses, microsegregation, anisotropy, undesirable phases, layered structure, and poor surface quality are the challenges to be faced before LPBF-ed Inconel 718 parts can be industrially implemented. Although heat treatment of Inconel 718 after the LPBF process is widely discussed in the literature, an industrial approach has been neglected, or the proposed heat treatment processes do not address all the aforementioned challenges. Recently, extensive work has been done in the field of high-temperature heat treatment of this material, showing promise. Following current trends, a 4-stage high-temperature heat treatment, developed by considering the industrial approach, is proposed and evaluated in this paper. A structured, microstructural, and mechanical evaluation of the achieved material properties is presented. Several factors were taken into account, including testing orientation in relation to the build direction (0°, 45°, 90°), surface condition (abrasive blasted, machined), testing temperature (21 °C, 425 °C), specimen size (dia. 4.06 mm, dia. 6.35 mm), application of the HIP process (with, without). After eliminating the influence of surface, subsurface and internal defects, the material is characterized with isotropic properties, regardless of testing orientation in relation to the build direction. Isotropic properties are confirmed for both room temperature and elevated temperatures (425 °C). The obtained elongation is >1.5× higher than the literature reports for heat treated LPBF-ed Inconel 718, while maintaining high tensile strength at the average level found in the literature.