Laser Powder-bed Fusion Additive Manufacturing Research Articles

Mechanical behaviour and high strain rate deformation of stainless steel 316L processed by selective laser melting

With growing interest in metal additive manufacturing (AM) for aerospace and defence applications, it is important to understand the influence of processing conditions on high strain rate behaviour. This paper presents research on as-built stainless steel 316L AM parts processed using laser powder bed fusion (L-PBF) also referred to as selective laser melting (SLM). Two sets of varied process parameters were examined as well as different build orientations. Laser scan speed and point-to-point distance are shown to have influenced the high strain rate mechanical properties of 316L AM parts. Additionally, both wrought and SLM produced 316L showed strain rate sensitivity (hardening) at strain rates of 103 s1, but this effect was more pronounced in the wrought material. Findings from this study can be used to determine appropriate part orientations and AM process parameters and their influence on the high strain rate behaviour and impact performance of L-PBF AM parts.

Experimental and numerical investigation on the effect of layer thickness during laser powder-bed fusion of stainless steel 17-4PH

Layer thickness is one of the most important input process parameters in laser powder-bed fusion (LPBF) additive manufacturing (AM) since it directly affects the defects found in the printed products, such as porosity, cracks, and manufacturing rate. In this work, three-dimensional finite element heat transfer model was employed to compare and evaluate two different powder layer thicknesses (20 μm and 40 μm) at varying laser power and scanning speeds. A layer-thickness dependent laser absorptivity approach was considered to improve the prediction accuracy of the proposed model. Single track experiments with stainless steel 17-4PH were conducted to validate the simulation model. Simulation results show good agreement with the experimental results with different layer thicknesses. The corresponding averaged melt pool error for width and depth were 4.2% and 9.1%, respectively. It is found that the melt pool dimensions with different layer thicknesses are similar for the most part with slight variations in the melt pool dimensions using varying laser power and scanning speed. However, the morphology of the melt pool track shows visible changes between different thicknesses.

Study of powder recycling and its effect on printed parts during laser powder-bed fusion of 17-4 PH stainless steel

Abstract The freedom of design and ability to print complex parts have made laser powder-bed fusion (LPBF) additive manufacturing (AM) a suitable alternative to traditional metal manufacturing approaches. As the focus of metal AM is shifting from prototyping to large-scale production, cutting the cost of AM powder remains a major priority. The metal LPBF process is likely to be more economical if feedstock powders are reusable without sacrificing the physical and mechanical properties of final parts. This work introduces the use of dosage to study powder recycling without adding any virgin powder to the feeder. To understand the effects of powder recycling in LPBF of 17-4 PH stainless steel powders, this work studies the properties of both recycled powders and the printed parts after ten successive prints. Powder rheology, morphology, microstructure, and chemical compositions are investigated to study the effect of recycling. Compared to virgin powder, the average particle size increased slightly while powder flowability decreased significantly in the recycled powders. The effect of powder recycling on the microstructure, chemical composition, porosity, roughness and tensile properties of printed parts are also investigated. Compared to the Print 1, samples printed in the Print 10 showed an increase in pore size (54 %) and surface roughness at the Top surface (17 %) resulting in a ∼7 % reduction in ductility.

On thermal properties of metallic powder in laser powder bed fusion additive manufacturing

Powder thermal properties play a critical role in laser powder-bed fusion (LPBF) additive manufacturing, specifically, the reduced effective thermal conductivity compared to that of the solid significantly affects heat conduction, which can influence the melt pool characteristics, and consequently, the part mechanical properties. This study intends to indirectly measure the thermal conductivity of metallic powder, nickel-based super alloy 625 (IN625) and Ti-6Al-4V (Ti64), in LPBF using a combined approach that consists of laser flash analysis, finite element (FE) heat transfer modeling and a multivariate inverse method. The test specimens were designed and fabricated by a LPBF system to encapsulate powder in a hollow disk to imitate powder-bed conditions. The as-built specimens were then subjected to laser flash testing to measure the transient thermal response. Next, an FE model replicate the hollow disk samples and laser flash testing was developed. A multi-point optimization algorithm was used to inversely extract the thermal conductivity of LPBF powder from the FE model based on the measured transient thermal response. The results indicate that the thermal conductivity of IN625 powder used in LPBF ranges from 0.65 W/(m∙K) to 1.02 W/(m∙K) at 100 °C and 500 °C, respectively, showing a linear relationship with the temperature. On the other hand, Ti64 powder has a lower thermal conductivity than IN625 powder, about 35% to 40% smaller. However, the thermal conductivity ratio of the powder to the respective solid counterpart is quite similar between the two materials, about 4.2% to 6.9% for IN625 and 3.4% to 5.2% for Ti64.

Effect of ultrasonic excitation on the process of L-PBFAM

Abstract Laser-Powder bed fusion additive manufacturing (L-PBFAM) is a layer-by-layer metal manufacturing technique that allows unlimited freedom of design with no increase in manufacturing complexity. Currently, parts made through L-PBFAM need to be heat treated and annealed to prevent residual stress, anisotropy, low ductility, and low fatigue life inherent to the process. This paper demonstrates that the adoption ultrasonic excitation to the L-PBFAM process is able to alleviate some of the aforementioned problems. To determine the effects of ultrasonic excitation on the solidification process and microstructure formation, a phase field simulation model incorporating the effect of ultrasonic cavitation in the melt pool was created. Finite Element Method simulation in ANSYS was used to determine the initial macro thermal condition of the melt pool, while the phase field was used to simulate the nucleation and grain growth within the melt pool under ultrasonic excitation based on thermodynamic theory. Robust Design of Experiment (DOE), guided by insights on the L-PBFAM process, was conducted to determine the contributions of different process parameters on the grain size and configuration. The simulated grain structure showed close correlation to optical micrographs of fabricated 304 L Stainless Steel L-PBFAM samples. The ultrasonic excitation is shown to have a significant effect on the microstructure and the cooling rate of the melt pool is found to be the key process optimization parameter affecting the grain morphology.

Electrochemical Characterization of Ti6Al4V Components Produced by Additive Manufacturing

The investigation of the corrosion resistance of Ti6Al4V alloy components produced by additive technology is still lacking in the literature. This paper aims to study the electrochemical behaviour of Ti6Al4V components fabricated by laser powder-bed fusion additive manufacturing process. The metallographic analysis was carried out by an optical microscope. The electrochemical behaviour has been evaluated in 3.5 wt. % of natural aerated NaCl aqueous solution by potentiodynamic polarization test. The results have been compared to a conventionally manufactured Ti6Al4V component. The typical martensitic structure has been shown by the additive manufactured sample. As expected, the metallographic analysis revealed a martensitic microstructure. The electrochemical tests carried out on the surface of the as-received additive manufactured specimen showed an influence of its morphology on the values of passive current density, higher than that recorded for the conventionally manufactured sample, used as the control. After mechanical polishing, the electrochemical tests were repeated on the "bulk" of the samples. The open circuit potential values were higher than the value recorded for the conventionally manufactured sample. The conditions of the additive process affect the corrosion resistance of the components due to the roughness of the surface and to the microstructure created.

The effect of primary processing parameters on surface roughness in laser powder bed additive manufacturing

Metal additive manufacturing (AM) is a method to create complex components in a layer-by-layer process. Laser powder bed fusion (LPBF) provides the ability to fabricate complex designs. Components with internal geometries have surfaces that are difficult to improve through post-processing and have been shown to be detrimental to mechanical performance. This work investigates the relationship between primary processing parameters, and surface roughness of nickel-based superalloy 718 fabricated on a LPBF AM machine. The surface roughness of components built using varied contour processing parameters was characterized using both nondestructive and destructive measurement techniques. Statistical correlations are presented between contour processing parameters and surface roughness height metrics along with general trends in the relationships. Results show that the destructive measurements are required to expose notch-like features that are obstructed by powder particles attached to the surface. However, nondestructive methods more easily provide a statistically significant sample size. Understanding the surface roughness can be used for further process optimization and to inform the qualification strategy for AM components.

Passivation Layer Tailoring Using Laser Powder-Bed Fusion Additive Manufacturing: A Tribocorrosion Study

Laser powder-bed fusion is an additive manufacturing high-energy laser beam creating near-end shape parts directly from a digital computer model. This technique has revealed new trends in manufacturing industry by producing parts with complex geometry and with topology optimizing the weight to strength ratio of structures. In this research, we characterize the corrosion behavior of a Ti6Al4V and AlSi12 alloy fabricated by using laser powder-bed fusion (L-PBF) when they were exposed to corrosive and tribocorrosion conditions. Two synthesis pathways (vertical and horizontal) were employed to produce alloy disks by using high and low laser power resulting in four different thermo-history profiles. The superfine microstructure of L-PBFed Ti6Al4V was revealed using EBSD and XRD, identified as α’ structure (an oversaturated HCP phase) with an n-type titanium oxide passive layer. The surface property was tailored by different 3D printing parameter resulting in a 30% increase in hardness 60% in wear loss and 28% decrease in the oxygen vacancies. The difference in oxygen vacancies tuned the corrosion to wear ratio from 25% to 15% at 2N normal load in simulated body fluid. The corrosion behavior of L-PBFed AlSi12 was tested in simulated acid water. The two different melt pool microstructure (shown in Fig1 (a) and (b)) engineered by the L-PBF building direction results in a difference in the suspiciously of pitting corrosion. The horizontal built specimens show better in pitting resistance, but with a broader probability distribution, which increases the uncertainty in engineering structurer design. The coarse grain silicon precipitation at melt pool boundary result in localized cathode barrier that deflects the pit propagation resulting in shallow pits. However, the heterogeneity of grain boundaries leads to the weakness in forming a protective surface layer under Cl- attack. Figure 1 (a) Pit morphology and (b) melt pool structure and (c) accumulated pitting probability of AM-AlSi12 in dilute Harrison’s solution Figure 1

The influence of additive manufacturing processing parameters on surface roughness and fatigue life

Additive manufacturing (AM) is used to create metal components with complex geometries where the internal surfaces cannot easily be post machined. Fatigue behavior in additively manufactured metals is largely driven by defects in the material such as porosity and surface roughness. This work investigates the processing-structure-property-performance (PSPP) relationship for varied contour processing parameters, and the resulting surface roughness features for alloy 718 fabricated using the laser powder bed fusion AM process. Two nondestructive characterization methods are used to determine the relationships between contour parameters and surface roughness characteristics. The benefits of each characterization technique and metrics required for an accurate representation of the additively manufactured surfaces are discussed. Axial fatigue testing is performed to connect the entire PSPP relationship. An understanding of the PSPP relationship with respect to surface roughness features and fatigue life can be used to guide future process parameter optimization and to inform the qualification strategy for AM components.

Effect of structural support on microstructure of nickel base superalloy fabricated by laser-powder bed fusion additive manufacturing

INCONEL® 718 cubes with and without structural support were built by laser-powder bed fusion (L-PBF) additive manufacturing. The effect of support on the as-built microstructure was studied based on the microstructural characteristics and micro-hardness variations. Specifically, the microstructure was examined by optical microscopy, and scanning and transmission electron microscopy. The precipitates were identified via selected area diffraction supplemented by high-resolution energy dispersive X-ray spectroscopy. Micro-hardness distributions on cross sections parallel and perpendicular to the build direction were mapped. In addition, analytical equations, taking into account various laser processing parameters, material properties and support geometries, were developed to calculate the heat build-up and cooling conditions during L-PBF. The results of microstructure characterization and analytical calculation showed a marginal effect of the support on the local microstructure and hardness due to the low heat input in L-PBF. Moreover, the comprehensive set of microstructure data is useful for future work of modelling processing-microstructure relation as well as optimizing post-fabrication heat treatment.

On the effect of spatter particles distribution on the quality of Hastelloy X parts made by laser powder-bed fusion additive manufacturing

Laser Powder-bed Fusion (LPBF) is considered to be one of the main additive manufacturing (AM) methods for the production of functional metallic parts. Despite its numerous advantages, the spatter formation and distribution on the powder-bed surface, due to the melt pool instability and flow field around the process zone, is considered as one of the drawbacks of this process. In this study, a comparison has been performed on the virgin and spatter powder particles. Chemical composition, phases, microstructure, morphology and thermal behavior of virgin and spatter Hastelloy X (HX) powders have been studied to assess their contributions to the part quality. Moreover, the interaction of the spatter particles with the shielding gas and the recoater motion has been analyzed to determine the distribution and effect of spatter particles on the quality of LPBF-made parts at different locations in the build plate. In this regard, surface roughness was measured on various surfaces of the parts. It was found that the top surface roughness is increased from 14.4 μm to 28 μm in the spatter rich region of the build plate, thus this region is recommended to be avoided for the manufacture of high quality parts.

On the printability and transformation behavior of nickel-titanium shape memory alloys fabricated using laser powder-bed fusion additive manufacturing

Laser powder-bed fusion (L-PBF) additive manufacturing is regarded as an attractive alternative for producing parts with complex geometries for nickel-titanium shape memory alloys (NiTi SMAs). These alloys are known to pose challenges when processed using traditional subtractive or formative manufacturing technologies. Although L-PBF of NiTi has been investigated in some previous research efforts, very little emphasis has been placed on the manufacturability (or printability) of NiTi, where we use printability to refer to the capability of producing parts free of macroscopic defects.The current study elucidates challenges related to the printability of NiTi SMAs using L-PBF, and its interaction with their phase transformation behavior, responsible for their functional properties. More specifically, we conduct experiments and employ machine learning classification techniques to identify an adequate design parameter and an empirical rule for determining the printability of NiTi. Our results indicate that the linear energy density EL is a better design parameter for identifying satisfactory printability, while volumetric energy density, EV, is more relevant in controlling the transformation behavior of the processed material.

On the measurement of effective powder layer thickness in laser powder-bed fusion additive manufacturing of metals

In laser powder-bed fusion (LPBF), the actual thickness of powder particles that spread on solidified zones, so-called effective layer thickness (ELT), is higher than the nominal layer thickness. The source cause of this discrepancy is the fact that powder particles substantially shrink after selective melting, followed by solidification. ELT, as an unknown parameter, depends on process parameters and material properties. In this study, an effective method to measure ELT is proposed and applied to 17-4 PH stainless steel for a nominal build layer thickness of 20 µm. The measured ELT was larger than 100 µm, which is far beyond the values reported in the literature. Results obtained from the current study show the effect of applying the ELT rather than the nominal build layer thickness in numerical modeling studies as well as understanding the governing physics in the LPBF process.

3-Dimensional heat transfer modeling for laser powder-bed fusion additive manufacturing with volumetric heat sources based on varied thermal conductivity and absorptivity

In this article, a 3-dimensional heat-transfer finite element model for Laser Powder-Bed Fusion (LPBF) was developed for accurately predicting melt pool dimensions and surface features. The sole deployment of trial-and-error experiments for arriving at optimal process parameters is very costly and time-consuming, thus the developed model can be used to reduce the process/material development costs. A literature review of heat source models was presented. Eight commonly used heat source models are evaluated and compared. All of their simulated depths are smaller than the experimental result, which may be due to the melt pool convection and inconstant laser absorptivity in the reality during the experiment. In order to enable the numerical model to predict melt pool dimensions for different combinations of process parameters, a novel model including expressions of varied anisotropically enhanced thermal conductivity and varied laser absorptivity is proposed and verified by both the melt pool dimensions and track surface morphology. It is found that the heat source expressions can be linear while causing the simulation results to be in better agreement with both experimental melt pool dimensions and track surface morphology.

The use of nano-computed tomography (nano-CT) in non-destructive testing of metallic parts made by laser powder-bed fusion additive manufacturing

Computed tomography (CT) magnifies the spectrum of measurable internal features with minimal preparation and exceptional resolution. In this study, non-destructive nano-scale and micro-scale CT examinations of aluminum-based part manufactured by laser powder-bed fusion additive manufacturing (LPBF-AM) are compared to determine the extent of defect details (size and distribution of defects/pores) not detectable with micro-CT. Our findings show nano-CT system is able to provide insightful information for effective characterization of AM-made parts to predict their strength/durability more reliably.

Multivariate Calibration and Experimental Validation of a 3D Finite Element Thermal Model for Laser Powder Bed Fusion Metal Additive Manufacturing

Metal additive manufacturing (AM) typically suffers from high degrees of variability in the properties/performance of the fabricated parts, particularly due to the lack of understanding and control over the physical mechanisms that govern microstructure formation during fabrication. This paper directly addresses an important problem in metal AM: the determination of the thermal history of the deposited material. Any attempts to link process to microstructure in AM would need to consider the thermal history of the material. In situ monitoring only provides partial information and simulations may be necessary to have a comprehensive understanding of the thermo-physical conditions to which the deposited material is subjected. We address this in the present work through linking thermal models to experiments via a computationally efficient surrogate modeling approach based on multivariate Gaussian processes (MVGPs). The MVGPs are then used to calibrate the free parameters of the multi-physics models against experiments, sidestepping the use of prohibitively expensive Monte Carlo-based calibration. This framework thus makes it possible to efficiently evaluate the impact of varying process parameter inputs on the characteristics of the melt pool during AM. We demonstrate the framework on the calibration of a thermal model for laser powder bed fusion AM of Ti-6Al-4V against experiments carried out over a wide window in the process parameter space. While this work deals with problems related to AM, its applicability is wider as the proposed framework could potentially be used in many other ICME-based problems where it is essential to link expensive computational materials science models to available experimental data.Graphical Two-stage multi-variate statistical calibration of the finite element thermal model

Direct measurements of laser absorptivity during metal melt pool formation associated with powder bed fusion additive manufacturing processes

Direct calorimetric measurements are used to study the effective optical absorptivity at 1070 nm laser wavelength for bulk and powder-coated discs of industrially relevant metals. Effective absorptivity is plotted as a function of nominal laser power from 30 up to 600 W for scanning velocities of 100, 500, and 1500 mm/s. The absorptivity versus power curves of the bulk materials typically shows a reduction in effective absorptivity until the beginning of the formation of a keyhole-type surface depression that is associated with an increased absorption of the laser light in the growing keyhole until a saturation value is reached. For powders, an additional plateau of higher absorptivity can be observed for low laser power, until the curves qualitatively collapse when full melting of the powder tracks is achieved. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both room temperature powder layer measurements and liquid metal estimates from the literature.

Predictive model for porosity in powder-bed fusion additive manufacturing at high beam energy regime

Process consistency and control are bottleneck issues to wider insertion of powder-bed fusion additive manufacturing in the industrial shopfloor. Of particular interest is the porosity of the components, which remains the limiting factor to high-cycle fatigue performance. Recent experiments have shown that, with increasing energy density, a surge in porosity is seen in selectively laser melted metals. In this high-energy density regime, porosity must originate from mechanisms that are different from the well-known incomplete melting in the low energy density regime. To shed light on this interesting phenomenon, this paper first discusses the mechanism of bubble formation in the melt pool and possible trapping during the solidification, and then formulates a predictive model for porosity in this regime. To compare with experimental results, we perform computer modeling and simulations which have been fully validated by experiments to determine the parameters in the model. We show that the model predictions are in good qualitative and quantitative agreement with the experimental measurements. Hence, the proposed model can be used as a tool to predict the porosity, and further to control and possibly reduce porosity in laser powder-bed fusion additive manufacturing, paving the way for its wider adoption in manufacturing shopfloors.

Precipitation and dissolution of δ and γ″ during heat treatment of a laser powder-bed fusion produced Ni-based superalloy

One drawback of the laser powder-bed fusion additive manufacturing (AM) technique is the build-up of residual stresses during processing that requires a stress-relief heat treatment prior to components being removed from the build plate. Here, we demonstrate the coprecipitation of the γ″-phase alongside the known δ-phase, during stress-relief annealing of AM Inconel 625 at 870 °C. The unexpected precipitation of γ″ in the AM material is attributed to the local solute enrichment to the interdendritic regions of the as-built solidification microstructure. Dissolution of the δ and γ″ phases is achieved after annealing for 15 min at 1150 °C.

Corrosion assessment of Ti-6Al-4V fabricated using laser powder-bed fusion additive manufacturing

We describe the interfacial characteristics of a Ti-6Al-4V alloy fabricated using laser powder-bed fusion (L-PBF) upon exposure to a corrosive environment and tribocorrosion conditions. Electrochemical techniques in both the time and frequency domains demonstrated a correlation between the manufacturing process parameters and corrosion properties, and a titanium oxide passive layer produced specific semiconductor n-type properties. The influence of the microstructure on the titanium oxide passive layer was revealed by comparing L-PBF parts with a traditionally manufactured Ti-6Al-4V alloy in Ringer's simulated body solution. Tribocorrosion characterization reflected the endurance of the surface oxide when the process parameters were adjusted. In addition, wearing simultaneously reflected the mechanical condition as the surface repassivate. The production of fragmentation film during wear caused accumulation of the corrosion product and demonstrated the effect of aggressive ions on the chemical and mechanical properties of the passivation film.