Search

Porosity defects are currently a major factor that hinders the widespread adoption of laser-based metal additive manufacturing technologies. One common porosity occurs when an unstable vapor depression zone (keyhole) forms because of excess laser energy input. With simultaneous high-speed synchrotron x-ray imaging and thermal imaging, coupled with multiphysics simulations, we discovered two types of keyhole oscillation in laser powder bed fusion of Ti-6Al-4V. Amplifying this understanding with machine learning, we developed an approach for detecting the stochastic keyhole porosity generation events with submillisecond temporal resolution and near-perfect prediction rate. The highly accurate data labeling enabled by operando x-ray imaging allowed us to demonstrate a facile and practical way to adopt our approach in commercial systems.

In powder-bed-based metal additive manufacturing (AM), the visualization and analysis of the powder spreading process are critical for understanding the powder spreading dynamics and mechanisms. Unfortunately, the high spreading speeds, the small size of the powder, and the opacity of the materials present a great challenge for directly observing the powder spreading behavior. Here, we report a compact and flexible powder spreading system for in situ characterization of the dynamics of the powders during the spreading process by high-speed x-ray imaging. The system enables the tracing of individual powder movement within the narrow gap between the recoater and the substrate at variable spreading speeds from 17 to 322 mm/s. The instrument and method reported here provide a powerful tool for studying powder spreading physics in AM processes and for investigating the physics of granular material flow behavior in a confined environment.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals., 3D printing pore-free complex metal parts remains a challenge. Here, the authors combine in-situ imaging and simulations to show thermocapillary force can eliminate pores from the melt pool during a laser powder bed fusion process.

Additive Manufacturing (AM) has significantly increased the design freedom available for metal parts and provides significant flexibility within each build to produce multiple components of varying size and shape. In order to obtain the highest build efficiency, it is ideal to print multiple parts together spanning the entire plate with as little spacing as possible between the parts. Work has been performed to characterize the variance of materials properties as a function of location within the build volume as well as component density on the build plate. This work utilizes mechanical, chemical, and microstructural analysis techniques to expand on previous work by statistically evaluating the impact of build location, and nearest neighbor proximity on tensile performance in Electron Beam Melted (EBM) Ti-6Al-4 V. Mechanical results are then correlated to structural phenomenon and the effectiveness of various strengthening mechanisms are determined. Results show that properties span a small range regardless of build design and that interstitial strengthening and lath spacing are the driving factors for mechanical strength.

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution., The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts.

Powder spattering is a major cause of defect formation and quality uncertainty in the laser powder bed fusion (LPBF) additive manufacturing (AM) process. It is very difficult to investigate this with either conventional characterization tools or modeling and simulation. The detailed dynamics of powder spattering in the LPBF process is still not fully understood. Here, we report insights into the transient dynamics of powder spattering in the LPBF process that was observed with in-situ high-speed high-energy x-ray imaging. Powder motion dynamics, as a function of time, environment pressure, and location, is presented. The moving speed, acceleration, and driving force of powder motion that are induced by metal vapor jet/plume and argon gas flow are quantified. A schematic map showing the dynamics and mechanisms of powder motion during the LPBF process as functions of time and pressure is constructed. Potential ways to mitigate powder spattering during the LPBF process are discussed and proposed, based on the revealed powder motion dynamics and mechanisms.

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.

Purpose In the development of powder bed additive manufacturing (AM) process parameters, the characterization of mechanical properties is generally performed through relatively large mechanical test samples that represent a bulk response. This provides an accurate representation of mechanical properties for equivalently sized or larger parts. However, as feature size is reduced, mechanical properties transition from a standard bulk response to a thin wall response where lower power border scans and surface roughness have a larger effect. Design/methodology/approach For this study, samples of wall thickness varying between 4.0 and 0.25 mm were built in 304L on the selective laser melting (SLM) platform and Ti-6Al-4V on the electron beam melting (EBM) platform. Samples were then mechanically tested, and fractography was performed for analysis. Findings This study experimentally identifies the threshold between bulk and thin wall mechanical properties for 304L SS on the SLM platform and Ti-6Al-4V on the EBM platform. A possible method for improving those properties and shifting the transition from bulk to thin wall response to smaller wall thicknesses by manipulation of scan pattern was investigated. Originality/value This study is a novel investigation into the effect of reduced wall thickness on the mechanical properties of a part produced by powder bed AM.

Laser Powder-Bed Fusion (L-PBF), often called selective laser melting (SLM), is a powder-bed fusion process in Additive Manufacturing (AM) that uses a laser beam to selectively fuse layers of powder into near net-shape components with little porosity. However, inconsistencies in the part properties due to the presence of defects in as-built components have hindered the widespread adoption of L-PBF for industrial applications motivating researchers to study the sources of variation for quality control purposes. A critical area suspected of creating variation in the part properties is the feedstock, where batch-to-batch differences as well as changes in the powder properties with reuse have the potential to affect performance. During processing, laser spatter and condensate form and deposit into the powder-bed surrounding the built parts. These particulates, collectively known as ejecta, differ morphologically and chemically from the virgin powder, potentially compromising reusability. In this study, 304L stainless steel powder was recycled for a total of 5 times through a systematic approach aimed at accelerating powder reuse to reveal its influence on both the tensile properties and impact toughness. Through analysis of variance (ANOVA), it was found that tensile properties did not change with reuse, while the impact toughness showed a steady decline revealing the differences in static and dynamic part properties due to powder reuse.

Spatter causes defect formation, powder redistribution and contamination in laser powder bed fusion (LPBF) additive manufacturing process. It is critical to distinguish different types of spatter and understand their features and formation mechanisms. This work reveals the features and formation mechanisms of five unique types of spatter during the LPBF process by in-situ high-speed, high-energy x-ray imaging. Spatters observed during LPBF testing are quantified by their speed, size, and direction. Distinct quantifiable characteristics for each type of spatter are identified. Effects of the laser power, scan speed, and ambient pressure on spatter formation and features are unraveled. A spatter formation map for AlSi10Mg alloy is constructed.

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior.

Additive Manufacturing (AM) has significantly increased the design freedom available for metal parts. Many novel designs rely on the existence of surfaces that are not accessible and therefore rely on the surface finish of the parts directly from the AM equipment. Work has been performed to characterize the difference between AM, then machined tensile samples and AM tensile samples with an unimproved surface finish. This work utilizes surface analysis, fractography, and finite element analysis (FEA) to expand on this by investigating the effects of the unimproved surfaces on local tensile behavior and fracture mechanics in AM materials. Results show that measurement error in cross-sectional area is the main source of variation between unfinished and machined strength measurements. Results also indicate that a ductile material may demonstrate the same tensile strength regardless of post processing. Fractography shows that stress concentration near the surface of the samples leads to changes in fracture behavior likely explaining the difference in elongation of the samples. Finally, FEA work did not successfully show a difference in fracture initiation, though this is likely due to inaccurate representation of the samples surface.

The application of rigid polymeric foam for large investment casting patterns with complex geometries can improve the dimensional tolerances and the surface quality of the casting. However, these pattern materials have a tendency to promote crack formation in investment casting shells during pattern removal by firing. Experimental methods were combined with finite element modeling to predict stress in the shell. The model takes into consideration the thermal and mechanical properties of the pattern and the shell materials to determine the heat transfer and thermal expansion stresses developed in the shell during firing. The thermal and mechanical properties of the pattern and shell were obtained from experimental tests. A 3D nonlinear finite element model was developed to predict possible crack formation in the shells during pattern removal. The effects of the thermo-mechanical properties of the foam and the shell, as well as the firing process parameters were modeled, and extreme cases were experimentally validated. Recommendations for firing process parameters and pattern design to decrease stress and eliminate crack formation in the shell were formulated.

The prediction of materials properties and their variation within a specification or design space is key in ensuring reliable production uniformity. To capture the complex mechanisms that underpin materials’ performance, processing-structure-properties links are established using a “systems design” approach. QuesTek Innovations LLC has previously utilized multi-scale ICME modeling methodologies and tools (e.g., CALPHAD thermodynamic and kinetic databases, property models, etc.) and advanced characterization techniques to design advanced materials with improved performance. This work focuses on building an ICME infrastructure to predictively model properties of critical materials for energy and defense applications by optimizing existing materials, performing calculations to quantify uncertainty in material properties, and defining target specification ranges and processing parameters necessary to ensure design allowables. Focusing on two material case studies, 304L austenitic stainless steel and glass-ceramic-to-metal seals, we show how these ICME techniques can be used to better understand process-structure and structure-property relationships. These efforts provide pathways to novel, fully optimized alloys and production processes using the Accelerated Insertion of Materials (AIM) methodology within ICME. The AIM method is used for probabilistic properties forecasting to enable rapid and cost-efficient process optimization and material qualification.

During the investment casting process, the shell is subjected to high internal pressure and thermal stress, particularly during pattern removal and when pouring steel into the free standing ceramic shell. Most testing methods investigate the properties of the ceramic shell in flat regions while cracks typically form in the sharp corners and edge regions. The corners and edge regions have different structure and thickness when compared to flat regions and experience large mechanical stress during processing. In this study, experimental methods were combined with finite element modeling to predict failure stress in the internal corner regions of the shell. The model takes into consideration the mechanical properties of the ceramic shell to determine the stress developed during loading. The effect of shell porosity on stress concentration in sharp corners was evaluated. A general equation was developed to predict the force necessary for crack formation in the shell based on various geometric variables. The results from the model were experimentally verified and the failure stress in flat and corner regions of the shell were compared in order to develop an improved equation.

Austenitic stainless steels generally have good resistance to hydrogen-assisted fracture; however, structural designs for high-pressure gaseous hydrogen are constrained by the low strength of this class of material. Forging is used to increase the low strength of austenitic stainless steels, thus improving the efficiency of structural designs. Hydrogen-assisted racture, however, depends on microstructural details associated with manufacturing. In this study, hydrogen-assisted fracture of forged type 304L austenitic stainless steel is investigated. Microstructural variation in multi-step forged 304L was achieved by forging at different rates and temperatures, and by process annealing. High internal hydrogen content in forged type 304L austenitic stainless steel is achieved by thermal precharging in gaseous hydrogen and results in as much as 50% reduction of tensile ductility.