Showing papers in "Additive manufacturing letters in 2022"

Mitigating keyhole pore formation by nanoparticles during laser powder bed fusion additive manufacturing

Abstract: Keyhole pore formation is one of the most detrimental subsurface defects in the laser metal additive manufacturing process. However, effective ways to mitigate keyhole pore formation beyond tuning laser processing conditions during keyhole mode laser melting are still lacking. Here we report a novel approach to mitigate keyhole pore formation during laser powder bed fusion (LPBF) process by using stable nanoparticles. The critical keyhole depth for keyhole pore generation (i.e., the largest keyhole depth without keyhole pore formation) during LPBF of Al6061 increases from 246 µm to 454 µm (85% increase) after adding TiC nanoparticles. In-depth x-ray imaging studies and thermo-fluid dynamics simulation enable us to identify that two mechanisms work together to mitigate keyhole pore generation: (1) adding nanoparticles prevents the keyhole from collapsing by increasing the liquid viscosity to impede the protrusion development; (2) adding nanoparticles slows down the keyhole pore movement by increasing the liquid viscosity, resulting in the recapturing of the pore by the keyhole. We further demonstrate that adding TiC nanoparticles can also eliminate the keyhole fluctuation induced keyhole pore during LPBF of Al6061. Our research provides a potential way to mitigate keyhole pore formation for defect lean metal additive manufacturing.

Process Planning for Additive Manufacturing of Geometries with Variable Overhang Angles using a Robotic Laser Directed Energy Deposition system

Abstract: In the present work, a novel Laser Directed Energy Deposition (LDED) process planning methodology is proposed to build a dome structure with variable overhang angles. Overhang structures with different overhang angles were built where the maximum angle of 35° can be used to build overhang structures without the process and structure compromise. The thin-wall hemispherical dome built using the developed methodology shows the maximum deviation of 2% with respect to the diameter of the original CAD model data. The study paves a way for building high-value, lightweight thin-walled structures with complex cylindrical-based shape (e.g., storage tanks, nozzles, combustion chambers) for engineering applications.

Microstructure and Mechanical Properties of Ti6Al4V Alloys Fabricated by Additive Friction Stir Deposition

Abstract: The additive friction stir deposition (AFSD) of Ti6Al4V alloy was successfully performed over a wide processing window. The microstructures were examined based on the high atemperature β grain size, grain boundary α formation and the propensity of variant selection as a function of process parameters. The analysis revealed that a reduction in deposition temperature could be achieved by the change in AFSD parameters, resulting in major reduction of prior β grain size and refinement of as-deposited α phase structure. This resulted in significant improvements of ductility and strength in the as-deposited state with ductility values up to ∼20% and maximum yield and ultimate tensile strengths values of 1010 MPa and 1233 MPa, respectively.

Uniform Fatigue Damage Tolerance Assessment for Additively Manufactured and Cast Al-Si Alloys: An Elastic-Plastic Fracture Mechanical Approach

Abstract: • Influence of additively manufacturing and casting on microstructure and porosity characteristics as well as quasi-static and cyclic stress-strain behavior of Al-Si alloys. • Evaluation of Murakami-Noguchi approach and Shiozawa approach for fatigue limit and lifetime estimation, however, both approaches are not sufficient for AM Al-Si alloys. • Linking J integral (elastic-plastic fracture mechanics) including cyclic stress-strain curve by means of morrow parameter (K’, n’), defect size and position, stress amplitude and stress ratio. • New approach enables a uniform fatigue damage tolerance assessment of fatigue lifetime and limit for additively manufactured alsi10mg alloy as well as die-cast and sand-cast alsi7mg alloy. Lightweight Al-Si alloys are used in the automotive and railway industry due to their excellent strength-to-weight ratio and near-net-shape manufacturing. In additive manufacturing and casting, the near-net-shape manufacturing results in a non-homogenous solidification and cooling of the parts, leading to significant local gradient in microstructural and defect features as well as deformation behavior. In this paper, a uniform damage tolerance assessment was developed based on fracture mechanical approaches of Murakami (√area) and Shiozawa for a reliable defect-based mechanical design of fatigue-loaded structures. The linear-elastic fracture mechanical (LEFM) approaches of Murakami and Shiozawa were used to calculate defect-based lifetime curves, where the cyclic stress intensity factor (ΔK) at the failure-initiating defect (√area) was used to describe the local stress concentration conditions, called K-N curves, instead of nominal stress-based S-N curves. The LEFM-based K-N curves did not allow to describe the fatigue behavior in terms of a unified fatigue design of AM and casting material. Therefore, the cyclic stress-strain (CSS) behavior was used for a plasticity-modification of the LEFM approach by calculating the effective cyclic J integral (ΔJ eff ) to plot J-based K-N curves, called Kj-N curves. This elastic-plastic fracture mechanical (EPFM) approach allowed a uniform fatigue damage tolerance (FDT) assessment of AM and cast Al-Si alloys for the HCF regime by FDT law and fatigue limit by FDT limit. Exemplarily, the FDT limit could be used to predict synthetic Haigh diagrams for specific AM and cast batches and derive process- and service-relevant knowledge on the effect of tensile or compressive mean stress.

Micro Wire and Arc Additive Manufacturing (µ-WAAM)

Abstract: • A novel variant of WAAM is developed. • Increased accuracy, while maintaining high depositions rates. • µ-WAAM can overcome some limitations of powder-bed fusion additive manufacturing processes. In this work we explore the wire and arc additive manufacturing (WAAM) process scale limits by using a wire diameter of 250 µm and about 2 mm stickout. This WAAM variant, named µ-WAAM, aims at competing with laser powder bed fusion technology, by enabling the fabrication of smaller parts with significantly higher deposition rates. The main issues of descaling the WAAM process are discussed and an acceptable parameter window to fabricate thin walls is presented. Several depositions were successfully performed with ASTMA 228 steel using a wire feed speed ranging from 75 to 90 mm/s, travel speed from 7 to 10 mm/s, a current intensity of 16 A RMS and power of ≈ 35 W RMS.

Powder Spreading over Realistic Laser Melted Surfaces in Metal Additive Manufacturing

Abstract: Powder recoating is a key step in metal Additive Manufacturing (AM) processes where powder is spread across laser processed surfaces to add material for the next layer. Achieving the desired thin powder layers that are both sufficiently dense and uniform is essential for maintaining the requisite geometric tolerances and final part quality. In this study, we focus on the influence of the substrate surface topography by comparing spreading performance over a set of realistic surfaces. We simulate powder spreading over these surfaces using a calibrated non-spherical particle Discrete Element Model for Ti-6Al-4V that incorporates cohesion and Coulomb friction interactions between particles and surfaces. We identify the four key length scales of the recoating process determined by frictional contacts, powder size distribution, the layer thickness and the melted surface topography. We find that realistic AM surfaces show markedly different powder coverage compared to an idealised flat-plane. Rougher surfaces are found to be recoated with larger amounts of powder than smoother surfaces, as smaller particles get trapped by the grooves and valleys across the surface. Counterintuitively, we find that a finer more cohesive powder can achieve the best layer coverage over realistic surfaces - indicating that powder flowability is an incomplete measure of powder spreading performance on realistic AM surfaces. We also demonstrate how the recoating process can significantly size segregate the feedstock powder, favouring deposition of smaller sized particles on the melted surfaces.

Observations on comparable aluminium alloy crack growth curves: additively manufactured Scalmalloy® as an alternative to AA5754 and AA6061-T6 alloys?

Abstract: Scalmalloy® has been proposed for use as additively manufactured (AM) replacement aluminium alloy parts for civil and military aircraft, and for AM aluminium parts for use in satellites and space structures. This study builds on the results of a prior investigation to reveal that Scalmalloy® has a crack growth curve similar to those associated with aluminium alloys AA5754 and AA6061-T6, which are widely used in the automotive industry and in maritime vessels. The R = 0.1 da/dN versus ΔK curve for small naturally occurring cracks in LPBF Scalmalloy® is also predicted and the additive manufacturing community is challenged to undertake testing to validate or disprove this prediction.

Crack mitigation in laser engineered net shaping of WC-10wt%FeCr cemented carbides

Abstract: Laser engineered net shaping of a WC-10wt%FeCr cemented carbide showed cracking during deposition despite using a full factorial design of experiments matrix along with single and multiple objective optimization models to establish an optimal parameter set. In this study four techniques namely, laser re-melting, use of FeCr and Ni-alloy butter layers, and substrate preheating, were used in an effort to reduce the crack susceptibility of deposited samples and the resultant effects on microstructure and hardness were studied. Laser re-melting improved the surface morphology of the deposited samples and reduced the number of primary and secondary cracks, however the hardness decreased. The Ni-alloy butter layer reduced the formation of secondary cracking and led to an increase in hardness, while the FeCr butter layer resulted in increased primary cracks and a reduced hardness. Substrate preheating reduced crack formation and led to an increase in the hardness with the reduction in cracking being attributed to a reduction of the initial thermal gradient.

Semi-solid wire-feed additive manufacturing of AlSi7Mg by direct induction heating

Abstract: In this study, a novel process is presented in which direct induction heating with frequencies in the MHz range is used for the wire-feed additive manufacturing of the alloy AlSi7Mg. The high frequency of 1.5 MHz enables processing of 1.2 mm diameter wires without the need for indirect heating via a nozzle. The feasibility of the process is proven by the experimental identification of a proper process window regarding the influential parameters such as the distance between inductor and substrate, induction power and wire feed rate for the fabrication of single layers. Furthermore, a strategy for the successful fabrication of multi-layered cubes is developed. The microstructure of the cubes exhibits a characteristic variation along the build direction. Micro-computed tomography is used to reveal defects like lack of fusion and spherical pores in test cubes. The presented results are used to derive possible process improvements, which will allow the novel process to be used as a fast and powder-free alternative metal additive manufacturing route in future.

Multi-faceted monitoring of powder flow rate variability in directed energy deposition

Abstract: Powder flow rate is a key parameter in Directed Energy Deposition (DED) processes. During a typical build, if powder flow rate is reduced for just 1 second, 30 mm of melt track is affected. Consequently, even a small variation in powder flow rate can have significant implications on build quality. In this work, the powder flow stability for different types of 316 L steel powders was quantified using a combination of methodologies including offline weight measurements, flow imaging, in-situ build data and coaxial melt pool imaging. Flow rate oscillation was observed, correlated with the periodicity of powder hopper turntable rotation, at sufficient magnitude to cause build quality effects and be identifiable in coaxial melt pool imaging. The implications of flow rate variation on the use of melt pool imaging for closed-loop control are discussed.

Bulk-Like Ductility of Cold Spray Additively Manufactured Copper in the As-Sprayed State

Abstract: • As-sprayed CSAM Cu can exhibit a simultaneous high ductility and strength. • This is triggered by a specific topology of bimodal grain size. • The deposits have a high thermal conductivity comparable to bulk. • They are perfectly gas-tight with He leakage rate of less than 1 × 10 −7 mbar•l/s. • All these were achieved with 99% efficiency using inexpensive nitrogen. In recent years, cold spray process is increasingly used for additive manufacturing of metallic components, referred to as cold spray additive manufacturing (CSAM). Unlike the fusion-based AM processes, CSAM is achieved in a solid-state, bringing several advantages such as the absence of severe oxidation or phase composition changes. At the moment, the main limitation of CSAM is the generally low ductility of the as-sprayed deposits. In this paper, using Cu as a model material, we demonstrate a way to overcome this limitation. Importantly, a high ductility of the deposits in their as-sprayed state is achieved without a trade-off in mechanical strength. Furthermore, we show that without any post-heat treatment, the properties of CSAM Cu are comparable to bulk, non-AM Cu.

Manufacturing single crystals of pure nickel via selective laser melting with a flat-top laser beam

Abstract: • Fabrication of single-crystal structure using selective laser melting was studied. • A flat-top profile was utilized without the implementation of single-crystal seeds. • Parametric optimization resulted in planar melt pool formation. • A homogeneous near-{001}<100> texture was achieved in high building heights. • Suppressing strain accumulation was necessary to avoid high-angle grain boundary. The exploration of flat-top laser profile in fabricating a single crystal (SX) structure using selective laser melting (SLM) in pure Ni was investigated. Optimization of the parameters led to the formation of a planar melt pool. A homogeneous near-{001}<100> texture with suppressed high-angle grain boundary (HAGB) in high building heights of >20 mm was achieved without an SX seed. In addition, the planar melt pool suppressed the geometrically necessary dislocation accumulation and prevented strain-induced continuous dynamic recrystallization that could cause HAGB formation. Thus, an SX structure with homogeneous near-{001}<100> texture and suppressed HAGB was successfully achieved without an SX seed.

Refined microstructure and ultrahigh mechanical strength of (TiN + TiB)/Ti composites in-situ synthesized via laser powder bed fusion

Abstract: Titanium matrix composites reinforced by titanium nitride (TiN) and titanium boride (TiB) were in situ synthesized via laser powder bed fusion (L-PBF) using premixed boron nitride (BN) and titanium powders. The optimal BN content was determined as 0.5 wt%, and a near-full density was achieved with optimized process parameters. The microstructure of the printed composite consisted of α-Ti matrix doped with N, as well as nano-sized TiN and TiB precipitates. The ultimate tensile strength of the printed composite sample was enhanced by ∼85% from ∼590 MPa to ∼1100 MPa as compared with the pure Ti counterpart, whereas the elongation was reduced to ∼3%. This research provides insights into the additive manufacturing of metal composites with near-full density and enhanced mechanical strength.

Catalyzed frontal polymerization-aided 3D Printing of Epoxy Thermosets

Abstract: • Tailoring frontal polymerization of epoxy resin with carbon materials • Revealing the size and geometric effect of carbon materials on the catalyzed frontal polymerization of epoxy resin • New method in lowering frontal temperature while maintaining the frontal polymerization rate • 3D printing of epoxy thermoset with self-propagated in-situ curing Frontal polymerization is a self-propagating exothermic reaction and provides a rapid and energy-efficient way to manufacture thermosets. A critical issue for frontal polymerization is to concurrently maintain a low frontal temperature and a self-sustained frontal propagation, which significantly depends on the frontal velocity. In this work, carbon nanotubes (CNTs), graphene oxide(GO) and discontinuous carbon fibers (d-CFs) were incorporated into epoxy thermosets to tune the frontal polymerization. Their catalytic effects on frontal temperature and frontal velocity were studied. Both CNTs and GO were found to significantly reduce the activation energy of frontal polymerization, whereas d-CFs did not show any obvious effect on the activation energy. The real-time non-destructive characterization showed that 1wt % CNTs incorporation reduced the frontal temperature from 240 to 227°C while the front velocity remained the same (6.5 cm min −1 ), indicating effective decoupling frontal temperature from frontal velocity. The frontal temperature could be further reduced to 220°C or lower at an increasing loading of CNTs while the frontal velocity remained the same. In contrast, 1wt% GO incorporation reduced the frontal temperature from 240 to 220°C, but also decreased the frontal velocity from 6.5 to 5.1 cm min −1 (21.5% reduction). In addition, as-prepared CNTs-incorporated epoxy resins were used in the 3D printing process via frontal polymerization and their printability were demonstrated. This discovery opens a new pathway for additive manufacturing through catalyzed in-situ frontal polymerization.

Printability of low-cost pre-heat-treated ball milled Al7075 powders using compressed air assisted cold spray additive manufacturing

Abstract: A successful attempt at the cold spray additive manufacturing (CSAM) of low-cost ball milled Al7075 powders using air as the carrier gas is demonstrated. The irregular shape and hardness of the as-received powders are identified as a primary obstacle to the successful printability which demonstrated 0% deposition efficiency. Thus, different annealing procedures are suggested as an optimization technique to facilitate printing. These annealing procedures are compared and contrasted using differential scanning calorimetry (DSC) and the hardness/printability of the annealed powders are correlated exactly to the enthalpies in the low temperature i.e., <300 °C region. The annealing procedure of 300 °C for 4h (referred to as the C4 powder) is found to be the most optimal based on the combination of hardness, deposition efficiency, porosity and enthalpy values. On printing, the C4 powder also shows the best combination of good quality edge spraying, highest deposition efficiency of 59% and low porosity of 1.80%. Finally, while the microstructure of the C4 powder particles is retained in the microstructure of the printed part, the hardness of the printed part is higher than that of the C4 powder. This is attributed to a combination of deformation strain induced within the interparticle boundaries during the cold spray process and excellent interparticle interlocking.

Enabling Part-Scale Scanwise process simulation for predicting melt pool variation in LPBF by combining GPU-based Matrix-free FEM and adaptive Remeshing

Abstract: • A highly efficient GPU-based matrix-free finite element modeling with adaptive remeshing framework is developed to overcome the high computational expense of l -PBF thermal process modeling, making it feasible to do detailed scanwise thermal simulation for part-scale within days. • The effects of the scanning strategy, geometrical features, and proximity to the turning points on the melt pool size variability and lack of fusion porosity are investigated for two different parts. • The predicted variation in melt pool size and lack of fusion porosity has good correlation with experimental measurements. This work proposes to combine matrix-free finite element modeling (FEM), adaptive remeshing, and graphical processing unit (GPU) computing to enable, for the first time, scanwise process simulation of the Laser Powder Bed Fusion (L-PBF) process with temperature-dependent thermophysical properties at the part scale. Compared to the conventional FEM using the global stiffness approach and a uniform mesh running on 10 CPU cores, l -PBF process simulation based on the proposed methodology running on a GPU card with 5,120 Compute Unified Device Architecture (CUDA) cores enables a speedup of over 10,000x. This significant speedup facilitates detailed thermal history and melt pool geometry predictions at high resolution for centimeter-scale parts within days of computation time. Two parts consisting of various geometric features are simulated to reveal the effects of scan strategy and local geometry on melt pool size variation, which correlate well with melt pool and lack-of-fusion porosity measurements obtained via experiment.

Control of microstructure and shape memory properties of a Fe-Mn-Si-based shape memory alloy during laser powder bed fusion

Abstract: Fe-Mn-Si shape memory alloys are materials, whose functional properties strongly depend on microstructural factors such as grain size and orientation, phase fraction and chemical composition. The present study demonstrates the possibility of microstructure modification via process parameter variation on a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. By varying the scan speed, samples characterized by coarse elongated grains with strong <001> orientation along the build direction or by finer equiaxed grains without preferential orientation can be fabricated. Changes in the volume phase fraction of bcc-δ ferrite and fcc-γ austenite are also introduced by selective Mn evaporation. A direct correlation between the generated microstructure and achieved mechanical and shape memory properties is found.

Flexural Characteristics of Material Extrusion Steel 316L: Influence of Manufacturing Parameters

Abstract: This short communication paper reports the effects of manufacturing parameters (raster angle, layer height, and print orientation) on the flexural properties of material extrusion (ME) Ultrafuse Steel 316L. Flexural specimens were produced with four raster angles: ±30°, ±45°, ±75°, and 0°/90°, with each raster angle printed at three layer heights: 0.1, 0.15, and 0.2 mm. The former combinations were of ‘flat’ print orientation, and a limited amount of ‘on-edge’ print orientation specimens were included for a general comparison. Statistical analysis was performed on the flexural strength data. Furthermore, the digital image correlation (DIC) technique was employed to study the deformation modes/mechanisms of the specimens. Comparable flexural strength results were obtained for the two print orientations. However, the ‘on edge’ print orientation specimens outperformed their counterparts. The ±30° raster angled specimens outperformed all other specimens in both the print orientations, recording the highest flexural strengths. Uniform deformation modes of the specimens were evidenced from the DIC analysis, where the effects of internal voids/porosity on subsequent mechanical properties were found to be minor. Overall, the results presented herein will increase confidence in employing the ME process for Steel 316L for applications involving bending loads.

Directed energy deposition and characterization of high‐speed steels with high vanadium content

Abstract: High vanadium high speed steels (HVHSSs) are alloys that are often employed for hard-facing applications owing to their excellent hardness and wear-resistance. These properties, however, make them difficult to machine into functional parts, even when employing energy-intensive processes. In this work, we employ directed energy deposition (DED) as an alternative manufacturing route to produce defect-free HVHSSs builds. We study the printability of two alloy compositions—namely Fe-10V-4.5Cr-2.5C (V10) and Fe-15V-13Cr-4.5C (V15)—and confirm their exceptionally high microhardness, which ranges from 850 HV to 1000 HV. We ascribe the differences in mechanical properties between the two HVHSSs to the size and volume fraction of vanadium carbides in the microstructure as well as the phase content, all of which vary as a function of the cooling rate imposed by DED. Our work inspires new strategies to design near-net-shape parts for hard-facing applications via additive manufacturing.

Crack growth in conventionally manufactured pure nickel, titanium and aluminum and the cold spray additively manufactured equivalents

Abstract: It has recently been shown that when the da/dN versus ΔK curves associated with crack growth in conventionally manufactured, additively manufactured (AM), and cold spray additively manufactured (CSAM) 316L stainless steel are replotted with da/dN expressed as a function of the Schwalbe crack driving force (Δκ), then the various different curves collapsed onto a single master curve. This study reveals that this phenomenon also arises for crack growth in titanium, nickel, and aluminum. In each case, the da/dN versus Δκ relationship is shown to be independent of whether the specimen was conventionally manufactured or produced by CSAM.

Printability, mechanical and degradation properties of Mg-(x)Zn elemental powder mixes processed by laser powder bed fusion

Abstract: • Printability of binary Mg-Zn alloys with 0, 0.5, 1, 2, 5 and 8 wt% were investigated by laser powder bed fusion. • Density parameter maps for varying laser power and scan speed were recorded for each alloy composition. • Mg0.5 Zn and Mg1Zn possessed the highest density results. • UTS of Mg1Zn was 126 MPa at a strain at failure of 5.7% and Mg0.5 Zn averaged 113 MPa at 5.4%, but showed better reproducibility. • Mg0.5 Zn and Mg1Zn possessed similar degradation behavior but were inferior to a LPBFed WE43 reference.

3D Tracking Velocimetry of L-PBF Spatter Particles using a Single High-Speed Plenoptic Camera

Abstract: Particle spatter is an unavoidable by-product of the Laser-Powder Bed Fusion (L-PBF) process, as the high intensity of the incoming laser beam generates high vapor fluxes on the meltpool, allowing metal particles to be ejected into the process environment. This is detrimental to the final manufactured part, as it risks the incorporation of defects. It is therefore important to study this spattering behavior and to apply the knowledge gained to further improve the L-PBF process. This work introduces the use of a high-speed plenoptic camera to acquire 3D spatter particle trajectories generated via L-PBF line tracks. Spatter particles are tracked in the volume above the laser-metal interaction zone at 1000 fps and their velocity calculated. It is found that the calculated speeds of the spatter particles are within the expected range for this process, and that the behavior is a complex 3-dimensional process.

Scan Path Resolved Thermal Modelling of LPBF

Abstract: • Laser powder bed fusion product quality depends on process and on the scan strategy. The accuracy by which a feature outline is printed defines the surface finish, workpiece fatigue behavior and the extent of post processing needed to achieve surface quality specifications. In this paper we develop a semi-analytic thermal model allowing the resolution of the scan strategy in detail while solving the energy equation to predict the thermal history of the printed component and its features. Experiments are performed using process parameters that have been shown to achieve an IN718 relative density larger than 99.5%. We study the influence of scan strategies on different features’ surface quality while keeping the process parameters unchanged. The model is driven by the build file provided to the 3D printer or using the more accurate in-situ process data describing the laser path and status. The model is verified using high-fidelity modes as well as experimental results. We demonstrate both experimentally and numerically how the printer controller changes the build strategy, slightly affecting the quality of fine features. Taking the real scan path into account, different scan strategies are studied. Laser powder bed fusion product quality depends on process parameters (e.g. laser power, scan speed, etc.) and on the scan strategy. The accuracy by which a feature outline is printed defines the surface finish, workpiece fatigue behavior and the extent of post processing needed to achieve surface quality specifications. In this paper we develop a semi-analytic thermal model allowing the resolution of the scan strategy in detail while solving the energy equation to predict the thermal history of the printed component and its features. Experiments are performed using process parameters that have been shown to achieve an IN718 relative density larger than 99.5%. We study the influence of scan strategies on different features’ surface quality while keeping the process parameters unchanged. The model is driven by the build file provided to the 3D printer or using the more accurate in-situ process data describing the laser path and status. The model is verified using high-fidelity models as well as experimental results. We demonstrate both experimentally and numerically how the printer controller changes the build strategy, slightly affecting the quality of fine features. Laser scan dynamics and timing, particularly sky-writing, was shown to have a critical effect on the melt pool morphology and necessary to include in the models.

Conformal 3D printing of a polymeric tactile sensor

Abstract: Conventional additive manufacturing processes are generally inadequate for printing electronics on a curved surface. When printing a curved functional structure, the typical way of generating the extrusion path only in a horizontal plane could cause various issues such as impreciseness and disconnect in the printed part. In this work, conformal 3D printing of a soft tactile sensor is presented in which curvilinear extrusion paths were generated for the printing of a curved sensor. An extrusion-based multi-material direct printing system was employed to print the sensor, and ultraviolet light was used to polymerize the printed layers. An ionic liquid–based pressure-sensitive polymer membrane, carbon nanotube-based conductive electrodes, and a soft polymeric insulation layer were conformally 3D printed to fabricate the curved sensor on a fingertip model. The conformally printed sensor was evaluated under different conditions. Sensors 3D-printed using conformal and planar slicing processes were compared to investigate the effect of curvilinear slicing on the printed parts. The results show that conformal 3D printing is able to overcome the fabrication limitations of conventional planar processing while also retaining the functionality of the printed structures.

Influence of Reduced Carbon Content on Microstructure and Mechanical Behavior of Inconel 718 Prepared by Laser Powder Bed Fusion

Abstract: • Successful production of a reduced carbon Inconel 718 alloy via SLM. • Significant changes in the precipitation behavior of NbC and γ’’ are achieved. • Reductions in NbC content and finer γ’’ precipitation verified By Thermocalc. • Reduced γ’’ size results in improved tensile strengths at both 25 °C and 650 °C. Inconel 718 powder with reduced carbon content was used to manufacture coupons via laser powder bed fusion (LPBF) to study the influence of carbon content on microstructures and mechanical properties of HIP'ed and heat-treated material. The carbon concentration in this material was kept deliberately low (0.01 wt%) relative to a typical commercial powder composition (0.04 wt%) of IN718 in order to minimize the precipitation of primary carbides during post-processing heat treatments, which have been observed to be deleterious to elevated temperature mechanical properties such as elongation, stress rupture ductility, and notch sensitivity. The final microstructure of the lean C alloy is shown to have a significantly reduced NbC population relative to the baseline material. Furthermore, other microstructural variations such as an increased δ phase population and decreased γ’’ particle size were observed. Empirical observations agreed with microstructural simulations calculated by the ThermoCalc/TC-PRISMA precipitation module. These changes to the microstructure resulted in a 10% increase in the yield strength at both 25 and 650 °C in the lean C alloy, bringing the strength of LPBF-printed material closer to that of wrought IN718. Overall, the reduction in carbon content in the raw material has significant effects beyond limiting the pre-solution treatment precipitation of NbC, primarily by freeing Nb for formation of δ and γ’’ phases.

Evaluating the Intrinsic Resistance to Balling of Alloys: A High-throughput Physics-informed and Data-enabled Approach

Abstract: • The ratio of the characteristic solidification and spread times can indicate balling • Said criterion can be calculated in a HTP manner for alloy design • Utility of criterion is validated against AM database consisting of 2496 observations To date, the vast majority of work on metal additive manufacturing (AM) has been framed in terms of the need to tune processing conditions for a particular AM technology in order to print conventional alloys, oftentimes developed for fabrication methods other than AM. This approach overlooks the fact that historically, many engineering alloy system has been designed with a particular processing route in mind, e.g., ingot metallurgy, powder metallurgy, rapid quenching, etc. There are thus significant opportunities to design alloys specifically for AM. A key challenge is that alloy design requires performance metrics that can be optimized by exploring the alloy chemistry space. Here, we present a study in which we examine how intrinsic thermophysical properties can be used to estimate performance metrics related to the behavior of a solidifying metal droplet under AM-relevant conditions. By identifying these intrinsic properties, it is possible to directly incorporate ‘intrinsic printability’, specifically ‘intrinsic resistance to balling’, into AM-focused alloy design.

Uniform Fatigue Damage Tolerance Assessment for Additively Manufactured and Cast Al-Si Alloys: Size and Mean Stress Effects

Abstract: The near-net-shape manufacturing in additive manufactured and cast of Al-Si alloys results in a heterogeneous solidification and cooling of the parts, leading to significant gradients in microstructural and defect features as well as deformation behavior. In this paper, an elastic-plastic fracture mechanical model for uniform fatigue damage tolerance assessment of Al-Si alloys was further qualified and extended for a uniform view of different testing volumes as well as various stress ratios between R = -2…0.5 based on the fracture mechanical approaches of Murakami (√area) and Shiozawa for a reliable main crack defect-based mechanical design of fatigue-loaded structures. The linear-elastic fracture mechanical (LEFM) approaches of Murakami, Murakami-Schijve and Shiozawa were used to calculate defect-based lifetime curves, where the cyclic stress intensity factor (ΔK) at the failure-initiating defect (√area) was used to describe the local stress concentration conditions (so-called K-N curves) instead of nominal stress-based S-N curves. The LEFM-based K-N curves did not allow a unified assessment of fatigue behavior. Therefore, the cyclic stress-strain (CSS) behavior (K’, n’) was used for a plasticity-modification of the LEFM approach based on the elastic-plastic fracture mechanical (EPFM) approach of Fischer by calculating the effective cyclic J integral (ΔJeff) to plot J-based K-N curves, called Kj-N curves. This EPFM approach could be qualified for a uniform and reliable fatigue damage tolerance assessment of AM and sand cast Al-Si alloys for the HCF regime.

Accelerating High-Fidelity Thermal Process Simulation of Laser Powder Bed Fusion via the Computational Fluid Dynamics Imposed Finite Element Method (CIFEM)

Abstract: The current work proposes a finite element method (FEM) to accelerate scanwise thermal process simulation of the laser powder bed fusion (L-PBF) process with computational fluid dynamics (CFD) resolution near the melt pool. Termed the CFD imposed FEM (CIFEM), the transient thermal fields from a high-fidelity CFD simulation and inferred by deep learning are imposed as temperature values rather than utilizing a conventional heat source model as in existing FEM-based process simulations. These fields are enforced only within a relatively small computational region encompassing the melt pool, while heat diffusion effects elsewhere are solved via the FEM. For a wide range of laser power and scan speeds covering the conduction, transition, and keyhole melting regimes, 29 of the 30 total CIFEM-simulated melt pool sizes lie within two standard deviations of the experimental melt pool sizes. Compared with the CFD simulations, the thermal fields obtained by CIFEM possess 7.44% mean absolute relative error (MARE), significantly less than the 43.76% MARE on three representative test cases simulated using the Goldak heat source model calibrated to the measured melt pool dimensions. In terms of computational efficiency, the CIFEM model running on a GPU card with 4,608 Compute Unified Device Architecture (CUDA) cores is 28.2× more efficient than the CFD simulations running on 24 CPU cores in parallel.

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

Abstract: • Powder bed homogeneity, contaminations, and printed surface quality are crucial in powder bedbased AM processes to obtain a defect-free part. • Standard process monitoring equipment does not provide a sufficiently high resolution for detecting small-scale defects. • The optical scanner mounted on the recoater can provide high-resolution images (1200 dpi) of the last printed slice and the powder bed for the whole build platform, without affecting process time. • In-situ surface topography deviations and powder bed contaminations can be observed and studied with this new monitoring setup. Powder bed homogeneity, contaminations, and printed surface quality are crucial in powder bed-based AM processes to obtain a defect-free part, but the scale at which these defects are seen is not compatible with the resolution of current industrial image-based monitoring solutions. In this work, we explore the implementation of an optical scanner in an industrial laser powder bed fusion (L-PBF) machine to detect powder bed and part-related defects. The sensor is mounted ”parasitically” on the recoater and exploits its movement to scan across the build platform before and after powder deposition to obtain high-resolution images. The acquisition seamlessly integrates with the process, without delaying the production as the acquisition occurs in parallel with the new layer deposition. The system was used to monitor test builds as well as longer builds (1000+ layers) to prove its robustness to the challenging L-PBF chamber environment. The in-situ powder bed images of the new monitoring system were compared to the acquisitions of a standard external camera setup. The improved image quality and resolution of the new system were demonstrated on both large-scale ( > 1 mm) and small-scale features. The new system proved to be capable of capturing printed surface topography anomalies and powder bed contaminations ( < 100 µm), opening a whole new range of possibilities for detecting small-scale defects via in-situ monitoring.

Influence of defects on high-temperature oxidation performance of GH3536 superalloys fabricated by laser powder bed fusion

Abstract: • Microstructure and oxidation behaviour of an LPBF fabricated GH3536 superalloy was investigated. • Oxidation resistance is closely related to defect density while micro-cracks significantly impair the oxidation resistance. • Oxidation resistance can be improved by optimising processing parameters and powder composition. This study investigates the formation of defects and their impact on oxidation behaviour of GH3536 superalloys fabricated by laser powder bed fusion (LPBF). Defects of six series of samples manufactured by different parameters is dominated by pores, micro-cracks, and lack of fusion, respectively. The superalloy is isothermally oxidised in 950 °C dry air for 500 h. The oxidation resistance of LPBF GH3536 depends on the types and quantity of processing defects, wherein the sample with micro-cracks as the main defect exhibit the worst oxidation resistance. Electron probe microanalysis of the superalloy revealed the impact of micro-cracks on diffusion process. Defects seriously deteriorate the oxidation resistance, especially micro-crack defects. Reducing manufacturing defects in the samples can improve the corrosion resistance.