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Poor surface quality has long been a significant challenge to the broad adoption of additive manufacturing (AM) in industry. This work deploys a customized electropolishing (EP) procedure to improve the surface finish of complex laser powder bed fusion Ti-6Al-4V parts with internal channels and various geometries using a substantially less hazardous, anhydrous, alcoholic solution, while maintaining the geometrical fidelity of the parts. A preliminary study on simple geometries was run to evaluate the effect of EP time on surface roughness and thickness loss of a part. By using a customized test setup and an artificial EP waveform to accurately control EP parameters, a high EP efficiency was achieved (maximum reduction of surface roughness up to 70% and a minimum thickness loss of 11%) with low treatment times. Subsequently, EP was conducted on the internal surfaces of various geometries in increasing order of complexity; the results showed the effectiveness of the proposed technique across different geometrical features, leading to maximum uniform surface roughness improvement of 77% with minimum thickness loss of only 7%. The proposed methodology from this work for a fast and cost-effective surface enhancement methodology in an environmentally safe solution may be used on real, complex AM parts to enhance both internal and external surface quality.

Electron beam powder bed fusion can offer advantages in terms of powder reuse by virtue of larger layer thickness and flexible recoater, thus lowering the economic barrier by reducing waste. Systematic studies focused on quantifying the effect of reuse on powder performance metrics such as size, rheometry and flowability remain scarce. This work presents results on the influence of the process on Ti-6Al-4V powder, by comparing virgin powder to reused powders over two processing cycles, benchmarking the most sensitive powder characteristics. Particle size, morphology, flow properties and rheometry are compared. It was observed that morphology, size distribution, flowability and spreadability were degraded due to partial sintering and powder recovery.

Additively manufactured parts produced via laser powder bed fusion (LPBF) have limitations in their applications due to post-processing requirements caused by high surface roughness. The characteristics of side-skin surfaces are generally assumed to be dominated by adhered powder particles. This work aims to analyze and interpret the effects of LPBF processing parameters on side-skin surfaces. As such, this work has two sections to investigate the effect of (i) core and (ii) border LPBF parameters on side-skin surface roughness for Ti–6Al–4V. The findings show that there is a robust correlation between both core and border LPBF parameters on side-skin surface morphologies. In terms of core LPBF parameters, an interaction between laser power and beam velocity is shown to influence side-skin surface roughness, resulting in Sa values in the range of 11–26 µm. Additionally, a preliminary investigation into the effect of melting mode phenomena at the border leads to a possibility of obtaining Sa values of

Additive manufacturing (AM) of titanium (Ti) and Ti-6Al-4V lattices has been proposed for bone implants and augmentation devices. Ti and Ti-6Al-4V have favourable biocompatibility, corrosion resistance and fatigue strength for bone applications; yet, the optimal parameters for Ti-6Al-4V lattice designs corresponding to the natural micro- and meso-scale architecture of human trabecular and cortical bone are not well understood. A comprehensive review was completed to compare the natural lattice architecture properties in human bone to Ti and Ti-6Al-4V lattice structures for bone replacement and repair. Ti and Ti-6Al-4V lattice porosity has varied from 15% to 97% with most studies reporting a porosity between 50% and 70%. Cortical bone is roughly 5–15% porous and lattices with 50–70% porosity are able to achieve comparable stiffness, compressive strength, and yield strength. Trabecular bone has a reported porosity range from 70% to 90%, with trabecular thickness varying from 120 to 200 μm. Existing powder bed fusion technologies have produced strut and wall thicknesses ranging from 200 to 1669 μm. This suggests limited overlap between current AM of Ti and Ti-6Al-4V lattice structures and trabecular bone architecture, indicating that replicating natural trabecular bone parameters with latticing is prohibitively challenging. This review contributes to the body of knowledge by identifying the correspondence of Ti and Ti-6Al-4V lattices to the natural parameters of bone microarchitectures, and provides further guidance on the design and AM recommendations towards addressing recognized performance gaps with powder bed fusion technologies.

A path to lowering the economic barrier associated with the high cost of metal additively manufactured components is to reduce the waste via powder reuse (powder cycled back into the process) and recycling (powder chemically, physically, or thermally processed to recover the original properties) strategies. In electron beam powder bed fusion, there is a possibility of reusing 95 - 98% of the powder that is not melted. However, there is a lack of systematic studies focusing on quantifying the variation of powder properties induced by number of reuse cycles. This work compares the influence of multiple reuse cycles, as well as powder blends created from reused powder, on various powder characteristics such as the morphology, size distribution, flow properties, packing properties and chemical composition (oxygen and nitrogen content). It was found that there is an increase in measured response in powder size distribution, tapped density, Hausner ratio, Carr index, basic flow energy and specific energy, dynamic angle of repose, oxygen, and nitrogen content, while the bulk density remained largely unchanged., Comment: 24 pages, 8 figures, 1 graphical abstract

Binder jetting additive manufacturing (BJAM) is a comparatively low-cost process that enables manufacturing of complex and customizable metal parts. This process is applied to low-cost water-atomized iron powder with the goal of understanding the effects of printing parameters and sintering schedule on maximizing the green and sintered densities of manufactured samples, respectively. The powder is characterized by using scanning electron microscopy (SEM) and particle size analysis (Camsizer X2). In the AM process, the effects of powder compaction, layer thickness, and liquid binder level on green part density are investigated. Post-process heat treatment is applied to selected samples, and suitable debinding parameters are studied by using thermo-gravimetric analysis (TGA). Sintering at various temperatures and durations results in densities of up to 91.3%. Image processing of x-ray computed tomography (μCT) scans of the samples reveals that porosity distribution is affected by powder spreading, and gradients in pore distribution in the sample are largely reduced after sintering. The resulting shrinkage ranges between 6.7 ± 3.0% and 25.3 ± 2.8%, while surface roughness ranges between 11.6 ± 5.0 μm and 32.1 ± 3.4 μm. The results indicate that the sintering temperature and time might be tailored to achieve target densities anywhere in the range of 64% and 91%, with possibly higher densities by increasing sintering time.

In the present research, structural analyses are conducted on Invar (64% Fe-36% Ni) samples manufactured by modulated laser powder bed fusion (LPBF) under different laser power regimes. Within the selected process window, the results obtained from X-ray computed tomography analysis indicate that the relative density of as-built samples increases with an increase in laser power. In addition, it is observed that irregular-shaped pores with the longest axis normal to the building direction are significantly reduced with increasing the laser power. Microstructural investigation suggests that with increasing the laser power, a transformation from conduction to transition and keyhole modes occurs. In X-ray diffraction patterns of the as-built samples, only fcc γ-phase is detected and no reflection of bcc α-phase is found. Texture measurements show that the intensity of 〈110〉 crystallographic orientation along the building direction of the as-built samples increases with increasing laser power. Coefficient of thermal expansion of the as-built samples is very low and comparable to that of conventionally manufactured Invar alloy. Furthermore, as-built samples fabricated at lowest (250 W) and highest (400 W) laser powers exhibit the lowest and highest thermal expansion displacement, respectively. Finally, the thermal expansion behavior and its correlation with structural integrity and texture are discussed. Keywords: Invar alloy, Laser powder bed fusion, Porosity, Microstructure, Thermal expansion

To facilitate functional part production in metal binder jetting additive manufacturing, the relationship between materials, process and sintering needs to be understood. This work relates sintering theory with process outcomes. For this, commercially pure titanium was deployed to study the effect of powder size distributions on green and sintered part qualities (bulk density, relative density, particle size, pore size, sinter neck size). The powders were uni- and bi-modal blends of 0–45 μm, 45–106 μm, and 106–150 μm. Computed tomography analysis was used to evaluate non-densifying (1000 °C) and densifying (1400 °C) sintering regimes. For green parts, the relative density and powder size distribution along the build direction followed a periodic fluctuation equivalent to the 150 μm layer thickness. The relative density fluctuation range was higher (±20%) for bi-modal blends with 0–45 μm, compared to all other blends (±8%) due to powder segregation. For non-densifying sintering, parts with 0–45 μm blends displayed both densifying and non-densifying behavior. For densifying sintering, powders containing 0–45 μm blends surpassed the 70% density threshold expected for this sintering regime. Overall, the finer particles improved bulk density of sintered parts, at the expense of higher levels of shrinkage and density anisotropy along the build direction. Keywords: Additive manufacturing, Binder jetting, Computed tomography, Commercially pure titanium powder blends, Sinter structure analysis, Particle distribution

Experimental studies in the literature have identified the powder-bed compaction density as an important parameter, governing the quality of additively manufactured parts. For example, in laser powder-bed fusion (LPBF), the powder-bed compaction density directly affects the effective powder thermal conductivity and consequently the temperature distribution in melt pool. In this study, this physical parameter in a LPBF build compartment is measured using a new methodology. A UV curable polymer is used to bind powder-bed particles at various locations on the powder-bed compartment when Hastelloy X was used. The samples are then scanned using a nano-computing tomography (CT) system at high resolution to obtain an estimation of the relative powder-bed compaction density. It is concluded that due to the interaction between the recoater and the variation in the powder volume accumulated ahead of the recoater across the build compartment, the relative powder-bed compaction density decreases along the recoater moving direction (from 66.4% to 52.4%.). This variation in the powder-bed compaction density affects the density and surface roughness of the final printed parts that is also investigated. Results show that the part density and surface quality decrease ~0.25% and ~20%, respectively, along the build bed in direction of the recoater motion. Keywords: Additive manufacturing, 3D printing, Compaction density, Selective laser melting, Laser powder-bed fusion, Hastelloy X

Spherical powders are almost exclusively deployed for metal laser powder bed fusion (LPBF) additive manufacturing (AM). In this work, low-cost irregular powder feedstock is studied for its potential in three key areas to meet minimum AM process requirements, namely: (1) hopper flow, (2) layer spreading, and (3) de-powdering. Irregular water-atomized Iron powder was used as the study population, while spherical plasma-atomized Inconel 625 powder was used as the control. Powder flow characteristics were obtained using a FT4 Powder Rheometer (Freeman Technology). Layer spreading was evaluated indirectly via powder bed density measurements. Measurements were quantified by using printed artifacts for captive powder and evaluated using isopropanol infiltration and three-dimensional computed tomography (CT) imaging (Zeiss Xradia 520 Versa). Powder clearing from fine channels was quantified through vision-based measurements of printed artifacts (Dino-Lite DinoCapture 2.0). The results from this work demonstrated that: (1) A larger opening and steeper hopper angle are necessary to maintain a mass flow regime with the irregular powder. (2) Powder bed density is similarly consistent across the bed indicating adequate spreadability. (3) Water-atomized Iron powder has better de-powdering characteristics in the smallest cleared 0.6 mm diameter features, likely due to its 15% lower bed density.

The data included in this article provides additional supporting information on our recent publication (Liravi et al., 2018 [1]) on a novel hybrid additive manufacturing (AM) method for fabrication of three-dimensional (3D) structures from silicone powder. A design of experiments (DoE) study has been carried out to optimize the geometrical fidelity of AM-made parts. This manuscript includes the details of a multi-level factorial DOE and the response optimization results. The variation in the temperature of powder-bed when exposed to heat is plotted as well. Furthermore, the effect of blending ratio of two parts of silicone binder on its curing speed was investigated by conducting DSC tests on a silicone binder with 100:2 precursor to curing agent ratio. The hardness of parts fabricated with non-optimum printing conditions are included and compared.

The feasibility of a hybrid additive manufacturing (AM) method combining material extrusion and powder bed binder jetting (PBBJ) techniques for fabrication of structures made of silicone (polysiloxane) is investigated in this paper. A full factorial experimental design was conducted to maximize the geometrical accuracy of the parts. The rheological and morphological properties of the silicone powders, the thermal characteristics of the liquid silicone binder, and mechanical characterization the additively manufactured parts are reported. Using this hybrid AM method, porous cylindrical structures (5 mm diameter (D) × 3 mm height (H)) with potential applications in biomedical industry were additively manufactured. The final structures are composed of ∼60% silicone powder, ∼ 30% silicone binder, and

Over the past two decades, additive manufacturing has opened a new window of opportunities in fabricating complex porous matrix structures such as cellular solids. Several factors including design, material and process parameters can selectively be varied to tailor the porous properties of products based on the intended application. This article addresses the effect of variable throughout layer thickness configuration in the binder-jet additive manufacturing of titanium structures for orthopedic applications. Two layer thicknesses of 80 and 150 μm are selectively controlled inside of each titanium sample with four different configurations. Several studies were performed, including shrinkage analysis, porosity measurements, and mechanical compression tests to quantify the effect of layer thickness on part quality and mechanical properties. The results of the porosity measurement revealed that there is about 5% variation among the samples with different layer thickness configuration. Bulk porosity values obtained from micro computed tomography (μCT) scan data placed the bulk porosity of the samples combining more than one layer thickness, in between of the results for control specimens, which were manufactured by applying a single layer thickness throughout the samples. Mechanical properties did not show any significant variation, which is attributed to the low range of the porosity deviation (less than 5%). The highest Young’s modulus of 3.50 ± 0.4 GPa and yield stress of 175 ± 25 MPa were obtained from analysis of the data achieved from the compression test.

Depending on processing conditions, laser powder bed fusion (LPBF) is known to have two operational regimes – conduction mode and keyhole mode. Heat conduction is the dominant heat transfer mechanism for conduction mode melting, whereas heat convection is the dominant heat transfer mechanism for keyhole mode melting. In addition, there exists a transition mode, which lies between the conduction and keyhole mode, wherein the dominance of conduction or convection depends upon the processing conditions. In this work, normalized processing diagrams are obtained to visualize the three melting modes - conduction mode, transition mode, and keyhole mode. The normalized processing diagrams obtained from this work are shown to be independent of material for specific classes of materials, of LPBF system, of laser modulation, and of powder layer thickness. Additionally, an analytical model is proposed to robustly predict the threshold between the three melting modes for two different classes of materials, (i) materials with low reflectivity and low thermal conductivity such as titanium, ferrous, and nickel alloys, and (ii) materials with high reflectivity and high thermal conductivity such as aluminium alloys. The normalized processing diagrams, alongside the identified melting mode thresholds, can provide a useful tool in diagnosing the origins of porous defects and enable accelerated process optimization efforts towards tailoring material properties in LPBF.

In this work, the tensile behavior of Inconel 625 parts fabricated via laser powder bed fusion (LPBF) at different laser power levels is examined, and correlated to bulk porosity as well as pore characteristics such as pore size, aspect ratio morphology, and polar orientation extracted from X-ray computed tomography (CT). Scanning electron microscopy (SEM) is employed to identify the fracture mode and origin of failure in the pulled samples. Microstructural examination on the as-built samples showed that increasing the laser power resulted in the transition of melting mode, from lack of fusion to keyhole, with an increase in part bulk density from 98.86% to 99.29%, respectively. It was found that the general bulk porosity level does not correlate directly with the Ultimate Tensile Strength (ranging between 780–820 MPa) and strain to fracture (ranging between 0.2–0.39) behavior of the parts. Detailed pore space characteristics obtained from CT datasets before and after the tensile test contributed to establishing a relationship between defects size, morphology, orientation and tensile properties of the samples. In general, it was found that strain to failure is directly influenced by pore space characteristics, while tensile strength is influenced by a combination of pore space and microstructural characteristics. This study also identified that there are systematic bias effects in the LPBF process, likely introduced by the combination of nuisance variables such as powder layer spreading and gas flow.

Additive manufacturing (AM) is a promising approach for fabricating structures to serve as bone substitutes, or as biomaterial components in biphasic implants for repair of osteochondral defects. In this study, the three dimensional printing (3DP) AM process was investigated to determine the effect of powder layer orientation on mechanical and structural properties of fabricated parts. Five types of standard cylindrical parts were manufactured via AM with 0°, 30°, 45°, 60° and 90° stacking layer orientations relative to the vertical z -axis of the print bed, using amorphous calcium polyphosphate (CPP) powder of irregular particle shape, average aspect ratio ≈1.70 and particle size between 75 and 150 μm. It was concluded that layer orientation had an effect on porosity and compressive strength, based on induced powder particle orientation in the green part during powder layering. The resulting bulk porosity values ranged between 30.0 ± 2.4% and 38.2 ± 2.7%, while the compressive strength ranged between 13.50 ± 1.95 MPa and 45.13 ± 6.82 MPa. The orientation with the highest compressive strength was 90°, while orientations with the weakest compressive strength were 0° and 45°. Based on these results, it was established that AM-made parts are structurally and mechanically anisotropic. The stacking layer orientation which results in the highest strength performance along a preferred loading orientation can be implemented to further optimize mechanical strength of constructs along the maximum loading direction.

An additive manufacturing platform was developed for fabricating constructs with heterogeneous properties. The liquid binder amount was investigated in calcium polyphosphate bone substitutes. Two part categories were produced with 150 and 190 μm layer thickness and 70, 80, 90, 100, and 90/70% gray scale. It was shown that the gray scale level had an effect on samples with 150 μm layer thickness where porosity ranged between 43 ± 1% and 49 ± 2% and the compressive strength varied between 4.8 ± 1.3 MPa and 15.5 ± 1.9 MPa. The results suggest that the binder gray scale level can be used to locally predict porous properties.

This work addresses the characterization of a UV-based micro-syringe deposition (μSD) system utilized in the micro-dispensing of photopolymers on particulate ceramic substrates. This methodology is used in embedding functionally graded and interconnected micro-features within constructs produced by a novel combined powder-based additive manufacturing (AM) and UV-based micro-syringe deposition (μSD) technique. The process is experimentally characterized using SEM and optical microscopy to study the effect of a wide range of process parameters on the geometrical quality of deposited tracks. Experimental data show that the system can produce features ranging from 200 to 575 μm in width and from 20 to 200 μm in height on particulate ceramic surfaces. To gain insight into the proposed micro-deposition process, a two-tier model is also developed. The first framework describes an analytical model for predicting the flow rate of the dispensed photopolymer fluid based on the piston displacement. The second model is a stochastic framework for predicting the line width of the features deposited on the substrate using a Monte Carlo probabilistic simulation to compensate for uncertainty in the system input parameters. A comparison between experimental and modeling line width predictions shows that the modeling results are 14–38% higher than the experimental results, depending on the system input variables. The proposed model is enhanced by introducing adjustment factors to compensate for UV exposure delay, fluid migration, and imbibition.