Laser Powder Bed Fusion Samples Research Articles

Effect of Temperature on Passive Film Characteristics of LPBF (Laser Powder-Bed Fusion) Processing on UNS-S31603.

The effect of temperature on the localized corrosion resistance and passive film characteristics of laser powder-bed fusion (LPBF) 316L (UNS S31603) was studied in a buffered 3.5 wt% NaCl solution at 25, 50, and 75 °C. DC techniques such as cyclic potentiodynamic polarization showed lower passive current densities, high breakdown potentials, and a higher resistance to initial breakdown compared with wrought 316L samples at all temperatures. However, LPBF 316L was more susceptible to metastable pitting at potentials before film breakdown and higher damage accumulation post film breakdown. AC techniques, such as Mott-Schottky analysis and electrochemical impedance spectroscopy, showed that the formed passive film was more robust on the LPBF 316L samples at all temperatures, accounting for the higher initial resistance to pitting. However, with increasing temperatures, the film formed had an increasing concentration of defect density. Passive compositions at the various test temperatures studied using X-ray photoelectron spectroscopy (XPS) showed that the LPBF samples showed higher amounts of Cr and Fe oxides and hydroxides compared with the wrought samples, which made the passive films on the LPBF samples more compact and protective. Investigation of the pits formed on the LPBF showed the preferential regions of attack were the melt-pool boundaries and cell interiors due to their being depleted of Cr and Mo when compared with the boundaries and matrix.

Wet chemical surface functionalization of AA2017 powders for additive manufacturing

Laser Powder Bed Fusion (L-PBF) is an important route for processing aluminum alloys, nevertheless, the high laser reflectivity and thermal conductivity and lower powder flowability of the aluminum alloys remain a challenge for their processing. Aiming to overcome these issues, AA2017 powders were surface-functionalized using chemical etchings. The powders' physical and rheological characterization revealed a decrease in the laser reflectance and an improvement in the flowability of the surface-functionalized powders, when compared to the gas-atomized powder. The higher amount of keyhole porosity in the as-printed L-PBF samples using the surface-functionalized powders also suggests an improvement in the laser absorption of these powders. However, the formation of a thick oxide layer on the surface-functionalized powders led to oxidation porosity in the as-printed samples. Therefore, the chemical etching should be improved to avoid the formation of a thick oxide layer on the powder surface and oxidation porosity in the as-printed L-PBF samples.

In-situ EBSD study of the coordinated deformation behavior of 2205 duplex stainless steel fabricated via laser powder bed fusion during the tensile process

Duplex stainless steels (DSSs) produced via laser powder bed fusion (LPBF) exhibit excellent strength and plasticity compatibility. In this study, in-situ electron backscatter diffraction (EBSD) was employed to investigate the coordinated deformation behavior of 2205 DSSs fabricated by LPBF with heat treatment at 1050 °C for 1 h, including strain distribution behavior and mechanism of stress concentration release. The results indicate that the initial deformation of LPBF samples is primarily concentrated in the austenite phase. As the tension progresses, austenite gradually transitioned to hard orientation, and the strain in austenite can pass through the phase boundary into the ferrite grain. To coordinate the deformation, ferrite transitioned to soft orientation through rotation towards the slip systems of {110}<−111> and {1–21}<111>. The stress concentration in ferrite can be alleviated by forming dislocation walls and cells through planar and cross-slip dislocations. In contrast, the stress concentration in austenite can be released by stacking faults and deformation twins. Compared to the as-cast samples, the yield strength of LPBF samples is improved from 472 ± 13 to 641 ± 17 MPa due to fine grain strengthening and dislocation strengthening. This study offers guidance for further enhancing the strength and plasticity of materials prepared by LPBF.

Influence of annealing on enhancing soft magnetic properties in laser powder bed fusion processed Hiperco (Fe-49Co-2V)

Annealing of laser powder bed fusion (LPBF) processed Fe-49Co-2 V (Hiperco) samples at 865 °C for 4 h leads to a substantial improvement in its soft magnetic properties. While the as built LPBF samples exhibited relatively higher coercivities (Hc) ranging from 25.8 – 26.5 Oe, the annealed LPBF samples showed significantly lower coercivities (Hc) of 6.9 – 10.8 Oe. These lower Hc values can not only be attributed to the 15–20 times larger grain sizes, but also the higher degree of B2 ordering in the annealed condition. The enhanced degree of B2 ordering also increases the saturation magnetization (Ms), from 213 to 228 emu/g, in samples processed with a laser fluence (energy/density) of 3.4 J/mm2. These results reveal the underlying mechanisms leading to an enhancement of soft magnetic properties in LPBF processed Hiperco via annealing-induced microstructural control.

Simultaneous Pore Detection and Morphological Features Extraction in Laser Powder Bed Fusion with Image Processing.

Internal pore defects are inevitable during laser powder bed fusion (LPBF), which have a significant impact on the mechanical properties of the parts. Therefore, detecting pores and obtaining their morphology will contribute to the quality of LPBF parts. Currently, supervised models are used for defect image detection, which requires a large amount of LPBF sample data, image labeling, and computing power equipment during the training process, resulting in high detection costs. This study extensively collected LPBF sample data and proposed a method for pore defect classification by obtaining its morphological features while detecting pore defects in optical microscopy (OM) images under various conditions. Compared with other advanced models, the proposed method achieves better detection accuracy on pore defect datasets with limited data. In addition, quickly detecting pore defects in a large number of labeling ground truth images will also contribute to the development of deep learning. In terms of image segmentation, the average accuracy scores of this method in the test images exceed 85%. The research results indicate that the algorithm proposed in this paper is suitable for quickly and accurately identifying pore defects from optical microscopy images.

Influence of Fe content on the microstructure and mechanical properties of Al-7Si-0.7Mg-2.8Fe alloy processed by laser powder bed fusion

Iron contamination is a major concern in aluminum alloys recycling. It is considered the main deleterious contamination due to its low solubility in the aluminum matrix leading to the formation of Fe-rich intermetallic phases with platelet-like morphology, which act as stress risers deteriorating the mechanical properties. This paper investigates the effect of Fe contamination in the microstructure and mechanical properties of Al–7Si-0.7Mg-2.8Fe alloy processed by Laser Powder Bed Fusion (LPBF). Microstructure characterization was carried out by Optical Microscopy (OM), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscope (TEM). The mechanical characterization was performed by Vickers microhardness and tensile tests. The LPBF sample was analyzed in two directions, vertical and horizontal. The results show that the LPBF process provided a microstructural refinement and reduction of the aspect ratio of α-AlFeSi phase and silicon particles, leading to a strong improvement in mechanical properties when compared to the Fe-contaminated as-cast Al–7Si-0.7Mg-2.8Fe alloy.

From raw elements to 3D samples: An economical route for Co-Cr-Mo alloy fabrication

This study explores an efficient and feasible method for production of non-commercial Co-Cr-Mo powder feedstock for Laser Powder Bed Fusion (LPBF), employing mechanical alloying (MA) with following thermal treatment. Elemental Co, Cr, and Mo powders were subjected to MA to create a powder nanostructured composite. Initially, the mechanically alloyed powder had a mixed-phase state, with ε-phase as the main one (about 95 wt%) and impurities of Cr2O3. However, an additional 1.5-hour of isothermal annealing at 400 °C successfully reduced the Cr2O3 impurities and caused oxygen content to decrease, indicating a reduction in residual oxygen in the powder. This annealing process activated diffusion and induced relaxation of residual stresses, evidenced by decreased lattice microstrains: 0.8% for the Co-phase, 0.2% for both the Cr-phase and Mo-phase. Powder subjected to the thermal treatment was then utilized in LPBF to create bulk Co-Cr-Mo samples. The formed samples showed a typical LPBF microstructure represented by uniaxial cells and dendrites, with the distance between these structures not exceeding 3 µm. The samples showed homogenous elemental distribution, mixed-phase composition, and oxygen concentration not exceeding 5 wt%. In terms of mechanical properties, the LPBF samples exhibited a microhardness value of 4300 ± 200 MPa, ductility of 0.85, and yield strength of 1.4·103 MPa. Our method yields results comparable to those obtained from commercially spherical powder, indicating that our cost-effective approach does not sacrifice product quality. It represents a substantial cost reduction, approximately 4.5 times lower than that of commercial spherical powder, thereby highlighting the economic efficiency of our powder feedstock preparation for LPBF. Our findings underscore the potential of mechanically alloyed and thermally treated powder in LPBF. This worthwhile and efficient method of preparation powder feedstock breaks ground for more comprehensive facilities in 3D printing, cut down on cost of production with maintaining the high-quality output.

Designing size-controlled air cavities to reduce iron losses of 3D printed ferromagnetic parts: Modelling and experimental results

We studied the effect of cavity insertion to reduce iron losses into Fe-2.6wt.%Si ferromagnetic components fabricated by Laser Powder Bed Fusion (L-PBF) 3D printing process. Finite Element Method (FEM) simulation was used to understand the impact of various cavity configurations on eddy current losses and to identify design guidelines. Results highlight the dependence of losses on eddy current path length, which can be expressed by simple geometrical design parameters. Finally, L-PBF samples were magnetically characterized up to 500 Hz. A good correspondence between experimental data and modelling results is observed. A torus designed with six external cavities allowed the reduction of the losses by 60 % compared to a bulk specimen (0.5 T; 500 Hz).

Unraveling composition graded microstructure of in-situ alloyed Ti-15Nb alloy through laser powder bed fusion

The novelty of the present work lies in applying an in-situ alloying procedure to obtain a metastable Ti-15Nb (wt.%) alloy through laser powder bed fusion (LPBF). Mixing elemental powders of c.p. Ti and c.p. Nb were subjected to different parameter combinations through LPBF and these conditions led to different results regarding microstructure homogeneity, porosity, and properties as a composite of Ti-15Nb matrix with unmelted Nb powder particles. The phases found in the LPBF samples were ά martensite (hcp) and β-Ti matrix (bcc). The lowest hardness of 262 HV was obtained by microhardness test while the highest was 318 HV. However, porosity and unmelted Nb exhibit notable variations and show a distinct trend based on energy density, scanning speed, and laser power. TEM analysis together with ASTAR mapping showed α' martensite needles of 50 to 100 nm width dispersed through the β-Ti matrix. Processing of elemental powders such as Ti and Nb to form an in-situ alloying of Ti-15Nb alloy through laser powder bed fusion with a low Young’s modulus of 64 GPa opens new perspectives for biomedical applications.

Additive manufacturing of Mn-Al permanent magnets via laser powder bed fusion

Permanent magnet materials based on Mn demonstrate high potential to fill the gap between lower-performance bonded ferrites and high-performance magnets based on rare-earth elements. Mn-Al permanent magnets were processed into bulk via the laser powder bed fusion (LPBF) technique. Gas-atomized Mn54Al46 precursor particulates were fused into cube-shaped magnets of the high-temperature ε phase with ∼93 % bulk density. A 450 °C anneal for 30 min transformed the ε phase into largely the ferromagnetic τ phase. The magnetic properties were compared to an as-cast reference bulk Mn54Al46 sample prepared from ε phase via the same heat treatment. The LPBF sample exhibited a finer grain size, a ∼45 % gain in coercivity, and higher remanence due to grain elongation and a weak <001>τ texture parallel to the printing direction. Despite the loss of density, the LPBF sample had a ∼70 % improvement in (BH)max compared to the as-cast bulk reference sample. The success of this process demonstrates that the LPBF technique is a viable method to densify the metastable Mn-Al τ phase from particulate into bulk while retaining a fine grain size, developing magnetic anisotropy, and preventing phase decomposition. Optimal properties of the LPBF magnet were Mr = 41 Am2/kg, Hci = 129 kA/m, and (BH)max = 7.2 kJ/m3 along the print direction.

The effect of support structure on residual stress in laser powder bed fusion of Ti6Al4V alloy

Support structures are widely used in laser powder bed fusion (LPBF) for overhanging features. However, the majority of works carried out on support modeling were to obtain the best trade-off between production time, cost, and accuracy. A notable absence in the existing literature pertains to investigate support structure modeling for optimizing heat dissipation and residual stresses for improving the support structure, thus, enhancing the final part quality. In this study, the effects of the support structure on residual stress were investigated systematically. The influences of the support on residual stress in LPBF sample were categorized into the effect of the constraints and heat conduction area. The rigidity and area of the support structure are used to describe the effect of the support structure on the residual stress. The results showed that the constrain of LPBF samples mainly depends on their inherent characteristics, with limited impact from external constraints. The residual stress in the samples increased as the support area increased. Therefore, when designing the support, priority should be given to reduce its thermal conductivity area. In addition, the hardness of the support added sample were lower than that of the sample without the support, but the hardness distribution is uniform along the deposition height regardless of whether the support is added or not. Therefore, the introduction of support does not result in performance irregularities along the sample height direction.

Pit growth kinetics of additively manufactured MoNi over-alloyed type 316L stainless steel

The microstructure and corrosion performance of wrought, laser powder bed fusion (LPBF), and MoNi over-alloyed LPBF type 316L stainless steel is compared. Adding Mo and Ni to LPBF 316L stainless steel resulted in ẟ-ferrite. Potentio-dynamic polarisation was used to rank all microstructures, with LPBF + MoNi has more resistant than LPBF and wrought material. Bipolar electrochemistry was applied, with LPBF samples having the lowest critical pitting potential and highest pit growth kinetics. The best corrosion performance was for LPBF + MoNi stainless steel. Different pitting corrosion resistance as function of process directions were found. The tiny pores negatively impacted the pitting performance.

Oxidation kinetic behavior and microstructure mechanism of GH3536 alloy fabricated by laser powder bed fusion

This study comprehensively investigates the oxidation kinetics of laser powder bed fusion (LPBF) processed GH3536 superalloy at 950 °C. The oxidation kinetics and oxide film microstructure characteristics are analyzed and the underlying mechanism is discussed, by comparing with the conventionally manufactured wrought GH3536 samples under the exactly same test condition. The oxidation rate of LPBF samples is faster at initial stage and then tends to flatten out with increase of heating duration time. The finer and equiaxed chromia crystals formed in LPBF GH3536 surface can effectively improve the oxidation resistance comparing to the columnar structure chromia in wrought GH3536. The oxide film with excellent protection in LPBF GH3536 provides an important contribution to the application of GH3536 at high temperature.

Influence of high temperature oxidation on mechanical properties and in vitro biocompatibility of WE43 magnesium alloy fabricated by laser powder bed fusion

Laser powder bed fusion (L-PBF) has been used to fabricate biodegradable Mg implants of WE43 alloy, but the degradation is too fast compared with the term bone reconstruction. Previous studies show that high temperature oxidation (HTO) can successfully inhibit the degradation of WE43 alloy. In this work, the influence of HTO on L-PBF samples of WE43 alloy was investigated regarding tensile, compressive, and abrasive resistance, as well as in vitro cytotoxicity, cell proliferation, hemolysis, and osteogenesis. Compared with the as-built L-PBF samples, HTO increased grain size and grain texture, stabilized and coarsened precipitates, and caused discontinuous static recrystallization in the matrix. The oxide layer at the surface of the HTO samples improved surface roughness, hydrophilia, hardness, and abrasive resistance. The tensile strength decreased slightly from 292 to 265 MPa, while the elongation substantially increased from 10.97% to 16.58% after HTO. The in vitro cell viability, cell proliferation, hemolysis, and osteogenic effect were considerably enhanced due to the improvement of surface quality and the initial inhibition of excessive Mg2+ releasement. Overall, HTO is of great benefit to the surface performance, ductility, and biocompatibility of WE43 alloy fabricated by L-PBF for biodegradable applications.

Influence of volumetric energy density on the tribological behaviors of pure nickel fabricated by laser powder bed fusion

ABSTRACT Laser powder bed fusion (L-PBF) enables the fast fabrication of pure nickel parts with complex structures. Tribological behaviors of the printed pure nickel are crucial to its application, and highly dependent on the volumetric energy density (VED), due to the variation of wear mechanisms. In this study, to investigate the wear mechanisms and evaluate the tribological behaviors of L-PBF pure nickel, samples fabricated with different VEDs were tested by a ball-on-disc tribometer under different representative loads. Unlike tribofilms formed on casting samples during sliding, L-PBF samples suffer abrasive wear, and their wear resistance decreases initially and increases afterwards with increasing VED. It suggests that the preferred VED provide good densification behaviors, high microhardness, and sufficient thin columnar subgrains, which decrease the abrasive wear. As a result, the wear resistance of the preferred L-PBF pure nickel can outperform cast counterparts under a load close to the yield strength of pure nickel.

Quantifying microstructural contribution to yield stress and strain hardening of Ni20Cr alloy manufactured by laser powder bed fusion with different volumetric energy densities

Laser powder bed fusion (LPBF) process is an additive manufacturing technique that focuses on intricate metal fabrication using laser processing of metallic powder. Various processing parameters like laser power, scanning speed, and hatch spacing giving out unique applied volumetric energies are involved in such fabrication. Those varied energies bring about microstructural changes leading to modifications in mechanical response such as yield stress and strain hardening behaviour. In this work, we investigated the influence of volumetric energy density on the aforementioned mechanical properties of a Ni-20 wt%Cr alloy manufactured via LPBF. First, an analytical model was employed to study the contribution of each microstructural feature on yield stress of LPBF samples. Dendritic cellular structures (and their sizes) are found to be the most important feature to govern this parameter. The Kocks-Mecking model was further extended to associate the different strain hardening mechanisms with dislocation production and interaction mechanisms via different channels like dendritic cellular structures, grains and forest dislocations. The production of dislocation via dendritic cellular structures is also the most significant mechanism for unique hardening behaviour in LPBF alloys. A modified equation of dislocation production mechanisms is finally proposed to simplify the application of this model for modelling the mechanical behaviour in tension of LPBF Ni20Cr.

Study on additive and subtractive manufacturing of high-quality surface parts enabled by picosecond laser

Low dimensional accuracy and high surface roughness induced by the inherent powder adhesion and staircase effect are the critical challenges for the application of parts fabricated by laser powder bed fusion (LPBF). In this study, an innovative hybrid additive manufacturing process by adding the ultrafast laser machining to the LPBF process was proposed. After LPBF process, the additively manufactured samples were micromachined using a picosecond laser. The effects of parameters (i.e., laser frequency, and scanning speed) of the ultrafast laser machining on the surface morphology were investigated by the single-pass ablation experiments. The impact of the scan counts on the cutting depth was analyzed by the large-area cutting experiments. Subsequently, samples with different tilt angles were fabricated by a hybrid additive manufacturing process with alternating processing of LPBF and the ultrafast laser machining. The side surface roughness (4.98 ± 0.44 µm) of the samples fabricated by hybrid additive manufacturing process was lower than that of as-built LPBF samples. The processing characteristics and the ablation morphology of the picosecond laser were investigated. The factors affecting the quality of hybrid manufacturing were systematically discussed. This work may provide a new way to manufacture parts with high dimensional accuracy and low surface roughness by LPBF.

Effects of Powder Characteristics and Chemical Composition on the Properties of 25Cr7Ni Stainless Steel Fabricated by Laser-Powder Bed Fusion and Evaluation of Process Simulation

The 25Cr7Ni stainless steel alloy system is gaining increasing interest in the oil and gas industry because of its combination of high strength and corrosion resistance properties. However, very few studies on the effects of starting powder attributes and chemical composition on the as-printed properties of 25Cr7Ni stainless steel fabricated through laser-powder bed fusion (L-PBF) exist in the literature. This study examined the influence of powder attributes and chemical composition on the samples from gas atomized and water atomized 25Cr7Ni stainless steel powders, fabricated through L-PBF, on their as-printed microstructure and properties. The mechanical properties that were examined included ultimate tensile strength (UTS), elongation (%), and hardness. The corrosion behavior was also studied using linear sweep voltammetry in 3.5 wt.% NaCl solution. The evolved phases were characterized using optical and scanning electron microscopy, as well as through X-ray diffraction. The gas atomized powders, with their spherical and uniform morphology, yielded as-printed parts of higher relative densities when compared to water atomized powders, with irregular morphology due to better powder bed compaction. The higher densification obtained in the L-PBF samples from gas atomized powders translated into the highest UTS, hardness, and yield strength among the L-PBF samples from water atomized powders and wrought–annealed 25Cr7Ni stainless steel. The presence of higher amounts of N and Mn in the chemical composition of the gas atomized powders over water atomized powders promoted the presence of retained austenite in the corresponding L-PBF samples. Higher amounts of Mo, combined with austenite content, yielded a higher corrosion resistance in the L-PBF samples from the gas atomized powder than in the L-PBF samples from the water atomized powders. The latter part of the work is focused on the evaluation of simulation parameters for analyzing the fabrication procedure for the L-PBF process using Simufact software. For a given set of process parameters, Simufact provides the distortion and internal stresses developed in the printed parts as output. The present study sought to evaluate the process simulation by comparing the experimental observations in terms of the part distortion achieved in a stainless steel cube fabricated through L-PBF with Simufact process simulation obtained using the same set of process parameters.

Structure and Properties Evolution of AZhK Superalloy Prepared by Laser Powder Bed Fusion Combined with Hot Isostatic Pressing and Heat Treatment

The structure and properties of samples obtained by the laser powder bed fusion (LPBF) method from the AZhK alloy, intended for the manufacture of heavily loaded body parts with operating temperatures up to 800 °C, have been studied. The optimal mode of LPBF, ensuring the attainment of the minimal residual porosity of 0.02%, was identified for the superalloy AZhK. Additionally, the evolution of the microstructure of LPBF samples after hot isostatic pressing (HIP) and heat treatment (HT) was studied using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The macrostructure of LPBF samples is represented by columnar grains oriented in the direction of predominant heat dissipation, perpendicular to the build plate. At the microlevel, the structure consists of colonies of columnar dendrites. Nb4AlC3 and Nb6C4 carbides, as well as the Mo2Hf Laves phase, are precipitated in the interdendritic region as a result of doping element segregation. The low strength of the LPBF samples (σ = 967 ± 10 MPa) is caused by the fact that there are no reinforcing particles and by high internal stress due to high crystallization speed. HIP and HT were found to have a favorable effect on the structure and properties of the LPBF samples. The post-treatment resulted in uniform distribution of γ′-phase precipitates sized up to 250 nm in the matrix bulk and carbides at grain boundaries, as well as Laves phase dissolution. Therefore, the strength characteristics were significantly improved: by 45% at room temperature and by 50% at elevated temperatures. High strength and ductility were attained (at 20 °C, σ20 = 1396 ± 22 MPa and δ = 19.0 ± 3.0 %; at 650 °C, σ650 = 1240 ± 25 MPa and δ = 15.8 ± 1.5%; at 750 °C, σ750 = 1085 ± 23 MPa and δ = 9.1 ± 2.3%). An ejector-type part was fabricated, and its geometric parameters coincided with those in the electronic models. After conducting computed tomography, it was found that there are no defects in the form of non-fusion and microcracks within the volume of the part; however, it was observed that the pore size is ≥20 μm.

Irradiation damage and corrosion performance of proton irradiated 304 L stainless steel fabricated by laser-powder bed fusion

The irradiation-induced microstructure and corrosion performance of proton irradiated 304 L stainless steel (SS) fabricated by laser powder bed fusion (LPBF) is investigated. Irradiation-induced dislocation loops are observed in both LPBF and traditionally manufactured (TM) 304 L SS following 2 MeV proton irradiation at 360 °C. A comparison analysis of the oxide scales formed on proton irradiated LPBF and TM 304 L SS is conducted along with the unirradiated counterparts. The irradiation-induced defects accelerate the corrosion by providing a rapid diffusion path to form a thicker and more porous film with a double-layer structure and herein the oxide film thickness of TM 304 L SS is approximately twice that of the LPBF 304 L SS. The unique microstructure of high dislocation densities, cellular sub-grains boundaries and dispersed nanoscale inclusions provides large amounts of interfaces in LPBF samples, which is benefit to absorb the irradiation-induced point defects. The irradiation induced fast diffusion paths and the nucleation points of oxides are thus reduced, resulting in a thinner oxide film thickness and a better corrosion resistance revealed by the more Cr enrichment of inner scale in LPBF 304 L SS.