Laser Powder Bed Fusion Parts Research Articles

Laser additive manufacturing of Zn metal parts for biodegradable implants: Effect of gas flow on evaporation and formation quality

Zn based metals have exhibited promising applications for biodegradable medical implants. Although additive manufacturing has been widely investigated for many medical metals, only a handful of very recent reports can be found on laser powder bed fusion (L-PBF) of pure Zn cubes. Severe evaporation occurred during laser melting of Zn powders due to the low boiling point of Zn metal. The formation quality was either not satisfactory or the obtained processing window was too narrow. In this paper, a specially designed gas flow system was used in order to eliminate the negative effect of Zn metal evaporation on the processing of L-PBF. Numerical analysis was used to simulate the velocity distribution of shielding gas flow and the interaction between the gas flow and the evaporation fume based on computation fluid dynamics with considering different shielding flow designs and laser energy inputs. Pure Zn metal parts were obtained with density over 99.90% after the optimization of shielding gas flow and laser energy input. The areal surface roughness (Sa) of L-PBF parts was about 10 μm at side surfaces for as-melted status, and reduced to 4.83 μm after sandblasting. The effects of gas flow and laser energy input on evaporation and formation quality were discussed. An effective method was provided to deal with metal evaporation during L-PBF processing, which was not only beneficial to additive manufacturing of Zn based metals, but also to other active metals with high evaporation tendency like Mg and Al alloys.

Microstructure-property relationships of 420 stainless steel fabricated by laser-powder bed fusion

In this study, we report the mechanical properties, microstructure and corrosion behavior 99+ % dense 420 stainless steel parts fabricated by laser-powder bed fusion (L-PBF). As-printed L-PBF parts exhibited an ultimate tensile strength of 1050 ± 25 MPa, yield strength of 700 ± 15 MPa, an elongation of 2.5 ± 0.2% and a hardness of HRC 55 ± 1. After the heat treatment operation at 315 °C, the ultimate tensile strength improved significantly to 1520 ± 30 MPa, yield strength increased to 950 ± 20 MPa, and elongation increased to 6.3 ± 0.2% respectively whereas hardness remained at HRC 53 ± 1. These properties are higher than previously reported literature values for 420 stainless steel fabricated by L-PBF, metal injection molding and powder metallurgy routes. The microstructure indicated a mixture of martensite and retained austenite phases in the as-printed parts. After heat treatment, the microstructure was richer in tempered martensitic laths that is consistent with the enhancement of ultimate tensile strength, yield strength, and elongation without appreciable change in hardness. The corrosion behavior of the as-printed L-PBF parts exhibited a corrosion current of 2.85 ± 0.4 mA.cm−2, a polarization resistance of 17,100 ± 520 Ω.cm−2 and a corrosion rate of 28 ± 2 μm/year which were comparable to the corrosion properties of wrought 420 stainless steel. A significant increase in pitting potential (170 mV) was obtained after the heat treatment.

Defect Detection and Monitoring in Metal Additive Manufactured Parts through Deep Learning of Spatially Resolved Acoustic Spectroscopy Signals

Abstract Laser powder bed fusion (LPBF) is an additive manufacturing (AM) process that promises to herald a new age in manufacturing by removing many of the design and material-related constraints of traditional subtractive and formative manufacturing processes. However, the level and severity of defects observed in parts produced by the current class of LPBF systems will not be tolerated in safety-critical applications. Hence, there is a need to introduce information-rich process monitoring to assess part integrity simultaneously with fabrication so that opportune corrective action can be taken to minimize part defects. Spatially Resolved Acoustic Spectroscopy (SRAS) is a uniquely positioned nondestructive acoustic microscopy sensing approach that has been successfully used to probe the mechanical properties and assess the presence of defects in LPBF parts. However, the technique is sensitive to extraneous phenomena, such as surface reflectivity, which occur within the LPBF system and may occlude identification of surface breaking and subsurface defects. With a view to applying the SRAS technique for in-process monitoring in a production-scale LPBF environment and to overcome the foregoing challenge, this study proposes the use of a deep learning convolutional neural network that is termed Densely connected Convolutional Block Architecture for Multimodal Image Regression (DCB-MIR), which invokes SRAS-derived acoustic velocity maps of the part as input data and translates them to an output resembling an optical micrograph. Through this approach, we demonstrate that defects, such as porosity and surface imperfections in titanium alloy and nickel alloy specimens made using LPBF, which were not clearly discernable in the as-measured SRAS acoustic map and were obscured by artifacts in the optical image, are accurately identified. To quantify the accuracy of the approach, the cosine similarity between the predicted output images and target optical images was used as the objective function of DCB-MIR. The cosine similarity between the acquired SRAS signatures and the corresponding as-measured optical micrographs of samples typically ranged between –0.15 and 0.15. In contrast, when the optical micrograph-like images derived from DCB-MIR proposed in this work were compared with the optical signatures, the cosine similarity improved in the range of 0.25 to 0.60.

Layer-by-layer combination of laser powder bed fusion (LPBF) and femtosecond laser surface machining of fabricated stainless steel components

Post-processing techniques for surface machining of laser powder bed fusion (LPBF) fabricated parts offer limited access to internal surfaces in complex shapes, precision surfacing, side-wall morphology, and material removal rates, thereby considerably limiting their applicability. In this work, a two-step fabrication involving the initial fusion of a powder layer in a LPBF set-up followed by the selective ablation of layer contours by rastering normally incident femtosecond laser pulses, is repeated for every layer of the built. For 1 kHz repetition rate, pulse fluence of 6.7 J/cm2 and super-Gaussian beam of shape factor 16, an experimentally obtained groove-free surface on laser machined walls was correlated with scan speeds when the pulse accumulated fluence distribution profile was horizontally flat or constant. Increasing the line overlap in raster scans did not yield any noticeable change in the machined wall surface. Groove-free surfaces, uniformly textured with sub-micrometer spaced laser-induced periodic surface structures (LIPSS), were successfully extended over side-walls of several unidirectionally machined consecutive layers. Ultimately, laser machined multi-layered LPBF parts possessing average side-wall roughness of Rt=4.7±1μm and side-wall taper angle of as low as ∼3°±1° were fabricated.

Characterization of Inconel 625 fabricated using powder-bed-based additive manufacturing technologies

The purpose of this study was to perform a comparative analysis of powder-bed-based additive manufacturing (AM) technologies during the production of metallic components using Inconel 625 powder material. The AM technologies explored in this study include electron beam powder bed fusion (EPBF), laser powder bed fusion (LPBF), and binder jetting technology. Samples were fabricated in two build directions (X and Z build orientations) for this evaluation process, where all specimens underwent a hot isostatic pressing (HIP) post-process. The comparison was made in terms of microstructure and mechanical properties including ultimate tensile strength (UTS), yield strength (YS), percent elongation, and modulus of elasticity (E). Microstructural characterization showed evidence of equiaxed grain formation for binder jetting and LPBF parts, whereas EPBF parts displayed a more columnar grain formation parallel to the build direction. Six specimens were tested per technology, three built in the X orientation and three built in the Z orientation. All six specimens were built in a single run of each AM machine. Results indicated that all three technologies are capable of meeting the minimum requirements of the ASTM F3056-14 standard for parts produced in the X orientation, with properties that are similar to wrought Inconel 625. In the Z orientation, however, only LPBF was able to meet the minimum standard requirements. Through the comparative analysis of the mechanical properties, this work showed that LPBF outperformed the other technologies in a majority of the evaluated properties, followed by EPBF and binder jetting. An analysis of the fracture surfaces of tensile specimens was also performed, and it indicated ductile fracture (dimple rupture) for the specimens produced with all three of the AM technologies studied. Nevertheless, the characterization also showed certain differences in the fractured surfaces, such as the presence of un-sintered powder particles for the binder jetting processed Inconel 625, or the development of the so called woody structure for the EPBF processed material. This study can be used to determine distinct characteristics between the three powder-bed-based technologies for the fabrication of Inconel 625 that can further include other technologies and materials using similar approaches.

Process Mapping and In-Process Monitoring of Porosity in Laser Powder Bed Fusion Using Layerwise Optical Imaging

The goal of this work is to understand the effect of process conditions on lack of fusion porosity in parts made using laser powder bed fusion (LPBF) additive manufacturing (AM) process, and subsequently, to detect the onset of process conditions that lead to lack of fusion-related porosity from in-process sensor data. In pursuit of this goal, the objectives of this work are twofold: (1) quantify the count (number), size and location of pores as a function of three LPBF process parameters, namely, the hatch spacing (H), laser velocity (V), and laser power (P); and (2) monitor and identify process conditions that are liable to cause porosity through analysis of in-process layer-by-layer optical images of the build invoking multifractal and spectral graph theoretic features. These objectives are important because porosity has a significant impact on the functional integrity of LPBF parts, such as fatigue life. Furthermore, linking process conditions to defects via sensor signatures is the first step toward in-process quality assurance in LPBF. To achieve the first objective, titanium alloy (Ti–6Al–4V) test cylinders of 10 mm diameter × 25 mm height were built under differing H, V, and P settings on a commercial LPBF machine (EOS M280). The effect of these process parameters on count, size, and location of pores was quantified based on X-ray computed tomography (XCT) images. To achieve the second objective, layerwise optical images of the powder bed were acquired as the parts were being built. Spectral graph theoretic and multifractal features were extracted from the layer-by-layer images for each test part. Subsequently, these features were linked to the process parameters using machine learning approaches. Through these image-based features, process conditions under which the parts were built were identified with the statistical fidelity over 80% (F-score).

Effects of powder characteristics and processing conditions on the corrosion performance of 17-4 PH stainless steel fabricated by laser-powder bed fusion

Powder characteristics and processing conditions are known to strongly influence the densification of parts fabricated by the laser-powder bed fusion (L-PBF) process. However, the influence of powder and L-PBF process parameters on corrosion performance of parts has not been studied extensively. In this paper, the effects of processing conditions (energy density) and powder characteristics (shape and size) on the corrosion performance of 17-4 PH stainless steel parts produced by L-PBF were investigated. The corrosion performance of the L-PBF parts was evaluated using corrosion current, polarization resistance and corrosion rate values from the potentiostatic polarization curves. It was observed that the density and consequently corrosion performance of L-PBF parts using coarser water-atomized (D50 = 24 and 43 µm) powders increased when the energy density was increased from 64 to 104 J/mm3. However, the density and subsequent corrosion performance of the L-PBF parts was relatively higher for the finer gas (D50 = 13 µm) and water-atomized (D50 = 17 µm) powders when fabricated using the same range of energy densities. At an energy density of 104 J/mm3, the corrosion performance of L-PBF parts fabricated using water-atomized 17-4 PH stainless steel powders all powders exhibited higher polarization resistance (28,000 ± 500 Ω) than the wrought sample (25,000 ± 1000 Ω) in the 0.5 M NaCl environment, indicative of better corrosion properties.

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

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

Glass forming ability, flexural strength, and wear properties of additively manufactured Zr-based bulk metallic glasses produced through laser powder bed fusion

An evaluation of low-cost, high-oxygen content Zr-Cu-Al-Nb bulk metallic glasses (BMGs) produced through laser powder bed fusion (PBF) was performed. Four-point bending and wear resistance tests were used to compare the mechanical properties of the printed alloy with laboratory grade cast parts. It is shown that the laser PBF parts, while not being able to be cast as a bulk glass, can be printed amorphous up to at least several millimeters thick and yet still have reasonable mechanical properties.

Effects of particle characteristics on the microstructure and mechanical properties of 17-4 PH stainless steel fabricated by laser-powder bed fusion

The effects of powder characteristics (powder shape, size and type) and processing conditions (laser power and scanning speed) on the mechanical properties and microstructures of laser powder bed fusion (L-PBF) 17-4 PH stainless steel were studied using four types of powders. The % theoretical density, ultimate tensile strength, hardness of L-PBF parts are sensitive to energy density and starting powder shape, size and type. The density and mechanical properties of both water and gas-atomized powders increased with increased energy density. The gas-atomized (D50 = 13 μm) powders which are spherical in shape and water-atomized (D50 = 17 μm) powders of high tap density produced low-porosity and high-density (~97% density) L-PBF parts at low energy densities of 64 and 80 J/mm3. The increase in energy density to 104 J/mm3 resulted in high dense (97 ± 0.5%) water- and gas-atomized powders L-PBF parts. However, even at a high % theoretical density (97 ± 1%), the properties of L-PBF parts varied over a relatively large range (UTS: 500–1100 MPa; hardness: 25–39 HRC; elongation: 10–25%). This large variation in mechanical properties could be attributed the martensite and austenite phase as well as grain size in the L-PBF parts. Furthermore, the martensite and austenite phase content and of the L-PBF parts were also sensitive to the energy density and starting powder type.

Elucidating the Relations Between Monotonic and Fatigue Properties of Laser Powder Bed Fusion Stainless Steel 316L

The laser powder bed fusion (L-PBF) technique builds parts with higher static strength than the conventional manufacturing processes through the formation of ultrafine grains. However, its fatigue endurance strength σ f does not match the increased monotonic tensile strength σ b. This work examines the monotonic and fatigue properties of as-built and heat-treated L-PBF stainless steel 316L. It was found that the general linear relation σ f = mσ b for describing conventional ferrous materials is not applicable to L-PBF parts because of the influence of porosity. Instead, the ductility parameter correlated linearly with fatigue strength and was proposed as the new fatigue assessment criterion for porous L-PBF parts. Annealed parts conformed to the strength-ductility trade-off. Fatigue resistance was reduced at short lives, but the effect was partially offset by the higher ductility such that comparing with an as-built part of equivalent monotonic strength, the heat-treated parts were more fatigue resistant.

Fatigue and fracture behaviour of laser powder bed fusion stainless steel 316L: Influence of processing parameters

The laser powder bed fusion (L-PBF) process involves a large number of processing parameters. Extending the intricate relationship between processing and structure to mechanical performance is essential for structural L-PBF materials. The high cycle fatigue properties of L-PBF parts are very sensitive to process-induced porosities which promote premature failure through the crack initiation mechanisms. Results from this work show that for stainless steel 316L, porosity does not impinge on the high cycle fatigue properties when processing is kept within a ±30% tolerance band. In this ‘optimum’ processing region, crack initiation takes place due to defects at the solidification microstructure level. Beyond the ‘optimum’ processing region, over-melting and under-melting can lead to porosity-driven cracking and inferior fatigue resistance. In addition, regardless of the processing condition, fatigue resistance was found to follow a direct linear relationship with ductility and tensile strength in the low and high stress fatigue regimes respectively.

Multiple Sensor Detection of Process Phenomena in Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is an additive manufacturing (AM) process in which a high power laser melts metal powder layers into complex, three-dimensional shapes. LPBF parts are known to exhibit relatively high residual stresses, anisotropic microstructure, and a variety of defects. To mitigate these issues, in-situ measurements of the melt-pool phenomena may illustrate relationships between part quality and process signatures. However, phenomena such as spatter, plume formation, laser modulation, and melt-pool oscillations may require data acquisition rates exceeding 10 kHz. This hinders use of relatively data-intensive, streaming imaging sensors in a real-time monitoring and feedback control system. Single-point sensors such as photodiodes provide the temporal bandwidth to capture process signatures, while providing little spatial information. This paper presents results from experiments conducted on a commercial LPBF machine which incorporated synchronized, in-situ acquisition of a thermal camera, high-speed visible camera, photodiode, and laser modulation signal during fabrication of a nickel alloy 625 AM part with an overhang geometry. Data from the thermal camera provides temperature information, the visible camera provides observation of spatter, and the photodiode signal provides high temporal bandwidth relative brightness stemming from the melt pool region. In addition, joint-time frequency analysis (JTFA) was performed on the photodiode signal. JTFA results indicate what digital filtering and signal processing are required to highlight particular signatures. Image fusion of the synchronized data obtained over multiple build layers allows visual comparison between the photodiode signal and relating phenomena observed in the imaging detectors.