Laser Powder Bed Fusion Technique Research Articles

Quality enhancement of microstructure and surface topography of NiTi parts produced by laser powder bed fusion

Laser powder bed fusion (LPBF) is a robust metal additive manufacturing technique for generation of complex geometries using a high-energy laser beam. NiTi is a promising material for porous biomaterials applications. The use of LPBF for such applications is justified by the complex architecture of porous biomaterials and by the current difficulties in NiTi manufacturing. Elimination of unwanted secondary phases after LPBF of NiTi has been an important research topic in recent years. In this research work, the effect of LPBF process parameters on phase composition, porosity, and surface topography of NiTi parts has been studied using different techniques, including X-ray diffraction, scanning electron microscopy and micro X-ray computed tomography. Three different regimes of process parameters have been proposed for obtaining the single phase B2-NiTi by LPBF technique. The obtained results revealed that high laser power adjusted to high scanning speed reduced the surface roughness and the porosity content in LPBF NiTi products.

Effects of heat treatment and applied stresses on the corrosion performance of additively manufactured super duplex stainless steel clads

The super duplex stainless steel (SDSS) cladded low carbon steel (LCS) substrates manufactured via laser powder bed fusion technique (LPBF) have been recently developed to enhance the service life of the structural components in highly corrosive environments. As-printed (AP) SDSS clads showed predominantly δ-ferrite matrix with allotriomorphic austenite precipitating at the ferrite grain boundaries. Increasing scan speeds resulted in decreasing austenite phase fraction with a dominant widmannstatten morphology at higher scan speeds. Cladded composites were heat-treated to reduce the high residual and thermal stresses induced by the LPBF process, which decrease the corrosion resistance. Heat treatment restored the δ-γ phase balance, thereby increased the corrosion resistance of the heat-treated (HT) clads as compared to AP clads. Increasing scan speeds resulted in decreasing corrosion resistance for the AP and HT clads. Subjecting the SDSS-LCS composite to tensile stresses increased the corrosion rates; however, these applied stress effects were eliminated by post stressing heat treatments, which restored the corrosion resistance of the clads. The SDSS clads produced at all scan speeds showed no signs of delamination from the LCS substrate, even at failure strains.

Scratch and wear resistance of additive manufactured 316L stainless steel sample fabricated by laser powder bed fusion technique

This study investigates the structural and tribotechnological properties of a 316L stainless steel sample additively manufactured by the laser powder bed fusion (LPBF) technique. The tribotechnological tests have been performed on the top surface in the prepared samples. Structural analysis at the top and the side surface shows that surface textures (100), (111) and (001) are strongly built. The scratch test is performed at three different loads to assess the effect on penetration depth and friction response, wherein some deformation is observed only at the highest load. These loads are further used for the ball-on-disk dry sliding tests to quantify the changes resulting from linear/continuous motion. The obtained values of sliding friction and wear are slightly higher than the scratch test results, but the overall effect is similar. The role of the peculiar microstructural feature of the LPBF samples on wear mechanism is described. Also, the influence of pore and tribo-film formation on the wear mechanism is validated by the Raman analysis. The comparison of tribotechnological properties of additive manufactured and conventionally manufactured (cold rolled) samples shows significant results in the additive samples.

On the solidification characteristics, deformation, and functionally graded interfaces in additively manufactured hybrid aluminum alloys

AlSi10Mg powder is deposited on top of a cast AA2618 substrate via laser powder bed fusion technique to form hybrid aluminum parts for additive repair purposes. The characterization of the additively manufactured (AM) hybrid aluminum alloys is conducted through advanced electron microscopy techniques across the interface, mechanical testing, validation of elasticity models, and crystallographic orientations analysis. In addition, the fracture behavior of the hybrid samples is investigated through electron microscopy analysis and the results are compared with those of AlSi10Mg and cast AA2618. The microstructural and physical properties of the hybrid bi-material are also discussed based on the solidification and diffusion phenomena. Finally, the dependency of elastic and shear moduli on the distance from the interface and solute concentration are separately evaluated via elastic-field mathematical equations. It is observed that an integrated bond is formed at the interface of the two dissimilar alloys showing suitable mechanical properties and shear strength, as well as a modified microstructure compared to the substrate material. The outcome of this work can be employed as a first step to use hybrid aluminum alloys in the blow molding industry and for repair and maintenance purposes.

Tribological and Mechanical Characterization of Nickel Aluminium Bronze (NAB) Manufactured by Laser Powder-Bed Fusion (L-PBF)

To cope with the increasing demands of high-performance materials, efforts were made using Laser Powder Bed Fusion technique (L-PBF), taking advantage of its extremely short solidification time to fabricate copper alloy with highly refined microstructure. Copper and Copper alloys are reputed for their high reflectivity resulting in low laser absorption, which makes their 3D fabrication process very challenging. This study aims to provide high density samples in order to obtain high mechanical and tribological properties. This objective is realized through the optimum energy density value which is obtained by monitoring laser power P, scan speed v, hatch space h and layer thickness t. Upon mapping of optimum parameters, the best combination is adopted to manufacture the samples of this study. Mechanical and tribological characterization of nickel aluminium bronze alloy confirmed the efficiency of laser powder-bed fusion (L-PBF) in providing high performance materials, higher properties such as wear resistance, tensile strength and hardness were obtained compared to other manufacturing techniques.

Quantification of porosity and microstructure and their effect on quasi-static and dynamic behavior of additively manufactured Inconel 718

The objective of this investigation is to quantify the evolution of microscopic features, including porosity, precipitates, and grain structure with respect to post-processing heat-treatments and their influence on the quasi-static and dynamic behavior of additively manufactured Inconel 718 built by laser powder bed fusion technique. In laser powder bed fusion, porosity, phase formation, and grain configuration could arise from different sources, including building powder or from the processing (e.g., laser pattern and solidification). The effect of these microscopic features on mechanical properties has drawn the scientific community's attention to developing thermal post-processing procedures targeting porosity closure, microstructural recrystallization, and precipitation of strengthening phases, thus improving material performance. One major challenge is to understand whether one can generalize the effect of certain microscopic features on mechanical properties under different states of stress and whether observed trends remain the same across strain rates? To address these questions, a comprehensive characterization agenda, which includes X-ray Micro-Computed Tomography, Scanning Electron Microscopy, Electron Backscatter Diffraction and mechanical testing at two strain rates (10−4 s−1 and 103 s−1) were implemented on as-built, stress relieved, hot isostatically pressed, and solution treatment plus aging test samples. Results show that, for the given specimen size, precipitate morphology and distribution changes from heat-treatment have a more significant effect than the porosity. In addition, it is demonstrated that there is a considerable enhancement in the magnitude of the plastic flow stress after applying the stress relief heat-treatment, while the subsequent heat-treatments do not have the same effect. When the state of stress is changed to compression, a different trend is observed that is relatively consistent increases in the magnitude of the flow stress. However, an opposite observation is seen under dynamic loading conditions indicating compressive mechanical behavior is more sensitive to the underlying microstructure than tensile behavior. Furthermore, strengthening mechanisms as a result of implemented heat-treatment is also discussed.

Process-Dependent Composition, Microstructure, and Printability of Al-Zn-Mg and Al-Zn-Mg-Sc-Zr Alloys Manufactured by Laser Powder Bed Fusion

Additive manufacturing (AM) technology for metallic alloys such as laser powder bed fusion (LPBF) brings tremendous opportunities for development of novel alloys specifically designed for AM that would desensitize the inherent process variability and requires a refined understanding of processing–structure–property relationship that would contribute to future alloy development. In this study, two different alloys, Al-6Zn-2Mg and Al-6Zn-2Mg-0.7Sc-0.3Zr in wt. pct, representing traditional and AM-specific novel aluminum alloys, respectively, were manufactured by LPBF technique using pre-alloyed gas atomized powders. The Al-Zn-Mg ternary alloys exhibited cracks at various LPBF processing parameters, primarily along the grain boundaries of the large columnar grains that extended across multiple melt pools. The severity of cracking in LPBF Al-Zn-Mg alloys was process-dependent and could be correlated to the change in alloy composition due to evaporation of Zn and Mg with high vapor pressure. The Scheil–Gulliver non-equilibrium solidification calculations showed that the Al-Zn-Mg alloys with lower Zn and Mg concentrations had smaller solidification range (i.e., ΔT) and steepness values (i.e., $$ \left| {{\text{d}}T / {\text{d}}f_{\text{s}}^{1/2} } \right| $$ near $$ f_{\text{s}}^{1/2} = 1 $$), which corresponded to a lower cracking severity. On the other hand, no cracks were observed in Al-Zn-Mg-Sc-Zr alloys, although their solidification range and steepness values were similar to the ternary Al-Zn-Mg alloys. The microstructure of Al-Zn-Mg-Sc-Zr alloys exhibited a much refined heterogeneous microstructure containing small equiaxed and columnar grains within each melt pool, owing to the heterogeneous nucleation upon primary Al3(Sc,Zr) particles. Process-dependent microstructure was observed as a result of variation in thermal gradient and cooling rate associated with LPBF parameters. Findings from this study provide guidance for the future design of AM-specific aluminum alloys and insights into the microstructural control by LPBF.

Additive manufacturing of metal-bonded grinding tools

Grinding tools with superabrasive grains can be manufactured from different bond materials. In several industrial applications, metallic bond systems are used. In general, these show good grain retention and offer a high thermal conductivity, when compared to the other widely used bond types such as vitrified and resin bonds. One drawback of the metallic bond is the lack of pores in the grinding layer. This is caused by the manufacturing processes that are typically used, like brazing or hot pressing. These generally produce very dense layers. The high density and low porosity lead to comparatively little space for the transport of lubricant, coolant, and chips. One approach to eliminate this disadvantage is to introduce cavities into the grinding layer, using the laser powder bed fusion technique (LPBF). In order to evaluate the general suitability of LPBF in combination with the bond material and diamond grains, grinding layer samples with a nickel-titanium bond were produced. The abrasive behavior of these samples was tested in scratch tests on cemented carbide to verify the applicability as grinding tools. While the diamond grains in the powder mixture are not part of the fusion process, they also did not interfere with the manufacturing process, and the scratch tests showed promising abrasive capabilities. The grinding layer itself withstood the process forces, and no grain breakout could be observed. This indicates that the grain retention forces are high enough for the grinding process and that NiTi has a high potential as a bonding material for the manufacturing of grinding tools via LPBF.

Non-destructive characterization of process-induced defects and their effect on the fatigue behavior of austenitic steel 316L made by laser-powder bed fusion

Additive manufacturing (AM) offers a high potential for light weight applications due to its possibility to generate complex structures with a high freedom of design compared to conventional techniques. However, the mechanical characterization of additively manufactured materials has become an essential topic in research over the last years to use AM parts for structural components especially in the automotive and aerospace industry. In the current research, specimens for fatigue tests made of the austenitic stainless steel AISI 316L processed by laser-powder bed fusion technique with three different building orientations were investigated. The gauge sections of the fatigue specimens were scanned by microfocus computed tomography (µ-CT) to detect and identify process-induced defects. The 3D information about the defects gained by µ-CT were compared with the crack-initiating defect on the fractured surfaces. The fatigue data were used for a model-based description of the fatigue strength based on the √area-parameter model. The results depict that the fatigue life is significantly influenced by the orientation of the inherent pores caused by the building direction causing a significant scatter in fatigue life. The µ-CT data allow to estimate the fatigue strength using the √area-parameter model based on the identified critical defects and are in good accordance with the results from the fatigue tests and the data obtained by fractography. Additionally, it could be shown that the √area-parameter model is applicable for additively manufactured 316L steel. It can also give an explanation for the anisotropic fatigue behavior, which supports the assumption that mainly the orientation of the pores and their √area-parameter are influencing the fatigue strength.

Additive manufacturing of maraging steel-H13 bimetals using laser powder bed fusion technique

In this paper, maraging steel powder was deposited on top of an H13 tool steel using laser powder bed fusion (LPBF) technique. The mechanical properties, microstructure, and interfacial characteristics of the additively manufactured MS1-H13 bimetals were investigated using different mechanical and microstructural techniques. Several uniaxial tensile tests and micro-hardness indentations were performed to identify the mechanical properties of the additively manufactured bimetal. Advanced electron microscopy techniques including electron backscatter diffraction and transmission electron microscopy were used to identify the mechanism of interface formation. In addition, the microstructure of the additively manufactured maraging steel along with the conventionally fabricated substrate-H13 were studied. It was concluded that, a very narrow interface was formed between the additively manufactured maraging steel and the conventional H13 without forming cracks or discontinuities. The first deposited layers possessed the highest hardness due to grain size refinement, solid solution strengthening, and cellular solidification structure. Finally, under uniaxial tensile loading, the additively manufactured bimetal steel failed from the underlying tool steel, indicating a robust interface.

Mechanical properties characterisation of AlSi10Mg parts produced by laser powder bed fusion additive manufacturing

AbstractAdditive manufacturing refers to a wide class of manufacturing processes based on the progressive building of functional parts through the addition of material layer upon layer. These technologies were first confined to prototyping, but the subsequent development of additive manufacturing processes for further materials, such as metals, has encouraged their worldwide industrial spread, from the biomedical field to the automotive and the aerospace industries. Additively manufactured parts are required to meet high and stable performance, at least comparable to that of conventional wrought materials, so as to comply with strict and well-defined international standards. This paper presents an investigation into the mechanical properties of AlSi10Mg parts produced by laser powder bed fusion technique, using different spatial orientations within the build volume. The effects of the part position and orientation on the static (tensile) properties of the produced parts were assessed by means of the two-way analysis of variance technique. The build angle was found to be the most effective parameter, while the variability ascribable to the effect of part position resulted mainly as physiological. The fatigue resistance showed a globally decreasing trend with increasing build angle.

Multiaxial fatigue strength under non-proportional loading of additively manufactured notched components of Ti-6Al-4V

In this study fatigue tests have been conducted on additively manufactured notched specimens made of titanium alloy Ti-6Al-4V under uniaxial loading and multiaxial non-proportional loading. Laser powder bed fusion technique has been employed to fabricate cylindrical specimens which have been successively machined to obtain a circumferential notch. Four varieties of specimens were fabricated depending on the build orientation and the application of a post-fabrication stress-relief heat treatment. The influence of layer orientation and heat treatment on the material microstructure has been analyzed. Strain controlled tests were performed with both proportional loading represented by a push-pull strain path and non-proportional loading represented by a circle strain path. The number of cycles to failure under both proportional and non-proportional loadings seemed not depending on layer orientation and heat-treatment. The cyclic plastic behavior of the material has been verified through the observation of the softening and hardening curves. The surface of the crack has been finally analyzed to verify the crack initiation position which has been detected on voids or defects located in the vicinity of the notch tip.

Additive Manufactured A357.0 Samples Using the Laser Powder Bed Fusion Technique: Shear and Tensile Performance

New aluminium alloys, with lower silicon content than in the first-developed formulations, have recently been introduced in the field of Additive Manufacturing and are dedicated to automotive applications. As they are relatively new, mechanical characterization under standard protocols of the automotive field are of utmost scientific as well as industrial relevance. The paper addresses the mechanical properties and microstructure of A357.0. Static tensile and shear tests of samples built by Laser Powder Bed Fusion, with different orientations in the machine work volume, have been performed. The aim was to identify possible anisotropy in the tensile and shear behaviour of this innovative alloy. Particularly for shear, the effect of adhesion between the layers onto shear strength was studied. Results analysis, by means of statistical tools, allows for the affirmation that no tensile modulus or yield strength anisotropy is observed. Instead, a small (yet statistically significant) increase in both shear- and tensile strength and a decrease in ductility are obtained as the direction of the specimens approaches the growth direction. Scanning Electron Microscope (SEM) observation of the failure mechanisms assisted in the interpretation of the results, by relating different failure modes to the relative orientation of loads versus the directions of inherent anisotropy in Laser Powder Bed Fusion processes.