Additive Manufacturing Specimens Research Articles

An acoustic emission study of anisotropy in additively manufactured Ti-6Al-4V

This investigation applies acoustic emission (AE) to study the microstructural features of additively manufactured (AM) Ti-6AL-4V parts fabricated through direct energy deposition (DED). Microstructural features including anisotropy, porosity, and grain size were tailored by processing parameters such as laser power, powder feed rate, and scan speed. Additionally, rolled and annealed Ti-6Al-4V specimens were used as a baseline. AE features were measured for specimens subjected to cyclic loading via three-point bending tests in two orientations. Higher levels of AE counts and energy were observed in the specimens fabricated with higher laser power while lower levels of AE counts and energy were observed in the specimens fabricated with lower laser power. For AM specimens fabricated with low laser power, the AE features were similar in two different testing orientations where microstructures were columnar for both orientations. However, for AM specimens fabricated with high laser power, the AE were different along different testing orientations, with higher indications being along the growth direction. This may be due to the microstructural anisotropy where AM specimens exhibited columnar grains aligned along the growth direction and a mixed columnar-equiaxed grains in a direction perpendicular to the growth direction. The AE features in the baseline rolled and annealed Ti-6Al-4V were considerably higher than those in AM samples. This may be related to fine recrystallized (α + β) microstructure for rolled and annealed Ti-6Al-4V compared to coarse microstructure in AM samples. The AE observations were further studied by AE waveform analysis.

Microstructure and Ratcheting Behavior of Additive Manufactured 4043 Aluminum Alloy

In this paper, 4043 aluminum alloy was printed using additive manufacturing (AM) with robot cold metal transfer (CMT) technology. The microstructure and mechanical properties of different parts, including the bottom, middle and upper portions of the printed AM 4043 aluminum alloy, were analyzed. Ratcheting tests were performed using an asymmetric load, and the tensile properties of the pre-ratcheting and un-ratcheted specimens were tested. The results indicate that the dendrites in the middle region are larger than those at the top and bottom regions and that the coarse microstructure associated with the interlayer junction resulted in the decline of the mechanical properties. The tensile strength of the AM specimens is 165 MPa, and the yield strength is 50 MPa. The printed 4043 aluminum alloy exhibits mechanical properties similar to those for conventional cast and welded aluminum alloys. The ratcheting behavior indicates that an increase in the mean stress or stress amplitude leads to an increase in the ratcheting strain, and the accumulation of ratcheting strain causes plastic damage to the materials. Pre-ratcheting behavior causes the tensile and yield strengths to increase compared to the behavior of the un-ratcheted specimens.

Significance of hot isostatic pressing (HIP) on multiaxial deformation and fatigue behaviors of additive manufactured Ti-6Al-4V including build orientation and surface roughness effects

Additive manufacturing (AM) technology has gained significant attention in recent years due to several important advantages. However, design of critical load carrying parts using this technique is still at its infancy, partly due to the inferior performance and lack of sufficient understanding of cyclic deformation and fatigue behaviors of AM metals as compared to their wrought counterparts. Similar to most other components in different industries, AM parts typically undergo cyclic loadings through their service life; therefore, fatigue performance is a key performance criterion. In addition, due to common presence of multiaxial stress states at fatigue critical locations, multiaxial fatigue is of special interest. In this study, thin-walled tubular specimens made of Ti-6Al-4V alloy fabricated with powder bed fusion (PBF) process were used to investigate their cyclic deformation and fatigue behaviors. The main focus of this study was on the effect of hot isostatic pressing (HIP) on fatigue performance under uniaxial, torsion, combined in-phase, and 90° out-of-phase axial-torsion loads. To study build orientation effect, both vertically built and diagonally built (at 45°) specimens were included. However, no substantial difference on the fatigue behavior of vertical and 45° built specimens was observed. Although the majority of specimens had a machined surface, some torsional tests with as-built surface were also included to evaluate surface roughness effect. It was found that even after HIP treatment, the rough surface is the dominant factor in shortening the fatigue life. In addition, to evaluate the effectiveness of the HIP treatment, torsion fatigue performance of the HIPed specimens were compared to torsion fatigue performance of annealed AM specimens, as well as to the multiaxial fatigue behavior of the conventional wrought material. Fatigue failure mechanism of the HIPed material was shear, similar to the wrought material, and fatigue performance was found to be similar to the wrought alloy. Fatigue test results were correlated well using a shear-based critical plane parameter approach, also similar to the wrought material. Comparison of multiaxial fatigue data in this study and a similar previous study by the authors indicates that fabrication machine-to-machine and the associated build parameter differences can result in substantial differences in the observed defect size and population, resulting in different failure mechanisms and subsequently different fatigue performance.

Corrosion behavior of additively manufactured 316L stainless steel in acidic media

Recently, additive manufacturing has got tremendous attention due to ease in the production of complex metallic parts for different applications i.e. aerospace, petrochemical etc. However, there is a scarcity of literature, addressing the corrosion behavior of additive manufactured (AM) alloys. This study presents, the chemical composition and corrosion response of the passive oxide film formed on the AM 316L stainless steel in acidic regime (pH ≤ 3) and its comparison to wrought counterpart, by applying X-ray photoelectron spectroscopy (XPS) and electrochemical analysis, respectively. Microstructural characterization of AM specimen revealed the presence of nanometer-ranged ripples type sub-granular structure confined within the macro grains. XPS analysis indicated the formation of mono layered and bi layered passive oxide film in pH 1 and 3 electrolytes, respectively. Interestingly, higher charge transfer resistance (50 times) and significantly decreased corrosion current density (2 order of magnitude) in aggressively acidic solution (pH 1) has been observed by AM specimens compared to conventional wrought 316L stainless steel. The higher corrosion resistance has been attributed to the development of fine sub-granular structure, which most likely regulates the stability of the passive oxide film and the raid solidification rate (approx. 107 K/s) involved in the additive manufacturing process rationalizing the reduction of MnS inclusions. In comparison, a significantly higher corrosion resistance by the AM 316L stainless steel in highly acidic environment (pH ≤ 3) has been recoded, surpassing the conventional wrought material.

On the effect of shot-peening on fatigue resistance of AlSi10Mg specimens fabricated by additive manufacturing using selective laser melting (AM-SLM)

The effect on fatigue resistance of additively manufactured (AM) AlSi10Mg specimens fabricated by selective laser melting (SLM) following surface treatment by shot-peening was investigated. Specimen surface was shot-peened with either steel or ceramic balls. Nano-indentation measurements revealed that shot-peening caused surface hardening, with the hardness profile from the surface to the interior of the bulk disappearing 50 μm below the surface. Surfaces polished before shot-peening or following removal of about 25–30 μm from the surface after shot-peening by either mechanical or electrolytic polishing showed improved fatigue resistance and fatigue limit. Fractography of broken specimens demonstrated that for shot-peened specimens, the site of fatigue crack initiation was deeper than that for specimens that had not undergone shot-peening. The fracture area of AM-SLM AlSi10Mg specimens before and after shot-peening displayed a ductile fracture with relatively deep dimples. In contrast to AM specimens, the final fracture area of die-cast samples exhibited a brittle fracture surface, containing numerous cleavage facets and micro-cracks.

Instrumented Impact Testing of Miniaturized Charpy Specimens of AM Ti-6Al-4V.

An investigation on the impact toughness properties of wrought and additively manufactured (AM) Ti-6Al-4V was conducted at National Institute for Standards and Technology (NIST) Boulder by means of instrumented impact tests on miniaturized Charpy specimens. Full transition curves for absorbed energy and lateral expansion were obtained by performing tests in the temperature range between -196°C and 700°C. The effect of various parameters was investigated for AM specimens, namely specimen orientation, hot isostatic pressing (HIPing), and notch configuration (printed or machined). Our results indicate that AM specimens exhibit equivalent or better impact toughness than wrought material after HIPing and that the material is more resistant to cracks growing in the plane perpendicular to the build direction than in the plane containing the build direction. HIPing has a significantly beneficial effect for the AM material, while no effect of notch configuration was observed from the results obtained.

Characterizing the tensile behavior of additively manufactured Ti-6Al-4V using multiscale digital image correlation

The tensile behavior of an additively manufactured (AM) Ti-6Al-4V alloy is examined at multiple length scales using two-dimensional digital image correlation (DIC) techniques. The work focuses on characterizing the relationship between microstructure and bulk mechanical properties of the AM Ti-6Al-4V specimens and understanding how unique microstructural features pertaining to AM metals, such as columnar grain morphology and nonequilibrium phase structures, influence deformation behavior at the microscale (i.e., the length scales of those features). Lower magnification in situ DIC experiments were performed at the macroscale during tension experiments to assess the bulk properties, and a higher magnification ex situ DIC study was performed at the microscale using a microscope to observe heterogeneous strain accumulation at that scale. At the macroscale, the AM Ti-6Al-4V exhibited greater strength than conventionally processed Ti-6Al-4V, due to its acicular, or needle-like, grain structure. At the microscale, alternating regions of high and low strains were observed to accumulate along these needles, or laths. The effects of build orientations (related to microstructure anisotropy), powder bed layering thicknesses (30 and 60 µm), and heat treatment, on the properties were also examined. A decrease in strength was observed in samples subjected to annealing heat treatment compared to as-built specimens with no heat treatment. Also, the AM specimens exhibited higher tensile strength when the direction of applied load was perpendicular to the AM build orientation. This observed anisotropy is thought to be related to interfaces of consecutively deposited layers because regions of high strain at the microscale were observed to accumulate at changes in structure associated with the deposition of layers.

Role of Si in the Anodizing Behavior of Al-Si Alloys: Additive Manufactured and Cast Al-Si10-Mg

In this work, the role of the Si phase microstructure in the anodizing behavior of Al-Si alloys is studied. Experiments were conducted using an additive manufactured (AM) Al-Si10-Mg specimen and a cast alloy of approximately the same chemical composition. A systematic characterization of the anodic oxide film was conducted using a variety of surface analysis techniques. Clear evidence is provided through X-ray photoelectron spectroscopy analysis for the almost entire oxidation of Si during the anodizing of the additive manufactured specimens, which can explain the lower anodizing efficiency observed in the AM material compared to the cast alloy. Additionally, a more detailed characterization of the oxide layer revealed that a slightly thinner anodic oxide film is formed at the borders of the characteristic melting tracks resulting from the metal additive manufacturing process. Furthermore, the features observed in the voltage–time response during the anodizing of the AM specimens seem to be closely associated with the dimensions of the Al/Si cells present in the plane parallel to the direction of the oxide front growth.

Fatigue behavior and cyclic deformation of additive manufactured NiTi

The aim of this study is to experimentally investigate the fatigue behavior of additively manufactured (AM) NiTi (i.e. Nitinol) specimens and compare the results to the wrought material. Additive manufacturing is a technique in which components are fabricated in a layer-by-layer additive process using a sliced Computer-Aided Design (CAD) model based on the desired geometry. NiTi rods were fabricated in this study using Laser Engineered Net Shaping (LENS), a Direct Laser Deposition (DLD) AM technique. Due to the high plateau stress of the as-fabricated NiTi, all the AM specimens were heat-treated to reduce their plateau stress, close to the one for the wrought material. Two different heat treatment processes, resulting in different stress plateaus, were employed to be able to compare the results in stress- and strain-based fatigue analysis. Strain-controlled constant amplitude pulsating fatigue experiments were conducted on heat-treated AM NiTi specimens at room temperature (∼24°C) to investigate their cyclic deformation and fatigue behavior. Fatigue lives of AM NiTi specimens were observed to be shorter than wrought material specifically in the high cycle fatigue regime. Fractography of the fracture surface of fatigue specimens using Scanning Electron Microscopy (SEM) revealed the presence of microstructural defects such as voids, resulting from entrapped gas or lack of fusion and serving as crack initiation sites, to be the main reason for the shorter fatigue lives of AM NiTi specimens. However, the maximum stress level found to be the most influential factor in the fatigue behavior of superelastic NiTi.

Multiaxial fatigue behavior of wrought and additive manufactured Ti-6Al-4V including surface finish effect

Additive manufacturing (AM) technology has enabled efficient fabrication of parts with complex geometries and laser-based powder bed fusion (L-PBF) is one of the most commonly used AM fabrication process. In many applications, the loading condition of AM parts is multiaxial. Even under uniaxial loading conditions, due to the geometry complexity or interaction of residual stresses, the stress state may still be multiaxial. Therefore, an understanding of cyclic deformation and fatigue behaviors of AM materials under multiaxial stress states is critical to the expected performance of such parts. These behaviors were investigated in this study using thin-walled tubular specimens of Ti-6Al-4V alloy made of a common PBF process. To compare with the conventional material performance, wrought Ti-6Al-4V alloy was also investigated. The loadings considered included axial, torsion, in-phase axial-torsion, and 90° out-of-phase axial-torsion loads. The surface roughness effect was also studied by considering both the as-built and the machined and polished surface conditions of the AM specimens. The ductility of the AM material was found to be significantly lower than the wrought material due to the martensitic microstructure as well as the presence of defects. AM specimens had significantly shorter lives compared to the wrought specimens under all loading conditions and regardless of the surface finish. However, machining improved the fatigue performance of AM specimens. For all loading conditions brittle fracture of AM specimens was observed with cracking on maximum tensile plane, and ductile fracture of wrought specimens with shear cracking. Consequently, fatigue test results of the wrought material were correlated using a shear-based critical plane model, while the AM specimen test data were correlated based on the maximum principal stress criterion.

Additive manufacturing of fatigue resistant materials: Challenges and opportunities

This overview focuses on the current state of knowledge pertaining to the mechanical characteristics of metallic parts fabricated via additive manufacturing (AM), as well as the ongoing challenges and imminent opportunities in fabricating materials with increased fatigue resistance. Current experimental evidence suggests that the mechanical properties of laboratory AM specimens may not be representative of those associated with parts, due primarily to differences in geometry/size which influence the thermal histories experienced during fabrication, and consequently, microstructural features, surface roughness, and more. In addition, standards for mechanical testing methods, specimen design procedures, post-manufacturing treatments, etc., may need to be revised for AM parts. Standardizing the AM process may only be accomplished by strengthening the current understanding of the relationships among process parameters, thermal history, solidification, resultant microstructure, and mechanical behavior of the part. Having the ability to predict variations in mechanical behavior based on resultant microstructure, or matching the best conceivable properties of a part in accordance with the loading critical plane, are some possible solutions for making AM a more reliable means for producing functional parts. Developing microstructure-property models is arguably the first necessary step toward design optimization and the more efficient, accurate estimation of the structural integrity of AM parts.

Fatigue Properties Of Additively Manufactured AlSi10Mg – Surface Treatment Effect

Space industry strives to constantly increase performance and reduce costs of space missions. When dealing with producing space components, Additive Manufacturing (AM) is seen as a game changing technology also towards these goals. The applicability of this technology has already been demonstrated for space products and parts are currently flying. However, further studies are still needed to better understand the process parameters-final properties relationship.With this aim, a cooperative R&D activity was designed by EOS, RUAG and ESA, in order to characterise the fatigue behaviour of AlSi10Mg processed via AM. This activity comprised the study of the influence of building direction, platform temperature, powder layer thickness, surface finish and heat treatment on the fatigue properties of the AM specimens. As a complementary assessment, the defect population of the samples was studied through X-ray Computed Tomography (XCT).Results showed that there is a direct correlation between the process parameters and the fatigue properties of AlSi10Mg parts, having received the same post processing. Observations indicate that changing the parameters influences the characteristics of the defect population, determining the fatigue life of the specimens. In addition, the different surface finishing methods have, as expected, a strong impact on the fatigue behaviour of AM AlSi10Mg materials. This study shows that an optimised combination of process parameters and surface finish can improve fatigue performance, without the need for machining of the AM parts.

Galvanostatic Anodizing of Additive Manufactured Al-Si10-Mg Alloy

The galvanostatic anodizing behavior of additive manufactured (AM) Al-Si10-Mg alloy was studied in H2SO4 electrolyte. The analysis of the voltage vs time response was complemented with a systematic characterization of the anodic oxide layer using a variety of techniques. In addition, a cast alloy of approximately the same chemical composition as that of the AM specimens was used as a reference in this study. Significant differences were found in the voltage–time characteristics of the samples analyzed. Besides, an anisotropic anodizing behavior was observed in the additive manufactured specimens. Due to the fine silicon microstructure present in the additive manufactured samples, the anodic oxide growth was much more obstructed than for the cast alloy. Nevertheless, even though the oxide layer was generally thinner in the AM samples for the same conditions and anodizing time, a much more continuous and uniform oxide layer was found in the additive manufactured specimens compared to the cast alloy. The porous structure was found to be greatly affected by the fine distribution of the silicon phase in the AM parts.

Microstructure, Fatigue Behavior, and Failure Mechanisms of Direct Laser-Deposited Inconel 718

Inconel 718 is considered to be a superalloy with a series of superior properties such as high strength, creep resistance, and corrosion resistance at room and elevated temperatures. Additive manufacturing (AM) is particularly appealing to Inconel 718 because of its near-net-shape production capability for circumventing the poor machinability of this superalloy. Nevertheless, AM parts are prone to porosity, which is detrimental to their fatigue resistance. Thus, further understanding of their fatigue behavior is required before their widespread use in load-bearing applications. In this work, the microstructure and fatigue properties of AM Inconel 718, produced in a Laser Engineered Net Shaping (LENS™) system and heat treated with a standard heat treatment schedule, are evaluated at room temperature. Fully reversed strain controlled fatigue tests were performed on cylindrical specimens with straight gage sections at strain amplitudes ranging from 0.001 mm/mm to 0.01 mm/mm. The fracture surfaces of fatigue specimens were inspected with a scanning electron microscope. The results indicate that the employed heat treatment allowed the large, elongated grains and dendritic structure of the as-built material to break down into smaller, equiaxed grains, with some dendritic structures remaining between layers. The AM specimens were found to possess lower fatigue resistance than wrought Inconel 718, and this is primarily attributed to the presence of brittle metal-carbide/oxide inclusions or pores near their surface.

Preliminary Design and Analysis of Tensile Test Samples Developed by Additive Manufacturing

Additive Manufacturing (AM) is starting to replace conventional manufacturing processes where complex parts with small lead-time and lot sizes are needed. As conventional test methods are not suitable for AM parts, new standard specimens and test procedures have to be defined. This work undertakes some efforts to progress the design of specimens for mechanical tests. A methodology for tensile tests of AM specimens made from one layer is proposed and verified on an example. It identifies challenges during the design and manufacturing of Fused Layer Modeling single layer specimens.