Improving the Fatigue Performance of Binder Jet Manufactured 316L by Severe Shot Peening Surface Modification

This study explores the findings concerning the impact of severe shot peening (SSP) on 316L stainless steel, both in laser powder bed fusion (PBF-LB) and traditional manufacturing routes. Microstructural changes induced by SSP are examined through electron backscatter diffraction (EBSD) analysis. The study demonstrates that SSP treatment leads to grain refinement in both materials, but martensitic transformation was only observed in sheet metal. Mechanical testing revealed significant increases in yield strength and ultimate tensile strength in SSP-treated 316L, and fatigue testing under axial and bending loading conditions showed substantial improvements in fatigue limits, with the PBF-LB material benefiting more from SSP treatment. These findings highlight the potential of SSP for enhancing the mechanical properties of 316L stainless steel, making it valuable for applications demanding superior strength and fatigue resistance.

An in-situ crosslinking binder for binder jet additive manufacturing

Binder jetting is an additive manufacturing (AM) technology that has gained popularity and attention in recent years for production applications in tooling, biomedical, energy, and defense sectors. When compared to other powder bed fusion-based AM methods, binder jetting processes powder feedstock without the need of an energy source during printing. This avoids defects associated with melting, residual stresses, and rapid solidification within the parts. However, one of the challenges of this process is the relatively lower densities which impacts part density, and subsequently, sintering and mechanical properties. In this study, we investigated the influence of bimodal powder size distributions (a mixture of coarse to fine particles) as a method for increasing part density and mechanical strength, and used stainless steel (SS) 316L bimodal mixtures in this case. Four unimodal and two bimodal groups were evaluated under similar AM processing conditions for sintered density measurements and flexural strengths. Our results demonstrated that bimodal size distributions showed a statistically significant increase in density by 20% and ultimate flexural strength by 170% when compared to the highest performing unimodal group. In addition to experimental findings, reactive molecular dynamics simulations showed that the presence of finer powders along with coarser particles in the bimodal particle mixture contribute to additional bonds that are stronger across the particle interfaces. Findings from this study can be used to design bimodal particle size distributions to achieve higher density and better mechanical properties in binder jetting AM process.

In this study the effect of laser heat treatment (LHT) and severe shot peening (SSP) on laser powder bed fusion manufactured AISI 316L stainless steel is investigated. The effect of LHT and SSP on the hardness of the surface of the PBF-LB 316L is studied performing microhardness measurements. Microstructure is evaluated in the EBSD investigation. The residual stresses will be measured to determine the influence of LHT and SSP. The effects of LHT and SSP on tensile and bending fatigue strength will be evaluated. LHT altered the microstructure 200 µm from the surface. The grain structure on the surface was more ordered and no substructure or local strains were present. Finer grain features adjacent to the sample surface were found, which are most likely caused by effective recrystallization and fast cooling. The grain morphology was left relatively unchanged when SSP was applied on LHT surface. However, local deformation has occurred on the surface, and clear orientation gradient within grains is seen. LHT had no effect on the hardness. SSP increased the surface hardness by 205%. LHT decreased the yield or tensile strength of the PBF-LB 316L. Residual stress measurements showed that SSP induced a high compressive stress in the PBF-LB 316L. LHT and SSP significantly improved the fatigue strength of the PBF-LB manufactured 316L.

Additive manufacturing of cementitious composites: Materials, methods, potentials, and challenges

Additive manufacturing (AM) of hot-work tool steels such as H13 offers unique opportunities for producing complex, conformally cooled tools with reduced production time and material waste. In this study, five metal AM technologies—Fused Deposition Modeling and Sintering (FDMS, Desktop Metal Studio System and Zetamix), Binder Jetting (BJ), Laser Powder Bed Fusion (LPBF), and Directed Energy Deposition (DED)—were compared in terms of microstructure, porosity, and post-processing heat treatment response. The as-printed microstructures revealed distinct differences among the technologies: FDMS and BJ exhibited high porosity (6–9%), whereas LPBF and DED achieved near-full densification (<0.1%). Samples with sufficiently low porosity (BJ, LPBF, DED) were subjected to tempering and quenching treatments to evaluate hardness evolution and microstructural transformations. The satisfactory post-treatment hardness was observed in both tempered and quenched and tempered BJ samples, associated with secondary carbide precipitation, while LPBF and DED samples retained stable martensitic structures with hardness around 600 HV0.5. Microstructural analyses confirmed the dependence of phase morphology and carbide distribution on the thermal history intrinsic to each AM process. The study demonstrates that while FDMS and BJ are more accessible and cost-effective for low-density prototypes, LPBF and DED offer superior density and mechanical integrity suitable for functional tooling applications.

Nitriding duration reduction without sacrificing mechanical characteristics and fatigue behavior: The beneficial effect of surface nano-crystallization by prior severe shot peening

Fatigue life of AW 7075 Aluminium Alloy after Severe Shot Peening Treatment with Different Intensities

The effects of shot peening on fatigue crack growth in 7075-T735 single-edged notch bend (SENB) and three point bend (TPB) specimens are discussed. Fifty-one total fatigue tests were conducted with SENB and TPB specimens, which were machined from an 8.1 mm thick 7075-T7351 aluminum stock plate. All specimens were tested in the as-received or shot peened conditions, with crack perpendicular to the rolling direction, i.e., in the L-T direction. Fatigue tests of SENB specimens were conducted at a constant maximum load, stress ratios of R = 0.1 or 0.8, and to a pre-determined fatigue cycle or to failure. Crack growth rate was determined from the tunneling crack profile or by fractography. Peened surface roughness, subsurface microstructure, and micro-hardness profiles were also examined to evaluate their effect on crack growth rate. The TPB specimens were fatigued to failure at stress ratios of R = 0.1 or 0.8 and a S-N diagram constructed. Both the SENB and the TPB test results showed that severe shot peening (0.016A) slightly increased the fatigue life at low stress ratio of R = 0.1 but was found to be negligible at a high load ratio of R = 0.8. The stress concentration factor due to dimples generated by severe shot peening was estimated to be 1.14 and had negligible effect on the fatigue life. Grain distortion was observed to a depth range of 130–410 μm indicating the depth of the residual compressive stress layer developed for peening intensities of 0.004–0.016A, respectively.

3D printing of fine-grained aluminum alloys through extrusion-based additive manufacturing: Microstructure and property characterization

This paper reports on the fatigue life of objects manufactured by additive manufacturing (AM) using Type 316L Stainless Steel (Type 316L SS), an austenitic stainless steel commonly used in the nuclear power industry. Fatigue tests were performed with axial strain control from low cycles to high cycles of about 1 million times, and the strain amplitude εta for the target fracture life was set using a prediction equation for conventional materials. Powder Bed Fusion (PBF) and Directed Energy Deposition (DED) were used as AM methods to manufacture the test objects. In addition, the effects of heat treatment conditions (with or without Heat Isostatic Pressing (HIP)), test direction (horizontal/vertical), and surface condition (polished/as-built) were evaluated as factors that are thought to affect fatigue life. As a result of conducting fatigue tests on specimens manufactured using PBF or DED, the surface condition of the as -built surface showed a shorter life than the polished surface, but no clear significant differences were observed for other factors. In this paper, a prediction equation for conventional materials was used, but the predicted values for specimens manufactured using AM were equivalent to those for the conventional material, indicating that this prediction equation may also be applicable to AM specimens. In addition, both the PBF and DED specimens had fatigue lives that satisfied the rules on design and construction of the Japan Society of Mechanical Engineers (JSME).

Binder jetting is a layer-based additive manufacturing process for three-dimensional parts in which a print head selectively deposits binder onto a thin layer of powder. After the deposition of the binder, a new layer of powder is applied. This process repeats to create three-dimensional parts. The binder jetting principle can be adapted to many different materials. Its advantages are the high productivity and the high degree of freedom of design without the need for support structures. In this work, the combination of binder jetting and casting is utilized to fabricate metal parts. However, the achieved properties of binder jetting parts limit the potential of this technology, specifically regarding surface quality. The most apparent surface phenomenon is the so-called stair-step effect. It is considered an inherent feature of the process and only treatable by post-processing. This paper presents a method to remove the stair-step effect entirely in a binder jetting process. The result is achieved by controlling the binder saturation of the individual voxel volumes by either over or underfilling them. The saturation is controlled by droplet size variation as well as dithering, creating a controlled migration of the binder between powder particles. This work applies the approach to silica sand particle material with an organic binder for casting molds and cores. The results prove the effectiveness of this approach and outline a field of research not identified previously.

In automated de-powdering of binder jetting additive manufactured parts, robots must quickly and accurately calculate how to grasp fragile, brittle green parts without shattering them. Previous work has used non-linear models to find a grasp configuration (specifying the robot gripper position, orientation, and distance between gripper fingers) that minimizes part stress and the likelihood of shattering under the gripper force. However, this process is time-intensive and impractical for additive manufacturing applications, where robots must work quickly. This paper proposes a novel method for accurately and quickly calculating safe grasps for fragile green parts. Our method involves modeling parts using a finite element linear elastic model and calculating part deformation caused by a surface traction force from the gripper’s fingers in each grasp configuration. The grasp configuration resulting in the lowest part deformation is chosen as the least likely to shatter the part. For an elastic and brittle material, part stress is approximately proportional to strain before fracture, meaning the grasp with the lowest strain for the elastic model corresponds to the lowest stress for the brittle part with the same geometry. We integrated our algorithm into a prototype binder jetting de-powdering robot and used the algorithm to choose grasps for binder jetting manufactured green parts. With optimizations such as parallelization, our algorithm takes 18 minutes to calculate grasps per part. During testing, our algorithm’s chosen grasp configuration was used to pick up and hold test green parts without shattering them at 80 N, as measured by the gripper’s contact force sensors, despite the parts shattering under forces as low as 2N in some grasps. Handling fragile green parts is one of the main barriers hindering the automation of the de-powdering process in binder jetting. Our grasping algorithm’s success in enabling the safe handling of green parts will drive progress toward automated de-powdering.

Inconel 718 is a nickel-based superalloy known for its excellent combination of high-temperature strength, corrosion resistance, and weldability. Additive Manufacturing (AM) has revolutionized traditional manufacturing processes by enabling the creation of complex and customized components. In this work, three prominent AM techniques: Laser-Based Powder Bed Fusion (PBF), Wire Direct Energy Deposition (DED), and Binder Jet (BJ) processes were explored. A thorough metallographic analysis and comparison of samples was conducted after short-term creep testing originating from each of the three aforementioned techniques in addition to wrought material. Detailed electron microscopy unveiled equiaxed grains in both BJ and wrought samples while PBF samples displayed elongated finer grain structures in the build direction, characteristic of PBF. The DED samples revealed a more bimodal grain distribution with a combination of smaller equiaxed grains accompanied by larger more elongated grains. When assessing the three processes, the average grain size was found to be larger in the BJ samples, while the PBF samples exhibited the most significant variation in grain and sub-grain size. Number density, size, and shape of porosity varied between all three techniques. Post-creep test observations in PBF samples revealed the occurrence of wedge cracking at the failure point, accompanied by a preference for grain boundary creep void formation while BJ samples exhibited grain boundary creep void coalescence and cracking at the failure location. In the DED samples, void formation was minimal however, it seemed to be more prevalent in areas with precipitates. In contrast, the wrought sample showed void formation at the failure site with a preference for areas with primary carbide formation. Despite BJ samples demonstrating similar or even superior rupture life compared to other AM techniques, a noteworthy reduction in rupture ductility was observed. While a coarse, uniform grain size is generally linked to enhanced creep resistance and rupture life, the combination of pre-existing voids along grain boundaries and the formation of new voids is hypothesized to accelerate rapid fracture, resulting in diminished ductility. This research shows careful consideration is needed when selecting an AM technology for high- temperature applications as creep behavior is sensitive to the large microstructural variations AM can introduce.

• Selective laser melted Ti6Al4V with three different surface conditions was tested. • Quasi-static, high- and low-cycle fatigue behaviours were characterized. • The theoretical fatigue limit in ideal surface finish conditions was found. • The effect of roughness on fatigue limit was quantified through reduction factors • Fracture mechanics analysis provided further insight about failure mechanisms. Ti6Al4V has emerged as one of the most promising materials for Additive Manufacturing (AM). In this work, three series of AM Ti6Al4V samples were employed to assess the material fatigue behavior and its dependency on surface finish. One series was left in as-built conditions, one was machined, and one was finely polished. No samples underwent heat treatment, and their dimensions were chosen to be representative of the characteristic strut size of AM reticular structures. The as-built and machined series were characterized in terms of roughness, monotonic, Low-Cycle Fatigue (LCF) and High-Cycle Fatigue (HCF) behaviors. The polished series was intended to determine the fatigue limit under ideal surface finish conditions, undergoing roughness and HCF tests only. This allowed the determination of the fatigue limit reduction factors due to surface finish, providing designers with a practical tool for evaluating the effect of surface roughness on the fatigue performance of AM Ti6Al4V. Besides, scanning electron microscopy and fracture mechanics analysis through Kitagawa-Takahashi diagrams, based on Murakami’s theory, were performed. Different laws correlating defect size and fatigue limit of defective components were compared. By transforming roughness into an equivalent defect, further insight into the effect of surface finish on fatigue performance was gained.