Papers by Keyword: Fatigue Strength

Abstract: This study investigates the influence of layer thickness and surface condition on the microstructure and mechanical performance of Laser Powder Bed Fusion (PBF-LB) manufactured Ti6Al4V. Specimens were produced using two layer thicknesses, 40~µm and 80~µm, under identical process parameters. Characterization included tensile and axial fatigue testing, supported by microstructural analysis using field-emission scanning electron microscopy (FE-SEM) and electron backscatter diffraction (EBSD). Both processing conditions produced fully martensitic α′ microstructures, with the 40~µm builds showing finer lamellae and smaller prior-β grains due to higher cooling rates. Tensile tests revealed higher ductility for the 40~µm specimens while maintaining similar strength levels. Axial fatigue tests revealed better performance for lower layer thickness, electropolished surface and diagonal orientation. The results confirm that fatigue performance in PBF-LB Ti6Al4V is primarily governed by surface integrity and defect population rather than changes in microstructural morphology. Overall, finer layers and surface finishing enhance endurance strength, though at the cost of reduced build productivity.

Abstract: We report on a comprehensive study of tensile strength and fatigue behaviour depending on process parameters and ambient temperature. For this, Polyamide-12 components are fabricated using Selective Laser Sintering. Firstly, different process parameters like, e.g, scan-speed, laser power, and applied energy density are varied. Secondly, the ambient temperature during testing is varied, evaluating the impact of decreased, respectively increased test temperatures on the characteristics of the Polyamide-12 components. For all components, the static and dynamic mechanical load behaviour is investigated, quantifying the changes in the Ultimate Tensile Strength (UTS) and the endurance limit. As the applied energy density transpires as a decisive parameter, a variation leads to significant changes in UTS and endurance limit, whereas an adjustment of scan-speed and laser power at a constant energy density do not affect the mechanical properties. Finally, the ambient temperature during testing is evaluated, demonstrating different ambient application conditions. The impact of an active cooling of the component as well as increased temperatures on the mechanical behaviour is tested, providing fundamental findings on the operating life of Polyamide-12 components.

Abstract: Wire Arc Additive Manufacturing (WAAM) is an emerging technology for producing large scale metal components, offering significant advantages in material efficiency and reduced production time compared to conventional methods. This study investigates the microstructure and mechanical properties of carbon steel produced using two different WAAM systems: the Fronius TransPuls Syn ergic 2700 CMT and the Kemppi X5 500 Pulse+ systems. Both systems utilized similar operating parameters, yet exhibited subtle differences in microstructure, including grain size and phase distri bution. Due to slight microstructural variations, the mechanical properties, such as tensile strength, hardness, and fatigue performance, were nearly identical for both materials. The findings demonstrate the potential of WAAM to produce high-quality carbon steel components with consistent mechanical properties, highlighting its suitability for applications requiring large, custom metal parts.

Abstract: This study investigates the surface roughness, hardness, and fatigue performance of AISI 316L walls produced via wire arc additive manufacturing (WAAM) under three different surface conditions: as-built, severe shot peened (SSP), and machined. The WAAM-printed walls exhibit typical layered structures with some surface irregularities due to thermal cycling. Surface roughness measurements show that the machined surface has the lowest roughness values (Ra = 0.43 µm), while the as-built surface displays significant roughness (Ra = 37.56 µm), which is decreased by SSP (Ra = 34.67 µm). SSP slightly improves overall smoothness but does not eliminate major surface irregularities. Hardness measurements indicate that the base material has uniform hardness across the wall, ranging from 200 to 220 HV, while SSP significantly increases surface hardness to 450 HV near the edges due to localized work hardening. SSP-treated surfaces improve fatigue resistance by inducing compressive residual stresses, with a fatigue limit of 198 MPa, compared to 75 MPa for the as-built surface. However, machined surfaces exhibit the best fatigue performance, with a fatigue limit of 223 MPa, owing to the elimination of surface defects and stress concentrators. While SSP enhances surface hardness and fatigue performance over the as-built condition, machining remains essential for achieving superior fatigue life and surface quality.

Abstract: This study investigates the bending fatigue strength of ultra-high-strength steel (UHS) steel manufactured using wire arc additive manufacturing (WAAM) technology. Hardness evaluations, conducted in the built direction, demonstrated a remarkable consistency with an average hardness of approximately 292 HV throughout the entire deposited component. Further hardness measurements across printed layers revealed uniformity, indicating a lack of hardness variation within the interlayer region. Macrostructural analysis revealed fine grains characterized by an equiaxed morphology, showcasing a distinctive crystallographic arrangement. This microstructural configuration plays a pivotal role in shaping the mechanical behavior and properties of the material. While a detailed microstructure study was not conducted, the macro-level investigation confirmed the absence of visible pores or defects in the printed material, affirming its structural integrity and resilience. Tensile tests conducted on samples extracted from the WAAM part unveiled anisotropic behavior, with tensile strength in the built direction approximately 35 MPa higher than that in the deposition direction. The maximum yield strength reached an impressive 846 MPa in the built direction. Although the yield strength of WAAM UHS was lower compared to the yield strength promised by the welding wire manufacturer, the differing heat input in the WAAM process accounted for this variation. Fatigue strength of the WAAM UHS steel was significantly better compared to WAAM carbon steel used as reference material. The WAAM UHS sample exhibited a robust fatigue limit of 350 MPa.

Abstract: Next generation rolls such as super-cermet rolls and all-ceramic rolls can be manufactured using only sleeve assembly type rolls, which have the advantage of being able to reuse the shaft by replacing the damaged sleeves. However, in some cases, failures with unknown causes may occur such as circumferential slippage, shaft pull-out or residual bending deformation at the shrink-fit interface. Such slipping failures cannot be prevented by conventional design concept. This is because even if the resistant torque is greater than the motor torque, the circumferential slippage will occur. Through numerical simulation and miniature roll experiment, the following results are obtained. 1) Even under free rolling condition without motor torque, the circumferential slippage occurs. 2) The slippage is caused by the accumulation of irreversible slip during the roll rotation. 3) The motor torque accelerates the sip amount significantly. 4) The geometry of slippage defect can be identified experimentally. 5) The fatigue strength of sleeve assembly rolling rolls can be evaluated by using √area parameter characterizing the identified slip defects. 6) By preventing the slip damage, the fatigue strength of sleeve rolls can be nearly equal to that of conventional solid rolls without shrink-fit.

Abstract: The examination of WAAM UHS steel laser welds revealed effective material penetration, with desirable geometry showcased by a nearly I-shaped structure. Minor deficiencies were observed at the weld face, while excessive penetration was evident at the weld's root. Cross-sectional analysis indicated no discernible porosity or defects within the weld. Microstructural analysis highlighted fine-grained structures with dispersed precipitates in the WAAM UHS steel base material. Laser welding induced changes in the grain structure, resulting in finer grains and a mixture of ferrite and martensite in the weld zone. Significant increases in hardness were observed in the weld metal and HAZ near the fusion line, attributed to martensite prevalence induced by rapid cooling rates. The hardness of the base material measured around 294 HV, significantly rising in the weld metal, exceeding 401 HV. Mechanical properties altered post-welding, with yield strength decreasing from 749 MPa to 732 MPa. Laser welded WAAM UHS steel had 4% higher tensile strength compared to base material. However, ductility reduced from 27% to 22.5%. Bending fatigue tests revealed a considerable reduction in fatigue limit for laser-welded samples (80 MPa) compared to the base material (419 MPa), with fractures originating from the fusion line between the HAZ and the base material. Notably, the notch sensitivity of ultra-high-strength steels significantly reduces fatigue resistance.

Abstract: This study investigates the mechanical properties and microstructure of 316L stainless steel fabricated using laser powder bed fusion (PBF-LB) additive manufacturing with different layer thick nesses and orientations. Impact toughness is evaluated under various conditions as well as bending fatigue performance to understand the influence of layer thickness and surface quality on fatigue lim its. Microstructural analysis using scanning electron microscopy (SEM) provides insights into grain structure. Key findings include the superior impact toughness of the vertical orientation, particularly notable in specimens with a layer thickness of 40 µm. Bending fatigue tests revealed distinctive behav ior influenced by layer thickness and surface quality, with the 80 µm thickness and vertical orientation demonstrating lower fatigue limits. These insights contribute to optimizing manufacturing processes and enhancing material suitability for diverse applications.

Abstract: This study provides a comprehensive investigation into the microstructure, hardness, tensile strength, and bending fatigue behavior of a Wire Arc Additively Manufactured (WAAM) component composed of dissimilar materials—Carbon Steel (CS) and 316L stainless steel. Microscopic analysis reveals distinct microstructural characteristics, such as equiaxed ferrite grains in WAAM CS and a coarse columnar structure with delta-ferrite phases in WAAM 316L. A macroscopic phase map indicates a predominantly Body-Centered Cubic (BCC) structure near the interphase, suggesting element migration between CS and 316L due to high heat input. Higher magnification scans highlight martensitic structures on both sides of the interphase, with the CS side exhibiting larger grain sizes. Hardness assessment along the built direction shows a peak hardness of 407 HV near the interphase on the 316L side, contrasting with the CS side's average interphase hardness of 316 HV due to larger grain sizes. The yield strength of both WAAM CS and WAAM dissimilar material was consistently measured at 392 MPa. In comparison, WAAM 316L exhibited a slightly lower yield strength of 359 MPa. Notably, WAAM 316L demonstrated the highest tensile strength among the materials, reaching 656 MPa. Meanwhile, WAAM CS displayed a robust tensile strength of 503 MPa, and the WAAM dissimilar material exhibited a yield strength of 520 MPa. In terms of elongation, WAAM CS and WAAM 316L showcased values of 44.9% and 49.6%, respectively. On the other hand, WAAM dissimilar material exhibited a somewhat lower elongation of 20.4%, suggesting a different mechanical behavior in terms of ductility. Bending fatigue tests on WAAM 316L, WAAM CS, and the dissimilar material reveal a fatigue limit of approximately 225 MPa for WAAM 316L, 210 MPa for WAAM CS, and approximately 210 MPa for the dissimilar material. In the low-cycle and medium-cycle regimes, the dissimilar material exhibits slightly superior fatigue strength, potentially due to its marginally higher static strength. Notably, consistent fractures on the CS side during fatigue tests underscore a recurring behavior in the dissimilar material.

Abstract: Binder jetting is a rapidly evolving additive manufacturing technique, challenging the dominance of laser powder bed fusion in metal fabrication. This study focuses on the material properties of austenitic stainless steel 316L produced via binder jet technology. Porosity remains a significant challenge across additive manufacturing methods, adversely affecting material properties and fatigue life. To address this issue, we propose a novel approach employing severe shot peening as a post-processing treatment to enhance the material's characteristics. Microstructural analysis, including electron backscatter diffraction (EBSD), coupled with tensile testing, was conducted to evaluate the mechanical properties. Additionally, fatigue behavior was investigated under both axial and flexural bending loading conditions. The results revealed a substantial increase in material strength achievable through the post-treatment. Notably, the fatigue limit of the material in bending fatigue was elevated from 120 MPa to 190 MPa, indicating a significant enhancement in fatigue performance. This study contributes new insights into the enhancement of fatigue resistance in binder jet-manufactured 316L stainless steel through surface modification techniques. The findings underscore the potential of severe shot peening as an effective strategy to improve material properties and expand the applicability of binder jet printing in demanding industrial applications.