Papers by Author: Markku Keskitalo

Abstract: This study investigates the microstructural and mechanical integrity of the interface between Wire Arc Additive Manufactured (WAAM) carbon steel (CS) and an S355 structural steel (Wrought CS) plate, with emphasis on the suitability of WAAM for repair and reinforcement of structural components. A wall structure was deposited on a Wrought CS plate using a Cold Metal Transfer (CMT) process and subsequently characterized through microstructural analysis, hardness measurements, tensile testing, and bending fatigue testing. The microstructural observations revealed a smooth and defect-free transition across the interface, consisting of a fine-grained heat-affected zone (HAZ) formed by partial recrystallization. The hardness profile exhibited a continuous gradient, with slightly elevated values near the interface (~210 HV), indicating grain refinement and the absence of softening effects. The tensile results showed that the WAAM-deposited CS possessed higher strength and ductility than the Wrought CS, while the hybrid WAAM CS–Wrought CS specimens displayed intermediate properties. Fracture consistently occurred within the Wrought CS plate rather than at the interface, confirming a metallurgically sound and mechanically robust bond. Under bending fatigue loading, the WAAM CS demonstrated the highest fatigue limit (~250 MPa), followed by the hybrid (~205 MPa) and Wrought CS (~162 MPa). All hybrid specimens fractured on the Wrought CS side, indicating that the interface remained intact under cyclic stress.

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: This study was initiated to investigate the material characteristics of binder jet (BJ) manufactured austenitic stainless steel 316L, focusing specifically on the less studied bronze infiltrated version of this material. While BJ technology offers a compelling alternative to the current market leader laser powder bed fusion, all additive manufacturing methods are susceptible to porosity, which adversely affects the fatigue properties of parts, resulting in inferior fatigue life compared to traditionally manufactured counterparts. In this study, we explore the novel application of severe shot peening (SSP) as a post-processing method to enhance fatigue life. Through comprehensive microstructural analysis utilizing EBSD, mechanical properties testing via tensile testing, and fatigue life analysis using flexural bending fatigue testing, we demonstrate that SSP treatment induces surface modification, leading to increased material strength albeit with a trade-off in ductility. Moreover, our findings reveal a significant improvement in the fatigue life of the material. Utilizing SSP, we observed that the fatigue limit of the material more than doubled, surpassing the performance of the sheet metal counterpart of the same material. These results underscore the potential of SSP as an attractive method for property enhancement in additive manufacturing.

Abstract: In this paper, the effect of printing parameters on the surface roughness and mechanical properties of wire arc additive manufactured (WAAM) carbon steel is evaluated. WAAM has become increasingly popular as an additive manufacturing method, particularly for producing large parts. Utilizing welding equipment with cold metal transfer (CMT) technology in WAAM production ensures high-quality parts. However, printing parameters play a crucial role in determining material properties. This study evaluates the impact of five different printing parameters on these properties. Microhardness measurements were conducted in the deposition direction of the printed walls, while optical microscopy was used to assess the surface roughness of the printed carbon steel. Tensile tests were performed to determine the mechanical properties of the WAAM-printed carbon steel. The results indicated uniform hardness across all printing parameters, with no observable defects such as pores. Significant differences in surface roughness were noted between the various printing parameters. Although the printing parameters did not significantly affect the tensile strength of the printed carbon steel, they did result in noticeable differences in elongation.

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: This investigation evaluates the influence of surface quality on the bending fatigue strength of Wire Arc Additive Manufacturing (WAAM) Ultra-High-Strength (UHS) steel, The study is focused on structural integrity, hardness, surface roughness, tensile strength, and bending fatigue performance. Cross-sectional analysis reveals slight variations in wall thickness, averaging 5.5 mm, with an absence of discernible pores or defects, affirming the process's capability to yield high-quality results. Hardness assessments indicate uniformity across the deposited component, with an average hardness of 303 HV, emphasizing consistent material properties. Surface roughness analysis highlights superior fatigue strength in polished samples compared to machined and as-built ones, with roughness inversely impacting fatigue resistance. Tensile tests confirm satisfactory yield and tensile strength, with favorable ductility characteristics. Bending fatigue tests show that surface quality significantly influences fatigue strength, with polished samples exhibiting the highest fatigue limit. Conversely, as-built surfaces display the lowest fatigue strength due to increased sensitivity to crack initiation, emphasizing the critical role of surface quality in UHS steel fatigue performance. These findings underscore the suitability of WAAM-printed UHS steel for structural applications while emphasizing the importance of surface quality in determining fatigue resistance and overall component performance.

Abstract: Additive manufacturing, specifically Laser Powder Bed Fusion (PBF-LB), has gained prominence for its capability to produce complex near-net-shaped components. While PBF-LB offers advantages such as lightweight construction and cost-effectiveness, post-processing remains crucial to meet specific design requirements. This study investigates the post-processing technique of severe shot peening (SSP) on PBF-LB-manufactured 316L stainless steel, a material widely used for its favorable mechanical properties and corrosion resistance. The research focuses on the enhancement of bending fatigue properties through SSP treatment, examining the influence of material thickness on fatigue behavior. Comparative analysis reveals the effectiveness of SSP in significantly improving fatigue strength irrespective of variations in material thickness. Mechanical properties are explored for different thicknesses subjected to SSP treatment. Electron Backscatter Diffraction (EBSD) is employed to scrutinize the surface properties of the samples, providing knowledge on the microstructural changes induced by SSP. The study contributes to the understanding of the role of material thickness in the context of SSP treatment, offering a comprehensive exploration of the mechanical and fatigue characteristics of PBF-LB-manufactured 316L stainless steel.