Enhancement and underlying fatigue mechanisms of laser powder bed fusion additive-manufactured 316L stainless steel

In this study, the enhancement of additively manufactured (AM) 316L, by annealing, to the fully reversed tension-compression fatigue performance, in terms of fatigue life and fatigue damage, were investigated under two conditions: as-built (AB) and heat-treated (HT) at 900 °C. The underlying fatigue mechanisms were comprehensively characterised through intensive microstructural observations of cyclic-strained microstructures and fracture surfaces using laser confocal scanning microscopy (LCSM) and secondary electron imaging using scanning electron microscopy (SEM). The experimental results showed that the fatigue resistance of HT 316L was significantly enhanced by 100% as the fatigue limit was increased from 75 to 150 MPa for AB and HT 316L, respectively. The fatigue cracking mechanism in AB 316L is mainly related to two imperfections of the AM-induced microstructural components: residual stresses, which cause highly localised deformation, and dendritic cellular structures, which possess a weak link in their grain boundaries against crack propagation. Upon heat treatment at 900 °C, the residual stresses and dendritic structure were effectively reduced. Consequently, the fatigue life of AM 316L was significantly enhanced by promoting the formation of high-angle boundaries. More precisely, the cyclic deformation processes in fatigued HT 316L involve persistent slip bands and strain hardening.publishedVersionPeer reviewe

The Effect of Severe Shot Peening on Fatigue Life of Laser Powder Bed Fusion Manufactured 316L Stainless Steel

Severe shot peening (SSP) was used on additive manufactured 316L by laser powder bed fusion. The effect of the post processing on the surface features of the material was analyzed through residual stress measurements, tensile testing, hardness-depth profiles, and fatigue testing by flexural bending. The results showed that SSP can be utilized to form residual stresses up to −400 MPa 200 µm below the surface. At the same time, a clear improvement on the surface hardness was achieved from 275 HV to near 650 HV. These together resulted in a clear improvement on material strength which was recorded at 10% improvement in ultimate tensile strength. Most significantly, the fatigue limit of the material was tripled from 200 MPa to over 600 MPa and the overall fatigue strength raised similarly from a low to high cycle regime.publishedVersionPeer reviewe

Effect of high-pressure torsion on microstructure, mechanical properties and corrosion resistance of cast pure Mg

© 2018, The Author(s). High-pressure torsion (HPT) processing was applied to cast pure magnesium, and the effects of the deformation on the microstructure, hardness, tensile properties and corrosion resistance were evaluated. The microstructures of the processed samples were examined by electron backscatter diffraction, and the mechanical properties were determined by Vickers hardness and tensile testing. The corrosion resistance was studied using electrochemical impedance spectroscopy in a 3.5% NaCl solution. The results show that HPT processing effectively refines the grain size of Mg from millimeters in the cast structure to a few micrometers after processing and also creates a basal texture on the surface. It was found that one or five turns of HPT produced no significant difference in the grain size of the processed Mg and the hardness was a maximum after one turn due to recovery in some grains. Measurements showed that the yield strength of the cast Mg increased by about seven times whereas the corrosion resistance was not significantly affected by the HPT processing

Reversed Microstructures and Tensile Properties after Various Cold Rolling Reductions in AISI 301LN Steel

Heavy cold rolling is generally required for efficient grain size refinement in the martensitic reversion process, which is, however, not desirable in practical processing. In the present work, the influence of cold rolling reductions of 32%, 45% and 63% on the microstructure evolution and mechanical properties of a metastable austenitic AISI 301LN type steel were investigated in detail adopting scanning electron microscopy with the electron backscatter diffraction method and mechanical testing. A completely austenitic microstructure and a partially reversed counterpart were created. It was found that the fraction of grains with a size of 3 µm or larger, called medium-sized grains, increased with decreasing the prior cold rolling reduction. These grains are formed mainly from the shear-reversed austenite, transformed from slightly-deformed martensite, by gradual evolution of subgrains to grains. However, in spite of significant amounts of medium-sized grains, the tensile properties after the 32% or 45% cold rolling reductions were practically equal to those after the 63% reduction. The austenite stability against the formation of deformation-induced martensite in subsequent straining was reduced by lowering the cold rolling reduction, due to the larger grain size of medium-sized grains and the shift of their orientation towards {211} <uvw>

Processing and Properties of Reversion-Treated Austenitic Stainless Steels

Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has been utilized industrially in a very limited extent, but apparently, it could provide a feasible processing route for strengthened austenitic stainless steels

Reversed microstructures and tensile properties after various cold rolling reductions in AISI 301LN steel

Abstract Heavy cold rolling is generally required for efficient grain size refinement in the martensitic reversion process, which is, however, not desirable in practical processing. In the present work, the influence of cold rolling reductions of 32%, 45% and 63% on the microstructure evolution and mechanical properties of a metastable austenitic AISI 301LN type steel were investigated in detail adopting scanning electron microscopy with the electron backscatter diffraction method and mechanical testing. A completely austenitic microstructure and a partially reversed counterpart were created. It was found that the fraction of grains with a size of 3 µm or larger, called medium-sized grains, increased with decreasing the prior cold rolling reduction. These grains are formed mainly from the shear-reversed austenite, transformed from slightly-deformed martensite, by gradual evolution of subgrains to grains. However, in spite of significant amounts of medium-sized grains, the tensile properties after the 32% or 45% cold rolling reductions were practically equal to those after the 63% reduction. The austenite stability against the formation of deformation-induced martensite in subsequent straining was reduced by lowering the cold rolling reduction, due to the larger grain size of medium-sized grains and the shift of their orientation towards {211} <uvw>

Assessing the compressive and tensile properties of TPMS-Gyroid and stochastic Ti64 lattice structures: A study on laser powder bed fusion manufacturing for biomedical implants

In the field of additive manufacturing, the design and fabrication of lattice structures have garnered substantial attention, particularly for their potential in advanced material applications. This study focuses on the mechanical properties of titanium alloy Ti64 lattice structures fabricated by Laser Powder Bed Fusion. It examines the mechanical features of two structure types, TPMS Gyroid and Stochastic Voronoi, analyzing their design and fabrication interplay. To evaluate mechanical properties, two methodologies to compute stress were employed: Method A, nominal diameter calculations of the cross-sectional area as a bulk material, and Method B, averaged cross-sectional area measurements along the open-cell porous structures. Method A generally showed lower Elastic Modulus and Yield Stress than Method B, highlighting the importance of geometric accuracy in lattice design mechanics. Both TPMS gyroid and stochastic lattices, within a 0.1–0.5 relative density range, displayed mechanical properties similar to human bone, suggesting their potential in orthopedic use. This could help reduce stress shielding and improve implant lifespan. The findings were further complemented by Scanning Electron Microscopy and Digital Image Correlation evaluations, highlighting the intricate relationship between fabrication parameters, lattice morphology, and the emergent microstructural characteristics

Evolution of magnetic properties during tempering

Quality control in heat treatment of steel is often conducted after the treatment. Failure to confine within the specified range of mechanical properties may lead to wasted energy and production resources. Performing quality control in-line in the heat treatment process allows for early detection and possibility to react to changes in the process. The prospects of utilizing the change in the electromagnetic (EM) properties of steel, as means for quality control, are investigated in this paper. The focus is on the tempering process of hardened SS2244 (42CrMoS4) steel. The tempering takes the hardness of the steel from approximately 600 HV down to around 400 HV. The EM signature of the steel is recorded during the tempering process. This is later compared with results from more traditional means of material characterization, such as laser scanning microscopy, X-ray diffractometry and Vickers microhardness measurement. This initial study shows clear indications of precise detection of the hardness through EM properties during the tempering process of selected material

Processing and properties of reversion‐treated austenitic stainless steels

Abstract Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has been utilized industrially in a very limited extent, but apparently, it could provide a feasible processing route for strengthened austenitic stainless steels