RESUMEN
The friction stir welding (FSW) process was recently developed to overcome the difficulty of welding non-ferrous alloys and steels. In this study, dissimilar butt joints between 6061-T6 aluminum alloy and AISI 316 stainless steel were welded by FSW using different processing parameters. The grain structure and precipitates at the different welded zones of the various joints were intensively characterized by the electron backscattering diffraction technique (EBSD). Subsequently, the FSWed joints were tensile tested to examine the mechanical strength compared with that of the base metals. The micro-indentation hardness measurements were conducted to reveal the mechanical responses of the different zones in the joint. The EBSD results of the microstructural evolution showed that a significant continuous dynamic recrystallization (CDRX) occurred in the stir zone (SZ) of the Al side, which was mainly composed of the weak metal, Al, and fragmentations of the steel. However, the steel underwent severe deformation and discontinuous dynamic recrystallization (DDRX). The FSW rotation speed increased the ultimate tensile strength (UTS) from 126 MPa at a rotation speed of 300 RPM to 162 MPa at a rotation speed of 500 RPM. The tensile failure occurred at the SZ on the Al side for all specimens. The impact of the microstructure change in the FSW zones was significantly pronounced in the micro-indentation hardness measurements. This was presumably attributed to the promotion of various strengthening mechanisms, such as grain refinement due to DRX (CDRX or DDRX), the appearance of intermetallic compounds, and strain hardening. The aluminum side underwent recrystallization as a result of the heat input in the SZ, but the stainless steel side did not experience recrystallization due to inadequate heat input, resulting in grain deformation instead.
RESUMEN
In the present study, the hydrogen embrittlement (HE) susceptibility of an additively manufactured (AM) 316L stainless steel (SS) was investigated. The materials were fabricated in the form of a lattice auxetic structure with three different strut thicknesses, 0.6, 1, and 1.4 mm, by the laser powder bed fusion technique at a volumetric energy of 70 J·mm-3. The effect of H charging on the strength and ductility of the lattice structures was evaluated by conducting tensile testing of the H-charged specimens at a slow strain rate of 4 × 10-5 s-1. Hydrogen was introduced to the specimens via electrochemical charging in an NaOH aqueous solution for 24 h at 80 °C before the tensile testing. The microstructure evolution of the H-charged materials was studied using the electron backscattered diffraction (EBSD) technique. The study revealed that the auxetic structures of the AM 316L-SS exhibited a slight reduction in mechanical properties after H charging. The tensile strength was slightly decreased regardless of the thickness. However, the ductility was significantly reduced with increasing thickness. For instance, the strength and uniform elongation of the auxetic structure of the 0.6 mm thick strut were 340 MPa and 17.4% before H charging, and 320 MPa and 16.7% after H charging, respectively. The corresponding values of the counterpart's 1.4 mm thick strut were 550 MPa and 29% before H charging, and 523 MPa and 23.9% after H charging, respectively. The fractography of the fracture surfaces showed the impact of H charging, as cleavage fracture was a striking feature in H-charged materials. Furthermore, the mechanical twins were enhanced during tensile straining of the H-charged high-thickness material.
RESUMEN
In this work, 2 mm thick medium-Mn austenitic stainless steel (MMn-SS) plates were joined with austenitic NiCr stainless steel (NiCr-SS) and low-carbon steel (LCS) using the gas tungsten arc welding technique. A precise adjustment of the welding process parameters was conducted to achieve high-quality dissimilar joints of MMn-SS with NiCr-SS and LCS. The microstructural evolution was studied using laser scanning confocal and electron microscopes. Secondary electron imaging and electron backscatter diffraction (EBSD) techniques were intensively employed to analyze the fine features of the weld structures. The mechanical properties of the joints were evaluated by uniaxial tensile tests and micro-indentation hardness (HIT). The microstructure of the fusion zone (FZ) in the MMn-SS joints exhibited an austenitic matrix with a small fraction of δ-ferrite, ~6%. The tensile strength (TS) of the MMn-SS/NiCr-SS joint is significantly higher than that of the MMn-SS/LCS joint. For instance, the TSs of MMn-SS joints with NiCr-SS and LCS are 610 and 340 MPa, respectively. The tensile properties of MMn-SS/LCS joints are similar to those of BM LCS, since the deformation behavior and shape of the tensile flow curve for that joint are comparable with the flow curve of LCS. The HIT measurements show that the MMn-SS/NiCr-SS joint is significantly stronger than the MMn-SS/LCS joint since the HIT values are 2.18 and 1.85 GPa, respectively.
RESUMEN
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.
RESUMEN
In this study, ultra-high-strength steels, namely, cold-hardened austenitic stainless steel AISI 301 and martensitic abrasion-resistant steel AR600, as base metals (BMs) were butt-welded using a disk laser to evaluate the microstructure, mechanical properties, and effect of post-weld heat treatment (PWHT) at 250 °C of the dissimilar joints. The welding processes were conducted at different energy inputs (EIs; 50-320 J/mm). The microstructural evolution of the fusion zones (FZ) in the welded joints was examined using electron backscattering diffraction (EBSD) and laser scanning confocal microscopy. The hardness profiles across the weldments and tensile properties of the as-welded joints and the corresponding PWHT joints were measured using a microhardness tester and universal material testing equipment. The EBSD results showed that the microstructures of the welded joints were relatively similar since the microstructure of the FZ was composed of a lath martensite matrix with a small fraction of austenite. The welded structure exhibited significantly higher microhardness at the lower EIs of 50 and 100 J/mm (640 HV). However, tempered martensite was promoted at the high EI of 320 J/mm, significantly reducing the hardness of the FZ to 520 HV. The mechanical tensile properties were considerably affected by the EI of the as-welded joints. Moreover, the PWHT enhanced the tensile properties by increasing the deformation capacity due to promoting the tempered martensite in the FZ.
RESUMEN
Additive manufacturing (AM) is an emerging fabrication technology that offers unprecedented potential for manufacturing end-to-end complex shape customized products. However, building products with high performance by AM presents a technological challenge. Inadequate processing parameters, fabrication environment or changes in powder properties may lead to high defect density in the part and poor mechanical properties. Microstructure, defect structure, and mechanical properties of AISI 316L stainless steel pieces, additively manufactured by the laser powder bed fusion method using three different volume energy densities (VEDs), were investigated and compared with those of a commercial wrought AISI 316L sheet. Scanning and transmission electron microscopies were employed for characterization of grain and defect structures, and mechanical properties were determined by tensile testing. It was found that the number of defects such as pores and lack of fusion in AM specimens did not affect the strength, but they impaired the post-uniform elongation, more significantly when processed with the low VED. Twinning was found to be an active deformation mechanism in the medium and high VED specimens and in the commercially wrought material in the later stage of straining, but it was suppressed in the low VED specimens presumably because the presence of large voids limited the strain attained in the matrix.
RESUMEN
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.