Effect of the shot peening process on the fatigue strength of SAE 5160 steel

The fatigue behaviour of SAE 5160 steel was evaluated before and after applying a shot peening process by using different Almen intensities and surface coverings (uncovered, partial coverage and total coverage). In the high-cycle fatigue tests, maximum stresses of 0.8Sut, 0.7Sut and 0.6Sut were applied in the three-point bending test on an Instron 8872 servo-hydraulic machine at a frequency of 10 Hz and a constant stress ratio of Smin/Smax¿=¿0.2 for all tests. The fatigue tests were performed based on the ASTM E8, the specimens were classified into six groups for each stress evaluated, and each group consisted of three standardised specimens (ASTM E466). Also, yield strength, ultimate strength, hardness and microhardness were obtained. The Wilcoxon’s non-parametric test was used to statistically compare all the mechanical properties obtained from the base material with those obtained after the application of the shot peening, for the different surface coverings and Almen intensities. The results showed that the shot peening process significantly increases the fatigue strength of the material, with a 94% increase in fatigue strength of the fully coated specimens. However, no significant increase in fatigue strength was found due to a change in the Almen intensity value. A high correlation factor was found between the increase in the ultimate resistance and the increase in Almen intensity; however, for the yield stress the correlation was medium and inverse. For hardness and microhardness, the correlation factor was very low. Finally, the microhardness values revealed a 3% increase in Vickers microhardness of the shot peening specimens compared to the untreated specimens.Peer ReviewedPostprint (author's final draft

Fatigue of friction stir and GTAW welded AZ31B magnesium alloy

The mechanical behavior of butt welds made on AZ31B magnesium alloy plates by solid-state friction stir welding (FSW) and gas tungsten arc welding (GTAW) is presented. Fatigue, tensile strength, and hardness tests were performed. Also, fractographic analyses of the weld microstructures were conducted. Tests results show that the fatigue performance of FSW joints was superior to that of conventional welding (GTAW).Peer ReviewedPostprint (published version

Numerical modeling of flow stress and grain evolution of an Mg AZ31B alloy based on hot compression tests

Magnesium alloys offer a wide range of applications in modern lightweight structures, although the correct forming parameters need to be found to reach a good combination of fine microstructure and the required mechanical properties. Several discrete and statistical methods have been proposed to simulate the dynamic recrystallization process and adopted to study microstructural evolution. However, the materials parameters necessary to develop these models are not widely available. Hence, industrial evaluation of these parameters is complex, unpractical for several types of material and time consuming for daily industrial applications. In that way, the thermomechanical behavior and grain size evolution modeling of the AZ31 alloy are proposed using isothermal compression data. Parameters to calculate coupled stress–strain–temperature parameters, dynamic recrystallization, volume fraction and grain size were obtained from the stress–strain curves. Then, the data were input in Deform-3D software to simulate the hot deformation process and verify with experimental data the consistency of the values obtained. Measured grains size agreed with the conducted modeling, showing the reliability of strain–stress and grain size data on predicting dynamic recrystallization phenomena.Postprint (author's final draft

Mechanical and fatigue strength assessment of friction stir welded plates of magnesium Alloy AZ31B

Light-alloys play a significant role in saving weight in automotive and aerospace industries; however, a few joining methods guarantee mechanical and fatigue strengths for high performance application. Even conventional arc welding processes do not offer constant quality joints. Therefore, this study uses an alternative solid-state welding process, friction stir welding (FSW), to analyze post processing microstructures and assess mechanical and fatigue strength. Magnesium alloy AZ31B plates were welded using different welding parameters in a dedicated FSW machine. The effect of the spindle speed (¿) and welding speed (¿) on the microstructure, the tensile strength and fatigue were studied. The stirred zone (SZ) at the FS-welded joints presented a microstructure composed by homogeneous equiaxial grains, refined by dynamic recrystallization. A rise in grain size, weld bead width, tensile and fatigue strengths with the increase of speed ratio (¿/¿) were observed. Results of the fatigue and mechanical strength here presented outperformed results from welds made with conventional milling machines.Postprint (published version

Synthesis and characterization of aluminum titanium carbonitride TiAlCN via mechanical alloying

This study presents synthesis of titanium aluminum carbonitrides alloy (TiAlCN) by mechanical alloying in Attritor ball mill from elemental powders of titanium, aluminum and graphite in nitrogen atmosphere. TiAlCN was characterized by SEM, XRD, DSC and FTIR techniques. XRD showed presence of titanium aluminum nitride, aluminum nitride, titanium carbide and titanium aluminum carbon nitride, while SEM showed existence of micro and nano particles with high agglomeration. Energydispersive spectroscopy (EDS) analysis shows a homogeneous distribution of elements, and mapping analysis from X-rays confirms distribution of elements.Peer ReviewedPostprint (published version

Investigation of material flow and thermomechanical behavior during friction stir welding of an AZ31B alloy for threaded and unthreaded pin geometries using computational solid mechanics simulation

In the friction stir welding process, the tool role in the material flow and its thermomechanical behavior is still not entirely understood. Several modeling approaches attempted to explain the material and tool relationship, but to this date, insufficient results were provided in this matter. Regarding this issue and the urgent need for trustful friction stir welding models, a computational solid mechanic's model capable of simulating material flow and defect formation is presented. This model uses an Arbitrary Lagrangian-Eulerian code comparing a threaded and unthread pin profile. The model was able to reproduce the tool's torque, temperatures, and material flow along the entire process, including the underreported downward flow effect promoted by threaded pin's. A point tracking analysis revealed that threads increase the material velocity and strain rate to almost 30% compared to unthreaded conditions, promoting a temperature increment during the process, which improved the material flow and avoided filling defects. The presented results showed the model's capability to reproduce the defects observed in real welded joints, material thermomechanical characteristics and high sensitivity to welding parameters and tool geometries. Nevertheless, the outcomes of this work contribute to essential guidelines for future friction stir welding modeling and development, tool design, and defect predictionPeer ReviewedPostprint (author's final draft

Dependence of wear and mechanical behavior of nitrocarburized/CrN/DLC layer on film thickness

Diamond-like carbon (DLC) films are amorphous metastable carbon form that provide interesting mechanical and tribological properties. The role of film thickness influence upon wear and mechanical properties is of interest and not yet fully reported. In this study, two samples of previously plasma nitrocarburized, quenched and tempered H13 steel were duplex treated. First, a physical vapor deposition (PVD) chromium nitride (CrN) layer was applied, followed by a top final diamond-like carbon layer applied by plasma-enhanced chemical vapor deposition (PECVD). To evaluate thicknesses influence on mechanical and wear properties of coatings, samples were treated using two different thicknesses of both layers. In this study, the thickest CrN and DLC case presented higher hardness and better tribological properties, however, its failure occurs in brittle fashion.Peer ReviewedPostprint (published version

Evaluación de la resistencia mecánica de uniones soldadas mediante proceso SFA y GTAW de la aleación de magnesio AZ31B

En el presente trabajo se muestran los resultados de la evaluación del comportamiento mecánico de las uniones soldadas de la aleación de magnesio AZ31B usando el proceso de soldadura fricción-agitación (SFA) y GTAW, consistente en la evaluación de la resistencia a la tensión y microdurezas de las uniones soldadas; realizando un análisis de tipo explicativo de su correlación con los cambios de microestructura, tamaños de grano presentes en las diferentes zonas, y los defectos de calidad, adicionalmente se realizó el análisis fractográfico para determinar la causa de falla en las probetas soldadas y ensayadas. Los resultados obtenidos mostraron una mayor resistencia a la tensión de las uniones soldadas de GTAW, en comparación con las uniones de SFA.Peer ReviewedPostprint (published version

Guide for recommended practices to perform crack tip opening displacement tests in high strength low alloy steels

Fracture mechanics approach is important for all mechanical and civil projects that might involve cracks in metallic materials, and especially for those using welding as a structural joining process. This methodology can enhance not only the design but also the service life of the structures being constructed. This paper includes detailed consideration of several practical issues related to the experimental procedures to assess the fracture toughness in high strength low alloy steels (HSLA) using the crack tip opening displacement (CTOD) parameter, specifically pipeline steels for oil and gas transportation. These considerations are important for engineers who are new in the field, or for those looking for guidelines performing different procedures during the experimentation, which usually are difficult to understand from the conventional standards. We discuss on topics including geometry selection, number of replicate tests, fatigue precracking, test procedure selection and realization, reports of results and other aspects.Peer ReviewedPostprint (published version

Investigation of building orientation and aging on strength–stiffness performance of additively manufactured maraging steel

The interaction between building orientation and heat treatment may change the strength–stiffness behavior of maraging steel manufactured by powder bed fusion (PBF). Further investigations about the correlation of the fracture characteristics and microstructures with the measured properties are needed to improve the process findings. Thus, this study investigated the way an interaction between building orientation (0°, 45°, and 90°) and specimen condition (as-built and aged) may change the defects interaction mechanism and the mechanical properties of the maraging 300 steel manufactured by PBF. Aging treatment, microstructure characterization, density measuring, tensile and microhardness tests, and fractography were performed on the specimens. The aging treatment strengthened the material, which also reduced the probability of defect coalescence. The interaction of building orientation and aging affected the stiffness, rising this in¿˜¿70.5 GPa for the 0° aged specimens. Furthermore, this interaction changed the melt pool stretching direction, and the behavior of the defect coalescence rises the elongation at break in¿˜¿16.5% for 0° as-built specimens. The investigated mechanisms show the importance of considering the interaction between the building orientation and condition (as-built and aged) for the mechanical performance of additively manufactured maraging steel 300.Peer ReviewedPostprint (author's final draft