Microstructural Evolution of Dissimilar Metal Welds Involving Grade 91

Metallurgical and Materials Transactions A - Dissimilar metal weld failures between low alloy Cr-Mo ferritic steels and austenitic stainless steels made with Ni-base filler metals are typically...     

Microstructural Evolution During Friction Surfacing of Dissimilar Aluminum Alloys

The microstructural evolution during friction surfacing of an aluminum alloy 6082-T6 rod on an aluminum alloy 2024-T351 substrate was characterized using the electron backscatter diffraction technique. Crystallographic data were obtained from several regions in the consumable material and in the deposited material. From the results, it can be deduced that the grain structure formation was a complex process governed by the geometrical effect of strain and the superposition of continuous and discontinuous dynamic recrystallizations.     

Microstructural Evolution and Mechanical Properties of Fusion Welds in an Iron-Copper-Based Multicomponent Steel

NUCu-140 is a copper-precipitation-strengthened steel that exhibits excellent mechanical properties with a relatively simple chemical composition and processing schedule. As a result, NUCu-140 is a candidate material for use in many naval and structural applications. Before NUCu-140 can be implemented as a replacement for currently used materials, the weldability of this material must be determined under a wide range of welding conditions. This research represents an initial step toward understanding the microstructural and mechanical property evolution that occurs during fusion welding of NUCu-140. Microhardness traverses and tensile testing using digital image correlation show local softening in the heat-affected zone (HAZ). Microstructural characterization using light optical microscopy (LOM) revealed very few differences in the softened regions compared with the base metal. Local-electrode atom-probe (LEAP) tomography demonstrates that local softening occurs as a result of dissolution of the Cu-rich precipitates. MatCalc kinetic simulations (Vienna, Austria) were combined with welding heat-flow calculations to model the precipitate evolution within the HAZ. Reasonably good agreement was obtained between the measured and calculated precipitate radii, number density, and volume fraction of the Cu-rich precipitates in the weld. These results were used with a precipitate-strengthening model to understand strength variations within the HAZ.     

Microstructural Evolution During Normal/Abnormal Grain Growth in Austenitic Stainless Steel

The grain growth behavior of 304L stainless steel was studied in a wide range of annealing temperatures and times with emphasis on the distinction between normal and abnormal grain growth (AGG) modes. The dependence of AGG (secondary recrystallization) at homologous temperatures of around 0.7 upon microstructural features such as dispersed carbides, which were rich in Ti but were almost free of V, was investigated by optical micrographs, X-ray diffraction patterns, scanning electron microscopy images, and energy dispersive X-ray analysis spectra. The bimodality in grain-size distribution histograms signified that a transition in grain growth mode from normal to abnormal was occurred at homologous temperatures of around 0.7 due to the dissolution/coarsening of carbides. Continued annealing to a long time led to completion of secondary recrystallization and the subsequent reappearance of normal growth mode. Another noticeable abnormality in grain growth was observed at very high annealing temperatures, which may be related to grain boundary faceting/defaceting. Finally, a versatile grain growth map was proposed, which can be used as a practical guide for estimation of the resulting grain size after exposure to high temperatures.     

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.     

Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds

The microstructure in the heat-affected zone (HAZ) of multipass welds typical of those used in power plants and made from 9 wt pct chromium martensitic Grade 92 steel is complex. Therefore, there is a need for systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds manufactured using the traditional arc welding processes in order to understand possible failure mechanisms after long-term service. In this study, the microstructure in the HAZ of an as-fabricated two-pass bead-on-plate weld on a parent metal of Grade 92 steel has been systematically investigated and compared to a complex, multipass thick section weldment using an extensive range of electron and ion-microscopy-based techniques. A dilatometer has been used to apply controlled thermal cycles to simulate the microstructures in distinctly different regions in a multipass HAZ using sequential thermal cycles. A wide range of microstructural properties in the simulated materials were characterized and compared with the experimental observations from the weld HAZ. It has been found that the microstructure in the HAZ can be categorized by a combination of sequential thermal cycles experienced by the different zones within the complex weld metal, using the terminology developed for these regions based on a simpler, single-pass bead-on-plate weld, categorized as complete transformation, partial transformation, and overtempered.     

Dissimilar Spot Welds of AISI 304/AISI 1008: Metallurgical and Mechanical Characterization

This paper aims at investigating structure‐properties relationships in dissimilar resistance spot welding of AISI 304 austenitic stainless steel (SS) and AISI 1008 low carbon steel (CS). Microstructural characterization, microhardness test and the tensile‐ shear test were conducted. It was shown that the shape of the SS/CS fusion zone (FZ) is unsymmetrical and the final fusion line shifts from sheet/sheet interface into the higher resistivity side (i.e. AISI 304). FZ microstructure was ranged from ferrite‐austenite to full martensite depending on the dilution ratio of the base metals. The variation of SS/CS dissimilar welds failure mode was explained in terms of hardness/microstructure characteristics. It was concluded that to ensure pullout failure mode, welding parameters needed to adjust so that the FZ size is sufficiently large and dilution is sufficiently high to produce a martensite FZ. Fusion zone size at CS side proved to be the most important controlling factor of SS/CS peak load and energy absorption. Finally, the mechanical properties of SS/CS dissimilar welds were compared with SS/SS and CS/CS similar welds.     

Influence of Rotational Speed on Microstructure and Mechanical Properties of Dissimilar Metal AISI 304-AISI 4140 Continuous Drive Friction Welds

 Fundamental investigation of continuous drive friction welding of austenitic stainless steel (AISI 304) and low alloy steel (AISI 4140) is described. The emphasis is made on the influence of rotational speed on the microstructure and mechanical properties such as hardness, tensile strength, notch tensile strength and impact toughness of the dissimilar joints. Hardness profiles across the weld show the interface is harder than the respective parent metals. In general, maximum peak hardness is observed on the stainless steel side, while other peak hardness is on the low alloy steel side. A trough in hardness distribution in between the peaks is located on the low alloy steel side. Peak hardness on the stainless steel and low alloy steel side close to the interface increases with a decrease in rotational speed. All transverse tensile joints fractured on stainless steel side near the interface. Notch tensile strength and impact toughness increase with increase in rotational speed up to 1500 r/min and decrease thereafter. The mechanism of influence of rotational speed for the observed trends is discussed in the torque, displacement characteristics, heat generation, microstructure, fractography and mechanical properties.     

Deformation Behaviour and Microstructural Evolution During Hot Compression of AISI 904L

The hot deformation behaviour and microstructural evolution of AISI 904L super‐austenitic steel has been investigated by means of hot compression tests. The tests were carried out on a Gleeble 1500D thermo‐mechanical simulator in the temperature range from 850 °C to 1150 °C and at strain rates range from 0.001 s^?1 to 5 s^?1. The microstructure evolution was examined by means of light optical microscopy (LOM). The results show that after an initial deformation hardening, softening mechanisms occur. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation with the activation energy for deformation of 463 kJ/mol. The steady state was achieved at maximum strain of 0.9 only at the lower strain rates (under 1 s^?1) and the higher temperatures (above 1100 °C). Microstructural analyses showed a gradual increase in the dynamically recrystallized area with an increase of the temperature and a decrease of the strain rate. The grain size did change, as expected, correlating to the deformation conditions.     

Static Electropulsing-Induced Microstructural Changes and Their Effect on the Ultra-Precision Machining of Cold-Rolled AZ91 Alloy

The effects of electropulsing on the phase transformations of a cold-rolled Mg-9Al-1Zn alloy were studied using X-ray diffraction, back-scattered electron microscopy, transmission electron microscopy, and optical microscopy techniques. The results indicated that with increasing frequency of electropulsing, the decomposition and precipitation of β phase were tremendously accelerated sequentially. Electropulsing accelerated the decomposition of β phase by a factor of approximately 3600 times. The effects of the electropulsing-induced microstructural changes on machinability of the alloy, by single-point diamond turning, was discussed.     

Global and Local Mechanical Properties and Microstructure of Friction Stir Welds with Dissimilar Materials and/or Thicknesses

This article studies the properties of a wide range of friction-stir-welded joints with dissimilar aluminum alloys or thicknesses. Two aluminum alloys, namely, 2024-T3 and 7075-T6, are selected for the study and are welded in ten different combinations of alloys and thicknesses. The welding parameters are optimized for each configuration, and a systematic study of the effects of material and thickness combinations on the microstructural features, global and local mechanical properties, and fracture mechanisms of the welds is carried out. It is shown that dissimilar alloys are extruded into each other, the texture is heterogeneous in the weld zone, and that there is no significant diffusion of alloying elements between the alloys. For most configurations, the local and global mechanical properties decrease as the thickness ratio increases. The local yield strength and plasticity parameters substantially vary next to the weld centerline, hence requiring their implementation in finite element method (FEM) models. Machining to obtain a constant thickness significantly influences the mechanical properties of the welds. The fracture mechanism is found to be a mixture of ductile and brittle fractures and to qualify as “quasi-cleavage.”     

铜/钛异种金属焊接微观组织及力学性能研究

研究采用电子束偏置铜侧的方法完成了T2铜和TC4的异种金属焊接,并采用光学显微镜（OM）、扫描电镜（SEM）、显微硬度及抗拉强度测试等方式分析其微观组织及力学性能特征。研究结果表明：铜侧焊接具有成形较好的焊缝,焊缝表面鱼鳞纹较均匀,成形较好,无明显的微裂纹、气孔以及夹渣现象,电子束焊接可实现单侧焊接双面成形。钛合金侧焊缝可观察到宽度25～40μm的金属间化合物层,金属间化合物层由多种反应产物组成,金属间化合物层的存在将恶化接头的力学性能。在偏铜侧1.5 mm焊接时,抗拉强度可达到152 MPa,相当于T2 Cu抗拉强度的66%,拉伸断口表现为脆性解理断裂,引起断裂的主要物相为CuTi相。     

Influence of Metal Properties on the Formation and Evolution of Metal Coatings During Mechanical Coating

Powders of Cu, Ti, Ni, Fe, and Zn metals were used to prepare coatings on the surfaces of Al₂O₃ balls by the mechanical coating technique. The coated Al₂O₃ balls were characterized with XRD and SEM. The results showed that all the metal powders except Ni formed continuous metal coatings. The evolution of metal coatings during mechanical coating was also investigated. The analysis indicates that as long as continuous metal coatings can be formed, the evolution can fall into the following stages: nucleation, formation and coalescence of discrete islands, formation and thickening of continuous coatings, and exfoliation of continuous coatings. Electronegativity of the metal was shown to have a major effect on the adhesion of the tiny metal particles on the surfaces of the Al₂O₃ balls during the initial stage of mechanical coating. The lower the electronegativity of the metal, the greater the coverage of the metal on the Al₂O₃ ball and the easier the adhesion of the tiny metal particles. Further, the better the plastic deformability of metal, the easier the cold welding among metal powder particles and the greater the thickness of the continuous metal coatings.     

Microstructural and Mechanical Studies of T91/T22 Welded Joints

Comparative studies have been performed to decide an appropriate combination of welding process and filler material by virtue of microstructural evolution, micro-hardness studies, tensile strength and fractographic analysis. Manual arc welding and tungsten inert gas welding processes are used along with different filler materials to manufacture T91/T22 welded joints. Studies with the purpose of comparison and evaluation of different zones of the weldments have been carried out. The highest value of micro-hardness observed on the T91 HAZ of the weldments may be attributed to martensitic structure of the region. The fracture morphology of both the weldments obtained from T22 BM has revealed the ductile fracture. Comparatively higher tensile strength (578 MPa) of T91/T22, GTAW combination is noticed by virtue of lower heat input. The better performance of T91/T22, GTAW weldment can be quoted on the basis of better joint integrity, tensile strength and ductility (26.4%).     

Microstructural Evolution During Friction Stir Welding of Mild Steel and Ni-Based Alloy 625

Microstructure evolution during friction stir welding (FSW) of mild steel and Ni-based alloy 625 was studied. Regarding the Ni-based alloy, the welding process led to grain refinement caused by discontinuous and continuous dynamic recrystallization, where bulging of the pre-existing grains and subgrain rotation were the primary mechanisms of recrystallization. In the steel, discontinuous dynamic recrystallization was identified as the recovery process experienced by the austenite. Simple shear textures were observed in the regions affected by the deformation of both materials. Although the allotropic transformation obscured the deformation history, the thermo-mechanically affected zone was identified in the steel by simple shear texture components. A new methodology for the study of texture evolution based on rotations of the slip systems using pole figures is presented as an approximation to describe the texture evolution in FSW.     

Influence of Cold Work on the Microstructural Evolution and Hardness During Aging of AA6061 Alloy

It is well known that the kinetics of precipitation is altered with the degree of cold working and this study aims to bring out the effect of cold working on the aging response of the alloy AA6061 along with the studies of microstructural evolution. The as-received samples were initially solutionized at 530 °C and subsequently aged at 160 °C for varying time from 2 to 28 h at intervals of 2 h. A peak hardness of 124 HV was observed at 15 h beyond which the hardness decreased due to overaging. The solutionized samples were also subjected to various degrees of cold working and the increase in hardness by virtue of strain hardening was observed. The samples that were cold worked to various levels (10, 20, 30% etc.) were subsequently aged at 160 °C for 4 h. A peak hardness of 134 HV was obtained for the sample cold worked to 75% and subsequently aged for 4 h and this was attributed to the increase in the dislocation density that improved the kinetics of precipitation. The evolution of microstructure for various samples during the course of aging was observed using transmission electron microscopy and the changes in the properties were correlated to the obtained microstructure.