Direct quenching has recently been used to develop cost-effective novel high and ultra-high strength steels. In weldments made of these steels, a distinct soft zone, which can lower the ultimate ...strength and deformation capacity of the welded joint, can form in the heat affected zone of the joint. In order to accurately estimate the static strength and deformation capacity of a welded steel structure fabricated from these steels, the effect of this softening must be taken into account. In this study, the effect of the softened zone is explored based on round tensile test specimens with metallurgical constraints using experiments and numerical calculations. The applicability of the quasi-static part of the Johnson-Cook material model and failure model for fracture assessment under given conditions was hereby examined. Firstly, the material parameters were determined for direct-quenched armor steel and cold-formed structural steel using notched tensile test specimens and FE models combined with an optimization routine. A good agreement was achieved between the tests and the results from the FE models for the notched specimens. Specimens with metallurgical constraints were manufactured from the materials using rotational friction welding, and the corresponding FE models were created using the material parameters obtained from optimization. Two different fracture mechanisms were observed in the tensile tests conducted for the specimens with metallurgical constraints, namely cup and cone fractures at the center of the specimen and fractures near the weld interface. In the corresponding FE models, the fracture initiations were observed in the same areas. A good agreement between the FE models and experimental tensile tests was thus achieved in the case of the cup and cone fractures. However, the generated FE models underestimated the fracture displacement in the specimens where the fracture occurred near the weld interface.
•The effect of metallurgical constraint to the strength and ductility of friction welded tensile specimen were studied.•Two distinct fracture mechanism was identified.•Quasi-static Johnson-Cook material and fracture model was successfully applied in the failure assessment of the specimens.
•Dual laser-beam bilateral synchronous welding (DLBSW) experiments of Ti6Al4V double lap-fillet joints have been carried out.•The relationship of thermal cycle and microstructural evolution is ...studied.•Narrow HAZ width and fine acicular α'-martensite for low welding speed is observed.•Residual stress distribution has a great influence on fracture behavior of lap-fillet joint.•Slower welding speed is in favor of higher tensile strength.
Employing prefabricated “boss” on the substrate to replace wire feeding, an attempt is made to explore the non-filler welding process of Ti6Al4V double lap-fillet welded joints with the technology of dual laser-beam bilateral synchronous welding (DLBSW). The weld quality is evaluated in terms of bead geometry, microstructural morphology and mechanical property. An optimal range of welding speed (1.3 m/min) with constant unilateral laser power of 950 W is identified to achieve satisfactory weldments with no underfill defect and acceptable porosity. Phase transformation and microstructural evolution occurring through thermal history in different regions of the welded joint are clarified. The average length of α'-martensite in the weld seam (WS) decreases significantly as welding speed increases. A connection between dimensional variation of α'-martensite and tensile strength is developed, and the fracture mechanism of the lap-fillet joint is discussed. The existence of fine α'-martensites helps to attain high-performance joints with strength comparable to the base metal (BM). The fracture position of tensile specimens is located in the region near the weld toe where stress concentration occurs. Fractography analysis indicates that the failure of weldments is in ductile mode with the feature of a great number of dimples.
Offshore structures are generally fabricated of welded joints, which are considered as potential spots for crack initiation owing to the degree of stress concentrations imposed by the weld geometry ...and the effects of residual stresses introduced by welding processes. There are significant numbers of the current and anticipated offshore installations coupled with the use of newer materials and fabrication techniques. It is therefore important to understand the crack growth mechanisms in these structures accompanied with the effects of mean or residual stresses for a safe estimation of their service lives. In this paper, crack growth results of HAZ and weld materials similar to those used for offshore installations are presented. Tests were conducted in air and in simulated free‐corrosion conditions at loading frequencies of 0.3 Hz in seawater, 5 Hz in air and at loading ratios of 0.1, 0.5 and 0.7. Results showed that crack growth rates were influenced by mean stresses, materials microstructure accompanied by welding procedure and environment. Crack growth results showed good agreement when compared with those obtained from other steels used for offshore structures.
This research involves studying the mechanical properties and corrosion behavior of “low carbon steel” (0.077wt% C) before and after welding using Arc, MIG and TIG welding. The mechanical properties ...include testing of microhardness, tensile strength, the results indicate that microhardness of TIG, MIG welding is more than arc welding, while tensile strength in arc welding more than TIG and MIG. The corrosion behavior of low carbon weldments was performed by potentiostat at scan rate 3mV.sec-1 in 3.5% NaCl to show the polarization resistance and calculate the corrosion rate from data of linear polarization by “Tafel extrapolation method”. The results indicate that the TIG welding increase the corrosion current density and anodic Tafel slop, while decrease the polarization resistance compared with unwelded low carbon steel. Cyclic polarization were measured to show resistance of specimens to pitting corrosion and to calculate the forward and reveres potentials. The results show shifting the forward, reverse and pitting potentials toward active direction for weldments samples compared with unwelded sample.
Electron beam welding is conducted on the as‐polished Ni‐base superalloys. The wrought–solidified–wrought (W–S–W) complex structure was machined from the central part of the weldments, and the hot ...compression test of W–S–W was then conducted. The DRX activation energy was calculated, and a constitutive equation which describes the DRX occurrence in hot working process was established. The deformed substructures such as dense dislocation networks were observed in the deformed dendrites in the welding seam of W–S–W. When the solidified dendrite in the welding seam is entirely covered with the recrystallized grains, the first necklace microstructure is formed. The dislocations drive the occurrence of DRX again in the first necklace microstructure. When several necklaces were formed, the solidified dendrite in the welding seam disappeared. The area fraction of DRX increases with increasing height reduction. The solidified dendrite could be greatly eliminated and refined after severe plastic deformation due to the repeating occurrence of DRX.
Electron beam welding is conducted, and wrought–solidified–wrought complex structure is machined from the central part of the weldments. Dislocations drive the occurrence of DRX. Solidified dendrite can be greatly eliminated and refined after hot deformation due to repeating occurrence of DRX.
The present investigation is concerned with high‐cycle axial fatigue testing of a 2‐mm AA6060‐T6 hybrid metal extrusion & bonding (HYB) butt weld produced in the solid state using AA6082 filler metal ...addition. The results complement the three‐point bend testing and the tensile testing done in two previous studies. In this study, optical microscope and scanning electron microscope examinations have been carried out to reveal the joint macro/microstructure and document possible surface and root defects deemed to affect fatigue life. In the as‐welded condition, the HYB weld suffers from surface irregularities at the weld face and ‘kissing’ bond formation in the root region. Despite of this, the subsequent testing shows that the fatigue properties exceed those reported for comparable AA6082‐T6 gas metal arc butt welds and matching those reported for corresponding high‐strength laser beam and friction stir weldments.
Considering the problem of the weak bonding interface structure between the rolling mill oil and film bearing bushings of Babbitt alloy and steel substrate, a numerical simulation of the layered ...bimetallic ZChSnSb8Cu4/steel by tungsten inert gas (TIG)-metal inert gas (MIG) hybrid welding process was carried out using Simufact Welding software (version 2020). In this study, the TIG-MIG hybrid welding process was simulated to obtain the temperature field and the stress field distributions. The residual stress and the deformation of the weldment were also analyzed using the calculated results. The results showed that the temperature gradient and the thermal stress were reduced in TIG-MIG hybrid welding compared to the conventional MIG welding preparation of layered bimetal ZChSnSb8Cu4/steel, which resulted in an improvement in the structural stability of the weldment. The temperature field and deformation of TIG-MIG hybrid welding of Babbitt alloy were studied under different controlled electrode spacings and TIG welding currents, and it was found that as electrode spacing increased, so did heat loss. Furthermore, with increased TIG welding current, compressive stress increased and tensile stress at the weld decreased, and the maximum thermal efficiency of welding was with a preheating current of 60 A.
In the present study simulated and the actual microstructures of the coarse grain, fine grain and intercritical heat-affected zones of P91 steel weldment are characterized using electron ...backscattered diffraction (EBSD) technique. Both the simulated and actual heat-affected zone (HAZ) microstructures showed profound substructural recovery after post-weld heat treatment/tempering. The subgrain size distributions between the simulated and the actual HAZs are comparable. The actual HAZ microstructures exhibited greater low angle grain boundaries and local strain distributions compared to the simulated HAZ microstructures due to the complex microstructural geometry of the actual weldment. The similarity in the variations of grain size, precipitate size and hardness of the simulated and actual HAZs validates the simulation of HAZ microstructures through furnace heat treatment.
PDF
