Ductility Of Joints Research Articles

Experimental study of plate-buckling type earthquake-resilient prefabricated steel beam-column joints with replaceable double flange cover plates

The damage control and post-earthquake repair of beam-column joints is a hot research topic in the seismic field. Therefore, a new type of earthquake-resilient prefabricated steel beam-column joint with replaceable double flange cover plates (RFCP-ERPSJ) was proposed. The yield load of this joint can be controlled by the buckling or slip of plates, so it can be divided into plate-slipping type and the plate-buckling type. Considering that the yield load of the plate-buckling type RFCP-ERPSJ is stable, firstly, a practical design procedure and design theory of the newly plate-buckling type joint is proposed in this paper. Then, to study the seismic performance and post-earthquake repairability of this RFCP-ERPSJ, a total of 6 full-size specimens are designed and subjected to a low-cycle reciprocating loading test and constant-amplitude low-cycle fatigue test. The effects of the thickness of FCPs, the material strength of FCPs, dog-bone width, amount of FCPs connecting bolts, and bolt spacing at the middle row on the seismic performance and energy dissipation performance of the joint are studied. The experimental results are also compared with the theoretical results. The research showed that the plastic hinge of the joint could be concentrated on the replaceable FCPs, so as to protect the beams and columns. By replacing damaged FCPs and bolts, the repaired joint had the same performance as the original joint. After comparison, the theoretical yield load is in good agreement with the experimental yield load. Besides, the small thickness and dog-bone width of FCPs can reduce the load carrying capacity and energy dissipation capacity of joints, and improving the material strength of FCPs will reduce the ductility of joints and increase the risk of plastic damage to the beams and columns. If the amount of FCPs connecting bolts is reduced, the specimens mainly dissipate energy by the slip of FCPs, and the load carrying capacity will descend. Excessive bolt spacing at the middle row will reduce the load carrying capacity and energy dissipation capacity.

Design of higher temperature copper brazing filler metals with reduced brittle phase content

Anneal-resistant copper alloys have been developed to address the need for stronger yet lighter fin and tube parts in heat exchangers, in order to facilitate greater efficiency and increased temperature capability. As such, brazing as opposed to traditional soldering has become the preferred joining method for these materials, due to the enhanced high-temperature properties of such joints. Copper-based filler metals are used extensively for the brazing of these materials, generally based on the copper-phosphorus system with alloying additions. In the present study, the microstructural issues arising from the current commercially available filler metals are discussed and addressed through CALPHAD-supported alloy design. Three selected novel compositions were used in the brazing of pure copper as a test of the design concept. Alloy compositions in weight percent of Cu(bal.)− 7Ni-4Sn-5 P and Cu(bal.)-10Ni-2Sn-6.5 P were found to result in a reduction of brittle Cu3P phase while increasing the fraction (0.78 and 0.77 respectively) of copper-rich solid solution in joints post-braze, comparing favourably to commercial Meta-Braze™ 077 under similar conditions (0.61). Furthermore, the copper-rich solid solution was leaner in tin, predicted to impart an increased melting temperature while microhardness profiles indicate increased joint ductility. The implications for mechanical properties of filler metals in this alloy family are discussed.

Impact of ultrasonic vibration power and tool pin profile on mechanical and microstructural behaviour of friction stir welded dissimilar aluminium alloy joints

PurposeThe purpose of this paper is to investigate the impact of ultrasonic vibration (UV) and tool pin profile on mechanical properties and microstructural behaviour of AA7075-T651 and AA6061-T6 joints was analysed.Design/methodology/approachThe joints were fabricated using three different tool pin profiles such as cylindrical, square and triangle. For each tool pin profile, two different UV powers of 1.5 kW and 2 kW were used.FindingsOn both the advancing and retreating sides of the weld, the thermo-mechanically affected zone has the lowest microhardness. In all joints, the tensile fracture locations match to the minimum hardness values. Field emission scanning electron microscope fractography of tensile tested specimens reveals heterogeneous modes of brittle, shear and ductile fracture. Three-point bending analysis was performed to determine the ductility and soundness of the weld joint. The acoustic softening effect of UV, as well as the static and dynamic ratio of tool pin profile, plays an important role in determining the material flow and mechanical behaviour of the joint.Practical implicationsDissimilar aluminium joining fascinates many applications like aircraft, aerospace, automobiles, ship building and electronics, where fusion welding is a very intricate process because of the deviation in its physical and chemical properties.Originality/valueFrom this study investigation, it is found that the square pin profiled tool with 2 kW UV power produces metallurgical defect-free and mechanically sound weld with maximum tensile strength, hardness and bending load of 297 MPa, 151HV and 3.82 kN, respectively.

Effect of Process Parameters on Weld Quality in Vortex- Friction Stir Welding of 6061-T6 Aluminum Alloy.

Vortex- friction stir welding (VFSW) utilizes a stir bar made of an identical material to the workpiece to rub the workpiece's top surface, which avoids the keyhole defect in conventional friction stir welding. It presents great potential in the repair field of aluminum alloys. In this study, the effect of stir bar diameter, rotation speed, and welding speed on the weld formation was investigated in the VFSW of 6061-T6 aluminum alloy. The weld macrostructure, penetration, and mechanical properties were characterized. The results show that a large diameter of the stir bar can enhance the vortex material flow, increase the heat input, and eliminate the incomplete-penetration defect. The increase in rotation speed within limits can enhance the weld penetration and the mechanical properties of the weld nugget zone (WNZ). However, too high a rotation speed reduces the weld penetration and weakens the mechanical properties of the WNZ. The increase in welding speed reduces the weld penetration but enhances the mechanical properties of the heat affected zone. The incomplete-penetration defect significantly weakens the ductility of the VFSW joint. It can be eliminated by enlarging the stir bar diameter and choosing a moderate rotation speed and a lower welding speed.

Review on the Solid-State Welding of Steels: Diffusion Bonding and Friction Stir Welding Processes

Solid-state welding (SSW) is a relatively new technique, and ongoing research is being performed to fulfill new design demands, deal with contemporary material advancements, and overcome welding defects associated with traditional welding techniques. This work provides an in-depth examination of the advancements in the solid-state welding of steels through diffusion bonding (DB) and friction stir welding (FSW). Considerable attention was given to DB of steel, which overcame the difficulties of segregation, cracking, and distortion stresses that are usually formed in liquid-phase welding techniques. The defects that affected DB included two types: two-dimensional defects of a metallic lattice, i.e., phases and grain boundaries, and three-dimensional defects, i.e., precipitation. FSW, on the other hand, was distinguishable by the use of relatively low heat input when compared to fusion welding processes such as tungsten inert gas (TIG), resulting in the formation of a limited heat-affected zone. Moreover, fine grain structures were formed in the FSW interface because of the stirring tool’s severe plastic deformation, which positively affected the strength, ductility, and toughness of the FSW joints. For instance, higher strength and ductility were reported in joints produced by FSW than in those produced by TIG. Nevertheless, the HAZ width of the specimens welded by FSW was approximately half the value of the HAZ width of the specimens welded by TIG. Some defects associated with FSW related to the diffusion of elements, such as C/Cr atoms, through the weld zone, which affected the local chemical composition due to the formation of rich/depleted regions of the diffused atoms. Moreover, the lack-of-fill defect may exist when inappropriate welding conditions are implemented. On the other hand, the stirring tool was subjected to extensive wear because of the high hardness values, which negatively affected the economical usage of the FSW process. A summary of the results is presented, along with recommendations for future studies aimed at addressing existing difficulties and advancing the solid-state technology for steel.

Finite element analysis to predict the cyclic performance of GFRP-RC exterior joints with diagonal bars

This paper presents a finite element modeling (FEM) approach to predict the cyclic response of glass fiber-reinforced polymer (GFRP) reinforced concrete (RC) exterior beam-column joints with different joint details. Four GFRP-RC exterior beam-column joints detailed with U or L-shaped anchorages at the end of the longitudinal bars of the beam, horizontal stirrups and additional diagonal bars (Z or U-shaped) at the joint region were modeled and analyzed under reversed cyclic loading. The FEM cyclic behavior in terms of load-drift ratio, cracking patterns, joint shear stress and strain evolutions were compared with the experimental behaviors. The FEM predicted load-drift ratio response, cracking patterns and dissipated energy showed good correlations with the experimental results. A parametric study was conducted to evaluate the influence of compressive strength of concrete, column axial load ratio and size of the additional diagonal bar on the performance of the GFRP-RC joints. It was found that an increase in the column axial load significantly degraded the performance in terms of strength, maximum joint shear stress and ductility of the GFRP-RC joints. Also, GFRP-RC joints with additional U-bars were able to resist higher joint shear stress and drift ratio than GFRP-RC joints with additional Z-bars, respectively.

An experimental study of FRP truss side plate joint

Ultra-high performance concrete (UHPC) slab - fiber reinforced polymer (FRP) truss hybrid beam has been proved as an effective and advanced bridge system. However, the limited shear strength and the significantly lower transverse properties compared with its longitudinal counterparts result in low rigidity and load carrying capacity of bolted FRP truss joints, which governs the overall performance of the UHPC slab- FRP truss hybrid beam. A FRP truss side plate joint was proposed which integrates a steel U-shaped gusset plate (SUGP) to reinforce the traditional bolted joint. The proposed joint outperformed the conventional bolted joints in terms of the failure mode, load carrying capacity, and stiffness. The load carrying capacity of the proposed joint can be improved by up to 63% and the ultimate deflection was reduced by up to 33%. The effects of a variety of key design parameters, including adhesive, bolt diameter, pre-torque, and length and thickness of the gusset plate, on the performance of the SUGP joint were evaluated. Test results showed that: (i) the adhesive between the pultruded FRP profiles and the steel gusset plate could increase both the initial stiffness and ultimate load carrying capacity of the steel SUGP joint; (ii) length and thickness of the steel gusset plate did not result in notable difference in the behavior of the steel SUGP joint for the range of parameters investigated herein; and (iii) application of pre-tightening torque within the thread of the bolts slightly improved the load carrying capacity but significantly improved the ductility of the steel SUGP joint.

Strategies for solder joints with high shear strength, slow interfacial consumption, ductile intermetallic compounds using hybrid crystalline-amorphous Co-P coatings

Crystalline and amorphous cobalt-phosphorus (Co-P) coatings have their own advantages in interfacial diffusion resistance and mechanical ductility of solder joints, respectively, but it cannot be better at both. In this work, the hybrid crystalline-amorphous Co-P coating (Co-9.1 at.% P) was fabricated and applied on the Cu/Sn interface by controlling the crystallinity through compositional design. Combining the advantages of crystalline and amorphous Co-P, strategies for solder joints with high shear strength, ductile intermetallic compounds (IMCs) and slow interface consumption was proposed. The evolution, microstructure, phase, and mechanical properties of the Co-Sn, Co-Sn-P and P-rich layers in solid-state diffusion were systematically characterized and tested by means of SEM, EPMA, EBSD, TEM, nanoindentation, and shear test. It was found that the hybrid Co-P coating achieved a low consumption rate, 14.2 nm/h, attributing to the partially crystalline structure and the Co-Sn-P exfoliation behavior. In addition, solder joints containing large-thickness ductile Co-Sn IMCs, 3.2–3.6 GPa of hardness, was established. The shear strength after aging reached 75.4 MPa relying on the large thickness of Co-Sn IMCs. Severe mechanical loads would be able to be withstand by virtue of the plastic deformation of ductile IMCs. Hybrid crystalline-amorphous Co-P would be a promising interface material in microelectronic packaging.

Retrofit of exterior reinforced concrete beam-column joint using pre-tensioned stiffened steel angles

This paper presents an innovative retrofit technique to strengthen the non-seismically designed reinforced concrete beam-column joint. For this purpose, five half-scale exterior beam-column joints were prepared and tested. Four of the joints were strengthened with stiffened steel angles under various pre-tensioning levels and stiffened steel angle sizes. The columns were loaded under constant axial compression, and the beams were loaded under quasi-static cyclic load. The experiment test results showed that this retrofit technique can relocate the plastic hinge from the critical joints toward beams. This improved the sudden joint shear failure mode to a ductile beam failure mode. Besides, this retrofit technique improved the seismic behaviours, including load-carrying capacity, ductility, energy dissipation, and stiffness of the beam-column joints. This paper showed that the pre-tensioning level affects the bond behaviour between the reinforcement steel and the concrete, whereas stiffened steel angle size affects the failure mode and seismic behaviours.

Nonlinear Numerical Analysis and Restoring force Model of Composite Joints with Steel Reinforced Recycled Concrete Columns and Steel Beams

Cyclic loading tests were conducted for eight composite joints (five interior joints and three exterior joints) with steel reinforced recycled concrete (SRRC) columns and steel beams under cyclic loading. The axial compression ratio and recycled coarse aggregate (RCA) replacement percentage were considered as the main design parameters for the above composite joints. The seismic behavior of the composite frame joints with (SRRC) columns and steel beams were studied by numerical simulation and the constitutive models of recycled aggregate concrete (RAC), steel and reinforcement were given to the corresponding elements. The experimental results were used to validate the finite element (FE) model. The results indicated that the FE model can accurately predict the deformation and stress load-strain curves of the composite joints. Then, a FE parameter analysis was carried out on the seismic behavior of the composite joints. The RAC strength, steel strength and column steel web thickness were considered as the main parameters. Results pinpointed that the increase in RAC strength and steel web thickness of the columns significantly affected the horizontal bearing capacity of the composite joints. However, with the increase in RAC and steel strength, the deformation ability and ductility of composite joints were almost unaffected. The reasonable increase in column steel web thickness can improve the seismic ductility of composite joints, evidently. On this basis, considering the stress characteristics of joint specimens, a four broken line restoring force model for steel reinforced recycled concrete column steel beam composite frame joints was established. This model includes the skeleton curve model, the stiffness degradation pattern and hysteresis rules. The calculation curve was in good agreement with experimental curves, which can well reflect the hysteretic behavior of joints subjected to low cyclic repeated loading. The above results can be used as a reference for the seismic design of composite frame joints.

Shear behaviour of a novel vertical grouting joint in precast concrete structure for wind turbines

Precast concrete structure (PCS) towers have been widely used as support structures for onshore wind turbines owing to their high stiffness, good stability, and low maintenance cost. Owing to the limitations of production, transportation and construction, the PCS towers must be vertically segmented, and the vertical segments of precast concrete are mainly connected by filling the joints with high-strength grouting materials (HSGMs). The shear performance of the vertical grouting joints significantly affects the mechanical behaviour of PCS towers for wind turbines. To study the shear performance of the vertical grouting joints of PCS towers, six vertical grouting joint specimens were designed and tested under shear loads, and the failure modes and shear force–slip curves of the specimens were examined. The experimental results indicated that the failure modes of the specimens involved the interface peeling failure of the new and old concrete and the shear failure of the HSGM. U-shaped shear reinforcement can increase the ultimate and residual shear capacities of joints and significantly improve the ductility of vertical grouting joints. Vertical reinforcement has a limited effect on the ultimate shear capacities of vertical grouting joints but can limit the development of internal cracks and improve the residual shear capacities of joints. According to the experimental results and force–slip curves of the specimens, the shear force mechanism of the joints was analysed, and a three-stage shear force model of the joints was proposed. Moreover, the distribution of shear strain on the concrete surface along the height of the specimens under different load levels was analysed. Finally, the shear capacities of all the vertical grouting joint specimens were calculated using the existing design methods, and correspongding design suggestions were proposed according to the calculation results.

Experimental Study on Seismic Behavior of Precast Bolt-Connected Steel-Members End-Embedded Concrete (PBSEC) Beam-Column Connections

With the rapid development and research of precast concrete frame structures, it is not difficult to find that the structural form and seismic performance of dry-connected precast joints have always been the focus of research. Since this type of structural system is complex, the construction is inconvenient in practical application, and many additional parts need to be installed, this paper develops a kind of precast bolt-connected steel-members end-embedded concrete (PBSEC) beam-column connection to solve the shortcomings of the current dry-connected precast joints. There is no wet work in the assembly process, and all-dry construction and assembly methods are used. There is no need to pour concrete and support formwork, which significantly improves construction efficiency compared to wet and cast-in-situ connections. Low cyclic reversed loading tests were conducted to obtain test data, such as failure mode, hysteresis curve, skeleton curve, stiffness, ductility, and deformation capacity of the precast concrete joint. The failure mode of the PBSEC joints is the buckling failure of the connecting steel plate, leading to a perfect seismic capacity and collapse resistance of the structure. The hysteresis curves of the PBSEC joints are bow-shaped and full in shape, showing high energy dissipation capacity. The bearing capacity of the joints begins to rise rapidly at the initial loading stage and then decreases slowly after reaching the peak, which is an ideal shape. By summarizing the average peak load, strength degradation coefficient, loop energy per cycle, loop energy per level, and cumulative energy damping coefficient, it is found that the joint using 10 mm thick Q235 steel can obtain the most suitable failure mode and obtain the best energy dissipation performance. When the strength of the steel plate material increases, the energy dissipation performance of the joint drops.

Experimental Investigation of Exterior Reinforced Concrete Beam-Column Connections Subjected to Reversal Loading

Eight full-scale reinforced concrete beam-column subassemblages, 4 for each non-ductile and ductile detail, were tested under quasi-static cyclic loading. All test specimens were mainly designed for gravity service loads. Four specimens named as non-ductile connections were constructed with typical reinforcement details using in Thailand. The others were assembled with details described by the seismic design building code of Thailand, known as the 1301/1302-61 code. Their performance was examined in terms of lateral load resistance, ductility index, stiffness, energy dissipation, failure modes, including joint shear strength. The test results indicated that the specimens with ductile details achieved better seismic performance in every aspect, particularly specimens with adequate joint shear strength designed according to the SST method. The ductile details have no effect to retard the stiffness degradation in the elastic range, but helped to slow the rates of stiffness degradation after the specimens reached their peak loads. The transverse reinforcement in the joints was more efficient to enhance the ductility of joints with lower joint shear strengths (J1 and J2) than the sufficient joints (J3 and J4). Based on the SST method, the connections with low shear strength capacity, J1, J1D, J2, J2D, J3, and J4 suffered with severe joint shear failure (JS), although their beam sections reached the flexural moments. On the other hand, specimens with high joint shear strengths, J3D and J4D, experienced the beam failure (BF) with mild joint shear failure (JS). Test data also demonstrated that a specimen with lesser amount of joint reinforcement exhibited satisfactory seismic behavior, as long as the joint was provided with the adequate shear strength designed by the SST method. Finally, the ACI 318 Building Code overestimated the shear strength of exterior beam-column joints. Moreover, the simplified SST model much better predicted the demand joint shear strength than the ACI 318 Code.
 HIGHLIGHTS
 
 Numerous surveys of RC buildings after earthquakes also confirmed the inadequate detailing of beam-column joints such as presence of splices, lack of hoops, deficiency of beam bar anchorage, and no steel joint stirrups in the joint
 The ductile details have no effect to retard the stiffness degradation in the elastic range, but helped to slow the rates of stiffness degradation after the specimens reached their peak loads. The transverse reinforcement in the joints was more efficient to enhance the ductility of joints with lower joint shear strengths (J1 and J2) than the sufficient joints (J3 and J4)
 The ACI 318 Building Code overestimated the shear strength of exterior beam-column joints. Moreover, the simplified SST model much better predicted the demand joint shear strength than the ACI 318 Code
 Based on the SST method, the beam-column connections with low shear strength capacity, J1, J1D, J2, J2D, J3 and J4 suffered with severe joint shear failure (JS), although their beam sections reached the flexural moments. On the other hand, specimens with high joint shear strengths, J3D and J4D, experienced the beam failure (BF) with mild joint shear failure (JS)
 The deterioration of beam-column joints under seismic could be effectively restrained by the ductile details according to 1301/1302-61 code with adequate shear strength capacity determined by using the equation of SST model
 
 GRAPHICAL ABSTRACT

Friction stir welding of tube-to-tubesheets for thermoplastic shell and tube heat exchangers

Recently, there is a real demand for thermoplastic shell-and-tube heat exchangers because of their antifouling and chemically inert properties. A novel non-conventional joining technique is developed for creating efficient thermoplastic tube-to-tubesheet joints (TTJs) using the friction stir welding (FSW) process. The carbon black reinforced high-density polyethylene sheet and tube materials are used. A highly ductile joint with a maximum strength of 517 N (42 % greater than adhesives), and a maximum leak path of 3 mm (77 % of remaining tubesheet thickness) was achieved using FSW. The proposed technique was found promising and reliable for developing TTJs for thermoplastic shell-and-tube heat exchangers.

Connection behaviour of through reinforced concrete beam to concrete-filled steel tube column after exposed to heating

The experimental study presented in this paper investigate the post-fire performance of through reinforced concrete (RC) beam to concrete-filled steel tube (CFST) column connection. A total of twelve connection specimens were tested under hogging moment. The connection specimens are grouped into two groups; group A-connections, without fire exposure and group F-connections that are exposed to fire. Two classical connection types were used in this study (type A and B) and one new connection type were proposed herein (type C) by associating additional steel plates to the connection zone. In addition, different beam reinforcement ratios were used. These parameters were considered to evaluate the strength, stiffness, joint classification, ductility, failure modes and strain response of the composite joints. Three failure modes of the connections were found. The connection specimens without fire exposure were found to be a rigid connection. The fire exposure significantly influenced the connection performance, and that influence is varied based on the connection type and beam reinforcement ratio. The proposed connection type is found to have the highest residual capacity among the other connection types. However, a comprehensive analysis of the post-fire behaviour of each connection type is illustrated.

Investigating the multiscale effects of a novel post–weld composite processing treatment on minimizing weld softening in AA6061-T6 Al–alloy

The fusion welding of heat-treatable AA6061-T6 aluminum alloy results in significant joint softening which results in the ultimate tensile strength (UTS) and elongation (El) of the welded joint only reaching up to 60–70% and 40–50% of the base metal (BM), respectively. A post–weld composite processing treatment (PWCPT) that uses solution treatment (ST), artificial aging treatment (AT) and cold rolling (CR) has been proposed in this study to mitigate this issue. The effect of different composite process sequences and parameters on the microstructural characteristics and mechanical properties of the joints were investigated. The results show that the formation of the fine-grained microstructure in the fusion zone (FZ) and abnormal sandwich microstructure in the heat-affected zone (HAZ) was highly dependent on the dislocation density and fraction of the insoluble phases. By clarifying the strain characteristics of the joints, a strategy was developed to optimize the homogeneity of the microhardness distribution which improves joint ductility. The optimal PWCPT involved the ST and AT for the AA6061-T6 joint that was produced by tungsten inert gas (TIG) welding with filler wire, with subsequent cold rolling of the weld reinforcement to achieve a rolling reduction of 1.2 mm. The PWCPT could completely eliminate joint softening and improved the joint UTS and El to 100% and 88.6% of the BM, respectively. By systematically expounding the evolution law of microstructure and mechanical properties of the welded joint under the coupling of thermal and mechanical, the strengthening mechanism of the PWCPT is revealed.

Prediction of electron beam weld quality from weld bead surface using clustering and support vector regression

Destructive manual experiments are primarily used for quality assessment of electron beam welded components, which consume the resources and time significantly. Vision-based and other NDT-based quality monitoring in EBW is rarely reported, and consequently, more investigation is required. An inexpensive quality evaluation technique for the welded joints has been proposed using the weld bead surface attributes. These attributes have a good correlation with quality indicators like aspect ratio and mechanical properties of the joint. Here, electron beam welding of CuCrZr alloy was conducted to get varying weld profiles, and the surface weld attributes were measured. The model correlating top and bottom bead widths with the process variables was established using support vector regression. The predicted unlabelled weld attributes were employed as inputs for the unsupervised clustering algorithms, namely fuzzy C-mean clustering and density-based clustering. The clustering algorithms provided different clusters of the joints, namely poor, fair and good. FCM was found to be more precise for the clustering of welding data. Poor category of the joints was represented by that with partial penetration and lower aspect ratio, and the corresponding process variables’ combination could be avoided in the next iteration through the feedback. The fair category represented the joints with a high aspect ratio and keyhole-based profiles with moderate strength and ductility. The good category signified the weld joints having the maximum joint strength and ductility, and an acceptable aspect ratio with the conduction mode of welding. Keyhole-based weld profile could be obtained using high beam power (5.5 kW to 6.3 kW) with high welding speed (1000 mm/min and above). A combination of 0.1 mm oscillation amplitude and 900 Hz oscillation frequency was found to be the most favourable one, in this study.

Effect of pulse mode and frequency on microstructure and properties of 2219 aluminum alloy by ultrahigh-frequency pulse Metal-Inert Gas Welding

Based on self-developed ultra high frequency pulse Metal-Inert Gas Welding (UFP-MIG) power source, the ultra-high frequency pulse (UFP) current was studied as an effective way for improving the microstructure and properties of 2219 aluminum alloy joints. The welding pores reduction performance, microstructures, microhardness, and tensile properties of conventional pulsed MIG(CP-MIG) welds and the UFP-MIG welds with different modes and frequencies were investigated. The results indicated that the UFP-MIG process could almost completely eliminate welding porosity in the joints of 2219 aluminum alloy. Compared with CP-MIG welding process, the grain size of the UFP-MIG welding zone was reduced and gradually decreased as the frequency of UFP increased. Similarly, the grain refinement effect also appeared in the heat affected zone (HAZ) near the welds, and the grain size could be reduced by up to 14.85%. Meanwhile, the morphology and uniformity of θ phases and α+θ eutectics along grain boundaries of UFP-MIG welds were also improved. The microhardness in the weld zone and partially melted zone (PMZ) of UFP-MIG weld joints with different modes and frequencies were significantly improved. The highest joint average microhardness occurred at peak pulse mode (PP) with 40 kHz(75.17HV), which was 27.9% higher than that of CP-MIG (58.74HV). The strength and ductility of welded joints were simultaneously improved by UFP-MIG weld process. Fracture analysis results showed that the large reduction of weld pores, the refinement of weld grains, and the lower segregation of Cu element in the eutectic were responsible for the joint tensile properties improvement.

Study on microstructure evolution and mechanical properties of high-strength low-alloy steel welds realized by flash butt welding thermomechanical simulation

Defects would occur in the weld joint of the wheel rims during the post-flash butt welding (FBW) process suffering from poor plasticity, which will deteriorate the quality and lifecycle of finish products. Therefore, the FBW process of the 440CL high-strength-low-alloy (HSLA) steel was physically simulated and the influence of flash parameters on FBW joints was systematically evaluated in this study. The results showed that the width of heat affected zone increased with accumulated flash allowance (δf) while declined with accelerated flash speed (vf). The recrystallization level would be intensified with increased δf. Meanwhile, the acceleration in vf populated the WZ with a more homogeneous microstructure, higher recrystallization degree and lower dislocation density. The hardness in WZ slightly reduced (202 → 195 HV) as increased δf but obviously dropped (192 → 177 HV) as increased vf. All tensile samples were fractured at the BM location and the tensile properties of FBW joints exhibit a good match with those of BM, with a slight increase in strength (UTS: 468 ~ 493 MPa; YS: 370 ~ 403 MPa) but a mild decrease in plasticity (EL: 39 ~ 44%; RA: 74 ~ 79%). Furthermore, both the joint strength and ductility showed a downward tendency with the increment of δf. However, the strength slightly decreased while the ductility increased with the advancement of vf. These findings would be valuably referential to the real FBW of HSLA steels with optimized microstructure and mechanical performance.

Microstructure, mechanical and corrosion properties of differently cooled friction stir welded joints of Al and Mg alloys

Current analysis emphasizes on microstructural, mechanical, and corrosion behaviour of friction stir welding (FSW) joints of AA6061-T6 Al alloy and AZ31B Mg alloy using various cooling mechanisms i.e., air cooling, forced water cooling, and underwater cooling. The air-cooled, forced-water cooled, and underwater cooled FSW are abbreviated as Air-FSW, FCFSW, and UWFSW respectively. The x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray (EDS) analysis established the formation of intermetallic compounds (IMCs) layers of varying thickness with composition of Al12Mg17 and Al3Mg2 at the bonding interaction of the joints. The surface appearance of Air-FSW joints was observed rougher and darker due to generation of more heat while the appearance of surface of UWFSW joints was smoother and brighter because the generated frictional heat was taken away by the water. Due to reduction of the heat and so the temperature in FCFSW and UWFSW joints, the development of IMCs got reduced, resulting in improved weld strength and joint efficiency. Occurrence of weld crack in the weld stir region was reduced in FCFSW joint as compared to Air-FSW joint. Whereas, very few or no cracks were observed in weld stir region of UWFSW joints. Maximum joint’s strength of Air-FSW joints was observed 120.74 MPa, which increased to 151.56 MPa and 182.7 MPa with FCFSW and UWFSW respectively. Air-FSW joint exhibited a brittle fracture mode but in water medium, their fracture path was shifted to ductile mode which demonstrated the formation of enhanced ductility of the UWFSW joints. The corrosion resistance got improved when the joints were prepared in underwater medium.