Tensile Joint Research Articles

Tensile strength and failure of dissimilar friction stir welded joints between 6061-T6 and 2014-T6 aluminum alloys

The present study is attributed towards better tensile and impact properties of friction stir welded joints between AA6061-T6 and AA2014-T6 alloys with minimum or no void formation. Two different tool pin profiles, threaded with three intermittent flat faces (TIF) and truncated square pyramidal shaped (TPZ) tool pins were selected. Dissimilar joining was carried out using different combinations of tool rotation (660, 900, 1200 rpm) and welding speeds (36, 63, 98 mm/min). Results revealed that for TIF tool pin, the maximum joint tensile strength was achieved at tool rotational speed of 1200 rpm with welding speed of 98 mm/min. On contrary, the maximum impact energy was recorded for joints prepared with TPZ tool pin at 900 rpm tool rotation and 63 mm/min welding speed. Detailed metallurgical characterization of different weld zones and fractographic studies were also performed and discussed.

Analytical study on mechanical behavior of bolted tensile joint applied to cast iron deck module's connection

Analytical study on mechanical behavior of bolted tensile joint applied to cast iron deck module's connection

Joint strength and failure studies of dissimilar AA6061-AA7075 friction stir welds: Effects of tool pin, process parameters and preheating

Abstract The present study aims at bracketing the effect of tool pin geometry, process parameters e.g. tool rotational speed, feed rate, and use of additional heat source (preheating) during dissimilar friction stir welding (FSW) between 6061-T6 and 7075-T651 aluminum alloys. Experimental designs following Taguchi orthogonal array and subsequent statistical analysis are carried out to assimilate the influence on joint tensile strength and flexural load. Detailed micrographic observations of the weld cross-section, joint failure studies are performed in addition to the fractographic analysis. Tool pin profiles and tool rotational speed are observed most prevailing factors and when suitable tool pin profiles are used, the tool rotational speed and feed rate are observed most significant. For the dissimilar FSW joints where a sound weld is obtained using suitable tool pin and process parameter combinations, the tensile failure of the joints observed from the base metal of the advancing side (AA6061-T6). On the flipside, failure is observed from the side of the stir zone when material flow, mixing and joining is inadequate. During flexural loading, initiation of failure is mostly observed from the vicinity of the void/tunnel region of the weld nugget. Study of fracture surfaces using scanning electron microscope shows the presence of numerous dimples in the sound bonded joints ensuring ductile failure of the joints under tensile loading. However tearing, rupture and brittle failure is observed for poorly bonded joints. Energy dispersive spectroscopy of fracture surface also shows the presence of oxides in aluminum matrix for poorly bonded joints resulting in brittle failure of the joints.

Shear and tensile joint strengths of carbon fiber-reinforced thermoplastics using ultrasonic welding

The shear and tensile strengths of carbon fiber-reinforced thermoplastic (CFRTP) laminate joints by ultrasonic welding were investigated by lap shear tests (LSTs) and cross tensile tests (CTTs). The adherends were the cross-ply and the twill woven CFRTP laminates, and the flat energy director (FED) was used to clarify its effectiveness. A modified jig and testing method for CTTs were newly proposed. In each test, the strengths were calculated in two ways: by indicators of “welding efficiency” and “welding quality.” From the experimental results, the strengths of the cross-ply laminate joints were saturated at high welding energy, and the “welding quality” was improved by FED. On the other hand, higher strengths of the twill woven laminate joints were obtained as the welding energy increased, and the effectiveness of FED was confirmed mainly for the “welding efficiency.” The fracture surfaces were then observed to clarify the fracture mechanisms of the joints.

Microstructure Evolution and Properties of Friction Stir Welding Joint for 6082-T6 Aluminum Alloy

To investigate the evolution of grain morphologies, grain boundary features, and dislocation structure in upper part of nugget zone during friction stir welding of 6082-T6 aluminum alloys, electron backscattered diffraction and transmission electron microscope techniques were employed. The results showed that the metal on retreating side of the weld zone produced thermoplastic deformation during the welding process. The substructure of the lamellar dislocation wall formed at retreating side. Motivated by the welding thermal cycle, geometric dynamic recrystallization occurred in the substructure of the lamellar grains, leading to the lamellar array of grains. Discontinuous dynamic recrystallization occurred in the lamellar grain, leading the grain to be arranged in a necklace form. Simultaneously, the coarse grain of base material was refined. Due to the inhomogenous distribution of stress during the welding process, the alternating presence of large grain and small grain regions on advancing side took place. During the process of welding joint tensile, the fracture results from dislocation piling-up at the Mg2Si interface in large grains with a (111)[110] orientation. Thus, the advancing side becomes the weakest part of the mechanical properties in the welding joint,the tensile strength of the joint was 275MPa with a break elongation of 6%.

The influence of process parameters in friction stir welding of Al-Mg alloy and polycarbonate

The Role of rotating and travelling speed of the friction stir welding (FSW) tool on the dissimilar lap-joining of an AA5058 aluminum alloy and polycarbonate (PC) sheets has been investigated. The Relation of the processing parameters with tensile, bending strength, material flow and hardness of the joints were studied to attain defect-free joints with proper mechanical strength. The results showed both the size and the contribution of PC and aluminum alloy on the formation of stir zone are strongly related to the tool's rotational speed. Tool's rotational speed mainly controls the joint strength and the fracture mode, while traverse velocity affects the size of aluminum fragments within the PC matrix. The most suitable dissimilar lap-joining microstructure is achieved at 1600 rpm and 45 mm/min tool speed. Under this condition, the maximum joint tensile and bending strengths attained are 68.2% (46 MPa) and 69.4% (60 MPa) of PC raw material respectively. Fractography indicated that during the tensile test, the rupture occurs below and in the middle of the stir zone on AA5058 alloy/PC interface during the bending test. The hardness of the PC after FSW decreased by the molecular weight reduction and AA5058 micro-hardness increased because of the fine grinding after suffering thermo-mechanical cycle.

Fatigue Properties of Ti-6Al-4V Titanium Alloy Friction Stir Welding Joint

Friction stir welding was used to consolidate 2-mm-thick Ti-6Al-4V titanium alloy sheets, and the static and fatigue properties of the joint were investigated. The results reveal that uniform microhardness distribution of the nugget zone (NZ) after post-weld heat treatment was associated with the uniform equiaxed microstructure of the NZ. The joint fatigue limit predicted by the S–N curve reached 640.84 MPa, up to 64.76% of joint tensile strength. Fatigue specimens fractured at the transition zone between the base material and NZ, and the positions of fatigue crack initiation sites were relatively random. The fracture surface of the fatigue crack propagation region was composed of radial strips and striations, and numerous dimples were observed on the final fast fracture surface.

Microstructure and Mechanical Properties of Rapidly Cooled Friction Stir Welded Ti-6Al-4V Alloys

Two mm-thick Ti-6Al-4V alloy plates were friction stir butt welded under natural- and rapid-cooling conditions. The microstructures and mechanical properties of joints were investigated and compared. The rapid cooling via spraying cooling liquid nitrogen on the plate top surface was beneficial to obtaining a narrower cross section and finer and more uniform grains in the stir zone than those obtained by natural cooling. Joint tensile strength of 1021 MPa and elongation of 1.88% under the rapid-cooling condition respectively reached 102.7 and 150.4% of those under the natural-cooling condition, respectively. The uniform-and-compact equiaxed grains led to equiaxed dimples on the fracture surface, which demonstrated a ductile fracture of the welding joints.

Numerical simulation and experimental investigation on electron beam welding of Ti-22Al-25Nb alloy with electron beam oscillation

Ti-22Al-25Nb (at%) alloy with the thickness of 3.5 mm is welded by using electron beam welding (EBW) based on electron beam oscillation. Numerical simulation analysis of EBW process, and microstructure and mechanical properties of welded joint are systematically investigated. Microstructure analysis shows that base metal consists of phases α2, B2 and O. The fusion zone (FZ) of joint in as-welded condition consists of single phase B2. During EBW process, the transformation from phase B2 to phase α2 or phase O is restrained in FZ due to the rapid cooling rate. Results of numerical simulation demonstrate the thermal change in EBW process and verify the microstructural evolution of weldment. Electron beam oscillation can stir the molten pool and stabilize the keyhole, thus the mechanical properties of welded joint are greatly improved. Under the experimental condition, the ratio of joint tensile strength varies from 85.2% to 91.9% and the failed locations of joints are in FZ or heat-affected zone (HAZ). Tensile fracture morphology of joint presents the pattern of quasi-cleavage fracture.

Investigations on joining of stainless steel tailored blanks by µ-PTA process

ABSTRACTPresent work is focused on the autogenous joining of tailored blanks, that is, sheets of different thickness but of the same material (austenitic stainless steel grade SS 316L). A thin sheet (1mm) and a thick sheet (3 mm) were joined autogenously using Indigenously developed micro-plasma transferred arc (µ-PTA) process. Effects of current, torch travel speed and stand of distance (SOD) were investigated to determine their optimum levels of joint tensile strength. The produced joint was characterized using different methods such as optical microscopy, scanning electron microscopy, uniaxial tensile tests, Vicker’s microhardness, and energy dispersive X-ray analysis. It was found that (19.4 A, 63 mm/min, 1.3 mm), (19.6 A,80 mm/min, 1.3 mm) and (19.8 A, 100 mm/min, 1.3 mm) with values of plasma flow rate at 0.4 Nl/min and shielding gas flow rate at 5 Nl/min, yielded sound weld Joint. Absence of heat affected zone was confirmed by very small variation in Vickers microhardness values across the fusion zone. This study establishes the capability of microplasma transferred arc process for joining stainless sheets of different thickness without any filler material and producing very good quality joint using much less power as compared to conventional fusion joining processes.

Microstructure and mechanical performance of ultrasonic spot welded open-cell Cu foam/Al joint

Open-cell copper (Cu) foam has comprehensive application prospects in the thin-film electrode production of high-capacity batteries. However, the lack of high-quality welding techniques for open-cell Cu foam has thus far limited its further application. High porosity open-cell Cu foam and 1A99 aluminum (Al) sheet have been reliably joined utilizing ultrasonic spot welding under very low welding-energy conditions. The real-time temperature, three-dimensional surface morphology, tensile strength, weld interface morphology and composition, fracture morphology, and electrochemical performance of the joints were systematically evaluated. The structure and properties of the foam copper were preserved to the greatest extent, and the optimum joint tensile strength was about 89% of the Cu foam base metal. All the joints presented good toughness characteristics and favorable corrosion-resistance quality, fractures occurred at the edge of the weld spot on the Cu foam side. The peak temperature of the welding interface was much lower than the Al-Cu solid-solution temperature. No intermetallic compounds were found at the weld interface. The open-cell Cu foam and solid Al sheet were joined through atomic diffusion and mechanical locking. USW have been demonstrated to be a feasible method for reliably joining open-cell Cu foam and Al sheet.

Microstructure and Mechanical Properties of Thixowelded AISI D2 Tool Steel

Rigid perpetual joining of materials is one of the main demands in most of the manufacturing and assembling industries. AISI D2 cold work tool steels is commonly known as non-weldable metal that a high quality joint of this kind of material can be hardly achieved and almost impossible by conventional welding. In this study, a novel thixowelding technology was proposed for joining of AISI D2 tool steel. The effect of joining temperature, holding time and post-weld heat treatment on microstructural features and mechanical properties were also investigated. Acceptable joints without defect were achieved through the welding temperature of 1300 °C, while the welding at lower temperature resulted in a series of cracks across the entire joint that led to spontaneous fracture after joining. Tensile test results showed that maximum joint tensile strength of 271 MPa was achieved at 1300 °C and 10 min holding time, which was 35% of that of D2 base metal. Meanwhile, tensile strength of the joined parts after heat treatment showed a significant improvement over the non-heat treated condition with 560 MPa, i.e., about 70% of that of the strength value of the D2 base metal. This improvement in the tensile strength attributed to the dissolution of some amounts of eutectic chromium carbides and changes in the microstructure of the matrix. The joints are fractured at the diffusion zone, and the fracture exhibits a typical brittle characteristic. The present study successfully confirmed that by avoiding dendritic microstructure, as often resulted from the fusion welding, high joining quality components obtained in the semi-solid state. These results can be obtained without complex or additional apparatuses that are used in traditional joining process.

Effect of wire electrode and shielding gas compositions on the mechanical properties of DOMEX 700 steel welded by the GMAW-P process

High-strength steels such as DOMEX 700 combine high mechanical strength and great ductility. However, when processed by welding their microstructure could present grain growth and deleterious phase formation. Nevertheless, using high-power GMAW-P the effects of electrode and shielding gas composition on the mechanical and microstructure properties of DOMEX 700 welded joints need to be understood. Thus, wire electrodes such as AWS ER 90S-D2 (A1) and AWS ER 120S-G (A2), and shielding gases such as Ar + 15% CO2 (G1) and Ar + 8% CO2 (G2) were used and the microstructure of welded joints was analyzed through optical and scanning electron microscope (SEM). Mechanical properties of joints was characterized through joint tensile test, impact test from 20 to − 40 °C, and microhardness in the joint cross-sectional. It is possible to highlight the increase in strength and elongation values with the use of electrode A2, and reduction in impact energy values for specimens welded by gas mixture G2.

Improving Joint Formation and Tensile Properties of Dissimilar Friction Stir Welding of Aluminum and Magnesium Alloys by Solving the Pin Adhesion Problem

Friction stir welding (FSW), as a solid-state welding technology invented by TWI in 1991, has potential to join dissimilar Al/Mg alloys. In this study, the pin adhesion phenomenon affecting joint quality during FSW of 6061-T6 aluminum and AZ31B magnesium alloys was investigated. The adhesion phenomenon induced by higher heat input easily transformed the tapered-and-screwed pin into a tapered pin, which greatly reduced the tool’s ability to drive the plasticized materials and further deteriorated joint formation. Under the condition without the pin adhesion, the complex intercalated interlayer at the bottom of stir zone was beneficial to mechanical interlocking of Al/Mg alloys, improving tensile properties. However, the formation of intermetallic compounds was still the main reason of the joint fracture, significantly deteriorating tensile properties. Under the welding speed of 60 mm/min without the pin adhesion phenomenon, the maximum tensile strength of 107 MPa and elongation of 1.2% were achieved.

Friction Stir Welding of Al-B4C Composite Fabricated by Accumulative Roll Bonding: Evaluation of Microstructure and Mechanical Behavior

In this investigation, friction stir welding (FSW) of Al-B4C composite fabricated by 10 cycles accumulative roll bonding was conducted. In order to investigate the influences of pin geometry on microstructure and mechanical properties, four different pin geometries (cylindrical, square, triangular and hexagonal) were selected. It was found that FSW parameters had a major effect on the fragmentation and distribution of reinforcement particles in stir zone. When the tool travel speed was increased, the distribution of B4C particles was become gradually uniform in the aluminum matrix. The effect of tool rotational speed on the peak temperature was determined to be greater than the tool travel speed. The attained data of tensile properties and microhardness tests showed that the tool travel speed had bilateral effect on the tensile strength. The maximum tensile joint efficiency was obtained as 238% for FSWed of Al-2%B4C composite to annealed base Al sheet.

Study on the effect of stiffener and bolt position on joint strength of high strength bolted tensile joint by FEM analysis

Study on the effect of stiffener and bolt position on joint strength of high strength bolted tensile joint by FEM analysis

Investigation of tungsten/steel diffusion bonding with Ni–Fe cladding on tungsten substrate

In this work, a diffusion bonding approach, for the joining of W and steel using the Ni–Fe cladding layer, was developed for nuclear component application. First, a 10μm thick Ni–Fe alloy cladding layer was deposited on the pure W substrate through high temperature cladding technology. Then the diffusion bonding was carried out in vacuum at 1050°C for 1h with 10MPa to join the cladded W and steel. The bonded specimens were characterized a typical W–Ni–Fe alloy with continuous element distribution at the joining interface, which strengthens the bonding interface. Joint tensile strength at different testing temperatures was investigated and results showed the fracture all occur in tungsten side near the joining interface due to residual stresses concentration. The hardness distribution across the joining interface was also determined.

Stress relief and purification mechanisms for grain boundaries of electron beam welded TZM alloy joint with zirconium addition

The microstructure and mechanical properties of molybdenum-titanium-zirconium alloy joints papered by electron beam welding with and without zirconium filler metal were evaluated. The distribution of precipitates was identified by transmission electron microscope. The stress concentration was confirmed by electron backscatter diffraction analysis and stress state was further confirmed by nanohardness test. In the weld zone of the joint without zirconium addition, Mo/Ti-dioxides were prone to segregate at grain boundaries of the weld zone, resulting in tensile stress concentration at grain boundaries. When zirconium was introduced into the weld metal, grain boundaries with segregation of Mo/Ti-dioxides would be purified by the preferential formation of ZrO2 in the grain interior in the weld zone. Tensile stress concentration was eliminated by purification of grain boundaries, leading to the enhancement of joint tensile strength. The joint welded with zirconium interlayer was fractured in a fully transgranular cleavage fracture mode, instead of a mixed fracture mode featured by the intergranular and transgranular cleavage fracture of the joint welded without zirconium addition.

Effect of welding speed on microstructural evolution and mechanical properties of laser welded-brazed Al/brass dissimilar joints

Laser welding-brazing process was developed for joining 5052 aluminum alloy and H62 brass in butt configuration with Zn-15%Al filler. Effect of welding speed on microstructural characteristics and mechanical properties of joints were investigated. Acceptable joints without obvious defect were obtained with the welding speed of 0.5–0.6m/min, while lower and higher welding speed caused excessive back reinforcement and cracking, respectively. Three reaction layers were observed at welding speed of 0.3m/min, which were Al4.2Cu3.2Zn0.7 (τ′)/Al4Cu9/CuZn from weld seam side to brass side; while at welding speed of 0.4–0.6m/min, two layers Al4.2Cu3.2Zn0.7 and CuZn formed. The thickness of interfacial reaction layers increased with the decrease of welding speed, but varied little at different interfacial positions from top to bottom in one joint. Tensile test results indicated that the maximum joint tensile strength of 128MPa was obtained at 0.5m/min, which was 55.7% of that of Al base metal. All the joints fractured along the weld seam/brass interface. Some differences were found regarding fracture locations with three and two reaction layers. The joint fractured between Al4Cu9 and τ′ IMC layer when the interface had three layers, while the crack occurred between CuZn and τ′ phase in the case of two layers.

Tensile resistances of L-stubs

L-stubs are commonly used to model the tensile parts of ring flange bolted connections. Contact between opposite flanges has a significant effect on the elastic and plastic behaviour of these tensile joints. In the present paper, the effects of contact between flange plates are modelled via a refined beam model which has been previously applied with success to represent the elastic behaviour of non-preloaded L-stubs and T-stubs. Taking into account the behaviour of the flange in the contact area and bolt bending, a model is developed to predict the plastic resistance of L-stub joints. Simplifications are proposed to facilitate the determination of the contact zone and the ultimate resistances corresponding to failure modes involving flange and bolt. The comparison between analytical results with 3D finite element elasto-plastic analyses and experimental tests indicates that the proposed model provides good capacity predictions. Effects of contact, and therefore of the prying force, are particularly important in the case of relatively thin flanges.