Hook Defect Research Articles

External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants.

Tissue folding is a central building block of plant and animal morphogenesis. In dicotyledonous plants, hypocotyl folds to form hooks after seedling germination that protects their aerial stem cell niche during emergence from soil. Auxin response factors and auxin transport are reported to play a key role in this process. Here, we show that the microtubule-severing enzyme katanin contributes to hook formation. However, by exposing hypocotyls to external mechanical cues mimicking the natural soil environment, we reveal that auxin response factors ARF7/ARF19, auxin influx carriers, and katanin are dispensable for apical hook formation, indicating that these factors primarily play the role of catalyzers of tissue bending in the absence of external mechanical cues. Instead, our results reveal the key roles of the non-canonical TMK-mediated auxin pathway, PIN efflux carriers, and cellulose microfibrils as components of the core pathway behind hook formation in the presence or absence of external mechanical cues.

Prediction and validation of temperature distribution and material flow during refill friction stir spot welding of AZ91D magnesium alloy

Temperature distribution and material flow behaviours during refill friction stir spot welding (RFSSW) of AZ91D magnesium alloy were studied by a combination of numerical modelling and experimentation. The coupled Eulerian–Lagrangian (CEL) formulation was adopted to analyse processing thermal history and plastic deformation mechanisms of the RFSSW joint more accurately. Welding thermal cycles of varying positions in joint were measured by thermocouples so as to validate accuracy of this simulation process. The FEA (Finite Element Analysis) simulation to cover mechanical results was found to be consistent with the experimental values. Moreover, simulation analysis presented that material flow caused the clear hook defects and bonding ligament, and such material flow behaviours were experimentally verified via the macrostructure and microstructure of the RFSSW joint.

Influence of probe length on the formation of an interface in friction stir welded T-lap joints

ABSTRACT The dissimilar aluminum alloy of the T-lap joints between 7075 and 5083 was Friction Stir Welded (FSW). The attention was focused on the influence of probe length on the formation of a bonding interface between skin and stringer. The bonding interface of the joints was clarified by using skin and stringer tensile tests. Both the morphology and bonding strength of the interface were significantly influenced by the pin length. The hook defects could be eliminated by applying the short probe length but led to enhance the bonding line defect. The kissing bond defects at the two corner fillets seemed to be uneliminable and unaffected by the probe lengths. A suitable pin length was proposed to maximize joint efficiency for both the skin and stringer tests.

Influence of Softening Mechanisms on Base Materials Plastic Behaviour and Defects Formation in Friction Stir Lap Welding

The AA6082-T6 and AA5754-H22 aluminium alloys were selected as the base materials to fabricate similar and dissimilar friction stir lap welds. Three lap configurations, AA6082/AA5754, AA5754/AA6082 and AA6082/AA6082, were produced using three pin profiles and tested to analyse the role of the plastic behaviours of the base materials on the welding conditions. The macrostructural characterisation was carried out to understand the material flow response and hook defect formation. The mechanical characterisation of the joints was done by microhardness and lap tensile shear testing. The finite element analysis and phase simulation were conducted to predict the phase dissolution temperatures and the softening kinetics. The welding torque and axial forces registered were analysed to quantify differences in the alloy’s flowability during welding. The analysis of the welding machine outputs enabled to conclude that higher axial forces were registered when the AA5754 alloy was placed at the top of the dissimilar lap joint, showing that the non-heat-treatable alloy has lower flowability than the heat-treatable alloy. These results were associated with the flow-softening of the AA6082 alloy in plastic deformation at high temperatures. The coupled experimental and numerical analysis revealed that the plastic behaviour of the base materials strongly influenced the material flow and, in this way, the hook defect formation and the shear tensile properties of the welds.

Effects of Shoulder Geometry on Microstructures and Mechanical Properties of Probeless Friction Stir Spot Welded Aluminum 7075-T651 Sheets

In this work, three types of probeless tools (archimedes, involute, and concave tools) were designed. A 7075-T651 sheet of 1.0 mm thickness was welded using the designed probeless tools. It was found that the stir zone and hook defect varied a lot for different joints. Plunge depth was the dominant process parameter for joint property. The joint’s maximum failure load (5.73 kN) was obtained with the concave tool when the target plunge depth was 0.55 mm. Two typical joint fracture modes (shear fracture and plug fracture) were found using three designed tools with different plunge depths, rotation speeds, and dwell times. Shoulder geometry presented little influence on heat generation. Compared with involute grooves, archimedes grooves showed to be more effective on the material flow. The properties of the joints with the archimedes tool were more sensitive to welding parameters. Compared with the flat tool, the concave tool decreased the escape of plasticized material, improved the forge force, and optimized the metallurgical bond at the interface.

Interface Characteristics and Mechanical Properties of Ultrasonic-Assisted Friction Stir Lap Welded 7075-T6 Aluminium Alloy.

In this work, friction stir lap welding (FSLW) and ultrasonic-assisted friction stir lap welding (UAFSLW) was applied to 6-mm-thick 7075-T6 alloy sheets using three welding tools with the same process parameters. The joint formation, microstructural characteristics, and mechanical properties of the resulting lap joints were then investigated. The results showed that ultrasonic vibration significantly promoted the flow of metal at the interface, enlarged the size of the stirred zone (SZ), and reduced the angle between the hook defect and the interface. During lap shear testing, the FSLW and UAFSLW joints fractured in a similar manner. The fracture modes included tensile fracture, shear fracture, and a mixture of both. Cold lap and hook defects may have served as crack-initiation zones within the joint. Under configuration A (i.e., upper sheet on the retreating side (RS)), all joints failed in the shear-fracture mode. The effective lap width (ELW) of the joint welded using tool T2 was the greatest. This resulted in a higher shear fracture strength. The maximum shear fracture strength of the UAFSLW joint was 663.1 N/mm. Under configuration B (i.e., upper sheet on the advancing side (AS)), the shear fracture strength was greatly affected by the fracture mode. The highest shear fracture strength of the UAFSLW joint, 543.7 N/mm, was welded by tool T3. Thus, under otherwise identical conditions, UAFSLW joints can withstand a greater fracture shear strength than FSLW joints, as ultrasonic vibration helps to mix the material at the interface, thus, enlarging the SZ and diminishing the cold lap defects.

Joining dissimilar grade aluminum alloy sheets using multi-hole dieless friction stir riveting process

A recently developed spot joining process namely dieless friction stir riveting is explored for understanding the effect of multi-hole configurations on the joint formation in dissimilar grades of aluminum alloy sheets. These spot joints are strengthened by mechanical interlocking together with metallurgical bonding and possesses the absence of hook defect as well as pin hole defect. This contributes to superior lap shear fracture load than conventional spot joints. The mechanical load performance tests, detailed macro/micro structure analysis, the hardness measurement, external morphological study, and fracture mode analysis are conducted. Single-hole configuration yields appreciable mechanical performance than multi-hole configurations by achieving lap shear fracture load accounting about 7.42 kN and cross-tension fracture load accounting about 2.89 kN. The farther the holes in multi-hole configurations, the more imperfect the mechanical interlocking becomes. The dynamic recrystallization, associated grain growth, and its relation with the extent of plastic deformation in distinct zones of the joint are also identified. The frictional heat flux and severe plastic deformation have significantly affected the hardness of the two sheets. Critical weak zones leading to various fracture modes of the joint are also identified.

Micro-characteristics of High-Entropy Alloy Weld Formed by Friction Stir Lap Welding of 1060 Al and Al0.1CoCrFeNi

1060 Al and Al0.1CoCrFeNi are friction stir lap welded in this study. The limited flow of high-entropy alloy avoids the occurrences of hook and cold lap defects in the lap joint. Coarsening of large-size Al(Cr)-rich particles and dissolution of small-size Al(Cr)-rich particles simultaneously take place in the heat-affected zone of high-entropy alloy during the welding. The plastic deformation of high-entropy alloy induced by welding tool refines and homogenizes the microstructures in thermo-mechanically affected zone and stir zone. Friction stir lap weld of high-entropy alloy presents higher hardness than the base metal in each zone. The hardness of stir zone exhibits a maximum of 600 Hv, higher than the maximum hardness of AlxCoCrFeNi traditionally obtained. This should be attributed to the microstructure refinement as well as the enhancement of solid-solution strengthening and the occurrence of body-centered-cubic structures due to Al content improvement.

Tensile-Shear Mechanical Behaviors of Friction Stir Spot Weld and Adhesive Hybrid Joint: Experimental and Numerical Study

In this work, the tensile-shear mechanical behaviors of friction stir spot weld and adhesive hybrid joint were performed from both numerical and experimental viewpoints. Weld through (WT) and flow in (FI) processes were studied in this research. The focus was to evaluate joint defects, tensile-shear failure load (TSFL), failure energy, failure mode and stress distribution of the joint. In FI joints, keyhole and hook defects appeared in the weld zone and the areas of material removed from the base metal were filled with adhesive. In the WT joints, the adhesive layer close to the weld zone was carbonized due to the welding heat. Meanwhile, under the rotating movement of welding tool, the adhesive impurities entered the stirring zone (SZ) and heat affected zone (HAZ) of the weld, which decreased mechanical performances of WT joints. Compared to the friction stir spot welding (FSSW) joint, the TSFL value, stiffness and failure energy of FI joint were increased by 2.7, 1.1 and 8.14 times, respectively. In order to study the stress distribution of the joints, a finite element (FE) model, which considered the weld structure and mechanical properties of weld regions, was implemented. Moreover, the adhesive layer was simplified by the cohesive zone model (CZM). FE results show that the FI process effectively decreases the stress concentration of the weld edge from 243.09 to 15.5 MPa, under the 2 kN tensile load. The weld can block the adhesive crack propagation, and the adhesive optimized the stress distribution of FI joints through a synergistic effect. So, the use of FI process for aluminum alloy connection is strongly recommended, especially in crucial structure areas.

Pin plunging reinforced refill friction stir spot welding of Alclad 2219 to 7075 alloy

Abstract A Pin-plunging reinforced refill friction stir spot welding (P-RFSSW) technique was introduced in this study in order to solve the problem of the central void defect and the alclad residues in the weld center associated with conventional refill friction stir spot welding (C-RFSSW) of dissimilar alclad aluminum alloys. The P-RFSSW process is based on promoting the plastic deformation in the weld center owing to the addition of an initial pin plunge stage. The microstructure and mechanical properties of the C-RFSSW and P-RFSSW joints were compared. Results have shown that the P-RFSSW process could eliminate the central void defect and promote the metallurgical bonding and intermixing in the weld. Weld strength was improved when using P-RFSSW compared with the C-RFSSW process, which was attributed to the specific downward extended hook defect into the stronger, thicker 7 series aluminum alloy.

Dieless friction stir lap joining of AA 5050-H32 with AA 6061-T6 at varying pre-drilled hole diameters

The role of hole diameter on mechanical behavior and joint formation of dieless friction stir lap joints in dissimilar alloy grades of aluminum is investigated in the present article. Concurrent occurrence of mechanical interlocking along with metallurgical bonding reinforce the fabricated joints. Keyhole defect and hook defect of conventional friction stir spot welding are apparently suppressed. Hole diameter range from 3 to 3.5 mm is found to be favorable, while considering the macrostructure analysis, joint morphology analysis and load performance analysis. Appreciable fracture load up to 7 kN is obtained in lap shear test throughout the hole diameters employed. Zones present in the joint are identified, in addition the effect of size of the hole on mechanical interlocking is exposed through macrostructure analysis. The extent of recrystallization induced by frictional heat generation along with plastic deformation has affected the grain structure of the joint. Critical zones resulting in the failure of the joints are stir spot circumference and neck of the pin. A tool diameter to hole diameter ratio for successful DFSL joint fabrication is also established through the present work.

Friction stir lap welding of AA6061 aluminium alloy with a graphene interlayer

ABSTRACTThe present study aims to enhance the strength of friction stir lap welded aluminum alloys by using an interlayer of graphene nanoplatelets (GNPs) at the weld interface. With GNP interlayer, the weld strength and percentage elongation increased by 121 and 53%, respectively, as compared to the weld without GNP interlayer. The interlayer also changes the mode of fracture from brittle in the weld without GNP to ductile mode. Grain size in the weld with interlayer decreased by ~38% as equated to the weld without GNP. The height of the hook defect (HD) and cold lap defect (CLD) decreased by 26% and 41%, respectively, in the weld with GNP interlayer as compared to the weld without interlayer. In weld with interlayer, the bottom of the top plate on the retreating side acts as the potential site for fracture due to the presence of an interfacial defect, and undeformed GNP layer. The strengthening of the weld is attributed to various primary strengthening mechanisms like thermal mismatch, grain refinement, Orowan looping, and load transfer. Moreover, GNP interlayer also prevents the formation of the Al2O3 layer at the lap interface and thus contribute significantly in the weld strengthening.

Friction stir lap welding of AA2024-T4 with drastically different thickness

Friction stir lap welding is a promising technology to obtain high-strength joints of Al alloys in aviation and aerospace industries. Despite extensive studies performed on thin-wall structures, few works were conducted to join the plates with drastically different thickness, such as the joining of aircraft skins and panels. In this paper, 2.5-mm and 25-mm-thick AA2024-T4 plates were joined with a circumferential notches shape pin. The influence of pin length on joint features and mechanical properties was investigated. The pin improved the material flow behavior and stirred the materials violently, which were beneficial to reducing the interface migration and facilitating the joining of interfaces. The hook defects were eliminated. The failure load firstly increased with the pin length from 2.6 mm to 2.7 mm and then decreased when the pin length increased from 2.7 mm to 3.1 mm. The failure load reached 7.97 kN due to sufficient material intermixing with strong mechanical joining and large bearing area with metallurgical bonding. The selection of pin length should consider the deformation of thick plates, which acted as backing plates with lower elasticity modulus than typical backing plates made of steel.

Characterizations of weld defects, intermetallic compounds and mechanical properties of friction stir lap welded dissimilar alloys

This experimental investigation describes the effect of tilt angle and rotational speed on the characteristics of welding defects, intermetallic compounds, weld strength and micro-hardness in friction stir welding of thin sheets of AA6061-T6 and AISI304. Qualitative and quantitative assessments of the welding defects have been made by using X-ray micro computed tomography and metallographic study. Three-dimensional tomography and special distribution of internal voids (pores) have been discussed by correlating it to different values of tool rotational speed and tilt angle. Internal defects were localized in the stir zone. Welding at a tilt angle of 1° reduced both surface and internal defects. Non-uniform thickness of intermetallic compound (FexAly) layer has been found in the weld zone whose thickness varied with the values of process parameters. Higher weld porosity, bigger steel hook defect and increased layer thickness of intermetallic compound caused a significant reduction in the joint strength and elongation.

Fatigue Behavior of Friction Stir Welded Lap Joints for Dissimilar AA7150-AA2524 Aluminum Alloy

The detail fatigue rating (DFR) value of friction stir welded (FSW) lap joints for dissimilar AA7150-AA2524 aluminum alloy was experimentally investigated. The metallographic analysis was carried out on by optical microscope (OM). Fatigue fractography was observed under scanning electron microscope (SEM). The results indicate that DFR value of AA7150-AA2524 FSW lap joints is 191.54MPa. There is no distinct different boundary between HAZ and BM of the upper AA7150 sheet. The grains in the upper NZ of AA7150 are coarser than those in the bottom NZ of AA2524. Fracture morphologies shows that lots of hook defects in the FSW lap joint. Fractography indicates that fatigue crack is initiated from the AA2524 bottom surface around the weld center, where weak-bonding defects can be seen. The hook defects and weak-bonding defects have obvious influence on the fatigue property of friction stir welding lap joints, which seriously reduce the fatigue strength.

Influence of Tools Diameters and Plunge Depth on Mechanical Properties of Micro Friction Stir Spot Welding Materials A1100

Welding on thin aluminum alloy material is still relatively difficult, so that appropriate welding technology is needed to joint the ultra-thin aluminum material. One of the processing welding capable of solving to ultra-thin aluminium is micro friction stir spot welding μFSSW). The micro FSSW process is an adaptation of the FSW process where solid-state processes allow the welding process without changes in the basic characteristics of the workpiece. The purpose of this study was to analyze the effect parameters used in this study are pinless tool diameter and plunge depth on micro FSSW. The material used in this study was A1100 with a thickness of 0.42 mm using lap joint. The welding parameters used in this study are tool diameter and plunge depth, where each parameter has three levels. While the response variables observed were mechanical and fractures properties. To analyze maximum shear strength/load using response surface methodology (RSM). Experimental results and analysis of influence tool diameter and plunge depth on micro-friction stir spot welding μFSSW) analysis show that tool diameter and plunge depth are directly proportional to maximum shear load. Furthermore, the smallest hook defect occurs on the tool diameter 5 mm with plunge depth 500 μm.

Improving mechanical properties of friction-stir-spot-welded advanced ultra-high-strength steel with additional water cooling

ABSTRACT1.2 mm thick DP1180 advanced ultra-high-strength steel were successfully friction stir spot welded (FSSWed) with additional water cooling. No hook defect and obvious keyhole were observed in the spot joints when using a specially designed WC-Co tool. Softening of the heat-affected zone was substantially suppressed and the lap shear strength of the joints was obviously enhanced via water cooling. The enhanced strength of water-cooled joints was attributed to the grain refinement, the formation of the untempered martensite and the effective restraint of tool softening underwater. Accordingly, the maximum lap shear strength of 17.9 kN was obtained for the water-cooled joint. This study provides a novel strategy to enhance the strength of the FSSW joints for high-strength steels.

A Comparative Investigation on Conventional and Stationary Shoulder Friction Stir Welding of Al-7075 Butt-Lap Structure

Both conventional friction stir welding (C-FSW) and stationary shoulder friction stir welding (S-FSW) were employed to join the Al-7075 butt-lap structure, then the microstructural evolution and mechanical characterization of all FSW joints were systematically studied. The C-FSW joint exhibited a rough surface with flashes and arc corrugations, while the surface of the S-FSW joint became smooth. Moreover, for the S-FSW joint, the shoulder-affected zone got eliminated and the material flow mode during FSW was changed owning to the application of stationary shoulder. Furthermore, in comparison to C-FSW, the lower welding heat input of S-FSW decreased the average grain size in the nugget zone and inhibited the coarsening of strengthening precipitates in the heat-affected zone, elevating the overall hardness for the S-FSW joint. In addition, the tensile strength of the S-FSW joint became higher compared to the C-FSW joint, and all the FSW joints failed inside the nugget zone attributing to the existence of hook defect. The sharp-angled hook defect deteriorated the plasticity of the C-FSW joint further, which was only 70% that of the S-FSW joint.

Refill friction stir spot welding of AA6082-T6 alloy: Hook defect formation and its influence on the mechanical properties and fracture behavior

Process parameters optimization was carried out to evaluate the individual effect of rotational speed, feeding rate and plunge depth on the weld strength of an AA6082-T6 aluminum alloy produced by Refill Friction Stir Spot Welding. Statistical analysis allowed to maximize the weld peel and shear strength and a strong correlation between plunge depth and weld resistance was found. It was shown that plunge depth has a strong effect on the formation and shape of the hook defect. Moreover, the fracture behavior of the welds is strongly dependent on the hook defect configuration. By applying the so-called one-factor at a time (OFAT) method combined with macro and microstructure characterization it was possible to identify three possible hook defect configurations. A comprehensive description of the different hook configuration formation is presented by analyzing the microstructure of samples from interrupted welding tests. The configuration of the hook defect depends on the position and direction of the material flow around the sleeve and the drop region. Macro and microstructure analysis of interrupted mechanical test specimens allowed the characterization of the crack initiation site and crack propagation path according to qualitative observations of hook configuration, bonding ligament and the interface region between the heat affected zone and the thermal mechanically affected zone. It was found that the fracture mode is also strongly related to the configuration of the hook defect. The optimized welding condition was reached using intermediate values of plunge depth, which produces a hook pointing down configuration.

Strengthening and toughening mechanisms in refilled friction stir spot welding of AA2014 aluminum alloy reinforced by graphene nanosheets

Refilled friction stir spot welding is proposed to replace friction stir spot welding to solve the problem of keyhole in joining of light metals. However, it is rather challenging to ensure the mechanical property of refilled friction stir spot welding joints because of hook defect. In this study, a novel method was proposed to use graphene nanosheets to strengthen the tip of hook defect and improve overall mechanical properties of joint. The joints of AA2014 aluminum alloy with 0.6%wt graphene nanosheets were fabricated by refilled friction stir spot welding. The results show that the tensile/shear strength of joint increases by 31% and the fracture toughness also is improved by nearly 20% with graphene nanosheets. Extension in fatigue life was achieved on joint with graphene nanosheets. Microstructural observations, fracture characteristics and electron back scattered diffraction analysis were systematically investigated to clarify the mechanism for improvement of strength and toughness. The synergetic effect including grain refinement, load transfer, dislocation strengthening and crack bridging, caused by graphene nanosheets, leads to hardening behavior of graphene nanosheets pinned zone and a significant crack deflection during fracture process, which contributes greatly to improving strength and toughness of joint.