Kissing Bond Defects Research Articles

Influence of FSW process on dissimilar T-joint of aluminum alloy to pure copper

ABSTRACT This work investigated the influence of friction stir welding (FSW) parameter on forming the dissimilar T-joint of aluminum alloy 6061 (AA6061) to pure copper (C1100). The experimental outcomes indicated that the hook formation trended to move toward the center of welding line at low welding and/or high rotational speeds. In all cases, the kissing bond (KB) defects were significantly observed in advancing (AS) compared to retreating sides (RS). The thickness of diffusion and intermetallic compound (IMC) layers decreased with increasing welding and/or decreasing rotational speeds. A highest joint efficiency was attained with approximately 74% at 500/125 rev/mm of the skin test and 34% at 500/100 rev/mm of the stringer tests. The fracture process under the stringer test initiated from the AS to the RS along the interface while that of the skin test cracked in the heat affected zone. Improving dissimilar T-joint of AA6061/C1100 was impacted not only by controlling the KB defect but also the IMC layer thickness.

Topology optimisation for robust design of large composite structures

The manufacturing of composite structures can lead to material defects. Some of these defects, due to their small size, can go undetected by current non-destructive testing techniques; especially in very large and geometrically complex components. Therefore, it is of paramount importance to design composite components to resist defects by accounting for them as early as possible. Topology optimisation is a methodology whereby complex features, such as cracks and debonds, can be accounted for in the design. Therefore, this work proposes a new approach for the design of very large composite structures. This new approach is enabled by a solid shell element formulation with boundary tracking capabilities (through the floating node and the level-set methods) and optimisation capabilities (through design sensitivity analysis of an energy release rate objective function). Furthermore, this work proposes a new algorithm and modelling workflow that has the ability to handle large multi-scale models. The proposed methodology shows advantages as the ability to track the topology explicitly, and the ability to increase defect tolerance of the design. This works shows the new methodology is capable of handling complex structures, such as a multi-scale stringer run-out model from an aircraft wing-box, while a kissing bond defect is present.

Optimization of Friction Stir Welding Parameters for AA6061-T651 Aluminum Alloy: Defect Analysis and Process Improvement


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Friction Stir Welding (FSW) is an eco-friendly process known for high-quality joints without filler metal. This study investigated the physical properties of FSW-welded joints in AA6061-T651 aluminum alloy by varying the concave shoulder angle. Optimal process parameters were determined through Taguchi optimization using an Orthogonal Array design. Macro and micro testing revealed overlap defects, kissing bond defects, and wormholes in some samples. Systematic experimentation identified key parameters to avoid defects, including tool rotation speed, welding speed, tool tilt angle, tool indentation angle, and shoulder depth. Visual inspection and microstructural analysis played a crucial role in assessing weld quality. Optimizing welding parameters, such as rotational speed, welding speed, temperature, and tool geometry, was highlighted as crucial for defect-free joints. The study offers valuable insights for researchers and professionals in the field, promoting the advancement and application of FSW in aluminum alloy welding.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Highlight:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 FSW: An environmentally friendly welding process with good joint quality.
 Defect Analysis: Identification of overlap defects, kissing bond defects, and wormholes in welded joints.
 Process Optimization: Determination of optimal parameters to avoid defects, including tool rotation speed, welding speed, tool tilt angle, tool indentation angle, and shoulder depth.
 
 Keyword:
 Friction Stir Welding, AA6061-T651 Aluminum Alloy, Welded Joint Analysis, Process Optimization, Defect Prevention

Advances in friction stir welding of Ti6Al4V alloy complex geometries: T-butt joint with complete penetration

In this work, the friction stir weldability of Ti6Al4V T-joints has been investigated. Its aims are: (i) to study the influence of tool and welding parameters on weld quality, (ii) to assess the joints’ mechanical strength to foresee future applications, and (iii) to characterize Co-based FSW tools’ wear by following the wear during the tests. Welds’ defectivity is studied by cross-section macrographies analysis. Independently from welding parameters and tools, internal voids are avoided, and a suitable weldability window is identified. Microstructure observations have corroborated temperatures below the β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta $$\\end{document}-transus point even in the nugget zone, guaranteeing joints’ maximum mechanical strengths at 96% and 87% of the base material for UTS and Y, respectively. Contrarily, elongation at break is very low, without reaching 20% of the base material. The failure is linked to section thinning and kissing bond defects at the joints’ corners. Additionally, tool wear proved to be a critical issue while friction stir welding Ti6Al4V. The inner part of the shoulder is the most sensitive to wear. The consequent high wear rate might be a problem for mass production. The work established the pertinence of assembling complex geometries of Ti6Al4V using friction stir welding, considering weld quality and the mechanical strength achieved. However, critical factors such as section thinning, kissing bond, and tool wear must be carefully addressed to avoid joints’ low elongation at break and to guarantee their mechanical strength.

Discrimination between weak adhesion, delamination and kissing bond at the interfaces of composite bonded assemblies

In this work, the impact of delamination, weak adhesion and kissing bond defects at the interfaces of trilayer composites structures, are numerically investigated in order to differentiate these three phenomena that are usually encountered in composite bonded assemblies. The structure is composed by two carbon-epoxy composites with 1.8 mm bonded by an adhesive epoxy of 0.5 mm. The Stiffness Matrix Method (SMM) is used to calculate the transmission and reflection coefficients of bulk waves likely to propagate in a trilayer structure immersed in water. Then, the behaviors of the symmetric and antisymmetric Lamb modes are analyzed. Four different states of interfaces are considered: (a) nominal interfaces with good adhesion, (b) interfaces with weak adhesion, (c) interfaces with delamination and (d) interfaces containing kissing bond defects. The carbon-epoxy composite is considered as an orthotropic layer and the epoxy adhesive interface as an isotropic layer. Results show that, only the antisymmetric Lamb modes are affected by the presence of kissing bond defects while the symmetric and antisymmetric Lamb modes are affected by the presence of delamination at the interfaces. In addition, weak adhesion only partially preserves the symmetric modes (with a shift in their frequencies) and introduces a reduction in amplitude compared to kissing bond like defect. The potential of the proposed method for the qualification of bonded composite structures is clearly demonstrated, with the ability to discriminate among three categories: weak adhesion, delamination and kissing-bond-like defect.

Effect of friction stir welding parameters on microstructure-based defect formation and mechanical properties of Tailor Welded Blanks

This maiden work on friction stir welding (FSW) of unequal thickness dissimilar (UETD) materials with plate thickness ratio (PTR) of 2.54 presents a comprehensive analysis of the effect of FSW process parameters on mechanical properties and microstructure-based defect formation of tailor welded blanks (TWBs). In contrast with equal thickness butt joints, the thickness mismatch in TWBs creates various challenges in the process requisite, influencing the process mechanics leading to inadequate heat input and improper mixing of base materials, making the joint prone to various defects. Defects such as tunnel defect, kissing bond defect, voids, and incomplete root penetration (IRP) defect observed and discussed comprehensively in this study are largely attributed to the complexities in the process requisites and mechanics, selection of unfavourable combinations of FSW process parameters, variation of plate thickness, and occurrence of surface oxide layers at the abutting surfaces. The present work included and discussed microstructural features around defects, temperature history, and mechanical properties of joints such as tensile strength and hardness to understand the effect of FSW process parameters on failure mechanism of TWBs in detail. Based on the results obtained from mechanical properties of joints discussed in this study, it was revealed that IRP defect deteriorated the mechanical properties the most followed by tunnel defect, kissing bond defect and voids. Figure: Illustrates (a) FSW set up (b) macrostructure of welded joint (c) tensile specimen and (d) specimen indicating positions of microhardness readings

Assessment of Composite Aluminum Adhesive Joints Using Digital Image Correlation

Traditional nondestructive evaluation (NDE) methods present significant challenges to detecting and characterizing kissing or weak bonds in adhesively bonded structures. These kissing or weak bonds also cannot transmit shear stresses or handle complex loading modes and, if not detected, can present a significant threat to the structural integrity of the components or systems. This paper demonstrates the digital image correlation (DIC) technique for evaluating adhesively bonded dissimilar materials joints subjected to kissing or weak bonds. The study employed four adhesively bonded carbon fiber reinforced plastics and aluminum (CFRP-Al) lap-shear test coupons with varied bond quality (i.e., with no contamination and three simulated kissing bond defects). The novelty of the approach presented in this paper was that this technique could detect and demonstrate changes in the normal strain (εyy) contour map of the contaminated specimens at relatively lower load levels. This load level corresponds to 15% of the failure load for the silicone and hydraulic oil contaminated sample and around 30% for the polyvinyl alcohol (PVA) contaminated sample. In addition, higher compressive strains along the overlap edges were observed in the strain map for the single lap joints due to the higher peeling stresses of the adherend and the stress concentration at the edges of an adhesively bonded joint.

Thermal cycles, microstructures and mechanical properties of AA7075-T6 ultrathin sheet joints produced by high speed friction stir welding

0.5 mm thick AA 7075-T6 ultrathin sheets were butt jointed by conventional friction stir welding (C-FSW) and high speed friction stir welding (HS-FSW) under a constant rotational speed ( ω )/welding speed ( v ) ratio of 6.67 rad/mm using a pinless tool. In this study, the conventional ω of 2000 rpm and conventional v of 300 mm/min were adopted by C-FSW, and the high ω of 8000 rpm and high v of 1200 mm/min were adopted by HS-FSW. Compared with C-FSW, HS-FSW presented a fast heating and cooling rate thermal characteristic with a slightly higher peak temperature and significantly shorter elevated-temperature exposure time. Kissing bond defect was appeared in the weld bottom for C-FSW. While, no obvious defect was found in the weld obtained by HS-FSW. Compared with C-FSW, the proportion of deformed grains in the center of nugget zone (NZ) fabricated by HS-FSW was increased. For the joint fabricated by HS-FSW, the upper NZ was dominated by recrystallized grains, and the lower NZ was dominated by deformed grains. The ultimate tensile strength of the joint fabricated by HS-FSW can reach 87% of base metal (BM), and its elongation, yield strength and ultimate tensile strength were increased by 58.8%, 23.3% and 22.8% respectively compared with C-FSW. Both the joints fractured in the center of the NZ. The joint fabricated by C-FSW broke due to the kissing bond defect. While the joint fabricated by HS-FSW may be broken due to the weld thickness reduction. • The thermal cycle characteristics of HS-FSW were revealed. • The effect of the thermal cycle on microstructure was revealed. • The microstructures of the joint produced by HS-FSW were investigated by EBSD. • The tensile properties of the joint produced by HS-FSW were superior to by C-FSW.

Numerical Investigations Into Nonlinear Vibro-Ultrasonics and Surface Vibration Comparison Method for Detection of Defects in a Composite Laminate

Abstract This paper begins with a numerical study based on earlier experiments of nonlinear vibro-ultrasonic behavior of a composite laminate with a delamination defect upon sinusoidal linear sweep signal excitation. A methodology to model laminates with cross-ply layup is presented which can be extended to any layup if desired. In comparison to experiments where it is challenging to visualize the fine details of vibrations, simulations make it easier to visualize and help in optimizing the defect probing methods. The paper goes on to discuss with the help of numerical results that a separation gap between the delamination surfaces occurs to be a common cause for the failure of nonlinear vibro-ultrasonic methods to detect delamination defects. This constraint can often be overcome with application of higher excitation amplitudes as has been demonstrated in several experimental works. However, in this study, a new approach named surface vibration comparison method to probe delamination defects in the absence of contact acoustic nonlinearity is proposed as a proof-of-concept. The technique is then evaluated for detection of weak kissing bond defects in composite beam specimens. Both the experimental and simulation results show potential of the method as damage detection technique in thin composite structures.

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.

Evaluation of Optimization Parameters of Semi-Solid Metal 6063 Aluminum Alloy from Friction Stir Welding Process Using Factorial Design Analysis

The semi-solid-metal 6063 aluminum alloy was developed for the automotive industry. The objective of this research was to optimize parameters in friction stir welding process that can provide the highest tensile strength. The ANOVA factorial design was used to analyze rotation speed, welding speed, and tool geometry at different factor levels of experimentation. The results showed that the optimized tensile strength was 120.7 MPa from the cylindrical tool, rotation speed was from 1300 to 2100 rpm, and welding speed less than 75 mm/min in the coefficient of determination R2 was 95.09%, as can be considered from the regression equation. The examination of the stir-zone and thermal mechanical affected zone using SEM and EDX showed that the new recrystallization of the microstructure causes fine grain in the stir-zone, coarse grain in advancing-side thermal mechanical affected zone, and equiaxed grain in the retracting-side thermal-mechanical affect zone. The intermetallic compounds of β-Al5FeSi phase transformation phase were formed to three types, i.e., β″-Al5Fe, Mg2Si, and Al8Fe2Si phase were observed. Moreover, in the stir-zone and thermal-mechanical-affected zone, defects were found such as flash defects, void or cavity defects, crack defects, lack of penetration defects, tunnel defects, kissing bond defects, and dendrite formation defects affecting weldability.

Study on fatigue strength of FSW joints of 5083 aluminum alloy with kissing bond defect

This work studied the effect of kissing bond defects on the fatigue strength of friction stir welded (FSW) specimens and assessed fatigue life prediction approaches for the fatigue strength evaluation of friction stir welded specimens. For specimens with inherent kissing bond defects prepared by friction stir welding process made of AA5083 aluminum alloy thin sheets, constant amplitude fatigue tests under different loading levels were carried out on MTS test frame. Profiles of kissing bond defects at weld toes as well as the fatigue fracture morphologies were observed with scanning electron microscopy. According to the experimental observations, finite element models based on both the virtual notch radius theory and the crack propagation conception were built, respectively, to calculate the stress and strain in the weld zone. Correspondingly, fatigue lives were predicted by these two methods and compared with experimental results.

Influence of tool offsetting and base metal positioning on the material flow of AA5052-AA6061 dissimilar friction stir welding

This study examines dissimilar friction stir welding of AA5052-AA6061 aluminum alloys with varying tool offsets. The base metals were positioned and fixed at a slight diagonal positioning such that varying tool offset position from the centreline can also be varied along the length of the weld. After the fabrication process, microstructural and mechanical property characterization was subsequently conducted. The results show that, above a certain threshold for tool offset, incomplete consolidation (i.e. kissing bond defects) will occur. Regardless of the base material positioning, a zero tool offset shows optimum intermixing in the stir zone. EDX mapping confirms the presence of a distinct interface between both materials in the stir zone region. However, enhanced material intermixing and better elongation are observed when AA6061 alloy is positioned at the tool advancing side.

Defect formation during dissimilar aluminium friction stir welded T-joints

AA8011 remains one of the largest used aluminium alloys due to excellent ductility, corrosion resistance and sufficient strength in H14 conditions. The 5754, due to high strength and corrosion resistance finds key applications in marine, process industry, automobile and nuclear applications. A T-joint combination of the two alloys (when stronger forming is stiffeners) results in significant weight savings. The T-joint between 8011 and 5754 (with the latter forming the stringer) is highly prone to defect formation due to the stronger material being placed away from the heat source. In this paper, friction stir welding of AA5754 and AA8011 aluminium alloys in T-lap configuration is performed. Effects of processing parameters namely tool transverse speed, rotational speed and shoulder diameter on the weld properties are studied with the help of micro-hardness, macrostructure and microstructure analysis. Analysis of defects such as tunnel and kissing bond, and mechanism of defect formation is also discussed. Tunnel defect is observed on advancing side in all the samples, which is attributed to high heat flux on advancing side, insufficient material flow in the vertical direction and asymmetric flow of plasticized material from advancing side to retreating side. The size of the defect is found to increase with increase in shoulder diameter keeping the rotational speed constant. Kissing bond defect was also observed in some samples owing to the presence of oxide layer between the joining surfaces.

Role of tool design on thermal cycling and mechanical properties of a high-speed micro friction stir welded 7075-T6 aluminum alloy

AA7075-T6 sheet welds that were 0.5 mm thick were fabricated by micro friction stir welding (μFSW) using a pinless tool as well as a pin tool. The experiments were conducted at a higher rotational speed of 6000 rpm and a higher welding speed of 1200 mm/min than those that are used in conventional FSW. The heat generation, peak temperature and elevated-temperature exposure time (t150) of the joints fabricated by the pin tool were higher than those fabricated by the pinless tool at the same shoulder penetration depth. The minimum ratio of the thickness reduction at the center of the friction stir weld was 0% for the pinless tool at a penetration depth of 0.14 mm. In comparison to those for the joints fabricated by the pin tool, the joint fabricated by the pinless tool showed less softening and better tensile properties. For the welds fabricated by the pin tool, the tensile properties increased with increasing penetration depth. In the case of welds made by the pinless tool, the tensile specimens failed at the transition region between the stir zone and the thermomechanically affected zone on the retreating side due to the local thickness reduction. However, the tensile specimens failed at the center of the stir zone due to kissing bond defects in the joints produced by the pin tool.

High-Speed Friction Stir Welding of T6-Treated B4Cp/6061Al Composite

T6-treated 20 wt% B4Cp/6061Al sheets were joined under welding speeds of 400–1200 mm/min by friction stir welding (FSW) with a threaded cermet pin. The macro-defect-free FSW joints could be achieved at high welding speeds up to 1200 mm/min, but larger plunge depth was required at the welding speeds of 800 and 1200 mm/min to eliminate the tunnel defect. In the nugget zone (NZ) of the joints, the B4C particles were broken up and uniformly redistributed. The NZ exhibited lower hardness than the base metal (BM), and the hardness value almost did not change with increasing welding speed, attributable to the dissolution of precipitates. Compared with the BM, the joints showed lower tensile strength. As the welding speed increased from 400 to 800 mm/min, the joint efficiencies were nearly the same and up to ~ 73%. When the welding speed increased up to 1200 mm/min, the tensile strength significantly decreased, due to the occurrence of kissing bond defect at the bottom of the NZ. With increasing welding speed, the fracture location of the joints transferred gradually from the heat-affected zone to the NZ due to the kissing bond defects.

Effect of Welding Parameters on Junction Structure and Fracture Behavior of Friction-Stir-Welded Joints of 2024 Aluminum Alloys

In this paper, the effect of welding speed and tool rotation speed on the junction structure and fracture behavior of friction-stir-welded joints was investigated with use of 2024 aluminum alloys of 3.2 mm thick. Results show that the formation of junction structure is more affected by tool rotation speed than by welding speed due to the differences of heat input levels. When welding speed is at 100 mm/min and tool rotation speed is in the range of 400-600 r/min, the joint junction structure causes complete bonding with no welding defects. While under improper welding parameters, the junction structure causes the precipitate aggregation on the bonding line and the partial bonding with the appearance of both tunnel defects and kissing bond defects. Meanwhile, the results of tensile tests show that the tensile fracture of all FSW joints occur at the weld center. The failure mode of joints with defects-free junction structure characterized by shear fracture is ductile. While the failure mode of joints with defects-existent junction structure characterized by layered fracture surface is brittle. Furthermore, that heterogeneous microstructure, precipitate aggregation and welding defects converge inside the junction structure is the main factor affecting the tensile properties and fracture behavior of the friction-stir-welded joint.

Investigation of AA5083 T-lap joint fabricated by friction stir welding

Transportation industries are faced to big maters that the scientists focus on, such as energy saving and ecologically sustainable products. Therefore, many innovative solutions are delivered that will support environmental preservation but meet industries’ requirements for greater productivity and minimized operational costs. Aluminium alloys have successfully contributed to meeting the rising demand for lightweight structures. Recently, notable developments in aluminium welding techniques have resolved many welding related problems, although some problems need to be addressed. In this paper, 5083 aluminium alloy T-lap-joints were successfully fabricated by friction stir welding with various welding regimes. The defects morphology in the T-joints was experimentally observed and analyzed by a high magnification microscope. The roles of the grain microstructure and the effects of defects morphology in the mechanical behavior of the T-joint was investigated. In addition, the fracture locations and the fracture surface of the failure specimens were observed and discussed as well. Results indicate that the fracture of T-joints along the stringer is attributed to the bonding line defects, kissing bond defects and the tunnel defects. The result also shows, in the T-joints of 5083 aluminum alloy, the welding parameters influence significantly on the features and sizes of the defects.

Defect detection of composite adhesive joints using electrical resistance method

Mechanical joint and adhesive bonding are two typical joining methods for composite materials. The bolt fastening method is reliable but increases the weight of the structure and cuts the composite fiber, which may cause stress concentration and strength degradation. The adhesive bonding method has a higher bonding strength than the mechanical fastening, but the bonding strength can deteriorate depending on the various parameters that influence the environmental conditions and manufacturing process. In addition, bonding defects due to the presence of foreign substances on the bonding surface or immature surface treatment is called a Kissing Bond, and can significantly reduce the bonding strength. The electrical resistance method is a very promising technique for detecting kissing bond defects by measuring the electrical characteristics after the dispersion of CNTs in the adhesive.In this study, composite-to-composite single-lap joint specimens with CNTs were fabricated and the defect detection capability and the static strength of composite joints were evaluated using the electrical resistance method. A carbon/epoxy composite was used as an adherend and the electrical resistance of the adhesive joint was measured by varying the content of CNTs to 0.5 wt%, 1.0 wt%, and 2.0 wt%. The AC and DC resistance values of composite adhesive joints with artificial defects were measured using an LCR meter and a high-resistance meter, and their lap shear strengths were evaluated.

Microstructure and Mechanical Properties of Thick Plate Friction Stir Welds for 6082-T6 Aluminum Alloy

Abstract 6082-T6 aluminum alloy plate with thickness of 42mm was butt welded by friction stir welding (FSW) from two sides. The microstructures of the joints exhibited different grain sizes because of unequal frictional heating and plastic flow during FSW process. The transition from the heat affected zone (HAZ) to the nugget zone (NZ) in thermos-mechanical affected zone of advancing side (AS-TMAZ) was more sudden than thermos-mechanical affected zone of retreating side (RS-TMAZ). Kissing bond (KB) defect throughout the entire FSW joint was displayed both at the grain boundary and in the interior of the grain with semi-continuous bands. KB had no direct effect on tensile properties. Vickers hardness of the FSW joint was lower than the BM because its high heat input, dissolved and coarsened precipitates and little to the grain size after FSW. Hardness distribution of double-sided welding joint showed X-shaped area softening characteristics, that is to say the lowest hardness was the junction of two welding joint of NZ and the junction of TMAZ and HAZ. The tensile fracture position occurred in the lowest hardness region of the FSW joint, and it did not occur in the KB defect position.