Promising Joining Technique Research Articles

Experimental characterisation and statistical modelling of woven carbon fibre weld conductors for electrical resistance welding of thermoplastic composites

Resistance welding of high-performance thermoplastic composites is a promising joining technique for the structural assembly of aerospace components. Knowledge of the weld conductor resistance as a function of its dimensions, as well as external boundary conditions such as contact preparation and pressure, can be used to scale welding parameters. This paper presents a comprehensive review on the weld conductor contact preparation, length, width and contact pressure dependent resistance with respect to the Toray 5HS T300JB carbon woven prepreg 281 gsm fibre architecture. The study is based on a 4-wire resistance measurement of the carbon fibre fabric weld conductors and was conducted to provide a solid basis for defining the weld conductor’s resistance, considering the dependence on length, width and contact pressure. The experimentally determined resistance values were approximated using linear and non-linear regression functions and combined into a general formulation with respect to the investigated carbon fibre fabric architecture. The least squares fit of experimental versus model data confirmed a very high model confidence. Thus, the model allows a simplified transfer of electrical properties for process pre-design and scaling considering weld conductors with the same fibre architecture.

Effect of Adherend Thickness on Near-Field Ultrasonic Welding of Single-Lap CF/LMPAEK Thermoplastic Composite Joints

Ultrasonic welding is a fast and promising joining technique for thermoplastic composite parts. Understanding how changing the part thickness affects the process is crucial to its future upscaling and industrialization. This article presents an initial insight into the effect of the adherend’s thickness on the near-field ultrasonic welding of CF/LMPAEK thermoplastic composites. Different thicknesses of the top and bottom adherend were welded and analyzed using the output data of the welding equipment, temperature measurements, and other visual characterization techniques. Increasing the thickness of both the top and the bottom adherends showed to increase the power consumed during welding. An overshoot in the power needed at the onset of the welding process for increased thickness of the top adherend precluded welding beyond a threshold thickness of 4.72 mm. In the case of the thicker top adherends, there was also melting of the energy director and early fiber squeeze-out within the top adherend as a result of increased bulk heating. Increased bulk heating was hypothesized to be caused by increased hammering, as indicated by the amplitude readings for thicker adherends. Welding with a higher force, which is known to reduce hammering, corroborated this hypothesis as fiber squeeze-out within the top adherend was not observed. It is believed that hammering contributes to heating by causing an oscillatory impact excitation that is close to the natural frequencies of the system, which would result in amplification of the cyclic strain and subsequent increase in the viscoelastic heating in the adherend.

WITHDRAWN: A critical review on influencing parameters for adhesively bonded joints in composite laminates for structural applications

This review article critically examines the influencing parameters that play a pivotal role in the performance of composite laminated adhesive bonded joints for structural applications. Adhesive bonding has emerged as a promising joining technique for composites, offering numerous advantages such as weight reduction, improved fatigue endurance, and enhanced load-bearing capacity. However, achieving optimal adhesive bonding requires a comprehensive understanding of the key parameters that influence bond strength and durability. The review begins by outlining the fundamental principles of adhesive bonding in composite laminates and discusses the importance of interfacial strength between the adhesive and composite substrates. It delves into the various adhesive materials and their properties, highlighting the significance of selecting the right adhesive for specific applications. The impact of environmental conditions, including temperature, humidity, and exposure to aggressive media, on adhesive bond performance is thoroughly assessed. Special attention is given to the influence of surface preparation techniques, such as mechanical abrasion, chemical treatments, and surface coatings, in promoting adhesion and mitigating potential failure modes. Moreover, the review presents an in-depth analysis of the effects of joint geometries, such as overlap length, bond line thickness, and joint configurations, on the mechanical strength of composite laminated adhesive bonded joints. In conclusion, this comprehensive review article synthesizes the existing knowledge on influencing parameters for composite laminated adhesive bonded joints and identifies critical areas for future research and development. By gaining insights into these influencing factors, researchers, engineers, and practitioners can enhance the performance and reliability of composite structures in various structural applications.

Uncovering the high-temperature microstructural evolution and creep-fatigue damage mechanism of CMSX-4 brazed joints

Brazing is a promising joining technique for nickel-based single-crystal superalloy (Ni-SXs) intended for turbine blades, which endure the creep-fatigue conditions. In this work, the high-temperature microstructural stability and creep-fatigue damage mechanism of brazed joint are studied. The microstructural evolution of brazed joint, especially Zone I, where the creep-fatigue failure occurs, is influenced by the aging duration. The governed creep-fatigue damage mechanism is correlated with the strain amplitude and microstructural evolution in the brazed joints. The results obtained will be beneficial to improve the joining quality of Ni-SX brazed joints used for turbine blades.

The effects of misaligned adherends on static ultrasonic welding of thermoplastic composites

Ultrasonic welding is a promising joining technique for thermoplastic composite parts. On the way towards upscaling and industrialisation of this technology, it is crucial to understanding how it is affected by manufacturing tolerances. The objective of the research presented in this paper was to investigate the influence that misaligned adherends have on static ultrasonic welding of thermoplastic composites. Different angles were created by changing the clamping configuration. The results showed that increasing the angle between adherends decreases the power consumed and increases the process time while decreasing the weld uniformity and increasing the risk of overheating. These effects were associated to the impact of the different clamping configurations on the cyclic strain in the energy director and adherends during the welding process. Decreasing the top adherends bending stiffness by increasing the clamping distance was found to significantly reduce the adverse effects of adherend misalignment on weld quality.

Hybrid laser arc welding of dissimilar TWIP and DP high strength steel weld

High manganese TWIP steels have a remarkable potential to produce impact resistant car body parts in the automotive industry. Hybrid laser arc welding (HLAW), i.e. simultaneous action of an arc and laser source, can be regarded as the most promising joining technique for such components due to, among other factors, the possibility to use filler metals to avoid improper microstructures and Mn chemical segregations in dissimilar welds of TWIP with other steel grades. In this regard, this work studies the influence of the main welding settings during HLAW of 1.4 mm thick TWIP and Dual Phase (DP) steel sheets used in the automotive industry, which were joined with the use of an austenitic steel filler. Quality of TWIP-DP welds was investigated based on welding parameters, tensile and hardness properties, and microstructures.TWIP and DP steel sheets can be properly joined with a linear heat input in the 50–75 kJ/m range. Welded samples exhibit an average tensile strength and elongation at fracture roughly of 650 MPa and 13%, respectively. These values are similar to those of DP steel, but significantly lower than TWIP steel. The tensile welded samples failed within the heat affected zone of DP steel, being weldments and TWIP steel more resistant. This work confirms that HLAW can be considered a promising solution to join vehicle components made of TWIP and DP sheets when fillers are needed.

Crack growth analysis of adhesively-bonded stepped joints in aluminium structures

Adhesive bonding has become a promising joining technique, by satisfying the needs of diverse industrial fields. Several joint configurations are available for the designer, and the most common and exhaustively studied are single-lap, double-lap and scarf joints (Petrie in Handbook of adhesives and sealants. McGraw-Hill, New York, 2000; Moreira and Campilho in Eng Struct 101:99–110, 2015). Stepped-lap joints consist of a finite number of steps in the bonded region, and they are characterized by having stress concentrations at the end of each step rather than only at the bonded length edges, which leads to a higher load transfer through the interior part of the overlap compared to lap joints and, consequently, to a higher performance. The present work experimentally and numerically evaluates the static tensile strength of stepped-lap joints bonded with three adhesives, Araldite® AV138 (brittle), Araldite® 2015 (moderately ductile) and Sikaforce® 7752 (ductile), considering different overlap lengths (LO). A finite element analysis was undertaken to compare through-thickness normal (σy) and shear (τxy) stresses in the adhesive layer between joint configurations. Additionally, a cohesive zone model (CZM) analysis was considered to study the failure modes and the CZM damage variable during failure and to evaluate the CZM technique’s accuracy. It was possible to conclude that the recommended adhesive depends on the joint geometry, since strong and brittle adhesives are better for small LO, while for higher LO ductile adhesives with less strength can give better results. The CZM technique was accurate in the strength prediction, although with slight under-predictions for highly ductile adhesives.

A study on amplitude transmission in ultrasonic welding of thermoplastic composites

Ultrasonic welding of thermoplastic composite materials is a promising joining technique that is now moving towards up-scaling, i.e. the assembling of large industrial parts. Despite its growing technological maturation, the assumed physical mechanisms underlying ultrasonic heating (viscoelastic heating, friction) are still insufficiently understood and modelled. In particular, the hammering phenomenon, resulting from the periodic loss of contact between the sonotrode and adherends due to the high frequency vibration caused to the former, directly impacts the heating efficiency. We propose in this work an original experimental and modelling approach towards a better understanding of the hammering effect. This approach makes combined use of: (i) an experimental static welding setup provided with a high-frequency laser sensor to analyse the vibration amplitude transmitted to the adherends and (ii) an improvement of the multiphysical finite element model already presented in previous works. Results show it is possible to obtain a good estimation of the vibration transmitted to the upper adherend from laser measurements close to the sonotrode. The hammering effect is shown to decrease during the welding process, due to the heating of the interface which directly affects further heat generation. Quantitative introduction of this hammering effect in the existing numerical model results in improved predictions in terms of dissipated power in time.

Ductile adhesively-bonded timber joints – Part 1: Experimental investigation

In the field of timber load-bearing structures, adhesive bonding is a promising joining technique that may increase the structural stiffness and capacity of timber joints and structures. The use of ductile adhesives may furthermore allow designing ductile joints, which can compensate for the material ductility that timber lacks. To demonstrate the potential of this approach, adhesively-bonded double-lap timber joints were manufactured using a ductile acrylic adhesive and then subjected to axial tension and compression loading. The load-displacement responses were measured and compared to those of the same joint configuration for which a brittle epoxy adhesive was used. The effect of the different adhesives on the joint capacity and ductility has been studied and quantified. Strain field measurements using the Digital Image Correlation (DIC) technique and a quadratic strain interaction criterion provided a better understanding of the mechanical behavior of the two different joint types.

Influence of Mechanical surface treatment on the strength of mixed adhesive joint

Adhesive bonding is a promising joining technique in modern transport industries which provides for lightweight connection of dissimilar materials. However, available adhesive types have reached their limits in terms of durability and strength. A potential solution to this problem is the utilization of mixed adhesives. However, the influence of the surface preparation method on the performance of mixed adhesive joints has not been investigated thoroughly yet. The objective of this study is therefore to analyze such influence on the strength of mixed adhesive single lap joints. In this regard, five mechanical surface treatment methods were examined on aluminium alloy AA6082 using combinations of three different types of adhesives by means of static tensile tests. The results show significant increase in joint strength and ductility for DP 8005-AV138 compared to AV138 alone for sanding surface preparation. The roughness value in the range of Ra=1.7 to 3.1µm and Rm=36.5 to 44.60µm provide the highest shear strength.

Laser Welding of BTi-6431S High Temperature Titanium Alloy

A new type of high temperature titanium alloy, BTi-6431S, has recently become the focus of attention as a potential material for aircraft engine applications, which could be used up to 700 °C. Pulsed laser welding was used to butt join the BTi-6431S titanium alloy in order to understand the feasibility of using fusion-based welding techniques on this material. The effect of laser energy on the microstructure and mechanical properties of the joints was investigated. The microstructural features of the joints were characterized by means of microscopy and X-ray diffraction. Tensile testing was conducted at both room temperature and high temperature to simulate potential service conditions. The results show that the microstructure of the laser welded joints consists of primary α phase and needle α’ phase, while the microstructure of the heat affected zone consists of α, β, and needle α’ phases. The tensile strength of the welded joints at room temperature was similar to that of the base material, despite a reduction in the maximum elongation was observed. This was related to the unfavorable microstructure in the welded joints. Nonetheless, based on these results, it is suggested that laser welding is a promising joining technique for the new BTi-6431S titanium alloy for aerospace applications.

Properties of Friction Stir Welded Blanks Made from DC04 Mild Steel and Aluminum AA6016

Due to increasing demands for lightweight structures in automotive applications the use of sheet metal components made from aluminium alloys is a promising approach for weight reduction. The combination of steel and aluminium in car bodies may be an interesting alternative compared to a monolithic material design. The weight of structural parts of a car body shell can be reduced if dedicated parts consist of aluminium instead of steel. This approach allows for an optimal exploitation of the material properties of both materials, bringing high strength into highly loaded areas while areas subject to lower loads are equipped with lower strength and weight. However, a multi-material design combining steel and aluminium demands for suitable joining methods, especially if a forming operation is applied to the welded sheets. In conventional fusion welding processes the formation of intermetallic phases due to the metallurgical affinity of aluminium and iron is a serious problem. Recent developments in regulated cold metal transfer (CMT) welding technologies at the Institute of Welding Technology and Joining Technology (ISF) at the RWTH Aachen promise an appropriate solution to this problem. Due to a digitally regulated arc technology, the heat input in CMT is reduced to a minimum. However, the inevitable formation of a welding bead in arc processes with filler material is a criterion of exclusion in the case of production of welds for car body shells. To achieve an optimal appearance of the body shell, the welding beads need to be removed from both sides of the sheet in a second manufacturing step. Hence, to avoid further costs, it seems expedient to search for alternative welding technologies. Friction stir welded (FSW) joints show relatively even welding beads. Furthermore, this joining method is characterised by a low process temperature, which is considerably below the melting temperature of the base materials. Hence, FSW is a promising joining technique to produce tailored blanks out of aluminium and steel. The main objective of the present paper is the evaluation of suitable process parameters for the production of FSW butt joints with a thickness of 1 mm made from the aluminium alloy AA6016-T4 and the mild steel DC04. Welding experiments using a varying rotational speed, tool offset, tool velocity, tool plunge depth and tool tilt angle were carried out. To identify the best parameters in terms of the strength of the joint, tensile tests were performed. It is shown, that an amount of approximately 85% of the tensile strength of the base material AA6016 can be achieved. Using SEM the formation of the fracture surfaces was analysed. Different fracture types were identified and the possible reasons for their occurrence are discussed. It is shown that in the case of optimal joining procedure the failure occurs in the thermomechanically affected zone in the aluminium sheet, were the plastic deformation is low. Additionally, thermography has been employed to evaluate the temperature distribution during the process. In metallographic investigations it was found that during welding the microstructure of the aluminium base material changes due to plastic deformation and temperature increase in the area of the weld seam. Using hardness measurements the change of the mechanical properties in the contact zone of both base materials and in the heat affected zone was examined. Finally, an outlook is given with respect to the possibilities of producing FSW welded sheets that can be formed using conventional deep-drawing.

Tool Path Design in Friction Stir Spot Welding of AA6082-T6 Aluminum Alloys

In the paper, a variation of the Friction Stir Spot Welding (FSSW) process has been considered. In particular, a particular tool path is given after the sinking phase nearby the initial penetration site. The process mechanics was highlighted and the joint strength was considered at the varying of the most relevant process parameters. Furthermore macro and micro analyses were developed in order to highlight the process mechanics and the local material microstructure evolution. The investigated technology appears a promising joining technique in order to develop effective spot joints.