3rd International Conference on Advanced Joining Processes 2023

In this paper, experimental studies on the load-bearing behaviour of tensile test pieces, bolts, and lockbolts subjected to combined tension and shear loading are presented. The results are compared to the design approaches from different standards and evaluated in terms of reserves and uncertainties. In general, a notch-independent behaviour is observed for the specimens with smooth, threaded, or grooved shank. It turns out that the description of the load-bearing behaviour by linear interaction models is unreliable. Although the testing results are arranged in an arc shape in the interaction diagram, they are located on the uncertain side of the quadratic interaction model, which is valid according to the standard VDI 2230 part 1. Especially due to the lack of consideration of the tensile load-bearing capacity of the cross section in the shear plane, the load-bearing potential is not fully utilised by the interaction approaches of some standards.

To determine the dependence of the test velocity on the friction coefficient (µ) in high-strength preloaded connections, experimental investigations are performed on bolted joints typical of rail vehicle construction. This is achieved through slip load tests following the procedure outlined in Annex G of EN 1090-2, well as high-speed tensile tests, subsequently comparing the resulting friction coefficients. Furthermore, the influence of a previously acting cyclic load for two different friction surface conditions is presented. As a result, an influence of velocity on the friction coefficient could not be definitively determined. However, the increase in the friction coefficient by applying a previously acting cyclic load as well as through the use of friction-increasing films was successfully demonstrated.

As a technology successfully employed on joining aluminum and steel, Self-Piercing Riveting (SPR) is yet to be established for magnesium alloys. Due to magnesium’s limited slip systems at room temperature, the main issue with the development of magnesium SPR technology is the magnesium button cracking. Several techniques have been proposed by researchers to address this problem, including pre-heating magnesium, applying rotation to rivets during SPR, using a sacrificial layer and padded SPR. All of these methods require installation of new equipment or additional processing steps. The objective of the current work is to introduce a SPR technique which employs current existing SPR machines with minimal modifications to existing processes to create crack free joint between magnesium and aluminum. Experiments were carried out to join coupons of 1.0, 2.0 and 3.0 mm thick aluminum to AM60B magnesium high pressure die casting (HPDC) samples. Through the results, it was shown that the joint is formed which is crack-free both externally and internally with a joint strength of >4 kN. With the flexibility of material selection and the compatibility with existing SPR process lines, this SPR process has great potential to be an enabler for more SPR applications on materials with limited ductility.

Friction Stir Welding (FSW) is a solid-state welding process, which has strongly impacted welding technology, particularly for aluminum alloy applications. Due to its high-quality welds in all aluminum alloys, comparatively low specific heat input at high energy efficiency and ecological friendliness, FSW is used in a rapidly growing number of safety critical applications. Currently destructive and non-destructive testing methods are added as a separate process step to verify weld seam quality, adding complexity, cost, and time to the production. Imperfections are detected late in the production process and require costly rework or discarding of the assembly. Several studies have shown the possibility of using Deep Neural Networks to evaluated data recorded during the FSW process. Analyzed data includes thermal measurements, acoustic measurements, image or video data and most commonly the comparably large and distinctive process feedback forces. This study is a continuation of efforts by the research group. In this study Convolutional Neural Networks (CNN) based on the DenseNet architecture were successfully trained to classify FSW process force recordings supplemented with weld meta-data to detect volumetric subsurface defects. The data-sets were generated while welding different aluminum alloys in multiple sheet thicknesses over a wide range of spindle rotational speeds and feedrates. The networks classification accuracy as well as the ability to generalize across the different welded aluminum alloys, sheet thicknesses and corresponding welding tools was evaluated. Achieving a classification accuracy of 98.37%, the development aims to provide a reliable and cost-effective quality monitoring solution with a wide range of applicability to replace the required expensive and time intensive ultrasonic, x-ray or macro-section weld seam testing.

The economic joining of large pipes with large sheet thicknesses is becoming increasingly important in wind turbine construction, not least due to the defined and adopted expansion targets for wind energy. Currently, only conventional arc welding processes such as the gas metal arc welding (GMAW) or the submerged arc welding (SAW) are used for joining these large pipes. Process-induced, a large number of weld passes are required for large sheet thicknesses. This results in considerable costs, which are composed of the energy requirements, weld filler metal and production time, among other things. By combining the laser beam process with the conventional submerged arc welding process in a common process zone, it was possible to develop a high-performance welding process at the Institute of Welding and Joining Technology at RWTH Aachen University that enables more efficient joining in the thick sheet area. With the laser beam submerged arc hybrid welding process (LUPuS Hybrid), it has already been possible to join sheet thicknesses of up to 50 mm using the layer/counterlayer technique in just two weld passes. Therefore, this paper presents the further development of the LUPuS single wire process into the LUPuS tandem process in order to increase the robustness against component tolerances such as joint gaps and offsets. The welds carried out with the newly developed LUPuS tandem hybrid process show that both joint gaps (up to 1.5 mm) and offsets (up to 3 mm) can be performed with high weld quality and reproducibility.

The increasing popularity and rapid advancements in Fused Deposition Modelling (FDM) have greatly expanded the spectrum of available materials for this technology. Among these materials, fully organic biodegradable PLA-based composites, enriched with a variety of organic additives, have emerged as a fully green and cost-effective alternative to conventional commercial FDM filaments. However, it is crucial to acknowledge that the incorporation of organic additives significantly influences the mechanics of the printing process and introduces complexities into the intricate process-structure–property interrelationship. This increased complexity may give rise to various challenges, such as the formation of defects, nozzle clogging, and even the failure of components during fabrication. Therefore, there is an urgent imperative to advance our understanding of the FDM process when utilizing biocomposite materials. Equally important is the development of computational tools capable of predicting and mitigating defects in biocomposite components produced through FDM. In response to this demand, the present study introduces a mesoscale numerical model specifically tailored for biocomposite materials. This innovative numerical model successfully captures the formation of defects in PLA/wood composites manufactured using varying layer heights. Importantly, the numerical results align closely with experimental findings. Moreover, the model identifies the formation of wooden sublayers at the peripheries of deposited layers as a potential factor contributing to reduced layer bonding and the overall occurrence of defects.

Laser joining of GFRP (nylon 66–30% glass fiber reinforced) to titanium is a promising method that can reduce product weight and enable multi-material development feasibly in miscellaneous industries such as aerospace. However, this joining is challenging due to the mismatch of their chemical and physical properties. Thermal degradation of the polymeric material due to the high energy input of the laser beam and the lack of proper adhesion are common defects in overlap configuration which can reduce the mechanical strength and permeability. To address the former, a structured and controlled manner design of experiments (DoE) is applied for statistical analysis to gather the optimized parameters of the joining. Studying a range of laser power, laser scanning velocity, and beam oscillation of laser beam shows there is a threshold that can affect and improve the joining quality and mechanical strength. To resolve the latter, laser surface treatment of the Ti prior to the laser joining process, which is a precise and non-contact procedure, is applied to strengthen the bonding. Microscopic observation and mechanical tensile-shear tests are used for analysis of the joints and mechanical strength measurement.

Resistance welding (RW) of high-performance carbonfiber-reinforced (CF) thermoplastics such as polyphenylene sulfide (PPS) have high potential for the use in aircraft primary and secondary structure assembly due to its suitability and scalability for high-volume production. In this study, we investigate the application of electrical RW for CF/PPS using a CF-based weld conductor. We focus on the validation of temperature homogeneity during the heating phase and the characterization of the resulting welding properties using non-destructive and destructive testing methods. Two methods of applying power with constant voltage and pulsed voltage were investigated and compared with regard to the temperature homogeneity of the weld. Experiments were conducted to characterize the properties of the welds and validated using a Design of Experiments (DoE) based screening test plan. The welds were validated for heating time, attenuation losses during water coupled ultrasonic testing (acc. to AITM 6- 4010) and single lap shear strength (acc. to ASTM D1002). Furthermore, the weld results were examined in detail by means of reflected-light and scanning electron microscopy for a clear interpretation of the DoE results. Thus, the present investigations served to further understand important process variables in relation to RW, paving the way for the detailed design of a robust process cycle.

Due to their corrosion resistance, nickel alloys are increasingly used in environmental and power engineering. Incorrect preparation, execution and finishing of welds on nickel alloys can lead to problems such as cracking, porosity or inclusions in the weld metal and heat affected zone. Nickel is prone to pore formation during welding, with a sharp change in the solubility of hydrogen and oxygen as the metal passes from the solid to the liquid state. In order to weld nickel alloys and nickel 99.2 sheet efficiently with predictable results, different welding processes need to be evaluated and optimized with respect to the influences that affect the welding process. A sound technical solution for the efficient and flawless welding of Ni 99.2 and nickel alloys must therefore be found. The following article presents an investigation of the influence of different seam preparations welded by different welding processes (beam welding, arc welding) to determine the most effective welding process for Ni 99.2 (2.4068) thin sheets. For this purpose, thin sheets with different edge preparations are produced for linear welding. In addition to the optical and mechanical properties, the microstructure of the weld is also investigated. The results show that it is possible to establish a stable and efficient welding process for Ni 99.2. Cross sections show dendrite growth towards the center of the seam. Preliminary tests show a dependence of the grain orientation on the welding speed. The influence of the grain orientation and the characteristic of the dendrite growth direction on the mechanical-technological properties of the welded material is investigated.