The torsion test is rarely used for resistance spot-welded joints since they are not subjected to torsion in applications. Normal, shear, and/or peel loads are usually the main stresses. Extensive scientific investigations in the context of Kunsmann’s dissertation date back more than 50 years. These investigations are still the basis of ISO 17653 and the German guideline DVS 2916-1. Recent scientific investigations only use torsion tests, but do not describe the reason for its use. A decisive advantage of the torsion test over the other standardized destructive testing methods lies in the types of fracture modes that occur and the properties of the fracture surfaces. Torsional loading results in either interfacial or button-pulled fracture modes. No material residues occur on the fracture surfaces for ductile and advanced high-strength steels. Hence, the measurement of weld diameter is achievable with minimal constraints, resulting in reduced variability and facilitating objective assessments of spot welds. This article delineates these attributes through a comparative analysis of various destructive testing methods employing statistical approaches. Additionally, the article expounds on the design concept of the developed rig for conducting torsion tests on spot welds.

The weld of resistance projection welded joints is not visible from outside. Therefore, visual evaluation is restricted, and visual inspection is not possible at all. So far, the parameters of the welding process are monitored and controlled. In addition, the welds are periodically tested destructively to determine the quality of the welds. No industrial standard has yet been established in the field of non-destructive testing (NDT) for projection welded joints. This study focuses on NDT of projection welds using two different ultrasonic imaging inspection systems and the passive magnetic ux density testing (pMFT) method. The ultrasonic inspection systems commercially available and established in the field of NDT of spot welds. Both systems are originally designed for the NDT of spot welds and not for projection welds. Unlike the ultrasonic systems, the pMFT is still in laboratory status. The method has originally been developed to evaluate spot welds and is also used in this study to evaluate projection welds. The applicability of the investigated systems to projection welding is investigated in order to derive mandatory development steps to achieve reliable results. The pMFT method shows also good results for NDT of spot welds In this contribution, the measurement and evaluation concept of the three NDT systems for projection welded joints is presented. The NDT results are discussed in the context of the corresponding destructive results in terms of tensile forces and fracture areas. Advises for further development of all investigated systems are given.

Capacitor discharge welding (CDW) is characterized by a pulsed electrical current profile. It is primarily utilized for resistance projection welding tasks, offering high power densities and short welding times. According to the latest findings, the welding process can be divided into different phases: contacting, activating, material connection, and holding pressure. During the activation phase, high-speed video-imaging reveals the generation of metal vapor which effectively eliminates impurities and oxide layers from the contact zone. The result of this is an activated surface. The purpose of this paper is to describe the physical effects of the bonding mechanism during short-time resistance welding. The Chair of Joining Technology and Assembly at the Technische Universität Dresden has a laboratory facility that can interrupt the welding current at any desired time during capacitor discharge welding. This allows different welding current profiles with always the same current rise time to be scientifically investigated. The experimental findings were supplemented with simulative analyses to clarify the bonding mechanism in resistance projection welding. Three different surface conditions are considered to generalize the findings on the bonding mechanism. Temperature and current density distributions were assessed to provide a physical description of the activation phase. The power density in the joining zone at different interruption times is determined, which gives an indication of activation by metal vaporization. The material connection is determined experimentally for the same interruption times. The numerical simulation model can be used to describe the bonding mechanism in short-time resistance welding. In resistance welding, the bond is formed due to the molten phase (solidification structure). In short-time resistance welding, the bond is formed due to surface activation by metal vaporization.

Resistance spot welding (RSW) of galvanized steel sheets brings a risk of faulty welds in the form of stick-welds. These differ from high-quality spot welds in the way that only the coatings are joined with a material bond, but the base materials are not fused together. Stick-welds can occur, among other things, in critical material combinations with greatly differing sheet thicknesses or with increased wear of the welding electrodes. Due to the significantly different material-specific strengths of zinc and steel, stick-welds do not meet the required weld strengths. Therefore, it is important to reliably prevent or detect the stick-welds. However, the established and most frequently used non-destructive (NDT) method for spot welds by manual ultrasonic testing reaches its limits in this respect, as the fused zinc layers are almost as sound-conductive as proper spot welds. A promising alternative is the method of passive magnetic flux density testing (pMFT) developed at the Technische Universität Dresden, Chair of Joining Technology and Assembly. With this NDT method, the spot welds are first magnetized and their residual flux density on the surface is measured. Due to the different magnetic properties of the used steel alloys and their zinc-coating, clear differences can be measured between correct and stick-welds. This paper shows the experimental design and the developed procedure for evaluating the spot welds on different galvanized steel sheet alloys. In addition, the high potential of integrating this NDT method into fully automated production lines is shown in order to bridge the gap between the highly automated RSW process and NDT, which can only be carried out manually up to now.

The amount of aluminium sheets in future body-in-white concepts is still on the rise. There is a need for optimizing the joining techniques, caused by the different characteristics compared to the established steel components. Especially the electrode life for resistance spot welding as a reliable and established process needs to be improved. One reason for the short electrode life when welding aluminium is the insulating effect of the aluminium oxide layer. One possibility to reduce the electrode wear is the mechanical destruction of the oxide layer before the welding. This paper describes the influence of a translational and rotational electrode movement on the electrode wear. The oxide layer destruction is detected by resistance measurement. It could be shown that the destruction of the oxide layer already occurs at low movements. However, a homogeneous, large-area destruction is necessary for a wear reduction.

Conventional resistance spot welds are not visible from the outside. Therefore, it is not straightforward to evaluate the joint quality non-destructively. The pulse-echo method of manual ultrasonic is widely used for non-destructive testing. Another option is the passive magnetic flux density testing, which is being developed at Technische Universität Dresden, Germany. The spot weld is magnetized in the normal direction and the residual magnetic flux density is measured on top of the surface of the joint. This method is suitable for spot welds on typical car body steels. Previous investigations show that the magnetic properties of the materials influence the test result. In order to develop this new non-destructive testing method further, it is necessary to know the magnetic properties of the different microstructure regions of a spot weld. This article focuses on methods to measure and evaluate the magnetic properties of these regions, especially of the base material and the weld. Different measuring methods and approaches are presented and compared with each other. Based on the results, recommendations for future measurements for magnetic characterizations are given.

Resistance spot welding is used as a high-productivity joining process for the increasing use of lightweight materials in industrial applications. The main challenge for the welding of aluminum alloys is the wear of the electrodes. Due to the natural oxide layers on the metal surface, high contact resistances exist between the sheet metal and the electrode. This results in increased heat generation at this interface, which leads to increased alloyage on the electrode. Breaking the insulating natural oxide layers can be achieved by a friction-free or friction-assisted motion overlay and leads to a reduced electrode wear. The complex physical correlations involved are currently being investigated in a basic research project at TU Dresden. In addition to welding tests on a test rig, simulation-based analyses are also carried out. These allow a physic-based modeling of the effects and can be used later on for an extrapolation of any resistance welding process with motion overlay. In this paper, first experimental and analytical considerations for the mechanical destruction of the oxide layer of aluminum are discussed. Subsequently, the developed extended simulation model is presented and compared according to the quality of simulation of the welding process with motion overlay. The simulation method used is a multiphysics FEM simulation with ANSYS.

Capacitor discharge welding primarily applies to projection welding. Components with ring projections up to 200 mm diameter can be welded with peak currents up to 1000 kA and welding times less than 10 ms. Weld nuggets are expected to occur, as the CD-Welding belongs to resistance welding. Although the required strength is given, welding nuggets in cross-sections cannot invariably be verified. According to recent researches, joining occurs without a welding nugget, but with metal vaporisation and linked activation of the surfaces. This process is called short-time welding with high thermal current density. The type of the welded joint depends on the heating properties in the weld zone. Different welding energies, electrode forces and welding times result in different welded joints. The process-specific advantages can be taken, and new application areas for CD welding can be developed by knowing the cause-and-effect relationships.

Non-destructive testing (NDT) of resistance spot welding compounds continues to pose a great challenge. The nugget diameter is the most important quality criterion. Established ultrasonic-based methods reach their limits when evaluating the nugget diameter of combinations with significantly different sheet thicknesses and with deep electrode indentations. In these cases, the imaging analysis of residual flux density shows high potential. Previous results of this NDT method concentrate on spot weld combinations of ferromagnetic steels. Hence, latest research at Technische Universität Dresden focuses on the application of the imaging analysis of residual magnetic flux density on spot welds using austenitic steels. Due to the high cooling rates after welding delta ferrite with ferromagnetic properties is generated inside the nugget, and therefore the residual magnetic flux density can be measured. The talk will show the measuring concept of the NDT method for spot welds and presents the results of its application on two and three sheet metal combinations of austenitic steels.

Resistance spot welding is one of the most frequently used joining methods in automotive and rail vehicle engineering due to its high proces s reliability and economic efficiency. In consequence of the process characteristics, the pot weld is localized within the overlapping area of the welded plates. For the nondestructive quality assurance, the concealed position of the joint is problematic. Recent research at Dresden University of Technology focuses on a non-destructiv e testing (NDT) method for resistance spot welding of ferromagnetic steel plates. Th joint is magnetized by coils and the remanent magnetization is analyzed afterward s. The talk introduces this new NDT method, compares it to established testing systems for NDT in resistance spot welding and presents the results of i application on two and three sheet metal combinations.

The continuing trend of lightweight construction within the industry requires joining methods for lightweight materials, such as aluminium alloys, that provide a safe and reliable result. One of these processes is resistance spot welding. Its high level of automation and efficiency lead to being one of the most widely used joining methods. The current problem in resistance spot welding of aluminium alloys is the high wear of the electrodes. One reason is the insulating effect of the aluminium oxide layer. The high electrical resistance of the oxide results in local temperature rise and an increased wear on the electrodes. In the extreme, after less than 100 spot welds, the process reliability is no longer guaranteed. A way to reduce the wear is the mechanical destruction of the oxide layer. One possibility is to rotate the welding gun and thus the electrodes around the z-axis, as KUKA’s RoboSpin realizes it.

GMA welding is one of the most frequently applied welding techniques in industry. Particularly the joining of aluminium, high alloyed steels or titanium requires a cover of shielding gas in order to provide a low PPM concentration of oxygen. The result of the welding process depends essentially on the chemical and thermophysical properties of the process gas used. Consequently, it is necessary to be able to describe and to analyse its flow with respect to various influencing variables. However, it is very difficult to realize this during arc welding processes; a poor access is predominant due to the covered areas inside the welding torch and temperatures of up to 20 000 K cause the strong radiation of the arc and electromagnetic fields. This paper deals with experimental and numerical methods for visualization and quantification of process gas flows in arc welding and gives examples for their technical applications. Unlike previous work, the described methods consider the arc as a dynamic element which determinates the gas flow. Advanced Particle Image Velocimetry (PIV) and Schlieren measurement were used for characterization of the flow field in the direct vicinity of the arc in GTA and GMA welding. Furthermore, a numerical model including magneto-hydrodynamics and turbulence models was used for a detailed visualization of the flow in the free jet and in the hidden interior of the torch. It is based on a commercial CFD code which allows to model complex 3-D geometries of torch and workpiece design. Mixing effects and turbulence model were validated by oxygen measurements in the gas shield.

Gas metal arc welding (GMAW) is one of the most important joining processes in today's industry. GMAW with pulsed current is a highly dynamic and unsteady welding process, in particular. In pulsed GMAW, the welding current, the temperatures, the geometry of the electrodes, the plasma composition (argon and vaporized metal) and the shielding gas flow change permanently. In this paper, transient simulations of pulsed GMAW processes are presented, which calculate the complex arc properties as a function of time-dependent boundary conditions of electrical current and the vaporization at the wire tip (mass flow and location). The calculated temperature and iron distribution inside the arc are in good agreement with spectroscopic measurements. The transient behaviour of current and voltage in phase space diagrams shows a very good agreement as well. Finally, a purpose-built particle image velocimetry (PIV) system is presented, which is able to measure the flow in welding arcs and to validate numerically the calculated flow characteristics continuously.

The electric arc is widely-used for welding and cutting of metals. The knowledge of the plasma and shielding gas flow as well as its interaction with the electric arc and the energy transport in the workpiece is essential for optimizing torches and the welding and cutting process. For the simulation of electric arcs the commercial computational fluid dynamics code Ansys CFX can be used. Additionally a MHD model for calculation of the electromagnetic effects and large datasets of the plasma gas properties have to be implemented. By explaining the solution of a simulated hand-guided torch for plasma-welding the influence of process parameters, shielding gas properties and nozzle design is presented.

In modern vehicle bodies, light metals are frequently used in combination with steel sheets. Hot pressure brazing constitutes a suitable material-locking thermal joining process since the fusion welding of these combinations of materials leads to the formation of brittle intermetallic phases. One important prerequisite is the elimination of the oxide skin on the component surface. This can be carried out prior to the actual brazing operation by preplacing braze metal on the joining members by means of plasma brazing surfacing or non-vacuum plasma spraying. It is thus possible to manufacture similar and dissimilar joints consisting of high-strength aluminium alloys (such as AC120) or magnesium alloys (such as AZ31) as well as of galvanised steel sheet (DC04).