Solid-state joining processes

A central task in the further development of solid-state joining processes is monitoring and the guarantee of process reliability. For this reason, interactions from the joining process are increasingly being used to derive conclusions about the quality or condition of the joint. These include, among other things, topographical properties of the joint or measurable process feedback effects such as the acting forces, torques or vibrations. The use of artificial intelligence can also help to identify patterns that indicate instabilities or irregularities. These include contamination in the joining area and process-related wear.

Resistance spot welding is the most established process for spot-shaped sheet metal joints as well as for electromechanical applications. The advantages of resistance spot welding are its high degree of automation, low process costs, short process times, and the fact that it does not require joining elements or filler materials. The research of the department focuses on the application-oriented electrical and material adaptation of the electrodes. In order to achieve suitable contact conditions, the electrodes are optimized with regard to a variety of aspects. This makes it possible to produce hybrid joints as well as mixed metallic joints.

The operating principle of the ultrasonic and friction stir welding process is fundamentally based on friction-induced heat input, which plasticizes the materials to be joined. However, continuous application causes process-related wear to occur on the contact surfaces of the tools. A quantitative and qualitative analysis of the wear initially enables the localization of wear-relevant areas as well as the detection of the acting mechanisms. Based on this, methods for reducing, predicting and preventing wear are currently being identified.

The design of friction stir welding tools in industrial applications is usually based on empirical values or empirical considerations. Oversizing of shoulder and welding pin causes high process forces to occur, whereas under sizing increases the susceptibility of welding pins to breakage. A suitable design of the tools according to the joining task can therefore be considered a challenge. In current investigations, a methodology for the design of friction stir welding tools is being developed based on the real stresses occurring in the process. This makes it possible to design the welding tools individually depending on the joining task.

Friction stir welding can be implemented using different tool concepts. The main distinguishing feature here is the control and the direction of rotation of the shoulder and welding pin. The application of the respective tool concepts can ultimately be justified by the necessary weld seam requirements, loads in the process or the respective joining task. For this reason, current research projects are investigating the combination of different tool concepts, such as the standing shoulder, the conventional design and a variant with the shoulder and welding pin rotating in opposing directions. The different tool concepts are realized by means of a spindle attachment developed at the TU Ilmenau.

The advantages of friction stir welding result from the excellent mechanical and metallurgical weld seam properties, high seam reproducibility and the realization of similar and dissimilar joints. A prerequisite for the production of such joints is the application of a suitable heat input, which is ensured, among other things, by the axial force. At the same time, the acting process forces represent a challenge for the application of friction stir welding, with reference to deformation-prone and complex component parts (e.g. hollow sections). This results in increasing demands on support and clamping structures. One way of reducing process forces can be achieved by selectively reducing the shoulder and welding pin diameter (scaling). The process forces can be reduced by up to 64% using the tool scaling approach.

Future challenges for industrial products are increasingly determined by resource and sustainability requirements. At the same time, aspects of cost-efficient manufacturing must be taken into account. In this context, lightweight construction applications are gaining widespread use across all sectors. In the automotive sector, for example, compounds made of aluminium and copper (Al/Cu) are increasingly being used, as this allows the vehicle weight and the required material costs to be reduced. Simultaneously, the formation of brittle intermetallic phases must be reduced. Al/Cu or Al/steel joints produced by friction stir welding, resistance spot welding or ultrasonic welding are therefore subject to special requirements. Aluminium and copper can be reliably joined by selecting the appropriate joining process and carefully analysing the process control.

The reliable application of pressure welding processes requires an advanced understanding of the interactions between the material and the joining process. These can be detected using suitable measurement procedures and analysis methods. These include the forces, torques, temperatures and vibrations acting throughout the process. Optical methods such as thermography, laser vibrometry or high-speed imaging allow the interaction of joining method, material and process feedback to be reliably analysed.

Ultrasonic welding is a widely used joining process that is characterized by fast process times. In addition, it can be used to produce similar as well as dissimilar joints. Ultrasonic welding has proved particularly successful in the joining of cables or stranded wire/conductor connections. However, the assessment of the joint quality of such joints has so far been based exclusively on the achieved strength. For this reason, further evaluation methods are currently being analysed which will allow an advanced understanding of such joints.

The thermal joining of metals with thermoplastics is a new type of process that can be carried out by means of laser joining as well as resistance spot welding. Applications range from the automotive industry to machine and plant construction to "white goods" in household appliance technology. Thermal joining enables the direct production of a hybrid composite without additional joining elements or adhesives and allows the simultaneous use of both materials for the implementation of optimized component structures. The main focus of the research is on linking process design and the resulting material properties, aging and fatigue behaviour, as well as fundamental considerations of the interface and bonding mechanism.

Diffusion bonding has practical applications in the manufacturing of internally contoured components for tempering and in the production of mixed joints. It also represents a model process for other solid-state joining processes, allowing the recrystallization- and diffusion-related processes at the material interfaces to be analysed under adjustable boundary conditions.