Thermal Joining Engineering

The research project investigates the application of induction technology for pre- and post-heating in manual wet arc welding. Due to the influence of the media, there are high levels of hydrogen input and, due to the strong convection, high cooling rates after welding. As a result, critical material properties and cracks can result. The effective introduction of energy by means of induction is intended to be used for the practical compensation of subsea-specific risks during wet welding and consequently, also enabling the safe joining of high-strength steels. This is necessary for the economical and high-quality repair of structures in hydraulic steel engineering. Application guidelines for the use of induction heating engineering are in development.

The welding processing of thick-walled sheets is necessary for many constructions in the maritime sector in constrained positions and under construction site conditions. ES duct welding is a welding process for single- layer welding of vertical seam connections which is particularly economical and with low distortion. However, the inadequate level of knowledge about welding metallurgy and the lack of system technology mean that it is rarely used. The aim is therefore to develop a mobile ES duct welding system for use on butt and T-joints, as well as to adapt the system technology, welding engineering and welding metallurgy of ES duct welding to European manufacturing specifications.

In steel construction, it is necessary to weld attachments without a primary load bearing function to structures subject to vibrational stress, which has an influence on the service life and design of high-strength steels. The project investigated whether arc brazing with copper-based solders is suitable for the substitution of welding processes in steel construction. The investigations show that the use of arc brazing to join attachments can extend the service life of the entire construction by up to 500% without the need for post-weld treatment. For example, there is a saving of 50 tonnes in weight on a tubular steel tower for wind turbines without reducing the static load-bearing capacity of the payload parts.

When joining aluminium, there is sometimes a great deal of distortion of the assemblies due to welding. In order to subsequently compensate for the distortion, straightening processes such as flame straightening are used. For steel constructions, annealing colours are used as a reference to estimate the temperatures required to achieve the desired straightening effect. This is not possible with aluminium, as annealing colours do not form in the relevant temperature range. The planned non-contact temperature monitoring system, which can be implemented in conventional straightening systems, is a basic requirement for increasing the reproducibility, reliability and degree of mechanisation in the flame straightening of aluminium, thus offering the possibility of qualifying the flame straightening process and expanding the business fields of interested SMEs.

In order to utilise the potential of high-strength steels for lightweight construction, the use of post-weld treatment processes is essential. In contrast to mechanical  engineering, a regulatory basis for the use of corresponding procedures is necessary in the highly regulated field of construction. In this regard, DASt Guideline 026 enables the use of high frequency metal impact (HFMI) treatment processes as post-weld treatment methods in German steel construction. Currently, the guideline is restricted with regard to material strength and the applicability of the methods to constructions with geometric imperfections outside the usual limits. Based on the results of this project, the guideline shall be extended to allow the use of ultrahigh-strength materials and to specify how to address geometric imperfections.

Thanks to the energy transition, wind turbines have become an indispensable part of modern power supply. The rotor bearings or rolling bearings are one of the most important components of such plants. The material used here must have a high degree of purity, hardness, wear and corrosion resistance in order to achieve the required load-bearing capacity and the longest possible service life. A general material characterisation with regard to the above-mentioned properties is the main component of this project and will be carried out according to the standards provided for this purpose. For the purpose of the load investigation, a mechanical-thermal coupled simulation model will also be created. This helps to determine the stress distribution and thus the geometric optimisation of the rotor bearing.

The importance of LNG and hydrogen as fuel is growing strongly worldwide. Transport and storage often take place in the liquid state at cryogenic temperatures. For this purpose, steels with high nickel content are usually used. Due to the increased demands on low-temperature toughness, the materials used are reaching their limits. One approach to solving this problem is the use of new types of medium- and high-manganese steels. However, there are currently no reliable welding processing instructions to bring these materials into widespread use for the growing market in storage/transport of cryogenic raw materials. Therefore, medium and high manganese steels will be investigated in this project and recommendations on processing strategies, modified filler materials and work safety will be developed.

There is currently little experience with the transport of large quantities of liquid hydrogen (LH2) due to the developing market. Tank designs relate to standard onshore storage and transport applications with vacuum-insulated, double-walled austenitic stainless steel structures, which have comparatively high thermal diffusivity and conductivity and increased weight. This currently reduces efficiency due to increased boil-off and unfavourable gravimetric storage density.

New tank concepts are therefore necessary for the maritime production and transport of LH2. Innovative technical approaches from space travel (fibre windings) and for high-temperature applications (thermal barrier coatings "TBC") are being taken up and combined.

The development of new manufacturing technologies is opening up new perspectives for the production of components for maritime systems. Additive manufacturing (AM) technologies have developed rapidly not only because of their ability to produce near-net-shape components with complex geometries, but also because they offer various advantages over conventional processes in the area of individual component production. As part of the current research project, in addition to the development of a robot-assisted machining cell and the design of the ship's propeller suitable for production, an adapted hybrid process chain is to be developed so that large-format components can be manufactured additively while ensuring high manufacturing accuracy and freedom from defects.

Due to the constant expansion of offshore wind farms and other offshore structures, the demand for repair and maintenance concepts is also increasing. Characterising the fatigue strength of wet-welded construction details is of enormous necessity in order to be able to perform fatigue verifications for cyclically stressed underwater repair welds in the future. Both WTG operators and commissioned engineering companies need knowledge about the possible results and consequences of a welding repair below the waterline. A direct classification of the data obtained in this research project into the notch case classes of Eurocode 3 thus enables a remaining service life calculation for the repair case.

In steel construction, joining long joints using MSG welding is a labour-intensive and time-consuming production step. Mobile welding carriages are often used here with the aim of increasing productivity with reproducibly high seam quality. In practice, however, weld defects and process instabilities often occur for various reasons, e.g. varying seam preparation. The aim is therefore to develop an algorithm and control system that detects and prevents process instabilities in mechanised MSG welding before they occur. For this purpose, a prognosis model is to be developed, which serves as a reference variable for a downstream control system.