A process with dual benefits: quality in manufacturing, efficiency in recycling

The demand for cast aluminium components is enormous given their wide range of applications. At the same time, issues such as resource scarcity, sustainability, recycling and the circular economy are becoming increasingly important alongside pure component quality. These changes necessitate the development of innovative and open, automated melting process technologies. This is precisely where the team at the Steinbeis Innovation Center Intelligent Functional Materials, Welding and Joining Techniques, Implementation comes in: together with its partner SMP Schüßler Modell- und Prototypenbau, the Steinbeis enterprise has developed a universally automated plasma arc melting torch as part of a project funded by the German Federal Ministry for Economic Affairs and Energy. Equipped with an integrated polarity reversal system and modular functional components, it enables the continuous melting of metallic materials using a large-area, high-power plasma arc.

The trend towards environmentally friendly and energy-efficient process technologies for manufacturing functional components and products is leading to the development of new lightweight construction concepts. These focus on reducing component weight whilst maintaining or increasing performance. Cast aluminium components are used in numerous applications across many sectors, such as energy generation (transformers, generators and electric motors), the automotive and aerospace industries, and mechanical engineering, and play a key role in the success of the energy transition. The quality of the components plays a decisive role in this: casting defects must be reduced or avoided. These include, amongst others, surface and volume defects (such as cracks, pores, defects or voids, microstructural defects, and poor dimensional and shape stability), which limit or prevent the usability of the components.

The issue of sustainability and resource efficiency in the manufacture of aluminium cast components is also coming increasingly into focus. To meet the demand for metal raw materials, conserve resources and reduce emissions, scrap and residual parts, as well as metal shavings, should be recycled and returned to the economic cycle. This allows, for example, aluminium raw materials to be recovered for industrial applications. In this way, a sustainable and economical use of metal raw materials can be achieved throughout the entire manufacturing chain. This growing market offers great potential, particularly for small and medium-sized enterprises, but requires innovative, automated and open melting process technologies.

Plasma arc melting torch: flexible, sustainable, high quality

Against this backdrop, the team at the Steinbeis Innovation Center Intelligent Functional Materials, Welding and Joining Techniques, Implementation and its project partner SMP Schüßler Modell- und Prototypenbau are developing a universally automated plasma arc melting torch with an integrated polarity reversal system and functional modules. It enables the continuous melting of metallic materials using a large-area, high-power plasma arc. The molten mass produced is continuously processed in a specially developed open casting module system under a protective gas atmosphere and transferred via a melt chute into a crucible system for the casting process. The functionality of the developed process technology has been demonstrated through the production of aluminium casting prototype parts and the melting of various recycled materials.

The application of this process in metal melting technology – particularly for the production of aluminium cast components and the recycling of metallic materials – achieves several technical, technological and economic objectives:

• high process flexibility in terms of material input, implementation and component geometry,
• high-quality production of simple and complex aluminium cast component geometries with definable mechanical and metallurgical properties,
• energy-efficient recycling of metallic materials through the melting of scrap and off-spec parts as well as metal swarf with reduced energy and technological input,
• recovery of metal raw materials,
• lower acquisition and operating costs due to reduced process steps and manufacturing costs, lower energy consumption and sustainable material utilisation, as well as
• an overall sustainable production process.

From concept to complete system

The first step in developing and constructing the torch system was to draw up a concept. A plasma torch with transferred arc technology, an integrated power source, plasma and shielding gas supply, and a pendulum unit for the torch’s lateral movement was designed for an electrical power output of 12 kW. To this end, the project team developed the necessary functional parameters as well as materials and auxiliaries based on defined technical target parameters to ensure a safe process sequence. Using the polarity switching system, the electrode/cathode could be set to positive or negative polarity depending on the type of material.

Subsequently, a dual-circuit water-cooling system was developed for the torch head, enabling separate cooling of the plasma nozzle and electrode, as efficient cooling is essential for the high-power melting process. The correct selection of raw materials and consumables was a prerequisite for achieving optimal and safe operation of the entire system, as well as the required electrical and thermomechanical properties of the system’s functional components. A highly focused plasma arc with high energy density was used to melt aluminium and other recycled materials. An intensively cooled plasma nozzle (anode) was developed for this purpose.

In the next step, the team tested the burner prototype on a test bench and evaluated it in terms of parameters such as ignition behaviour, leak-tightness and gas supply. The burner was tested in direct current mode with a positive electrode and negative potential at the component and vice versa (with a positive electrode), as well as in alternating current mode at a current of 120 A (alternating current frequency of 80 to 120 Hz). A stable arc was successfully ignited, ensuring a reliable melting process. The dual cooling circuit provided sufficient cooling capacity, ensuring that no thermal damage occurred. The planned wider arc width of up to 50 mm with a deeper melting effect was achieved using a built-up plasma nozzle configuration. An even larger area of arc melting was achieved through the integrated torch oscillation unit on the torch system, which allows the torch to be moved and controlled in the transverse direction along the width of the material to be melted at a defined feed rate using an electric cylinder. This cylinder was integrated for vertical positioning to ensure a constant distance between the torch and the material.

The team also carried out metal melting trials using the fully developed system technology. The plasma arc melting torch, including its associated equipment, was technically linked to and integrated into the overall casting system. By varying the process parameters (current, voltage, process gas volume, material feed rate, etc.), melting rates of up to 0.4 kg/min were achieved. This demonstrated that the burner functionality and its melting rate can be significantly influenced by differentiated polarity (direct or alternating current operation), variable burner oscillation width and adapted process parameters. Alternating current operation – like direct current operation – resulted in a stable plasma arc and thus a reproducible and uniform melting process, but with lower thermal stress on the electrode and simultaneous removal of aluminium oxide layers from the material surface.

To avoid excessive thermal stress on the electrode, the use of alternating current is particularly suitable for machining aluminium materials. Using the plasma arc process implemented, aluminium materials were successfully melted with a defined melting rate under various process parameters in both direct and alternating current modes. In direct current operation, the set current ranged between 130 and 250 A. The torch also operated in alternating current mode under varied current parameters, thereby enabling the determination of optimal process parameters. The aluminium material was melted using alternating current at a frequency of 100 to 120 Hz with a negative AC balance of up to 10 per cent. The project team subsequently used the aluminium material melted by the plasma torch in the casting process to manufacture cast components.