Steinbeis experts develop process technology for thermal joining of multimaterial components and composite structural components
The demand for multimaterial components, also known as multimaterial mixes, is rising continuously. This is particularly the case with lightweight materials made from a combination of metals and plastics, now used in lightweight construction, apparatus, vehicles, and aircraft. The key aim with such materials is to significantly minimize the amount of energy needed in transportation by making them lighter. The increasing use of multimaterial mixes also places demands on thermal joining processes, especially in terms of material use and technology. For example, processes may need to be automated to achieve defined levels of seal stability. Such composite material requirements now make it necessary to come up with new types of arc welding technology, along with the corresponding hardware. Experts at Intelligent Functional Materials, Welding and Joining Techniques, Implementation, a Steinbeis Innovation Center based in Dresden, decided to take on this challenge – and were successful!
The new process solves some key technological problems encountered with materials: insufficient strength, breakdown of heat-sensitive parts due to excessive thermal loads, and material delamination due to exposure to extreme heat. The team members ensured the project also looked at gas metal arc welding (GMAW) with non-transferred arcs. Their work benefited from experience gathered on a previous R&D project with the welding equipment manufacturer Weber.
The focus of this follow-on project lay in developing a form of process technology representing a hybrid of arc soldering and GMAW welding, with corresponding torch technology for joining thermally sensitive multimaterial mixes. A compact welding torch technology was developed, based on a non-transferred arc with a non-melting and melting electrode, to provide a kind of torch prototype designed and built so that it could be tested and evaluated. Based on these technical and material concepts, the team defined a list of requirements for the MSG hybrid soldering torches developed and for the corresponding peripheral technology.
A basis for MIG and TIG welding
The first concept developed by the team was based on the principle of metal inert gas (MIG) welding with a non-transferred arc. The filler metal – a wire electrode – was defined as the cathode (positive polarity) with a cooled copper nozzle as the anode (negative polarity). The second concept was based on the principle of cold wire tungsten inert gas (TIG) welding with a non-transferred arc. With this concept, the arc burns between the tungsten electrode (cathode, negative polarity) and a cooled gas anode (copper ring nozzle, positive polarity).
To develop the required wire-feeding unit, again the Steinbeis team joined forces with its partner from industry, Weber. The first step was to develop torch heads. These had to meet a number of requirements: faultless arc ignition, reliable arc stability, a reproducible and functionally reliable arc-soldering process, and reliable torch operation by building an effective cooling system and compact torch head. For the torch head variant to be used for MIG welding, a tungsten electrode was mounted along the side of the contact tip to deliver simple ignition and a stable arc process. For the torch variant with an external wire feed, the torch head was designed in such a way that the cooled copper ring nozzle acts as the anode, with the tungsten electrode acting as the cathode. The filler material is fed from the side and melts into the arc area. This variant delivers reliable arc stability. Since this subjects the anode to high thermal loads, it has to be cooled.
The research allowed the project team to evaluate process data from a number of angles. They were able to look at the physical effects of the non-transferred arc for both torch concepts, the impact on the melting process of adjusting certain process parameters, droplet transition/flow and corresponding thermal distribution, the cumulative energy balance, and the relationship between arc emissions and component surface activation. By carefully synchronizing the wire feed and joining speeds, as well as other parameters influencing quality such as the welding current, welding voltage, and droplet temperature, it was possible to optimize the performance of the welding process under marginal conditions.
Testing the torch prototypes
The two prototype torches were tested for ignition, electrical insulation, sealing of the cooling system, and water/gas flow. The results were impressive, with no ignition problems when using the power source as a high-frequency device, and a stable arc process at low power levels. The watertight seal produced by the torch heads was flawless and the cooling system functioned well. Arc stability is strongly dependent on the wire feed and joining speed. Wire properties such as stiffness, electrical conductivity, and diameter are also important and have a strong influence on the stability and effectiveness of the soldering process. The project team managed to achieve high soldering speeds of up to 1.5 m/min with excellent solderability.
The soldered test specimens based on the two soldering wire types (SnCu3 and CuAl8) and the TIG torch variant produced defect-free soldering joints. When CuAl8 was used as a filler metal with different diameters, the soldered samples had excellent joints with high tensile strengths of up to 90% of the base material. A solid bond was produced with high adhesion properties in the soldered area on the substrate material. When multimaterial parts or sandwich materials with a variety of seal designs were soldered to the same multimaterial or galvanized steels, the polymer layer was preserved without damage.
With the MIG torch variant, the arc ignition process was based on an electrical source with a high-frequency ignition unit. With this approach, keeping the preset distance constant between the tungsten electrode and the wire resulted in the arc igniting quickly and evenly, such that the distance between the anode and the cathode no longer had a critical impact on the ignition process. The experiments and torch tests that were conducted demonstrated that the cooling system on the torch is effective and watertight, and the level of process gas flow meets functional requirements. Using welding current and voltages of 150 A and 27 V makes it possible to achieve soldering speeds of up to 1.5 m/min. At low torch power levels (30-50 A and 20-30 V), the arc is stable, but the joining speed has to be adjusted. Ideally, it should be set right at the beginning of the process and synchronized carefully with the rate of wire feed. The soldering tests looked at different types of seams such as I seams, blind seams, and flanged seams. None of the soldered seams had brazing defects in the microstructure or seam joints, and the polymer material remained intact.
The wire feed system developed jointly by the Steinbeis experts and Weber showed that the welding filler material feed easily met technical fault-free feed rate requirements, achieving up to 10 m/min for diameters ≤ 0.8 mm as well as diameters ≥ 0.8 mm. In such cases, arc stability is strongly dependent on the positions of the wire ends and their dimensions, making it essential to ensure that wire feed rates remain constant. Using the wire feed system developed for the project resulted in good arc stability and correct synchronization of molten droplets with the joining speed.
Both of the torch head prototypes that were developed and constructed on the basis of a non-transferred MSG arc function flawlessly in process terms and are easy to connect to welding robots. The torch cooling systems are highly effective and meet torch performance requirements, and ignition processes are faultless using straightforward welding power on high-frequency ignition units. The new wire feeding system offers good arc stability with uniform and continuous wire feeding, paving the way for excellent soldering joints.
Associate Dr.-Ing. habil. Khaled Alaluss (Autor)
Steinbeis Innovation Center – Intelligent Functional Materials, Welding and Joining Techniques, Implementation (Dresden)