High-performance materials are increasingly being used in car body construction in particular. In addition to optimized material selection and mixing, the focus is also on the joining solution, especially when it comes to joining aluminium and high-strength steel with a tensile strength of >1000 MPa. One method of joining both materials is friction element welding. ARNOLD UMFORMTECHNIK GmbH & Co KG from Forchtenberg is increasingly using this joining process for the multi-material mix. “We are expanding our product portfolio and want to be able to offer our customers services at all levels of car body construction, including when it comes to joining light metal sheet with high-strength steel,” says Emanuel Heinle, an employee in the Research & Development Fastening Systems department at ARNOLD UMFORMTECHNIK. He has been with the company since 2016 and is a specialist in the friction element welding process.
High connection performance thanks to a material-locking connection
In friction element welding, for example, an aluminum sheet and a high-strength steel sheet are joined together using a friction welding element. The method is based on a heat process that develops when the friction welding element rubs against the high-strength sheet steel.
The process is divided into four steps. In the first step, the hold-down device is placed on the assembly and presses the sheets together. The tip of the element sits on the aluminum cover plate. In the second phase, this sheet is penetrated: the fastener is subjected to a defined speed and axial force and thus penetrates the aluminum cover sheet. It is heated and plasticized as a result. At the same time, the fastener displaces the cover sheet material against the joining direction. Once the fastener has completely penetrated the cover sheet and makes contact with the high-strength steel sheet, the third phase of the process begins: rubbing. This involves mechanically cleaning the surface of the steel sheet. The fastener rubs against the sheet steel and the friction heats up the joining zone. The sheet steel and fastener plasticize. This shortens the fastener. If the shortening is sufficient, the final stage of the process begins: upsetting. The speed is reduced to zero and the joint is compressed with high axial force, causing it to cool down. The steel sheet and connecting element form a material bond and the cover sheet is held positively and non-positively between the steel sheet and the element head.
“The biggest unique selling point of friction element welding is that the process can be used for high-strength sheet metal with a strength of over 1000 MPa. There are few universally applicable solutions that have established themselves in automotive engineering for joining a high-strength steel and aluminum sheet,” says Heinle. There is also no limit to the strength of the steel material. The only thing that needs to be ensured is that the respective sheet thickness allows for reliable plasticization during the friction process. According to Heinle, accessibility from both sides is also required. As the joining direction is always from aluminum to steel, the joining element must be inserted on the side of the aluminum sheet. “In principle, the process is suitable for joining light metals to high-strength steel. No pre-punching is required and a very high joint strength is achieved,” says Heinle, summarizing the advantages.
ARNOLD UMFORMTECHNIK has developed its own connecting element for friction element welding, which is characterized by distinctive design features.
Application for components with high crash loads
Friction element welding is suitable, for example, for the production of vehicle assemblies such as the passenger cell, which is exposed to high crash loads in an emergency and must therefore guarantee high component rigidity. A base plate, which is made of high-strength steel, and a cover plate, which is made of aluminum or another light metal material, are always joined. High-strength steels such as hot-formed steel 22MnB5 can be used. The sheet thicknesses of the base sheet range from approximately 0.80 mm to 2.00 mm and those of the aluminum sheet from 1.00 mm to 3.00 mm.
Geometry of the connecting element for a reliable process
In collaboration with BMW AG, ARNOLD UMFORMTECHNIK has developed its own connecting element for friction element welding, which is characterized by various distinctive design features:
- The drive is designed in such a way that the axial force and speed can be optimally transmitted.
- The element flange guarantees a head support so that the joint offers the lowest possible point of attack for corrosion.
- The underhead groove provides space for the material rising from the cover sheet so that the element head can rest on the sheet.
- The shank provides enough material to penetrate the cover sheet and to create a welded joint when rubbing.
- The tip geometry is characterized by wave-like indentations, which provide the necessary process stability and reduce the axial force.
Plant technology tailored to the processing method
The processing technology was specially developed for this process by Harms & Wende GmbH & Co. KG from Hamburg in cooperation with BMW. Thanks to a cooperation between ARNOLD UMFORMTECHNIK and Harms & Wende, both companies are now marketing this technology together. For the Forchtenberg-based fastener specialist, the cooperation also means that a complete system including service can be offered to customers for friction element welding. The system itself consists of a feed unit, a joining unit and the control cabinet for the control system. The system differs from competitor products in that the joining parameters are more variable. These are the hold-down force, the axial force and the speed, whereby the hold-down force can be up to 4 kN, the axial force up to 10 kN and the speed up to 25,000 rpm. The joints can be tested using both destructive and non-destructive testing methods. The chisel test with fork chisel and the subsequent evaluation of the fracture pattern are used for rapid testing. Shear tensile and head tensile forces are used to make statements about strength. And in metallographic micrographs, the welding lens can be made visible as an important evaluation criterion by etching the micrograph surface.
Concrete customer projects speak for the advantages of the process
Initial customer projects show that the friction welding process is justified in lightweight automotive construction. The process is to be used in series production at BMW for the first time in 2021. Laboratory tests are also underway with other car manufacturers. And last but not least, suppliers are interested in the technology. For ARNOLD UMFORMTECHNIK's customers, the use of the process has a number of advantages. For example, the material mix meets high demands in terms of the stability and joint strength of the car body. No preparatory process steps such as decoating, cleaning or pre-punching are necessary. Narrow flange widths can also be processed in the non-cutting process. Last but not least, friction element welding is characterized by short cycle times, low complexity of the joining program and a wide process window. ARNOLD UMFORMTECHNIK is expanding its solutions for complex joining applications with friction element welding. Thanks to ARNOLD UMFORMTECHNIK's global presence in Europe, Asia and America, customers worldwide can benefit from this joining process.
About ARNOLD UMFORMTECHNIK
The ARNOLD GROUP, internationally renowned for innovative fastening technology, has developed into a leading supplier and development partner for complex fastening systems thanks to its extensive expertise in the production of intelligent fasteners. Under the umbrella of “BlueFastening Systems”, the company positions itself as a comprehensive provider of engineering, fasteners, functional parts, feeding systems and processing technology. This holistic combination of experience and know-how offers efficient, sustainable solutions on an international level. ARNOLD has been part of the Würth Group since 1994.
