Laser welding/brazing for joining tailored blanks
A new process furthers the development of dissimilar material joints
For several years, new concepts for producing lightweight, high-strength joints of dissimilar materials have been under development in the field of traffic engineering. The introduction of tailored blanks is one of the first solutions based on the idea of an optimized material usage in respect to its mechanical properties. The development of dissimilar material joints is a further advance on the concept of tailoring blanks.
The idea of laser joining dissimilar materials was initiated at BIAS (Bremen, Germany) eight years ago. Since then it has been defined as one of the company's main research projects.
Starting with the metallurgical considerations, which are based on the Fe/Al system, a laser process was developed with respect to its time-temperature behavior. The main focus of this process is the minimization of the phase layers that are a few micrometers in thickness in the joining area, because the intermetallic phases deteriorate the ductility and the dynamic mechanical properties of the joint. Furthermore, it was possible, for the first time, to accomplish the joining of titanium in combination with aluminum with different joining geometries.
Figure 1. Tactile seam tracking system.
Due to the fact that dissimilar materials need a high standard on clamping techniques and process control, a conceptual, simple and effective clamping and positioning technique was developed, which is now available on the market. For joining the overlap geometry using the thermal heat conduction process, a special working head for process stabilization was developed (see Figure 1). The working head allows tactile seam tracking and constant pressure on the sheets during joining. Using this working head it is possible to achieve an optimized heat transfer between the two sheets being welded. With the optimized process it is possible to use the working head for the production of tailored blanks or for the direct joining of three-dimensional parts, such as aluminium constructions with steel frame parts.
Figure 2. Combined welding brazing process.
Figure 3. Temperature field simulation of the Fe-Al-joint.
The joining of tailored blanks in butt-joint geometry was achieved by a combined welding/brazing process (see Figure 2). During this process the sheets are positioned in a butt-joint geometry using a pneumatic-hydraulic clamping device. The beam of a high-power laser system is focused and positioned in such a way that excessive melting of both materials is avoided. One material has to be molten so that the liquid material can wet the surface of the solid material (see Figure 3). For better process stabilization a filler wire metal is used.
Figure 4. Joining area of the tailored blanks with different material combinations and joint geometries.
With the optimized joining technology the intermetallic phases in the joining area can be repetitively produced with a thickness of less than 2 µm. Figure 4 shows a cross-section of tailored blanks with different material and joining geometry combinations. In the joining area it is obvious that regular wetting of the liquid material occurs. With respect to tensile strength, the heat affected zone near the joining area in the aluminum base metal is a limiting factor. For the Fe/Al-butt joint configuration, a tensile strength of about 70 percent (166 N/mm2) and for the titanium overlap joint 91 percent (220 N/mm2) of the tensile strength of the aluminum base material (240 N/mm2) has been reached.
Figure 5. Deep drawn titanium-aluminum overlap joint, distribution marked with arrows.
To characterize the cold forming behavior of the joined tailored blanks, deep drawn tests with the Multibran process were carried out at the Lehrstuhl für Umformende und Spanende Fertigungstechnik (LUF) at the University of Paderborn, Germany. Figure 5 shows a deep drawn sample of a titanium-aluminum joint. The failure of the deformed sample occurs in the area from the bottom zone to the cylindrical skirt. Using cold forming tests it is possible to demonstrate the anisotropic flow behavior of the different materials. For the steel-aluminium-overlap configuration comparable results have been reached. In the course of the project funded by the state of Bremen the influence of the weld seam position in correlation to the forming behavior will be further evaluated for both material combinations (Ti/Al, Fe/Al) and joint geometries (overlap and butt configuration).
Michael Kreimeyer, Florian Wagner and Gerd Sepold are with the Bremer Institut für angewandte Strahltechnik. For more information, e-mail Kreimeyer@.bias.de.