Jennifer Heßmann, Kai Hilgenberg and Marcel Bachmann, of BAM, explain a new joining process that could help reduce the weight of vehicles
The automotive industry is facing the necessary reduction of CO2 emissions. Due to increasing safety requirements, customer demands in comfort and motor performance, as well as the use of heavy batteries in e-mobility, the weight of vehicles has grown rapidly in recent years.
To save weight without affecting the functional properties of vehicles, heavy materials must be replaced with lighter alternatives.
Modern lightweight concepts have therefore been developed, featuring multi-material designs that require the joining of dissimilar materials.
Besides steel, aluminium is one of the most important construction materials for the mass production of automobiles. The joining of this material combination, especially by thermal processes, is a challenge to overcome. Issues are caused by different material properties such as melting temperatures and thermal expansion coefficients. Furthermore, steel and aluminium are not dissolvable within each other, thus leading to the formation of brittle intermetallic phases. These phases reduce the load capacity of the joint and often act as crack initiation points. It is therefore still necessary to develop joining technologies to reduce these problems.
Electromagnetic melt pool manipulation offers many possible ways of optimising laser welding. At the Bundesanstalt für Materialforschung und-prüfung (BAM) in Berlin, many possible use cases were investigated. For example, supporting the mixing process when using filler materials in thick plate welding or the degassing of pores while welding aluminium die cast. Another application is melt pool support, when welding thick plates to avoid the drop out of the melt pool due to its own weight.
One new possible approach for the joining of dissimilar metals is based on laser welding and melt displacement by contactless induced electromagnetic forces. This joining technology is still in the experimental stage but has already shown promising results. The process steps are illustrated in figure 1.
Figure 1: Single process steps of the new approach for joining dissimilar metals based on laser welding and melt displacement by induced electromagnetic forces
The joining partners, in this case steel and aluminium sheet metal, are placed in an overlap configuration whereby the upper joining partner needs to have the higher melting point, as well as a hole leading to the lower joining partner. The laser beam melts the lower sheet through this hole. An oscillating magnetic field, placed below the overlap configuration, induces an electromagnetic force. This Lorentz force FL is directed upwards and works against the gravity force Fg. Thus, the molten material is pushed upwards into the hole and results in a material- and form-fitting joint.
For this joining technology no auxiliary elements are necessary, and it can be used for spot-shaped or line-shaped joints. However, a controlled heat input delivered via laser beam is needed to reduce the formation of brittle intermetallic phases.
At BAM, it was possible to successfully create a spotshaped lap joint between steel and aluminium by using the presented new approach. The lower joining partner was an aluminium wrought alloy (EN AW 5754) with a thickness of 2mm, while the upper joining partner was a ferromagnetic steel (DC01) with a thickness of 1mm. In the steel sheet a hole with a diameter of 1.6mm was drilled and the specimens were fixed mechanically to ensure a technical zero gap. An IPG ytterbium fibre laser was used with a wavelength of 1,070nm and a spot diameter of 570µm. A laser power of 2.5kW and a laser duration of up to 200ms were used with argon (20l/min) as shielding gas. The penetration depth of the oscillating magnetic field was limited to the thickness of the lower joining partner according to the skin effect. This limitation should minimise the influence of the ferromagnetic steel on the induced electric currents. Therefore, a frequency of 3.75kHz was chosen. The magnetic field power was varied from 0W to around 2kW.
The experiments were supported by numerical analysis to improve the understanding of the new joining technology, especially to reveal the temperature distribution of the joining partners and to find the optimal moment for the laser to shut down, to minimise the heat input. For this, a 2D model of the overlap configuration was created with Comsol Multiphysics FE software.
Results
The experimental results of the presented approach show promise. An example of a created spot joint is shown in the cross section in figure 2a. Here, a laser power of 2.5kW, a laser duration of 200ms, a frequency of 3.75kHz and a magnetic field power of around 2kW were used. Reproducible joints can be generated although cracks or process pores are formed in a few cases.
Figure 2: a) Example of a spot-shaped lap joint, b) formation of intermetallic phases at the interface between steel and aluminium alloy after melt displacement
When the aluminium melt moves upwards, the reduced heat conduction in the steel layer results in heat accumulation in the aluminium melt. The changed heat flow results in an increasing width of the aluminium melt pool directly below the steel sheet. The steel sheet is heated by the displaced melt, leading to the formation of a heat-affected zone. The formation of intermetallic phases is exemplary, shown in figure 2b for the same process parameters.
Analysis by scanning electron microscope shows an intermetallic phase seam at the interface of steel and aluminium, with an average width of about 7µm. This width is lower than the critical value of 10µm for laser beam joining of steel and aluminium reported in literature. Next to this seam, further needle-shaped phases grow into the aluminium melt. Some micro cracks were detected in the intermetallic phase seam caused by the shrinkage of the aluminium melt during solidification.
The calculated required time for the melt displacement and the occurring temperatures of the joining partners are shown in figure 3a. The numerical results show the effective range of magnetic field power is between 600W and 2kW for a complete melt displacement. With higher magnetic field power, the displacement needs less time compared to lower magnetic field power.
Figure 3: a) Results of the numerical analysis of the effective range of the magnetic field power and the required time for a completed melt displacement, b) experimental results of shortened process time of 100ms in the case of 2kW magnetic field power
By increasing the magnetic field power to around 2kW, the displacement process can be shortened to 100ms (see figure 3b). The experimental results confirm the numerical predictions of the required process time and effective range of the magnetic field power.
Conclusion
This work shows it is possible to create a spot-shaped lap joint between steel and aluminium by electromagnetic melt displacement. The numerical analysis improves the understanding of this new joining technology and shows a good agreement to the experimental results. The numerical analysis helps to save experimental time and find the required time for a complete melt displacement, so the laser beam can be shut down as early as possible to minimise the heat input in the joining partners. The calculation of the thermal distribution improves the understanding of the formation of the intermetallic phases. These results are a first step for the further development of this new joining technology. In ongoing research, the focus lies on the creation of line-shaped joints and the analysis of the mechanical properties of test joints.
This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), grant number HI 1919/2-1; 646941. Financial funding is gratefully acknowledged.
Jennifer Heßmann is a research assistant at BAM’s additive manufacturing of metallic components division. Kai Hilgenberg is the head of BAM’s additive manufacturing of metallic components division. Marcel Bachmann is the team leader of welding simulation at BAM’s welding technology division.