ANALYSIS: DISSIMILAR METAL WELDING
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
magnetic field, placed below the overlap configuration, induces an electromagnetic force. This Lorentz force FL
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 spot- shaped 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
is
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. 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
WWW.LASERSYSTEMSEUROPE.COM | @LASERSYSTEMSMAG
“This joining technology is still in the experimental stage but has already shown promising results”
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. 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.l
Jennifer Heßmann is a research assistant at BAM’s additive manufacturing of metallic components division. Marcel Bachmann is the team leader of welding simulation at BAM’s welding technology division. Kai Hilgenberg is the head of BAM’s additive manufacturing of metallic components division
AUTUMN 2021 LASER SYSTEMS EUROPE 13
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46
