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Prospects and Restrictions of the Metal Magnetic Memory Technique
The introduction of an external magnetic field in laser beam welding can bring a lot of beneficial effects e. g. a homogeneous element distribution and grain refinement. However, the physical mechanism has not been fully revealed because it is difficult to describe the weld pool behavior quantitatively. In this work, the influence of an oscillating magnetic field in wire feed laser beam welding is studied using a three-dimensional transient multi-physical model. The magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics and element transport are all taken into consideration. Under suitable electromagnetic parameters, a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force. It shows important influence on the keyhole stability and the fluid flow. The keyhole collapses more frequently at the upper part under the oscillating magnetic field. The forward and downward flow along the central plane of the weld pool is enhanced, which can bring the additional element from the filler wire to the root of the weld pool. There is a good agreement between the numerical results and the experiment results from the high-speed imaging, the optical micrograph and the energy dispersive X-ray spectroscopy. This work provides a fundamental understanding of the relationship between electromagnetic control, weld pool behavior and weld property.
The macro and micro benefits from a magnetic field have been well recognized in the laser beam welding, but the influence mechanism, especially on the local keyhole dynamics, is yet not fully understood. A combination of experimental approach and numerical modeling was conducted to study the highly transient keyhole dynamics under the magnetic field. It was found that the magnetic field gave a direct impact on the laser energy absorption at the keyhole front wall. The keyhole-induced porosity was suppressed under the influence of the magnetic field, which was confirmed by the micro-X-ray computed tomography. The porosity mitigation comes from multiple physical aspects, including keyhole stabilization, widening of the bubble floating channel, and the electromagnetic expulsive force. The predicted penetration depth and weld pool profiles with/without magnetic field show a good agreement with the experimental measurements, including optical micrographs and in-situ metal/glass observation.
Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase in the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are ana lyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain size after welding verify the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains, that have been solidified and formed previously, from further growth and generates some small cel lular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.