Control of geometry and metallurgy in laser beam microwelding by influencing the melt pool dynamics via locally and temporally adjusted energy input
In laser micro welding, the molten phase, in interaction with the stability of the vapor capillary (“keyhole”), has a significant influence on the precision and quality of the resulting weld seam. This shows in geometrical dimensions, such as seam depth and width, metallurgical dimensions, such as formation of intermetallic phases, and functional dimensions, such as strengths.
According to the current state-of-the-art in research, there is no methodology as yet that remedies or compensates for the process instabilities in laser micro welding, which are due to the dynamics of the molten phases. Here, an approach using temporally and locally adapted energy deposition offers the greatest potential. The aims of the project include an analysis of the individual precision-determining aspects of laser beam micro welding, a model-based evaluation of different influence factors, and, as a result, the derivation of methods and process control strategies to significantly improve quality and precision. The project has a two-stage objective:
- Determination of precision-determining time constants and process boundary conditions using high-resolution process visualization at a high temporal resolution and, at the same time, with increased process understanding
- Increase in weld seam precision with regard to geometric properties (weld penetration depth constancy ≤ 1 %, weld depth control ≤ 5 µm) and functional quality (roughness RA ≤ 10 µm)
These objectives are to be attained by modulating energy deposition, thus manipulating both the melt pool size – and with it the weld depth and weld seam width – and the dynamics of the molten pool with the convective energy transfer. Different local and temporal performance modulation strategies are to provide insights into the controllability of seam geometry and molten pool dynamics and contribute to stabilizing the vapor capillary.
In order to obtain a thorough understanding of the physical processes for an evaluation and weighting of quality-determining factors, a detailed analysis of energy input, the melting process, and the molten pool dynamics through innovative methodological diagnostics approaches in laser beam micro welding of technologically relevant metal alloys (in particular, Cu and Al materials).
A first focus of the project is to analyze the weld process using such modulated laser radiation techniques, with special consideration of molten pool dynamics and both phase transitions. The insights gained will be immediately used to refine the model and simulation in order to attain the best possible prediction of process results.
Local power modulation
Functional principle of local power modulation in laser beam microwelding. In contrast to conventional laser beam welding, the laser beam is moved spirally over the surface of the component to be joined. The form of movement is created by a circular superposition of the linear feed motion. In addition to stabilizing the welding process, the local power modulation leads to an increase in efficiency and the connection cross section.
3D welding depth
Realization of a three-dimensional measurement of a weld seam by means of a TargetMaster of the company Struers procured in the SFB1120. A precise geometry of the weld seam can be determined by the creation of precise longitudinal grindings of a weld seam and the continuous ablation and recording of additional planes. The distance between the measured planes is approx. 20 µm, which makes it possible to investigate the variation in welding depth during laser beam microwelding.
By observing the molten pool during laser beam microwelding, the process stability and size of the molten pool can be investigated in the case of a parameter variation of the local power modulation. The course of the keyhole, which is characterized by a spiral modulation due to the local power modulation, can also be easily recognized.