In-situ diagnosis and control of the melting and solidification dynamics during laser beam cutting

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Reinhart Poprawe

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+49 241 80 40419

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While in machining processes with geometrically defined cutting edges - such as turning and milling - the tool cutting edge and its orientation are decisive in determining the resulting ablation geometry, thermal cutting processes generate the ablation by local heating of the workpiece. The ablation effect is produced by heat conduction and phase transformation of the material - in the case of fusion cutting processes dominantly by transformation of the volume to be ablated into the molten phase - and the removal of at least parts of this melt from the cutting joint that is formed. The ablation geometry cannot therefore be derived directly from a well-defined cutting edge. Rather, the distribution of the locally coupled energy, the heat conduction, the melting process, and the processes of partial material expulsion on the one hand and of residual melt adhesion and solidification on the other hand determine in particular the precision (reproducibility and regularity) and consequently also the accuracy to be ensured (dimensional and shape accuracy) of the resulting cutting edges.

In laser cutting, instabilities of the laser cutting front cause undesirable quality losses in the form of ablation and solidification striations. They can even lead to the formation of dross due to solidified melt. The analysis of the causal phenomena has been hindered so far mainly due to limited diagnostic possibilities resulting from domains that are difficult to access in terms of time and space. Thanks to new diagnostic methods, the mechanisms of scoring and dross formation, the two most important precision features of a laser cut edge, have now been analyzed in detail in the first two phases of the subproject and are much better understood.

A primary goal of this subproject remains the consistent implementation and sustained testing of the methods found for increasing the precision of oxide-free laser beam fusion cutting of sheet materials. The knowledge gained about multiple reflection contributions, beam shaping influences and acoustic resonances is to be transferred into reliably effective, spatial and temporal modulation methods for homogenizing the absorbed power density distribution and for controlling the melting and solidification dynamics.

In the third phase, the melting process on the cutting front and the solidification process on the cutting flanks are to be controlled by feeding back the sensor signals associated with the sub-processes to the manipulated variables of the laser beam and the cutting gas flow in order to ensure damping of instabilities and precise cutting flank shaping. Photodiodes and signal processing tools are used to detect geometric and thermal process information in real time. To adjust the nozzle/joint impedance, increase the flow efficiency and flow stability, the completely new findings on the stabilizing effect of acoustic resonances in the cutting joint and the chances of their amplification by appropriate adjustment of nozzle design and laser modulation, also found in the second phase, will be used. Ultimately, the characteristic sheet thickness-dependent frequency in the joint is not only to be amplified but tuned to maximum effect. On the way to closed control loops for the compensation of intrinsic process instabilities and external disturbances, the third phase will be divided into the following work packages:

• System design for online process monitoring using targeted cycle amplifying and minimally invasive cutting gas or laser beam modulation; Detection of process response signals and identification of process states in real time supported by new machine learning (ML) methods

• Compensation of instabilities on the cutting front and in the cutting joint by adapting the gas and laser beam modulation parameters

• Feedback of process signals into closed control loops, if necessary, also with ML-generated inference technology

The main objective is to minimize irregularities, i.e. to generate dross-free cut edges using the example of 6 mm thick stainless-steel sheets with averaged roughness depths Rz below 10 μm when cutting with fiber and disk lasers. Furthermore, mastering the process will enable the further functionalization of cut edges with defined geometries and textures, which was started in the second phase, thus significantly enhancing the value of correspondingly manufactured products without extending the process chain by subsequent process steps.

  left: Principle of in situ diagnostic setup for incisions; right: Principle of streak recordings Copyright: © SFB 1120 left: Principle of in situ diagnostic setup for incisions; right: Principle of streak recordings
 
  left: Principle of the cut depth dependent melt wave detection method for a cut depth of 2 mm and a selection of 40 frames; right: Cut depth dependent analysis of melt wave frequencies (fmw) Copyright: © SFB 1120 left: Principle of the cut depth dependent melt wave detection method for a cut depth of 2 mm and a selection of 40 frames; right: Cut depth dependent analysis of melt wave frequencies (fmw)
 
  left: Nozzle design of the cutting whistle; right: plot of spectral peak frequencies vs. cavity length. Theoretical frequencies are represented by the solid lines and experimental data by the dots. Copyright: © SFB 1120 left: Nozzle design of the cutting whistle; right: plot of spectral peak frequencies vs. cavity length. Theoretical frequencies are represented by the solid lines and experimental data by the dots.
 
  Validation of the acoustic resonance of the cutting whistle by comparing optical and acoustic measurements (schlieren and microphone measurements, respectively). To determine the frequency from the schlieren recording, one-pixel stripe was extracted from Copyright: © SFB 1120 Validation of the acoustic resonance of the cutting whistle by comparing optical and acoustic measurements (schlieren and microphone measurements, respectively). To determine the frequency from the schlieren recording, one-pixel stripe was extracted from
 
  Use of acoustic resonances stabilizes the melt dynamics in laser beam cutting. Copyright: © SFB 1120 Use of acoustic resonances stabilizes the melt dynamics in laser beam cutting.
  Schlieren-optical high-speed video recording of supersonic gas flow from the cutting whistle during free expansion
  Schlieren-optical high-speed video recording of supersonic gas flow from a standard conical nozzle during free expansion