Syringe Pump

An in-process measurement of the geometry of the laser cutting front and cutting kerf using high-speed x-ray imaging was demonstrated. The presented three-dimensional determination of the time-averaged geometry makes it possible to apply ray-tracing for calculating the distribution of the absorbed irradiance and the overall absorptance.

The local inclination of the lower part of cutting front with respect to the laser beam increases, the angle of incidence decreases with increasing feed rates. This leads to an increase in the locally absorbed irradiance, resulting in an increase in the overall absorptance.

Syringe needle

Cutting of stainless steel was performed using a disk laser with a wavelength of 1.03 μm in combination with a Precitec ProCutter cutting head. The focal lengths of the collimating and focusing lenses were 100 and 150 mm, respectively. In combination with the used optical fiber with a core diameter of 100 μm and the Precitec EdgeTec beam shaping module, this resulted in a quasi-ring-shaped intensity distribution with an outer diameter of approximately 900 μm in the focal plane. Figure 1 shows this intensity distribution measured with a Primes FocusMonitor.

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Syringe bedeutung

To visualize the cutting front and cutting kerf in a side-view, online high-speed x-ray imaging was performed with a camera at a frame rate of 1000 fps, with a spatial resolution of 37 pixels/mm and an image size of 261 × 314 pixels. The acceleration voltage of the x-ray tube was set to 140 kV with a tube power of 90 W. The obtained x-ray videos were postprocessed with a flat-field correction and Kalman filtering17 in order to enhance the image contrast and reduce the noise. It is noted that lead apertures were positioned at the upper and lower edges of the sample in order to avoid overexposure of the camera. As a result, only 8 mm of the 10 mm thick sample could be observed in the z-direction.

In this report, we present the investigation of the cutting front and cutting kerf during laser beam cutting of 10 mm thick stainless steel as a function of the feed rate by means of online x-ray imaging. In addition, the time-averaged three-dimensional geometry of the cutting front and the cutting kerf is reconstructed, and the distribution of the absorbed irradiance is calculated as a function of the geometry.

Furthermore, the gray scale value for each pixel contains information about the thickness of the irradiated material. The higher the absorption of the x rays on their way through the sample, the lower the gray scale value. Using the Lambert–Beer law, the attenuation coefficient of the sample material for the x rays, and the known gray scale value of the unprocessed sample, this allows calculating the local width of the cutting kerf, assuming that the geometry is mirror-symmetric to the x-z plane. This allows a virtual, three-dimensional reconstruction of the cutting front and cutting kerf. An example is shown in Fig. 4.

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This also leads to an increase in the overall absorptance with increasing feed rates as shown in Fig. 8. The overall absorptance was calculated as the ratio of the laser power absorbed on the cutting fronts and the total incident laser power.

Figure 7 shows the distribution of the absorbed irradiance in the different geometries obtained at the different feed rates, which are mapped in a logarithmic color scale. The locally absorbed irradiance was low in the upper part of the cutting front due to the high angles of incidence of the laser radiation in these areas. As the inclination in the lower part of the cutting front increased and, therefore, the angle of incidence of the radiation decreases with increasing feed rates, and the locally absorbed irradiance is enhanced under these conditions.

Average images from x-ray videos for different feed rates. The contour of the center line of the cutting front is marked with colored lines. P = 8 kW (enhanced online).

Reconstructed cutting kerf in front view (left), top view (upper right), and isometric view (lower right) for P = 8 kW and v = 2 m/min.

For depths of about <3.5 mm, the local inclination of the cutting front with respect to the laser beam was very small for all feed rates. For depths >3.5 mm, the local inclination of the cutting front with respect to the laser was significantly larger especially with increasing feed rate.

The local angle of incidence has a direct influence on the locally absorbed irradiance which is given by the angle-dependent absorptivity and the effective area of the irradiated surface. The distribution of the absorbed irradiance on the cutting front and inside the cutting kerf was calculated with the ray-tracing software developed at the Institut fuer Strahlwerkzeuge (IFSW).20,21 The time-averaged three-dimensional geometry of the cutting kerf reconstructed from the averaged images of the x-ray videos was used for this purpose. The rays for the tracing calculation were defined according to the specifications of the laser beam used in the experiments. The intensity distribution of the beam was approximated with a ring-shaped intensity distribution following the measured intensity distribution as shown in Fig. 1. The complex refractive index nc = n − ki was chosen for liquid steel, i.e., with Re(nc) = n = 3.6 and Im(nc) = k = 5.0.22,23 The propagation and reflection of 500 000 randomly polarized beams was calculated, considering up to 12 reflections of each beam.

Figure 3 (Multimedia view) shows a time-averaged image of an x-ray video recorded during the cutting process performed with a feed-rate of 2 m/min. The images were taken from the middle of the 40 mm long cut to make sure that the process is in steady state. During their propagation through the sample, the x rays are subject to absorption depending on the thickness of the irradiated material. A clear contrast between the solid sample material (dark, high absorption of x rays) and the cutting kerf (light, low absorption of x rays) is, therefore, visible in the gray scale images. From these images, the geometry of the cutting front (line at the center of the cutting front) can be derived, as marked by the purple line in Fig. 3. The origin of the coordinate system is set at the intersection point of the beam axis and the surface of the sample. The position of the beam in the x-ray images was determined by applying a short laser pulse (1 ms at 8 kW) on the sample. The symmetry axis of the resulting hole was defined as the position of the beam axis, which is represented by the white dashed-dotted line in Fig. 3.

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The geometry of the cutting front and the cutting kerf was measured with an online high-speed x-ray diagnostic system. X-ray videos from fusion cutting of 10 mm thick stainless steel samples were recorded with a frame rate of 1000 Hz. A three-dimensional reconstruction of the time-averaged geometry of the cutting front and cutting kerf out of these images made it possible to apply ray-tracing for calculating the overall absorptance and the distribution of the absorbed irradiance at the cutting front. When increasing the feed rate, it was observed that the local inclination of the lower part of the cutting front with respect to the laser beam increased as well as the locally absorbed irradiance on the cutting front. This also leads to an increase in the overall absorptance with increasing feed rates.

It is noted that small structures moving downward on the cutting front11–13,18 could not be observed in the x-ray images due to the spatial and temporal resolution limits of the x-ray diagnostic sytem.19

Iud syringe

A significant increase in the maximum cutting depth and the maximum feed rates has been demonstrated in the last few years for laser beam cutting. This is due to the increase in the available average laser power. However, the quality achieved when cutting of thick materials with solid-state lasers is still often unsatisfactory. This mainly manifests in dross adherence and formation of striation, leading to rough surfaces of the cutting edges.1 The resulting cut quality is determined by the local melt flow inside the cutting kerf.2,3 The distribution of the absorbed irradiance is one of the driving factors of the melt flow. The locally absorbed irradiance is determined by the absorptivity of the material, the geometry of the cutting kerf near the front, and the intensity distribution of the incident laser beam.4,5 Knowledge of the geometry of the cutting front is, therefore, important for evaluating methods to improve the cut quality. Unfortunately, the in-process observation of the geometry of the cutting front is difficult to realize with conventional diagnostics. Different approaches to measure the cutting front during the process have already been demonstrated, including cutting behind glass,6–8 evaluation of the thermal emission,9–11 and the use of high-speed cameras.3,12–14 Furthermore, the applicability of high-speed x-ray imaging for laser cutting was demonstrated for cutting of 4 mm thin sheets of mild steel and cutting of 10 mm thick aluminum samples.15,16

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Figures 5(a)–5(c) (Multimedia view) show time-averaged images of the cutting process for three different feed rates. The contour of the center line of the cutting front is marked with colored lines. A complete cut of the material can be observed up to a feed rate of 2.0 m/min, Figs. 5(a) and 5(b) show the front at 1.0 and 1.5 m/min. For a feed rate of 2.5 m/min, the 8 kW of power is not sufficient for a complete cut, resulting in a loss of cut which can be clearly seen in Fig. 5(c).

Reconstructed cutting kerf in front view (left), top view (upper right), and isometric view (lower right) for P = 8 kW and v = 2 m/min.

The ray-tracing calculations show that 22% of the incident laser power was absorbed at a feed rate of 1 m/min. At a feed rate of 2 m/min, the overall absorptance increased to 40%. For a loss of cut, an overall absorptance of 81% was calculated.

A sketch of the experimental setup is shown in Fig. 2. The cutting nozzle with an outlet diameter of 2.5 mm was positioned 1 mm above the 10 mm thick sample. The feed rate was varied between 1.0 and 2.5 m/min. The laser power was 8 kW, the focus position was set to 1 mm below the sample's surface, and nitrogen with a pressure of 12 bar was used as the cutting gas. A cut with a length of 40 mm was produced in a sample with a width of 6 mm.

Jannik Lind, Florian Fetzer, David Blazquez-Sanchez, Jens Weidensdörfer, Rudolf Weber, Thomas Graf; Geometry and absorptance of the cutting fronts during laser beam cutting. J. Laser Appl. 1 August 2020; 32 (3): 032015. https://doi.org/10.2351/7.0000024

Average images from x-ray videos for different feed rates. The contour of the center line of the cutting front is marked with colored lines. P = 8 kW (enhanced online).