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Cut Edge Surface Roughness

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Introduction

The laser beam removes material by locally heating the workpiece surface. Optical energy is absorbed when the beam strikes the surface; most of this absorbed energy is converted into locally dissipated heat, raising the temperature under the beam to very high values. In metals the workpiece becomes molten from the absorbed energy. The molten material is blown out by an assist gas, forming a kerf of a given depth, and the residual molten metal is removed from the cut edge. It is essential that this process runs stably.

The preparation for laser cutting can be divided into three categories:

  1. Category one — studying intrinsic material characteristics, dealing with defects caused by poor-quality rolled stock, preparing the metal, and cleaning the sheet surface.
  2. Category two — selecting appropriate cutting parameters and exploiting the capabilities of the laser machine.
  3. Category three — continuous monitoring of the laser cutting process.

To improve cut quality and productivity, we mainly work on process parameters and control or eliminate abnormal events.

Several studies in the laser-cutting literature have shown that the quality aspects of the laser cut edge — dross adherence, surface roughness, and the striation pattern — strongly depend on melt-flow dynamics.

Cut surface roughness, denoted as Rz, is a key indicator of cut quality.

Cut front

Three distinct regions can be identified through the sheet thickness:

  • Upper region (~2 mm), zone (I). Characterised by thin, regular striations that are the main cause of relatively high roughness. The striations have the smallest depth and form by periodic erosion from the top edge into the material.
  • Middle region (~4 mm), zone (II). A transition from the typical wavy profile to the smoother region below; roughness usually reaches its highest values here. Striations show curvature and become slightly tilted backwards. They are deeper because the melt is acted upon simultaneously by the laser beam and the assist gas.
  • Lower region (~9 mm), zone (III). The process stabilises and the cut edge becomes smoother. The striations show the largest lag (tilt) opposite to the cutting direction. They form mainly because of the action of the flowing molten metal and assist gas on the cut surface.

Because three equivalent roughness measurement lines are present in the lower region, their values are averaged for comparison with shearing/trimming results.

The cut surface roughness depends on the depth, repetition frequency, and tilt of the striations, and it varies through the sheet thickness. As cutting speed and power density rise, striation depth decreases over the entire cut surface — provided the gas pressure is appropriate. The depth of the heat-affected zone depends on the same parameters as the kerf width: primarily the focused beam diameter and the cutting speed, which must match the assist-gas flow (see the "speed by sparks" rule).

Cutting speed "by sparks"

  1. Correct cutting speed. Sparks scatter downward; the result is a smooth cut surface with no residue at the bottom.
  2. Excessive cutting speed. The cutting sparks are deflected.
  3. Insufficient cutting speed. Sparks do not scatter; they are few, grouped together, or deflected away from the cutting head's motion.

Roughness evaluation parameters

There are several methods for evaluating surface roughness:

  • Ra — arithmetic mean deviation of the profile (mean roughness);
  • Rz — mean height of irregularities;
  • Ry — maximum profile height.

The first two are most commonly used. For laser cut edges, Ra is a practical metric.

Since laser-cut roughness is distributed layer-by-layer through the thickness (the closer to the lower surface, the rougher and worse it gets), the measurement is usually taken at 1/3 of the edge height from the bottom.

Upper region — zone (I)

Defect causes:

  • wrong nozzle choice — nozzle diameter too large;
  • incorrect gas pressure — striations burnt by excessive pressure;
  • incorrect cutting speed — burning caused by too low or too high speed.

Remedies:

  • change to a smaller-diameter nozzle;
  • reduce gas pressure to improve the cut area quality;
  • adjust the cutting speed so that the laser power matches the speed.

Zones (II) and (III)

The main parameters are laser power W, cutting speed V, assist-gas pressure P, and the material thickness t.

In real-world cutting, you must understand how the cutting parameters — primarily laser power and cutting speed — depend on the sheet thickness.

The first step is to find the optimal focal-spot position with respect to the metal surface for the chosen power. This parameter strongly affects the width and geometry of the kerf. The next step is to find the optimal cutting speed V.

Pulse frequency also affects the surface roughness — it can either decrease or increase depending on the parameter combination. At higher pulse frequency the melt zone becomes narrower and more controllable. However, with poorly chosen parameters, "overheating" effects and higher roughness can appear.

When the melt front advances, monitor its parameters — its thickness and velocity. Fluctuations in the absorbed laser power and in the velocity of the high-speed gas jet, together with poorly chosen parameters, can disturb the melt, in turn producing fluctuating striation patterns on the cut edge.

After laser cutting, the cut surface shows semicircular grooves or ripples, known as striations. They appear because of the focusing characteristics of the laser beam, the influence of cutting speed on the kerf formation, and the way liquid metal is removed from the cut cavity.

Defect causes:

  • nozzle too small; cutting focus misaligned;
  • gas pressure too low or too high;
  • cutting speed too high;
  • low-quality plate, poor sheet surface; small nozzles have trouble removing slag.

Remedies:

  • replace the nozzle with a larger-diameter one;
  • adjust the focus and the nozzle–sheet stand-off to the appropriate position;
  • increase or decrease the gas pressure until the gas flow is appropriate;
  • use higher-quality metal.

Additional notes

  • Vortices and grooves on the cut face of thick metal are linked to the physics of laser cutting. Such irregularities are typically caused by gas-jet detachment acting on the metal outside the beam's influence zone. They can be avoided by adjusting the gas outlet pressure at the nozzle.