Fibre Laser Cutting

Introduction
In the Fibre Laser Cutting experiment we were faced with a High Power Laser and required to design a series of experiments to determine how certain parameters affect laser cutting

Aim
To gain a basic understanding of the operation of High Power Class 4 Industrial Lasers in regards to the cutting of sheet metal, and how varying parameters affects the cuts finish.

Objectives 1. Learn basic components of a high power (Class 4) laser system and the basic safety issues involved

2. Learn principles and characteristics of laser cutting

3. Design and perform an experiment to investigate how three process parameters; Laser power, Type of gas and gas flow rate affect cut quality by the analysis of samples

4. Learn basic procedure of sample preparation and analysis using an optical microscope and digital camera system for the examination of the samples

5. Reach evidence based conclusion as to how the three varied process parameters affect laser cut quality

Background Information and Safety Procedures

A Laser is a device the emits light through a process of optical amplification, this is generated by transitions between high and low energy states in atoms, ions and molecules (species) through various media in the infrared, visible and ultraviolet portions of the electromagnetic spectrum. A Laser works on the principle of light amplification where stimulated emission causes a photon to collide with another excited species, there by causing it to release its photon prematurely; the photons travel in the same direction until the next collision thereby building a stream of increasing density. Laser cutting works by directing the output of a high-power laser at the material to be cut, the laser either melts, burns or vaporises away or is blown away by a jet of gas. Gases possess a number of properties that account for their popularity in industrial lasers. The noble gases are the active media in neutral atom gas lasers, fusion cutting incorporates relatively weak discharges and have moderate gain and power output, therefore they tend to be used for fine scale, low power, precise material processing. Reactive fusion cutting uses a laser to cause a reaction between the gas and the material to aid in the cutting; they tend to be used for large scale, high power situations where high precision is not necessary. When working with high powered lasers there are many dangerous elements to the process thus strict safety regulations are required in order to operate the equipment safely. The main consideration is involving eye damage since relatively small amounts of laser light can lead to permanent eye injuries. Moderate and high-power lasers are hazardous as they can burn the retina of the eye or the skin, to control the risk of injury lasers are defined under classes depending on the power and wavelength. Class 4 lasers are the highest classified category and have outputs of more than 500mW, the beam used in our experiment was 1000W, most industrial, scientific or military lasers are in this category. For our experiment all laser cutting took place behind a large partition to prevent any visual contact with the laser beam.

Apparatus

For our experiment the laser cutting system comprised of:

1. An IPG YLR-1000-SM 1kW single-mode fibre laser (1070nm wavelength) with optical fibre delivery (14 micron core)

2. A Precitec cutting head

3. A 1D CNC motion stage

4. An interlocked safety enclosure

We also used a control bench with a computer system for setting the Laser power and turning on the laser and a microscope and digital camera system for sample examination.

Experiment Design

In the laboratory session we were required to design the experiment to be performed. Three variables were identified:

1. Type of Gas (Oxygen or Argon) 2. Gas Flow Rate (Varied by changing pressure) 3. Laser Power (Low- 0.5kW or High-1kW)

The two types of laser cutting to be investigated are Fusion cutting (melt and blow) and Reactive fusion cutting. Fusion cutting considers a moving laser which melts a small area of the sheet metal and then an inert gas immediately blows the molten material away and out of the cut ‘kerf’.
Reactive fusion cutting considers when a gas such as Oxygen that reacts exothermically with the work piece thus producing another heat source to aid in the cutting; the gas also drags the molten metal away.

Therefore a simple experiment was drawn up to vary these three parameters.
|Experiment Order |Type of gas |Gas flow rate/ bar |Laser Power/ W |
|1 |Argon |1 |500 |
|2 |Argon |5 |500 |
|3 |Argon |5 |1000 |
|4 |Argon |1 |1000 |
|5 |Oxygen |1 |500 |
|6 |Oxygen |5 |500 |
|7 |Oxygen |5 |1000 |
|8 |Oxygen |1 |1000 |

Method

1. Taking experiment 1 as an example; the Argon gas cylinder was connected so that it was incident on the cutting area and the gas flow rate set to ‘low’. 2. The 0.9mm metal sheet was placed in the CNC vice and the door to the safety enclosure secured. 3. The control bench software was used to set the laser power to 500W and to begin the cutting. 4. Once the cutting was over the safety barrier was removed and the sheet shifted to allow for a new cut to be performed. 5. The parameters were changed (experiment 2); gas flow rate increased to ‘high’ and the steps 2 and 3 repeated. 6. Once all eight experiments have been performed varying the three parameters so all combinations have been undertaken the metal sheet was removed from the CNC stage. 7. The metal sheet was then placed under a digital camera/microscope system for analysis and examination. 8. The ‘Keyence’ system allowed a high resolution, high magnification image of the metal sheet to be seen for measurement using the software’s tools. Measurements of the cut width on both sides of the sheet, heat affected region and dross width were all taken with detailed images in each case. 9. The metal sheet was then cut using a saw to allow the striations to be seen under the ‘Keyence’ system once more. Measurements of the distance between striations was noted.

Results

Below are the results obtained from the 8 experiments, a table shows the measurements taken and the dross width, heat affected width, Striation width and taper dimensions.

24 screenshots were taken (top, bottom and side views of each cut) however the ones to be analysed are the ones which demonstrate unique characteristics depending on the varying parameter.

The results will be divided into two sections, those which used Argon as a gas and those that used Oxygen.

The main characteristics to be analysed in this section can be noted from Figure 1 below.

[pic]

Figure 1

Fusion cutting and Reactive Fusion cutting

Four cuts were done under each conditions; the noble gas used was Argon. With Argon no heat affect zone was obtained due to the nature of fusion cutting. For Reactive Fusion cutting Oxygen was used.

Taper

The first two cuts were performed with the same laser power of 500W, from the screenshots and measurements taken we get some unexpected values.

The taper values for the first two cuts show that the there was a negative taper meaning the width of the cut was greater at the bottom surface, the taper was calculated by simply subtracting the top cut width from the bottom, for cut one and two the taper values were -60.27µm and -0.99 µm respectively. This is contradictive to what is expected, although the second cuts negative taper is very minute. All other cuts demonstrated a positive taper.

This suggests that in Fusion cutting, using Argon at a low laser power of 500W we obtain a negative taper, for high laser powers of 1000W we get a positive taper (cut 3- 38.59 µm, cut 4- 71.51 µm).

Regarding tapers we see that generally higher power increases the positive taper. For Fusion cutting the relationship between gas flow rate and taper varies depending on the power. The taper in cut 1 with low flow rate and low power (taper- -60.27 µm) gives a value less than when the flow rate is high with the same power (cut 2, taper- -0.99 µm)), however at high powers the taper is greater when the flow rate is low, for cut 3 with high flow rate the taper is 38.59 µm whereas for low flow rate it is 71.51 µm. For Reactive Fusion cutting the relationship is more consistent, the order with increasing taper is as follows; Low laser power- high flow rate (61.96µm), Low laser power- low flow rate (101.51µm), High laser power- high flow rate (134.35 µm), High laser power- low flow rate (161.71 µm). This demonstrates that in Reactive Fusion cutting taper is laser power predominantly followed by the inverse of pressure.
Kerf Width

The relationship between flow rate and laser power is consistent with both gas types for Kerf width, for Fusion cutting the smallest kerf width is for cut 2 and is 109.61µm; laser power of 500W and a flow rate with 5bar pressure. This was followed by cut 1 with 132.49µm; laser power of 500W and a flow rate with pressure at 1 bar. Cut 3 with kerf width 183.96µm is next with laser power set to 1000W and a high flow rate, finally cut 4 has the largest kerf width at 193.51µm; laser power of 1000W and low flow rate. This shows that Kerf width varies according to increasing laser power and decreasing gas flow rate. The values for Reactive fusion cutting are similar and increase in the order cut 6, 5, 8, 7. This means that at higher laser powers in Reactive Fusion cutting Kerf Width increases with both Laser power and Gas flow rate.

There is a large variation in kerf width with the smallest value (Cut 2) being 109.61µm and the largest being 300.24µm.

Below are two screen shots of the largest kerf width in both the Fusion and Reactive Fusion cutting cases.

Figure 2. Cut 4, Fusion cutting, Kerf width 193.51µm

Figure 3. Cut 7, Reactive Fusion cutting, Kerf width 300.24µm

Dross

Dross is the left over material on the bottom surface after a laser cut; it is melted material that has solidified before the gas has removed it. The following figures have been chosen as they demonstrate the trends of the experimental results.

The dross was quantified by measuring its width either side of the cut at different locations, averaging the results and then halving this to obtain the average in one direction.

For fusion cutting the dross width increased with gas flow rate at a low laser power, with cut 1 providing a value of 131.69µm and cut 2 giving 238.64µm. At high powers the trend was reversed with higher dross width for the lower gas flow rate, cut 3’s width was 239.05µm and cut 4’s was 322.77µm. Below are figures for cut 1 and cut 4, they demonstrate the maximum and minimum amount of dross for Fusion cutting.

For Reactive Fusion cutting only the cuts performed at low gas flow rates provided a notable amount of dross, this was cut 5 and cut 8. Two images have been included the one with a large dross width (cut 5) and one demonstrating little to no dross. Cut 5’s dross width was comparable to the largest dross width in the Fusion cutting case (310.39µm).

Heat affected zone

The heat affected zone is a result of Reactive Fusion cutting, the discoloration is due to the exothermic reaction produced by using oxygen. A heat affect zone is produced when the temperature rises above the critical transformation point, this is localised near the cutting region.

The variation

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Figure 5. Cut 4, bottom surface

Figure 4. Cut 1, bottom surface

Figure 7. Cut 6, bottom surface

Figure 6. Cut 5, bottom surface