Laser Process Parameters And Surface Roughness
Laser Process Parameters

There are several important considerations when it comes to laser cleaning. When the laser cleaning methodology is compared with other cleaning  methods such as sand-blasting or water-jetting one should think of the following criteria1,2:

How effective the material removal has been using the method?

What is the impact on substrate material?

Suitability of the treated substrate for the follow up coating?

For the first question, a research study1 which used a pulsed YAG laser (24 kHz pulse frequency, 83 ns pulse duration) on stainless steel reports a maximum speed of 19 ft2/hour on a 10 mil (0.25 mm) coated sample using a 7.6 cm scan width. This speed is slower than some other competing methods such as sandblasting but laser cleaning offers the huge benefit of environmental cleanliness.

As for the effect on the substrate material, the same research study1 shows that above a certain fluence level (3 J/cm2) there will be a melting and a substantial rise in temperature.

As far as the third question and the adhesion behavior is concerned, it is closely related to the surface roughness. How the surface roughness of the metal substrate changes after laser cleaning is a very relevant question. Figure 1 shows the surface roughness of a stainless steel substrate without  any paint layers. 


Figure 1: Surface roughness of stainless steel substrate3 

After application of paint layers and usage for several years, the paint needs to be removed. The question is how does laser cleaning affect the substrate’s roughness after removing the paint. Some other cleaning methods such as water jetting remove all surface roughness, so the subsequent layers of paint can’t be applied directly. Sandpaper needs to be used to roughen the surface. That is not the case with laser cleaning since it could also increase the initial surface roughness depending on the process parameters.

One has to consider several factors to examine the effects of different laser process parameters on surface roughness. These parameters are listed below and are examined one by one:

  • Cleaning speed

  • Cleaning times

  • Laser power

  • Pulse frequency

  • Overlap ratio

  • Energy density

1. Cleaning Speed and Cleaning Times

One has to distinguish between the cleaning speed and the scan speed. The scan speed is produced by the galvo scanners and basically the laser spot forms a line of a certain width. The cleaning speed is perpendicular to this line and is along the direction of the movement of the laser head (sweep direction). The cleaning speed direction is shown in figure 2.

Figure 2: Cleaning speed direction  is along the sweep direction

According to one research study for the aluminum alloy substrate4,  at low cleaning speeds (18cm/min) the roughness first increased with cleaning times and then decreased with further increase in cleaning times. At higher speeds (27 cm/min and 36 cm/min) the surface roughness increased with cleaning times. This is shown in figure 3.

As for the same cleaning times, the change of roughness was different at different cleaning speeds. After one cleaning run, the roughness first decreased sharply with cleaning speed and then slowly increased with further increase in cleaning speed. At twice the cleaning times, the surface roughness decreased with increasing speed although sharply at first and then decreased more slowly with further increase. At higher cleaning times of 4, the roughness first increased and then decreased with increasing speeds. At 18 cm/sec and 36 cm/sec the surface roughness value was the same with a maximum value at a speed of 27 cm/min. This behaviour is once again shown in figure 3.

Figure 3: At different Atr cleaning times, the roughness behaviour varies with cleaning speed4

2. Laser Power and Pulse Frequency

The general behaviour of how the surface roughness varies with laser power is increasing roughness with higher laser power which has been shown in figure 4  for four different pulse frequencies4
As for the pulse frequency effect, the surface roughness decreases with increasing pulse frequency at fixed laser power. This is probably due to the fact that at a fixed laser power, increasing the pulse frequency will decrease the energy per pulse and therefore will cause less heating effect.


Figure 4: Change of surface roughness with laser power and pulse repetition rate for Aluminum substrate4


3. Energy Density and Overlap Ratio

Surface roughness shows a linear increase with increase in energy density4 as shown in figure 5. It is also observed that although two samples (A3 and A5) had different process parameters (80 W-240 kHz and 40 W-120 kHz) since their energy densities were similar (17.0 J/cm2 using  spot diameter of 50 μm and the equation F=P/(f.π.d2) where F is energy density, P is the power, f is the pulse frequency and d is the laser spot diameter), they produced similar roughness of nearly 6 μm. Figure 5 shows the plot of surface roughness vs. energy density.

Figure 5: Linear dependence of surface roughness on energy density

This proves that a single parameter can not affect the roughness parameter and the surface roughness mainly changes through the variation in energy density.

Overlap ratio also affects energy density4 as shown in figure 6. The figure shows that at higher energy densities (17.0 J/cm2 and 21.2 J/cm2), the surface roughness generally increases with overlap ratio upto 50% but for lower energy densities (8.5 J/cm2), the surface roughness is independent of overlap ratio below 50%, peaks at 50% and then decreases with further increase with overlap ratio. 
Figure 6: Dependence of surface roughness on overlap ratio

In a few other controlled experiments which were done in the same research study4, it was found that samples with similar energy density and overlap ratios showed similar roughness. Therefore it is concluded the changes in cleaning speed, pulse frequency, power, etc., change the surface roughness mainly through changing energy density and overlap ratios. Changes in these two parameters mainly control the changes in the value of roughness 

Allied Scientific Pro offers laser cleaning systems with different power levels (100 Watt, 200 Watt, 500 Watt systems). For further information please refer to the following link:
In addition, meeting industrial standards to evaluate surface profile/condition and  surface roughness is also important. Generally, the standards which are used to evaluate surface preparation include NACE NO.4/SSPC-SP7 Brush off, NACE NO.8/SSPC-SP14 Industrial, NACE NO.3/SSPC-SP6 Commercial, NACE NO.2/SSPC-SP10 Near-White Metal, as well as NACE NO.1/SSPC-SP5 White Metal. The requirements of each standard can be found in the figure 7.

Figure 7: Requirements of each standard5

All of these require removing all loose material from substrates. such as visible oil, grease, and other contaminants. As for the tight material (material which cannot be removed by dull putty knives, such as, rust and old paint) and stains (light shadows and streaks), the requirements are different. NACE NO.1/SSPC-SP5 White Metal requires the greatest degree of cleanliness among these standards, requiring the surface to be free of any loose material, tight material, stains or shadows. Based on the projects that were done by Allied Scientific Pro in the past, laser cleaning equipment can meet the requirements of NACE NO.1/SSPC-SP5 White Metal in an effective and efficient way, and can also provide a desired surface roughness at the same time by adjusting the laser parameters. 


References:
1- Effects of Laser Ablation Coating Removal (LACR) on a steel substrate: Part 1: Surface profile, microstructure, hardness and adhesion, M. Shamsojjoha et.al, Surface & coating Technology, 281, 2015
2- Laser cleaning of steel for paint removal, G.X.Chen et.al, Apply Phys A (2010)
4- Effect of laser cleaning process parameters on the surface roughness of 5754-grade aluminum alloy, G.Zhang et.al, The international journal of advanced manufacturing technology (2019).
5- Surface Preparation Standards, NACE International.

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