A Lugan, P A Hilton and D W Taylor
Paper presented at ICALEO 2002, 14 - 17 October 2002,
Scottsdale, Arizona, USA.
Abstract
The first experiments on gas assisted laser cutting were performed
in 1967. Since then, approximately 20,000 commercial laser sheet
metal cutting systems have been installed worldwide. At present,
steel up to 12mm in thickness is routinely cut on a commercial
basis, for applications in shipbuilding, structural steel work,
off-highway vehicles and many other industry sectors. Market
drivers for these industries include the need to improve cut
quality, maximise cutting speed and reduce rejection rates. In the
last few years, special steels (known as laser grade steels) have
been developed with compositions claimed to be beneficial for laser
cutting. Much anecdotal evidence has suggested improvements to
cutting speed, edge quality and reproducibility for these steels.
In this work, cut edge quality has been established for the CO
2 laser cutting of laser grade, mild and C-Mn
steels of 6 and 12mm thickness. The influence of the plate
composition on laser cut edge quality has been studied by measuring
the edge surface roughness and squareness. Using a statistical
analysis method, the most significant elements affecting cut
quality have been determined. The analysis indicates the important
role of silicon which, for the range of materials evaluated, has a
positive effect on surface roughness and a negative effect on edge
squareness.
Introduction
The first experiments on gas assisted laser cutting were performed
by TWI in 1967,
[1] using a prototype slow flow 300W pulsed
CO
2 laser. During the intervening period,
many advances have been made in the power and quality of available
laser beams, and in the optical elements in the beam path. In
addition, special 'laser grade' steels have been developed
with compositions which are claimed to be beneficial for laser
cutting, and the thickness of materials which are cut on a
production basis has increased significantly. These 'laser
grade' steels, are steels marketed as providing improved
cutting speed, quality and reproducibility.
Laser users have four basic requirements for laser cutting: cut
quality, cutting speed, cutting reproducibility and material cost.
Currently, most of the evidence for factors that affect cuttability
is anecdotal, from end users and, more particularly, from steel
suppliers who have launched 'laser grade' steels. The
compositions and manufacturing processes used by other steel makers
may also provide benefits for laser cutting, but these steels are
not always promoted on this basis. The issue of laser cutting
quality is complex, with a variety of parameters that can affect
the process, [2-6] some of which are listed below:
- Variable laser related parameters - including power, speed,
assist gas pressure, lens focal length.
- Fixed laser parameters - for example laser beam quality or beam
polarisation direction.
- Machine performance - such as focus position control or
stability of motion.
- Operator influence - both at individual and company level.
- Material composition - such as levels of carbon, manganese,
silicon, phosphorus and sulphur.
- Surface condition - such as mill scale and surface preparation
methods.
- Material dimensional effects - such as flatness and material
thickness control.
Greater understanding of the influence of the above factors
should allow steel makers to supply steel plates with improved
cuttability, leading to greater consistency and reproducibility of
the laser cutting process. To provide some of this understanding, a
study of how the material composition, and the surface condition of
carbon and C-Mn steels, can affect the quality of laser cut edges,
has been implemented. The results of this work are compared with a
survey of cutting capability trials carried out in a series of UK
based laser cutting jobbing shops.
Experimental approach
The first stage of the work consisted of an industry survey of
steel makers, equipment suppliers and end users, to establish
current market views and identify critical issues. A series of
industrial trials were then carried out using a laser grade steel
of two thicknesses (6 and 12mm), to determine the variability of
laser cutting performance due to machine and operator effects.
Standard test pieces were then assessed to determine the cut
quality using the DIN 2310 standard.
[7] At present this
is the most common standard used to determine laser cut quality.
This standard has two quality levels, quality I and quality II.
Although this is a subjective assessment, samples meeting the
quality II levels can be generally classed as good, acceptable
cuts, with a good compromise between speed and quality. In general,
it might not be expected to meet quality I levels, unless this was
a specified requirement, as this would often involve a slower
cutting speed. Following this, a systematic investigation of plate
composition and surface condition was carried out to establish the
importance of the various alloying elements, surface oxides and
surface treatments. A range of 12 different carbon and C-Mn steels
were all cut using the same laser parameters. The DIN 2310 standard
was again used to establish cut quality for these samples. The cut
quality results were then analysed to determine the statistically
significant elements and produce an optimised model based on these
elements.
Table 1 lists the ranges of composition for some
of the important constituents in the steels evaluated.
Table 1: Composition range for the 12mm thickness steels
studied.
| |
C |
Mn |
Si |
P |
Mo |
| Min %wt |
0.09 |
0.5 |
0.006 |
0.007 |
<0.003 |
| Max %wt |
0.14 |
1.39 |
0.48 |
0.024 |
0.016 |
Results
The industry survey
The industrial survey confirmed that steel makers, laser cutting
system suppliers and end users all believed that the material
composition and surface condition had a strong influence on CO
2 laser cutting performance. It also revealed that
there is very little published data available to confirm these
views and that most of the work in this area has been carried out
by the steel companies themselves, who have a vested interest in
the results.
The industry trials showed a high level of consistency in laser
cut quality between different operators, job shops and laser
cutting systems. All samples easily met the requirements of DIN
2310 quality II for both 6mm and 12mm thickness laser grade steels,
although only one job shop produced a class I roughness cut (on 6
mm thick material). Squareness results showed that it was much
easier, at both 6 and 12 mm thickness, to achieve a quality level I
cut. This work also established a baseline for comparison with the
trials carried out at TWI. Results are shown in Figures 1,
2 and 3.
Work at TWI
Fig. 3. Laser cut edge squareness measurements on 12mm thick
laser grade steel. Results of the industry trials (cutting speed:
0.8m/min), with S1, 2 and 3 representing the three different laser
cutting machines used, and O1, 2,3, 4 and 5 representing the five
different laser cutting job shops which took part in the work.
|
Surface roughness measurements made on 12 plates, each of differing
material composition, are presented in
Fig.4. Two of the
12 steels represented in this figure are marketed as 'laser
grade steels' and they are marked with an asterix in
Fig.4. The first 12 results (from left to right) shown in
Fig.4, were all made with the same set of cutting
parameters on the same equipment and, in all cases, the roughness
measurements were made using a plate with surface mill scale. The
last six results show the effect of surface preparation on four of
the chosen steels (m/c: machined surface, s/b: shot blasted
surface).
Table 2 lists, for all samples produced, the
range of measurements, for surface roughness and edge squareness,
as a function of operator variability, machine variability and
material variability, on the 6 and 12 mm thick materials. It is
evident that, at 12mm thickness, material composition and surface
condition have a greater effect on roughness and squareness, than
the combined effect of machine and operator variables.
Table 2: Variability of laser cutting quality due to operator,
machine and material effects.
Fig. 4. Surface roughness measurements on 12mm thick C and C-Mn
steels for the different compositions and surface conditions
evaluted.
*Refers to laser grade steels.
|
Statistical analysis
| Variable |
Quality parameter |
Range of measurements for all
samples |
| 6mm thickness steel |
12mm thickness steel |
| Operator |
Surface Roughness |
6µm |
10µm |
| Edge Squareness |
0.04mm |
0.09mm |
| Machine |
Surface Roughness |
10µm |
8µm |
| Edge Squareness |
0.04mm |
0.04mm |
| Combined Machine & Operator |
Surface Roughness |
16µm |
25µm |
| Edge Squareness |
0.06mm |
0.12mm |
| Material |
Surface Roughness |
N/A |
59µm |
| Edge Squareness |
N/A |
0.24mm |
A statistical analysis was performed in an attempt to relate the
material composition of the 12 chosen steels to the observed
surface roughness and squareness. The analysis has allowed a
compositional Cutting Quality Factor, CQF, to be proposed. For the
steels investigated, CQF
R (R = roughness)
was determined by the equation below:
CQF R = 24P + 21Mo - Si
[1]
Statistically, none of the other elements contributed to the
roughness observed. The allowable surface roughness to meet DIN
2310 quality II requirements for 12 mm thickness steel is
108µm. Figure 5 shows the graph of surface roughness
against CQF R for 12 mm steel, complete with
the equation shown below defining the trendline:
R Z = 108CQF R + 68 [2]
At this thickness the quality II requirements of DIN 2310 limits Rz
to 108µm. This suggests that CQF R
Fig. 5. The effect of steel
composition, as assessed by the Cutting Quality Factor R, on edge surface roughness
|
should not exceed 0.37 in any chosen steel in order to meet the
quality II requirements (ref Fig.
[5] ). Due to high
level of scatter of the values, this does not guarantee, however,
that any steel with a CQF
R <0.37 will
meet these quality levels.
For the twelve steels investigated, the CQF R values ranged from 0.024 to 0.487. The CQF R values of the two laser grade steels studied in this
project were 0.248 for material ref. L6217, and 0.321 for material
ref. L4665. The CQF R values for the laser
grade steels are relatively high due to their low silicon content
and suggest that the makers of these laser grade steels were also
considering factors other than roughness when establishing the
material composition.
The results were also analysed to determine whether the effects
of the individual elements were statistically significant in terms
of their effect on edge squareness. This analysis showed that only
silicon had a significant effect on edge squareness. The effects of
the remaining elements were therefore considered as not
significant. The equivalent CQF model based on edge squareness is
as shown in the equation below:
CQF S = Si
[3]
The value of CQF S should be minimised to
produce the squarest laser cut edges.
Surface effects
Figure 6 shows the effect on surface roughness of laser
cutting with and without plate surface mill scale. For this
comparison the mill scale was machined away to leave a bright clean
surface (L4665 and L6217 are laser grade steels). It is clear that
the mill scale has little effect on resultant surface roughness.
Figure 7 shows that shot blasting of the plate surface
(for the steels investigated) had a detrimental effect on edge
roughness.
| Fig. 6. The effect of machining off the plate mill
scale on edge surface roughness |
Fig. 7. The effect of shot blasting and mill scale on
edge roughness |
| |
Discussion
Care should be taken when considering the composition and surface condition of material to be used for CO
2 laser cutting. If the requirements are to meet the quality II levels specified in DIN 2310, then the guidelines proposed here can be used to assess the material composition and surface condition. It should be noted however, that this standard has very specific requirements and does not consider the presence of defects. It is therefore important to establish a criteria for judging laser cut quality, whether it is DIN 2310 or not, before an assessment can be made of the suitability of a material with respect to quality and reproducibility of the cutting process. This work also showed that there is a requirement for an improved method of determining laser cut edge quality to replace the current standard DIN 2310. This should allow some assessment of the level of defects produced in a section of laser cut edge.
In this work the laser grade steels did not always achieve higher quality levels or higher cutting speeds than some of the non-laser grade, carbon and C-Mn steels. The results of the industrial trials and the trials at TWI showed that the laser grade steel did achieve good results with a range of laser cutting machines and operators. This could be a result of the wider processing window available with these steels. The results of this project have confirmed that, if cut edge quality is considered paramount, the silicon levels in steel should be considered as an important factor in assessing the suitability of a material for laser cutting.
This work was designed to provide a practical guide to the effects of steel composition and surface quality on laser cutting. In doing so it has raised a number of important issues which were not fully resolved. A greater understanding of composition on the mechanisms of the laser cutting process would assist greatly in taking this work further and help to provide users with better information in the selection of materials.
Conclusions
- Silicon is the most important element affecting laser cut edge quality. Silicon was shown to have a positive effect on surface roughness and a negative effect on edge squareness.
- A Cutting Quality Factor has been proposed which can provide a guideline for selection of steels. To meet the quality II requirements of DIN 2310 for edge roughness, it is suggested the value of CQF R should not exceed 0.37.
- The effect of material composition and material surface condition had a greater influence on overall laser cut quality than the combined effects of the laser cutting machine and operator. The range in cut quality for a series ofdifferent material compositions was twice that found with the same material processed by different operators on different laser cutting machines.
- Industrial laser job shops were easily able to meet the quality II requirements of DIN 2310 using laser grade steel of both 6mm and 12mm thickness.
- Assessment of surface preparation methods showed that machining of the mill scale layer had no significant effect on laser cut quality. However, shot blasting of the mill scale layer did affect laser cut quality. In this work, shotblasting produced improved squareness, but with rougher laser cut edges, when compared to the 'as rolled' condition.
Acknowledgements
This work was funded by the Industrial Members of TWI, as part of its Core Research Programme. The authors would like to acknowledge the efforts of Mr R Lombardi and Mr G Muggridge who carried out some of the laser cutting trials. Mr I Jones is thanked for his assistance with the statistical analysis work.
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