Steel Composition and the Laser Cut Edge Quality

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 cutt ability 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 cutt ability, 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.

Fig. 1. Laser cut edge surface roughness 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, andO1, 2, 3, 4 and 5 representing the five different laser cutting job shops which took part in the work.

Fig. 2. Typical edge quality on 12mm thick laser grade steel 

a) Laser cut edge at a speed of 0.8m/min
b) Laser cut edge at a speed of 0.9m/min
c) Laser cut edge at a speed of 1.0m/min

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.

Work at TWI

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.

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.

Table 2: Variability of laser cutting quality due to operator, machine and material effects.

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

Statistical analysis

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]

Fig. 5. The effect of steel composition, as assessed by the Cutting Quality Factor _R, on edge surface roughness

At this thickness the quality II requirements of DIN 2310 limits Rz to 108µm. This suggests that CQF R 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 of different 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, shot blasting 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.

References

1. Sullivan A B J and Houldcroft P T. 'Gas-jet laser cutting'. British Welding Journal 1967 Vol. 8; p. 443.
2. Green M. 'Steels for improved laser cutting'. Editorial, The Industrial Laser User, 1997 Vol. 8; p. 16.
3. Pope M. 'Materials for Laser Processing'. Proc conf 'Make it with Lasers(', November 2000; pp. 21-22.
4. Indren H. 'Steels for Laser Cutting'. Proc AILU conf 'CO2 Laser Cutting for Users', May 1998, pp. 13.
5. Price D 'Steels made to be cut'. Engineering Lasers, 1999, Vol. 9; pp. 6-8.
6. Woollen W.G. 'Modern steels for shipbuilding'. Proc conf 'International Conference of the International Institute of Welding (IIW)'. Hamburg, 1998, DVS berichte 195, pp. 64-68.
7. DIN 2310: 'Thermal Cutting'. Parts 1-6, 1986.

Meet the authors

Ariane Lugan first graduated in Physics (France, Toulouse). She then completed a course of Materials Engineering and received a Masters in Materials Science in 1999 (France, Nantes and Lille). She has been working as a Project Leader in the Laser and Sheet Processes Group at TWI since March 2000, involved in projects dealing with a wide range of materials for laser cutting and welding applications. She is also looking at developing these techniques for medical applications, and has recently had some important activity in laser welding of Ni superalloys.

Paul Hilton is Technology Manager-Lasers at TWI in the UK, where he has specific responsibility for TWI's strategic development in laser materials processing. He is also a founding vice president of the UK's Association of Industrial Laser Users.

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