Quality...at the cutting edge

Just how much does the effect of steel composition govern laser cut edge quality?

Ariane Lugan worked as a project leader in the Laser and Sheet Processes Group at TWI until March this year. She was involved in projects dealing with a wide range of materials for laser cutting and welding applications as well as laser repair of nickel super alloys. In April 2003 she transferred to TWI's Defect Assessment section.

Paul Hilton is Technology Manager-Lasers at TWI, where he has specific responsibility for TWI's strategic development in laser materials processing.

David Taylor joined TWI in 1998 with a BEng (Hons) in Mechanical Engineering from the University of Birmingham. He is a Professional Member of the Welding Instuitute and a Chartered Engineer. He has now left TWI. As senior project leader in the Laser and Sheet Processes Group he was involved in R&D projects on laser materials processing.

The first experiments on gas assisted laser cutting were performed by TWI in 1967, 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. As Ariane Lugan, Paul Hilton and David Taylor report these 'laser grade' steels are steels generally marketed as providing improved cutting speed, quality and reproducibility.

The issue of laser cutting quality is complex, with a variety of parameters that can affect the process. Some of these parameters are listed below:

• Variable laser related parameters - including but not limited to, 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 cutting characteristics, 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 surface condition of carbon and C-Mn steels, can affect the quality of laser cut edges, has been implemented at TWI. In this paper, the results of this work investigating composition are presented along with the results of trials conducted at a series of UK based laser cutting jobbing shops.

Cutting machines have characteristics that can affect the quality of laser cutting. Laser parameters such as beam quality or pulsing capability can have a major impact on cut quality. In addition to the laser beam properties, machine characteristics, such as focal position control, motion stability and gas nozzle characteristics, will also influence laser cutting quality. The influence of the laser cutting machine operator is also a factor in laser cut edge quality. Although modern systems reduce the reliance on the operator through improved process automation and control, the skill of the operator will have some effect on the laser cut edge quality that can be achieved. Variable laser parameters, such as power, speed, gas pressure and lens focal length, can be altered by the operator to maximise cutting quality or cutting speed. This compromise between cutting speed and quality is difficult to optimise and is the key to successful laser cutting. The 'operator effects' described in this paper relate to how different individual operators and different laser job shops select these parameters to balance speed against quality and is an assessment of what is subjectively determined to be acceptable quality. Laser system operators affect laser cut quality at both an individual level and a company level, through the subjective determination of acceptable cut quality. This paper combines these two factors and describes them as 'operator effects'.

In assessing the effect of the material being cut, there is evidence that some types and grades of steel exhibit improved quality, cutting speed and reliability, over others. Material factors affecting cut quality can include the composition, surface condition and dimensional effects. The laser grade steels currently on the market are promoted, based on control of alloying elements and dimensional properties, to provide consistently improved cutting speeds, edge quality and reliability. A premium is often charged for these products, but if the result is enhanced productivity then laser grade steels might still increase profitability.

Currently, most of the evidence for factors that affect how well these steels perform is anecdotal, from end users and, more particularly, from steel suppliers who have launched 'laser grade' steels.

Industrial Trials

In this work, the series of industrial trials was carried out using a laser grade steel of two thicknesses (6 and 12mm). To attempt to quantify any variability due to machine and operator effects, five laser job shops were supplied with material and asked to reproduce a standard test piece. All lasers used in the trials were fast axial flow CO ₂ lasers in the power range 2-3kW, some using RF excitation and some with DC excitation. The 6mm thickness material was selected to provide a wide processing window well within the capacity of the laser cutting equipment. The 12mm thickness material was selected because of its generally much narrower processing window, approaching the maximum thickness capacity of some of the lasers used. The cutting procedures and parameters used were not defined or monitored. The job shops were selected to allow three comparisons to be made:

• Variability between three different operators using the same laser cutting machine.
• Variability between two different laser cutting machines located at the same operator.
• Variability between three different laser cutting machine/operator combinations.

The resultant components were then assessed at TWI to determine the laser cut edge quality using the DIN 2310 standard. At present this is the most common (qualitative) standard used to determine laser cut quality. Although the standard covers several aspects of quality, only surface roughness and edge squareness are covered here. The standard has two quality levels, quality I and quality II. In the DIN standard, edge squareness, u, is defined in Fig.1 below and is measured in millimetres.

For a given material of thickness a, the limit for a cut with a squareness u, being classed as quality level I, is given by the equation:

u = 0.1 + 0.015a

Here a and u are measured in millimetres. For edge roughness Rz, the equation providing the limit for a quality I cut is:

Rz = 30 + 3a

With 'a' measured in millimetres and Rz in microns.

The two corresponding equations defining the limits for quality level II cutting are:

u= 0.25 + 0.025a    and    Rz = 60 + 4a

Samples meeting the quality II levels can be generally classed as satisfactory 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.

Work at TWI

Following this work at the job shops, a systematic investigation of plate composition was carried out to establish the importance of the various alloying elements on cut quality. 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, in terms of squareness and roughness, for these samples. Table 1 lists the ranges of composition for some of the elements thought to influence cut quality, in the steels evaluated.

Table 1 Composition range for the 12mm thickness steels studied.

Results

The industry trials showed a high level of consistency in laser cut quality between different operators and laser cutting systems. All samples easily met the requirements of DIN 2310 quality II for both 6-12mm thickness laser grade steels, although only one job shop produced a class I roughness cut (on 6mm thickness material). Squareness results showed that it was much easier, at both six and 12 mm thickness, to achieve a quality level I cut. Typical results are shown in Fig.2-4.

A summary of the surface roughness measurements made at TWI on 12 steels, each of differing material composition, is presented in Fig.5. Two of the 12 steels represented in this figure are marketed as 'laser grade steels' and these are indicated with a star in Fig.5. The results shown in Fig.5 were all made with the same set of cutting parameters, at a speed of 0.8m/min, on the same equipment and, in all cases, using plates in the as received condition, with surface mill scale. The range of the surface roughness measurements for the 12 steels was 42 microns and the range of the squareness measurements was 0.22mm.

Discussion

Table 2 presents a summary of the industrial trials for measurement of Rz and u.

Table 2 Summary of the results of the industiral trials for Rz and u

The industrial trials included one make of laser cutting machine (S1) in three different job shops (01, 02, 03) and a second make of laser cutting machine (S3) in two different job shops (04, 05). In this set of results, for the 6mm thickness laser grade steel, the range in results, on either of these two types of machine, was 6µm for surface roughness and 0.04mm for edge squareness. For the 12mm thickness laser grade steel, the range in results for surface roughness was 10µm and for edge squareness 0.09mm. For both materials this represents a high degree of consistency and suggests that the individual operators/job shops had a limited effect on laser cut quality.

One trial was carried out using the same operator (01) and different laser cutting machines (S1, S2). In this set of results, for the 6mm thickness laser grade steel the range in these results for surface roughness was 10µm, and for edge squareness was 0.04mm. For the 12mm thickness laser grade steel the range in results for surface roughness was 8µm and for edge squareness 0.04mm. This also represents a high degree of consistency between the two different makes of laser cutting machine and suggests that the machine also had a limited effect on laser cut quality.

Overall, the three laser cutting machines studied in the industrial trials and the laser system used at TWI for the final trials (not a commercial laser cutting machine), showed good consistency in terms of the results. Including the operator effects, all machines met the quality II requirements for surface roughness in both the 6mm thickness and 12mm thickness laser grade steel. All machines met the quality I requirements for edge squareness in both 6mm thickness and 12mm thickness laser grade steel.

The variations in roughness and squareness recorded in the industry trials (on the same material) can be seen in Table 2. For the 12mm thickness material, the range in all the results obtained was 25 microns for roughness and 0.12mm for squareness. These figures can be compared to the range of results obtained at TWI using the 12 steels of different compositions. The variations due to material composition are nearly twice those representing the combination of machine and operator variations. A summary of these results for the 12mm thick material is presented in Fig.6.

In this work the laser grade steels did not always achieve higher quality levels than some of the non-laser grade steels in the trials. 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 work also confirmed, that if cut edge quality is considered paramount, the silicon levels in the steel should be considered as an important factor in assessing the suitability of a material for laser cutting. Increasing silicon content was shown to have a positive effect on surface roughness and a negative effect on edge squareness. The silicon levels in the two laser grade steels used in these trials were 0.009 and 0.006%wt.

This work was designed to provide a practical guide to the effects of steel composition on laser cutting. In doing so it has raised a number of important issues which were not fully resolved. A greater understanding of chemical 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

The effect of material composition had a greater influence on overall laser cut quality, in terms of edge squareness and roughness, than the combined effects of the laser cutting machine and operator. The range in cut quality for a series of different material compositions was almost 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.

The level of consistency in results between the different job shops and machines taking part in the trials was encouragingly high.

There was significant evidence that it is easier to meet the DIN requirements for squareness than for roughness, on both 6 and 12 mm thickness material. A revision of the quality levels I and II for roughness would therefore be recommended.

Acknowledgements

This work was funded by the Industrial Members of TWI, as part of its Core Research Programme.