TWI Industrial Member Report Summary 920/2009
By H Walker and P A Hilton
Cutting is an important application for materials processing with lasers, dominated by the use of CO2 gas lasers for the cutting of steels. Worldwide sales of CO2 lasers were expected to total more than $1,000 million in 2008, with the majority of this market being lasers used for cutting flat plate. Although CO2 lasers are well suited to producing high speed and high quality cuts in flat plate, the moving mirror beam manipulation systems required with CO2 laser technology become quite complicated for cutting 3D shapes.
Fibre delivered Nd:YAG lasers allow more flexible manipulation, and have proved useful for laser welding in three dimensions. However they have not been seriously considered for mass market cutting applications, as they are more expensive per kW than CO2 lasers, and also lack the beam quality for high quality cutting of thick sections at relatively high powers. However, some use of these lasers for cutting hydroformed tube in the automotive industry has occurred for sections up to about 3mm in thickness. Three dimensional cutting of thicker sections is possible using flame or plasma cutting, but the cut quality is poor compared to laser cutting. The recent advent of high power, high beam quality, Yb:YAG disc and Yb:fibre lasers, employing fibre optic delivery of the laser light, has provided a possible alternative to the use of CO2 lasers for cutting, providing results without the beam transmission difficulties that need to be overcome for three dimensional work or very large area processing in CO2 laser cutting.
The main difference between disc or fibre and CO2 lasers, other than the capability to deliver the beam through a fibre optic, is the wavelength of light produced. The fibre delivered laser beams have shorter wavelengths, which should be better absorbed by the metallic materials usually cut by lasers. For welding applications, it is clear the coupling of the laser light with the parent material is much improved at this wavelength. However, the behaviour of materials when cut using different laser types is much less well understood.
Preliminary results of cutting with fibre lasers showed that cutting with a 1µm wavelength laser beam was quite different to cutting with the 10µm wavelength beam from the CO2 laser, both for oxygen and inert gas assisted cutting. The work reported here, concentrated on a comparison of the cutting capability of a high beam quality disc laser and that of a very modern, commercially available, high quality CO2 laser cutting system, for cutting stainless steel from 0.6 to 6.0mm in thickness, using high pressure inert gas. Laser cut stainless steel is used in many industry sectors in applications such as the manufacture of food processing equipment, medical products, cryogenic equipment, white goods, architectural structures and works of art. The thickness range was chosen to provide the best comparison data, given that there was already some evidence for different performance on thin and thick materials when using the 1µm wavelength laser source. Inert assist gas was chosen, as opposed to oxygen assist gas, so as to not involve the additional exothermic energy available from the oxygen and its effects on the cut quality.
To compare the cutting performance of a high beam quality disc laser, with optical fibre delivery of the beam, with that of a commercially available CO2 laser cutting system, for inert gas cutting of stainless steel in the thickness range of 0.6 to 6mm.
The CO2 laser cutting machine had a full set of cutting parameters pre-programmed into its control system for a given material and thickness, so these were used for all trials. In addition, cuts were carried out at speeds above and below that specified by the machine, to examine the process window.
The trials as described above produced a set of samples cut at different speeds for each material and cutting condition. Each set of samples was first assessed visually in terms of dross, and any cuts with large amounts of dross were rejected. The cut with the lowest dross in each set was selected as the best cut, and used for edge roughness measurement. This roughness data was then used for comparison between samples, and to plot the effect of changing laser focus position and assist gas pressure when using the disc laser.
In addition, one set of samples for each material and laser were selected as giving the best combination of wide cutting speed process window and low cut edge roughness. Every sample in these eight sets had the cut edge roughness measured, to establish the effect of cutting speed on roughness, and a comparison was made of the process window for each of the different material thicknesses and lasers. The best cut from each set also had the edge squareness measured.