TWI Industrial Member Report Summary 742/2002
A Lugan
Background
The first experiments on gas assisted laser cutting were performed by TWI in 1967, using a prototype slow flow 300W pulsed CO 2 laser. In 1997, this early work was re-reported and it was estimated that by this time, approximately 12 000 commercial laser sheet metal cutting systems were installed worldwide. The value of these systems was then estimated at $4.5 billion. Although the early work at TWI was very encouraging, certain aspects of the results were puzzling. Speed discrepancies, for a given power and material thickness, were observed when the TWI work was compared with work in the USA, and in some instances the cut kerf did not appear perpendicular to the material surface, even though the laser beam, and plate surface were at right angles. In 1979 Olsen explained these observations by the nature in which polarised light interacts with a metal. The way in which the early CO 2 lasers were constructed led to some general polarisation of their beams. With these lasers, the best cuts were produced when processing in a direction parallel to the plane of polarisation of the laser beam. Poorer cuts were observed cutting perpendicular to this direction and, if circles were cut, inconsistent cut quality around the circumference of the circle resulted. Once the phenomenon had been understood, optical systems were developed to induce uniform (circular) polarisation in the laser beam, which resulted in the ability to cut with the same quality in all directions. It is interesting to note that the figures for sales of laser profiling systems begin in 1980, ie about the same time as the introduction of circular polarisation of the laser beam and when two-dimensional cutting became a reality. Advantages, in terms of increased cutting speed, were reported in laboratory work, when cutting with a plane polarised laser beam, in the direction parallel to the plane of polarisation, when compared to speeds achievable with a circularly polarised beam. Since these early results, there has been little work published on the effects of using linearly polarised laser light for cutting metals. During the intervening period, advances have been made in the power and quality of laser beams, and 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.
Objectives
To establish:
- Tolerance windows and resultant quality, for the laser cutting of laser grade, mild and C-Mn steels, using a circularly polarised laser beam.
- Any benefits of laser cutting the same materials, using linearly polarised light.