Cutting copper sheets using CO₂ lasers

Abstract

In this work various problems concerning cutting copper sheets using CO₂ laser are reported. First, all copper thermophysical properties, that regulate the process dynamics, and then the weight of each parameter has been evaluated numerically, even though only approximately. The surface absorption value of copper at room temperature and near the melting point and the order of laser power that is necessary to cause a gradual raise of the workpiece temperature from room to melting have been estimated. Then, the order of the cutting speed at which a sheet of a known thickness can be cut has been calculated. The analysis of all these problems, and the process dynamics and state of the art seem to confirm the validity of the current thesis on the impossibility of cutting copper by CO₂ laser. In the second part of the work the experimental data relating to the first ever tests on 0.2–4.0 mm thick copper sheet cutting by 2 kW CO₂ laser are reported. These first interesting results have been obtained thanks to the possibility of making overlapped layers of cupric oxide CuO, mixed with a small quantity of cuprous oxide Cu₂O grown under laser beam irradiation (CuO and Cu₂O, together, allow the laser cutting to be carried out). This has been confirmed by the analyses of the cutting edges with a computerized X-ray diffractometer. We have also seen that the per cent absorption of laser radiation at 10.6 micron does not increase in the presence of just cuprous oxide while, when the experimental conditions allow it, the growth of cupric oxide increases the absorption value to around 52–58 per cent, giving rise to the loop process with three variable quantities (temperature-oxide-absorption) that has been the winning clue of cutting process. The behaviour of the critical cutting speedV, the cutting widthsb and the productV·b versus the thickness for 2 kW CO₂ laser using a 4″ ZnSe and 3.5″ KCl focusing lens have been tested. Moreover, the influence of different gases and flows on the cutting process have been experimented. The work-speed turned out to be significant and various micrographic sections, performed on the workpieces have shown that the laser cutting quality is quite good. A first analysis of the results has shown that laser cutting is not comparable to the one of steel, so much so that the mathematical formalism developed for steels has proved to be unsuitable for copper.