Beam shaping for optimal control of co₂laser surface hardening

Some industrial laser applications demand high quality and repeatability levels, and therefore precise control of induced effects. In the case of CW CO2 surface hardening, the microstructural transformation depends on the initial structure and on thermal cycles, and has been widely studied. In order to control these thermal cycles accurately, optimal intensity distributions have been computed with the help of a two dimensional finite element code. The choice of the cost function to be minimized appeared to be quite important, in agreement with theoretical considerations. The cost function is minimized under a set of constraints chosen to describe quenching of a 0.35% weight Carbon low alloyed steel. Numerical results exhibit distributions constituted of several "strips". Such intensity distributions have then been realized and characterized with ETCA beam analysers on the Laser Lab workstation devoted to laser-material interaction studies. A kaleidoscope has been used either for top hat intensity shaping or for interference patterns creation. Both Spectra-Physics 5kW beam and CI1000 after amplification by the 5kW cavity have been used in experiments. Temperature measurements have been performed during treatment both in the samples bulk and on their surface. The microstructure after transformation has been characterized by hardness measurements.