Conventional laser cladding systems use a focusing nozzle to direct the beam onto the work piece. And by using either pre-placed or blown powder techniques, the powder is fused to the objective surface. Cross sectional analysis of the produced ‘tracks’ illustrates their parabolic nature. This cross section is due in part to the incident laser beam profile being circular and having a ‘Gaussian’ or ‘top-hat’ profile. The use of conventional optics provides no opportunity to modify this situation and so little improvement in the matching of laser process to industrial requirement has occurred.
Custom designed diffractive or holographic optics provide a significant step change in the laser beam delivery system, enabling the matching of the incident optical flux to the ‘actual’ design and material requirements. A popular build requirement for the rapid prototype, component repair and deposition workers, is for a track to be laid down which has a rectangular cross section and predetermined metallurgical structure. Predictive knowledge of these demands enables suitable thermal models to determine the ideal heat distribution necessary to accomplish these process requirements. It is now possible to use models of the thermal conduction, likely to take place in the substrate and deposited molten powder, to derive the necessary incident laser power distribution to produce this optimal situation.
Such a process has been undertaken, leading to the design and production of a suitable computer generated holographic optical element. This element converts the incident Gaussian 1.2 kW CO2 into a 3x5mm rectangular spot with a predetermined semi-idealised intensity cross-section. The optical element used is a derivative of the traditional holographic optic known as a ‘kinoform’, which enables approximately 90% of the incident laser beam to be completely reorganised into this required profile. We report the construction of such an optical element and the initial results for producing uniform rectangular 3mm tracks of 0.5mm deep stainless steel powder onto 1.2mm stainless steel substrate. Lack of porosity suggests such tracks could offer a novel means of producing 100% dense rectangular tracks for fully functional rapid prototype testing. The lack of ‘shoulder rounding’ from convention lay-up procedures suggests a significant improvement in build precision.