Development of an observation and control system for industrial laser cladding

摘要

Laser cladding has become an important surface modification technique in today’s industry. It is not only appliedudfor coating new products but also for repair and refurbishment as well as in rapid prototyping.udA laser clad workstation has been developed. It uses a 4 kW Nd:YAG fibre coupled laser as heat source. Audspecially developed optical system combines the benefits from a Top-hat energy distribution with a practicaludworking distance. The clad material is supplied to the melt pool by a lateral powder nozzle.udA camera based monitoring system for the laser cladding process has been developed. This system determinesudthe main dimensions of the melt pool in real-time.udA developed FEM model of the laser cladding process accurately predicts the shape and temperature of the cladudlayers by including the interaction between the laser beam and the powder jet. The model results are in goodudcorrespondence with experimental results.udAn extensive set of cladding experiments has been performed with variable spot size, laser power, claddingudspeed and powder mass rate. From the experimental work, a clear correlation between the dilution and the widthudof the melt pool was found. This correlation was found to be independent of the substrate temperature, enablingudreal time control of the dilution by adjusting the laser power.udA feedback control strategy was developed and implemented based on the melt pool width information from theudcamera. As a result, the energy input into the substrate and consequently thermal distortion of the products isudminimized, while a good metallurgical bonding and minimal dilution are obtained. Due to this minimal dilution,udthe hardness of the clad layer can be controlled and maintained to be uniform.udHigh temperature gradients and different material properties may cause high residual stresses or even cracks. Toudinvestigate this effect, a simple and fast method based on deflection measurements has been developed. Theudresidual stress values obtained by this procedure have been compared with stresses from X-ray measurements.udThe results show tensile stresses of large magnitude develop in the layer, which increase with the cladding speed.