Laser cladding has become an important surface modification technique in today’s industry. It is not only applied for coating new products but also for repair and refurbishment as well as in rapid prototyping. A laser clad workstation has been developed. It uses a 4 kW Nd:YAG fibre coupled laser as heat source. A specially developed optical system combines the benefits from a Top-hat energy distribution with a practical working distance. The clad material is supplied to the melt pool by a lateral powder nozzle. A camera based monitoring system for the laser cladding process has been developed. This system determines the main dimensions of the melt pool in real-time. A developed FEM model of the laser cladding process accurately predicts the shape and temperature of the clad layers by including the interaction between the laser beam and the powder jet. The model results are in good correspondence with experimental results. An extensive set of cladding experiments has been performed with variable spot size, laser power, cladding speed and powder mass rate. From the experimental work, a clear correlation between the dilution and the width of the melt pool was found. This correlation was found to be independent of the substrate temperature, enabling real time control of the dilution by adjusting the laser power. A feedback control strategy was developed and implemented based on the melt pool width information from the camera. As a result, the energy input into the substrate and consequently thermal distortion of the products is minimized, while a good metallurgical bonding and minimal dilution are obtained. Due to this minimal dilution, the hardness of the clad layer can be controlled and maintained to be uniform. High temperature gradients and different material properties may cause high residual stresses or even cracks. To investigate this effect, a simple and fast method based on deflection measurements has been developed. The residual stress values obtained by this procedure have been compared with stresses from X-ray measurements. The results show tensile stresses of large magnitude develop in the layer, which increase with the cladding speed.
|Place of Publication||Enschede|
|Publication status||Published - 13 Feb 2009|