Research On The Behaviour Variation And Influence Process Of Graphite In Bonding Zone In Laser Cladding With Grey Cast Iron

Utilizing laser cladding technology to modify the surface of grey cast iron can improve the usability of grey cast iron and prolong the service life of grey cast iron equipment, which has drawn lots of attention. However most of the researches focus on the fabrication of cladding layer and there is little research about phase variation during laser cladding process. Graphite is one of the most important phases in grey cast iron. Its behavior and variation have significant effect on microstructure transformation and microcracks initiation in laser cladding process. The research work in this dissertation aims at investigating graphite behavior and control strategy in laser cladding process. The effect mechanism of graphite to microstructure and microcracks is analyzed profoundly and a suitable cladding process is designed to improve the quality of cladding layer. The main research contents are as follows.Based on the mechanism of laser thermo effect, a numerical model of laser cladding process with grey cast iron is established. The nonlinear thermal-mechanical indirect coupling method is used with ANSYS as the simulation platform. Laser heat source and boundary conditions are modified and improved. Thermocouples and temperature collection module are used to acquire experiment temperature and the simulation precision of the model is amended. The thermal response mechanism of laser cladding process is investigated thoroughly and the temperature and stress variation process is analyzed.Laser cladding process with grey cast iron is carried out and the microstructure of cladding zone, bonding zone and heat affected zone is analyzed. Graphite behavior and variation in different zones are studied. The microstructure, graphite morphology variation and microcracks at graphite tips in bonding zone are investigated profoundly. Graphite numerical model of bonding zone is established with discrete elements from laser cladding model. The thermal-mechanical characteristics of graphite area in bonding zone are described quantificationally and microcracks initiation and propagation mechanism is explored.The effect of laser power and scanning speed on microstructure and graphite variation is investigated. Diffusion behavior of carbon atoms in graphite is analyzed. The environment temperature of graphite area in bonding zone under different process parameters is studied using numerical analysis approaches. Parallel with numerical models, relevant experiments are performed to investigate microstructure transformation and graphite refining degree. The suitable and appropriate process parameters combination is acquired.Laser rescanning process strategy is designed. The temperature circulation process, transient thermal stress variation and residual tensile stress distribution are investigated by numerical analysis methods. The microstructure of cladding zone and bonding zone is analyzed and the effectiveness of laser rescanning is verified. The effect of scanning speed in laser rescanning process is studied with numerical models and experiments and the suitable laser rescanning parameters are investigated.