Studies Of Key Technologies On Modification Of Metal Surfaces By Laser Hardening And Laser Cladding

In this thesis, with the purpose of industrial applications, several key techniques relevant to surface hardfacing and surface modification for metal materials were investigated. The investigations mainly refer to two aspects: laser surface hardening for metal materials and laser cladding on metal substrates. In the technical investigations on laser surface hardening for metal materials, new processes and techniques were developed to effectively improve the surface properties of some special equipment or component parts made of steel and meet the practical requirements. The work is to extend the application yield and to accelerate the application process of laser processing technology. In the investigations on laser cladding, experiments and analyses of laser clad nickel-based alloys and nickel-based ceramal composite alloys on steel substrates were performed to improve wear-resistance and corrosion-resistance properties of the coatings. Crack generation in laser-clad coatings is the most difficult problem to be solved before massive application of this technology. In order to seek out effective methods for eliminating or reducing crack generation, we carried out systemic experiments and mechanism analyses on the design of laser cladding materials, the exploration of laser cladding technique, the interaction between laser and materials, the microstructure and properties of the coatings. I. Investigation on laser surface hardening for metal materialsIn this work, the effect of the process parameters in laser surface hardening on the properties and surface feature of the hardened layer was studied. The action of overlapping manner on the microstructure and properties of the hardened layer was analyzed and some fundamentals were discussed. Systemic experiments and laser processing of the mould-roller used in packaging machine, the screen-changers used in plastic extruder, the fiber-cutting roller used in chemical plant and the rail used for railway were accomplished. Furthermore, the technique of laser hardening for the component parts was obtained and some key technologic problems were successfully solved. The main results and conclusions are shown as the following:1 . Many factors determined the effect of laser hardening. They can be divided into four kinds: parameters of the laser used, properties of materials the component parts made of, processing parameters and environment conditions in laser hardening. The primary factors in practice are the laser power and processing parameters in laser hardening. It is shown that rationalizing laser processing parameters is very important for laser hardening any workpiece.2. The technique of laser surface hardening entirely for the mould-roller used in packaging machine was successfully obtained at first. To treat the surface of mould-roller by means of this technology can overcome the disadvantage presented by traditional heat treatment. It has the advantages of processing speed, easy operation and low expenses. In laser surface hardening for the mould-roller, the hardened depth was controlled by laser specific energy density (P/DV), and proper overlapping ratio is the technical key.3. The research results on laser hardening of the fibre-cutting roller showed that using laser surface hardening process instead of thermal spraying to strengthen the surface of fibre-cutting roller of 4Crl 3 stainless steel is practicable. Not only this technique overcame the disadvantage of larger heat distortion by using thermal spraying but also possessed the advantages that the operation is simply and that expense is easy. Thus it is worthy of popularizing it in industry and would yield great economy.4. Success in laser surface hardening for a screen changer used in plastic extruder was obtained at first and application yield of laser processing was expended. The practical results showed that the hardened screen changer possessed excellent performance and that its employing life was close to two times of that by traditional heat treatment. Hence, it is worthy of ap...