Study On Technology And Mechanism Of Laser-Induction Hybrid Cladding

The laser-induction hybrid cladding represents a novel surface treatment technology, which combines the induction heat source and laser beam simutaneously. This technology not only can overcome the weakness of heat distribution by laser cladding with single heat source, but also has the advantages such as higher scanning speed, bigger powder deposition rates, better processing stability and better metallurgical bond state. Especially, it is easy to fabricate the free-cracking cladding coating with high performance. However, laser-induction hybrid cladding processing is not a simple plus of laser and induction heat source. Compared with the individual laser cladding or pure induction cladding, the microstructures and properties of the hybrid coatings have been changed greatly due to the complexity of physical and chemical reactions in the processing of hybrid cladding. However, there have been few reports about the mechanisms of laser-induction heating hybrid cladding up to now. In this dissertation, the equipment, technology and mechanisms of laser-induction hybrid cladding have been studied systematically. The following are the main results:Firstly, a system of laser-induction hybrid cladding was set up, which was comprised of a CO₂ laser system, a powder auto-feeding apparatus, a 4-aixis working table with CNC controller and a high frequency induction heater, etc. Based on the system, the effects of processing parameters on the quality of the hybrid cladding coatings were studied systematically. The results demonstrate that the induction energy has big influence on the dimensions of hybrid cladding coating. With the increase of the induction energy, the coating width increases and the height keeps almost unchanged. Furthermore, the dimensions of hybrid coatings have a linear relationship with the powder feeding rate, laser specific energy and inducton energy density.It is significant to study the interaction mode of two heat sources between the laser and induction heating source in the laser-induction hybrid cladding processing. The results demonstrate that the interaction modes of two heat sources are totally different from each others for different cladding methods, i.e. pre-placed coating by thermal spraying, pre-placed coating by organic binder and powder feeding in laser-induction hybrid cladding. As far as the laser-induction hybrid cladding with thermal spraying pre-placed coatings, the induction eddy current is mainly produced in the coatings. The induction eddy current moves from the top to bottom of the coating in the course of induction heating, while the laser is directly absorbed by the surface of coating and then is conducted to the substrate subsequently. As far as the laser induction hybrid cladding with the pre-placed coatings by organic binder, the induction eddy current generates primarily on the surface of substrate, while the laser is absorbed by the surface of coating. As far as the laser induction hybrid coating with powder feeding, however, the induction eddy current is entirely produced on the surface of substrate, while the laser beam irradiates on the surface of molten pool simultaneously. This energy action mode improves the wetting capability of interface and increases powder deposition rates greatly, and thus has the highest cladding efficiency and the best bond strength with the substrate among above three hybrid cladding processing methods.A quantitative analysis method was developed to express the interaction extent of the laser and indution heat source during the laser-induction hybrid cladding processing. A dimensionless parameterΦwas introduced to denote the changes of the effective cladding energy. The dimensionless parameterΦwas defined as energy increase rate of laser-induction hybrid cladding. The biggerΦ, the stronger influence extent of cladding parameter on hybrid cladding energy. The results of quantitative analysis demonstrate that the induction heat enhances the cladding deposition rates distinctly. For low induction energy, the parameterΦand surface temperature of workpiece increase with the increasing of induction energy. When the inducton energy exceed a critical value, the parameterΦand surface temperature of workpiece will keep unchanged. On the other hand, the parameterΦincreases evidently with the increasing of powder feeding rate. When the powder rates exceed a critical value, the parameterΦwill be kept unchanged. In addition, compared with the individual laser cladding, the hybrid cladding energy is greatly enhanced and theΦcan exceed 341 %, which means that the powder feeding and scanning speed are obviously increased.The dilution expression of laser-induction hybrid cladding was put forward. Moreover, the relationship between the dilution and process parameters of hybrid cladding was investigated. The dilution is a saddle shape with the increasing of scanning speed. In addition, the dilution increases with the increasing of induction energy density. For high induction energy density, the dilution retains invariable. The above conclusions are crucial for controlling the dilution and dominating the quality of cladding coating in laser-induction hybrid cladding.The microstructures and properties of Ni-based coating prepared by laser-induction hybrid cladding were studied in detail. The results show that the basic microstructures of hybrid cladding coating are dendrites. The microstructures of the hybrid coating become finer with the increasing of scanning speed and the decreasing of laser specific energy. At the same time, the microhardness and tensile strength increase. Furthermore, the microstructures of hybrid coating are similar to Ni-based coating prepared by individual laser cladding. One very important experimental phenomena is that for individual laser cladding performed on steel substrate, the microstructures of the heat affected zone are martensite as the cooling speed is beyond the critical quenching speed. However, only normalization zone can be found in the heat affected zone in laser-induction hybrid cladding, which is highly helpful to reduce the cracking of cladding coating.Usually, the crack susceptibility is a serious problem in laser cladding due to the high heating and cooling speed. In laser-induction hybrid cladding of Ni-based coating, cracking is relatively low due to the preheating effect by induction energy. Furthermore, the appropriate hybrid cladding energy can fully eliminate the cracking of Ni-based coating in laser-induction hybrid cladding.