Application of Laser Pretreatment Technology in Thermal Spraying Process

       In order to further improve the performance of the coating, the traditional pretreatment process needs to be improved. As mentioned above, the commonly used traditional pretreatment method consists of two steps. The first is surface purification treatment. Solubility reagents are often used to remove stains and oil stains on the surface of the substrate. However, in this process, due to the recycling of reagents and containers, it is easy to cause Surface contamination; secondly, sandblasting and roughening treatment, although it can enhance the bonding between the coating and the substrate, it is easy to introduce residual particles and residues during the treatment process, which will destroy the performance of the coating, not only weakening the adhesion of the coating, but also The fatigue resistance of the matrix is reduced, and compressive stress is introduced, which will deform the matrix if it is basically thin. In addition, the design of the spraying equipment did not take into account the time interval between the roughening of the sample and the start of the spraying process. If the dwell time was longer, the sprayed surface was sufficient to form an oxide layer on it, although its thickness was not large, but it will have a significant impact on the bond strength of the coating. Especially for easily oxidized metals and their alloys, such as Ti, Al, Mo and other matrix materials that are easy to form oxides, the interval time should be strictly controlled. In order to overcome the above-mentioned shortcomings of the traditional pretreatment process, people are studying the use of laser to pretreat the surface of the substrate.



Basic principle of laser pretreatment process



The process is to connect the laser head and the spray gun at a special geometric angle, so that the spraying process follows the laser process or overlaps the two processes, that is, the pretreatment process and The process of spraying process. The laser used can be a YAG laser with a pulse of 10ns and a frequency of 120Hz. In order to ensure that the laser can effectively clean the surface of the metal substrate, two laser light sources can be used to obtain a rectangular emission source and obtain a uniform and good irradiation area . The process parameters involved in the laser pretreatment process include: laser energy density, laser distribution intensity and beam shape. The shape of the area that the laser can cover once fired at a fixed position determines the efficiency of laser processing, and laser beams with square and rectangular shapes can achieve higher processing efficiency.



Advantages of the laser pretreatment process



The use of laser pretreatment process to replace the traditional pretreatment process can not only remove the contamination layer and oxide layer on the surface of the substrate, and improve the bonding strength of the coating and the substrate, but also the continuous laser irradiation can prevent the coating from being regenerated by dust and condensed gas. Contamination and re-formation of oxide layers, this simultaneous laser treatment process allows molten droplets to strike an oxygen-free surface, potentially resulting in chemical bonding.



3. Application and development of laser technology



On the aluminum substrate, the traditional method and the laser process were used for pretreatment respectively, and then Cu powder was sprayed. The metallographic observation showed that the interface of the coating treated by the laser process was clearer and smoother than the interface obtained by the traditional process, and the interface strength of the coating was obtained. Significantly increased. The energy density of the laser and the relative speed of the laser head and the substrate have a certain influence on the performance of the coating.



The original laser processing equipment is only equipped with a laser head that can obtain uniform energy, and the coating can obtain good performance only when the spray diameter of the plasma jet on the surface of the sample is similar to the diameter of the laser spot. For the currently widely used supersonic flame spraying and arc spraying processes, when spraying large-sized surfaces, the laser beam cannot completely cover the spray beam spot, and some powder particles are sprayed on the surface of the workpiece without laser treatment, resulting in the coating and the The matrix does not produce a strong bond. To overcome this deficiency, a device with two laser heads was designed, thereby increasing the surface area that can be processed in one sweep. The experimental results show that when Ni-AI is sprayed on the 2017 substrate, the interfacial strength of the coating obtained by using two laser heads for pretreatment is higher than that when using one laser head.