Experimental Study On Laser Repair Of Crack Tip

With the development of modern science and industry technology,the service environment of the components and parts becomes more and more complicated,and the requirement of properties of the parts is also getting higher and higher,which greatly increase the rejection rate of components.However,parts only with micro-crack defect could be repaired under the condition that the whole performance of the parts meets the working conditions.If the scrapped parts are repaired,we can not only can save huge costs and time,but also improve the utilization of resources.Laser repair technology takes advantage of the high energy density of laser to make the rapid melting between addition materials and base material,resulting in metallurgical combination to achieve the purpose of repairing cracks and crack arrest.Since 304 stainless steel is commonly used in the production of a wide range of equipment and components due to its superior strength and outstanding mechanical properties,it is of great significance to repair 304 stainless steel parts by laser repair technology.In this paper,304 stainless steel compact tensile specimens were taken as the research object,and the laser repair with various nano-powders were carried out at the crack tip by using the 5 kW transverse flow CO2 laser.The results showed that the suitable addition of single factor nano-WC powders were about 5wt.%.At the corresponding loading stage,fracture property of laser repair specimen improved by 10%~30% compared with that of the untreated specimen.In the microstructure,metallurgical bonding occurred between the repaired layer and the substrate.Grains in the repaired layer showed good equiaxed grains,and some of the undissolved WC particles were homogeneously distributed in the repaired layer.In the single factor experiment,when the mass fraction of nano-Al2O3 powder was1 wt.%,the specimen showed good fracture property.There was no crack and pore in the interface between the repaired layer and the substrate.Moreover,the microstructure of the repaired layer was dense and spherical particles included with nano-Al2O3 particles were uniformly distributed between fine grains.In the end,the laser repair experiments at the crack tip were carried out by using a variety of micro-nano-powders according to the principle of orthogonal test.When the ratio of composite powder was 3-7wt.% nano-WC,1-3wt.% nano-Al2O3,0.5-1.5wt.% V and the balance of 304 stainless steel powder,the fracture parameters of the specimen were the smallest in orthogonal test,which showed the best fracture performance.The interfacebetween the repair layer and the heat-affected zone was metallurgical bonded with no obvious defects,and fine particles containing a small amount of nano particles existed in the repair layer to refine the grains and improve the densification of the microstructure.However,excessive addition of nano-powders or V powders led to the tendency of separation in the interface between the repaired layer and the substrate,and there were obvious crack defects in the repair layer.