Research On Mechanical Impact On Controlling The Crack In Laser Cladding Of High Hardness Coating

With all kinds of k W-level laser pushed to the market, laser cladding with high power density and light source of highly stable output, whose cost is relatively lower, thus gradually become a new means of remanufacturing.That high-speed of the light source,rapid cooling of the molten pool and tensile stress because of solute cooling in the process of laser cladding,make internal and the surface of cooled cladding layer finallly have a large number of pores, cracks and other defects.Compared with base material,high hard powder coating has bigger difference of thermal physical parameters,and high hard cladding after cladding is more easier to produce cracks and other defects than other coating. The existing defects seriously affect the quality of cladding and make the combination of coating and the service life greatly reduced.The extened crack, even lead to peeling coating, fracture of workpiece and industrial accidents’ s appearing.So the control of the crack becomes the key issue in the industry badly needs to be solved.This thesis describes the research of laser cladding-layer crack control by mechanical impact mainly on theoretical analysis, numerical simulation and experimental research.The cotents are as following.It has designed a mechanical impact device which strikes the shaft workpiece on high frequency in the process of laser-cladding and analyzed the divice’s influence on cladding layer which is mainly embodied in two aspects:on the one hand, it has vibration effect on molten pool indirectly, which could increase the exchanging speed of convection heat in the molten pool, increase the degree of solution and enhance the breaking effect of dendrite arm in the cladding process, etc., thus reduce the organization segregation, bulky dendrite and intergranular crack phenomenon;on the other hand, the device strike the established but still solidified structure in high temperature conditions with high frequency,which creats compressive strain,and extend the larger plastic tensile stress of the tissue to reduce the cracks.Through theoretical analysis,the mechanical impact has great influence for the crystal nucleation rate, growth rate,solute distribution and temperature gradient factor and so onin the rapid solidificating process of the molten pool,which enhance the molten pool convection, reduce the organization segregation.In a word,it has been verified that the mechanical impact for the cladding layer could decrease the amount of crack.This thesis has established numerical analysis model with the mechanical impact of high frequency during the process of laser cladding for different hard coating in the process of laser-cladding with the mechanical impact of different high frequency.Through the method of thermal-structure coupling analysis,and the thermal stress under different frequency comparison,it proved that the mechanical impact change cladding layer stress effectively.It has made highly hardened layer of laser cladding of the different materials and anlysis the surface crack of cladding layer, microstructure, yield strength and microhardness by comparation.Besides, from view of the experimental analysis of the mechanical impact of cladding layer,it analysis the change of the microstructures and properties of micro and macro, and explore the causes of the decrasing hot and cold cracks of cladding layer.