Mechanism And Modeling Method Of Ultrasonic Vibration In Laser Cladding Forming

The manufacturing of high-performance metal components is the foundation of major equipment manufacturing industries.Additive manufacturing and remanufacturing of key metal components is conducive to the transformation and upgrading of China's manufacturing industry and the intensive use of resources.Laser cladding technology is a kind of advanced manufacturing technology that uses high-energy laser beams to melt powder on the surface of the substrate and solidify,ans it's the basis of laser additive manufacturing and remanufacturing technology.Due to the limitation of materials anr equipment,the metal components produced by traditional laser cladding often have pores,cracks and stress concentration.Therefore,multi-energy field composite laser manufacturing has become an important trend.In this paper,ultrasonic vibration was introduced into the process of laser cladding.The mechanism of ultrasonic energy field in metal forming was studied by numerial simulation and experiment,which provides a new idea for shape and performance control in ultrasonic vibration assisted laser cladding.This research was supported by the National Natural Science Foundation of China(NSFC)(Grant No.51705460)and the Key R&D Program of Zhejiang Province,China(Grant No.2019C04004).In this paper,the stress state of cavitation bubbles in the melt was analysed,the numerical simulation of the morphological evolution of single cavitation bubbles was carried out,and the mechanism of acoustic cavitation in molten pool was speculated.Then,a numerical model of ultrasonic vibration assisted laser cladding was established by COMSOL Multiphysics software,which includes fluid heat transfer,laminar flow,deformation geometry and pressure acoustics modules.Finally,an ultrasonic vibration assisted laser cladding of 316 L stainless steel was performed.Combined with the solidification parameters obtained by numerical simulation and metallographic structure,the mechanism of ultrasonic energy field in laser cladding was stuied.The properties of the cladding coating obtained by ultrasonic vibration assisted laser metal forming were studied by mircohardness tester and electrochemical workstation.The main research work and conclusions of this paper are as follows:(1)A multiphysics numerical model of ultrasonic vibration assisted laser cladding was established.The acoustic pressure gradient of the molten pool calculated by pressure acoustics was introduced into the Navier-Stokes equation as a source term.In this way,the acoustic field,temperature field and flow field were coupled,and the boundary conditions of the model and the thermophysical parameters of the material were reasonably considered.(2)Ultrasonic vibration has a significant effect on the geometry of the laser cladding coating.Under the action of ultrasonic vibration,the melted height decreases,the melted width increases,and the wetting angle of the cladding coating decreases,and this trend becomes more significant with the increase of ultrasonic power within a certain range.The reduction of the wetting angle can improve the energy density absorbed by the unit curved surface on the outer contour surface of the cladding coating,and at the same time,it can make the melt better fill the bottom corners of the cladding coating,and reduce the incidence of pores during multiple tracks.(3)According to the acoustic cavitation model,it can be known that the cavitation bubble would produce a mirco-jet directed at the bottom of the molten pool at the moment of collapse,and the mirco-jet would generate impact pressure on the primary dendrite and the newly formed solid crystal network.Through analysis,it can been seen that appropriate ultrasonic strength can cause the fracture of the primary dendrites or the fracture of the solid crystal network to form many fine grains.The fine grains are dispersed in the molten pool and become nucleation points to refine the grains.In addition,the metallographic structure obtained from the experiment shows that the thick and long dendrites in the middle became short and disorderly.(4)The ultrasonic vibration assisted laser cladding technology can appropriately improve the performance of the cladding coating.Compared with the cladding coating without ultrasonic vibration,the maximum microhardness can be increased by 23.26% under the process parameters used in this paper.The results of the corrosion resistance experiments show that the self-corrosion potential of the coating is increased and the corrosion current density is decreased under the appropriate ultrasonic power.The change of self-corrosion potential and the corrosion current density means that its tendency to corrode is reduced and the corrosion rate is slowed down.The reason is that the cooling rate is accelerated by ultrasonic vibration,and the grain size is refined to improve the performance of the cladding coating.