Study On The Key Technology Of Selective Laser Melting Forming Of Thin-walled Superalloy

Selective laser melting(SLM)process is not restricted by the complexity of parts,and can realize the manufacturing of thin-walled,complex cavity and medullary cooling channel parts which are difficult to process by traditional manufacturing methods.As a typical thin-walled structure,ramjet has a large depth to diameter ratio,which is difficult to realize by traditional manufacturing process.The local temperature field of thin-walled parts is characterized by dynamic change and transient nonuniformity.Compared with the solid structure,the heat dissipation area is small,the temperature change is large,and the structural rigidity is poor.Therefore,the thinwalled structure is very sensitive to the change of manufacturing process and structural characteristics,In the forming process of thin-walled parts,every slight fluctuation may accumulate layer by layer,which will eventually affect the forming accuracy and quality,and even lead to forming failure.Therefore,it is urgent to study and optimize the process parameters and typical structural characteristics that affect the forming quality of thin-walled parts in the process of selective laser melting,so as to improve its manufacturing capacity.Based on the finite element method,a three-dimensional transient finite element temperature field model and stress field model are established.A reasonable heat source model is selected to solve the problems of latent heat and material nonlinearity.Selective laser melting(SLM)of GH3536 nickel base superalloy is carried out by using"birth and death element" technology and subroutine parameterization language,Temperature field simulation of SLM forming process.On the basis of the finite element analysis of temperature field,the indirect thermal mechanical coupling method is adopted to realize the finite element simulation of stress field.Firstly,the effects of process parameters on the bath size and temperature gradient of GH3536 superalloy powder in single pass laser selective melting were studied.The results show that there is a positive correlation between the size of the molten pool and the laser power,and a negative correlation between the size of the molten pool and the scanning speed;moreover,the cooling rate of the molten pool is lower at lower laser power and higher scanning speed.On the basis of exploring the influence of process parameters on the temperature field distribution of single pass forming,the influence of process parameters and typical structure of thin-walled parts on the temperature field distribution of multi-layer single pass forming process is further studied.The results show that the cooling rate of molten pool on the lower surface of thin-walled parts is lower than that on the upper surface of thin-walled parts,and the surface roughness is larger.The cooling rate of the molten pool decreases with the decrease of the tilt angle,and the size of the molten pool increases gradually in the first few layers of SLM,and then tends to be stable.According to the results of numerical calculation of temperature field,the stress field distribution at different time and the influence of process parameters on the stress field distribution of single pass were studied by indirect coupling method.The results show that the thermal stress in the forward direction of laser scanning is larger than that in the reverse direction of laser scanning;with the increase of laser power and the decrease of scanning speed,the cooling rate,temperature gradient and thermal stress of molten pool decrease;with the decrease of laser power and scanning speed,the laser energy density decreases.The results show that the effect of thermal cycling annealing is gradually weakened.In the middle of the laser scanning area,the thermal stress keeps large,and on both sides of the area far away from the laser scanning line,the thermal stress decreases rapidly;along the direction of the laser scanning line,the thermal stress at the beginning and end of the scanning line is large,and the thermal stress in the middle area is small.Based on the research of single pass stress field,the research of process parameters and typical thin-walled structure on the stress field of multi-layer thin-walled parts with single pass is carried out.The results show that the upper surface of the thin-walled part has a larger cooling rate than the lower surface,and the thermal stress of the thin-walled part decreases with the increase of the number of forming layers.Based on the finite element simulation,the SLM forming experiment of GH3536 nickel base superalloy powder was completed,and the results of the simulation was verified.The effects of process parameters and typical structural characteristics on forming defects,forming accuracy and roughness of thin-walled parts were explored.The results show that at low and high scanning speed,the forming defects such as warpage,discontinuity and end coarsening of melt channel are easy to occur,and the forming defects such as serration,warpage and high porosity are easy to occur in single pass multi-layer forming of thin-walled parts.The SLM angle forming error of thinwalled parts decreases with the increase of laser power,the number of surface cracks increases,and the surface quality first improves and then decreases;the SLM angle forming error of thin-walled parts increases with the increase of scanning speed,the number of surface cracks decreases,and the surface quality first improves and then decreases.Under certain process parameters,the surface roughness decreases with the increase of the inclination angle and increases with the increase of the number of forming layers.