Research On Key Technologies Of Thermo-mechanical Evolution And Deformation Control During Laser Coaxial Powder Feeding Additive Manufacturing

Aiming at the stress control and deformation cracking prevention in the additive forming process of large aviation structural parts,this paper carries out the research on the key technologies of thermo-mechanical evolution and deformation control of laser coaxial powder feeding additive manufacturing,so as to lay a technical foundation for the integration,largescale and high-precision preparation of China’s advanced equipment.Laser coaxial powder feeding additive manufacturing technology takes laser as the heat source and uses carrier gas powder feeder to transport metal powder.The powder flow is coaxial with the laser beam.With the movement of the light spot on the workpiece surface,it grows and forms point by point,line by line and layer by layer.It is an integrated manufacturing technology that takes into account the requirements of forming and forming.However,due to the process characteristics,the forming accuracy of laser coaxial powder feeding additive parts is insufficient,and it is very easy to deform,even crack.Therefore,it is difficult to guarantee the forming quality of parts,which is a bottleneck restricting the development and application of laser additive manufacturing technology.In this paper,the space-time characteristics and evolution mechanism of physical quantities such as heat,stress and deformation in the process of adding material are studied by the combination of theoretical analysis,finite element simulation and experimental verification,so as to provide a theoretical basis for realizing the stress / deformation control in the process of adding material,improving the forming quality of parts and realizing the additive manufacturing of large components.The specific research contents and conclusions are as follows:(1)As the basic unit of additive manufacturing,the thermo-mechanical coupling behavior of thin-walled structure is also the basis of thermo-mechanical control of large components.Based on this,the size effects of thermal evolution in additive manufacturing of thin-walled structure are studied in this paper.The results show that with the increase of the height and size of the thin-walled structure,the molten pool temperature increases circularly,but there is no obvious regularity;During additive manufacturing,the stress value of the first layer is the largest,and the increase of height leads to the decrease of stress,but it will rise rapidly when cooling;With the increase of height and size,the longitudinal warpage deformation of the substrate is unstable and fluctuates greatly.When the length of the thin-walled structure increases,the cycle period of the molten pool temperature curve increases,the overall temperature of the specimen is higher,and the temperature gradient is greater after cooling for the same time;With the increase of length and size,the stress value of the first layer increases,and the range of low stress zone in the manufacturing process and high stress zone in the cooling process expand;The increase of the length and size of the thin-walled structure leads to the increase of the residual deformation of the substrate.(2)For cuboid specimens with different three-dimensional dimensions,the growth direction will inevitably affect the thermo-mechanical coupling of additive manufacturing,especially for large components with larger aspect ratio.Therefore,this paper studies the different effects of thermo-mechanical evolution under the two growth directions.The results show that the peak value of temperature curve is low,the variation range is large,the cooling rate is slow,and the temperature is high at the end of addition.But when cooling,the cooling rate is fast.During short-term growth,the temperature of molten pool is low and the range of heat affected zone is small.However,due to large cross-sectional area and large heat transferred to the substrate,the substrate temperature is high and the distribution area of high-temperature zone is wide.After cooling for the same time,the overall temperature of the short growth block is low.Comparing the stress evolution under the two growth directions,the fluctuation range of short growth stress curve is greater and the growth rate is slower in the later stage.However,in the cooling process,the distribution range of short growth high stress area is wider,and the peak value of residual stress rises faster.For the two growth modes,the warpage deformation of short growth substrate is significantly greater than that of long growth,and converges earlier.(3)Based on the investigation of the configuration characteristics of large aviation structures,it is found that the lamellar contour can be divided into several characteristic units.Therefore,it can be manufactured discretely according to the characteristics to control the stress and deformation.In this paper,the thermo-mechanical distribution characteristics and influencing factors of typical component additive manufacturing under characteristic zoning are studied.For the specific characteristic zoning unit,the effects of the deviation scanning path and overlapping scanning path on the thermal behavior of the temperature field in the region are studied.It is found that the deviation scanning path can reduce the molten pool temperature and narrow the range of high temperature region.Therefore,the deviation scanning path is preferred in the region.For a single layer of large components,it generally includes several feature zones.The effects of three different interval scanning sequences: continuous scanning,interval scanning and remote scanning are studied.The results show that continuous scanning has the highest bath temperature and substrate temperature,the largest stress peak and the most obvious deformation,while remote scanning is most conducive to controlling stress and deformation.(4)Aiming at the technical bottlenecks such as size limits and machining limits in the manufacturing of large components,the laser additive material connection technology is studied in this paper.That is,it is formed in sections and then connected as a whole,so as to discrete stress and reduce deformation.The finite element simulation is used to simulate the heat,force and deformation of large components in the connection process.It is found that the high temperature areas are concentrated near the connection joints and present the gradient distribution outward.The stress at the joint is not high in the connection stage,but after cooling for a certain time,a concentrated high stress area will be formed.Restricted by the boundary conditions,the longitudinal warping deformation is obvious when the frames are connected,while the rib plates deflect at different angles and directions.Through the reasonable layout of the connection areas and the optimal design of the sections,the segmented forming and additive connection of the overall structure of titanium alloy with a total length of 3.7 m are realized.