The Study Of Mathematical Modeling And Physical Behaviors Of Wire-based High Energy Beam Additive Manufacturing

Wire based high energy beam additive manufacturing is the key technology of large complex structure metal parts forming,which has important application prospects in aerospace major equipment manufacturing.At present,process optimization mainly relies on complex trial and error test,with long cycle and high cost,and some complex parts are still difficult to form.The research and development of process aided by numerical simulation has become an important research direction.However,numerical simulation is challenged by rapid material constitutive switching and multi-scale modeling.There is no mathematical model to realize the process simulation of wire based high energy additive manufacturing.Therefore,with the support of the National Key R&D Program of China,this paper carried out the research on the mathematical model,numerical calculation method and physical behaviors such as metal transition.(1)The first macro-meso-microscopic multi-scale mathematical model which can directly simulate the whole part forming process by wire depositon path by path is established.The immersion wire feeding method and mixed phase is proposed to treat the fluid solid coupling problem.The method overcomes the problem that it is difficult to calculate the whole part forming process in current research.The dynamic heat source model of laser and electron beam is established by path tracing method.The mathematical model can be used to simulate accurately the physical behaviors of wire based high energy beam additive manufacturing process,such as energy absorption,wire melting,metal transition,capillary flow in molten pool,grain growth and macro deposit morphology evolution.(2)Aiming at the problem of multi-scale simulation from a single molten pool to the whole part forming process,a high-performance computing method combining octree grid adaptive and dynamic load balancing method is proposed and implemented.The first software i Lwd which can directly simulate the whole forming process of part is developed.Compared with the static grid technology,the adaptive grid algorithm reduces the amount of simulation calculation by 1?2 orders of magnitude.The space filling curve method is established to realize the load balancing of multi-core parallel computing.The load balancing rate is more than 99%,which solves the problem of idle core waiting in parallel computing.Based on the simulation software and algorithm,the evolution behavior of temperature,flow and deposit stacking morphology of aviation blade was simulated on supercomputer for the first time.(3)The complete transition behavior of the metal from melting at the end of the wire,contacting the substrate molten pool,to solidification in the molten pool is simulated visually.During the transition of liquid bridge,the surface tension and recoil pressure drive the liquid metal to flow rapidly,and the flow speed can reach 1.5 m/s.During the droplet transfer,the surface tension makes the liquid metal spheroidize,the thermocapillary force drive the fluid to flow to the region far away from the laser,and the gravity makes the droplet break down.Under the mode of droplet and liquid bridge transition,the molten pool oscillates.Under the liquid bridge transition,the oscillation frequency of molten pool is 100Hz and the amplitude is small.Under the condition of droplet transfer,there is a large amplitude oscillation,which is 2 times larger than that of liquid bridge transfer,resulting in the uneven surface morphology of the deposit.(4)The physical image of metal transition from droplet to liquid bridge is revealed quantitatively.It is found that with the increase of wire feeding speed,the transition form gradually changes from droplet to liquid bridge.With the increase of power level,the critical wire feeding speed increases when the transition from droplet transfer to liquid bridge transfer.It is clarified that the energy mismatch is the reason for the transition mode from liquid bridge transition to droplet transition.The mechanism of fracture instability of slender liquid bridge due to Plateau-Rayleigh instability is revealed,and the idea of controlling forming stability based on the flow properties of molten pool is put forward:the flow characteristics of liquid metal can be adjusted to meet the requirements of We?O(10⁰?10¹),Ca?O(10^-2?10^-3),Bo?O(10^-2),Sl?(3.17-4.57),so as to maintain the stable liquid bridge transition,so as to realize the control of the shape accuracy of the deposit.(5)Through numerical simulation,the interaction of flow and heat transfer between layers and paths was revealed.The results show that there is a general lateral bias flow in the multi-path deposition,and its velocity is about 0.02 m/s,and the Pelect number is in the order of 10¹,which indicates that the lateral convective heat transfer is an important factor to promote the fusion between the multi-path.The surface tension effect caused by the wetting of liquid metal with side deposits is the cause of side biased flow.In multi-layer deposition,the uneven morphology of the deposit results in uneven surface tension,thermal capillary force and other mechanical effects,which makes the stability of the molten pool worse.The simulation reveals the mechanism of lack of fusion defects caused by the overlap of liquid metal at the end of the wire and the side deposit.The formation time of this kind of lack of fusion defects is about 40 ms,and the size is between 0.2mm-1mm.It can promote the melting of the side deposit by reducing the interval between the paths,so as to inhibit the generation of lack of fusion defects.The research results can provide data basis and simulation tools for the forming theory of laser and electron beam additive manufacturing process.