Finite Element And Meshfree Simulation Of Complex Parts By Plasma Deposition Manufacturing

The plasma deposition manufacturing(PDM) technology has the features of higherenergy density, rapid heating and solidification, widely-used materials, better metallurgicalquality, larger-dimensioned parts and lower cost of equipments. In the process ofmanufacturing, the excess energy due to the continuous heat input of plasma arc cumulatesrapidly, which usually induces the uneven temperature field and severe staircase effect thatis one of the most significant manifestations of part inaccuracy in liquid-based rapidtooling. Therefore, the key problem of advancing this technology into commercial marketis to control the temperature field, to reduce the hot crackability induced by thermal stressand to improve the formability and yield of metal pans. For the purpose of full scientificevidence to optimizing technical design, the nonlinear transient temperature and thermalstress fields of complicated parts are simulated in this thesis through both the finiteelement method(FEM) and meshfree methods.According to the features of moving heat source and adding material manufacture inthe PDM, The ANSYS Parametric Design Langrage(APDL) is used to develop the virtualmanufacturing process of elemental activation and load step by step following the movingheat source. This program is then used to investigate the influences of radiation andconvection, plasma power voltage(U_p), transferred arc current(I_PO), scanning speed ofplasma torch(v_s.), mean flow rate of cooling water in the substrate(ν_im) and the powderflow rate(u_p) on the transient temperature field of the superalloy thin wall. Moreover,three types of temperature control schemes, including the submerging substrate withcooling water, the preheating with a stove under the substrate and the cooling water insidesubstrate are compared and validated using above manufacturing program. Computationalresults show that the great majority of excess energy in the PDM is taken away throughthe substrate by the way of heat conduction, and about five percent of heat is released byradiation and convection. The suitable processing specifications can be obtained: U_p=30V,I_PO=70A,ν_s.=0.008m/s,ν_im=1.6m/s and u_p=0.627g/s together with Q235B as the substratematerial, and then the temperature control system of locally preheating in terms of thegeometry shape of metal part before fabrication and circulating water cooling inside the substrate at the outset of fabrication are proposed. Using the optimized process scheme,temperature distribution and its evolution principle of a car fender mould(including apunch and die mould) manufactured by the PDM are further investigated, and three typesof scanning path are evaluated in terms of hot crackability and formability. Computationalresults show that, compared with parallel Z-shaped and criss-cross Z-shaped scanningstrategies, the in-to-out scanning strategy can give much less peak temperature and itsgradient as well as much more uniform temperature distribution. Finally, a geometricallycomplicated turbine and a blade are manufactured successfully, in which the good surfacequalityand small deformation and warps are found during and after the fabrication.It is suggested from the foregoing analysis that a regular, uniform and fine meshshould be used for accurate selection to simulate the deposition process. In addition, largeelemental deformation and distortion usually take place especially for the tetrahedrons inthe transition region between the part and substrate. This kind of integrated initial meshcan not only result in the significant increase of manpower and cost of the analysis butalso strongly deteriorate the convergence rate, accuracy and reliability of the numericalsolutions. In recent decades, meshfree methods that are not sensitive to or even immunefrom elemental mesh have been developed as the powerful alternative techniques to theFEM. Remarkable progress has been made to provide more effectively solutions forproblems, such as large deformation and dynamic propagation of cracks that cannot beefficiently solved using the traditional FEM.Meshfree smoothed PIMs including the Node-based Smoothed PIM(NS-PIM) andthe Edge-based Smoothed PIM(ES-PIM) are developed recently by Professor Liu GR.They use the generalized gradient smoothing technique to modify the gradient fields.Based on the G space theory and the generalized variational principle, the dynamicincremental smoothed nonlinear mathematical formula of temperature and residual stressfields is established in this dissertation from conventional meshfree NS-PIM and ES-PIM.The meshfree codes are then developed to analyze three-dimensional nonlineartemperature and thermal stress fields of geometrically complicated parts. Numerousexample problems have been investigated in details using both NS-PIM and ES-PIM.They have been proven to be stable, works well with triangular and tetrahedral meshes respectively for two and three dimensions, free from volumetric locking, much moreresistant to mesh distortion, and capable of giving a close-to-exact stiffness, ultra-accurate,and even upper bound solutions compared with FEM. In the present work, thecomputational cost and efficiency are systematically investigated for methods of NS-PIM,ES-PIM, FEM, EFG and MLPG. It is indiciated that two types of meshfree methods ofNS-PIM and ES-PIM developed are much more efficient than the well-developed EFGand MLPG as well as the standard FEM.It is well-known that, up to the present, no any meshfree software has been developed.Based on the procedure of process-oriented programming, the dynamic incrementalnonlinear meshfree in-house code has been developed using FORTRAN 90. The detailedaspects of their programming are given in Chapter 5, and some kernel subroutine codesare given for the comparison and reference. A turbine part with both complicatedgeometry and boundary conditions are then simulated based on this software. Numericalresults show that present software is very stable numerically, and the computationalconvergence, accuracy and efficiency are better than the traditional FEM using the sameset of nodes and triangular cells. This indicates that present meshfree NS-PIM andES-PIM are promising and potential to the wide applications into practical problems witharbitrarily complicated geometries and boundary conditions.