Research On Mechanism Of Temperature Modulating On Titanium Alloy Manufactured By Laser Deposition Shaping

Laser deposition shaping is an innovative additive manufacturing method and it is of great importance in the manufacturing of complex components made of difficult-to-machine material.The complex geometry structures is formed by the accumulation of melting metal powder by laser beam,which is the basic theory of laser deposition shaping.Titanium alloy is widely applied in the aviation industry and biology implant industry because of its excellent mechanical properties and outstanding biology compatibility.However,titanium alloy is a kind of typical difficult-to-machine material,which is material and time consuming when manufactured by conventional forging and cutting method.The manufacturing time and costs of titanium alloy could be reduced remarkable by laser deposition shaping.Besides that,the worn titanium component could be repaired by laser deposition shaping.At the same time,the huge temperature gradient is the result of the fast laser beam movement,leading to the residual thermal stress and the nonuniformity of microstructure which is the reason of distortion and cracking.This thesis focused on the problem of microstructure and mechanical property nonuniformity in deposited titanium alloy.The technique of substrate preheating and delayed cooling is introduced into laser deposition shaping and its mechanism of action is analyzed.The main content of this thesis is as follows.To begin with,the mechanism of material solidification under different conditions is analyzed.The solidification process during laser deposition shaping is near-rapid nonequilibrium solidification process which is accompanied by complex phase and thermodynamics transformation.The temperature gradient and solidification speed of the melting pool is changed simultaneously by the substrate preheating.So the solid-liquid interface stability factor is not change dramatically,resulting in the enhancement of grain size without transformation in grain type.Based on the theory of the formation of defect in laser deposited layer,substrate preheating decreases the solidification speed in melting pool resulting in the decrease of defects in deposited layer.By summarize the microstructure evolution of Ti-6A1-4V under different temperatures,predicting the influence of substrate preheating on the microstructure evolution of titanium alloy.Besides that,the temperature variation characteristic of titanium plate with preheating treatment is modeled by FEM method and the heating equipment is developed in this thesis.Then,the single bead Ti-6Al-4V laser deposition experiment is performed and the macro-morphology,microstructure and the micro-hardness under different heating conditions is measured and analyzed.As a result,the macro-morphology of deposited layer is dramatically influenced by substrate preheating.With the increasing of preheating temperature,the depth of melting pool,the width of deposited bead,dilution rate increase,and the height of deposited bead decrease.The defects in deposited bead is inhibited by substrate preheating and the pore-free deposited bead is obtained under the preheating over 300 C°.With the substrate preheating,the average grain size of deposited bead increase and the columnar grain across the bead-substrate interface present.The microstructure of deposited bead is composed of laminar a phase and ?+?Widmanstatten structure which become coarser with the increasing of preheating temperature.The deposited bead is much harder than the substrate and the average micro-hardness of deposited layer decrease with the increasing of preheating temperature.The delayed cooling process is less influential to the microstructure and micro-hardness compared with preheating process.What's more,the multi-bead laser deposition experiment is performed and the residual stress in deposited layer surface is measured and analyzed.Then,the optimum process parameters of laser deposition is achieved by a well-designed orthogonal experiment.A titanium thin wall frame component is deposited in the optimum process parameters and the substrate distortion and surface cracks is measured.It is observed that residual stress on deposited titanium component surface decreased with the preheating process and the component deposited with preheating process is crack-free.It is safe to say that substrate preheating is beneficial in laser deposition manufacturing.