Study On Material Property Integrity Of Titanium Alloy Fabricated By Additive And Subtractive Manufacturing

The manufacture of high-performance part with integrated material,structure and function represents the important future direction of the manufacturing process.Additive and Subtractive Manufacturing(ASM)combines both additive manufacturing process(AM)to form complex geometries and control the grain structure of the part,and subtractive manufacturing process(SM)to guarantee the accuracy and surface quality of the parts.ASM is a high-performance part manufacturing method that meets the requirements of 'Shape/Property Integration Control'.Material property integrity is a general term of grain structure,state of residual stress,and distribution of internal and external defects,which is directly related to the performance of the part.The ASM process is mainly applied to the manufacture of high-performance metal parts with high material property integrity requirements.However,research on ASM process in terms of material property integrity is still in the initial stage.Key issues of ASM process such as the mechanism of grain structure evolution during the AM process,and the residual stress generation and distribution in ASM workpieces under the interaction of AM and SM need further investigations.In addition,it is easy to introduce surface and internal defects in the AM process.Although the ASM process has the ability to remove defects,currently there is no suitable defect detection method.To solve these problems,the main research contents and conclusions of this thesis are listed as follows:(1)The finite element model(FEM)of Ti-6Al-4V direct laser deposition(DLD)process is built to study the relationship between DLD process parameters and temperature history of the parts by Abaqus software.Temperature measurement experiments are conducted for FEM result verification.The simulated results show that most of the deposited material undergoes the remelting process after the first solidification due to the reheating of the post-deposited layers.The heat-affected zone causes solid grain coarsening of the pre-deposited layers.Therefore the evolution of solid grain structure is an important factor affecting the final grain structure of the workpiece.(2)Based on the analysis of the grain structure evolution process of DLDed workpieces,the finite element method,cellular automaton method and Monte Carlo method are combined to establish the AM grain structure multi-scale numerical model considering remelting and solid grain coarsening by MATLAB.The model is experimentally verified.The formation mechanism of grain structure in DLD forming is revealed.Based on the mechanism and numerical prediction results,the DLD experiment is designed to control the grain structure by single-track and multilayer DLD.The results show that with the increase of the number of deposited layers,the average grain width of DLDed workpieces increases from 118?m to 152?m;the area of remelting zone increases continuously,but the increase rate decreases gradually.The grains in the remelting zone of the AM workpiece are relatively small,while the grains in the heat-affected zone adjacent to the remelting zone show a certain coarsening trend.Therefore,the grain size in the middle region is coarser,and in the top is smaller.By controlling the process parameters,the relationship between the lift distance for single layer ?Z and the depth of the heat-affected zone d can be adjusted,and hence the grain morphology and grain size of the single-track and multi-layer DLDed workpiece can be controlled to some extent.(3)In order to study the generation and distribution of residual stress on the surface of ASM workpieces under the effect of AM and SM,with regard to the widely used subtractive milling process in ASM,the residual stress generation and distribution of simplex milling are studied firstly by the finite element model of milling residual stress.Then,according to the residual stress measurement after the milling experiment of the AMed workpieces,the characteristics of the simplex DLD and milling induced residual stress distribution are analyzed,and the residual stress distributions of the ASMed workpieces under different milling parameters are obtained.The results show that the final residual stress of ASMed workpiece is the result of the interaction between AM and SM processes.Within the depth range of 400 ?m below the surface,the initial AM residual stress in the experiment is tensile stress.Under the milling parameters of this thesis,the side milling is mainly affected by the ploughing effect of the flank face,and thus the compressive stress is induced.When the milling residual stress is large enough,the residual stress near the ASMed surface can be transformed into a compressive stress state.However,due to the relatively small affected zone by the milling,the tensile stress is dominating in the ASMed workpiece as the depth increases.(4)The method combining Eddy Current Detection(ECD)and ASM process is proposed for manufacture and defect detection,in which the eddy current detection process and the ASM process are discretized to extend the detection range.The feasibility of ECD detection of internal defects in ASM specimen is verified.The influence of detection parameters such as excitation frequency,sample temperature and defect size on ECD output signal is studied experimentally.The numerical model of eddy current distribution around internal defect is used to optimize the detection parameters.The results show that ECD can detect the subsurface narrow slit defects with a depth of 1.2 mm and a width of 0.2 mm.The reactance signal is increased linearly with the increase of sample temperature.ECD signal is increased with the increase of defect width.The limitation of boundary effects can be eliminated by using a scanning path parallel to the sample edge.