Study On Mechanism Of Microstructure Evolution And Mechanical Properties Of Ti-6Al-4V Alloy Deposited By Pulsed Plasma Arc Additive Manufacturing

Ti-6Al-4V alloy components have been widely used in aerospace industry,such as wind-shield frames,low compressor disks and blades,intermediate compressor cases of EJ2000 and tail cone of Boeing 777.The buy-to-fly ratio of these components fabricated by traditional manufacturing methods is as high as 40:1.Currently,to remanufacture the damaged Ti-6Al-4V parts or additive manufacturing?AM?processes based on laser beam or electron beam are mostly applied.However,titanium alloys fabricated by AM usually show anisotropy in mechanical performance due to the existence of coarse prior?grain formed under the effect of high cool gradient,which makes post heat treatment always required.In the depositing processes based on conventional arc sources,high heat input usually results in low forming precision and formation of coarse grains,so the mechanical performance decreased.While plasma arc with high energy density,high efficiency and low equipment cost is regarded as a potential heat source for additive manufacturing or remanufacturing titanium alloys.However,in manufacturing process,different thermal cycles result in different microstructures which have an impact on mechanical properties in as-built condition.In this thesis,firstly,the effect of processing parameters on deposition property and microstructure were studied,followed by an investigation on the evolutional mechanism of microstructure and mechanical properties of Ti-6Al-4V parts deposited by different pulsed frequencies plasma arc AM?PPAM?processes with different heat input.Finally,by optimizing the processing parameters,as-built parts were manufactured and the mechanical properties were superior to the standard for forged titanium alloys.The results and conclusions are listed as follow:Effect of processing parameters on deposition property and microstructure of Ti-6Al-4V components fabricated by pulsed plasma arc AM has been studied by the method of quadratic regression orthogonal.The results and conclusions can be summarised as:1)Heat input per unit volume has the most significant influence on the deposition property while the pulse frequency shows the least.2)Prior?columnar grain is affected mostly by impulse frequency and heat input per unit volume shows the second significant effect.3)Related response value of single pass and single layer can be accurately predicted by the established quadratic regression orthogonal model,with an error of 3%.The relationship between predicting accuracies of the three predicted objects is:bead height>bead width>prior?grain width.The microstructural evolution of components deposited by PPAM with constant heat input was investigated.Martensite?'microstructure and Widmanst?tten microstructure with basket weave were observed.Results show that the phase transformation obeys the Burgers orientation relationship.After several thermal cycles,the nanometre-size particles of secondary?phase grew along the<112 0>direction and the intersecting crystal axis of different crystal planes finally form lamellar structure with the triangular stars or rhombic patterns on the pole figures.The colony?structure located in layer band was coarsened by repeated heat inputs from at least three thermal cycles.The microhardness value ranges from 351-368HV_0.1,which has a negative correlation with the lamellar size of the colony?structure and a positive correlation with the volume fraction and size of the secondary?phases.The mechanisms of the microstructure growth and the evolution of mechanical properties with gradient-changed heat inputs were discussed.The calculated minimum value of G/R?temperature gradient ratio G and grain growth rate R?ranges from0.236×108 to 0.275×108 Ks/m².Rosenthal model was selected for calculation based on the actual growth rate of?phase and the value of G×R was 401-565 k/s.The three growth modes of crystal nucleus in front of metallic solidifying interface are the main explanations that how the pulse frequency affected the prior?grain growth.Under the effect of repeated thermal cycles,the growth of?phase in grain boundary obeys four growth mechanisms,resulting in different growth behaviours of?phase.The characteristic tensile property of samples manufactured by low-frequency PPAM is high strength with low plastic,which can be explained by the growth mode of crystal nucleus in front of metallic solidifying interface and the growth orientation of?phase.The sample with the best comprehensive mechanical performance was obtained with the pulse frequency of 70Hz,and the yield strength Rp0.2,elongation A and reduction area are 794.5MPa,11%and 39%,respectively,which is slightly lower in strength and higher in plasticity compared with forging standard.The effects of the microstructure on the anisotropy in mechanical properties were studied by alternating heat cycle strategy of successive deposition and interval cooling deposition.The deposition property increased to 88%with constant heat input while the value with gradient-changed heat input was 78%,and the error of deposition accuracy was approximately 1.4mm.The samples in transverse direction from the as-built parts showed superior average mechanical properties to standard forged titanium alloys.The tensile strength,yield strength,the ductility and the reduction area were 902MPa,826MPa,11.6%and 28%,respectively.The samples in longitudinal direction had the similar mechanical properties to the standard forged titanium alloys.The tensile strength,yield strength,the ductility and the reduction area were 896MPa,802MPa,11%and 27%.The as-built components deposited with alternating heat cycle can maintain equivalent thermal balance during the deposition process and change the heat transfer direction,which results in decrease in temperature gradient in the depositing direction.The dominating growth crystal plane of?phase changed both in the directions of depositing and scanning.As a consequence,the size of colony?grain in layer band was 0.41-0.55?m finer than colony?grain in unobserved layer band.The average thickness of?lamellar is0.41-1.21?m and the average size of prior?grain is 0.64-1.72?m,which effectively weakens the anisotropy in properties.