| Precipitated-phase strengthened Al–Cu alloys,due to their excellent thermal stability,light weight and corrosion resistance,have been widely employed as large-scale structural materials in aerospace and automotive lightweight industries.In recent years,wire and arc additive manufacturing(WAAM)has emerged as a suitable technique for realizing the rapid and efficient manufacturing of Al –Cu alloy components and has been applied to manufacture various large-scale structural components.In the process of WAAM for 2219 Al–Cu alloy,the formation of high porosity,columnar crystals and grain boundary segregation result in the decrease of the strength and toughness of the deposited samples.The addition of ceramic particles is often used as a mean of material modification.In this dissertation,a new method by adding TiC particles into the deposited components with assisted vibration energy was proposed from the perspective of homogenizing microstructure and inhibiting grain boundary segregation.The microstructure of the deposited specimens with the addition of various TiC particles was investigated,aiming at improving its performance.Besides that,the dynamics for TiC distribution inside the matrix,the precipitation of precipitates,and the mechanism of strengtheding and toughening have been revealed.Firstly,in view of the difficulty for droplet transition and poor fluidity of the molten pool after coating TiC particles on the surface of deposited layer,wire vibration was attended to improve the molten pool flow behavior,promoting the distribution of TiC particles in the molten pool.Meanwhile,the droplet transfer mode changed from large-drop shape to continuous liquid-bridge.What role played by vibration energy on droplet transition and spreading process was analysed.The deposition layer forming was optimized under different process parameters and vibration source positions.The influence exerted by process parameters on porosity and microstructure uniformity was discussed in detail.The effect of welding current on the porosity and grain refinement was more obvious.When the welding current was 110 A,the deposition speed was 200 mm/min,the wire feeding speed was 2.0m/min,and the mass fraction of TiC particle was 1.5wt.%,the porosity defects were eliminated and the grain was refined obviously.The microstructure and properties for deposited specimens with addition of TiC particles in different sizes were investigated.The microstructure characteristics,grain boundary segregation and grain distribution characteristics w ere analyzed.The results showed that the addition of nanoparticles in the deposited layer inhibited the grain boundary segregation and promoted the formation of sub-micron cell in the matrix.With the addition of micron-sized TiC particles,the average grain size was refined to 18 μm,and the ultimate tensile strength was over 400 MPa for the deposited specimen.Due to the prominent fine grain effect derived from the characteristics of micron-sized TiC particles and the formation of sub-micron cell in matrix promoted by nano-sized TiC particles addition during the solidification process of Al–Cu alloys,a new way was proposed to solve the problems of uneven microctructure and low strength-toughness by adding micro-and nano-double-sized TiC particles to the deposited specimen.The vertical interlacing distribution of θ’phase and dispersion distribution of θ’’ were precipitated near the grain boundary.Many fine spot-like θ’’ precipitates were observed within the grains.The maximum ultimate tensile strength and elongation for the deposited specimens were around407.3 MPa and 24.2%,respectively.The force condition of TiC particles with different scales at the solid-liquid interface during solidification process was analyzed,and the migration kinetics models for TiC particles in the molten pool on the micro and nano scales were established.The moving speed of the solid-liquid interface after the addition of various particles was obtained by accessing the average grain size and the solidification time.When the radius of the TiC particles greater than 3.9 μm or less than 0.8 μm,the migration rate for the particles at the solid-liquid interface front was reduced and the particles were easier to be swallowed by the interface front.The distribution characteristics of TiC particles on different scales in the deposited samples were distinguished to verify the accuracy of kinetic model and particle distribution in the matrix.In order to reveal the combined effect of micro-and nano-doube sized particles for the microstructural evolution,the solidification thermodynamics and grain refinement mechanism related to the particle size were further studied.The internal relationship between particle distribution and microstructure evolution was revealed based on the thermodynamic calculation.Then,the effect of particles on inhibiting the growth of columnar crystals and grain refinement was analyzed.The molten pool crystallization process with the addition of double-sized particles was revealed.The low nucleation free energy around the micron TiC particles made the crystal nucleus nucleate on the surfaces of the nanoparticles,promoting the generation of nano-polycrystals.Nano-sized particles that possess different lattice orientations generated lattice mismatch at the Al/TiC interface,which decreased the activatio n free energy for diffusion of the Cu solute.The dislocations that created pipe-diffusion paths for the Cu solute provided the driving force to precipitates coarsening.The nano-sized precipitates were evenly distributed within the grains.Multi-site nucleation combined with nucleation undercooling inhibited the grain boundary segregation,promoting the transformation of grain boundary from dendrite θ phase appearing in the samples without TiC addition to thin banded eutectic α+θ phase.Lastly,the strengthening mechanism for deposited specimens with the addition of single-scaled particles and double-scaled particles was analyzed.The improved strength thus appears to be related to the joint contributions arising from precipitation strengthening,Orowan strengthening,and grain boundary strengthening.Combined with the microstructural characteristics,the toughness improvement mechanism for the specimen with addition of double-sized particles was analyzed.The structure transformation of grain boundary and the formation of coherent interface between the matrix and grain boundary enhanced the toughness of grain boundary.On the other hand,the common lattice interface also provided a reliable path for load transfer.The above factors jointly promoted the excellent toughness for the deposited specimens. |
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