Al-based Amorphous Alloys Prepared By Cold Spraying And The Effect Of Preheating On The Deposition Of Particles

Al-based metallic glasses(MG)have attracted wide interest given their low density and excellent mechanical properties.However,application of Al-based MG is limited due to their relatively poor glass forming ability,narrow supercooled liquid region and low deformation ability.Cold spray technology can realize the deposition of particles in solid state and avoid the crystallization of particles.It has potential advantage in the area of preparing Al-based MG when compared with the commonly used melt-spinning and suction casting technique.In order to soften and accelerate particles,the suitable preheating temperature was applied during the cold spray process.Because the deformation mechanism between the amorphous alloys and the crystalline material was quite different,so it was essential to deeply investigate the effect of preheating on the deposition of the amorphous alloys particle and the crystalline material particle.In this thesis,the cold spray technology was adopted to prepare the Al-based MG coatings.The effect of processing parameters,especially the preheating temperature on the microstructure of the amorphous coatings was investigated.The Computational Fluid Dynamics(CFD)was employed to illustrate the crystallization mechanism of the coating.The deposition mechanism of the Al-based MG particle was explored.Then the suitable processing parameters were adopted to prepare bulk Al-based MG and its formation mechanism was also investigated.Besides,the hot isostatic pressure technique and the preparation of Al-based MG composites were taken to optimize the cold sprayed bulk Al-based MG,respectively.Then the finite element simulation method was utilized to investigate the effect of preheating on the deposition of single crystalline material particles and single amorphous alloys particles.The relationship between the preheating temperature,the critical velocity and deformation process was discribed.Finally,the multi-particles model was established with the Python script and the effect of preheating on the coating formation was studied.The results of this thesis were presented as follows:(1)As the preheating temperature rose,the coating's thickness of the single pass and the content of ?-Al increased.The suitable preheating temperature range of preparing Al-based MG coatings was 593 K-633 K.When the preheating temperature reached 713 K,some other crystalline phases except ?-Al appeared.The microhardness of the coating decreased as the preheating temperature increased.The porosity of the coatings was in the range of 3%-4%.The crystallization took place when the particle flight inside the Laval nozzle.According to the CFD simulation results,the particle's flight inside the Laval nozzle could be divided into three regions:rapid heating region,moderate heating region and rapid cooling region.The crystallization took place in the moderate heating region with the analysis of Kissinger equation.The CFD simulation results also showed that the particle temperature at impact was below the Tg of the Al-based MG,indicating the particles suffered from the inhomogeneous deformation.The Reynold theory could not be fully applied to explain the deposition mechanism of Al-based MG due to its narrow supercooled liquid region and poor glass forming ability.(2)The bulk Al-based MG was prepared by cold spray technology.But its compression strength was much lower than that prepared by conventional methods.The reason was attributed to the poor particle adhesion,leading to crack formation and propagation.The study about the formation of cold sprayed bulk Al-based MG showed that no obvious jet emerged at the interface between the particle and the substrate and particles experienced inhomogeneous deformation;the interaction of the particles with the deposited coatings showed that the ?-Al phase existing at the particle boundary could promote the particle deformation and decrease the porosity.Then three types of interaction among particles were put forward and proved via the finite element simulation method.Meanwhile,the simulation results showed that the ductile crystalline phase could hinder the concentration of shear stress at the particle boundary and promote the jet formation at the interface of particles.(3)The as-sprayed bulk Al-based MG was treated by hot isostatic pressing technique.The porosity declined obviously at high temperature due to the enhanced atomic diffusion capacity at the boundary of particles.But the compression strength did not obviously improve due to the severe crystallization.Then different types of Aluminum alloy powder(Al,A16061 and A17075)were mixed with the Al-based MG powder with different ratios by weight and the bulk Al-based MG composites were prepared by cold spraying,respectively.The compression strength of Al-based MG composites was the highest when mixed with A17075 powder,but its mechanical properties were still lower than that prepared by conventional methods.(4)The finite element method was adopted to investigate the effect of preheating on the deposition of single crystalline material particles and single amorphous alloys particles.For the same crystalline material combinations,the simulation results showed that there was a linear relation between the critical velocity and the minimum preheating temperature needed to promote jet formation at the particle boundary and it suits results reported by the previous study well.The preheating temperature did not affect the temperature distribution inside the particle.The equivalent plastic strain(PEEQ)value increased as the preheating temperature rose at a certain range.For the dissimilar crystalline material combinations,the hard particle became soft and jet emerged at the particle boundary when applied suitable preheating temperature.PEEQ values of the corresponding particle increased as well.The critical velocity obtained when taking preheating into consideration still had discrepancies with the reported results and this phenomenon could be explained by the theory of mixed speed of sound.The simulation about the deposition of single MG particles showed that inhomogeneous deformation was promoted when the particle temperature at impact located in the supercooled liquid region.Homogeneous deformation appeared when the particle temperature at impact was approximately 150 K higher than the initial crystallization temperature.The deformation extent of the MG particles has connection with the type of the substrate.More sufficient deformation could appear when impacted onto the hard substrate.When comparing the deposition process of the crystalline material particle and amorphous alloys particles,the results showed that the preheating has an effect on particle deformation behaviour,deformation mechanism and determination of processing parameters.(5)The multi-particle Eulerian model with finer meshing size was established based on the Python script.For the crystalline material particles,the deformation content of particles was enhanced by increasing particle velocity and preheating temperature and then the porosity inside the coatings was reduced.Simulation results about the residual stress of the coatings were closer to the reported results.For the amorphous alloys particles,coatings became much denser when the homogeneous deformation took place.The particle deformation and the coating density increased as the substrate hardness rose.The stress state at the interface changed from the tensile state to the compression state.