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Study On TLP Diffusion Bonding Of Magnesium-based Metal Matrix Composites (TiC_p/AZ91D)

Posted on:2008-05-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y GuFull Text:PDF
GTID:1101360212997763Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Magnesium alloys have been perfect materials to realize light weight due to the characteristic of low density, high damping capacity, good castability and mechanical properties, and also recycle properties. However, the disadvantages of low elastic modulus, low wear resistance, limited high temperature strength, low creep resistance at elevated temperatures and corrosion resistance severely restrict the use of magnesium alloy as engineering materials. Metal matrix composites provide a good way to solve the problem. In the academic research and industry application, magnesium-based metal matrix composites have caused more and more attention. According to the using circumstance, selecting different material as the different component of the structure, and changing them to a uniform system using bonding (welding) are effective way to reduce cost, and make use of different advantages of different materials. The recent research indicated that, transient liquid phase (TLP) bonding is one of the effective ways to bonding material. This dissertation studied the microstructure and mechanical properties of joints produced by TLP bonding, and the effect of processing parameters on the microstructure and mechanical properties of the joints. The studies will not only enrich the theory of bonding of materials, but also supply necessary theoretical basis of the technology of the practical utility of magnesium-based metal matrix composites.In this dissertation, AZ91D magnesium alloy was bonded by transient liquid phase bonding using pure copper and aluminum interlayers, respectively. The characteristics of the microstructure and mechanical properties of joints were revealed to provide theoretical instruction on the TLP bonding of magnesium-based metal matrix composites. On the basis of the study of TLP bonding of magnesium alloy, TiCP/AZ91D magnesium-based metal matrix composites containing two types of volume fraction (10vol.%, 30vol.%) of reinforced particulates(TiC) were bonded by TLP bonding using pure copper and aluminum interlayers, respectively. Microstructures and mechanical properties of joints were studied and the effects of bonding time, bonding temperature and volume fraction of reinforced particulates on them were investigated systematically.TLP bonding process of AZ91D magnesium alloy, which was bonded using pure copper interlayer with the thickness of 20μm, composed four stages: plastic deformation and solid diffusion, dissolution of the interlayer and base material, isothermal solidification, and homogenisation. The average concentration of Mg in the edges of the copper interlayer indicated that the edges were solid and the interlayer were almost not dissolved, but there was some plastic deformation in the interlayer during the stage of plastic deformation and solid diffusion. When the concentration in the interface of the interlayer/base material reached the concentration required for eutectic reaction, discontinuous liquid was formed on the interface. Since the composition of the liquid was not in equilibrium with its adjacent solid, the interlayer and base material dissolved. The dissolution of the interlayer and base material and the widening of the liquid were attributed to the inter-diffusion of Cu and Mg in the interface. During the stages of isothermal solidification, the liquid width decreased obviously. The bonding interface was not apparent and the grain coarsened. The composition in the joint tended to be uniform.The research demonstrated that the bonding time had obviously effect on the microstructure and mechanical properties (shear strength) of the joints. The shear strength of the joints was improved with the increase of bonding time. But, when the bonding time was too long, the shear strength would decrease. The mechanical properties of joints depended on the microstructure and composition of the joints principally. At the bonding temperature of 530℃, when the bonding time was 1min and 5min, the shear strength of the joints was low. This was because the interlayer was not dissolved (1min) or there was little liquid in the interface to realize metallurgical bonding. So, the increase of the shear strength was limited. The shear strength of joints decreasing when the bonding time was too long was due to the grain coarsening and the CuMg2 compound in the grain boundary. The maximum shear strength of the joint was 70.2 MPa at the bonding temperature of 530℃for the bonding time of 30min.TLP bonding process of AZ91D magnesium alloy, bonded using pure aluminum interlayer (10μm) was same as that bonded with pure copper interlayer. Early in the stage of TLP bonding, the shear strength of the joints was low, which was associated with large amount of Al12Mg17(L→Mg(α)+Al12Mg17(γ))brittle compound during cooling. The micro-crack formed at the Mg/Al12Mg17 interface and even in the Al12Mg17. This caused the joint fracture as dissociation or quasi-dissociation mode. The shear strength of the joints increased with the increase of bonding time. It was mainly due to the decrease of the amount of Al12Mg17 in the joints.It was found that the shear strength of the joints increased with the increasing bonding temperature. Since the liquid formed at interlayer/base material interface increased with the increasing temperature, isothermal solidification would be sufficient. It led to the increase of the shear strength. However, when the bonding temperature exceeded 480℃, the shear strength decreased due to the grain growth of Al12Mg17. On the other hand, the increase of temperature fastened the processing of TLP bonding. The start of isothermal solidification would be ahead of that at lower bonding temperature, which caused the decrease of the amount of Al12Mg17. This could increase the joint strength. But too high bonding temperature caused the decrease of the joints strength. The maximum shear strength of the joint was 76.1 MPa at the bonding temperature of 480℃for the bonding time of 60min.TLP bonding processing of 10vol.%TiCp /AZ91D was same as that of AZ91D magnesium alloy. The stages of dissolution of the interlayer and the base material, isothermal solidification and homogenisation were observed. However, the existence of the TiC reinforcement made the microstructure of joints different from that of AZ91D.Early in the stage of TLP bonding of 10vol.%TiCp/AZ91D,there was a diffusion zone at the interlayer/base material interface. And the width of the diffusion zone increased with the increase of temperature. The existence of the diffusion zone favored the dissolution of the interlayer and base material. At constant bonding temperature, the average concentration of Cu in the joint centerline decreased with the increase of the bonding time. When the bonding time exceeded 20min, the change of average concentration of Cu in the joint centerline was not apparent. The microstructure of joints were mainlyα-Mg,CuMg2 and TiC. It was found that there was little TiC particulates in the liquid, when the 10vol.% TiCP/AZ91D was bonded by TLP bonding using pure aluminum interlayer.Therefore, the dissolution of the interlayer and base material was not independent. The results indicated that the microstructures and composition of the joints were different at a constant bonding temperature with increasing bonding times. The microstructures and composition tended to be uniform. At the same time, the microstructures and composition also had a tendency to become uniform at a constant bonding time with increasing bonding temperatures. The processing of TLP bonding was faster at high bonding temperature than that at low bonding temperature.The experimental results demonstrated that the shear strength (the maximum shear strength was 68.19 MPa, at the bonding temperature of 530℃) of TLP joints of 10vol.TiCP/AZ91D bonded with pure copper interlayer were lower than that of AZ91D (the maximum shear strength was 70.2MPa, at the bonding temperature of 530℃).The shear strength (the maximum shear strength was 58.2MPa, at the bonding temperature of 480℃) of TLP joints of 10vol.% TiCP/AZ91D bonded with pure aluminum was also lower than that of magnesium alloy. It was mainly attributed to the appearance of the TiC reinforced particulates in the joints which caused the joints fracture through the interface of the TiC. Therefore, the strength of the joints was decreased.The experimental results indicated that the volume fraction of the reinforced particulates in the magnesium-base metal matrix composites had important effect on the microstructure and mechanical properties of the joints. The shear strength of TLP joints of pure copper (the maximum shear strength were 64MPa, at the bonding temperature of 510℃) 10vol.% TiCP/AZ91D interlayer were lower than that of 10vol.% TiCP/AZ91D (the maximum shear strength was 68.19MPa, at the bonding temperature of 530℃). It was principally because of the high volume fraction of the reinforced particulates. When the volume fraction was high, the joints would contain large amount of reinforced particulates. This would result in the decrease of the strength.On the basis of some hypothesis, the physical mode of the moving solid-liquid interface was established. The finite difference method was used to discrete the governing equation. A model about two phase moving boundary at constant temperature was obtained, and the model conserved solute. The realization of the numerical algorithm required. Solving a set of coupled non-linear equations. The fully implicit method and the up/down-wind approximations were employed to degrade the problem as solving a set of linear equations by iterative method. The numerical algorithm could converge to exact solution.Though the comparison between the simulated results and experimental results, it was approved that the simulated results could demonstrate the distribution of the solute concentration during the dissolution of the interlays and base material, and the isothermal solidification. Moreover, the results also showed that the numerical model could correctly explain the moving of solid-liquid interface. The numerical mode could also be used to select proper parameters of TLP bonding.
Keywords/Search Tags:Magnesium-based
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