Font Size: a A A

Research On High-intensity Ultrasonic Stirring Fabrication And Corrossion Resistance Of Nano-CeO2p/Zn-4.5%Al-RE-Mg-Ti Composites

Posted on:2008-10-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:H Z ZhouFull Text:PDF
GTID:1101360272976808Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
The steel protective covering materials nano-CeO2p/Zn-4.5%Al-RE-Mg-Ti (ZA) nanocomposites (ZACs), whose fabrication process has become one of the hot spots in metal matrix composites research, feature with good corrosion resistance, adhesion and excellent processing performance. In present study, the fabrication techniques of nano-particles reinforced metal matrix composites (PRMMNCs) were firstly introduced and compared. The high-intensity ultrasonic stirring technique, which could refine the molten alloy, disperse reinforcement particulates and enhance the wettability of the particulate/molten alloy couple, was selected to fabricate nano-CeO2 particulates reinforced Zn-based composites. Then, a suit of high-intensity ultrasonic stirring device was assembled. Moreover, the surface modification to nano-CeO2 particulates, wetting processes between particles and molten ZA alloy, mechanical-high intensity ultrasonic combination stirring processes, solidification characteristics, mechanical properties, as well as corrosion resistance and corrosion mechanism of the ZACs were systematically investigated by the help of SEM, EDS, TEM, XRD, DSC, TG, AES, XPS and EIS. Finally, the application of the ZACs in hot dipping is discussed.The tests show that the designed ultrasonic stirring device, which has the functions of antioxidation for molten metal and accurate temperature control, as well as a good stirring effect, can be used to fabricate MMNCs. The calculation results also exhibite that the vibration amplitude, ultrasonic intensity and pressure amplitude of the titanium stepped ultrasonic transformer with a transition section in molten are 53.41μm, 1.30×106 W/m2, 7.58 MPa, respectively.The TEM, FTIR and AES investigations show that nano-CeO2 particles are covered by a physical surface covering with its thickness about 20 nm by the help of ultrasonic agitation, and the distribution of the nano-particles is obviously improved. Besides, the TG result indicates that the carbonization of this covering layer can take place in the fabrication of ZACs. From classical thermodynamic point of view, reactions between the carbonization layer and zinc oxide may carry out at the interface, which promotes the wetting via getting rid of the block of the zinc oxide.According to the mechanics and dynamics discussion of the system's wetting process, it is very difficult for nano-CeO2 particulates to be directly dispersed into ZA melt by high-intensity ultrasonic. Thus, nano-CeO2 particulates are mechanically engulfed into ZA by machine stirring process, and subsequently dispersed by high-intensity ultrasonic stirring technique. By doing this, the ZACs with the nominal mass fractions of 1, 2, 3, 4, 5 and 6%, were obtained. Based on the FE-SEM and TEM observations, nano-CeO2 particles could be homogeneously distributed in the ZACs.The optical microscope and SEM observations clarify that the microstructure of ZACs at room temperature consists of island-like eutectoid structure ofα(Al)+β(Zn) and lamellar structure ofα+β. The investigation of the solidification process of the ZACs indicates that no reaction production exists on the clear and smooth interface between CeO2 and the matrix. Furthermore, nano-CeO2 are dispered in primary and eutecticβ(Zn), but no ctystallographic orientation relations are found amongβ(Zn),α(Al) and CeO2. Thereby nano-CeO2 particulates in ZACs are arrested by growingβgrains.An optimized amount of nano-CeO2 addition could improve the mechanical properties of ZA matrix. Compared with the matrix alloy, the tensile strength and elastic modulus of the composites increase obviously with the increment of mass fraction of nano-CeO2 particles, while the elongation decreases. However, the increased large mass fraction deteriorates the distribution of CeO2 particles in matrix alloy, which leads to the decrease of mechanical properties of ZACs. The tensile fracture also shows that the damage mechanism of the composites vary into a brittle fracture pattern.Furthermore, the corrosion tests results display that the ZACs exhibits better corrosion resistance than the ZA alloy. The mainly reason is that the oxygen, which is crucial for the corrosion process of ZACs, is consumed by nano-CeO2 particulates via the storage and release capacity (OSRC) and redox properties.And by doing so, the ZACs's corrosion resistance is improved.Finally, primary exploring works on of the ZAC layer with its mass fraction 2% reveal that hot dip quality is qualified in the condition of this study. The hot dip coating, which exhibits good adhesion with steel matrix, is about 30μm and almost has little surface defects, such as pretermission of plating, micro-cracks, and bubbles.
Keywords/Search Tags:Nano-CeO2 particulates, Surface modification, High-intensity ultrasonic stirring technique, Mechanical-high intensity ultrasonic combination stirring process, Zn-based metal matrix composites, Corrosion resistance
PDF Full Text Request
Related items