Basic Research Of Micro Forming Technology Of Zr-Based Metallic Glasses

Micro-parts and micro-devices have broad application prospect in many areas, such as micro-electro-mechanical-system (MEMS), communication, electronics, national defense, medical science, etc. The development of these industries put forward a higher requirement on micro-forming technology. Bulk metallic glass (BMGs) have lots of superior propertiesis and they are one of the most ideal micro forming materials, micro forming technology of BMGs have attracted wide attention in recent years. But the deformation and flow characteristics of BMGs during the micro forming, size effect and the influencing factors are still not clear; the selection of micro forming parameters and the design and manufacture theory of micro die are not perfect; In addition, whether the micro-structure and properties of BMGs will be influenced by superplastic forming is also unknown. In view of those, systematically basic research and technology research of superplastic micro forming technology of BMGs are carried out in this paper.First, the uniaxial compression deformation behaviors of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) in the supercooled liquid region and the micro-formability under different load mode are studied systematically, the results show that the deformation behavior of the BMG is strongly dependent on the temperature and strain rate, and there exists obvious stress overshoot when deformed at a low temperature or high strain rate. It exhibited non-newtonian behavior at a temperature of 690 K, but when deformed above 690 K, it will chang from a Newtonian flow at low strain rates to a non-Newtonian flow at high strain rates, the critical strain rate increased with the increasing in temperature and is about 10-2 s-1 at the temperature of 725 K. In addition, the study shows that vibration load can improve the micro-formability of MBGs by reducing the viscosity, higher amplitude or higher frequency is helpful to improve the micro-formability.The superplastic micro back-extrusion process of Zr55Cu30Al10Ni5 MBG supercooled liquid is investigated by using DEFROM software.The results shows that with the increasing of extrusion speed, the MBG supercooled liquid change form a Newtonian flow to a non-Newtonian flow, and points out that the optimum extrusion process parameters of Zr55Cu30Al10Ni5 MBG are:at a temperature of 725 K and extrusion speed above 4μm.s-1.Choosing cup-shaped parts with simple cross-section and three-dimensional complex variable cross-section as research objects, the micro back-extrusion process of Zr55Cu30Al10Ni5 and Zr65Cu15Ni10Al10 BMGs in their supercooled liquid region are studied under the guidance of numerical simulation, the influence law of the main process parameters, such as temperature and extrusion speed, on the forming quality and micro formability of the BMGs are analysed. A micro-cup-shaped object with an outer diameter of 2.2 mm and a single side wall thickness of only 0.05 mm, as well as a micro part with a three-dimensional complex variable cross-section are successfully extruded. Scanning electron microscopy shows that the parts, with good apparent quality, maintain amorphous structures, the shapes and sizes of the parts meet the design requirements.Two series of cup-shaped micro parts with the out diameter of 1.2 mm and 0.6 mm but different wall thickness were set as the objects, the size effect during the micro back-extrusion process of the Zr65Cu15Ni10Al10 BMG are systematically investigated. The results show that the apparent viscosity monotonously increases with the decreases of overall dimension or the wall thickness, and there exits obviously size effect when the ratio of inner diameter to outer diameter is greater than 0.83. Further analysis found that the rapid increase in the specific surface area resulted by the decreases in parts size and wall thickness causes the rapid increase in the micro friction and surface energy, and those contributes to size effect. Based on the Hagen-Poiseuille-equation, a equation for calculating the required extrusion load is deduced.The deformation characteristics and property changes of the BMGs during in the micro back-extrusion process were systematically analyzed by means of micro-hardness measurement and nanoindentation, the study found that the hardness of thick-walled parts is similar to this of as-cast amorphous alloy, but the nano-indentation hardness of thin-walled parts is smaller than that of as-cast amorphous alloys. In addition, the micro-hardness distribution of the longitudinal section is far from uniform, the largest hardness was found at the inside corner and the lowest hardness at the edge place, the relative difference value is about 14%. The free volume change caused by the complex stress state and local strong shear deformation is the main cause for the property changes and the uneven distribution of microhardness.These research results supply a credible theoretics bases for the development of BMGs micro forming technology, and also provide a stable academic foundation for expanding the industrialized application range of BMGs.