Research On Forming Rules Of Permanent Mold Casting For Microscale Components

Micro-precision casting is one of the most competitive processing technology for micro-parts in recent years. And it can prepare three-dimension complicated micro-metal parts with low-cost and mass. In this paper, a new micro-permanent mold casting processing technology is developed. And the involved basic science and theory, such as the filling flow law of fluid, the cessation mechanism and solidification forming in micro-scale, microstructure and mechanical properties of micro-castings, were studied deeply and systematically. Meanwhile, the micro-scale effects and microscopic mechanism about flow, solidification, microstructure and mechanical properties were revealed.When flow in micro-scale space, the viscosity of liquid metal decreases sharply and the gas anti-pressure and capillary force have significant effects on liquid metal flow . In this case, traditional macroscopic N-S equation cannot be applied. So amelioration N-S equation and related physical and mathematical model was established.Three obvious micro-scale effects which cannot occur under macroscopic state were observed when liquid metal fill in micro-pipe: increasion of boundry layer relative thickness, form of"convex effects"and generation of the gradient zone of negative pressure. The ratio between the boundry layer thickness and the pipe diameter increases with the decrease of the pipe diameter. In micro-scale, the stream-front is more prominent in comparision with that of macroscopic flow with increase of speed gradient. Then"convex effects"is formed, which increase significantly as the time increase and the pressure decreases. In addition, the gradient zone of negative pressure between the zone of flow front and upper zone is found. And the gradient zone becomes more and more obvious with the decrease of pipe diameter. The obvious micro-scale effects of the cessation of liquid metal flow in micro-pipe were observed. The cessation mechanism changed from the macroscopic front blocking mechanism to the composite cessation mechanism mixed with the front blocking mechanism and the semi-solid blocking mechanism. And the critical size of transition is 500μm.The results of numerical simulation of micro-flow, micro-heat transfer and micro-solidification of micro-gear castings illustrated that the second pressure head is formed by filling firstly melt metal into opposite gear axis to impact the back wall of mould cavity. Then the back flow is formed by reversible filling melt metal into each tooth of micro-gear. The gases in the mould cavity are stirred and cut by high speed motion of melt metal, and get out from discharge ports by the overflow of melt metal.The the plate microparts with different thickness and three-dimension microgears were prepared by micro-permanent mold casting. The minimum thickness of plate microparts is 200μm, and the length is 60mm. All the size of microgears reached the micron scale: the diameter of gear disk is 580μm, the thickness of gear disk is 350μm, the diameter of gear axis is 300μm, the length is 400μm and the minimum wall thickness is 50μm. This feasibility that three-dimensional complicate castings in micron-scale are produced by micro-permanent mold casting processing were proved.The microstructure of micro-parts exhibit obvious micro-scale effects. Firstly, compared with samples in macroscopic, the grain size is refined significantly as the size of micro-castings decrease. Secondly, the primary phaseβ-Zn decreases gradually, while eutectic structure increases. The morphology of eutectic structure of micro-castings is rod-like and different from lamellar on regular castings. Fine grain strengthening, solid solution strengthening and interface strengthening caused by rod eutectic were the main intensified factors.Mechanical properties of micro-castings also exhibit obvious micro-scale effects. Tensile strength and elongation of flake-like micro-castings are improved one time than that of castings of normal thickness of 2mm. Plentiful nano-scale particles in the dimples were found and hinder the beginning of slip systems in the intercrystalline and the extension of micro cracks. The hardness of micro-castings is higher than that of regular castings, by 1.7 times in the maximum. The sensitivity index m of strain rate of room temperature creep is apparently lower than that of macro-castings. This illustrates that micro-castings have higher creep resistance and longer service life.