Hole-forming Characteristics Of Array Micro-holes Material With Large Aspect Ratio In Aluminum Based Alloy By Casting Measures

With the development of advanced equipment and high-end equipment,hole structure material,as an important part of many parts and components,has been widely used in many fields.At the same time,it has also put forward more stringent and high standard requirements for hole material.Micro-hole with large aspect ratio is becoming the latest development trend in the direction of hole processing.However,in the field of hole material preparation,it is difficult to make micro-hole material with 0.1-3 mm diameter and larger aspect ratio by mechanical,special machining or casting methods.So it is the development trend of this direction to develop a new processing to prepare hole material with micro diameter and large aspect ratio.For this reason,the casting method was adopted in this paper.Taking advantage of the poor wettability and weak reaction between carbon and liquid aluminum,the carbon fiber hole core and the metal wire coated graphite coating hole core were respectively used to successfully fabricate array micro-hole with large aspect ratio on aluminum base.The minimum hole diameters are close to 0.2 mm and 0.5 mm respectively,and the maximum aspect ratio is about 170.And behavior of molten metal filled with space of prefabricate body in the process of hole forming,characteristics of micro-hole with large aspect ratio,and core-pulling force were studied.In the process of studying molten metal filling space of prefabricated body,filling coefficient was used to describe filling ability of molten melt,the results show that molten melt filling ability was related to pouring temperature,distance of adjacent two hole cores,hole core radius,arrangement of hole cores and hole core material.When these influence factors are alone,melt filling capacity increases with the increase of casting temperature,the hole spacing increases and the radius of the hole increases.At the same area,when the hole cores are arranged at an angle of 60°,the ability of filling is better than that of the angle of 90°.The characteristics of the micro-holes were described by aspect ratio,the arithmetic mean deviation of the hole wall outline(Ra)and the shape factor(K),and their influence factors were analyzed.The results show that under the influence of surface tension and metal static pressure,the aspect ratio and hole core spacing of micro-hole were increased with the increasing of pouring temperature when the diameter of micro-hole is constant.And the roundness and roughness of the micro-hole are mainly affected by the cross-sectional shape of hole core,surface smoothness of hole core and pouring temperature.So there is the better roundness of micro-hole obtained by hole core of metal wires coated graphite paint with better cross-section shape,and there is the smaller roughness of micro-hole obtained by hole core of carbon fiber with relatively smooth surface.Besides,there is no effect on morphology nearby micro-hole wall when carbon fiber was used as hole core.However,when the metal wire coated graphite coating was used as hole core,carbon elements appeared nearby the micro-hole wall and its content increased with the increase of casting temperature.Core-pulling resistance is mainly derived from the friction force formed on the interface between the hole core and the matrix.Therefore,a formula of core-pulling force is obtained according to Hooke's law.The results show that the core-pulling force is not only related to the friction factors on the contact surface,the area of the contact surface and the normal stress,but also closely related to the casting temperature,characteristics of micro-hole and type of contact surface.The core-pulling force increases with the increasing of pouring temperature,which is mainly due to the increase of the interfacial bonding force and occlusion force.In addition,to verify the feasibility of this process in the Al alloy,array micro-holes were obtained by selecting ZL102 alloy with good casting performance as base material under 800 C pouring temperature.Compared with pure aluminum,there is slightly difference in micro-hole distribution and micro-hole diameter.The minimum micro-hole diameter is 0.2mm and the maximum aspect ratio is 173.Si element in the alloy is beneficial to reduce the micro-hole roughness.The roundness of micro-hole is slightly improved,the shape factor of micro-hole obtained by metal wire coated with graphite coating hole core could reach more than 0.8.Under the same test condition,the core-pulling force is slightly reduced and it increases with the increase of aspect ratio.