ABSTRACT: Aluminum Alloy Ratchet wheel is a key component in high-speed railway overhead contact line system. The low-pressure casting process is mainly used to ensure the mechanical and fatigue properties of the ratchet wheel. The casting defects in low pressure casting of ZL114A aluminum alloy ratchet wheel were analyzed and the control measures were put forward to ensure the casting quality of ratchet wheel.

Key words: Ratchet; Aluminum Alloy; low pressure casting; casting defects; control technology

The ratchet wheel anchor device is a kind of mechanical compensator, which is an important part of the traction power supply catenary system of high-speed passenger dedicated line. In the course of daily operation, the suspension cable in the power supply catenary system will stretch and shrink due to the change of ambient temperature, which will cause the change of catenary tension. In order to ensure the constant tension of the overhead contact line, the ratchet wheel anchor device compensates for the variation of the catenary extension by adjusting the stroke and quantity of the weight. Fig. 1 shows the ratchet wheel anchoring device. The ratchet wheel is the most important part of the anchoring device. It is composed of rim, ratchet, hub, spokes and so on. Its profile is a typical revolving body structure with uneven wall thickness and complex contour. The quality of the ratchet determines the reliability of the anchor device and the constancy of the catenary tension compensation. This requires that the aluminum alloy ratchet wheel produced by low pressure casting not only has high strength and good fatigue resistance, but also has certain resistance to atmospheric corrosion and intergranular corrosion. To ensure these properties, the quality of low pressure casting ratchet should be obtained: casting defects should be less, surface quality should be high; casting grain size should be small, microstructure should be compact.

1. Low pressure casting of ratcheting wheel 1.1 ratcheting wheel materials at present, the ratcheting body of overhead contact line is generally made of aluminum alloy at home and abroad, and the cast aluminum alloy ZL114A (Zalsi7mg1a) is adopted at home and abroad, it is an Al-Si-Mg high strength cast aluminum alloy developed on the basis of Zl101a aluminum alloy with increased magnesium content. It not only has high mechanical properties, such as high strength, high toughness, but also has good foundry technology, such as good fluidity, air tightness and hot cracking resistance. The alloy is widely used in railway vehicles, aerospace and other fields. Table 1 shows the chemical composition of ZL114A aluminum alloy and Table 2 shows the mechanical properties of ZL114A Aluminum Alloy.

1.2 ratcheting low pressure casting is a casting method which uses electromagnetic pump system to fill the ratcheting die cavity under low pressure and low speed. It is between gravity casting and pressure casting, it is a kind of casting method which is developed on the basis of accumulating experience in Metal Mould Casting. The working principle is that the ratchet casting mould is placed on the top of an airtight smelting furnace, and the ratchet mould cavity is connected to the molten aluminium liquid in the smelting furnace through a certain size riser, after the aluminum alloy is heated and melted in the melting furnace, the molten aluminum is pressed into the ratchet mold cavity through the riser by using the compressed inert gas to exert certain pressure on the molten aluminum in the melting furnace When the molten aluminum enters the mold cavity and contacts the mold with the lower temperature, the molten aluminum gradually solidifies and forms under the pressure when it is cooled. As shown in figure 2 for Ratchet low-pressure casting schematic.

2. Defect analysis of aluminum alloy ratchet wheel in low pressure casting 2.1 hydrogen gas hole, crack, shrinkage cavity and oxide film inclusion are the main defects of aluminum alloy ratchet wheel in low pressure casting. 2.1.1 hydrogen pore in ratchet wheel is a main defect in low pressure casting process of aluminum alloy ratchet wheel, which is related to the solubility change and diffusion mechanism of hydrogen in Aluminum Alloy. Fig. 3 shows the solubility of hydrogen in aluminum alloy at different temperatures. As can be seen from the diagram, the solubility of hydrogen in aluminum alloy liquid is much greater than that in solid aluminum alloy, the solubility of hydrogen in liquid aluminum also increases with the increase of temperature. The solubility of hydrogen in solid aluminum is about 0.039 ML/100 G-A 1, and that of hydrogen in liquid aluminum is about 0.69 ML/100 G-A 1, the difference is about 20 times. In the casting process, if the mold temperature is lower and the solidification speed of liquid aluminum is faster, then more hydrogen will be released during the solidification process of liquid aluminum, which provides the conditions for the formation of hydrogen pores [3 ~ 5] . Most of the hydrogen gas holes were found on the surface of the casting, which were screw-shaped fracture.

2.1.2 cracks in the ratchet wheel of low pressure casting aluminum alloy not only affect the production cost and efficiency, but also affect the running safety of high-speed railway. The ratchet wheel is made of ZL114A aluminum alloy by low pressure casting. The linear expansion coefficient of the ratchet wheel is large, and the shrinkage rate is high when it solidifies. The cracks produced in the process of ratcheting casting are generally cold and hot. Aluminum Alloy casting cooling lower temperature, in its solid line temperature below, if the stress in the casting beyond the ultimate strength of the material itself, will form a cold crack. The cold crack of ratchet generally appears on the surface of ratchet teeth in contact with the die. The fracture surface is characterized by slight oxidation. ZL114A aluminum alloy is a eutectic alloy, and its solidification is carried out in a certain temperature range. The tendency of hot crack has a great relationship with this temperature range. The larger the range, the higher the crack rate. And the low-pressure casting speed can not be very fast, this temperature range is relatively large. In the later period of casting solidification, there is a little unsolidified liquid film between Dendrites, and the liquid film inside dendrites will be cracked under the action of large restraint stress, at this point if the liquid aluminum can not be added or not enough liquid aluminum to add, will form a hot crack. The surface of a hot crack will appear dark or black due to strong oxidation [3,6 ~ 7].

Fig. 6 shows the shrinkage cavity in the spoke of aluminum alloy ratchet in low pressure casting. Shrinkage porosity is characterized by large pores with irregular shape and unsmooth surface in microstructure, while shrinkage porosity is characterized by fine pores with smooth surface but dispersed distribution. Both shrinkage and porosity will reduce the elongation of the material, and have a great effect on the impact resistance and fatigue life of the product. 2.1.4 the oxide film of the Aluminum Alloy in the ratchet wheel is easy to oxidize. During the low pressure casting process of the ratchet wheel, the molten aluminum liquid will form a thin oxide film on the surface when it comes into contact with the air in the ratchet die, these oxide film will be entrained in the liquid aluminum affect the fluidity of the liquid aluminum. In the casting process, oxide film as an obstacle will lead to the return of liquid aluminum, resulting in partial under-pouring, and because of the oxide film folding, the casting will produce hot cracks and sometimes air holes. If the oxide film exists on the surface of ratchets, the surface of ratchets will peel off because the oxide film is Brittle and can not bear the stress of demoulding, thus the surface of ratchets is uneven, influence on surface quality of ratchet wheel and its subsequent treatment. Figure 7 shows an oxide film defect.

2.2 from the point of view of production management of ratchet casting, the factors that affect the quality of products are summarized as follows: personnel, equipment, materials, process method and production environment, abbreviated as: “Man, machine, material, law, ring.”. The influence of each element on different processes is very different and should be analyzed concretely. Figure 8 shows the impact of the ratchet wheel casting production of the five factors, and the above-mentioned major defects are related to the production of these five factors.

3. Control of defects in ratchet low pressure casting 3.1 control from the “Human”point of view (1) strengthen on-the-job training of workers to improve the level of practical operation skills; To ensure the smooth operation of the production. (2) the technician should be familiar with the important nodes of the whole production process, and give the workers a process lecture by regularly combining the cases in production, so that the workers can master the process rules in theory. (3) when pouring liquid aluminum, the worker should control the flowing state of liquid aluminum, and the best pouring state is laminar flow. 3.2 from the “Machine”point of view to control (1) the ratchet casting system to carry out technical transformation, mainly lifting device. In general, the riser can be improved by reducing the length of the riser, increasing the thickness of the insulation layer and increasing the diameter of the riser. (2) ratchet mold, should be appropriate to increase the opening stroke. Mold opening stroke will affect the worker on the mold inspection and the corresponding degree of cleaning, thus affecting the ratcheting rate. (3) adopt ceramic gate sleeve with better heat preservation performance. 3.3 control from the “Material”point of view (1) the raw material shall be clean and the melt shall be refined to eliminate the effects of non-metallic inclusions and hydrogen. (2) when melting aluminum alloy, adding 0.1% AlTi5B into the molten aluminum can improve the quality of molten aluminum and refine the grains. (3) to improve the purity of inert gas in the furnace.

3.4 control from the angle of “Method”(1) adopt reasonable casting technology to ensure the quality of casting products. (2) add a filter screen in front of the mould gate to avoid carrying too much oxide film in the molten aluminum. (3) for the ratchet wheel with complex shape, the casting defect should be avoided during casting. The speed of liquid aluminum pouring can be controlled, the heating temperature of die can be raised properly, the residence time of ratchet in die can be prolonged, and the cooling speed of casting can be reduced. (4) in the process of ratcheting casting, ultrasonic vibration technology is introduced to refine the microstructure of ratcheting, improve the density of microstructure and improve the mechanical properties of ratcheting. 3. 5 control from the angle of “Ring”(1) pay attention to the change of casting environment, especially the change of workshop humidity. (2) in the process of ratcheting casting, the pressure of compressed gas in different stages should be controlled to avoid the defects such as under-pouring and oxide film inclusion. Conclusion through the analysis of the common defects in the production of ZL114A aluminum alloy ratchet wheel low-pressure casting in high-speed railway contact wire, and combined with the five elements of “Man, machine, material, method and ring”in the casting production management of China Railway Construction Company, the measures to control the common defects in low pressure casting of aluminum alloy were put forward. According to the feedback data from the Quality Inspection Department of the Railway Construction Company in the past one year, the ratcheting yield of ZL114A aluminum alloy was greatly improved and the quality of the ratcheting casting was relatively stable after these corresponding control measures were adopted in the production process of low pressure casting, the mechanical properties and fatigue resistance were improved greatly.

 Source: Intercontinental Casting, author: Yin Rongxing, Li Dongsheng