Research on Magnesium Alloy Gypsum Pressure Coagulation under Casting Technology Vacuum Casting ZHAO Yu-feng, BAI Ji-qin (Henan Polytechnic University, Jiaozuo, Henan 454000, China) Relationship between tensile properties of magnesium alloys. The results show that under the conditions of high vacuum and high pouring temperature, magnesium alloy liquid can be well filled in the gypsum type. In the solidification process of castings, the higher the solidification pressure and the shorter the operating time, the denser the microstructure of the resulting castings and the higher the tensile properties.
At present, large-scale thin-walled castings are widely used in the aerospace, automotive, electronics, weapons, communications and computer industries. The shape and inner cavity of these castings are complex, and their dimensional accuracy and surface quality are high. They are basically made of aluminum alloy casting and die casting. The difficulty in casting is how to improve the dimensional accuracy of the casting and reduce the surface roughness. In addition to its low density, high specific strength, specific stiffness, and shock absorbing properties, magnesium alloys have excellent castability and processability, making them particularly suitable for the production of complex thin-walled castings.
However, magnesium alloy castings are mainly produced by die-casting and sand casting, and there have been reports of using magnesium alloy gypsum-type investment casting simple castings. Because the magnesium alloy is easily oxidized and burned, and the solidification rate is fast, and the permeability of the gypsum type is poor, the magnesium alloy gypsum investment casting process is complicated, the yield is low, and the production cost is high. The use of magnesium alloy gypsum to manufacture larger, complex thin-walled shell castings has not been reported. In this paper, the plaster casting vacuum casting pressure solidification process is used to study the feasibility of casting thin cast magnesium alloy castings.
1.1 Experimental Materials 8, Talc 24.5, Quartz Powder 16.4 Fireclay 5.4 Additives 7.5, Water 50~1.2 Process Factors Affecting Casting Quality Under the conditions of gypsum vacuum casting and pressure solidification, the filling ability and mechanical properties of magnesium alloy castings are affected. The factors are: the degree of vacuum in the cavity, the pouring temperature, the temperature of the casting mold, the solidification pressure, and the operating time. According to the allowable conditions at the production site, the value range of the above factors is shown in Table 1. Table 1 Process Factors Value range Vacuum degree Pz/MPa Casting temperature Tj/°C Casting temperature Tx/°C Solidification pressure P/MPa Operating time t, . / S 1.3 Experimental process 1.4 Detection The spiral flow method was used to measure the flowability of magnesium alloy in the gypsum type. The universal tensile machine was used to test the mechanical properties of the casting.
2 Analysis and discussion of the 1994 study and experimental work / AeadeJalEleetroniePublishing2 vacuum 1 degree, pouring temperature (degrees of mold and mold temperature on the experimental process of magnesium alloy flow in different degrees of vacuum, pouring temperature and temperature of the mold, spiral The sample is poured by the method, its length is measured, and the relationship between the spiral length and the vacuum degree, the pouring temperature and the mold temperature is shown, seeing that, at the same pouring temperature, the higher the vacuum degree, the larger the spiral length value. When the vacuum degree is higher than 0.06MPa, the spiral length increases rapidly with increasing pouring temperature, and the higher the vacuum degree, the greater the long speed.When the vacuum degree is also shown by the figure, the lower the vacuum degree, the maximum spiral length corresponds to the pouring temperature. The maximum value is also lower.If the vacuum degree is 0.04MPa, the pouring length rises slowly with pouring temperature from 660C to 700C. At about 700C, the spiral length is 678mm maximum and the pouring temperature is 720C. The length is 646mm. The vacuum degree is 0.02MPa. When the pouring temperature rises from 660C to 680C, the spiral length rises slowly. At about 680C, the spiral length has a maximum value of 633mm. From 680C to 720C, the spiral length is reduced to 614mm. The reason for the above situation is analyzed. The author believes that under the conditions of magnesium alloy gypsum casting, the filling ability is affected by pouring temperature, vacuum degree, casting material and temperature. Due to the wide crystallizing interval of the magnesium alloy and the small latent heat of the crystal, the alloy liquid has a strong ability to form a skeleton during the filling process, and the solidification speed is fast, so the magnesium alloy forming ability is poor, and the thermal conductivity of the gypsum is poor, and the selected casting mold can reduce the alloy liquid. The solidification rate increases the filling ability of the alloy.The influence of the vacuum, the casting temperature and the mold temperature on the moldability of the magnesium alloy has a great influence on the fluidity of the alloy.At low vacuum, Increasing the pouring temperature will increase the volume of residual gas in the mold cavity and increase the back pressure of the magnesium alloy solution to prevent the alloy from flowing. Secondly, the high pouring temperature accelerates the oxidation of the magnesium alloy solution to make the front of the alloy solution. Oxidation and inclusions are numerous, and the fluidity is reduced. Therefore, from the figure, when the vacuum is low, the pouring temperature is high, and the filling of the magnesium alloy is rather low; the lower the vacuum, the lower the degree of pouring is. The higher the temperature is, the worse the filling of the alloy is.In the case of high vacuum, the amount of gas in the cavity is greatly reduced, on the one hand reducing the back pressure of gas encountered in the filling process of the alloy liquid; on the other hand, reducing the magnesium The oxidation of the alloy liquid reduces the solid fraction of the front end of the magnesium alloy liquid, which slows down the solidification rate and prolongs the holding time of the alloy liquid.The magnesium alloy liquid is not easily oxidized under high vacuum conditions, and at this time, the pouring temperature can be increased, and the temperature can be fully increased. The fluidity of the alloy liquid, the higher the pouring temperature, the stronger the fluidity of the alloy liquid, so increasing the pouring temperature under high vacuum conditions can greatly enhance the filling ability of the magnesium alloy liquid.
In summary, the vacuum degree is 0.06 ~ 0.08MPa, casting temperature 750K mold temperature 300 ~ 350 °C, can make magnesium alloy liquid in the gypsum certain feelings, type mold cavity in the true and pouring temperature in the charge is very strong The effect of solidification pressure and operating time on the tensile properties of magnesium alloys is the relationship between solidification pressure, operating time, and tensile strength, yield strength, and elongation of magnesium alloys. It can be seen from the figure that the application of pressure during the solidification process can increase the tensile strength, yield strength and elongation of the magnesium alloy castings, and the pressure is high. These several mechanical properties of the castings increase accordingly, and the pressure affects the quality of the castings. Key factor. At the same time, it can be seen from the figure that the operation time is also a key factor that cannot be ignored. Under the same pressure conditions, the shorter the operation time, the higher the mechanical properties of the obtained magnesium alloy castings, and vice versa.
It is further seen that the quality of castings is affected by the interaction of solidification pressure and operating time. When the operating time is 0.4MPa, the tensile strength, yield strength and elongation of magnesium alloy castings increase significantly. However, when the operating time t = 35s, the effect of pressure is significantly reduced. After the operating time t>45s, the large pressure had little effect on the tensile strength, yield strength and elongation of the magnesium alloy castings.
After studying the above phenomenon, the reasons are as follows: (1) The existence of solidification pressure prevents the formation of shrinkage of the casting and strengthens the feeding effect. According to the criteria for the formation of shrinkage considering the pressure: degree, cooling rate and pressure; Kc* criteria.
It can be seen that solidification under pressure can prevent the formation of shrinkage, resulting in a denser alloy structure, and further improve the tensile properties of the casting. With the increase of pressure, the solid phase skeleton formed during the solidification process is compressed and fractured, the skeleton gap becomes smaller or even disappears, and the metal liquid in the liquid-solid two-phase region enters into these solid phase framework gaps for feeding. The higher the pressure, the denser the resulting cast structure, and the higher the tensile properties.
Shorten the operation time, prolong the effective time of pressure, and the casting structure is dense, and its mechanical properties are high.
In the process of pressure solidification, a series of operations such as vacuuming, pouring, unloading vacuum and building pressure are required. The time taken for this type of operation is the operating time given the structural size of the casting, the solidification time of the magnesium alloy gypsum casting is a constant, and the effective pressure acting time is ty from (1), under the same pressure. The shorter the operating time t, the longer the effective pressure acting time ty. When the molten metal is filled, the casting is in liquid phase, solid-liquid two-phase and solid phase. At this time, it can be considered that the casting is Bingham body. The yield stress is the rheological equation in the hold pressure state: *Equivalent to holding pressure; n* viscosity of the alloy liquid; Tl alloy liquidus temperature; T alloy actual temperature is a decreasing function of time.
When the operation time t is short, the actual temperature T* value of the alloy decreases less, the volume fraction of the liquid in the casting is large, the volume fraction of the solid phase is small, and the yield stress T of the casting in the liquid-solid phase is small, and the applied pressure acts on the liquid-solid two at this time. On the phase body, it is easy to force the solid-phase skeleton to be broken to form smaller gaps. At the same time, the effective time for the metal liquid to be pressed into the gap for feeding is long, and the resulting structure is dense. The shorter the operating time, the effective time of the additional pressure effect I long castings, the finer the organization, the higher the tensile properties. If the operation time is long, the liquid metal exchanges heat with the mold, resulting in a decrease in the actual temperature T* of the alloy. The fraction of the solid phase in the liquid metal is large, and it can be known from the formula (2) that the yield stress T of the liquid-solid phase is large. From equation (3), it can be seen that the rheological rate Y, which reflects the deformation of the casting, decreases, and the compression deformation resistance is large. At this point, the solid fraction in the casting is large, and when the pressure is applied, the deformation resistance of the casting is large, the solid phase skeleton is not easily deformed and broken, and the metal liquid feeding effect is reduced. When the operating time t reaches a certain value, T = T7 = 0. At this point the casting solidifies, and then the large pressure does not affect the casting deformation.
At 25s, with the pressure, the tensile strength, yield strength and elongation of the castings are very fast; when the operating time is 035s, although the pressure is applied, the tensile index of the castings is not much longer; when the operating time At 45 s, the increase in pressure has little effect on the tensile properties of the casting. Therefore, in order to obtain dense fine-grained structure, in addition to increasing the pressure, the key is to shorten the operating time as much as possible. This paper suggests that the operation time is controlled within 25s and the pressure is taken from 0.7~0.8MPa. The 750K mold temperature is 300~350°C, which can make the magnesium alloy liquid get good filling in the gypsum type.
The use of vacuum casting can solve the problems of oxidation caused by magnesium alloy casting process and poor gas permeability of the gypsum type, resulting in insufficient watering caused by helium gas.
The higher the pressure, the shorter the operating time and the longer the pressure effective action time. The denser the casting structure is, the higher the tensile properties are. This article believes that the coagulation pressure of 0.7 ~ 0.8MPa, the operation time is controlled within 25s as well.
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