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Effect of vacuum on mechanical properties of thixotropic die
Pubdate date:2019-10-17Views:0
In the conventional die-casting process, the use of vacuum-assisted processes to improve the mechanical properties of castings has been a topic of interest to researchers and production departments. There are many reports on this aspect [1~4]. Vacuum applications have been shown to reduce porosity defects in castings. The pores in the casting have a small size and a uniform distribution on the section of the casting. Different from conventional die-casting, semi-solid metal has high viscosity and different flow characteristics. Therefore, the effect of vacuum on the performance of die-casting parts during semi-solid metal die-casting process, and the effect of casting system on vacuum is the focus of this paper.

      1 test process

      1.1 Test equipment

      Buhler injection control die casting machine (H-250 SC); Alpha (Alpha 1) single coil induction heating body; Foudarex vacuum system.

      1.2 Test materials

      Aluminized aluminum alloy (A356).

      1.3 Test process

      (1) The test was carried out using two different gating systems of fan-shaped single runners and branch runners with a wall thickness of 5 mm.

      (2) The aluminum alloy ingot is heated to a semi-solid state (60% solids), placed in a pressure chamber of a die casting machine, and injection molded.

      (3) Cut the specimen for tensile strength test, elongation test, and microstructure analysis.

      2 Results and analysis

      2.1 Effect of vacuum on metal flow

      In conventional vacuum die casting, special vacuum cannulas must be machined. The function of the cannula is to solidify the metal before it flows into the vacuum valve system, preventing fine aluminum metal particles from coalescing on the filter. Once these particles enter the vacuum tank, they can cause mechanical damage to the vacuum pump. Usually the cannula is placed between the valve and the cavity (see Figure 1). The purpose of this curved vacuum tube design is to avoid the two cases mentioned above.

      It is envisaged that semi-solid metal has higher viscosity laminar flow characteristics and lower heat capacity than conventional die-cast metal, and the casting solidifies faster in the vacuum tube. In addition, laminar flow of semi-solid metal will greatly reduce metal droplets.

      2.3.1 Fan-shaped single runner

      The microscopic images close to the surface area are shown in Fig. 3 and Fig. 4. It shows that the grain size of the castings is not much different in the presence or absence of the vacuum assisted process; under vacuum, the nearly spherical grains per unit area in the microstructure of the casting The number is less than that of no vacuum, and there are fewer pores on the casting. This indicates that the vacuum helps to reduce the pore content of the casting, but it does not show significant in thixotropic casting.

      2.3.2 Branch runner

      The microscopic images near the surface area are shown in Fig. 5 and Fig. 6. It shows that there is a high amount of pores in the casting sample, and the number of near spherical grains per unit area is small, whether it is vacuum assisted or vacuumless. This shows that the vacuum assisted process has little effect on reducing the amount of gas in the casting in the branch runner system, that is, the total amount of pores in the surface of the casting on the micrograph is almost the same.

      Comparing Figure 3 with Figure 5 (or Figures 4 and 6), we can also see that the castings prepared in the fan-shaped single runner have a finer microstructure than the castings prepared from the branch runners, the grain size is slightly smaller, and the number of pores obviously decrease.

      3 Conclusion

      In conventional die-casting, the vacuum-assisted metal is easy to flow into the cavity, especially in the thin-walled and complex-shaped parts. There are few casting defects (such as insufficient pouring), but it is usually necessary to process a curved vacuum tube system with high cost and complicated shape. To cure more liquid metal.

      The viscosity of the semi-solid metal is too high, and the effect of the vacuum on the semi-solid metal die-casting process does not cause significant adsorption of the metal material. Therefore, it is only necessary to process the linear vacuum tube to be attached to the mold, so that the metal can be solidified before reaching the vacuum system.

      In semi-solid metal vacuum assisted thixotropic casting, the fan-shaped single runner system increases the tensile strength and elongation by about 14% and 15%, respectively; the branch runner system increases the tensile strength by about 4% and the elongation is almost constant. .

      From the results of two runner systems (fan-shaped single runners and branch runners), it can be seen that the metal can quickly reach the position of the valve body and exhibit the same result in both vacuum and vacuum-free processes. At the same time, in the two runner systems of the fan-shaped single runner and the branch runner, the filling metal can reach the same casting part, and the cast surface quality of the casting does not change much under the condition of vacuum. These fully demonstrate that there is little or no vacuum assist in the thixoforming casting and that different runner systems have little effect on the filling ability of the metal.

      2.2 Mechanical properties

      The results shown in Table 1 illustrate the effect of vacuum on the mechanical properties of castings in the case of two different casting systems (fan-shaped single runners, branch runners).

      Table 1 Test results of tensile strength of samples

 

 
Vacuum assist
Sprue flow rate
Elongation
Ultimate tensile strength*(average value)
Number of samples
m/s
%
MPa
Fan-shaped single pouring
Road system
Have
9.55
9.55
4.07
4.68
207.9
237.4
4
6
Branch runner system
No
Hvae
11.72
11.72
1.73
1.61
196.14
204.56
4
6

      The test data in the table shows two conclusions. For the test pieces produced by the same gating system, the vacuum can increase the tensile strength of the casting by 14% (fan-shaped single runner), 4% (branched runner) and elongation of 15% (fan-shaped single runner). The elongation change of the sample under the branch runner system is not obvious; in the test pieces produced by different gating systems, the tensile strength of the fan-shaped single runner system casting is 6% higher than the tensile strength of the casting of the branch runner system (None Vacuum) and 16% (vacuum), while the fan-shaped single-pass casting has an elongation of 191% (vacuum) and 135% (no vacuum) compared to the branch runner. From the absolute values ​​of tensile strength and elongation, the fan-shaped single runner system produces castings that are much taller than the castings of the branch runner system.

      2.3 Effect of vacuum on microstructure

      The casting is heated to 540 ° C for a period of time, and the gas entrained in the casting expands, causing the sample to deform and generate surface bubbles. In the test, it was found that in the fan-shaped single sprue system, the vacuum has an effect on the amount of porosity of the casting, that is, the gas content of the casting is reduced; while the branch runner system has the effect of vacuum and no vacuum assisting on the amount of porosity of the casting, casting The porosity on the top is almost the same. This point indicates that the effect of the vacuum assisted process to reduce the gas content is only applicable to those runner systems that are suitable for semi-solid metal casting.

      Changes in microstructure can reflect changes in the mechanical properties of the material. By observing the microstructure of the polished sample surface, it is also possible to recognize the cause of the change in the macroscopic properties of the material. Figures 3 to 6 clearly show that vacuum does not affect the microstructure of the material during thixotropic casting. This situation exists in both runner systems.

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