Foreword
In the conventional die casting process, the use of vacuum-assisted process to improve the mechanical properties of the casting has been a topic of interest of the majority of researchers and production departments. Many reports about this aspect [1-4]. Vacuum applications has been proven to reduce stomatal defects of castings. Casting pore size is smaller and more uniform distribution in the casting cross section. Unlike conventional die casting, semi-solid metal high viscosity and flow characteristics, vacuum die casting performance in the semi-solid metal die-casting process the gating system vacuum impact what role is the focus of this study.
1 during the test
1.1 Test Equipment
Buhler (Buhler) injection control die casting machine (H-250 SC); Alpha (Alpha 1) single-coil induction heating body; Foudarex vacuum system.
1.2 Test Material
Broken grain processing aluminum alloy (A356).
1.3 Test process
(1) fan single runner and branch runner two different type of gating system pressure test specimen wall thickness of 5 mm.
(2) The aluminum alloy ingot is heated to a semi-solid (60% solids), placed in the pressure chamber of the die casting machine, injection molding.
(3) cut specimens tensile strength test, elongation test, microstructure analysis.
2 Results and Analysis
2.1 vacuum metal flow
The conventional vacuum die casting, machining special vacuum intubation. The cannula's role is to make the metal flows into a vacuum valve system before the solidified metal, to avoid the fine aluminum metal particles coalesce on the filter. Once these particles into the vacuum canister, will cause mechanical damage of the vacuum pump. Usually placed between the valve and the cavity of the cannula (Fig. 1). The purpose of this curve vacuum design, in order to avoid the above-described two situations.
Figure 1 conventional die casting vacuum valve curve intubation
1. Vacuum mold table 3. Vacuum valve 4. Cavity intubation
Envisaged that the semi-solid metals as compared with the conventional die-cast metal, with the laminar flow characteristics of the relatively high viscosity and low thermal capacity, and the castings in the vacuum within the solidified faster. In addition, the semi-solid metal laminar flow injection will greatly reduce the number of metal droplets. Thus, the coupling valve and the pressure-type vacuum tube is processed into a linear shape, as shown in Figure 2.
Figure 3 fan-shaped single runner production casting surface microstructure (vacuum)
Figure 4 fan-shaped single runner the production casting surface microstructure (no vacuum)
Figure 5 branch sprue produce castings near the surface microstructure (vacuum)
Figure 6 branch sprue produce castings near the surface of the microstructure (no vacuum)
2.3.1 fan single runner
Close to the surface area of the microstructure picture as shown in Figure 3, Figure 4, indicating that little difference in the case of whether the vacuum assisted process, the grain size of castings; under vacuum, casting the microstructure of nearly spherical grains per unit area number compared with no the less vacuum castings stomatal less. This demonstrates that the vacuum helps reduce the contents of the stomata of the casting was not obvious, but in the thixotropic casting.
2.3.2 branch runner
Pictures of the near surface region of the microstructure shown in Figure 5, Figure 6, the display either vacuum assisted or non-vacuum assisted, the higher the amount of the stomata in the casting sample, and nearly spherical grains per unit area on a small number. This shows that the vacuum-assisted process to reduce the casting the volume gas branch runner system has little effect on the casting surface the total stomatal content within almost consistent microscopic photograph.
Compare Figures 3 and 5 (or Figure 4 and Figure 6), we can also see the castings prepared in sector single sprue, its microstructure detail, the grain size is slightly smaller than the runner castings prepared by the branch, stomatal Quantity significantly reduced.
3 Conclusions
Conventional die casting, vacuum-assisted process metal easily into the cavity is filled thin-walled and shape complex parts, casting defects (such as misrun) less, but it normally takes to process a high cost, complex shape curve vacuum tube system to cure more of the liquid metal.
The viscosity of the semi-solid metal is too high, the influence of vacuum on the semi-solid metal die-casting process does not produce the phenomenon of a metal material is adsorbed, thus simply machining straight vacuum tube is connected to the mold, the metal solidified before it reaches the vacuum system.
Vacuum Assisted thixotropic semi-solid metal casting sector runner system so that the tensile strength and elongation were increased by approximately 14% and 15%; branch runner system to improve the tensile strength of approximately 4% elongation almost unchanged .
Figure 2 semi-solid metal die-casting vacuum valve straight intubation
A vacuum tube 2. Die stage vacuum valve 4. Cavity cannula
From two runner system (sectorial single runner and sprue) of the branch results can be seen: the metal can soon reach the position of the valve body, and, in the case of the vacuum and no vacuum two processes, showed the same results. Meanwhile, in fan single runner and the branch of the runner two runner system, the filler metal can to the same parts of the castings, and the castings cast the state surface quality change with or without vacuum conditions. These full description and thixotropic casting, with or without vacuum assisted and runner system has little effect on the filling capacity of the metal.
2.2 Mechanical Properties
Table 1 shows the results of the vacuum casting mechanical properties in the case of two different gating system (fan-shaped single runner, branch runner).
Table 1 the tensile strength test results
Vacuum
Auxiliary runner velocity elongation at ultimate tensile strength (average) of the specimen
Quantity
m / s% MPa
Fan-shaped single pouring
Road System
There are 9.55
9.55 4.07
4.68 207.9
237.4 4
6
Branch runner
System without
There are 11.72
11.72 1.73
1.61 196.14
204.56 4
6
The test data in the table shows the conclusion of the two aspects. The test pieces produced by the same casting system, the vacuum can enhance the tensile strength of the casting of 14% (fan-shaped single sprue), 4% (branch sprue), and elongation of 15% the (sectorial single sprue). Branch runner system changes in the elongation of the sample was not obvious; specimens produced by different gating system high tensile strength, the tensile strength of the fan runner system castings casting than the branch runner system 6% (no vacuum) and 16% (vacuum), the fan single runner castings elongation than the branch sprue 191% higher (vacuum) and 135% (no vacuum). From the absolute value of the tensile strength and elongation of view, the production of castings by the sector a single runner system is much higher than the branching runner system castings.
2.3 Vacuum microstructure
The casting is heated to 540 ℃, holding time, the expansion of the gas involved in the casting, the specimen deformation and generate surface bubbles. Test found that the fan runner system, vacuum have an impact on the amount of casting porosity, that is, to reduce the gas content of the castings; branch runner system, vacuum and vacuum-assisted little impact on the amount of casting porosity, castings is almost the same on the porosity. This demonstrates that the vacuum-assisted process to reduce the role of the gas content only applies to those runner system should be semi-solid metal casting.
The microstructure changes may reflect changes in the mechanical properties of the material. By observing the microstructure of the polished surface of the sample can also be aware of the causes of changes to the macroscopic properties of the material. Figures 3 to 6 clearly show that in the thixotropic casting vacuum does not affect the microstructure of the material. In this case the two runner systems are present.
