Research on die casting process of Aluminum cylinder without cast Iron cylinder

Research on die casting process of Aluminum cylinder without cast Iron cylinder

1.All-aluminum cylinder status
  
Aluminum is a light metal with low density (2.79/cm3), good strength and plasticity. Aluminum alloy has good strength. The strength of super-hard aluminum alloy can reach 600Mpa, and the tensile strength of ordinary hard aluminum alloy can reach 200. -450Mpa, aluminum alloy with tensile strength greater than 700Mpa has been obtained, and its specific strength is comparable to that of high-quality alloy steel, and its specific steel is even higher than steel. Aluminum has good thermal conductivity and can be used as a heat sink material. Aluminum also has good corrosion resistance and good plasticity, and is suitable for various pressure processing. Therefore, aluminum alloy is widely used in various fields of machinery and automobiles.
  Automobile engines have evolved from all-iron materials to all-aluminum materials, with the aim of reducing the weight of the car, and reducing the weight of the car means saving fuel. Generally, an aluminum cylinder engine is used, which can reduce the weight of about 20 kg. For every 10% reduction in the car's own weight, fuel consumption can be reduced by 6% to 8%. Now the cylinder head of the gasoline engine is basically made of aluminum alloy, and the cylinder material has also been replaced with aluminum alloy.
However, the aluminum cylinders we are talking about are not completely aluminum. Almost all cast iron cylinder liners or steel cylinder liners are used. That's because of the current engine, in order to reduce the inertia of the reciprocating components, increase the speed and response speed, the piston mostly uses high-silicon aluminum alloy as the material. If the cylinder wall is also aluminum. The coefficient of friction between aluminum and aluminum is relatively large, so the performance of the engine is greatly affected. There is no such problem with cast iron or steel cylinder liners.
  But it is undeniable that some high-end engines have now achieved true all-aluminum materials, such as the Audi 3.1L FSI V6 all-aluminum cylinder, which is not using cast iron or steel cylinder liners, but is used high through the inner wall of the cylinder. Silicon-aluminum alloy to overcome the friction coefficient. The processing process is very expensive, but the heat dissipation performance has been greatly improved.
In order to truly realize the economically viable steel-free aluminum alloy cylinder block, some domestic companies have taken the lead in the development of high-silicon rare-earth aluminum alloy to manufacture cylinder blocks without the need for steel sleeves. Compared with the conventional cylinder block, the new material automobile engine block has the advantages of reducing friction work, saving fuel, optimizing engine heat dissipation, reducing exhaust gas emissions, reducing weight, improving cylinder strength, and reducing cost.
At present, such a cylinder of a company has been successfully rolled off, and the mold is developed by the author company. The author mainly analyzes the difficulty and points of this type of cylinder in the molding process from the perspective of cylinder forming.
2.Difficult point analysis of casting structure
  
The solid model of the aluminum alloy die-casting part is shown in Figure 1. The outer dimension is 445mm x 378mm x 226mm. The largest wall appears on the cylinder wall, the thickness is 13.4mm, and the remaining average wall thickness is 4mm. The casting material is high silicon rare earth aluminum. The alloy is a hypereutectic AL-Si series die-cast alloy with a silicon content of 19-21% by weight. The size of the Si crystal is controlled below 50 microns. The alloy has good fluidity, air tightness, thermal crack resistance, high wear resistance and low expansion coefficient, and is suitable for making automobile engine cylinder liners and brake blocks. , pump and other parts and materials.
  The product quality is 19.2kg. The overall shape of the casting is complex, the structure is variable, and the wall thickness is uneven (as shown in Figure 1). Especially, the cylinder liner is thicker because it does not use the pre-inlaid cylinder liner.

Fig 1.All aluminum engine block and sectional view
3.Design and calculation of the gating system
  
The design of the gating system includes the design of the runner, the design of the overflow exhaust system, and the design of the cooling heating system.
  First determine the feeding method. From the analysis of the structure of the product, the one-side feeding method is more suitable. Such a design facilitates the venting of the casting during the casting process, avoiding the gas pockets in the cylinder, and eventually causing large air venting holes in the interior.
  Secondly, according to the die casting process and parameters, some necessary calculations are made to determine the dimensions of each part of the runner. First, the cross-sectional area of the ingate is determined according to the flow method and some empirical values.

  In the formula: A is the inner gate cross-sectional area mm2; G is the mass of the molten metal passing through the ingate, including the mass g of the overflow slag; ρ is the density of the liquid metal g/cm3; V is the metal at the gate The flow rate of the liquid is m/s; t is the filling time S of the cavity.
   Casting density is taken to 2.7g / cm3; mass overflow dross takes 30% by mass of the casting member to give G = 25000 g; for the cylinder, taking V = 60m / s, t = 0.08s, the calculated A = 2200mm2
Casting volume of 7117cm3, dross gate taking 45% of the casting, the total volume of 10319cm3. The casting is used on a 2500T bedroom cold chamber die casting machine. The expansion force calculated according to the projection surface is 1920T, in the safe zone. A cylinder length of 960 mm, that the material is pre-filled in the cartridge does not appear turbulent gas volume, the filling rate is preferably controlled in the cartridge in the 40-50% range, the upper limit is selected here. It is obtained by rounding calculation cylinder diameter 160 mm.
Because the cylinder casting has a large contour and a complicated structure, the conventional exhaust gas is very easy to be gas-filled during pouring. Moreover, the particularity of the cylinder casting is vacuum evacuated in the overflow exhaust. Vacuum die casting can significantly reduce pores, make the structure more compact, and improve the mechanical properties of the casting. In addition, the filling back pressure is reduced, the molding performance is good, and the surface quality is improved. As shown in Fig. 2, the overflow exhaust gas in the latter part of the casting is concentrated together, and finally remitted to two exhaust plates, and then the vacuum is extracted by the fixed-die side exhaust plate to realize the pumping mode of the mold.
Fig2.Flow path, slag bag arrangement, vacuum pumping
 
  Mold temperature is an important factor affecting the quality of die castings. Most of the shapes are simple, and the castings with good die-casting process have low requirements on mold temperature control, and the mold temperature can be changed within a wide range to obtain qualified castings. For complex castings, qualified castings can only be produced by controlling the mold temperature within a narrow range. Mold temperature control is achieved by a heated cooling system of the mold. Its main purpose is to improve the internal quality and surface quality of the die-casting parts, stabilize the dimensional accuracy of the die-casting parts, improve the production efficiency of the die-casting parts, reduce the thermal alternating stress of the molds, and improve the service life of the molds. From the product structure discussed above, the thick cylinder area is the hot joint area, and the solidification will certainly lag behind other areas, so a reasonable cooling system needs to be set here. Figure 3 shows the water cooling structure at the cylinder. Threaded cooling is arranged in the peripheral wall of the cylinder tube, and the cooling wall thickness is controlled to be about 12 mm. Such a design can effectively reduce the temperature here and cause the molten metal in the area to solidify in advance.

Fig3.Cylinder insert water cooling
4.Finite element numerical simulation analysis
  In order to validate the theoretical data effectively, the author uses the professional numerical simulation software Magma to simulate the casting process and solidification process of the casting. The following is a screenshot of the simulated data.
Figure 4 shows the state of the castings reaching 30%, 50%, 70% and 90% respectively during the filling process.

Fig4.Mold filling state
  It can be obtained from the simulated data that the flow rate at the feed is as fast as 63 m/s, which is basically consistent with the theoretical design value. It can be seen from the punching state that the flow state is fast in the middle, slow in both sides, and no aerated area appears. The runner design facilitates the venting of the casting.
  Figure 5 is a schematic diagram showing the state of castings reaching 30%, 70%, 90% and 99% respectively during solidification.

Fig5.Mold flow solidification state
  It can be seen from the above view that the four cylinders and the feeding area are post-solidification areas, which belong to the hot section area and conform to the original design intent.
5.Actual casting and debugging
  
After the mold was completed, the first commissioning was carried out, no vacuum was applied, and the water cooling at the cylinder was in working condition. The state after the casting is cast is shown in Figure 6. It can be seen that the casting is complete and the appearance can be.

Fig6.T0 casting part
  However, after dissection, a wide distribution area and a large air shrinkage hole appear inside the cylinder tube, as shown in Figure 7.

Fig7.Internal defect of T0 casting part
  After analysis, the reason was that the gas was not cleaned, and the water cooling effect at the four cylinders did not reach the effect. The reason is that in the first commissioning phase, there is no vacuum pumping device, and there are more internal gas residues. Moreover, the cooling water pressure used is low, 4 bar.
  The author has improved the previous debugging results in a targeted manner, increasing the total exhaust area on the exhaust plate from 1.32 cm2 to 3.08 cm2, and using a vacuuming device. The cooling water pressure was controlled at 8 bar. Then, the second debugging was carried out, and the X-ray transmission was carried out as shown in Fig. 8 (8). It can be seen that the shrinkage cavities at the cylinder are greatly improved and meet the casting quality requirements.

Fig8.The X-ray transmission at the casting cylinder of T1
6.summary
  In summary, with the development of the automotive industry, the development and application of the cylinderless aluminum cylinder has extremely important significance, and its development and application will become more and more extensive. Therefore, research on materials, preparation, and research on product molding processes have become more meaningful. Before the high-volume production of product castings, due to the characteristics of the product structure, it is necessary to continuously improve the mold process and structure. From the experience of the first domestic cylinderless aluminum cylinder developed by the author, the following points need to be focused.
  1. It is first necessary to solve the problem of the air shrinkage hole at the cylinder. Vacuum die casting and an efficient cooling system contribute greatly to the internal quality of the casting cylinder.
  2. Due to the excessive tightening force at the cylinder, the extraction resistance of the cylinder insert is often caused, and the conventional insert is easily damaged, which will affect the future mass production. It is necessary to design a reasonable and effective cylinder insert structure and to surface-treat it to reduce the friction coefficient of the surface, so that the mold is removed to reduce the damage to the mold.
 


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