Improvement of die casting technology for Al alloy vehicle cylinder head cover

Improvement of die casting technology for Al alloy vehicle cylinder head cover

  With the increasing popularity of cars, the demand for automotive parts is growing, competition is becoming more and more fierce, and the price of spare parts is also greatly reduced. How to improve production management and process technology in order to improve product qualification rate and reduce production costs, so as to win the praise of customers and win the market, is a difficult problem facing and urgently needed to be solved by foundry workers. It is the duty of technicians engaged in automobile parts production to improve the quality of automobile parts products through advanced casting technology. In this paper, the improvement of die casting process for aluminium alloy cylinder head cover of car four-cylinder engine is studied.
1. Overview of Automobile Cylinder Head Cover Parts
  The part studied in this paper is the aluminium alloy cylinder head cover of four-cylinder engine of Chang'an passenger car. It has large size, complex shape, uneven wall thickness and high requirement. It is not allowed to have air holes, shrinkage holes and shrinkage looseness inside the part. (There is no leakage under 0.2 MPa pressure by air tightness test). There are no defects such as shot blasting on the surface. Because the spark plug hole is deeper and the oil hole is thicker, it is easy to cause pinholes in the spark plug hole during die casting, and shrinkage holes appear after tapping the oil ruler hole. The qualified rate of initial die castings is only 20%. The product and defect situation is shown in Fig. 1.
Die Casting Defects of Cylinder Head Cover
Fig.1.Die Casting Defects of Cylinder Head Cover
2. Analysis of Die Casting Defects of Cylinder Head Cover and Improvement of Process
   The original inner gate setting of the cylinder head cover die casting is shown in Fig. 2. After analysis, gas porosity. and shrinkage holes in the spark plug hole of the die casting are caused as follows:
Cylinder Head Cover Gate Distribution of Die Casting
Fig.2.Cylinder Head Cover Gate Distribution of Die Casting
  (1) The design of casting system is unreasonable. Because the spark plug hole of cylinder liner is located in the deep cavity, if the inner gate is arranged and distributed according to the original technology, the inner gate 1 and 2 have no right spark plug hole convex platform. When the liquid metal enters the cavity from the inner gate 1 and 2 during the filling process, it is easy to cause the cavity near the spark plug to fill prematurely. Full, and the deep cavity of spark plug protrusion has not been filled, which makes it difficult to eliminate the deep cavity gas, and the supercooled metal liquid entering the cavity in the early stage can not eliminate the deep cavity, so that there are slag holes in the spark plug hole, which appear after machining, leading to the product unqualified.
 (2) Excessive machining allowance of spark plug hole, excessive thickness of spark plug hole wall and no feeding during cooling. The center line of the spark plug hole is not perpendicular to the parting surface, that is, the center line of the hole and the direction of the opening die are at an angle of 10 degrees. The original design scheme of the die is shown in Fig. 3. Because of the joint of the two cylinder cores in the fixed and dynamic die, there is a dislocation at the joint of the two cores, which leads to the processing allowance of about 4 mm on both sides of the joint. It is difficult to effectively compensate the thick and large hole wall of the spark plug hole during die casting. After the dense layer on the surface of the die casting parts is machined, the internal shrinkage appears.
Misalignment of cylindrical butt of original die core
Fig.3.Misalignment of cylindrical butt of original die core
  (3) The oil hole is at a certain angle with the parting surface of the die. The inner hole of the oil hole is composed of two model cores of static and dynamic models, which makes it impossible to add a slag ladle above the oil hole for discharging cold metals and gases. Moreover, the oil hole wall thickness is very thick, located in the deep cavity and far from the inner gate. Even if a shorter boosting time and a larger boosting pressure are used in the die casting process, it can not achieve good feeding effect, which makes the oil hole content prone to shrinkage and porosity defects.
Based on the above analysis, the gating system and die core are optimized and improved as follows:
  1) Increase the width of the inner gate and adjust the position of the inner gate so that the inner gate is directly opposite to the spark plug hole convex platform, so that the liquid metal fills the deep cavity of the spark plug first after entering the cavity, and the whole can be filled sequentially, and the deep cavity gas and cold metal liquid can be eliminated smoothly, as shown in Fig.4.
Widening and adjusting position of inner  gate
Fig.4.Widening and adjusting position of inner  gate
  2) Four spark plug hole cores are improved, from the original dynamic and static die to the dynamic and static die, so as to eliminate the dislocation, reduce the thickness of the spark plug hole and reduce the processing allowance. As shown in Fig.5.
Semi-circle butting of die core
Fig.5.Semi-circle butting of die core
  3) According to the product situation and the 3D simulation analysis of the die casting die, a horizontal runner is added on the side of the oil hole end, and two inner runners are added at the original slag ladle position to increase the flow rate of filling liquid and the effect of solidification and shrinkage for most of the oil hole thickness, so as to reduce the gas porosity. and shrinkage tendency, as shown in Fig. 6.
Increase cross runner and inner runner at oil hole end
Fig.6.Increase cross runner and inner runner at oil hole end
  After optimization and improvement of process and die, the die casting aliasing has been verified and good results have been achieved. The scrap rate of shrinkage and gas porosity. and oil holes in spark plug holes has been reduced from 80% to 4%, which meets the design requirements and is now in mass production.
3. Summary
  A. The pouring system plays an important role in the quality of die castings. The distribution of the inner runners is closely related to the exhaust of liquid metal during filling and the release of cold metal. How to arrange the gating system reasonably is related to the smooth progress of the die casting process and get qualified die castings.
  B. The gating system improvement of cylinder head cover described in this paper can be used for reference and popularization of thin-walled die castings with similar deep cavity and difficult to compensate.


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