Optimization of the die casting process of Al for vehicle shock tower

Optimization of the die casting process of Al for vehicle shock tower

ABSTRACT: This paper analyses the structural characteristics of automotive die-casting shock tower, designs its die-casting process, and then uses numerical simulation method to analyze the flow of liquid aluminium and filling process. The results show that the filling of liquid aluminium in the cavity of shock tower is relatively stable. The liquid aluminium with oxide inclusions at the front end and large volume of entrainment gas enters the overflow tank, which avoids the defects caused by entrainment gas inside the castings. In addition, through local quenching and other methods, the problem of shrinkage defects easily occurring in large parts of local thickness was solved, and good quality aluminum alloy shock absorber die castings were eventually obtained.
  At present, aluminum alloy die castings have been widely used in automotive, aviation, aerospace and electronic industries. Replacing steel with aluminium can achieve 40% - 50% weight loss, which is conducive to reducing energy consumption, and is an important way to achieve energy saving and emission reduction. Many automotive structural parts are thin-walled shell parts, so more and more automotive structural parts are made of aluminium alloy, but its complex structure and high stress requirements pose challenges to die casting process. The results show that when the wall thickness of the casting is less than 4 mm, the surface tension of the liquid metal will lead to the decrease of its fluidity, which will lead to the difficulty of filling thin-walled parts in the die cavity. Die casting can fill the cavity with liquid metal under pressure, which can not  only effectively solve the filling problem, but also make the liquid metal solidify quickly and refine.
  Alloy structure, get higher strength castings. Due to the complex shape of shell parts and the uneven local wall thickness, the flow process of metal liquid in the die cavity is also complex. With the development of computer simulation technology, numerical simulation software can more and more accurately reflect the flow process of liquid metal in die casting die, and can accurately predict the location of casting defects. Therefore, it is an efficient and cost-saving method to use numerical simulation software to simulate the filling and solidification process, and then design and optimize the die casting process based on the simulation results to analyze the quality of parts. The shock tower belongs to large and complex aluminium alloy die casting. Flow-3D software was used to simulate and design the die casting process of the castings, which verified the rationality and feasibility of the production process. According to the simulation results, the process plan was improved to obtain high quality die castings, which improved the production efficiency of the parts and reduced the production cost.
1. Structural Analysis of Damping Tower
  Fig. 1 is a three-dimensional solid modeling sketch of a shock tower. The maximum contour size of the castings is 530 mm x 345 mm x 313 mm, and the average wall thickness of the main body is 3 mm. The structure of the castings is complex, the whole shell is curved, and the surface of the castings is designed with crisscross reinforcement bars to improve the overall strength of the parts. There are many near-cylindrical convex platforms locally, with the maximum height of 20 mm, which makes the wall thickness of each part of the castings vary greatly. On one side of the castings, there is a large bulge structure with a height difference of 195 mm from the shell part of the castings. The shock absorber is formed by die casting of A380 aluminium alloy with a net weight of 2.9Kg.
Die Casting Shock tower 3D solid modeling
Fig.1.Die Casting Shock tower 3D solid modeling
2. Design of gating system, exhaust chute and overflow chute
2.1. Design of gating system
  According to the characteristics of castings, the maximum profile of castings is selected as parting surface to facilitate the demoulding of castings. In order to reduce the entrainment in the initial stage of die casting process, the inner gate is set on the flatter side in the direction of casting length. The section area of the inner gate is calculated according to the following formula:
Formula of calculating section area for Inner Gate         (1)
  V is the total volume of parts, overflow and exhaust system, 115742mm3; vgis the velocity of liquid aluminium at the entrance of inner gate. According to the design manual, the filling speed of liquid aluminium at the entrance of inner gate is 20-60m/s and 40m/s; t is the time of filling cavity with liquid aluminium, and its recommended value is determined by the average wall thickness. The average wall thickness is calculated according to the following formula:
Calculating average wall thickness Formula       (2)
  In the formula, b1,b2,b3…bn is the wall thickness of a part of the casting, in mm; s1,s2,s3…sn  The area of wall thickness b1,b2,b3…bn,in mm2. The average wall thickness of the absorber is 3 mm, the recommended filling time of the cavity is 0.05-0.1 s and 0.07 s. According to the design manual, the thickness T of the inner gate is 1.5mm and the total width W=Ag/T=261.25mm. Horizontal cold chamber die-casting machine is used. The cross-section area of the runner is Ar=(3-4)Ag, 1371.54 mm2, the thickness of the runner D=(8-10)T and 15 mm, and the flat trapezoid is used for the runner. According to the die-casting machine chamber size, the diameter of straight runner (chamber diameter) is 120 mm. According to the parameters of straight runner, transverse runner and inner runner, the gating system of the die-casting shock tower is designed, as shown in Fig. 2.
Casting Shock Tower with a casting system
Fig.2.Casting Shock Tower with a casting system
2.2. Design of overflow groove  and exhaust groove 
  According to the actual die casting process parameters, the simulation parameters are set. The liquid aluminium enters the horizontal gate and the inner gate at the slow injection rate of 0.6m/s. When the liquid aluminium fills all the inner gate, the injection speed is increased to 5m/s, that is to say, the liquid aluminium fills the cavity quickly.
Figure 3 shows the simulation results of temperature field and entrainment of castings with gating system. From Figure 3, it can be seen that the designed gating system can fill the cavity more smoothly with liquid aluminium. There are two circular structures on the left side of the part. According to the simulation of filling process, it can be seen that the liquid aluminum is easy to produce eddy current when filling here, which causes the volume of entrainment to increase. Therefore, overflow grooves should be designed on both sides of the circular structure. According to the temperature field and entrainment characteristics, it can be seen that there is a large area of low temperature liquid aluminium on the right side of the part, and there is entrainment phenomenon in varying degrees from the edge to the interior. Combining with Fig. 1, it can be seen that the structure of the circle out part is more complex. The liquid aluminium enters the cavity through the right inner gate and then directly impacts the cavity wall. After being blocked, the liquid aluminium reflux fills the right part of the cavity, thus causing air entrainment. The parts are filled from bottom to top in turn. There are a large number of low temperature and serious entrainment liquid aluminium in the final filling part. Sufficient overflow grooves should be set up here to receive the liquid aluminium.
Simulated results of temperature field and entrainment in gating system
Fig.3.Simulated results of temperature field and entrainment in gating system(double-click zoom
  According to the simulation results, in some places where the temperature is low and the volume of air entrainment is large, an overflow tank with sufficient volume should be designed, but too large overflow tank can easily lead to liquid metal backflow. Therefore, a number of separate overflow slots and thin connecting ribs should be set up in these parts to ensure their strength. The overflow groove mainly adopts trapezoidal structure which is easy to process. The volume of overflow groove should be increased appropriately in the parts where local entrainment is serious and the shape of overflow groove should be modified slightly according to the flow characteristics (see Figure 3c). According to the design manual, the cross-sectional area of the exhaust groove is set to 30% of the cross-sectional area of the inner gate. The designed overflow groove  and exhaust groove  are shown in Fig. 4.
Structural Design of Overflow Groove and Exhaust Groove of Shock Tower
Fig.4.Structural Design of Overflow Groove and Exhaust Groove of Shock Tower
3. Simulation analysis and process optimization
  Fig. 5 is the filling process of metal liquid in the die. It can be seen that during the filling process of liquid aluminium, part of the liquid aluminium which is located at the front of the liquid-gas interface with low temperature and serious entrainment gas enters the designed overflow tank. After filling the cavity with liquid aluminium (see Fig. 5D and Fig. 5h), there is very little gas left inside the parts. Therefore, the designed overflow groove and exhaust groove are suitable for the die casting production of the shock absorber parts.
  Fig. 6 is a casting after complete solidification of molten aluminium. It can be seen that there is a large hole defect in the upper part of the bulge structure of the shock tower. Observing its local magnification map, it can be found that there are two large near-cylindrical bulges at this place, the height of which is up to 20 mm. During the solidification process, the solidification speed of this thick and large part is slower, and shrinkage phenomenon will occur, forming holes.
Therefore, local quenching is adopted to accelerate the solidification speed of this part in order to obtain compact castings. The quenching copper block is added to the die to achieve the purpose of quenching. The simulation results are shown in Fig. 7. Finally, the qualified aluminium alloy shock tower was produced by this process, and the finished product rate reached more than 90%.
Simulated results of temperature field and entrainment with pouring system, overflow groove and exhaust groove
Fig.5.Simulated results of temperature field and entrainment with pouring system, overflow groove and exhaust groove(double-click zoom)
Simulation of die casting Shock tower solidification process
Fig.6.Simulation of die casting Shock tower solidification process
Casting Simulation results after local quenching
Fig.7.Casting Simulation results after local quenching
4. Conclusion
(1) Design and optimize the pouring, overflow and exhaust system of aluminium alloy shock tower.
(2) The location of entrainment gas in shock tower was analyzed by numerical simulation, and the types and locations of die casting defects were predicted. On this basis, the design of gating system was optimized. The air entrainment and shrinkage defects are easy to occur in circular structures with large wall thickness.


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