In order to solve problems of pores and shrinkage cavities in die-casting process of large complex thin-walled automobile doors made of high-pressure die-cast aluminum alloy, flow field and temperature field of high-pressure casting process were simulated and analyzed using SuperCAST Zhizhu Super Cloud CAE cloud computing platform, internal defect distribution of casting was predicted, process optimization and experimental verification were carried out. Results show that two-stage slow-speed injection process schemes A and B respectively have long filling time and uneven temperature distribution, while scheme C adopts a uniformly accelerated slow-speed injection process, its filling time and temperature uniformity are more reasonable. Solidification simulation analysis found that with increase of boost pressure, shrinkage content of scheme C was significantly reduced, and defects were completely eliminated at 90 MPa. Overall, scheme C performs better than scheme A because it does not have phenomenon of air entrainment in material tube, filling temperature is more uniform, and the overall defect control effect is better. Optimized die-casting production process is Scheme C, whose low-speed injection adopts uniform acceleration injection process, with the highest critical speed of 1.23 m/s, high-speed speed of 4.6 m/s, high-speed starting position of 900 mm, and boost pressure of 90 MPa. On-site die-casting test verifies feasibility of injection process.
Main development trends of lightweight automotive parts include structural weight reduction and material weight reduction. Aluminum alloys are widely used in various engineering fields, especially in the field of automotive parts, due to their advantages such as high specific strength, corrosion resistance and easy recycling. Due to its near-net shape and high production efficiency, die-casting has become main manufacturing process for aluminum alloy parts. With rapid development of new energy vehicle industry, aluminum alloy integrated die-casting can not only replace steel with aluminum to achieve material lightweighting, but also integrate multiple parts to maximum extent to achieve structural weight reduction. On the premise of ensuring strength of vehicle body, it significantly reduces weight of vehicle body to improve energy efficiency, gradually becomes one of important development directions of die-casting parts for new energy vehicles.
Since Tesla first applied integrated die-casting technology to development of Model Y rear floor assembly, aluminum alloy integrated die-casting technology has gradually been applied to large and medium-sized complex structural parts of new energy vehicles such as rear floor, front cabin and battery tray. However, structural characteristics of integrated products, such as complex shape and oversized size, lead to complex die-casting forming process and difficult quality control, and are prone to cold shut flow marks, stress deformation and other casting defects, which limit large-scale promotion and application of this technology. As a typical integrated die-casting part, car door not only has complex structure, thin main wall thickness, uneven thickness in local areas, but also requires required mechanical properties and appearance requirements to be achieved under as-cast condition. Forming conditions are harsh, high requirements are placed on its die-casting process design and production quality control. Air entrainment defects and coarse crystal structure are the most common product defects in die-casting process. Their formation and evolution are mainly closely related to die-casting filling and solidification cooling process. Therefore, studying filling and cooling process in die-casting and predicting casting defects are of great significance for optimizing process and improving casting quality.
This paper takes car door as analysis object, simulates die-casting process based on SuperCAST Zhizhu Super Cloud Die Casting CAE cloud computing platform to study effects of different injection speeds and boost pressures on filling temperature, solidification temperature and hot spots, analyzes conditions for generation of defects such as shrinkage, and optimizes die-casting process, so as to effectively reduce generation of die-casting defects, improve quality of castings, and reduce production costs, providing a reference for die-casting production of such castings.

In this study, car door is taken as research object, and die-casting process is simulated by multi-physics field coupling method. First, during filling process, liquid metal is injected into mold under high pressure. In order to accurately predict problem of air entrainment defects, lattice Boltzmann method (LBM) is used to describe flow characteristics of metal:
LBM is based on Lattice-Bhatnagar-Gross-Krook (LBGK) method. Basic LBM model is as follows:
 
Where: i is ith discrete velocity direction, x is spatial position coordinate, ci is ith discrete velocity component, t is current moment, and Δt is time step. fi(x+ciΔt, t+Δt) is density distribution function at x+ciΔt position and t+Δt moment, fi(x, t) is density distribution function at x position and t moment, and Ωi(x, t) is collision term.
After LBGK approximation, we get:
 
Equilibrium distribution function is defined as:
 
Where: ωi is density weight in ith discrete velocity direction, ρ is density of fluid, u is fluid velocity at current position moment, cs is lattice sound velocity, and τ is relaxation time.
Macroscopic field density is given by following formula:
 
Macroscopic field velocity is given by following formula:
 
Combined with VOF (Volume of Fluid) method to capture changes in liquid-gas interface:
 
Where: F is occupancy ratio of liquid (0 is no liquid, 1 is full liquid).
To accurately predict flow behavior of liquid metal. Then, after filling is completed, casting enters solidification stage, at which cooling rate and heat node distribution directly affect internal quality. To this end, energy equation is used to simulate temperature field changes during solidification process:
 
Where: ρCp is specific heat capacity, T is temperature, k is thermal conductivity, and Q is heat source term.
At the same time, Stefan equation is used to characterize movement of solid-liquid interface:
 
Where: L is latent heat, s is position of solid-liquid interface, kL and ks are thermal conductivity of liquid phase and solid phase, respectively.
And solid-liquid ratio of metal at different temperatures is calculated by solid-liquid phase fraction model:
 
Where: fs is solid phase fraction, T is current temperature, TL is liquidus temperature, and Ts is solidus temperature.
To further analyze solidification process in different regions and more accurately describe heat conduction behavior between casting and mold, this paper adopts a 4D interface heat transfer model by introducing dynamic heat transfer coefficient h (t, x, y, z):
 
Where f (t) is change of heat transfer coefficient over time, and g (x, y, z) describes spatial change of heat transfer coefficient.
Using 4D interface heat transfer model, dynamic change of heat transfer coefficient needs to be numerically discretized during simulation. Heat conduction equation between casting and mold is:
 
Where T mold is temperature of mold changing over time, and T casting is temperature of casting changing over time.
In each time step, heat transfer coefficient h (t, x, y, z) is dynamically updated, temperature distribution between casting and mold is solved. By dynamically adjusting heat transfer coefficient, heat exchange behavior between casting and mold during filling and solidification can be simulated.

This casting is a car door designed and developed for a certain automobile company. Three-dimensional structure is shown in Figure 1. Casting material is AlSi10MnMg aluminum alloy, and mold material is H13 steel. This paper calculates thermophysical parameters of two materials using Thermo-Calc software, as shown in Table 1. Casting has a contour size of 1 135 mm * 665 mm * 60 mm, with a complex geometry and uneven wall thickness. Main wall thickness is 2.5 mm, maximum wall thickness is 4 mm, and weight is 5.56 kg.

 

Figure 1 Schematic diagram of three-dimensional geometric model of car door

Table 1 Thermophysical parameters of materials involved in castings and molds

Car door is regarded as a special-shaped thin-walled shell part. According to its structural characteristics, inner gate is set in the middle of casting. In order to reduce gas entrainment and impact of core caused by high-speed filling of molten metal, to ensure that molten metal reaches end of casting as much as possible at the same time, inner gate adopts M-shaped ring pouring method. At the same time, in order to avoid generation of spraying at gate position, thickness of inner gate is set to be equal to wall thickness of product gate position. Design of pouring system and overflow system is shown in Figure 2.

 

Figure 2 Schematic diagram of pouring and overflow system design of car door

For die-casting mold, design of temperature control system is conducive to controlling heating and cooling of mold, so that internal heat can reach a dynamic balance state, improve service life of mold, and ensure quality of casting. In automobile door mold, water circuit, vacuum temperature control and empty oil cylinder oil collecting seat are set up respectively, which can make casting achieve more efficient and balanced temperature control. Distribution of temperature control system of die-casting mold is shown in Figure 3.

 

Figure 3 Temperature control system of automobile door die-casting mold

Grades and thermophysical properties of die-casting alloys and mold materials are shown in Table 1. Die casting process parameters are shown in Table 2, where heat transfer model is a 4D interface heat transfer model, model parameters and values are as follows: fitting parameters
γ¹h is 8.92, γ²h is 28.33, ε is 1.45, β¹h is -1.82, β²h is -5.32, material traction solid phase fraction is 0.7, heat transfer coefficient-solidification state fitting parameter is 1.05, heat transfer coefficient peak matching coefficient is 2.5, minimum heat transfer coefficient matching coefficient is 0.15, critical mold surface temperature is 275℃, critical solid phase fraction is 0.8, and heat transfer coefficient between molds is 3000W/(㎡·K). Minimum size of casting cavity grid is 0.65mm, the total number of grids is 190 million, and grid division diagram is shown in Figure 4, where yellow border area is casting cavity grid.

Table 2 Main die-casting process parameters

 

Figure 4 Grid division of automobile doors
Reasonable slow injection process and boost pressure can improve casting quality and optimize production efficiency. Therefore, it is of great significance to study influence of different injection processes on automobile doors. Low speed during injection process was set to 0.2 m/s (near two-stage speed), 0.5 m/s (near two-stage speed), 1.23 m/s (uniform acceleration), and high speed was 4.6 m/s. Specific injection process curve is shown in Figure 5. In addition, boost pressure was set to 40 MPa, 60 MPa, 80 MPa and 90 MPa.

 

Figure 5 Schematic diagram of three different injection process curves

SuperCAST Zhizhu Chaoyun was used to simulate flow state of molten metal in material tube under three different slow injection processes (A 0.2~4.6 m/s, B 0.5~4.6 m/s, C 1.23~4.6 m/s), and results are shown in Figure 6. In process of filling aluminum tube in Scheme A, movement of aluminum liquid is wavy, and aluminum liquid rolls, which is very easy to form an entrainment defect. At the same time, in Scheme A, aluminum liquid stays in material tube for a long time and metal liquid fluctuates irregularly during filling process, which easily leads to more heat loss of aluminum liquid and air entering metal liquid. In Scheme B, metal liquid fluctuates irregularly during filling process of aluminum tube, which easily causes air and oxide inclusions to be entrained in metal liquid. In process of filling material tube in Scheme C, interface front of aluminum liquid always maintains a forward tilt state, wave surface moves smoothly, and there is no irregular movement of metal liquid, which effectively avoids gas in material tube from being drawn into aluminum liquid. In summary, Schemes A and B have reflux or tumbling phenomena, which are prone to air entrainment, filling temperature of metal liquid is low and uneven. Scheme C has a stable movement in material tube, is not prone to air entrainment, and has a uniform metal liquid temperature.

 

Figure 6 Temperature distribution of melt during material tube filling process

Effects of three different injection processes (A 0.2~4.6 m/s, B0.5~4.6 m/s, C 1.23~4.6 m/s) on filling time were simulated respectively. As shown in Figure 7, filling time distribution can be divided into four parts: blue, dark blue, red and yellow according to filling order. Blue areas of Schemes A and B mainly include gate and area near gate of casting, dark blue area mainly includes thin walls on both sides of gate, red area mainly includes four corner areas of casting, yellow area is mostly located in overflow groove of casting and middle part of upper side of casting, and a small part is located in the middle part of lower side of casting. Blue and dark blue areas of Scheme C mainly include pressure chamber and gate, red area mainly includes four corner areas of casting, yellow area is mostly located in overflow groove of casting and middle part of upper side of casting. Overall, filling sequence of Schemes A~C follows rule from near to far, among which high-speed filling time is about 0.04s, low-speed filling time is 4.14s, 2.00s, and 2.94s respectively, the total time of the entire filling process is 4.18s, 2.04s, and 2.99s respectively. Local filling time gradient of aluminum liquid in Schemes A and B is quite different, which is easy to lead to poor filling quality. In the whole filling process of Scheme C, time gradient of aluminum liquid at far gate is slightly lower than that at near gate, but filling time gradient is small, and the overall distribution is reasonable.

 

Figure 7 Distribution diagram of filling time of automobile door

Effects of three different injection speeds (A 0.2~4.6 m/s, B0.5~4.6 m/s, C 1.23~4.6 m/s) on filling temperature were simulated respectively, and results are shown in Figure 8. In process of melt filling in Scheme A, gate filling temperature is uniform, but temperature from material pipe to inner gate drops rapidly, which has a certain impact on fluidity of metal. After filling, it is found that the overall temperature of melt is low, and a large local temperature gradient appears in upper half of casting. Filling temperature of Scheme B is not uniform, and it is obvious that gate temperature in lower right corner drops rapidly. After filling, it was found that overall temperature of melt tended to be consistent and most of temperature was around 640 ℃. Although melt could smoothly fill far end, it was easy to cause overheating or long cooling time. Scheme C has uniform filling temperature at the gate, small temperature gradient from material tube to inner gate, and good metal fluidity.

 

Figure 8 Distribution diagram of temperature field of automobile door filling
Overall, Scheme A has uniform filling temperature and good metal fluidity, but temperature drops quickly. Scheme B has uneven filling temperature. After filling, melt temperature tends to be consistent and most of temperature is around 640 ℃, which is easy to cause overheating or long cooling time. Scheme C has uniform filling temperature and good metal fluidity.

Solidification shrinkage of Scheme C under different boost pressures of 40 MPa, 60 MPa, 80 MPa and 90 MPa was compared and analyzed, and results are shown in Figure 9. When boost pressure is 40 MPa, shrinkage cavities are mainly distributed around gate, and there are more shrinkage cavities defects; when boost pressure is 60 MPa, shrinkage cavities are basically distributed only on upper and lower sides; when pressure is increased to 80 MPa, shrinkage cavities are distributed near gate and one on upper and lower sides; when pressure is increased to 90 MPa, there are no shrinkage cavities. It can be found from defect size Figures 9 (a) to 9 (d) that sizes of point defect A at 40 MPa, 60 MPa, 80 MPa and 90 MPa are 199.1 mm3, 154.06 mm3, 104.63 mm3 and 0 respectively, indicating that as boost pressure increases, point defect tends to decrease until it is compensated. Among them, three positions A, B and C on casting were selected, and size of shrinkage cavity defect changes with boost pressure as shown in Figure 10.

 

Figure 9 Distribution diagram of shrinkage cavities in scheme C casting

 

Figure 10 Size of shrinkage cavities at different positions of scheme C casting changes with boost pressure

Die casting test of automobile door was verified using LJ 6 800 T die casting machine. Die casting process adopts scheme C, that is, uniform acceleration injection process is adopted, accelerating from 0 to critical slow speed of 1.23 m/s, high speed speed is 4.6 m/s, high speed position is 900 mm, and boost pressure is 90MPa. After mechanical processing to remove process systems such as runner and slag bag, net weight of door casting is 5.56 kg. Surface contour of casting is clear, without defects such as cracks, flash and cold shut, and quality is excellent. X-ray flaw detection results of door casting are shown in Figure 11. There are no obvious pores and shrinkage defects inside casting, which verifies that optimization scheme can meet production requirements.

 

Figure 11 X-ray detection results of automobile door castings

(1) Simulation and comparative analysis of different slow injection process schemes show that: in scheme A, molten metal forms obvious gas entrainment in material tube, and filling time is long, resulting in a low temperature of molten metal at the end of filling, which is prone to defects such as cold shut flow marks; in scheme B, due to high punch speed, molten metal rolls in material tube, which easily leads to gas and oxide inclusions being drawn into melt; scheme C adopts a uniform acceleration injection process, its filling time and temperature distribution are more uniform and reasonable.
(2) Solidification simulation analysis of automobile door found that: with increase of boost pressure, shrinkage of scheme C is controlled, shrinkage volume is significantly reduced, and shrinkage defect is completely eliminated at 90 MPa.
(3) Experimental verification adopted process scheme C: uniform acceleration slow injection, accelerating from 0 to critical slow speed of 1.23 m/s, high speed of 4.6 m/s, high speed position of 900 mm, and boost pressure of 90 MPa. Results showed that test casting had a clear outline, high dimensional accuracy, and no obvious shrinkage defects in X-ray detection area.