In order to reduce carbon emissions, lightweight and electrification technologies are widely used in the field of new energy vehicles, at the same time they have considerable help in control and safety of automobile driving. Due to high specific strength and cost performance of aluminum alloy, it can be used in structural parts such as body, engine, and wheel, its use in automobiles is increasing year by year. Compared with steel longitudinal beams manufactured by stamping and welding process, aluminum alloy die-casting structure has advantages of lightweight and integrated integration. The thinnest thickness can reach 0.5mm, with strong mechanical properties and high production efficiency, which meets characteristics of large volume, complex structure and high strength of body structural parts.
Compared with traditional casting structures, integrated die-casting parts are larger and more complex, and are prone to problems such as metal liquid flow turbulence, exhaust difficulty, cooling retraction, and difficulty in coordinating strength. Product yield is difficult to guarantee and needs to be supported by process design. In order to ensure its quality, it is necessary to strictly control key technologies such as position and quantity of die-casting pouring, overflow system and thermal balance of mold to improve productivity of castings.
This paper mainly conducts theoretical and experimental research on the key technologies such as pouring and overflow system design and mechanical property testing of longitudinal beam aluminum alloy die-casting structural parts, studies key forming technology of longitudinal beam aluminum alloy die-casting structural parts of new energy vehicles, studies influence of its process and structure on forming quality of die-casting parts. It can promote improvement of efficiency of integrated die-casting production process, provide a reference for optimization design of body structural parts of new energy vehicles, play a positive role in reducing carbon emissions in automotive industry and achieving carbon peak and carbon neutrality goals.

Longitudinal beam of new energy vehicles plays a role in supporting body and is main safety structural part of chassis. Design and forming process of structural parts need to meet functional requirements and die-casting process, quality of castings is regulated by structural optimization and process optimization. Traditional steel longitudinal beams use stamping manufacturing processes to manufacture components and splice them into a whole. Size of longitudinal beam can reach more than 1.6m, requirements for material strength and elongation are also higher. In order to ensure assembly of parts on car body, die-casting machine required to manufacture longitudinal beam aluminum alloy die-casting parts has a large clamping force, and deformation control is very important during manufacturing process. In addition, longitudinal beam can collapse and absorb energy when a new energy vehicle is severely collided, which can improve safety of car body. Therefore, it is necessary to pay attention to structural design and optimization of energy-absorbing part.
As shown in Figure 1, integrated thin-walled die-casting longitudinal beam has a large volume and a complex structure, and die-casting process has high requirements. In die-casting, complex structure of longitudinal beam makes flow of aluminum alloy molten metal disordered, and it is difficult for molten metal to completely fill cavity during filling stage, resulting in large internal defects in casting. After filling, cooling and solidification stage is prone to deformation and shrinkage. Design and manufacturing process of aluminum alloy longitudinal beams face many difficulties.

 

When designing structure of longitudinal beam die-casting parts, many factors such as wall thickness, fillet radius, demolding slope and reinforcement ribs are usually considered. Reasonable wall thickness can often avoid defects in castings and reduce problem of local mechanical properties of castings. Appropriate fillet radius design can ensure smooth filling of molten metal and facilitate gas discharge. Reasonable control of demoulding slope can facilitate smooth removal of casting after forming and reduce difficulty of taking out parts. Rib design can improve strength and rigidity of casting, guide molten metal to flow in set direction during filling stage, reduce surface and internal defects caused by poor molten metal flow behavior.

A horizontal cold chamber die-casting machine is used in integrated die-casting process. Its sprue is composed of die-casting machine pressure chamber and gate sleeve on die-casting mold. Pouring system mainly includes sprue, cross runner and inner gate, which delivers molten metal to cavity during die-casting filling. In order to ensure normal progress of integrated die-casting process, it is necessary to pay attention to structural design of die-casting pouring system and overflow system.
This article takes longitudinal beam with a size of 1340mm×524mm×290mm as an example. Its length dimension is quite different from width and height dimension. Its average wall thickness is 3mm. In order to meet quality requirements of large-size thin-walled structures, it is difficult to fill die-casting mold. Different gating system location selection determines flow distance of molten metal. Slight differences in dimensions in different directions lead to large differences in filling behavior. Reasonable design of gating and overflow system directly affects quality of die casting.

Gate is channel for molten metal to enter die casting model cavity. Its function is to guide molten metal into cavity in the best flow state according to structure, shape and size of casting to form a high-quality die casting. Due to large difference between length and width of longitudinal beam aluminum alloy parts, if gate position is set at its head or tail end, flow distance of molten metal during filling process is long, and it is easy to have problems such as premature solidification of molten metal. Therefore, actual flow distance of molten metal should be minimized, and cross-sectional area of gate should be ensured to be sufficient.
In order to ensure filling effect of longitudinal beam aluminum alloy parts, make molten metal fill smoothly in cavity, and minimize flow distance of molten metal in cavity, selected gate position is located at the side end of longitudinal beam structure. After calculation, cavity filling time is about 0.05s, gate speed is 45-55m/s, when casting weight is 20kg, aluminum liquid density ρ=2.75g/cm3, it can be calculated that gate cross-sectional area is about 3200mm2, and specific layout of gate is shown in Figure 2.

 

Runner is structure between tail end of straight runner and gate. Its function is to receive molten metal from straight runner and guide it to gate. Runner has a large volume, can store more molten metal and preheat mold. When casting shrinks during cooling, molten metal in runner can transfer pressure and supplement shrinkage.
Due to structural characteristics of longer longitudinal beam, two main runners are designed to ensure molten metal flow of gates in each part. In order to ensure stable filling of molten metal in cavity, two main runners are set with different cross-sectional areas. As shown in Figure 2, main runner is connected to multiple inner gates by transition runner to achieve overall pouring, ensuring that molten metal is evenly filled in cavity. Sprue part is connected to two main runners. After calculation, injection pressure ratio is 42MPa, pressure chamber filling degree is 67%, inner diameter D of pressure chamber and gate sleeve is determined to be 180mm, which is connected to two sprues.

Overflow system is mainly composed of overflow grooves and exhaust grooves, which are used to discharge gas inside cavity and molten metal mixed with impurities. Overflow groove is generally located around casting. In addition to cooperating with exhaust groove to discharge gas in cavity, it also discharges condensed molten metal and contaminated molten metal from front end of cavity.
Exhaust groove is generally located at the rear end of overflow groove, and its function is to discharge air in pressure chamber, runner and cavity and gas from lubricant out of mold.
Overflow grooves and exhaust grooves are set at the end of longitudinal beam aluminum alloy parts. Structure of mounting seat in the middle section is complex, grooves are gathered and ribs are designed more, which is prone to air entrainment defects. A set of overflow grooves and exhaust grooves are designed. Cross-sectional area of exhaust port is generally 20%-50% of cross-sectional area of inner gate, and cross-sectional area of exhaust port can be taken to be about 780mm2.

After casting system and overflow system of longitudinal beam aluminum alloy parts are designed, rationality of above structural design can be verified by mold flow analysis, and structure can be optimized. Inner gate speed is 50m/s, and the other parameter settings are same as above. After die-casting simulation analysis model of longitudinal beam aluminum alloy parts is set and process parameters are determined, die-casting filling simulation of casting will be carried out. With filling rate of 100% as calculation termination condition, filling process is shown in Figure 3.

 

Molten metal starts to fill from inner gate through edge of longitudinal beam. The first filling position is middle section of longitudinal beam shock absorption buffer area. After filling this position, molten metal begins to spread to both ends. Middle part of area is not filled completely at 0.013s, which may form air rolls. After middle area of longitudinal beam is completely filled, right end area is filled first, then the entire longitudinal beam body is filled. After main body is filled, overflow system at both ends is then filled.

Structural design of die casting itself will also have a great impact on the results of integrated die casting. In longitudinal beam aluminum alloy part, corresponding defect model can be obtained under determined pouring overflow system design. It can be seen that location of defect is concentrated at one end of longitudinal beam.
In structure of longitudinal beam, this position is location where automobile longitudinal beam is connected to suspension. In order to ensure strength of this location, dense reinforcement ribs are also designed, and structure is more complex than other locations. As shown in Figure 4, cross-sectional area of molten metal flow at this position is small. During die-casting process, gas in cavity at this position is difficult to discharge and filling is difficult. At the same time, some gas will combine with molten metal at this position. In addition, average wall thickness at this position is larger than that at other positions of longitudinal beam, and maximum thickness reaches 7mm. When molten metal enters, it is easy to cause problems such as air entrainment, forming die-casting defects.

 

Through die-casting simulation of aforementioned longitudinal beam aluminum alloy casting, it can be determined that in main structure of longitudinal beam aluminum alloy part, structure at the end of longitudinal beam is prone to air entrainment and surface defects. Structural design at this location affects filling behavior in die-casting. Structural optimization of this location helps gas in cavity to be discharged smoothly at this location and improve die-casting filling effect.

Use Leeb hardness tester to conduct targeted hardness tests on the areas where defects may exist in longitudinal beam aluminum alloy parts, and compare them with other areas. Figure 5 shows specific hardness test area of longitudinal beam aluminum alloy parts.

 

Before hardness test, aluminum alloy parts need to be fixed first to ensure accuracy of hardness test. Thickness of some areas of longitudinal beam aluminum alloy part is less than 3mm. To ensure test effect, impact device of hardness tester needs to be replaced with a C-type impact device when testing these areas. Hardness test of each area is shown in Table 1. From results, it can be seen that position with the lowest hardness is at the right end of longitudinal beam aluminum alloy part, and corresponding mechanical properties are poor, which is consistent with die-casting filling simulation results. Positions where hardness of longitudinal beam aluminum alloy part is significantly lower are also at the left end and middle platform.
Minimum hardness area of longitudinal beam aluminum alloy part is at the right end. Compared with die-casting results, air entrainment defects and surface defects at this position are also relatively concentrated. Air entrainment defects and surface defects on the left side of longitudinal beam are also concentrated. Hardness at this location is 79HB, which is significantly lower than other positions.

This paper focuses on pouring and overflow system of aluminum alloy die-casting of longitudinal beam of new energy vehicles, studies influence of its structural design on casting quality, and combines hardness testing to evaluate performance of aluminum alloy die-castings.
Combined with characteristics of aluminum alloy die castings of longitudinal beams, structural design methods of gates, runners and sprues in gating system in die casting process were studied, flow behavior of molten metal in die casting filling process was predicted, final filling position of molten metal was determined, position and structure of overflow trough and exhaust trough in overflow system were designed.
Simulation of pouring overflow system was carried out, influence of structural design of pouring overflow system on quality of die castings was studied. Combined with hardness test results, local structure of longitudinal beam die casting was optimized.