Based on die-casting CAE cloud computing platform Zhizhu Chaoyun, die-casting process of front fork casting of electric scooters was designed and optimized, and influence of different injection processes on quality of die-castings was analyzed. At the same time, injection process test verification of actual casting production was carried out. Results show that simulation results are basically consistent with actual production results. Optimized injection process design is used to obtain front fork die-casting with clear outline, smooth surface, high dimensional accuracy and no defects, and T6 treatment of die-casting is realized, thereby further improving its mechanical properties. After actual measurement on machine, casting meets technical requirements of front fork parts of electric scooters, and can achieve "casting instead of forging".
Die casting production has advantages of high productivity, high dimensional accuracy, low cost and near net shape, and has been widely used in the fields of transportation, electronic communications, instrumentation, computers and electrical appliances. Traditional die casting technology has characteristics of high pressure and high speed, most of them fill cavity in the form of jet and turbulence, resulting in defects such as air entrainment, oxidation slag inclusion and shrinkage inside ordinary die castings. Not only does it reduce mechanical properties and air tightness of die castings, but it also cannot perform more excess machining, welding and heat treatment, thus limiting application range of die castings.
Electric scooters are simpler in structure, smaller in wheels, lighter and simpler than traditional electric bicycles, and can save a lot of social resources. It is a new type of green and environmentally friendly product used by modern people as a means of transportation and leisure and entertainment. Front fork component is in the front part of electric vehicle structure. Its upper end is connected to handlebar component, frame component cooperates with front tube, and lower end cooperates with front axle component to form guide system of electric vehicle. Due to particularity of position, quality requirements for front fork parts are relatively high, especially its strength. At present, main methods used are metal mold gravity casting or forging. Density and strength of gravity-cast electric vehicle front fork parts are not high, and production efficiency is low. Forging has good performance, but machining volume is large and cost is high. Die-casting production of structural parts mostly adopts semi-solid or vacuum die-casting, which will add steps to die-casting production process, make process more complicated, and increase production cost. To this end, this paper uses a low-gas content die-casting process to develop a front fork die-casting for a high-end electric scooter manufacturer. Die-casting process is simulated based on Zhizhu Chaoyun die-casting CAE cloud computing platform. Through simulation analysis, defects such as air entrainment, shrinkage, and shrinkage can be predicted, and process can be optimized, so as to effectively avoid generation of die-casting defects, shorten development cycle, improve quality of castings, and reduce costs, providing a reference for die-casting production of such castings.

Casting is a three-dimensional entity of front fork of an electric scooter produced by a company, as shown in Figure 1. Maximum overall dimensions are 197 mm * 103 mm * 76 mm, and blank weighs 520 g. Its structure is moderate, but wall thickness is uneven, with thin wall being about 4.5 mm, thick wall exceeding 12 mm, and average wall thickness being about 7 mm. Die casting is required to have no pores, be able to undergo T6 treatment, and pass roller test fatigue impact for 200,000 times, requiring no cracks, deformation, or fracture. Alloy material is A380, and chemical composition is shown in Table 1.

 

Figure 1 Three-dimensional diagram of front fork component

 

Table 1 Chemical composition of A380 aluminum alloy wB/%

According to product characteristics, ingates are set on one side of cylindrical hole. In order to reduce turbulence and impact of core caused by high-speed filling of molten metal, and to ensure that molten metal flows through the entire thickness of ingates as evenly as possible, ingates adopt a circular 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. Maximum projected area of product is 79 c㎡, and projected area of pouring system is about 81 c㎡. Product is a structural part with strength requirements, so a higher compaction pressure needs to be selected, so a 250 t die-casting machine is selected.

 

Figure 2 Front fork die-casting process diagram

Die-casting process of front fork is simulated and analyzed using die-casting simulation cloud computing platform (Zhizhu Chaoyun). Pouring temperature is 660 ℃, mold working temperature is set to about 200 ℃, maximum low-speed injection speed is 0.57 m/s, low-speed adopts a uniform acceleration injection process, and high-speed injection speed is 4 m/s. Effects of three different starting and high-speed positions on filling process are simulated respectively. High-speed position is shown in Figure 3. High-speed position 1 is set at ingrate, high-speed position 2 is moved forward to the end of cylindrical hole of front fork casting, and high-speed position 3 is further moved forward from high-speed position to bifurcation of casting. Simulation results of filling process of high-speed position 1 are shown in Figure 4. It can be seen from figure that due to uniform acceleration injection technology used in low-speed injection stage, aluminum liquid moves smoothly in pressure chamber without rolling or reflux, thereby avoiding pressure chamber gas from being drawn into aluminum liquid and finally brought into casting, as shown in Figures 4a, b, c and d. After high speed at ingrate position, aluminum liquid flows smoothly when entering cavity, and no spraying occurs due to use of a gate with equal wall thickness, as shown in Figure 4e. However, when aluminum liquid reaches the end of cylindrical hole of front fork casting, due to increase in cavity space, a certain spraying phenomenon occurs, and alloy liquid generates turbulence here, as shown in Figure 4f. After aluminum liquid completes filling of this part, subsequent filling process is relatively smooth, as shown in Figures 4g and h. It can also be seen from distribution of entrained air pressure during filling process at high-speed position 1 shown in Figure 5a that entrained air pressure is relatively large near the end of cylindrical hole of front fork casting, which is prone to defects such as pores and slag inclusions.

 

Figure 3 Schematic diagram of high-speed position

 

Figure 4 Simulation results of filling process at high-speed position 1
In order to eliminate entrained air at the end of cylindrical hole of front fork casting, it is considered to move high-speed position forward, as shown in Figures 3b and c. Simulation results of entrained air pressure distribution during filling process after high-speed position is moved forward are shown in Figure 5.

 

Figure 5 Distribution of entrained air pressure during filling process
It can be seen from figure that when high-speed position is moved forward to position 2, entrained air pressure at the end of cylindrical hole of front fork casting is improved, but it has not been completely eliminated, as shown in Figure 5b; when high-speed position is moved forward to position 3, entrained air pressure at the end of cylindrical hole of front fork casting is basically eliminated, as shown in Figure 5b. From temperature distribution diagram of high-speed position 3 filling process shown in Figure 6, it can be seen that after alloy liquid enters cavity, flow is smooth. Since high-speed position is moved forward to part cavity, a large jet flow is avoided at the end of cylindrical hole of casting, thereby eliminating air entrainment phenomenon at this location.

 

Figure 6 Simulation results of high-speed position 3 filling process
In addition, high-speed position forward will prolong filling time and increase risk of defects such as cold shut and insufficient pouring. However, it can be seen from its temperature distribution diagram that after cavity is finally filled, alloy liquid still has a relatively high temperature, which is basically 620 ℃, which is higher than liquidus temperature of A380 aluminum alloy (599 ℃). Low-temperature alloy liquid basically remains in barrel and does not enter cavity.

In order to verify feasibility of simulation optimization scheme, die-casting mold was developed using die-casting process for actual production. Specific injection process is: temperature of aluminum melt out of furnace is 660 ℃, low-speed injection adopts uniform acceleration injection process, its maximum critical speed is 0.57 m/s, high-speed speed is 4 m/s, and mold temperature is controlled at about 200 ℃. Production practice verification was carried out using high-speed positions 2 and 3 respectively, and front fork die-casting with clear outline, smooth surface and high dimensional accuracy was obtained. Physical picture of front fork die-casting is shown in Figure 7. Through X-ray transmission flaw detection of the entire casting, it was found that front fork die-casting produced by high-speed position 2 had more holes near the end of cylindrical hole, as shown in Figure 8a. Density of casting obtained by Archimedes method is 2.718 g/cm³ and porosity is 8.09‰. However, no holes were found inside casting produced by high-speed position 3, as shown in Figure 8b. After casting was cut from middle, no defects such as shrinkage, pores and oxidation inclusions were found. No holes were found on cross-section, and structure was dense, as shown in Figure 7b. Density of casting reached 2.739 g/cm³, and porosity was only 0.39‰. It can be seen that porosity of die casting is much less than 2% die casting standard, which also shows that gas content of die casting is very low.

 

Figure 7 Front fork die casting

 

Figure 8 Front fork die casting X-ray flaw detection image
In addition, casting produced by high-speed position 3 was subjected to 525 ℃ + 4 h solution treatment and 170 ℃ + 2 h aging treatment, and no bubbling was found on its surface. Mechanical properties of fork casting were tested by sampling body of casting. Tensile strength of cast state was 260 MPa, yield strength was 169 MPa, and elongation was 2.2%. After T6 treatment, strength and toughness were greatly improved, that is, tensile strength was increased to 342 MPa, yield strength was 222 MPa, and elongation was 3.6%. Organization of casting is shown in Figure 9. It can be seen from figure that α-Al phase in cast structure is granular or nearly spherical. After T6 treatment, silicon phase is granular or short rod-shaped. At the same time, Al2Cu and Mg2Si and other particle reinforcement phases are precipitated, and distribution in organization is also relatively uniform, which further enhances performance of casting. Finally, front fork die casting was actually loaded and tested for 200,000 fatigue impact strength tests without fracture or deformation, which meets technical requirements for front fork parts of electric scooter products. This part was produced from 2021 to 2023, and nearly 40,000 sets have been produced. Last batch of castings delivered have not broken after more than 1.15 million load fatigue tests (Figure 10), far exceeding technical requirements of front fork parts.

 

Figure 9 Microstructure of die casting

 

Figure 10 Load fatigue test report of front fork assembly

(1) Based on Zhizhu Chaoyun-die casting CAE cloud computing platform, injection process of different injection process schemes was simulated and analyzed. Results show that when high-speed position 3 is used for injection, alloy liquid flows smoothly after entering cavity. Since high-speed position is moved forward to part cavity, a large jet flow is avoided at the end of cylindrical hole of casting, thereby eliminating air entrainment phenomenon at this location.
(2) Die-casting production process is that aluminum liquid pouring temperature is 660 ℃, low-speed injection adopts uniform acceleration injection process, maximum critical speed is 0.57 m/s, high-speed speed is 4 m/s, mold temperature is controlled at about 200 ℃, high-speed starts at high-speed position 3, and a front fork die-casting with clear outline, smooth surface, high dimensional accuracy and no defects is obtained.
(3) Tensile strength of front fork die-casting is 260 MPa, yield strength is 169 MPa, and elongation is 2.2%; after T6 treatment, tensile strength is increased to 342 MPa, yield strength is 222 MPa, and elongation is 3.6%, which meets technical requirements of front fork parts of electric scooter products.
(4) Density of front fork die-casting reaches 2.739 g/cm³, and porosity is 0.39‰. Porosity of die-casting is much less than die-casting standard of 2%, which also shows that gas content of die-casting is very low, and low gas content forming of die-casting is achieved.