Aiming at defects such as isolated liquid phase, hot joints and porosity in low-pressure casting caused by compact structure, uneven wall thickness, complex shape of spiral sand core, and large proportion of sand core volume in low-pressure casting parts of electric vehicle subframe, AnyCasting software is used to simulate and analyze influence of factors such as temperature, pressure and mold structure of sub-frame low-pressure casting on quality of solidification and crystallization; comprehensive numerical simulations are performed by combining defect and probabilistic defect prediction functions to predict occurrence of isolated liquid phases, hot spots and porosity in castings. Study intelligent control method of AnyCasting in cooling of low-pressure casting molds, explore how to use opening and closing sequence of cooling water pipe to adjust cooling and solidification sequence of aluminum alloy, improve forming density of internal structure of sub-frame castings and production quality of low-pressure casting.
Foreword: In order to adapt to development trend of electric vehicle lightweight and casting integration, many car companies use aluminum alloy overall low-pressure casting for sub-frame of new energy electric vehicles. Sub-frame formed by integrated low-pressure casting has a simpler structure, weight reduction of sub-frame itself can also enhance load-carrying performance of vehicle, which is of great significance in improving handling stability of vehicle. Figure 1 shows rear sub-frame parts and low-pressure casting mold diagram of an electric vehicle. A356 aluminum alloy material is used for low-pressure casting integrated forming, which not only ensures crystal structure density requirements of sub-frame aluminum alloy, but also meets load index and high temperature resistance performance requirements of small electric vehicles, improving lightweight level of the vehicle. But at the same time, integrated compact structure of sub-frame makes sand core shape, pouring system and mold structure of low-pressure casting more complicated, adds many interference factors to filling process and sequential solidification and crystallization of aluminum alloy liquid, changes preset crystallization and solidification sequence from top to bottom, casting is prone to defects such as local isolated liquid phase, porosity and shrinkage cavity. In order to ensure compactness of crystal structure forming inside sub-frame casting, meet its high-strength and high-temperature-resistant performance requirements, in practice, combined defect prediction function of AnyCasting simulation software is used to conduct comprehensive numerical simulations to predict occurrence of isolated liquid phases, porosity and shrinkage cavities in castings, adjust and plan distribution of mold cooling water pipes, opening and closing sequence to control cooling and solidification sequence of aluminum alloys, etc., to improve quality level and production capacity of low-pressure casting. Specifically, research is carried out from following aspects.

 

Figure 1 Structure diagram of aluminum alloy subframe parts and molds

Shape and structure of sub-frame of electric vehicles are complex, they are subjected to long-term loads during driving. Therefore, requirements for internal structure density are higher, and it is necessary to carry out detailed simulation comparison analysis before determining process plan to ensure normal realization of preset low-pressure casting solidification sequence. With rapid development of computer software and hardware technology and its wide application in manufacturing industry, intelligent low-pressure casting simulation function of AnyCasting software is constantly enriched and powerful. Using AnyCasting and other intelligent numerical simulation methods can carry out reasonable mold design and accurate process design, so as to achieve a higher product qualification rate.

Sub-frame shown in Fig. 1 has dimensions of 1 050 mm*762 mm*365 mm, wall thickness of 4–8 mm, and weight of 25.6 kg. It is formed by low-pressure casting of A356 high-strength aluminum alloy. Aluminum alloy casting of sub-frame has a compact and complex structure, interior is formed by a large overall sand core. Mold is divided into three mold opening directions: upper mold, lower mold and side mold. All parts of mold are made of SKD61 heat-resistant mold steel. into, as shown in Figure 2. Its main shape part is designed to be formed on upper mold, lower mold is mainly distributed with gating system and used to place sand core for positioning. Before mold is closed, the overall sand core must be installed in cavity according to fixed direction and position requirements. Sub-frame low-pressure casting molten metal filling is a typical bottom injection type, through external pressure and solidification sequence from top to bottom to form a relatively dense crystalline structure inside casting. Under action of low-pressure casting air pressure, aluminum alloy liquid enters gating system through riser and then enters cavity. Preset low-pressure casting sequential solidification scheme is that position far away from runner and upper part solidify first, position close to runner and lower part solidify last.

 

Fig. 2 Design of gating system for sub-frame low-pressure casting
Since structural characteristics of subframe parts are that weight of lower part accounts for a large proportion, it diverges and extends in multiple directions, feeding effect of top part of part through external pressure feeding is not obvious, so it is necessary to set up multiple dispersed risers in subframe low-pressure casting gating system to strengthen feeding and exhaust simultaneously. At the same time, thickness and shape of resin sand core change a lot, volume is larger, and gas generation of sand core is larger, so risers are added in many places to strengthen feeding and exhaust.

Since structural characteristics of electric vehicle sub-frame parts are that mass proportion of top is small, bottom diverges and extends in multiple directions, feeding effect of liquid surface pressure applied externally by low-pressure casting on the top of part is difficult to be effective. In addition, when integral sand core participates in filling process, heat conduction and gas generation of molten aluminum are complicated, so it is easy to generate multiple isolated liquid phases and heat nodes scattered inside casting, making it difficult to realize preset solidification sequence from top to bottom. At the same time, metal liquid must fully wrap sand core in it to complete filling and solidification, which makes it difficult to remove gas generated by reaction, increases tendency of defects such as gas entrainment and slag inclusion. Specific defects are shown in Table 1.

Table 1 Defects of low-pressure casting of sub-frame

HDTD-800 digital liquid surface pressure controlled low pressure casting machine is used for production of low pressure casting of electric vehicle subframe, process parameters shown in Table 2 are set according to four process stages of filling, pressurization, pressure holding and decompression .

Table 2. Setting table of process parameters for low-pressure casting of sub-frame
AnyCasting numerical simulation process is carried out according to above parameters, velocity, pressure, and temperature values at specific locations are checked through analog sensors. Predict subframe filling, heat conduction and solidification process in advance, generate defect prediction cloud map and numerical change chart based on feedback of sensor, as shown in Figure 3. Material types and physical parameters of subframe parts, sand cores and mold components are input into AnyCasting, temperature conduction, filling speed, filling pressure and solidification sequence of mold filling crystallization process are simulated and analyzed. Among them, solidification completion time of product is 150 s, and solidification completion time of gating system is 412 s.

 

Fig.3 Simulation of temperature conduction, filling speed, filling pressure and solidification sequence
In low-pressure casting molten metal filling, the larger volume of sand core in cavity, the stronger heat conduction between molten metal and sand core, the more heat and pressure sand core absorbs weakened aluminum liquid. Through mold design software, it can be directly obtained that volume ratio of sand core in mold cavity is 36%, that is, sand core occupies 36% of mold cavity space on subframe; This makes it necessary for molten metal to distribute and transmit a large amount of heat energy to sand core during mold filling, and temperature gradient expands rapidly. As a result, location of hot spots increases, and an isolated liquid phase region is generated during solidification process, which increases tendency to cause shrinkage and porosity. Prediction analysis of isolated liquid phase region in different time periods of filling process is shown in Figure 4.

 

Fig.4 Prediction of isolated liquid phase defects in different filling stages
In addition, gas generated by reaction between resin sand core and molten aluminum is stirred and mixed with molten metal to form a turbulent flow, resulting in entrained air and slag inclusions, etc. After solidification, defects such as pores and cold shuts are prone to occur. Since there are many reasons for formation of defects, it is necessary to conduct a comprehensive numerical simulation through AnyCasting combined defect and probabilistic defect prediction functions shown in Figure 5 to predict occurrence of isolated liquid phases, hot spots and porosity in castings. And use reverse cloud image to observe closed area of molten metal during mold filling, so as to plan and take quality improvement measures to achieve preset solidification sequence.

 

(a) Defect probability prediction parameter setting (b) Combined defect prediction parameter setting
Figure 5 AnyCasting combined defect and probabilistic defect prediction function

Aiming at defects such as isolated liquid phase, porosity and shrinkage cavity in low-pressure casting of shape of electric vehicle sub-frame, when using AnyCasting software to carry out numerical simulation analysis of sub-frame low-pressure casting mold filling, focus on analysis of isolated liquid phase distribution area formed by deviation of solidification sequence during filling process, hot spot caused by uneven wall thickness of casting, and entrained gas generated at location where sand core has a large gas generation.
Provide a basis for subsequent optimization measures such as planning and designing mold cooling channel distribution and local insert position selection, and use AnyCasting's flow intelligent control method in low-pressure casting mold cooling to accurately calculate flow of cooling water pipes, reasonably plan cooling sequence control method of each part of casting, and realize preset solidification sequence of sub-frame low-pressure casting.

Due to characteristics of shape and structure of sub-frame parts of electric vehicles, it is difficult to effectively feed top of parts under external pressure, thickness and shape of internally formed resin sand cores vary a lot, it is easy to generate multiple isolated liquid phases and heat nodes inside casting, making it difficult to realize preset solidification sequence from top to bottom. AnyCasting's intelligent cooling scheme can adjust local cooling sequence of low-pressure casting mold and control solidification sequence of different parts of casting. Since most of forming positions of subframe are concentrated in cavity of upper mold, cooling scheme of upper mold is particularly important to realize preset cooling, crystallization and solidification sequence of subframe. In numerical simulation design of AnyCasting, formula (1) is used for precise calculation:

 

In formula: H is water head difference (m), and a pressure of 1 kg is equivalent to a water head difference of 10 m; L is length of water pipe (m); S is specific resistance of pipe (which can be obtained from table according to type of pipe material).
After simulation-assisted design of AnyCasting cooling waterway, water flow and waterway switching time, comprehensively considering pressure head loss along pipeline, cooling circuit design of upper and lower molds is shown in Figure 6.

 

(a) Upper mold cooling circuit design (b) Lower mold cooling circuit design
Figure 6 Cooling circuit design of upper and lower dies
Aiming at 13 places in upper cavity where dispersed local isolated liquid phases are likely to occur, a total of 3 sets of cooling circuits are designed for upper and lower molds, and a total of 6 water cooling circuits are used for circulating cooling. At the same time, using AnyCasting's pipeline aided design function, medium selection, switch sequence, temperature and flow control of cooling circuit were refined and designed. Specific content is shown in Table 3.

Table 3 Cooling sequence control table of upper mold and lower mold
In order to realize ideal sub-frame low-pressure casting from far to near and from top to bottom, cooling circuits of upper and lower molds are gradually opened after liquid aluminum enters cavity, and are cooled in sequence according to predetermined order until casting is solidified . Among them, area with the strongest feeding capacity of cooling system should be set in lower mold. In order to better exert follow-up heat feeding function of rising pipe connected to lower mold and gating system, achieve preset solidification sequence, start time of cooling circulation water circuit of lower mold should be delayed appropriately, time and flow of cooling water should be controlled sequentially through mold temperature controller. After aluminum alloy of upper mold is basically solidified and crystallized, gating system is cooled, so as to realize preset solidification sequence of low pressure casting from top to bottom.

On control panel of low-pressure casting machine, cooling control mode of each cooling channel can be selected by selecting input box. There are two options: time control and temperature control. Time control mode controls opening and closing of each cooling channel according to input waiting time and opening time. Temperature control method is to compare actual temperature of mold with set temperature. When mold temperature is higher than set temperature, cooling channel is opened, and when mold temperature is lower than set temperature, cooling channel is closed. Figure 7 shows HDTD-800 low pressure casting machine mold pressure and cooling temperature control monitoring operation panel diagram.

 

Figure 7 Cooling control setting and monitoring of low pressure casting machine
According to structural characteristics of sub-frame, four sensor measuring points are selected for left and right side molds to detect and control mold temperature. Cooling channel is opened when actual temperature of measuring points is higher than this value, and closed when it is lower than this value. At the same time, it is necessary to cooperate with reasonable circulating water pressure and flow value to obtain the best mold cooling effect and control sequence of solidification and crystallization of sub-frame castings.

Through use of AnyCasting software to design and optimize subframe low-pressure casting pouring system and process parameters, to use digitally controlled low-pressure casting machine cooling temperature monitoring operation function to adjust opening and closing sequence of cooling water pipe to achieve quality improvement in filling and solidification sequence, which proves that timing control cooling circuit opening and closing method of mold temperature machine can be used to ensure solidification and crystallization sequence of casting, auxiliary feeding function of riser, cooling insert and gating system can be used to achieve preset solidification sequence from far to near and top to bottom. In addition to above-mentioned process optimization measures, it is also necessary to strengthen quality control of drying process of resin sand core, minimize amount of gas generated when resin sand core contacts with high-temperature aluminum alloy liquid during mold filling. In specific production, CAE intelligent simulation and continuous application and summary of production practice are needed to produce more mature and serialized process technology, cover different types of sub-frames to meet mass production needs of new energy electric vehicle sub-frames .