For thin-walled castings of new energy fuel cell boxes, a low-pressure casting process for castings was designed, influence of mold preheating temperature on microstructure and mechanical properties of castings was studied. Results show that designed casting process scheme is reasonable, no defects such as insufficient pouring and shrinkage cavities were found in casting process simulation analysis and actual castings, and it can meet mass production needs of new energy fuel cell boxes. Without changing casting process, adjusting preheating temperature of model can not only change structural properties of material, but also affect forming performance of liquid metal.
Due to energy crisis and urgent need for environmental governance, our country has made a major strategic decision to reach carbon peak and become carbon neutral. Green and environmentally friendly new energy not only lays foundation for various industries, but also enhances industrial competitiveness, achieves high-quality sustainable development of economy and society. As a widely used new energy source, fuel cells directly convert chemical reaction energy into electrical energy and have made an indelible contribution to scale and industrialization of new energy vehicles. As main force-bearing component of fuel cell, fuel cell box puts forward higher requirements for its comprehensive mechanical properties.
This research is based on fuel cell box castings, which are cast and formed using low-pressure casting. Casting mold is placed above sealed crucible, and compressed air is used to apply pressure on the surface of molten metal, thereby forcing molten metal to rise from riser tube to fill casting mold. Castings prepared by low-pressure casting have dense structures and high metal yields. Liquid metal under pressure has fewer defects after filling and solidifying, giving it a great advantage in producing large, thin-walled complex castings. In production of aluminum alloy thin-walled shells, shrinkage porosity and shrinkage cavity defects are more likely to form, resulting in quality of castings not being guaranteed, mechanical properties of material being poor, and being unable to meet requirements of structural parts. Therefore, low-pressure casting processes are currently used for this type of castings. However, differences in structure of different castings and casting processes are also quite different. This article mainly studies influence of different preheating temperatures of metal molds on fuel cell box castings under low-pressure casting. By clarifying optimal low-pressure casting process for new energy fuel cell box castings, it provides a reference for the production of similar castings.

Casting material is A356 aluminum alloy, and its chemical composition is shown in Table 1. Raw materials are industrial pure aluminum, Al-20Si master alloy and Al-10Mg master alloy. Use a resistance furnace to heat industrial pure aluminum and Al-20Si master alloy to 720℃ to melt, add Al-10Mg master alloy and stir, then add modifier and refining agent respectively for modification treatment and degassing to complete smelting of A356 alloy in preparation for pouring. Metal casting mold is made of ductile iron QT500-7 material. Surface of casting mold is sprayed with paint. After drying, it is preheated to 280 ℃, 315 ℃, and 350 ℃ respectively.

Table 1 Chemical composition of castings (A356 alloy) wB/%

Shematic diagram of the overall pouring system design is shown in Figure 1. Parting surface is set at the largest cross-section of casting, 4 risers are set on bottom plane, and 8 concealed risers are set on the top of casting. Design concept of this process is smooth, rapid filling and sequential gradient solidification. Through design of liquid rising tube, liquid level can still rise smoothly while filling quickly. This is supplemented by riser design, which plays the role of overflow slag collection, exhaust and improvement of solidification temperature field.

 

Figure 1 Schematic diagram of pouring system
Schematic diagram of the overall mold frame structure is shown in Figure 2, which adopts a system structure of horizontal parting of main body and core pulling in four directions. It is characterized by high efficiency, convenience and easy realization of continuous production.

 

Figure 2 Schematic diagram of the overall mold frame structure

A certain type of low-pressure casting machine is used; mold plate size: 2500 mm * 1800 mm * 1600 mm; mold opening force: 17t; ejection force: 30~35t; crucible capacity: 800kg; mold structure: four-sided core pulling, upper core pulling and ejection; number of liquid riser tubes: 4. Aluminum liquid temperature: 705~720 ℃; holding furnace atmosphere temperature: 740 ℃. Low-pressure casting equipment and segmented filling pressure curve are shown in Figure 3, and fuel cell box casting mold is shown in Figure 4.

 

Figure 3 Low-pressure casting equipment and mold filling segmented pressure curve

 

Figure 4 Fuel cell box casting mold

Castings are milled to remove surface allowances. Surface of test casting was sampled, metallographic specimens were made after grinding and polishing. Pore defects and organizational structure were analyzed, observed using a metallographic microscope, and conventional mechanical properties were tested. Dimensions of tensile specimen are shown in Figure 5, and fracture surface was observed using a scanning electron microscope.

 

Figure 5 Tensile specimen

Microstructure of castings under different mold preheating temperatures is shown in Figure 6. Results show that as temperature increases, dendrite spacing of α-Al in A356 alloy becomes larger, but no obvious change in Si phase morphology is found. Eutectic Si phase is still granularly distributed around white dendritic α-Al solid solution. Generally speaking, the finer primary α-Al, the greater number of dislocation accumulations at phase boundaries and grain boundaries during deformation, and the higher the strength of casting. At the same time, fine structure not only improves its yield strength, but also ensures plastic toughness of A356 alloy. Therefore, from perspective of microstructure, it is predicted that when mold preheating temperature is 280 ℃, the overall performance of casting will be the best.

 

Figure 6 Microstructure of castings under different mold preheating temperatures
Figure 7 shows performance comparison of as-cast sample T6 after heat treatment at different mold preheating temperatures. Results show that as preheating temperature increases, comprehensive mechanical properties of A356 aluminum alloy gradually decrease. Breaking strength dropped from 341 MPa when preheated at 280 ℃ to 329 MPa when preheated at 350 ℃; yield strength dropped from 311 MPa to 301 MPa; elongation dropped from 14.1% to 8.5%. At the same time, tensile fracture analysis also proves that dimples on the fracture surface under preheating at 280℃ are small and dense, while size of dimples increases when preheated at 315℃. At 350℃, partial cleavage planes are found on fracture surface, showing quasi-cleavage. fracture characteristics. Results of mechanical properties show consistency with microstructure.

 

Figure 7 Comparative results and fracture analysis of tensile properties of A356 alloy at different mold preheating temperatures

According to the above analysis results, casting mold preheating temperature selected for trial production of castings is 280 ℃. However, insufficient pouring and cold shutoff defects were found in thin-walled position of actual casting, as shown in Figure 8. At this time, aluminum alloy liquid has not yet filled mold cavity, and liquid flow fronts cannot be welded together even if they merge. Reason is that mold cavity at this location is thin (about 2.5 mm), which causes temperature of aluminum alloy liquid to drop significantly during mold filling process when mold preheating temperature is 280℃. When flow front reaches this position, sufficient fluidity is lost. Therefore, a large number of metallurgical defects are left in thin wall of fuel cell box.

 

Figure 8 Defects on castings
In order to solve this problem, mechanical properties and formability of castings were comprehensively considered. Preheating temperature of metal mold was increased to 315℃, and casting was analyzed based on numerical simulation technology. As shown in Figure 9a, filling process of casting is smooth, there is no behavior such as splashing and slag inclusions. In Figure 9b, it is shown that solidification stage conforms to principle of sequential solidification, from bottom to top, final solidification area is located inside the riser, and the overall feeding of casting is good. Predict shrinkage and porosity defects of castings, as shown in Figure 10. Results show that shrinkage/cavity defects of fuel cell box casting are concentrated at riser and sprue locations, so they can be removed through post-machining.

 

Figure 9 Numerical simulation results of castings

 

Figure 10 Casting defect prediction
Final qualified casting is shown in Figure 11a. Surface quality of casting blank is relatively smooth, and roughness is Ra12.5, which meets requirements of roughness Ra25.4 of drawing. Size and shape are consistent with metal casting mold, and no pore defects are found on the surface of casting. Metallographic structure of casting is shown in Figure 11b. It can be seen that casting structure is dense and has no pore defects after solidification.

 

Figure 11 Appearance quality and internal structure of castings

(1) A low-pressure casting process for new energy fuel cell box was designed, its feasibility was verified using numerical simulation analysis and actual production. Surface of obtained casting is smooth, and no shrinkage, porosity, or pore defects are found inside, which meets requirements for mass production of new energy fuel cell boxes.
(2) Casting process design of fuel cell box needs to comprehensively consider mechanical properties of casting and forming ability of liquid metal. While ensuring complete casting of minimum wall thickness structure, performance of casting can be improved by reducing mold preheating temperature. Optimal metal mold preheating temperature for this trial casting is 315℃.