Automobile lightweighting is an important way to achieve energy conservation and emission reduction. "Replacing steel with aluminum" for parts is still main means for current automotive industry to achieve lightweighting. As an important structural part in chassis suspension system of an automobile, aluminum subframe can greatly improve lightweight degree of automobile. This paper briefly introduces forming process and application status of aluminum alloy subframes, covering five mainstream forming methods: casting, hydraulic forming, stamping + extrusion + welding, casting + extrusion/stamping + welding, steel-aluminum connection, compares and analyzes advantages and disadvantages of these forming processes.

Subframe is a structural part on chassis of an automobile, used to connect suspension device and body. Subframe is not a real frame, but an auxiliary frame used to support assembly bracket of axle and suspension. It is a special functional structural part on car that carries engine and axle.
Traditional subframes are generally made by welding after stamping and deforming steel plates. This process is used by major OEMs because of its simple and convenient production. With increasing requirements for lightweight vehicles and pursuit of maximizing energy efficiency brought by reducing vehicle weight, major OEMs have gradually developed aluminum alloy subframes. Weight of a single steel subframe is usually 10 to 25 kg. Use of aluminum subframes can reduce weight by 30% to 50%, with significant lightweight effects.

 

Compared with traditional split subframes, which require casting, pre-welding CNC processing, welding, post-welding CNC finishing and assembly, tolerance of each link can be superimposed up to 1.2 mm. Changan Chongqing Chassis Branch's aluminum alloy integral low-pressure casting subframe innovatively adopts an integral low-pressure casting process. Through an integrated molding design, it completely eliminates welding link and avoids risk of welding deformation from root. Metal liquid is filled at a stable pressure of 0.3-0.5 MPa, and combined with precise control of mold experts, internal porosity of product is less than 0.05%. Rib network designed based on topological optimization algorithm achieves weight reduction while improving torsional stiffness, perfectly balancing lightweight and safety performance.

European Union first began to use aluminum alloys instead of steel to produce automobile subframes. After decades of development, aluminum alloy subframes were initially mainly used in high-end cars. With maturity of subframe production technology and development of diversified production processes, application of aluminum subframes in mid- and low-end cars has gradually increased.
Compared with research and development and application of foreign aluminum alloy subframes, research and development of domestic aluminum alloy subframes is relatively backward. Structural design of aluminum alloy subframes mainly relies on introduction of foreign research and development results for adaptive transformation, and independent innovation capability is relatively weak. In terms of production, domestic aluminum alloy subframes mainly rely on imports and factories of foreign parts companies in China. In recent years, with advancement of domestic suppliers' technology and equipment levels, booming market of lightweight and new energy vehicles, great progress has been made in design, processing and manufacturing of aluminum alloy subframes.

Forming process of aluminum alloy subframes can be divided into four categories: casting, stamping, hydraulic forming and extrusion. However, due to complex structure of subframe, a single process is often difficult to meet forming requirements. Production of aluminum alloy subframes usually adopts a variety of forming processes, such as casting + extrusion/bending + welding process, extrusion/hydraulic forming + sheet stamping + welding process, tube profile + complex cross-section profile + welding process, etc.

Casting is mainstream process for production of aluminum alloy subframes. Compared with extrusion forming, stamping and other processes, casting can produce thin-walled parts with more complex shapes, it can be formed in one time, with high production efficiency, fewer subsequent machining and assembly processes, and low cost. Therefore, it is widely used in production of aluminum alloy subframes. According to different casting methods of subframe casting production, casting forming process can be divided into: metal mold gravity casting, low pressure casting and high vacuum die casting.

 

Low pressure casting process is widely used in production of aluminum alloy subframes. It is a form of anti-gravity casting. Because molten metal solidifies under pressure, organization is denser than ordinary metal mold castings and sand castings, mechanical properties are higher, and cost is low, which is recognized by domestic and foreign automobile companies. Commonly used aluminum alloy for low-pressure casting is AlSi7Mg alloy, which has advantages of good fluidity, no hot cracking tendency, small linear shrinkage, good air tightness, etc. Tensile strength of its low-pressure casting can reach more than 290 MPa, and elongation can reach more than 8%. As shown in Figure 1, some subframes formed by low-pressure casting process.

 

Figure 1 Aluminum alloy subframe produced by low-pressure casting process
High-pressure casting is to pour molten metal into pressure chamber, then injection rod pushes molten metal in pressure chamber into runner and cavity at high speed, solidifies it under high pressure to form a casting. However, due to fact that metal liquid is easily entrained in process of high-speed filling of cavity during high-pressure casting, castings produced have many internal pores, which cannot be heat treated, product elongation is very low, which is limited in application.
High vacuum die casting is based on ordinary die casting, and uses auxiliary high vacuum control systems, vacuum pumps, vacuum shut-off valves and other devices to extract gas in cavity before metal liquid fills mold cavity, so that a higher vacuum degree (<100 mbar) is formed in mold cavity and maintained until filling is completed. The first domestic high vacuum die-casting aluminum alloy subframe has a tensile strength of up to 240 MPa and a yield strength of 140 MPa, and is used in batches. High vacuum die-casting process can effectively avoid air entrainment during filling, and resulting castings have high density, can be heat treated and welded, have good mechanical properties and high elongation, and are valued by major OEMs. Figure 2 shows some examples of high vacuum die-casting aluminum alloy subframes.

 

Figure 2 Aluminum alloy subframes produced by some high vacuum die-casting processes
Metal mold gravity casting is also a common casting process for producing subframes. It is a process method in which molten metal enters metal mold through runner system and solidifies under action of gravity. Common gravity casting aluminum alloy materials are Al-Si series, especially AlSi7Mg alloy. At present, key research directions of this process are: process design and simulation technology with goal of achieving sequential solidification, quality control and deformation control of welding between castings/profiles, control of microstructure and properties of alloys and welds, and formulation of quality control standards. Figure 3a shows an aluminum alloy subframe used in a domestic model. Left and right brackets are gravity castings of AlSi11 alloy. Figure 3b shows a front subframe of an automobile with a size of 800 mm×500 mm×200 mm developed by Shanghai Huizhong based on a certain model of SAIC.

 

Figure 3 Aluminum alloy subframe produced by partial gravity casting

Hydraulic forming is different from traditional forming process. It uses water or oil instead of punch or die as force transmission medium. By applying pressure to force transmission medium, workpiece to be processed undergoes appropriate plastic deformation, thereby meeting design requirements. Hydroforming technology was first widely used in aerospace field. It can produce thin-walled parts with complex shapes, light weight, high rigidity and high precision. In production process, it can reduce types of parts, weld length, machining processes, product assembly processes, etc., has advantages of reducing production costs and shortening processing cycles. Since the 1990s, it has attracted great attention from automotive industry and has flourished. For production of complex parts such as automobile subframes, main processes of hydroforming include: bending, preforming, hydroforming and post-processing. Li Wuquan et al. used hydroformed pipe fittings and sheet metal stamping parts to weld rear subframe, reducing original rear subframe from 18.2 kg to 14.6 kg, achieving a weight reduction of 3.6 kg (19.7%), while maintaining same performance. Figure 4 shows subframe of BMW 5 Series produced by hydraulic production technology and pre-bent pipe products and subframe of Mercedes S series.

 

Figure 4 Aluminum alloy subframe and its components produced by hydraulic technology

Sheet metal stamping process generally uses medium-to-high strength Al-Mg-Mn alloys that are not heat-treated and strengthened. The most commonly used alloys are AlMg3Mn and AlMg3.5Mn. This type of alloy has high strength, good formability and weldability, does not require solid solution aging hardening, has good hot and cold rolling properties. Sheet metal stamping can prepare main beams, mounting brackets, reinforcing ribs, finally weld these parts into an integral subframe through welding methods and processes. Extrusion can produce parts with complex cross-sections and thin walls, with high dimensional accuracy, good surface quality, and high strength. Extruded parts on subframe are generally used as main beams, connection with other parts such as stamping parts is achieved through welding forming technology. These extruded profiles usually use Al-Mg-Si alloys represented by 6061, 6063 and 6082 aluminum alloys. This series of aluminum alloys has good extrusion formability, corrosion resistance, weldability, machining performance and forming performance, product surface quality is excellent. It is widely used in frames and subframes.
As shown in Figure 5a, rear subframe of Mercedes-Benz S-Class has a sheet stamping thickness of 2.5~3.5 mm, using AlMg3.5Mn alloy, and extruded structural parts use 6060 alloy. Weight reduction of the entire subframe can reach 40%. Figure 5b shows front subframe of a BMW model, which is welded by sheet stampings and extrusions.

 

Figure 5 Aluminum alloy subframe produced by stamping + extrusion + welding process

Extruded profiles and plates Stamping plates generally have advantages of high strength, high dimensional accuracy, and good surface quality. They are used to produce brackets or main beams of subframes, which can meet strength and stiffness requirements of subframes. Extrusion or stamping processes are not easy to form. Some OEMs often use casting to produce structural parts for body connection. Since they can be formed in one time, subsequent assembly and installation processes can be reduced, which improves production efficiency. Extruded parts and castings are connected together by welding to form automobile subframes.
Figure 6 shows an example of a partial automobile subframe formed by casting + extrusion/stamping + welding. Main beam of subframe shown in Figure 6a is an extruded part made of 6061 alloy and subjected to T6 heat treatment. Castings on both sides are made of AlSi7Mg0.6 alloy. Finally, they are welded together by MIG welding process. Compared with traditional steel subframes, weight is reduced by about 35%. Subframe shown in Figure 6b is welded from two castings and an extruded hollow profile, with a weight of only 5.8 kg, a weight reduction of about 30%. Subframe shown in Figure 6c is 1 130 mm * 675 mm * 268 mm in size, welded from castings and stampings, with a weight of only 13 kg, and is used in a certain luxury car.

 

Figure 6 Aluminum alloy subframe produced by casting + extrusion forming/stamping forming + welding process

Since cost of aluminum alloy is relatively high compared to processing cost of steel products, many OEMs have not completely aluminum-alloyed automobile subframe, but have used steel-aluminum connection method to realize aluminumization of some parts of subframe, which saves processing costs to a certain extent and realizes lightweight of subframe. Steel-aluminum connection generally uses IW (steel nail punch riveting aluminum plate and steel nail and steel plate spot welding), SPR (self-piercing riveting connection) and FDS (self-tapping thread connection) technology to achieve connection between steel structural parts and aluminum structural parts. Figure 7a shows steel-aluminum connected rear subframe of GAC Honda Accord. Due to strong demand for lightweighting by OEM, steel-aluminum connected subframe is not as lightweight as aluminum subframe and has now been gradually replaced by aluminum subframe. Aluminum subframe is shown in Figure 7b.

 

Figure 7 Steel-aluminum connected subframe and its aluminum subframe alternative