With the increasing demand for lightweight in the automotive industry, the material of chassis structural parts has gradually changed from traditional ferrous metals to non-ferrous metals, and has been widely used in steering knuckles, wheel brackets, control arms, sub-frames and other products. Among them, compared with traditional steel materials, aluminum alloy has the advantages of low density, light weight, excellent casting performance, good corrosion resistance, easy recycling and reuse, and absorbs more collision energy than steel materials in the event of a collision. It is widely used in chassis structural parts. Chassis structural parts mainly play the role of bearing and bearing, and are connected and assembled with multiple parts, which belong to automobile safety parts. This type of product usually presents the characteristics of complex structure, difficult forming, strict requirements on mechanical properties, etc., and has high requirements on manufacturing process. At present, low pressure casting, differential pressure casting and squeeze casting are mostly used in the industry to produce chassis structural parts. PINO’s gravity die-casting machine adopts the gravity tilting casting process, with special secondary refining treatment, to meet the performance of sub-frame bracket products, achieve the target yield, and realize the requirements of stable mass production.
1.Aluminum alloy gravity casting product introduction and performance indicators
The weight of the chassis structural parts is about 3 kg/piece, the size specification is 330 mm×260 mm×250 mm, the minimum wall thickness is 5 mm, and the maximum wall thickness is 36 mm. The product as a whole presents a special structure with thick walls around and thin walls in the center . Through the development of the gravity pouring casting process, the yield of finished products can reach more than 90%, and the annual production capacity of 300,000 pieces can be realized.
The sub-frame bracket is made of AlSi7Mg0.3. This alloy has the advantages of good fluidity, no thermal cracking tendency, small linear shrinkage, and good air tightness. The alloy composition is shown in Table 1.
1.1 Internal Quality Requirements
During the production process of the sub-frame bracket, 100% on-line X-ray flaw detection is required to judge internal shrinkage, pores and other defects. Products are divided into stress areas, functional areas and general areas, and different areas have different criteria for judging. For stress-bearing areas and functional areas, the diameter of a single defect is required to be ≤1 mm, and the porosity ≤5%. For general areas, the diameter of a single defect is required to be ≤3 mm, and the porosity ≤5%.
1.2 Surface quality requirements
100% appearance inspection shall be carried out on the sub-frame bracket, and defects such as air holes, cracks, and hot cracks on the casting surface shall be judged through non-destructive fluorescent penetration and artificial naked eye inspection methods. Judgment criteria are also divided according to force area, functional area and general area. According to the DIN EN 1371 standard, for the stressed area, the detected defects are required to meet the SP1/CP1 level (SP stands for nonlinear independent defects, CP stands for nonlinear aggregated defects). For functional areas and general areas, the detected defects are required to meet the SP2/CP2/AP2 level (AP stands for defects arranged in rows), and the maximum depth of defects can be 0.5 mm, but thermal cracks, thermal tears and impurity inserts are not allowed .
1.3 Mechanical property requirements
Because there is no steering knuckle and the subframe is bad in the working condition, the mechanical properties are lower than those of the steering knuckle and subframe. The casting body sampling is used for testing, and the sampling position is selected in the stressed area. The specific standards are yield strength R p0.2 ≥190 MPa, tensile strength R m ≥230 MPa, elongation A 50 mm ≥3%, hardness HBW80～ 110.
2.Process design
The production process of the sub-frame bracket is: molten aluminum smelting – secondary refining of molten aluminum – gravity tilting casting – trimming, removal of sprue riser – online X-ray flaw detection – T6 heat treatment – correction – crack detection – processing – cleaning – Assembly – Packing.
2.1 Refining and hydrotreating of molten aluminum
Aluminum alloy castings tend to shrink during the solidification process from pouring temperature to room temperature, and the shrinkage method is determined by the alloy composition, gas content and heat transfer conditions during cooling. Affect the generation of shrinkage cavity. In the production of some castings, in order to prevent local shrinkage cavity defects, gas is deliberately introduced into the molten metal before casting, such as automobile exhaust manifolds, wheels and other products have been used in batches.
According to industry research results, in aluminum alloy castings with poor feeding conditions, in order to prevent the formation of large shrinkage cavities and replace them with uniformly diffuse pores, a certain amount of hydrogen can be introduced into the alloy, and the amount depends on the specific alloy. Composition, and related to the liquidus temperature of the alloy. Hydrogen is the most easily soluble gas in liquid aluminum, and its solubility is very large, and increases with the increase of temperature. The solubility of hydrogen in the liquid and solid phases of aluminum is 0.65 cm 3 /100 gAl and 0.034 cm 3 /100 gAl, respectively, that is, the solubility of hydrogen in the liquid and solid phases is 19 times different, but hydrogen is almost insoluble in solid aluminum.
From a microscopic point of view, the reason why adding hydrogen to the aluminum alloy can reduce the shrinkage cavity defect is that when the liquid alloy enters the metal mold and starts to solidify, the volume shrinkage caused by the rapid cooling of the alloy by the mold, in the case of no feeding When the gas content in the channel and the alloy is particularly low, the isolated liquid will undergo single nucleation under the main influence of the hydrostatic tension generated by the shrinkage, and large pores will be formed in the subsequent shrinkage. However, if the hydrostatic tension or gas content is increased in the later stages of solidification, nucleation can occur in multiple places, depending on the core position occupied by the core and the initial gas content in the metal. The initial single hole has production advantages compared with the hole that grows after nucleation. Therefore, when the gas content of the alloy is low, a large shrinkage cavity will be formed in the casting. However, when the initial gas content in the alloy is further increased, under the joint action of shrinkage and gas precipitation, the tendency of multi-nuclei formation increases, resulting in the reduction of large shrinkage cavities, and the formation of a large number of small pores instead, and under a certain critical gas content, The casting will only appear multi-nuclear phenomenon, that is, the large shrinkage cavity disappears, and all are uniform small pores.
This product adopts the metal type gravity tilting casting process. In order to reduce the shrinkage tendency of the casting, the aluminum liquid treatment process is divided into two steps based on the above principle. The first step is to use a rotary degasser, use high-purity argon to degas and remove slag from the aluminum liquid, and use an online density meter to detect the density after degassing. After the degassing and slag removal is qualified, the aluminum liquid is processed in the second step again, and the rotary degasser is also used, but the hydrogen-nitrogen mixture is passed into the aluminum liquid to realize hydrogenation. After the second step of hydrogenation, use an online density meter and a hydrogen detector to test, and pouring can only be carried out after passing the test. During the production process, it is necessary to ensure that the aluminum liquid is stored for no more than 2 hours.
By adding hydrogen gas to the molten metal, the large shrinkage porosity (shrinkage cavity or hole) disappears and is replaced by uniformly dispersed pores, as shown in Figure 2, which meets the internal quality requirements of the subframe bracket.
Fig.2 Uniformly distributed pores formed after hydrogenation
2.2 Casting process design
The gravity tilting pouring method can effectively reduce the drop after the molten aluminum enters the mold cavity, reduce the oxidation slag generated when the molten aluminum splashes, and at the same time avoid the phenomenon of gas trapping (Figure 3). This production process is a mature and advanced casting process in the world, and has been applied in large quantities in products such as aluminum alloy cylinder heads. The auxiliary stable and reliable automatic control system in the production process can greatly improve the quality of castings and reduce the labor load of workers. The sub-frame support is not completely symmetrical left and right parts. The casting mold adopts the form of two parts in one mold, which are symmetrically arranged in the mold cavity. Road, respectively for the left and right pieces of independent filling. Because the product structure presents a thin-walled center and thick surroundings, risers are provided for the thick and large parts for feeding to reduce the tendency of shrinkage and porosity. MAGMA software was used to simulate and optimize filling, temperature, and shrinkage defects, and the final process yield was 37.5% (the product weight divided by the total weight of castings, runners, risers, etc.).
Figure 3 Gravity tilt casting process
Before pouring, preheat the mold with natural gas, and spray paint when the mold temperature reaches 220-260 °C. There are 4 types of coatings, including primer coatings, cavity thermal insulation coatings, runner riser thermal insulation coatings, and lubricating coatings. The purpose of the primer is to make the mold have better adhesion and improve the service life of the cavity insulation coating. After the primer coating is sprayed, use a watering can to spray the cavity insulation coating on the forming part of the casting. The particles of the cavity insulation coating are fine and meet the requirements of the surface finish of the casting. Apply another kind of thermal insulation paint on the runner and riser of the mold, and use a brush to manually brush it. The thickness of the paint can be thicker to increase the thermal insulation and improve the feeding capacity to achieve sequential solidification. Finally, spray lubricating paint on the parting surface and slideway of the mold. A variety of coatings are used together to ensure the quality of the castings.
The pouring temperature of molten aluminum is set at (735±5)°C, and the entire pouring process is completed by the cooperation of automatic tilting pouring machine and robot. The pouring robot takes aluminum from the holding furnace with a pouring spoon according to the instructions, and pours it into the sprue cup of the mold. The pouring machine drives the mold to tilt at a tilting speed of 6 s/90°, and the molten aluminum flows out of the sprue cup smoothly along the metal mold. into the mold cavity. Cooling begins after filling. 22 cooling water channels are designed inside the mold to conduct spot cooling on local locations. The cooling water temperature is 10-30 ℃, flow rate is 3-6 LPM, and pressure is 2-4 bar. At the same time, in order to ensure the sequential solidification, the mold needs to be heated locally. The cooling time for mold filling is 145 s. After cooling, the pouring machine returns to the horizontal position, the mold is opened, and the whole pouring process takes about 300 s per mold time. Each set of molds produces 1 piece of left bracket and 1 piece of right bracket at a time. After solidification, the casting is taken out of the mold by the robot and sent to the cleaning unit automatically.
The robot in the cleaning unit grabs the cooled casting and sends it to the cooling water tank for cooling. After cooling to room temperature, the robot grabs the casting and sends it to the edge trimmer to remove the riser, the saw to remove the sprue, and then print two Two-dimensional code and clear code numbers, so as to realize the identity definition of castings, and upload the casting process information to the IT server to ensure the traceability of products. During the entire pouring and cleaning process, the operator only needs to manually spray and clean the mold, and all other actions are controlled by the equipment itself according to the program settings, which greatly reduces the labor intensity of the operator.
2.3 Heat treatment process
The sub-frame bracket adopts T6 heat treatment process. The heat treatment furnace is a pass-through heat treatment furnace. Quenching treatment is carried out after solution treatment. The quenching medium is water. After quenching, the casting is cooled and then manually calibrated. First, the casting is placed on the size detector. Carry out dimensional inspection, measure the deformation of the casting, and correct the deformation position of the casting according to the inspection result. The calibrated casting will be inspected again for size, and after being confirmed to be qualified, it will be sent to the aging furnace for aging treatment. The heat treatment process is shown in Table 2.
Table 2 T6 heat treatment process
The heat treatment material frame needs to be specially designed to locate and support the easily deformed parts of the castings to prevent the castings from being deformed during the high temperature treatment process. At the same time, it is necessary to ensure that the castings are heated evenly during the heat treatment process and it is convenient for the operator to load and unload. The positioning tooling is shown in Figure 4 Show. After aging, the mechanical properties are tested, and the castings with unqualified heat treatment are allowed to repeat the heat treatment for up to 3 times. The whole process of heat treatment and calibration needs to scan and trace the casting, and store process information.
Figure 4 heat treatment positioning tooling
3.Performance analysis of castings
3.1 X-ray inspection results
Using an online X-ray flaw detector, setting multiple detection angles, covering all positions of the casting, through reasonable design of the pouring system, cooling water, mold temperature, and optimizing the paint spraying process, the internal quality of the casting’s stressed area, functional area and general area Qualified and meet the product requirements.
3.2 Metallographic examination and porosity
Sampling of the casting body. Figure 5 shows the metallographic structure observed under a metallographic microscope. It can be seen that the α-Al dendrites are uniform in size, and the eutectic silicon is in the form of fine particles. Circular second phase, there are second phases in irregular shapes such as strips and blocks, which are uniformly dispersed.
Figure 5 Metallographic structure
Because of the hydrogenation treatment process, tiny hydrogen pores are distributed inside the casting. The distribution of pores is determined by metallographic microscope. The results show that the pores are evenly distributed and meet the requirements of single pore size defects and porosity.
3.3 Mechanical properties
Mechanical samples are taken from the casting body, and two mechanical test rods are taken from each casting. The Zwick Z100 electronic universal material testing machine is used for testing, the test conditions refer to GB/T228 A224, the elastic modulus measurement speed is 0.000 25/s, the yield point and yield range speed is 0.000 25/s, the test speed is 0.006 7/s, room temperature (23 ±5) °C. The hardness test sample and the tensile test bar are taken from the same casting, and the test conditions refer to GB/T231.1-2009. After T6 heat treatment, the mechanical properties of the subframe can meet the product requirements, and the test results are shown in Table 3.
Fig.7 Schematic diagram of tensile specimen
3.4 Function test
The processed sub-frame bracket is assembled with bolts, and the sub-frame assembly is composed of the sub-frame and peripheral accessories, and the bench test is carried out. Bench tests include engine bearing test, stabilizer connection test, longitudinal force test, transverse force test and bolt connection test. After the rated load and test rounds, no defects such as breakage and local cracks were found in the subframe bracket, and it passed the test. The sub-frame bracket is subjected to the vehicle crash test and typical working condition test, which meets the experimental requirements and the product performance is qualified.
4.Conclusion
(1) The sub-frame bracket is a safety part of the chassis. The product structure is complex, casting and forming is difficult, and the requirements for internal quality and mechanical properties are strict. The product development is completed by adopting the metal type gravity tilting casting process and T6 heat treatment. The quality of the casting, the dimensional accuracy and The mechanical properties meet the requirements, and the yield rate is over 90%.
(2) Adopt advanced tooling equipment at home and abroad to realize automatic control of key processes, all processes can realize process parameter recording and uploading, meet the information traceability requirements of chassis safety parts, and the quality assurance ability has reached the international leading level, and has delivered more than 80 products to customers. Ten thousand pieces.