The heavy truck rear axle leaf spring support is the key component connecting the leaf spring and the frame. It supports the leaf spring and transmits the force from the leaf spring to the frame. Its structural strength directly affects the driving safety of the vehicle. Therefore, the ductile iron casting has complex shape and high technical requirements, so it is a ductile iron casting with great difficulty in development.
The support is made of nodular cast iron with a mass of 59 kg and an overall dimension of 637 mm × 799mm × 389 mm, the general wall thickness is 10 mm, the maximum wall thickness is 47.5mm, the wall thickness of nodular cast iron castings is uneven, and the structure of nodular cast iron castings is shown in Figure 1. Technical requirements: material grade qt800-2, spheroidization grade 1 ~ 3, graphite size 5 ~ 7, pearlite volume fraction ≥ 80%; Tensile strength ≥ 800 MPa, yield strength ≥ 380 MPa, hardness 245 ~ 335 Hb, elongation ≥ 2%; Dimensional tolerance of nodular cast iron castings: grade ct9; No casting defects such as cracks, cold shuts and burrs that affect the service performance are allowed.
1. Molding line
The static pressure horizontal parting green sand molding line is adopted, and the sand box size is 1000 mm × 800 mm × 350 / 300 mm, using sys-b hardness tester to measure the hardness of sand mold, the hardness is 85 ~ 95 g / mm2, the molding beat is 140 boxes / h, equipped with automatic pouring machine, with high production efficiency and automation.
2. Production process and defect location
The process layout of one box and one piece is adopted, with horizontal parting and cold core box core making. The semi closed gating system is adopted, and the sprue cross-sectional area ratio is Σ F straight: Σ F transverse: Σ F = 1:1.8:1.5, liquid iron is introduced from both sides of nodular cast iron casting, and the section size of sprue is φ 40 mm, cross section size of runner is 25 / 20 mm × 38 mm, 7 internal sprues in total. Place 4 cold irons at the upper and lower mounting holes of the frame, with the size of 35 mm × 35 mm × 30 mm, 2 pieces of 75 mm are placed on the runner × 75 mm × 22 mm foam ceramic filter to filter molten iron. The specific casting process is shown in Figure 2. The pouring temperature is 1390 ~ 1400 ℃ and the pouring time is 20 ~ 22 s. The main chemical compositions of ductile iron castings are: w (c) 3.6% ~ 3.75%, w (SI) 2.1% ~ 2.3%, w (MN) 0.4% ~ 0.55%, w (s) 0.005% ~ 0.02%, w (P) 0.03% ~ 0.05%, w (Cu) 1.4% ~ 1.6%. Through X-ray flaw detection, it is found that there are shrinkage defects in nodular cast iron castings, as shown in Figure 3. The defect rejection rate of shrinkage nodular cast iron castings is 30%. After machining, shrinkage defects are found in holes, with a rejection rate of about 8%, as shown in Figure 4.
Magma simulation software is used to simulate the original casting process scheme. The filling process of nodular cast iron casting is balanced. During solidification, there is a large isolated liquid phase area at the mounting hole of nodular cast iron frame. According to the isolated liquid phase area, the shrinkage and porosity position in nodular cast iron casting is predicted. The shrinkage and porosity position predicted by the software is basically consistent with the actual defect results, According to the basic principle of riser feeding, the original process was improved to eliminate the shrinkage defect in the frame hole of nodular cast iron casting.