1. Location and shape of crack defects in cylinder block castings
During the trial production process, cracks were found between the cylinder barrel after the sand falling of the cylinder block casting. The location and shape of crack defects are shown in Figure 1. It can be seen that the crack occurs between the vents and runs through the part between the two holes, which is a continuous straight line. The fracture of crack defect is cyan black, as shown in Figure 2. The crack defects of cylinder block casting were tested, and no abnormality was found.
2. Cause analysis of crack defects in cylinder block castings
According to different causes, the crack defects of cylinder block castings are divided into cold crack defects and hot crack defects. Cold crack defect is a crack defect caused by the local casting stress of the cylinder block casting is greater than the ultimate strength of the alloy when the cylinder block casting is cooled to the elastic state after solidification. Cold crack defects often propagate through the crystal to the whole section, in the shape of slender straight line or broken line with uniform width, and the fracture has metallic luster or slight oxidation color. Hot crack defect is a crack defect caused by the obstruction of solid-state shrinkage of cylinder block castings when the cylinder block castings are still in a state of low strength and plasticity at the end of solidification or shortly after final solidification. The fracture surface of hot crack casting defect is seriously oxidized and has no metallic luster. The crack defect initiates and expands along the grain boundary, showing an uneven, tortuous and irregular curve. The surface of the outer crack defect is wide and the inner one is narrow, showing tear shape; The internal crack defect usually occurs in the last solidified part of the cylinder block casting, and the section is often accompanied by dendrite.
Because the crack defect of cylinder block casting is basically linear, it conforms to the characteristics of cold crack; The fracture of cylinder block casting has obvious oxidation characteristics, which is consistent with the characteristics of hot crack defects. Therefore, the properties of crack defects cannot be accurately judged according to the above principle of crack defects and fracture morphology. The solidification state and cold crack criterion of cylinder block are simulated and analyzed with the help of numerical simulation software, as shown in Fig. 3 and Fig. 4. Figure 5 shows the relationship between temperature and tensile strength of the material.
It can be seen from Fig. 3 to Fig. 5 that the position where the crack defect occurs solidifies first and there is tensile stress; According to the criterion of cold crack defect, the location of crack defect is about 600 ℃, and the risk of cold crack is very high; According to the analysis of the relationship curve between the solidification temperature and tensile strength of the material, the tensile strength of the cylinder block casting is low (69 ~ 131 MPa) at about 600 ℃, and the crack defect of the cylinder block casting occurs when the cylinder block casting is at high temperature, which also explains the reason why the crack defect fracture of the cylinder block casting presents oxidation characteristics. Comprehensive analysis shows that the crack defect of cylinder block casting is cold crack defect.
3. Solutions to crack defects of cylinder block castings
After determining the attributes of crack defects, comprehensively analyze the quality requirements and production conditions of cylinder block castings, and take targeted solutions.
(1) Improve the structure of cylinder block casting to reduce stress concentration. Due to the great difference in wall thickness of cylinder block castings, the cooling speed of each part of cylinder block castings is too different in the cooling process, resulting in large shrinkage stress or stress concentration. Therefore, change the vent shape to disperse the stress. At the same time, increase the strength of the crack defect location and increase its cross-sectional area by 20%, as shown in Figure 6.
(2) Improve the concession of sand mold and sand core and reduce the shrinkage resistance to cylinder block castings. Due to the poor concession of sand mold or sand core, the shrinkage resistance of cylinder block casting is too large. Therefore, the amount of resin added is reduced from 1.5% to 1.2%, and the tensile strength of sand core is reduced by 20%; The round rod core is locally made into a hollow structure to increase the concession of the sand core.
(3) Adjust the melting process to reduce the shrinkage tendency of the material. Due to the large shrinkage tendency of the material, the pouring temperature is high. Select high-quality carburizing agent, increase the high-temperature purification time of molten iron, stay at 1500 ℃ for 15 minutes, increase the amount of W (c) from 3.12% to 3.20%, optimize the pouring system and reduce the pouring temperature by 20 ℃.
(4) Extend the sand falling time of cylinder block castings and control the temperature of cylinder block castings during sand falling. Due to premature unpacking of cylinder block castings, too fast cooling and too high sanding temperature, they are collided and squeezed during sanding and cleaning, and the residual stress causes the cracking of cylinder block castings. Therefore, ensure that the sanding time is ≥ 8 h and the casting temperature of cylinder block is ≤ 400 ℃.