Cause analyze and improve measures of mounting hole crack in die casting AL

Cause analyze and improve measures of mounting hole crack in die casting AL

Abstract:The die casting cylinder block was found to be broken when adding torque to a threaded hole during assembly in engine assembly shop after production. The defect rate is about 10%.In order to find the influence factors on the the oil pressure alarm mounting hole cracks of the aluminium alloy cylinder block, fracture analysis was used for observing the cracks and casting CAE analysis software was used for analyzing the casting defects. The results indicate as follows: the cracks were caused by stress concentration during installation process. The problem was solved by adding ribs to enhance the strength of casting, and the proposal is verified inproduction.
1.Problem background introduction
  Due to its complex structure and uneven wall thickness, the cylinder block of automobile engine is subjected to large thermal stress during operation. Therefore, the casting quantity of the cylinder block is required to be relatively high. The macro defects and micro metallographic defects such as slag inclusion, porosity and shrinkage porosity generated in the casting process may reduce the service life of the engine.
  The engine block of a car of our company is 409mm×396mm×207mm, made of aluminum-silicon alloy and die-cast.When the oil pressure alarm is tightened in the assembly workshop, when the torque reaches 50Nm, the installation hole cracks, and the location and crack conditions are shown in fig. 1 and fig. 2 below.However, for similar engine cylinder blocks, when the torque reaches 60Nm, the installation hole is still normal.After analysis and investigation by the project team, no obvious signs of strain were found in the installation of precast holes, and the preliminary suspicion was that fatigue cracks occurred under external force due to insufficient strength of the installation holes.
Location of mounting hole where crack occurs
Fig.1.Location of mounting hole where crack occurs
Macroscopic character of crack
Fig.2.Macroscopic character of crack
2.Crack analysis and detection
2.1.Fracture analysis and detection
  
The fracture was observed by a secondary electron image of Quanta250 scanning electron microscope: Figure 1 to Figure 4 show the low-magnification morphology at the crack source. It can be seen that the crack starts from the outer arc surface and expands radially. The crack source has cast holes and is not fused. Phenomenon; the fracture of the sample extension zone is the ductile fracture of the dimple, as shown in Figure 5 and Figure 6. Inferred from the fracture characteristics, the sample should be cast and unfused to cause stress concentration leading to cracking.
Fracture morphology (20×)
Fig.3.Fracture morphology (20×)
Crack source morphology (80×)
Fig.4.Crack source morphology (80×)
Fusible zone of crack source (500×)
Fig.5.Fusible zone of crack source (500×)
Crack source hole (300×)
Fig.6.Crack source hole (300×)
Extended zone morphology (1200×)
Fig.7.Extended zone morphology (1200×)
Extended zone morphology (2500×)
Fig.8.Extended zone morphology (2500×)

2.2.simulation analysis
  According to the results of fracture analysis, the location of the crack was found to be a casting defect.In order to confirm this point of view, we also carried on the simulation analysis to the cylinder block casting process.The materials and initial temperature used for simulation analysis are shown in table 1, the related physical properties of aluminum alloy are shown in figure 9,10,11 and solid phase line temperature 479℃,Liquidus temperature 578℃.and the injection process parameters are shown in table 2.
Table1.Simulation material and initial temperature

  Material Symbol Initial Temperature
Aluminium Alloy AlSi9Cu3-HPDC 675℃
Mold X40CrMoV5  180℃
Cylinder Liner  GJL-250 150℃
Punch CuCoBe 150℃

Die-cast AL density characteristics

Fig.9.Die-cast AL density characteristics
Die-Cast Aluminum Thermal Conductivity
Fig.10.Die Cast Aluminum Thermal Conductivity
Die casting aluminum specific heat capacity characteristics
Fig.11.Die casting aluminum specific heat capacity characteristics
Table 2  Simulated injection process parameters
   
Pressure chamber length 1217mm
Punch Diameter 150mm
Mold Cake Length 38mm
Start switching position at high and low speed 805mm
Low Speed  0.2m/s
High Speed 5.0m/s
  Fig.12 shows the temperature distribution of the casting at the end of filling. It can be seen that the temperature of the whole filling process of the casting is above the liquidus temperature (578 °C), and the probability of cold separation is low. Fig.13 shows the shrinkage volume prediction. It shows that there are shrinkage holes between the installation and the oil return passage, but the porosity is low, the volume is small, and the probability of defects is low; Fig.14 shows the mold temperature when the mold is opened, and the result shows that the mold opening temperature of the installation is low, and the pull occurs. The risk of injury is small; Figure 12 shows the analysis of the gas-filled gas. The results show that there is a helium phenomenon at the top of the installed patch during the filling process.
Casting temperature distribution after filling mold
Fig.12.Casting temperature distribution after filling mold
Prediction results of shrink cavity volume
Fig.13.Prediction results of shrink cavity volume
Mold opening temperature
Fig.14.Mold opening temperature
Gas porosity analysing
Fig.15.Gas porosity analysing
  From the results of the CAE analysis, there is a small amount of helium at the top of the mounting peg, and there is a risk of stomatal.Combined with the fracture detection analysis and the results of casting simulation analysis, it is known that the cause of the crack should be due to insufficient strength of the matrix caused by the casting defect and stress concentration during the installation tightening.
3.Measures to prevent cracks
  In response to this problem, the project team discussed and proposed the scheme shown in Figure 16: Adding ribs around the mounting lap, the purpose is: First, as an auxiliary flow path during filling, which is conducive to the filling of the lap and the casting of the product. The defects are greatly improved; second, increase the rigidity and strength of the mounting lap to prevent deformation. However, the unreasonable arrangement of the ribs may result in a large heat knot at the intersection, and shrinkage or shrinkage inside, which affects the internal quality of the product. Therefore, in order to verify the process feasibility of the scheme, we performed the casting CAE analysis again. The analysis results are shown in Figure 17 and Figure 18. It can be seen from the results that if the reinforcing ribs are added, the helium gas is slightly improved but not obvious, and the risk of other casting defects is not increased.
Die casting cylinders optimization
Fig.16./Die casting cylinders optimization
Casting porosity temperature distribution
Fig.17.Casting porosity temperature distribution
Shrink cavity prediction after optimization
Fig.18.Shrink cavity prediction after optimization
4.Conclusion
  The cause of the crack in the mounting hole of the engine block is due to insufficient strength of the base due to the casting defect and stress concentration during the tightening of the mounting. By adding reinforcing ribs around the mounting lap, the filling of the lap during the casting process is improved, the internal quality of the product is improved, and the rigidity and strength of the lap are increased to prevent stress deformation. After the implementation of the scheme, 1000 batches of batch verification were carried out in the production workshop. According to the feedback of production quality data, cracks were not found again here, indicating that the scheme is feasible.


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