Failures of Engine Cylinder Holes and Their Solutions

During the bench test of a certain type of engine, one cylinder hole and cylinder block penetrated and cracked, and the section showed the fatigue characteristic of the fracture. The crack originated from the cold shut on the outer surface of the cylinder hole. After analysis, the cold shut was generated when the nozzle angle of the corresponding position of the die-casting mold coating profiling spraying tool was adjusted to adapt to the structural optimization of the water channel mold core of the cylinder block in the manufacture and trial production process, causing a decrease in partial mold temperatures, and making the fluidity of the aluminum alloy liquid decrease when it reaches the mold. After adjusting the cooling of the mold and spraying process, perfecting the appearance inspection standards and quality control documents, and strengthening the inspection of die castings, this defect was resolved.
  
A cylinder block is one of the core components of automobile engines. The high-pressure die casting process (HPDC) is still the most widely used method in the industry to produce aluminum alloy cylinder blocks for automobile engines because of the requirements for high production efficiency, high dimensional accuracy of die castings, clean surfaces, and small machining allowance. With the development of automobile engines towards low fuel consumption, light weights, and precision, new technologies like cooling between cylinder holes have been applied to aluminum alloy engine cylinders. The cylinder block die castings have features of a complex thin wall and different positions with great wall thickness, and the die casting process is difficult. Vacuum die-cast technology can reduce the adverse effect of the gas on the cavity for the molding of die castings and improve the performance of die castings based on the advantages of ordinary die casting technology, which has attracted more and more attention and is used by foundry enterprises.
 
A certain engine aluminum alloy cylinder system was produced by a vacuum die casting process. During the endurance test, the end surface of the cylinder hole perforated and cracked. Took a sample of the cracked position of the cylinder and sent it for inspection and analysis.
 
See Figure 1 for the defect and surrounding macroscopic morphology. It can be seen that the crack penetrated the cylinder wall and extended in the longitudinal direction (Figure 1a). The outer surface of the cylinder hole at the cracked part showed typical casting cold shut, and small, narrow, irregular lines formed due to not well fused molten metal and the joint part sank obviously.
 
2. The analysis of the morphology
2. 1 Fracture morphology
Use stereo microscope and Qnanta 250 scanning electron microscope to observe the fracture morphology, and use the X-ray energy spectrometer to analyze the material of aluminum alloy cylinder block and cast iron cylinder liner near the defect. The stereomicroscope morphology of the macro-fracture is shown in Figure 2.
 
The morphological characteristics of the fracture under the scanning electron microscope are shown in Figure 3. The fracture shows the features of fatigue fracture. The crack originated from the bottom of the cold shut on the outer surface of the cylinder hole, and it had multiple sources and expanded to the inner surface. Fatigue steps were visible. The crack growth area was relatively flat. No old marks, gas holes, shrinkage porosity and other casting defects were found. The microtopography of the fatigue crack source and expansion zone showed the characteristics of compressed wearing (Figure 3c), and the high-magnification microscopic fracture morphology showed quasi-cleavage fracture, indicating that the cracking here was brittle fracture.
 
2. 2 Metallographic structure and composition analysis
Took the transverse section perpendicular to the crack; observed and analyzed the metallographic structure and composition. The aluminum alloy matrix structure of the cylinder hole is α-Al and eutectic Si (Figure 4a), which meets the technical requirements of the product. The metallographic structure of the cast iron cylinder liner (Figure 4h) was evaluated in accordance with GB/T7216-2009 Gray Cast Iron Metallographic Inspection. The inner side of the cylinder liner is type A graphite, and the amount of pearlite is greater than 98%. The outer side of the cylinder liner (being in contact with aluminum alloys) is D-type graphite, which meets the technical requirements of corresponding products for cast iron cylinder liners.
 
The chemical composition of the aluminum alloy matrix and cast iron cylinder liner at the defect is shown in Table 1 and Table 2, and both meet the technical requirements of the products.
  
Table 1 The chemical composition of the aluminum alloy matrix at the cylinder hole (Mass fraction)

 
Table 2 The chemical composition of the cast iron cylinder liner (Mass fraction)

 
3. Analyses of the causes of failures and casting defects
Through the above analysis of the morphology, metallographic structure and composition, it can be seen that the aluminum alloy matrix of the engine cylinder block, microstructure and chemical composition of the cast iron cylinder liner all meet the technical requirements of the product. The outer surface of the cylinder block hole has a defect of a typical casting cold shut, and the crack section is a fatigue fracture. The fatigue crack originates from the bottom of the cold shut, which has multiple sources, and expands to the cylinder hole. There are no macroscopic casting defects such as gas holes and shrinkage porosity in the fracture. The micro morphology of the fatigue crack source and the crack propagation zone is characterized by extrusion wear, and the micro morphology of the fracture is quasi-cleavage. In summary, under the action of complex alternating loads during the test, fatigue cracks occur at the stress concentration at the bottom of the cold shut due to the presence of cold shut on the outer surface of the cylinder hole, which gradually propagates and leads to failures.
 
After checking the production process of die-cast trial production of cylinder block castings, it was found that the temperature at the positioning core of the die casting mold cylinder liner is not stable and low in the die casting cycle, resulting in insufficient water evaporation after spraying the release agent and water droplets remaining in the water channel mold core. As a result, in the filling process of the next die casting mold, the fluidity of the partial aluminum liquid becomes worse, and the cold shut is formed near the small end cylinder hole of the die casting.
 
4. Improvement measures and effects
The following comprehensive measures are formulated concerning the low temperature in the manufacturing process of the cylinder liner positioning core, resulting in the cold shut at the small end of the die casting.
(1) Appropriately reduce the cooling flow of the cooling circuit of the cylinder liner positioning core, increase the mold temperature in the casting process of the cylinder liner positioning core, and promote partial moisture evaporation.
(2) Reduce the spray volume of the release machine at the position of the cylinder liner positioning core. Reduce the corresponding nozzle's flow rate of the release agent sprayer. Adjust the spray angle.
(3) Appropriately increase the pouring temperature of molten aluminum to reduce the overall trend of the cold shut of die castings.
(4) Specify the appearance inspection standards of die castings; strengthen the training of operators for appearance inspection, and prevent defective parts from flowing into the subsequent processing procedures.
 
The trial production continued to produce more than 10,000 pieces based on the above comprehensive measures, and the incidence of the cold shut of die castings has been reduced to 0.05%, and this problem has been solved.