Abstract: A certain automobile engine experienced abnormal noise at cold start during cold-resistant test. Cause was analyzed to be due to insufficient mechanical properties of cylinder block and excessive air leakage in cylinder block. By adding local extrusion technology to bearing hole and selecting an appropriate extrusion process; by optimizing die-casting process parameters, improving mold pouring system, adjusting spraying position, etc., mechanical properties of castings are up to standard, and air leakage rate of oil mark hole is reduced from 32 % is reduced to 0.4%, making engine air leakage lower than limit value, effectively solving problem of abnormal noise at cold engine startup.
In recent years, as process of lightweighting automobiles accelerates, aluminum alloys have replaced cast iron as preferred material for producing automobile engine cylinder blocks due to their low density, high strength, and good plasticity. Among them, pressure casting is the most common aluminum alloy casting process due to its high production efficiency, high dimensional accuracy of castings, and low surface roughness value. However, pressure casting also has problems such as short filling time and insufficient exhaust, resulting in pores in casting, thin-walled castings are prone to hot spots and cracks when melt solidifies, and thick-walled castings are prone to shrinkage cavities and shrinkage porosity. Research shows that shrinkage cavities, shrinkage porosity, and crack defects in die castings can easily cause air leakage in castings and affect mechanical properties of castings. In order to eliminate shrinkage cavities and shrinkage porosity defects, local pressurization processes are widely used in actual production. For crack defects, methods such as optimizing mold cooling system, adjusting mold retention time, and controlling melting process are generally used to improve or eliminate them. After a certain automobile engine was subjected to an extreme cold test of 30,000 kilometers, a problem of loud starting noise occurred during cold engine period. Investigation found that air leakage rate of engine in hydraulic pressure test was 32%, and amount of air leakage exceeded air leakage limit by 36.2%. By taking samples from castings and conducting tensile tests, tensile strength and yield strength of castings were measured to be 152.8 MPa and 104 MPa, which are both far lower than standard values: ≥200 MPa and ≥140 MPa. Therefore, it was determined that mechanical properties of engine cylinder block were insufficient, causing air leakage to exceed standard, which caused abnormal noise problem when engine was cold started.

Casting is a four-cylinder automobile engine block with a blank weight of 9.7 kg. It is die-cast using a Bühler 2800T cold chamber die-casting machine. Aluminum alloy grade is YZAlSi9Cu3. Alloy composition is shown in Table 1. Average wall thickness of casting is 15 mm, and maximum wall thickness is 50 mm. There are many lubricating oil channels, cooling water channels, oil scale channels and mounting threaded holes inside. Pins have a high temperature due to difficulty in cooling. In actual die-casting process, it is difficult to avoid defects such as shrinkage cavities, shrinkage porosity and cracks in thick wall of casting and near slender pins. In addition, because cylinder block bearing hole needs to withstand impact vibration caused by inertial force and inertial moment of piston's reciprocating motion, working conditions are harsh and requires high structural strength (requirements: tensile strength ≥ 200 MPa, yield strength ≥ 140 MPa) , and it is difficult to ensure that castings have such high strength using conventional die-casting processes. Since bearing hole is main force-bearing point of engine, there are many casting holes distributed nearby, structure is complex, and wall thickness is large, so bearing hole area of cylinder block is selected as a tie rod for mechanical experiments. Specific location is shown in Figure 1. By conducting tensile strength and yield strength tests on tie rods, results are shown in Table 2. Mechanical properties of cylinder block casting are insufficient. Observing fracture section of tie rod, section contains slag inclusions, grain size is 7.5, and section structure is loose, which is consistent with result of insufficient mechanical properties of casting.

Table 1 Chemical composition of YZAlSi9Cu3 aluminum alloy WB/%

 

Figure 1 Selecting position of experimental tie rod

Table 2 Mechanical property requirements of castings
Through water pressure test, it was found that cylinder oil mark pore pressure detected air leakage. When air leakage location was cut open, it was found that there were shrinkage cavities and shrinkage porosity of varying degrees near engine oil mark hole (Figure 2). Leakage location is thick wall of casting. During solidification process of aluminum liquid, because it is far away from surface of mold core and temperature is high, surrounding molten metal has completely solidified, and an isolated liquid phase area is formed in the center of thick wall, which cannot be fed during casting pressurization stage, thus forming shrinkage holes in casting. This is main reason for air leakage in cylinder block and unstable mechanical properties of casting. There are four main reasons for investigating unstable mechanical properties of castings: First, design of die-casting process is unreasonable, especially main control parameters such as high-speed speed, high-speed starting point, boost pressure and mold retention time; Second, design of mold pouring system is unreasonable, mold cooling system is abnormal, and release agent is improperly sprayed; third, YZAlSi9Cu3 alloy composition is out of tolerance, aluminum ingot smelting process is abnormal, impurities such as oxides and piston lubricating oil combustion materials are included in die-casting process; fourth, wall thickness of cylinder block casting is relatively large, shrinkage cavities and shrinkage porosity defects are prone to occur during die-casting process.

 

Figure 2 Air leakage location in hydraulic pressure test of castings

Partial extrusion mechanism is shown in Figure 3. It mainly consists of a working cylinder, an extrusion pin, an extrusion insert and other ancillary components. Extrusion mechanism is generally designed on mold frame or core of mold according to actual situation. In traditional local extrusion technology, due to unadjustable extrusion speed, extrusion action only exists at a certain moment, and cannot continue to pressurize throughout solidification stage of aluminum liquid, let alone adjust pressurization time period, making extrusion timing inappropriate. If extrusion is too early, extrusion pin becomes a fixed pin and cannot play a feeding role; if extrusion is too late, aluminum liquid has solidified, extrusion pin is subject to too much resistance and is easily broken. Therefore, traditional extrusion technology has little effect on eliminating or reducing shrinkage cavities in castings.

 

Figure 3 Schematic diagram of extrusion pin structure
At present, most companies adopt new extrusion technology. It adds a built-in oil cylinder to die-casting mold to link control signal of working oil cylinder with injection signal of die-casting machine. Extrusion time, extrusion delay, holding pressure and holding time can be adjusted on control panel of die-casting machine. Squeezing and pulling action of squeezing pin can make squeezing timing more appropriate. During solidification process of casting, squeeze pin exerts pressure on semi-solidified liquid phase, changes feeding sequence of aluminum liquid, and has a good feeding effect on central area in wall thickness direction of casting, which can effectively eliminate shrinkage cavities in casting, improve compactness of casting structure, and enhance mechanical properties of casting.

Combining previous experience in casting engine cylinder blocks and benchmarking EA211 cylinder block data, when extrusion pin solution is used near cylinder block bearing seat, casting structure is denser, there are no shrinkage holes at tie rod fracture, and mechanical properties are significantly improved. Since air leakage point of this engine is located near oil mark hole, bearing holes of the first and second cylinders, and each bearing hole is designed with a lubricating oil hole, which also has risk of air and oil leakage. Therefore, local pressurization technology is used on each bearing hole. Pressurized casting blank is shown in Figure 4. However, choosing appropriate extrusion process is crucial to quality of castings. In order to quickly obtain optimal extrusion process, orthogonal experiments were used to select three key parameters of extrusion process: extrusion pressure, extrusion delay and holding time as test factors. Three levels are selected for each factor, and standard orthogonal test L9 (3³) table is used. Test design is shown in Table 3. Test objective functions are tensile strength, yield strength and shrinkage cavity yield. Each group of tests consists of 5 die-casting pieces. Take 1 tie rod sample from bearing holes of the 1st to 3rd cylinders, and take 1 slice from bearing holes of 4th to 5th cylinders. In this way, each group of experiments consists of 15 experimental tie rods and 10 slices, average value of each group of experiments is taken as test result of group. Among them, shrinkage cavity yield takes into account X-ray inspection data.

 

Figure 4 Casting blank using local extrusion technology

Table 3 Orthogonal test plan table
Table 3 shows results of orthogonal test. Ranges of factor A (extrusion pressure), factor B (extrusion delay) and factor C (pressure holding time) are 8.7/9.0, 11.0/12.0 and 17/15 respectively. Factor C has the greatest influence on test results and is main factor causing unstable mechanical properties; factor A's range is 8.7/9.0, ranking second and the second most important factor; factor B has the smallest range and is a secondary factor. According to range analysis method, order of factors affecting casting qualification rate is determined to be C (holding time), A (extrusion pressure) and B (extrusion delay). In order to obtain optimal extrusion process plan, it is necessary to further determine levels of each factor based on objective function value. It can be seen from K value in Table 3 that tensile strength and yield strength show a similar change pattern, so only tensile strength is considered here. Maximum level 1 of factor A (extrusion pressure) is 227, followed by level 2, and level 3 is the worst; level of factor B (extrusion delay) is 123 from high to low, and level of factor C (holding time) is also 123. Therefore, it can be preliminarily determined that optimal process parameter combination is A1B1C1, that is, extrusion pressure is 140 bar, extrusion delay is 3 s, and pressure holding time is 10 s. In addition, shrinkage cavity yield rate of this group of extrusion casting castings is also very high.
Figure 5 is a slice of a casting using local extrusion process. Comparing Figure 2, it is found that shrinkage cavity problem near oil mark hole of casting has been significantly improved. Test data and engineering experience in Table 3 show that by improving peripheral equipment of die-casting machine and adjusting extrusion process, not only can shrinkage cavities and shrinkage porosity problems of castings be effectively eliminated, but mechanical properties of castings can also be improved. However, in actual production, issues such as installation position of extrusion pin, excessive extrusion pressure causing deformation or cracks in casting, and excessive hydraulic pressure causing oil pipes or joints to leak should be considered.

 

Figure 5 Casting slices using local extrusion process
Theoretical research shows that affected by filling sequence of aluminum liquid, mold temperature presents a gradient distribution of upper, lower, and higher, causing aluminum liquid to solidify sequentially from surface to inside, from high mold temperature to low mold temperature. If final solidification area does not receive feeding from aluminum liquid, shrinkage cavities will be formed due to insufficient feeding. Therefore, excluding abnormal factors, main cause of shrinkage cavities in aluminum alloy die castings is insufficient feeding of molten aluminum. However, solidification sequence of molten aluminum is affected by many factors, such as: casting structure, pouring system, exhaust system, cooling system and process parameters. Based on actual engineering experience, three main solutions were adopted to further improve shrinkage cavity problem of castings, namely adjusting die-casting process, optimizing mold gating system and adjusting spraying process.
Adjust process parameters. Aluminum liquid entering mold cavity from barrel generally goes through three stages: slow speed, high speed and pressurization. In addition, starting point of high speed is also an important parameter. Theoretically, high speed switching point should be located near arrival of aluminum liquid at inner runner, which can ensure that aluminum liquid can better fill cavity.
Studies have shown that if low speed is too high, aluminum liquid will oscillate during acceleration, forming gas entrainment, and castings will easily form gas shrinkage holes; if low speed is too low, temperature of aluminum liquid will drop quickly before filling, and castings will easily form a chilled layer. It was found through experiments that low speed was set to 0.28 m/s, starting point of high speed was 710 mm, high speed was 6.2 m/s, boost pressure was 1 050 bar, and boost pressure conversion adjustment was changed from speed conversion to pressure conversion, casting quality is better. Optimize gating system. Insufficient feeding of aluminum liquid is not only related to mold temperature distribution, but also related to filling direction and flow rate of aluminum liquid. Investigation found that in order to improve position tolerance of bearing hole, sprue of bearing hole was canceled. This may be part of reason for shrinkage cavity in casting, so sprue of bearing hole was restored. At the same time, thickness of gate is reduced to 5.5 mm, so that the total area of gate is approximately 1 800 mm 2 . Using a Φ150 mm injection piston, ratio of piston to gate area is approximately 9.81 (previously it was 6.9. Even if die casting machine reaches limit speed of 7.2 m/s at high speed, gate speed is only 49 m/s). Set speed is 6 m/s, and gate speed can reach 60 m/s, which increases high-speed filling capacity of molten aluminum. Through solidification simulation analysis, it was found that thick-walled area near oil marking hole has been well fed due to addition of a branch runner and increase in inner runner speed, and shrinkage cavity problem has been basically eliminated.
Adjust spraying process. Reasonable spraying time and spraying position can effectively maintain mold temperature and prevent cold shut, shrinkage cavities or crack defects in castings. Leakage location of this casting is close to oil mark hole. Pin is slender and temperature is high. An external water cooling device is added. Thermal imager was used to measure mold temperature after spraying to be 209℃, which is normal.

After adopting local extrusion process, mechanical properties of castings are significantly improved, tensile strength and yield strength are both up to standard, structure is denser, shrinkage cavities and shrinkage porosity defects are significantly reduced. However, as shown in Figure 5, shrinkage cavities and shrinkage porosity defects still exist in thick wall of casting. In order to completely eliminate hidden danger of cylinder block air leakage, die-casting process needs to be further improved.

By adding partial extrusion technology for bearing seat, adjusting die-casting process parameters, optimizing mold pouring plan, and improving spraying process, 500 pieces were die-cast verified, and all were air-tightly inspected. As a result, 2 pieces exceeded standard for air leakage, and air leakage rate was 0.4%; 100 pieces were sampled for tensile testing. Average tensile strength was 248.68 MPa, average yield strength was 182.83 MPa, and pass rate was 100%. This solved problems of air leakage and noise caused by unstable mechanical properties of castings in engine cylinder block.

(1) Automobile engine cylinder block casting has a large wall thickness and a complex structure. Defects such as shrinkage cavities, shrinkage porosity, and cracks are prone to occur during die-casting process, which affects mechanical properties of product. After using local pressurization technology, tensile strength and yield strength of castings increased by 62.7% and 75.4% respectively. Therefore, use of local pressurization technology can effectively improve or eliminate shrinkage cavities and shrinkage porosity defects in castings, and significantly improve mechanical properties of castings.
(2) Reasonable die-casting process and mold design have a greater impact on the overall performance of castings. However, in actual die-casting process, selecting appropriate process parameters is a complex and time-consuming task. Orthogonal experiments can comprehensively consider various aspects. Influencing factors of process parameters shorten time to find optimal process, which is a scientific optimal process selection method.