With need for lightweighting, energy conservation and emission reduction in automobiles, core engine components such as cylinder blocks and lower cylinder heads are increasingly being made of aluminum alloys instead of cast iron. High-pressure casting is used to achieve mass production. However, structural design often results in areas of thicker wall thickness due to functional requirements. During die-casting process, uneven wall thickness and large differences in solidification shrinkage times in thicker areas can easily lead to shrinkage cavities and porosity, which can cause leakage.
Lower cylinder block studied in this project has external dimensions of 415 mm * 325 mm * 112 mm and weighs 6.1 kg. It is made of aluminum alloy ADC12, complying with JIS H5302-2006. ADC12 is an aluminum-silicon alloy commonly used for die-cast cylinder blocks and cylinder heads due to its excellent fluidity, thermal cracking resistance, and airtightness. Casting sealing performance requires that the entire cavity leak less than 15 cm³/min at a pressure of 19.6 kPa, and high-pressure oil passage leak less than 3 cm³/min at a pressure of 343.2 kPa.

 

Cylinder block is a key component of an automotive engine, integrating engine's crankshaft and connecting rod mechanism, as well as oil supply, lubrication, and cooling mechanisms. Its quality directly impacts engine performance. Lower cylinder block is lower component of a two-piece cylinder block. Its structure and shape are shown in Figure 1. Function of lower cylinder block dictates complexity of its casting structure. It connects to upper cylinder block from above and mounts oil pan from below. An oil filter mounting hole is designed on one side of oil pan mounting surface, which also serves as an oil filter bracket. Lower cylinder block has a box-shaped overall structure with uneven wall thickness. Main wall thickness is 3.5 mm. Five crossbeams, 20 to 22 mm wide, are located in the center for mounting crankshaft. Oil filter holes are located on the sides, with uneven wall thickness. High-pressure oil passages are located within lower cylinder block. Casting's overall structure is zoned, with numerous hollowed-out areas. Furthermore, severely uneven wall thickness makes die-casting development challenging.

 

Figure 1: Lower cylinder block shape and defect locations
During trial production of a new product, sealing tests after machining casting revealed a leakage rate as high as 30% at 343.2 kPa in high-pressure oil passage on the side of lower cylinder block where oil filter is mounted. Shape and dimensions of high-pressure oil passage are shown in Figure 1c. M20 mm * 1.5 mm threaded hole for oil filter and threaded bottom hole are machined, while Φ15 mm lateral hole is not machined. A cross-section of defective part revealed varying degrees of shrinkage and porosity in oil filter mounting hole area, with the largest shrinkage cavity being 5 mm x 2 mm. Observation of defective area revealed varying degrees of buildup in outer surface of irregularly shaped area between M20 mm hole and Φ54 mm diameter. This led to conclusion that high-pressure oil channel leakage was caused by severe shrinkage in casting. After machining, shrinkage cavity and oil channel hole intersected. Furthermore, buildup of buildup destroyed dense surface layer of casting, leading to leakage.

During solidification process of a casting, shrinkage cavities often appear in last solidified area of casting due to alloy's liquid and solidification shrinkage. These cavities are irregular in shape, have a rough surface, are dark in color, vary in size. Some shrinkage cavities exist independently, while others are multiple, small, and dispersed. To analyze causes of shrinkage defects in aluminum alloy die-castings, a fishbone diagram was used to analyze defects from perspectives of die-casting alloy, die-casting mold, die-casting machine, die-casting process, and die-casting part structure, as shown in Figure 2.

 

Figure 2 Shrinkage Defect Fishbone Diagram

Based on aforementioned fishbone diagram for shrinkage defects, product structure and mold design were analyzed using UG design software and AnyCasting numerical simulation analysis software. Manufacturing process was analyzed using die-casting machine real-time control system and infrared imaging equipment. After item-by-item analysis and troubleshooting, cause of shrinkage in oil filter mounting hole area of lower cylinder block was determined to be as follows.
2.2.1 Excessive Wall Thickness in Local Castings
Cross-section of lower cylinder block at oil filter mounting hole is shown in Figure 1c. Wall thickness around high-pressure oil passage ranges from 8 to 22 mm, while main wall thickness is 3.5 mm. This area is excessively thick and uneven. After die casting, molten metal shrinks during solidification. Furthermore, this area is located at the end of filling process and is far from ingate, preventing effective pressure increase and shrinkage compensation, resulting in shrinkage defects.
AnyCasting software was used to numerically simulate solidification process of lower cylinder block. Results, shown in Figure 3a, show isolated liquid phase regions in illustrated area. These isolated liquid phase regions are highly likely to eventually form shrinkage cavities. Results of shrinkage cavity probability analysis based on retained melt modulus method are shown in Figures 3b and 3c. While other areas of casting are under pressure, scattered shrinkage cavities have little impact on casting. However, oil filter mounting hole contains a high-pressure oil passage, and shrinkage cavities within it directly affect sealing performance of casting. Numerical simulation results are consistent with theoretical analysis described above.

 

Figure 3: Numerical simulation of solidification
2.2.2 Localized mold temperature is too high
Observing defective area of casting revealed varying degrees of sticking on outer surface of irregularly shaped area between M20 mm hole and Φ54 mm dimension. This localized sticking is a sign of excessive mold temperature. Mold temperature monitoring using an infrared imager, as shown in Figure 4, revealed that mold temperature in oil filter mounting hole area was significantly higher than that of other forming surfaces. Due to structure of lower cylinder block, mold is distributed with a large number of precast cores, which affects layout of mold cooling channels. Furthermore, defective area of casting is a localized protrusion in movable die insert. During die-casting process, it was surrounded by a large amount of molten metal. During solidification, heat from casting could not be quickly dissipated through mold, resulting in excessively high local mold temperature. This is also cause of shrinkage cavities and sticking.

 

1. Casting 2. Extrusion sleeve 3. Extrusion rod 4. Moving mold insert 5. Water distribution plate 6. Cylinder coupling 7. Extrusion cylinder 8. Moving mold sleeve
Figure 4 Mold Temperature

Based on above analysis, following two measures were implemented for die-casting mold to address shrinkage porosity in oil filter mounting hole area: ① Without changing product's structural shape, M20 mm oil filter mounting hole was replaced with a local extrusion mechanism instead of a pre-cast core. When molten metal in mold cavity cools to point where liquidus and solidus coexist, a hydraulic cylinder is used to apply pressure to local area of casting to reduce shrinkage porosity in isolated liquid phase. ② Increase mold cooling in oil filter mounting hole area by using a water distribution plate to reposition local cooling water inlet and outlet. This addresses shrinkage porosity caused by excessive mold temperature due to poor local cooling. Specific mold optimization design scheme is shown in Figure 5.

 

Figure 5 Mold Optimization Design

Localized extrusion involves installing a hydraulic cylinder directly within mold to apply pressure to area where shrinkage porosity is occurring, suppressing shrinkage and producing high-quality die-castings. Localized extrusion structure of lower cylinder is shown in Figure 6. Extrusion sleeve is fixed to movable mold insert, extrusion cylinder is fixed to rear end of movable mold sleeve, and extrusion rod is connected to cylinder piston rod via a cylinder coupling. During extrusion, piston rod in rodless cavity of cylinder drives extrusion rod forward, squeezing molten metal in mold cavity and directly forming bottom hole of casting.
The key to design of localized extrusion mechanism is extrusion volume. If extrusion volume is too small, insufficient molten metal is squeezed in, and feeding effect cannot be achieved. If extrusion volume is too large, a large-diameter extrusion cylinder is required, which is not only wasteful but also impossible to install within limited space of mold. Extrusion volume is volume of molten metal that needs to be fed. Its size depends on localized forming volume of casting. Based on experience, feeding volume ratio for localized extrusion of aluminum alloys is generally 5% to 10% (reserved feeding volume/localized casting volume). Calculation shows that localized volume at oil filter mounting hole in lower cylinder is approximately 34 cm³. Designed reserved feeding volume V = 34 * 7% = 2.38 cm³. Based on casting structure, extrusion rod diameter d is designed to be 16 mm. Extrusion rod structure diagram is shown in Figure 6a. Extrusion stroke L is calculated based on extrusion volume as follows:

 

Substituting the above data into formula (1), we obtain L = 12 mm. In order to improve metal fluidity and expand extrusion influence range during extrusion, front end of extrusion pin is designed to be a cone with a length of 6 mm and a slope of 20°. At initial extrusion position, extrusion rod extends into mold cavity by 10 mm. At the end of extrusion, extrusion rod extends into mold cavity by 22 mm, located in the middle of extruded part, as shown in Figure 6b. This can expand extrusion range and improve extrusion shrinkage compensation effect.
According to Pascal's principle, given extrusion rod diameter and extrusion pressure, diameter of extrusion cylinder can be calculated:

 

 

Figure 6: Local Extrusion Mechanism
Where: P extrusion is extrusion pressure, generally set at least three times casting pressure and 400 MPa in lower cylinder; F extrusion is extrusion rod area (mm²); P cylinder is die casting system pressure of 16 MPa; F cylinder is rodless cavity area of extrusion cylinder (mm²); d is extrusion rod diameter; and D is extrusion cylinder diameter. Substituting this into above formula, extrusion cylinder diameter D = 80 mm. Considering a safety factor during design, final extrusion cylinder diameter is 1.25D = 100 mm.

Die-casting mold temperature is a key factor affecting die-casting quality. To ensure mold temperature remains within a reasonable operating range during continuous production, water cooling is often used to cool mold cavity. By providing cooling water channels within mold cavity, cooling water circulates to remove significant heat generated by die-cast alloy within mold, resulting in low cost and high efficiency. Cooling water channels are designed to be located in the highest mold temperature and heat-concentrated areas within mold cavity. This heat concentration is evident in the area surrounding M20 mm oil filter mounting hole in lower cylinder dynamic mold cavity. Local mold geometry is shown in Figure 7a. Due to numerous core holes and push rod holes surrounding M20 mm hole, a series cooling channel with horizontal circulation is not feasible. Therefore, vertical nozzle-based independent cooling is employed for cavity cooling. Referring to mold temperature graph in Figure 4, 10 nozzles were added to corresponding high-temperature areas, as shown in Figure 7b, to cool localized high-temperature areas. Based on local dynamic mold cavity geometry, water channel holes were sized to have a diameter of 10 mm and a depth of 8 mm from front forming area. Nozzles were copper tubes with an outer diameter of 6 mm and an inner diameter of 4 mm. Due to presence of an extrusion cylinder behind 10 newly added small nozzles, inlet and outlet water at rear end of nozzles could not be removed, making single-point independent nozzle cooling impossible. To address localized cooling water inflow and outflow issues, a "water distribution plate" structure was added, as shown in Figure 7c. A double-layer water channel was machined into water distribution plate. Copper nozzles were fixed to distribution plate, creating a series water channel for nozzles a1-4 and b1-6. Cooling water inlet and outlet holes for groups a and b were machined in locations that did not interfere with rear extrusion cylinder.

 

Figure 7 Cooling water channel design for water distribution plate structure

After local optimization of lower cylinder mold, mass production verification was conducted. Shrinkage defects in oil filter mounting hole area of casting blank were significantly reduced. X-ray nondestructive testing and cross-section inspection revealed no obvious shrinkage defects. Internal quality of optimized casting is shown in Figure 8. Optimized mold temperature was effectively controlled. After mold opening, heat concentration area of moving mold cavity was kept between 180℃ and 240℃, meeting operating temperature requirements for aluminum alloy die casting. Casting exhibited no noticeable sticking. Since internal and external quality of castings has been effectively improved, sealing test was carried out after product was processed, and leakage rate of high-pressure oil channel at oil filter installation location was reduced from 30% to less than 2%.

 

Figure 8 Internal quality of M20 mm hole area after optimization

(1) "Fishbone diagram" quality tool is an effective means to analyze shrinkage defects in castings.
(2) Use of local extrusion technology can effectively solve shrinkage and porosity defects and high-pressure oil channel leakage problems caused by excessive local wall thickness in aluminum alloy die castings.
(3) Sufficient cooling water channels need to be arranged in local thick and heat concentrated areas of casting. Water distribution plate structure can change inlet and outlet positions of cooling water channels to avoid interference with other structures in mold.