This article introduces structural characteristics and defect forms of an aluminum alloy lower cylinder die-casting part, and uses "fishbone diagram" to analyze local shrinkage defects of lower cylinder. Local extrusion technology is used to pressurize and shrink thick parts of casting, and at the same time, cavity cooling of defective parts is increased. "Water distribution plate" structure is used to change position of inlet and outlet of cooling water channel to avoid interference with extrusion cylinder. Through above measures, internal and external quality of lower cylinder has been effectively improved, and product qualification rate has been greatly improved.
With needs of automobile lightweighting, energy conservation and emission reduction, cylinder block and lower cylinder block, core parts of automobile engines, have gradually begun to use aluminum alloy instead of cast iron materials, mass production of products has been achieved through high-pressure casting. However, in structural design of products, local areas with large wall thickness often appear due to functional requirements. During die-casting process, due to uneven wall thickness of castings and large differences in solidification shrinkage time at thick wall areas, shrinkage cavities and shrinkage porosity defects are prone to occur, leading to leakage.
Outer contour dimensions of lower cylinder studied in this project are 415 mm * 325 mm * 112 mm, and weight is 6.1 kg. Material is aluminum alloy ADC12, and implementation standard is: JIS H 5302-2006. ADC12 is an aluminum-silicon alloy that is commonly used for die-cast cylinder blocks and cylinder heads because of its good fluidity, thermal crack resistance and good air tightness. Sealing performance of casting requires that leakage of the entire cavity is less than 15 cm³/min at a pressure of 19.6 kPa, and that leakage of high-pressure oil passage is less than 3 cm³/min at a pressure of 343.2 kPa.

Cylinder block is main component of automobile engine. Cylinder block connects engine crankshaft connecting rod mechanism and oil supply, lubrication, cooling and other mechanisms into a whole. Its quality directly affects performance of engine. Lower cylinder block is lower part of two-piece cylinder block. Its product structure and shape are shown in Figure 1. Since function of lower cylinder determines complexity of its casting structure, upper part is connected to upper cylinder, oil pan is installed below, and oil filter mounting hole is designed on one side of oil pan plane to realize function of oil filter bracket. The overall structure of lower cylinder is box-shaped with uneven wall thickness. Main wall thickness is 3.5 mm, and there are five cross beams with a width of 20~22 mm in the middle for installing crankshaft. Oil filter hole area is installed sideways. Wall thickness is uneven, and a high-pressure oil passage is arranged inside. The overall structure of casting is distributed regionally, with many hollow parts in the middle. At the same time, wall thickness is seriously uneven, which makes development of die castings more difficult.

 

Figure 1 Lower cylinder shape and defect location
During trial production of new products, sealing tests were conducted after casting processing, it was found that high-pressure oil passage where oil filter is installed on the side of lower cylinder has a leakage rate of up to 30% under a pressure of 343.2 kPa. Shape and size of high-pressure oil passage are shown in Figure 1c. Oil filter mounting threaded hole M20 mm * 1.5 mm and threaded bottom hole are machined holes, and lateral Φ15 mm hole is a non-machined hole.   Defective part of leaking part was cut and inspected, it was found that there were shrinkage holes and shrinkage porosity of varying degrees in oil filter mounting hole area, with maximum shrinkage hole being 5 mm * 2 mm. Observing appearance of defective part, it was found that in special-shaped area between M20 mm hole and Φ54 mm size, there were varying degrees of stickiness on outer surface. From this, it can be determined that cause of leakage of high-pressure oil channel is that there are severe shrinkage holes in parts of casting. After processing, shrinkage holes penetrate oil channel hole. At the same time, dense layer of surface layer is destroyed due to sticky material on the surface of casting, resulting in leakage of high-pressure oil passage.

During solidification process of casting, due to liquid shrinkage and solidification shrinkage of alloy, holes often appear in final solidified parts of casting to become shrinkage cavities. Shape of shrinkage cavities is irregular, surface is not smooth, color is dark, size is different. There are independent shrinkage cavities, multiple small and scattered shrinkage cavities. Aiming at causes of shrinkage cavities in aluminum alloy die-casting parts, fishbone diagrams are used to analyze causes of defects from aspects such as die-casting alloys, die-casting molds, die-casting machines, die-casting processes, and die-casting part structures, as shown in Figure 2.

 

Figure 2 Fishbone diagram of shrinkage cavity defect

Based on fishbone diagram caused by above-mentioned shrinkage cavity defects, product structure and mold design were analyzed using UG design software and AnyCasting numerical simulation analysis software, manufacturing process was analyzed with the help of real-time control system of die-casting machine and infrared imager. After item-by-item analysis and investigation, it was finally determined that causes of shrinkage cavities in lower cylinder oil filter mounting hole area were as follows.

Cross-sectional shape of lower cylinder oil filter mounting hole is shown in Figure 1c. Wall thickness of casting around high-pressure oil passage is 8~22 mm, while main wall thickness of casting is 3.5 mm. Wall thickness in this area is too large and uneven. After die-casting, volume of molten metal shrinks during solidification process. At the same time, this area belongs to filling end and is far away from inner runner. It cannot achieve better pressurization and feeding, so shrinkage defects occur.
AnyCasting software was used to numerically simulate solidification process of lower cylinder. Results are shown in Figure 3a. An isolated liquid phase region appears in casting in the area shown in figure, and probability of eventually forming shrinkage cavities in isolated liquid phase region is very high. Results of shrinkage cavity probability analysis based on residual melt modulus method are shown in Figure 3b and c. When other parts of casting are under pressure, scattered shrinkage holes have little impact on casting. However, there is a high-pressure oil passage at oil filter mounting hole, and internal shrinkage holes directly affect sealing of casting. Numerical simulation results are consistent with above theoretical analysis.

 

Figure 3 Solidification numerical simulation

Observing appearance of defective part of casting, it was found that in special-shaped area between M20 mm hole and Φ54 mm size, there are varying degrees of stickiness on outer surface. Generation of local stickiness is a manifestation of excessive mold temperature. An infrared imager was used to monitor mold temperature. Results are shown in Figure 4. It was found that mold temperature in oil filter mounting hole area was significantly higher than other forming surfaces. Since part structure of lower cylinder determines distribution of a large number of pre-cast hole cores on mold, this will affect layout of cooling water channels of mold. At the same time, defective part of casting is a local bulge in movable mold insert. It is wrapped by a large amount of molten metal during die-casting process. During solidification process of casting, a large amount of heat cannot be quickly dissipated through mold, causing local temperature of mold to be too high, which is also cause of shrinkage cavities and material sticking.

 

Figure 4 Mold temperature

Based on analysis of above reasons, following two measures are taken to solve shrinkage cavity in oil filter mounting hole area of die-casting mold: ① Without changing structural shape of product, oil filter mounting hole M20 mm is changed from original pre-cast hole core to partial extrusion. When molten metal in cavity is cooled to coexistence of liquidus and solidus, casting is pressurized and compressed locally through hydraulic cylinder to reduce occurrence of shrinkage cavities in isolated liquid phase area; ② Increase cooling of mold in oil filter mounting hole area, use water distribution plate structure to change inlet and outlet positions of local cooling water, thereby solving problem of casting shrinkage holes caused by excessive mold temperature caused by poor local cooling. Specific optimized design plan of mold is shown in Figure 5.

 

Figure 5 Mold optimization design

Partial extrusion is to install an oil cylinder directly in mold to directly pressurize parts where shrinkage cavities occur to suppress shrinkage cavities to obtain high-quality die castings. Partial extrusion structure of lower cylinder is shown in Figure 6. Extrusion sleeve is fixed in movable die insert, extrusion cylinder is fixed at rear end of movable die sleeve plate, and extrusion rod is connected to piston rod of oil cylinder through the cylinder coupling head. During extrusion, oil inlet piston rod of rodless cavity of oil cylinder drives extrusion rod to move forward, squeezing metal liquid in cavity and directly forming bottom hole of casting.
Key to design of local extrusion mechanism is extrusion volume. If extrusion volume is too small, that is, molten metal squeezed in is insufficient and feeding effect cannot be achieved; if extrusion volume is too large, a large-diameter extrusion cylinder needs to be designed, which not only causes waste but also cannot be installed in limited space of mold. Extrusion volume is volume of molten metal that needs to be fed. Its size depends on local forming volume of casting. According to experience, feeding volume ratio of local extrusion of aluminum alloy is usually 5% to 10% (reserved feeding volume/volume of forming part of casting). It is calculated that local volume of lower cylinder oil filter mounting hole is about 34 cm³, and designed reserved feeding volume V=34*7%=2.38 cm³. Based on casting structure, extrusion rod diameter d=16 mm is designed. Extrusion rod structure diagram is shown in Figure 6a. Extrusion stroke L is calculated based on extrusion volume as follows:

 

Substitute above data into equation (1) to obtain L=12 mm. In order to make metal fluidity better and extrusion influence range larger during extrusion, front end of extrusion pin is designed to have a tapered surface with a length of 6 mm and an inclination of 20°. At initial extrusion position, extrusion rod extends 10 mm into cavity. At the end of extrusion, extrusion rod extends 22 mm into cavity and is in the middle of extruded part, as shown in Figure 6b. This can make extrusion range larger and extrusion feeding effect better.
According to Pascal's principle, when diameter of extrusion rod and extrusion pressure are known, diameter of extrusion cylinder can be calculated:

 

 

Figure 6 Partial extrusion mechanism
In formula: P is extrusion pressure, which is generally more than 3 times casting pressure. In lower cylinder, it is 400 MPa. F is extrusion rod area (mm㎡). P cylinder is die casting system pressure of 16 MPa. F Cylinder is rodless cavity area of extrusion cylinder (m㎡), d is diameter of extrusion rod, and D is diameter of extrusion cylinder. Substituting above formula into calculated extrusion cylinder diameter D = 80 mm. Safety factor is considered during design, and final diameter of extrusion cylinder is 1.25D=100 mm.

Die-casting mold temperature is one of important factors affecting quality of die-casting parts. In order to ensure that mold temperature remains within a reasonable operating temperature range during continuous production, water cooling is often used to cool mold cavity. By setting up a cooling water channel in mold cavity, a large amount of heat generated by die-cast alloy in mold is taken away through cooling water circulation, which results in low cost and high efficiency. When designing cooling water channel, it is required to be arranged in the area with the highest mold temperature and relatively concentrated heat in mold cavity. M20 mm oil filter mounting hole area in lower cylinder dynamic mold cavity is the area where heat is concentrated. Local shape of mold is shown in Figure 7a. Since there are a large number of core holes and push rod holes arranged around M20 mm hole, it is impossible to set up a horizontal circulation series cooling water channel, so vertical nozzle type independent cooling is used for cavity cooling. Referring to mold temperature picture in Figure 4, 10 nozzles are added corresponding to high-temperature areas, as shown in Figure 7b, to cool local high-temperature areas. According to local shape of moving mold cavity, diameter of water channel hole is determined to be 10 mm, and depth is 8 mm from the front end forming point. Nozzle uses a copper pipe with an outer diameter of 6 mm and an inner diameter of 4 mm. Since there are squeeze cylinders on the backs of 10 newly added small nozzles, inlet and outlet water at the rear end of small nozzles cannot be led out, and single-point independent nozzle cooling cannot be achieved. In order to solve problem of local cooling water inflow and outflow, a "water distribution plate" structure is added, as shown in Figure 7c, a double-layer water channel is processed in water distribution plate, and copper nozzle is fixed on water distribution plate to realize series water channel of nozzle a1-4 and series water channel of b1-6. Cooling water inlet and outlet holes of group A and group B are processed at a position where rear extrusion cylinder does not interfere.

 

Figure 7 Cooling water channel design of water distribution plate structure

After lower cylinder mold was partially optimized and designed, mass production verification was carried out. Shrinkage cavity defects of casting blank in oil filter mounting hole area were significantly improved. Product has been inspected by X-ray non-destructive inspection and cross-section, and there are no obvious shrinkage defects. Optimized internal quality of casting is shown in Figure 8. Optimized mold temperature is effectively controlled. After mold is opened, temperature of heat-concentrated part of moving mold cavity is controlled at 180~240℃, which meets working temperature of aluminum alloy die-casting. There is no obvious sticking phenomenon in the appearance of casting. Since internal and external quality of castings has been effectively improved and sealing tests were conducted after product processing, leakage rate of high-pressure oil passage where oil filter was installed was reduced from 30% to less than 2%.

 

Figure 8 Optimized internal quality of M20 mm hole area

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