Aiming at structural characteristics of aluminum alloy lower cylinder block of a new automobile engine, an olecranon-type feed and 4-zone exhaust pouring system was used in development of its die-casting mold, which solved problems of difficult part forming, severe air entrainment, and internal pores. Installation method of 5 cast iron inserts and preheating temperature of 230 ℃ are designed to solve problems of installation positioning and separation of inserts. Long core with feeding alignment adopts YXR33/W360 high-toughness material, Dura-AR surface plasma treatment and high-pressure super-spot cooling, which solves problem of easy punching and burn of core. Extrusion pins are designed in high-pressure oil passage hole, and cooling plates are designed in extrusion cylinder, which solves problem of shrinkage cavity and ensures continuous production.

Lower cylinder block is to separate lower part of crankshaft from upper part of original cast iron cylinder block. Upper part is still made of cast iron, and lower part is made of aluminum alloy, which can not only maintain advantages of cast iron cylinder block, but also reduce quality of cast iron cylinder block. Separated cylinder crankshaft part is called lower cylinder block. A new lower cylinder introduced in this article is shown in Figure 1. Casting outline size is 390 mm*350 mm*170 mm, average wall thickness is 7 mm, casting weight is 6.05 kg, and material is A380 alloy. Five cast iron inserts are placed at joint between lower cylinder block and crankshaft, and these five cast iron inserts are processed as bearing seats after subsequent processing. 5 cast iron inserts should be installed in mold before closing mold for each production, become part of lower cylinder together with aluminum alloy after die-casting, and no separation is allowed. The total weight of casting including cast iron inserts is 7.41 kg.
Wall thickness of casting varies greatly, maximum wall thickness is 22 mm, minimum wall thickness is only 2 mm, and temperature of mold is extremely unbalanced; 5 cast irons are placed in the middle, as shown in Figure 1a, it is easy to separate in the case of different materials and temperature differences; thickness of aluminum material at the thinnest part of the two side walls of 5 casting parts of lower cylinder is only 2mm, which affects flow filling and feeding capabilities of aluminum liquid, and the two sides are processing surfaces. In order to avoid air hole leakage after processing, machining allowance on both sides should be as small as possible, and machining allowance is designed to be at least 0.35 mm; filter installation surface of casting is shown in Figure 1b, and there are two high-pressure oil passage holes, so internal quality requirements are strict. Inspection standard: under pressure of 0.29MPa, leakage is less than 2 mL/min. Among them, high-pressure oil passage hole I is an oblique hole. Due to limitation of mold structure, hole is not pre-cast, and it is machined in the later stage. High-pressure oil passage hole II is pre-cast.
Due to integration of high-pressure oil passages, filter installation and other functions, this type of lower cylinder is difficult to die-cast, so it is necessary to design corresponding solutions for key points of die-casting production.

 

Figure 1: Aluminum Alloy Lower Cylinder
1. Cast iron insert 2. High pressure oil passage hole Ⅰ 3. Filter installation surface 4. High pressure oil passage hole Ⅱ

Internal quality requirements of filter surface of lower cylinder and side of high-pressure oil passage hole are strict, and should be guaranteed first. Design adopts olecranon-style unilateral feeding. Olecranon-style feeding structure is shown in Figure 2a. It is a feeding method in which inner gate is olecranon-shaped and shoots to product wall. Since feeding angle is roughly consistent with shape of product, see Figure 2b, feeding is smoother, aluminum liquid is easier to rush to the bottom of product, which is conducive to discharge of gas, pressure transmission and feeding, ensures forming and internal quality of filter surface on moving mold side and high-pressure oil passage hole of product. In order to prevent product from collapsing into water outlet, an anti-collapse boss with a height of 2 mm and a shape 2 mm wider than inlet is designed at nozzle, as shown by arrow in Figure 3. According to above design, numerical simulation analysis of feeding is carried out, and analysis results are shown in Figure 4.

 

Figure 2: Olecranon feeding structure

 

Figure 3: Anti-collapse boss at feed inlet

 

Figure 4: Simulation of filling process on fixed mold side and moving mold side
From simulation results in Figure 4, it can be seen that feed inlet fills movable mold side of product first, then fills to water tail, the overall filling is relatively smooth, and no obvious turbulent flow is found, which meets purpose of design.

According to feed simulation results in Figure 4, overflow tank is designed at the last filling position of aluminum material or at intersection position of aluminum material. Figure 5 shows structural characteristics of lower cylinder. It can be seen that areas A and C are thick and large parts, where most of aluminum material of product is concentrated. Area B is a thin-walled position. After cast iron insert is installed, the thinnest part of aluminum wall thickness is only 2 mm, forming 5 narrow and long passages. Exhausting in area A and filling in area C are very difficult and easy to generate entrained air. Design of overflow exhaust system is shown in Figure 6. Ends of four areas are each connected to a tooth-shaped chill exhaust block. Slag collection effect of tooth-shaped chilling exhaust block is good, and use of a vacuum machine can solve problem of insufficient fluidity caused by large thickness differences. In the case that gas in each area can be discharged smoothly, on the one hand, it is beneficial to reduce internal pores of product, and on the other hand, it can reduce negative pressure of mold, which is beneficial to filling of aluminum material. Simulation results of gating system are shown in Figure 7. It can be seen that filling and exhausting of various parts of product are relatively smooth, which meets design purpose.

 

Figure 5: Schematic diagram of distribution of aluminum material in lower cylinder

 

Figure 6: 4 Zone Exhaust

 

Figure 7: Simulation process of gating system

5 cast iron inserts need to be accurately installed and repeatedly positioned on mold. In order to avoid falling, insert is placed on stationary side of fixed mold, see Figure 8a, and through hole on insert is used for positioning. In automatic production mode, robot is used to place 5 inserts in mold cavity at one time. Due to low accuracy of placement position, gap between positioning core on fixed mold side and through hole of insert needs to be appropriately increased. Through-hole gap should not only ensure that inlay can be placed smoothly, but also avoid loosening and falling due to excessive gap. After comprehensive consideration, designed fit clearance is 0.17 mm on one side, front end of positioning core is designed with arc and cone guides. Length of positioning section of positioning core on movable mold side is designed to be 2 mm, matching gap with through hole of cast iron insert is designed to be 0.025 mm on one side, and front end is designed to be inclined. When mold is closed, positioning core on the side of movable mold is inserted into through hole of cast iron insert to realize precise positioning of cast iron insert. The two ends of cast iron insert are respectively matched with movable mold and fixed mold to prevent aluminum material from entering through hole. Schematic diagram of installation structure is shown in Figure 8b.

 

Figure 8: Schematic diagram of installation of cast iron inserts
1. Cast iron insert 2. Positioning core on movable mold side 3. Positioning core on fixed mold side 4. End face of insert fits with movable die 5. End face of insert fits with fixed die
5 cast iron inserts cannot be separated from aluminum alloy. Temperature of mold insert after spraying is between 150℃ and 200℃, cast iron insert is heated to 230℃ in advance by using an electric heating box. After automatic installation of manipulators, mold clamping and other processes, temperature of cast iron inserts is basically same as that of mold during injection, which solves problem of separation of cast iron inserts and aluminum alloys due to temperature differences, and improves fluidity of aluminum liquid in die-casting process.

High-pressure oil channel hole I is an inclined hole, die-casting does not pre-cast hole, and hole is processed in the later stage. Wall thickness of casting here is more than 22 mm, which is prone to shrinkage cavity. In order to eliminate defects to the greatest extent, an extrusion structure is designed on fixed die side of high-pressure oil channel hole I. Designed extrusion pin diameter is Φ12 mm, extrusion stroke is 20 mm, and extrusion method is surface extrusion. This form of extrusion is to pressurize molding surface of casting, and pressurized part of casting is higher than actual height by a certain distance, so as not to squeeze cold material on the surface of casting into interior of casting. After extrusion casting produces a ring, see Figure 9, which is removed by post-processing. Designed extrusion cylinder is a square cylinder with a cylinder diameter of Φ80mm. Because oil cylinder is installed in fixed mold cover plate, installation space is closed, which is not conducive to heat dissipation and cooling. Cylinder sealing rings are prone to failure due to high temperature, affecting continuity of production. A water cooling plate is designed on the front mounting surface of oil cylinder to reduce temperature. Extrusion structure is shown in Figure 10.

 

Figure 9: Torus produced by extrusion

 

Figure 10: Schematic diagram of extrusion structure
1. Extrusion pin 2. Extrusion pin sleeve 3. Oil cylinder cooling plate 4. Oil cylinder 5. Design of high-pressure oil channel hole II core
Since horizontal direction is facing material inlet, core of high-pressure oil passage hole II is easily deformed and broken by high-speed impact of aluminum liquid. Due to high temperature of aluminum liquid, casting is prone to defects such as buckle damage and burns. Core design is made of YXR33/W360 high-toughness material, surface is treated with Dura-AR plasma, hardness (HRC) after heat treatment is 52-54, and hardness (HV) after surface treatment is 3600, which improves impact resistance of the core. A Φ6mm super-spot cooling hole is designed inside core (see Figure 11), and a stainless steel spot cooling tube with an inner diameter of only Φ3mm is installed for super-spot cooling (see Figure 12). Add high-pressure cooling equipment, and internal high-pressure cooling water pressure of core reaches 1.5 MPa. In order to enhance cooling effect and avoid being affected by other super-spot cooling, an independent water seat is designed, which is separately connected to super-spot cooling of high-pressure oil channel hole II core.

 

Figure 11: High-pressure oil channel hole II core

 

Figure 12: High pressure super point cooling

The total projection area of casting is 193 515 mm2, it is produced by Ube 16 500 kN die-casting machine, and diameter of injection punch is Φ130 mm. According to designed mold parameters and characteristics of parts, process parameters of die-casting trial production were formulated, as shown in Table 1.

Table 1: Die-casting trial production process parameter table
In trial production of die-casting, it was found that there were cold shut and black defects at water tail part. Mold temperature here was only about 120℃ after measurement, water-cooling flow rate of slider was reduced by 2/3 in this part for reproduction, and defects were eliminated. Both machined surface and internal quality of product meet technical requirements.

According to structural characteristics of lower cylinder body, gating system with olecranon-type feeding and regional exhaust vacuuming is designed, which effectively solves problems of difficult forming, internal air entrainment, shrinkage cavity and shrinkage porosity of lower cylinder casting; Reasonable installation and positioning of inserts ensures continuous and normal production of mold; preheating temperature of cast iron inserts is designed to be 230℃, which solves separation problem between cast iron inserts and aluminum alloy; design of extrusion pin solves shrinkage problem of unprocessed oil passage hole; designed core is made of YXR33/W360 high-toughness material, surface is treated with Dura-AR plasma and high-pressure super spot cooling is added, which improves service life of core.