Domestic new-generation CVT gearbox has good fuel economy, power, comfort and low cost, but complexity of integrated structure and diversification of functions have brought great challenges to manufacture of clutch housing. By applying and perfecting design of high-pressure point cooling core needle, cooling effect of hot joint can be effectively improved and service life of needle can be effectively improved; by improving shape design of blank and increasing diameter of extrusion pin, internal shrinkage porosity and shrinkage defects in thick and large positions can be effectively solved, internal quality and airtightness of product can be guaranteed.
Foreword: New generation of domestic CVT gearboxes can obtain the best match between transmission system and engine working conditions, but complex structure of gearbox, large changes in wall thickness, high requirements for internal quality, surface quality, machining dimensional accuracy, air tightness lead to difficulties in product manufacturing and high scrap rate. It is necessary to study and improve die-casting process.
Clutch housing produced by our company is made of ADC12 alloy, outline size is 545 mm*430 mm*230 mm, general wall thickness is 3.5 mm, and product quality is 10.47 kg. Peak annual output of this product is 200,000 pieces. Internal quality requirements are better than ASTM E 505 Class 2 standard. Appearance quality requires that diameter of pores after assembly and sealing position processing be less than 0.4 mm, interval is greater than 8 mm, and number is less than 3; appearance quality requires no defects such as casting spots, cold shuts, cracks, flashes, burrs, etc. on the surface of blank; air tightness requires that leakage of high-pressure oil passage is less than 3 mL/min under a pressure of 300 kPa.
Through design of die-casting process of clutch housing, cause analysis of die-casting defects and improvement of process, a solution to typical die-casting defects of similar products is proposed.

During die-casting, in order to ensure dimensional accuracy and internal quality of casting and prevent flying materials during die-casting process, parting surface must be locked. Therefore, the key to selection is calculation of clamping force of die-casting machine.
Casting total projected area:
A casting projected area = A product + A sprue + A overflow tank = 3 229 c㎡
F lock ≥ safety factor * injection specific pressure * A casting projected area = 1.1 * 75 * 3229 = 2 6640 kN
According to calculation results of clamping force and company's die-casting machine resources, IDRA's 27 000 kN die-casting machine was selected.

Ingate design should make filling distance of aluminum liquid as short as possible to reduce temperature drop of aluminum liquid; gate position should make aluminum liquid reach same distance from all parts of cavity, so as to realize simultaneous filling and solidification of remote parts; It should be installed at thick wall of die casting to realize sequential filling and pressure feeding. According to structural characteristics of product, interior of the two large bearing holes and interior of high-pressure oil passage have high quality requirements. According to needs of flow direction of molten metal filling, gating system adopts a comb-shaped design, thickness of gate is 4 mm, it is located at parting line with a wall thickness of 5 mm, and it is set on fixed mold surface. Gating system design is shown in Figure 1, mold design is shown in Figures 2 and 3.

 

Figure 1 gating system design

 

Figure 2 fixed mold design

 

Figure 3 Movable mold design

In order to improve wear resistance of front bearing hole of gearbox output pulley shaft, an insert structure is designed for product. Insert material is ADC14 alloy with a wall thickness of 3 mm, as shown in Figure 4. Position of insert on mold is required to be accurate, stable and reliable. During die-casting process, molten aluminum and insert are tightly combined without gaps, and hole is used as a machining positioning hole. Inner diameter of insert is φ78.57±0.03 mm, diameter of movable mold positioning boss is φ78.45±0.01 mm, and matching gap is 0.04~0.08 mm on one side. Insert is heated to 200 ℃ in an oven, it is manually picked up and placed on the front of mold. Positioning of insert on mold is shown in Figure 5.

 

Figure 4 Insert

 

Figure 5 Positioning of dynamic mold inserts

Casting pressure is 75 MPa, filling speed is 50 m/s; the total cross-sectional area of ingate is 1 490 m㎡; diameter of punch is 160 mm, and cross-sectional area of punch=πD2/4=20 096m㎡;  Ingate and punch speed ratio = punch cross-sectional area / ingate total cross-sectional area = 13.49; Punch speed = ingate speed / ratio of ingate and punch speed = 3.7m/s; G ingate passing weight = G product weight + G slag bag weight = 13 500 g; filling time = ingate passing weight / (liquid aluminum density x ingate sectional area x filling speed) = 68 s;
High-speed switching position is generally set based on position where molten soup reaches gate. According to product quality and functional requirements, switching position is adjusted back and forth to select the best position, which has a great impact on product quality. When a defect occurs in a certain position of product, product can be improved by adjusting switching position to observe change of defect, as shown in Figure 6.
High-speed stroke = G gate passing mass/cross-sectional area of punch/AI liquid density=(13500/20096)/2.64*1000=254 mm;
Thickness of handle is 35mm;
High-speed switching point = effective length of pressure chamber - material handle thickness - high-speed stroke = 915-35-254 = 626mm;

 

Figure 6 High-speed stroke and switching point position

(1) Question 1:M10 threaded hole on mounting and sealing surface of product variable shell has a diameter of 7.5 mm at large end of blank hole and a depth of 24 mm. After processing is completed, under air pressure of 300 kPa and high-pressure oil passage, leakage occurs, and occurrence rate reaches 50%, which seriously affects sealing and use function of product. See Figure 7 for location of air leak.

 

Figure 7 Air leak location
Reason is that position of M10 threaded hole is locally thick, reaching 20 mm, which belongs to a hot joint. During die-casting process, a shrinkage cavity of about 5 mm in size is generated, resulting in leakage of M10 threaded hole and high-pressure oil passage. Defect anatomy and X-ray inspection are shown in Figure 8.

 

Figure 8 Defect anatomy and X-ray
Aiming at problem of shrinkage cavity caused by local thickness of M10 threaded hole of product, it is solved by using a high-pressure core point cooling needle. Diameter of core needle is 7.5 mm, diameter of internal cooling channel is 3.5 mm, and wall thickness of one side is 2 mm. See Figure 9. During actual use, it was found that tip of point cooling needle sticks to aluminum, and cooling effect is poor. Main reason is that original design of bottom of cooling channel is 6 mm away from needle head, which cannot cool quickly and effectively. Improvement is that bottom of cooling channel is 3 mm away from needle head, , as shown in Figure 10, needle is not easy to break, cooling effect is good, shrinkage cavity defect problem is completely solved, and defect rate is 0.

 

Fig.9 Design of super-point cold needle before improvement

 

Figure 10 Improved super-point cold needle design

(1) Question 2: High-pressure oil channel hole of product has a diameter of 8 mm at big end of rough hole and a depth of 87 mm. After machining is completed, there is black skin residue in the front M10 threaded hole, and there is no thread in 1/4 of threaded hole. Proportion of this batch is as high as 47%, which seriously affects assembly and sealing performance of product, resulting in batch scrapping. See Figure 11 for bad threads.

 

Figure 11 Threaded hole shape, size and defect location
Reason is that size of large end of core pinhole is required to be φ8±0.3 mm, and measured result is 7.98 mm, which is qualified; depth of core pinhole is required to be 87±1 mm, and measured result is 86.79 mm, which is qualified. However, core needle began to bend at a distance of 40.83 mm from the top; at a length of 29.69 mm, surface of core needle was slightly strained, and surface coating was peeled off; at a length of 16.27 mm, it was seriously stuck to aluminum, as shown in Figure 12.

 

Figure 12 Bend position of core needle
Insufficient cooling of high-pressure spot cooling core pins, high surface temperature, and easy sticking of molten metal to mold, resulting in severe sticking of aluminum, difficulty in demoulding, and damage to core pins; insufficient thermal hardness and thermal stability of core pins, poor ability to resist thermal fatigue and sudden failure. Superposition of these two factors makes core needle prone to bending deformation.
Aiming at bending of high-pressure spot-cooled core needle, process improvement measures are as follows: ①Original design of high-pressure spot-cooled core needle, diameter of inner hole is 2.5 mm, diameter of central tube is 2.3 mm, unilateral gap is only 0.1 mm, drainage is small, it is easy to be blocked, and cooling effect is affected, see Figure 13; after design improvement, diameter of inner hole is 3 mm, diameter of central tube is 2 mm, unilateral gap is 0.5 mm, and drainage is smooth, which can effectively ensure cooling effect. After implementation of improvement measures, surface of needle has no defects of sticking aluminum and strain, and surface quality is good.

 

Figure 13 High pressure spot cooling core needle
②Because core needle has a small diameter and is relatively long, it is easy to bend and deform, resulting in a service life of only 200 pieces, which seriously affects continuity of production, material and life of needle need to be improved. Material is changed from SKD61 to W360 alloy, hardness (HRC) is 52~56, surface is treated with cyanide to improve surface hardness and wear resistance of core needle. After improvement measures were implemented, needle did not bend, but it would break when it was approaching its service life. According to statistics, service life is managed as 9,000 pieces, which can ensure continuous and stable production.

(3) Question 3: M6 threaded hole next to bearing hole of product has a depth of 21 mm. After processing is completed, under pressure of 300 kPa and high-pressure lubricating oil channel, leakage occurs, and occurrence rate is 35%, which seriously affects sealing and use function of product. See Figure 14 for location of air leak.

 

Figure 14 M6 threaded hole and high-pressure lubricating oil channel leakage position
Shape of high-pressure lubricating oil passage is complex, oil passage with a diameter of φ6 mm cannot be core-pulled and needs to be processed, resulting in shrinkage porosity and shrinkage defects inside oil passage area with a wall thickness of 15 mm, while the shortest distance between M6 threaded hole and high-pressure lubricating oil passage is 15.4 mm, prone to leakage. Aiming at position where shrinkage is prone to leakage, extrusion process is adopted. Diameter of blank boss is 14 mm, extrusion pin is designed to be φ6 mm, extrusion pin sleeve is φ12 mm, and extrusion stroke is 15 mm. Shrinkage cavity defect has been improved, but due to local thickness and small diameter of extrusion pin, feeding capacity of transferred aluminum liquid cannot completely solve the defect, resulting in serial leakage. Design of extrusion pin before improvement is shown in Figure 15.

 

Figure 15 Extrusion pin design before improvement
By modifying blank design, diameter of boss is increased from φ14 mm to φ22 mm, extrusion pin is changed to φ14 mm, extrusion pin sleeve is φ20 mm, and extrusion stroke is 15 mm, as shown in Figure 16. After measures were implemented, internal quality of product was good and no leakage occurred.

 

Figure 16 Improved extrusion pin design

(1) Applying and optimizing high-pressure spot cooling core needle can effectively strengthen cooling of hot spot in product hole, improve compactness of blank surface, avoid shrinkage cavities and leakage defects.
(2) By improving design of high-pressure point-cooled core needles improving material and surface treatment process of needles, problem of needle bending can be effectively avoided, and service life of core needles is extended from original 200 pieces to 8,000 pieces, application works well.
(3) By improving shape design of blank and increasing size of extrusion pin, shrinkage and shrinkage defects of internal tissue at thick and large positions can be effectively solved, internal quality and airtightness of product can be guaranteed.