1. Original Process of Ductile Iron Gearbox Housing
Pouring System: The original pouring system of the gearbox housing combination process. The end face has a designed process allowance of 4 mm. The tapping temperature is 1580 – 1600 °C, the pouring temperature is 1370 – 1440 °C, the pouring negative pressure is -0.06 – -0.04 MPa, and the negative pressure holding time is 900 s. The chemical composition of the gearbox housing is shown in Table 1.
Pouring System
Value
End Face Process Allowance
4 mm
Tapping Temperature
1580 – 1600 °C
Pouring Temperature
1370 – 1440 °C
Pouring Negative Pressure
-0.06 – -0.04 MP
Negative Pressure Holding Time
900 s
Chemical Composition (mass fraction, %)
Value
C
3.5 – 4.0
Si
2.0 – 3.0
Mn
0.45
P
0.05
S
0.025
Mg
0.02 – 0.06
RE
0.015 – 0.04
Defects: Y-type sample mechanical properties meet the requirements of QT450 – 10, and the spheroidization rate of 2 – 3 levels also meets the technical requirements of ductile iron gearbox housing. However, casting production of the gearbox housing by lost foam casting has defects such as wrinkling, shrinkage porosity, and shrinkage holes. In the small batch production process, a large proportion of the gearbox housing end face has wrinkling and shrinkage holes in geometric hot spots.
2. Formation Reasons of Wrinkling and Shrinkage Holes in Gearbox Housing
2.1 Wrinkling Formation Reason
Location and Appearance: Wrinkling defects often occur at the part where the molten metal finally flows or the “cold end” of the liquid flow, usually at the top, vertical side wall, or dead corner of the casting. The surface of the casting after cleaning shows wrinkling like an orange peel.
Cause in Lost Foam Casting: The copolymer foam pattern used for forming in lost foam casting decomposes under heat, producing a large amount of gaseous, liquid, and solid pyrolysis products. The state, composition, and discharge form of these pyrolysis products change during transmission, which is the fundamental reason for the diffuse carbon defects in lost foam casting.
Specific to Gearbox Housing: The original process of the gearbox housing is top pouring, which is actually a middle – bottom pouring process using the cavity as the runner. The casting is a rotary body with a thickness of 35 mm at the upper end and a sharp decrease in thickness below 50 mm to 13 mm. The original inner gate is designed on the top surface. During pouring, the high – temperature molten iron enters the top surface of the white pattern from the inner gate, not filling the mold gradually from top to bottom. Instead, it directly penetrates the thick – walled area at the top, uses the top of the cavity as the runner, and then fans down to fill the mold when reaching the thin – walled area. After reaching the bottom, it fills upward again, resulting in a turbulent filling mode of middle – bottom pouring. This causes the thick and large dead corner areas at the top and side to form “cold ends”, thus forming wrinkling defects.
2.2 Shrinkage Hole Formation Reason
General Reason for Shrinkage Holes in Ductile Iron Castings: The fundamental reason for the formation of shrinkage holes in ductile iron castings is that during the liquid shrinkage and solidification of the alloy, a certain part of the casting (usually the hot spot that solidifies last) cannot be supplemented with liquid metal in time, resulting in irregularly shaped holes with rough hole walls at that location.
Analysis of Gearbox Housing: Since the mechanical properties of the Y – type sample meet the requirements of QT450 – 10, the spheroidization rate of 2 – 3 levels also meets the technical requirements of the ductile iron gearbox housing, and the chemical composition of the gearbox housing meets the requirements. At the same time, from the surface quality observation of the ductile iron gearbox housing, there are no cold shuts or sand sticking problems, excluding the influence of spheroidizing treatment temperature, pouring temperature, and negative pressure on the shrinkage holes of the gearbox housing. The shrinkage hole defects of the gearbox housing mainly exist in the thick positions of the side processing holes, mainly caused by geometric hot spots. Therefore, to solve the shrinkage hole defects of the gearbox housing, it is necessary to start from the casting process, change the casting cooling structure, increase the heat dissipation surface area, reduce the local modulus, and eliminate the geometric hot spots of the casting.
3. Solutions and Verifications
3.1 Solution to Wrinkling
Problem Analysis: The wrinkling defect of the gearbox housing is mainly due to the unreasonable design of the pouring system, resulting in a middle – bottom pouring filling mode, causing turbulent flow of the molten iron during pouring and wrinkling defects on the casting surface.
Solution: Redesign the pouring system and change it to a bottom pouring system. The filling mode is pure bottom pouring. During the filling process, the high – temperature molten iron gradually fills the mold upward from the ground. Finally, the low – temperature molten iron at the front end and the incompletely gasified products of the white pattern stay at the processing allowance position on the top surface of the mold cavity, obtaining a casting with a sound surface.
Calculation and Design: Calculate the minimum cross – sectional area of the ingate:
The bottom injection average static pressure head height calculation (P=C): HP=HO-C/2=34cm. According to experience and reference data, design the inner gate. The new inner gate is a central pouring with four inner gates, each with a cross – sectional size of , and the total cross – sectional area of the four inner gates is 11.2-12.8cm^3. Design the length of the sprue as 480 mm according to the pressure head of 200 mm.
Verification: Produce 2000 pieces in batches according to the new casting process for verification. The results show that the surface quality of the castings produced by the new casting process is qualified, and no batch wrinkling defects occur on the surface.
3.2 Solution to Shrinkage Hole Defect
Traditional Solutions and Their Disadvantages: Traditional solutions to shrinkage hole defects include setting risers at hot spots to provide the required metal liquid compensation during casting formation to eliminate shrinkage holes, and using a chilling system, usually with chills to form an artificial end zone at the hot spot, changing the intermediate zone with almost no temperature difference into a chilled zone with a large temperature difference to form sequential solidification and eliminate shrinkage hole defects. However, these two methods are not suitable for lost foam casting of the gearbox housing. Adding risers will reduce the process yield of the gearbox housing casting and increase the overall process difficulty. Using the chill process, due to the characteristics of lost foam casting, the chill is easy to fall off during molding and may cause casting deformation, resulting in high process difficulty, having a large impact on the quality stability of the gearbox housing, and increasing the casting cost.
New Solution – Heat Dissipation Process: Develop a new lost foam casting process: the heat dissipation process. The core purpose of the heat dissipation process is to change the casting structure, increase the heat dissipation surface area, reduce the modulus at the hot spot of the casting, and the negative pressure gas takes away a large amount of heat during the solidification stage, achieving a chilling effect. Specifically, attach foam sheets (hereinafter referred to as “heat dissipation sheets”) at the hot spot positions of the casting, and then produce qualified castings through processes such as coating, drying, boxing, and pouring. During the pouring and solidification process of the lost foam casting iron process, negative pressure is continuously drawn. During the operation of the vacuum pump, cold air enters from the upper surface of the sand box, flows through the casting and the heat dissipation sheets to complete heat exchange and take away heat. The heat dissipation sheet has a large specific surface area, reducing the local modulus of the casting. The cold air takes away a large amount of heat, forming a microchannel flow heat exchange with the local molding sand in contact with the casting and the heat dissipation sheet, forming a chilled zone with a large temperature difference. The heat dissipation sheet acts as a chill, changing the casting from local solidification to a similar sequential solidification, eliminating casting shrinkage holes and shrinkage porosity defects.
Process Advantages: The heat dissipation sheet process has the advantages of simple and convenient operation, little impact on the casting process, almost no impact on the process yield of the gearbox housing casting, and simple post – processing of the gearbox housing.
Verification: The original process – produced gearbox housing casting has shrinkage hole defects in the bolt through holes. By analyzing the casting structure, it is found that the shrinkage hole position of the gearbox housing belongs to the geometric hot spot, with a large shrinkage hole tendency, which is consistent with the shrinkage hole defects in batch production. After comprehensive consideration, the heat dissipation process is used. During the blanking and bonding process, attach 12 heat dissipation sheets with a size of 50 mm × 30 mm × 7 mm at the hot spot area of the casting. The other process parameters remain unchanged. After trial production and machining verification, the bolt through holes of the gearbox housing are normal without quality problems. Batch production using the heat dissipation process, continuously producing 2000 gearbox housings, and no batch shrinkage hole defects occur at the bolt through holes after machining.
4. Conclusion
Wrinkling Defect Solution: By optimizing the pouring system, the filling process is stable, and the process allowance is used to collect the molten iron with impurities at the front end, solving the casting wrinkling defect.
Shrinkage Hole Defect Solution: For shrinkage holes caused by hot spots in lost foam casting of ductile iron castings, the traditional riser and chill processes have high difficulty. Therefore, a new process method, the heat dissipation sheet process, is designed. Setting a certain number of heat dissipation sheets at the hot spots of the casting can enhance the local heat dissipation speed, eliminate the geometric hot spots of the casting, and thus solve the casting shrinkage hole defect. The heat dissipation sheet process has passed the practical verification of the gearbox housing casting, proving that this process meets the design requirements and can solve the casting shrinkage hole defect. This is a new method for lost foam casting to solve shrinkage hole defects.
