Aluminum alloy castings have good surface gloss, corrosion resistance, low density, high specific strength and other characteristics, and have been widely used in industry. Aluminum alloy cover plate castings are conventional parts of gas insulated switchgear (GIS), with large annual consumption and relatively simple shape. They are generally produced by metal mold low-pressure casting process. Cover plate is made of ZL101A aluminum alloy. Shrinkage holes, shrinkage and inclusions are prone to defects during production process of cover plate.
With development of computer simulation technology, numerical simulation can achieve purpose of optimizing process design, ensuring casting quality, shortening trial production cycle, reducing R&D costs, improving production efficiency and product competitiveness. Using numerical simulation technology, probability of defects can be reduced by optimizing casting process system and rationally configuring chiller and riser, thereby achieving purpose of improving casting quality.
In this study, filling and solidification simulation analysis of a metal mold low-pressure casting aluminum alloy cover plate process for a gas insulated switch (GIS) was carried out by AnyCasting software. Causes of casting defects in cover plate sealing groove were studied and improvement measures were given. Finally, production verification was carried out. For casting shrinkage cavity and shrinkage defects of ZL101A cover plate sealing groove in metal mold low-pressure casting, simulation analysis was used to assist cause analysis and process optimization of problem, thereby improving casting quality, aiming to provide a reference for solving similar problems.
Material of aluminum alloy casting cover plate part for GIS studied is ZL101A alloy (composition shown in Table 1), which needs to be subjected to T6 heat treatment. Part weight is 13.2 kg, and three-dimensional diagram of cover plate part is shown in Figure 1. It can be seen that basic shape of cover is a "hat" shape, outer ring is a flange, there are 12 through holes on flange, there is a "roundabout" shaped reinforcement rib structure on wall, front of flange is a processing surface, inner ring has a sealing groove, and outer ring has a glue injection groove. Basic size of cover part is φ520 mm*83 mm, inner diameter of flange is 386 mm, flange thickness is 26 mm, cover wall thickness is 15 mm, and thickness of reinforcement position on wall is 20 mm. Cover casting is produced by metal mold low-pressure casting process with upper and lower mold structures. Basic process layout is shown in Figure 2. "Hat" is placed upside down, and sprue goes straight to top position of "hat", and casting is filled from this position; inner cavity of "hat" corresponds to flange surface position of four bosses, and a riser is set. Each riser is provided with an asbestos insulation sleeve. Asbestos insulation sleeve is mainly used to isolate aluminum liquid from direct contact with metal mold and solidify by rapid cooling, so that aluminum liquid in riser solidifies slowly and fills flange to ensure quality of flange surface. Inner diameter of contact position between insulation riser and flange surface is equivalent to width of flange surface, internal height is about 110 mm, and process yield is about 65%.

 

Table 1 Chemical composition of ZL101A alloy (%)

 

Figure 1 Three-dimensional diagram of cover plate parts
1. Glue injection groove 2. Sealing groove

 

Figure 2 Low-pressure casting process of cover plate original metal mold
Cover plate is prepared by metal mold low-pressure casting process, and smelting, pouring, cleaning and other processes are completely carried out in accordance with process card. When cover plate original metal mold low-pressure casting process is used to produce some parts, casting shrinkage and shrinkage defects appear in flange surface sealing groove at the root of riser, as shown in Figure 3. Since GIS products have strict requirements on airtightness of cover plate parts, such defects will directly lead to scrapping of castings.
Working temperature of mold is usually a range. In order to analyze cause of casting problem, possibility of shrinkage and shrinkage defects on flange surface during filling and solidification process is first confirmed through simulation when mold is at lower limit of 250 ℃ and upper limit of 350 ℃. Secondly, minimum height of riser that meets flange shrinkage compensation is confirmed through simulation. In process of finding cause of casting defects, it was found that actual height of some or all risers of some parts was relatively low, and lower riser height was less than 40 mm. According to experience, when riser height is low, heat node of flange surface at riser position is not fully introduced into riser outside flange surface, resulting in shrinkage of sealing groove at root of riser. Finally, process optimization and production verification are carried out.

 

Figure 3 Casting defects of original process of cover plate part

Table 2 Main simulation parameters
Original process mold temperature of cover plate casting is used for filling and solidification simulation analysis with lower limit (250 ℃) and upper limit (350 ℃), and results are shown in Figures 4 to 7 respectively. As shown in Figure 4, full filling time is 4.42 s. At this moment, temperature of casting with lower limit mold temperature (the lowest temperature is about 630 ℃) is slightly lower than temperature of casting with upper limit mold temperature (the lowest temperature is about 640 ℃), and casting temperature difference is about 10 ℃, both of which are higher than liquidus temperature of 614 ℃. As shown in Figure 5, solidification time of casting with lower limit mold temperature is 638 s, and solidification time of casting with upper limit mold temperature is about 714 s. Both castings with mold temperatures meet sequential solidification requirements, and thin wall solidifies first, then solidifies toward sprue and riser until sprue and riser solidify. As shown in Figure 6, residual melt modulus probability defects of castings with lower limit mold temperature and upper limit mold temperature exist in riser, wall and sprue. Sprue and riser will be cut off after subsequent cleaning, and existing defects are not considered. Probability of residual melt defects in wall is about 1%. There may be scattered point shrinkage, which does not affect quality of parts. Flange processing surface of focus part shows no defects. As shown in Figure 7, when lower limit mold temperature is used, cooling rate of casting is less than 0.2 ℃/s, all outside flange surface; when upper limit mold temperature is used, cooling rate of casting is less than 0.2 ℃/s, and there is a very small part of flange surface processing range, and the rest is outside flange surface. Usually cooling rate is low, alloy liquid shrinkage is relatively poor, solidification structure is coarse and density is poor.

 

Figure 4 Upper and lower limit mold temperature original process filling 100% temperature diagram (4.42 s)

 

Figure 5 Upper and lower limit mold temperature original process solidification time diagram

 

Figure 6 Upper and lower limit mold temperature original process residual melt modulus diagram

 

Figure 7 Upper and lower limit mold temperature original process cooling rate is less than 0.2 ℃/s
When upper limit mold temperature is used, shrinkage compensation effect of riser on flange is weaker than when lower limit mold temperature is used. In order to make on-site process more reliable and stable, upper limit mold temperature is selected to simulate different riser heights. Machining of parts with an effective riser height of less than 40 mm is tracked, and shrinkage defects appear in sealing groove of part. Therefore, riser height of 40 mm is considered as middle value of minimum riser height verified by simulation, which is about 1.24 times flange thickness. Thickness of flange surface is 32.3 mm. Riser height is 30 mm (about 0.93 times flange thickness) to verify whether there is insufficient flange shrinkage compensation, and to verify whether shrinkage compensation condition is significantly improved when riser height is 50 mm (about 1.55 times flange thickness).

 

Figure 8 Solidification sequence when riser height is 30, 40, and 50 mm

 

Figure 9 Temperature diagram of flange surface solidification when the riser height is 30, 40, and 50 mm

 

Figure 10 Residual melt modulus diagram at different riser heights
Usually, effective height of riser of cover blank part is lower than design height, mainly due to poor exhaust or blockage of mold exhaust plug above asbestos insulation riser; poor air permeability of insulation riser; moisture absorption of insulation riser causes gas generation, etc. These reasons lead to gas in riser not being discharged in time during filling process, and riser is suffocated to a certain extent, which reduces effective height of riser. In response to these situations, measures that can be taken include: using reasonable exhaust plugs and replacing them in time according to use of mold, punching ventilation holes on the top of insulation riser, baking insulation riser before use, and using inserts on the top of insulation riser of mold.
Conclusion
(1) Simulation results show that original process can fill mold and solidify and compensate for shrinkage well when upper and lower limits of mold working temperature are used, and there are no obvious problems in casting, indicating that defects of casting sealing groove are not related to mold working temperature range.
(2) Upper limit of mold working temperature is taken, and original process scheme with different riser heights is simulated. When flange riser height is 30 mm (i.e. 0.93 times flange thickness), there is a probability of more than 1% shrinkage at a depth of about 10 mm below flange surface at riser; when riser height is 40 mm (i.e. 1.24 times flange thickness), there is a probability of more than 1% shrinkage in riser outside flange surface, and there is no more than 1% probability of shrinkage defects in flange; when riser height is 50 mm (i.e. 1.55 times flange thickness), there is a probability of more than 1% shrinkage in riser 15 mm above flange surface, and there is no more than 1% probability of shrinkage defects in flange. Theoretical yield rate of original metal mold low-pressure casting process is about 65%, and riser height is designed to be 50 mm (i.e. 1.55 times flange thickness). At this time, casting process yield rate can reach 77%.
(3) After optimizing original process of cover plate, more than 100 pieces were trial-produced, and effective height of flange surface riser casting was no less than 50 mm, that is, riser height was no less than 1.55 times flange thickness, and trial production qualification rate was greater than 99%. Simulation results were consistent with actual situation.