At present, die-casting technology has been widely used in various fields such as automobiles, military industry, aviation, medical care, etc. Among them, automobile industry occupies a relatively important position in domestic die-casting industry. Main shell of rear-wheel drive electronic control box is an important part in new energy vehicles. Shell has its own circulating water chamber, is required to have good mechanical properties and air tightness. The overall parts are relatively small, belong to small and medium-sized parts, but structure of parts is relatively complex and the overall technical requirements of product are high. Only by combining early theoretical development and design with later actual process improvement can we produce qualified products that meet technical requirements.

Main housing of electronic control box is located in rear drive integration of new energy vehicle. Figure 1 shows its three-dimensional structure, with an outline size of 225mm * 284mm * 79mm and a weight of 1.24kg. It is a small and medium-sized part with a relatively complex part structure. As shown in Figure 2, the overall wall thickness of casting is about 3mm, and some of thicker parts have a wall thickness of about 8mm; wall thickness of some deep rib cavities is about 40mm, and point cooling is installed at wall thickness to prevent risk of shrinkage cavities.

 

Material selected for main shell parts is AlSi12Fe. Surface of parts is required to be shot peened; there must be no visible crack defects on the surface; there must be no grease and release agent residue; there must be no traces of oxidation. Sealing surface, supporting surface, and surface where heat dissipation gasket is attached shall be free of ejector pin marks, scratches, cracks, and burrs. Burrs at gate or slag bag should be less than 0.5mm, burrs at parting surface and other locations should be less than 0.1mm. There should be no sharp or detachable burrs; cavity air tightness test requirements: test pressure 0.5bar, leakage amount 2.5pa/s; water channel air tightness requirements: test pressure 3bar, leakage volume 2.5pa/s.

With rapid development of computer technology, die-casting simulation technology has begun to be widely used, which is of great significance to improving internal quality of castings, shortening casting development cycle, and reducing costs.

Main shell parts are small and medium-sized parts, but structure is complex, adopts a one-mold and one-cavity gating system. Analyze outline and size of main body of part, consider influence of length of aluminum liquid filling stroke, try to ensure that liquid aluminum does not directly hit core after entering cavity, and mainstream direction is facing important parts of part. Preset gating system of part is shown in Figure 3. Pouring system shown in Figure 3 has the shortest stroke, and mainstream direction of aluminum liquid in flow channel is facing direction of product water channel, which can ensure filling quality of water channel and meet leak test requirements of water channel of parts. Compared with initial part drawing, two bridges are added to filling end to enhance filling of molten metal at the end and prevent defects such as cold separation and poor molding at the end of part. Overflow tank is arranged at the end of molten metal filling and at preliminarily determined location where gas is likely to be trapped.

 

Use simulation analysis software to perform high-pressure casting simulation on preset parts pouring and drainage system, and analyze simulation results.

Simulation of aluminum liquid filling process is shown in Figure 4. Results show that filling process of molten aluminum is smooth, molten metal in multiple ingates is clearly layered, and there is no obvious intersection of molten metal hedging, which is a relatively ideal molten metal filling state.

 

Figure 5 shows simulation results of cavity filling time. It can be seen that cavity filling time is within 0.2s, which is within a reasonable range.

 

Figure 6 shows filling air pressure simulation results. It can be seen that most of gas is mainly distributed in overflow tank and product column position. Some column columns are connected with ribs to enhance exhaust and aluminum liquid filling to ensure internal quality of column. A small part is located in non-processing parts inside product.

Figure 7 shows simulation results of solidification of aluminum liquid at different times. It can be seen that middle wall thickness position of product cools slowly and there is a risk of shrinkage. Increase cooling at corresponding position to ensure that solidification sequence of product is consistent. Through analysis of simulation results, preset gating system can generally meet actual production requirements of parts, but there are still some risks. In view of risks caused by problem points, mold structure and part structure should be optimized in advance to prevent defects and conduct trial production. Further optimize pouring system, part structure, and mold structure based on actual production status to improve casting quality.

 

Front projected area of part is 50960mm2, side core-pulling projected area is 7350mm2, safety factor is set to 1.25, injection specific pressure is 75MPa, slider wedge angle is 10°, ratio of product's additional flow channel and slag bag projected area to product's front projected area is 1.3 for theoretical calculation. Expansion force is 6300kN, so a 630T die-casting machine is selected for production.

Based on previously designed full casting of die-casting mold and selected parameters corresponding to actual production die-casting machine model, theoretical calculations show that when punch of die-casting machine moves to 347mm, aluminum liquid just reaches inner gate position. Therefore, taking 350mm as theoretical high-speed switching point, three high-speed switching points of 330mm, 350mm, and 370mm were selected for actual production verification.

Figure 8 shows defects of products produced at different high-speed switching starting positions. When high-speed switching point is 330mm, product inlet mold buckling is obvious; when high-speed switching point is 350mm, material inlet mold buckling is significantly improved; when high-speed switching point is 370mm, there is no obvious mold buckling at material inlet, but water tail pores are serious. After verification, the most suitable actual production parameters were obtained: second fast start position is 350mm, second fast flow rate is 70%; boost start position is 470mm, and boost flow rate is 60%.

 

After verification of three sets of parameters in the early stage, 200 pieces were continuously produced with final confirmed parameters, after subsequent shot blasting and machining processes were completed, specific product production data obtained are shown in Table 1 (single defects are counted individually, and there are multiple defects in one product).
Table 1 Production scrap rate statistics

Through statistics, it was found that main problems of product were: pores, air leakage, shot blasting and peeling. In addition, during subsequent production verification process, batch molding was defective and was solved on site. Among them, air holes are mainly located in wall thickness near sealing groove as shown in Figure 9a, causing leakage during processing; air leakage is due to existence of shrinkage holes between lower core-pulling water channel hole and side threaded hole, resulting in cross-leakage, as shown in Figure 9b; location of shot blasting peeling is shown in Figure 9c.

 

The first trial production was 200 pieces, and 15 pieces were selected to detect internal quality with X-rays. Results showed that each product had shrinkage holes in blind holes. After processing, blind holes and core-pulling pinholes leaked in series. Analyzing reasons, we found that blind hole here is too small and no needle is inserted, resulting in uneven wall thickness of product and excessive local wall thickness, which can easily cause defects such as looseness and shrinkage cavities.
Solution:

(1) For this area, super-point cooling is added to movable mold for cooling, as shown in Figure 10, but shrinkage cavity is not significantly improved;

 

(2) Modify sprue, as shown in Figure 11. Figure 11a shows initial runner. Location of shrinkage cavity is where first runner is filled. After analysis and judgment, feed in the left half was too strong, so second runner was sealed and replaced with a slag bag. At the same time, first runner was widened to strengthen feed at defective part; after change, there is a certain improvement effect, but there are still leaking products, and other improvements need to be added.

 

(3) Lower core pulling increases local extrusion, as shown in Figure 12.

 

In die-casting industry, local extrusion has a very obvious improvement effect on solving product shrinkage cavities, and application of local extrusion technology is also quite mature. It is analyzed that lower core-pulling position corresponding to blind hole has enough space to install an extrusion cylinder, so local extrusion is used to address shrinkage defect of blind hole next to lower core-pulling pinhole.

 

After changing extrusion structure, in order to ensure that extrusion pin sticks to aluminum and affects extrusion effect, extrusion pin is equipped with core-pulling spray, extrusion parameters are set to a delay of 2s, and a pressure of 3s. The overall cooling of extrusion pin is better, there is no water-based release agent residue at needle sleeve, and the overall extrusion effect is better. Shrinkage cavity defect of blind hole next to core-pulling pinhole is solved. Figure 13 shows that there is no shrinkage cavity in X-ray, and there is no leakage after processing.

After adding lower core-pulling extrusion pin, one column platform of product has poor molding, as shown in Figure 14.

 

After analysis, column platform is above core-pulling hole of product. In actual production, core-pulling spray for lower core-pulling is set, that is, lower core-pulling is in driven state when spraying starts, which facilitates cooling of lower core pull and extrusion pin. However, when spraying with core pulled down and driven in, water-based release agent will remain in pillar hole blocked by core pulled out needle, making it difficult to blow out after a long period of air blowing after the spraying is completed. Residual moisture and supercooling of pillar hole are main reasons for poor molding defects caused by aluminum liquid filling cavity later.

 

Solution: Figure 15 is a schematic diagram of adjusting core spray water storage structure. After lower core-pulling spray is cancelled, and lower core-pulling is in a retreated state, without core-pulling needle being blocked, residual water in hole can be dried. At the same time, in order to ensure spray cooling of extrusion pin, a longer copper pipe needs to be installed to spray extrusion pin with core pulled down, and extrusion pin blowing copper pipe should be lengthened accordingly. After implementing remedial measures, defect of poor molding of product was resolved, as shown in Figure 16.

 

It can be seen from X-ray photos that there are pores in wall thickness of product, and some products leak after processing, making product unqualified. Cause analysis: During product filling process, metal liquid meets here, water tail is insufficient, and exhaust is not smooth, resulting in poor internal quality here. Following improvement plan is proposed for verification.
1) Increase thickness of two material bridges of water tail window from 2mm to 3mm to enhance material passing, but there is no obvious improvement effect;
2) Change three pinholes at the top of shrinkage hole into cylinder thimbles, use gap between needle and needle sleeve to vent air. As shown in Figure 17, defects have been improved, but improvement effect is not obvious;

 

3) Add sprues on the side to enhance feeding at defective locations. After the first two solutions had poor improvement results, it was analyzed that the overall feed on the right side of product was insufficient, and a side runner was added to enhance the overall feed on the right side. Specific plan is shown in Figure 18.

 

In the end, with joint implementation of these three solutions, internal quality of product was better improved. As shown in Figure 19, there are no external leakage holes during processing.

 

Uneven cooling of product causes flow marks on the surface of product and peeling after shot blasting.
Analysis of reasons: Main reason for casting peeling is that some areas of casting are specially designed with many pillars and grooves. Aluminum liquid fluctuates greatly during filling process. When filling mold at high speed, aluminum liquid flows to a place that cannot be filled by mainstream, that is, dead spot. Molten liquid splashes and its temperature drops quickly. For plane area surrounded by pillars and grooves, heat dissipates faster and solidifies faster.     Therefore, cold scar flow marks or pitting are formed on the surface of casting. Cold scar flow marks and pitting are main causes of shot blasting and peeling.
Solution:

(1) Add material passing ribs to enhance cold material discharge. Two material passing ribs are provided at plane and water tail, and two passing ribs of same thickness are added, as shown in Figure 20. It is verified that this measure can improve most of peeling, but there are still a few places where peeling occurs after shot blasting.

 

(2) Reasonably control thermal balance of mold, identify locations with serious product flow marks first, use oil cooling for locations where mold cools quickly and needs to be cooled to prevent this location from cooling too quickly. After implementation of this measure, improvement effect was not good, and peeling state still existed after shot blasting;
(3) In view of defect of shot blasting and peeling of product, in addition to reasonably controlling mold temperature, there is another way to better control shot blasting and peeling, which is to add etching patterns on mold surface. Principle is as follows: During flow of aluminum liquid in mold cavity, because mold surface is very smooth, if mold is etched in a location where cold scar flow marks and pitting are likely to occur on casting, mold surface will become uneven, forming high and low grooves, which can disperse flow direction of aluminum liquid cavity, form an air film between grooves to improve fluidity of aluminum liquid at this location and avoid or reduce occurrence of cold scars, flow marks or pitting.
After adding etching lines, shot blasting was verified and product showed no peeling, as shown in Figure 21.

 

Only through a multi-pronged approach can we produce high-quality products. Using simulation software to conduct early numerical simulation analysis of die-casting product gating system can effectively identify defect risks of actual product production, timely optimize gating system or mold structure to reduce or even avoid and solve defect risks; reasonable selection of process parameters can effectively ensure the overall quality of castings. When actual process parameter adjustment cannot improve product problem, cause of defect must be considered in a timely manner from pouring structure, overflow system, mold cooling, product structure and other aspects.