Development case of precision die-cast aluminum alloy motor housing

With the development of automobile industry, more and more auto parts are produced with aluminum alloy for light weight, and the casting structure is getting more and more complicated, the casting quality requirement is getting higher, and the development cycle is getting shorter. Because of the low density of aluminum alloy, the strength performance is similar to gray cast iron, but the toughness is higher than gray cast iron, and has good casting performance, can be formed into complex thin-walled auto parts, therefore, expanding the application of aluminum alloy can obviously reduce the weight of the car to meet the needs of the fierce competition in the automotive industry. Aluminum alloy is strengthened by alloy elements, and its strength is greatly improved, combined with light weight, good heat dissipation and other characteristics, which can well meet the requirements of the transmission shell, change shell, motor shell and other parts working in harsh environments. Aluminum alloy motor shell die-casting molding technology can control the material quality through purification, refining, refinement, densification, etc., and make the casting quality reach consistency and stability through precision die-casting molding.

This article takes a new energy vehicle motor housing as an example, and uses simulation software to simulate its die-casting process to shorten the development cycle of this product, and analyzes and improves it with the problems in the trial production and trial production stages.

The structure of the casting and the development of technical points

Automotive motor shell die-casting parts are shown in Figure 1. The part outline size is 459mm × 275mm × 281mm, die casting weight is 8.675kg, average wall thickness is 4mm, projection area is 90,296mm2, casting material ADC12 aluminum alloy. The casting has a number of thick-walled hot section parts (Figure 1 circle position), easy to cause shrinkage; require the product surface without burr flying edge and die-casting defects; all dimensions in line with the drawings and assembly requirements, especially flatness and positioning hole position degree must meet the requirements of the drawings; internal large cavity, bearing holes and other parts, can not appear air holes, shrinkage, loosening and other defects, so as not to reduce the mechanical strength of the product; and product requirements Airtightness test, specific airtightness requirements are: test leak pressure 200kPa, allowable leakage <5ml/min.

The part is produced on a 2700T (IDRA) horizontal cold chamber die casting machine equipped with a world-advanced die injection control and process monitoring system.

During the development of this product, process analysis was performed using die-casting simulation analysis software to shorten the development cycle. Due to the relatively high internal quality requirements of the motor shell part and the thick wall thickness relative to the large bearing hole location, the casting was poured by the side of the motor shell in a proper manner instead of the common horizontal placement of the casting as a whole, and the pre-designed pouring-overflow system was simulated. As shown in Figure 2, option 1 is for both sides of the pouring, option 2 is for the left, and option 3 is for the right.

Figure 3 shows the state of the three different scenarios at a pressure injection filling time of 2.61s and a cavity filling rate of 68%. From the simulation effect, it is concluded that Option 1 has rolled air at the location of the large circle of the motor case (marked circle in Fig. 3), which is not used because this location is a critical location and air holes are not allowed to exist.

Figure 4 shows the state of the three different schemes at the pressure injection filling time of 2.63s and cavity filling rate of 82%. The simulation confirms that schemes 2 and 3 are mainly concentrated in the filling end of the casting. Other key locations with less rolled air (Figure 4 scheme 2 and scheme 3 upper two marked circles). Because the temperature field of scheme 3 is too unbalanced when the aluminium liquid is filled (Figure 4 scheme 3 inside the lower circle, the temperature inside the circle The temperature range is from 580C° to 630C°), option 2 is chosen.

Analysis of the causes of deficiencies and countermeasures

Product water tail hole cause analysis and countermeasures

Cause analysis

For the simulation analysis of the filling end porosity of the casting of scheme 2, the inner sprue is widened by 13mm on the main sprue of scheme 2 (see Figure 5) to enhance the aluminium material at the location of the inlet sprue, so that the aluminium liquid can be filled more smoothly. At the same time, the volume of the slag collection package at the end of filling is increased.

Simulation analysis of the results in Figure 4 shows that.
1) the location of the product air hole is the farthest end of the inlet, and this location is easy to form cold material collection.
2) the product water tail part is the intersection of aluminium liquid in different directions, easy to form a roll of air, resulting in internal porosity. Although the casting adopts the vacuum-assisted moulding technology and the location of the air hole is close to the exhaust block, the gap between the exhaust block is too small and easy to form a low exhaust. It causes the casting to fill the end of the cold material convergence and air hole defects.

Measures

The solution measures are for the casting cold material convergence and porosity defects.
(1) increase the aluminium flow to the location of the inner gate area so that the aluminium fills more smoothly.
2) Increase the volume of the slag collection package at this location so that the package can hold more cold material.
The two methods are to reduce the material on the mould, increasing the area of the inner gate can be achieved by widening the inner gate; increasing the slag bag can also be achieved by increasing the width and thickness of the slag bag. Since both measures are easy to achieve, both methods are adopted simultaneously.

Analysis and countermeasures for the cause of large round lamination of motor shell

Analysis of causes

The casting using program 2 manufacturing mould, the die-casting trial system appeared in the motor shell large round position lamination defects, for the lamination analysis of the reasons is as follows.
1) narrow parting line gap, phi seam residue.
2) high-speed start position is too late to choose; the aluminium liquid slow speed stage is too long, resulting in the aluminium liquid surface layer forming a prematurely thin shell layer and the high-speed involved in becoming laminated.
3) poor aluminium liquid composition.

Measures

For the parting line gap is unreasonable, phi seam residue measures.
1) re-mould the parting line to closely match so that the product in the parting line position has no phi seam formation.
2) Pour a rounded corner in the mould parting line position (Figure 6 lower left circle position) so that the phi seam is not pulled off and taken out with the product when the mould is opened (as shown in Figure 6 right circle). Method 1 is too costly to be used for now.

The high-speed start position is chosen too late, and the slow-speed phase of the aluminium liquid is too long, leading to the premature formation of a thin shell layer on the surface of the aluminium liquid, which becomes entrapped into the interlayer at high speed. The reason for the formation of the interlayer needs to be solved by adjusting the high-speed starting point. The position of the high-speed starting point chosen earlier is 550mm; through simulation, it is found that due to the varying length of the casting sprue process, when the aluminium liquid is filled to all the inner gates, there is already some aluminium liquid filling inside the cavity. The filling area is roughly the same as the distribution area of the interlayer (Figure 7). In this regard, the high-speed start position is advanced, and the slow speed is increased. For the poor composition of the aluminium liquid, the following improvements were made:
1) Strengthen the aluminium liquid melting process (dross removal, gas removal and debris removal).
2) the need to clean up floating slag when adding material.
3)Cleaning of slag.
4)Add a cover plate to the material scoop.

Effect tracking

According to the above measures to optimize the parting line phi seam, adjust the high speed starting position and slow speed, and strengthen the control of aluminum liquid composition, after the tracking verification, the motor case large round position lamination was greatly improved.

Conclusion

Through the die-casting trial and improvement process of the aluminium alloy motor shell casing, the results show that
(a) Numerical simulation provides a strong guarantee for the pre-improvement of aluminium alloy die-casting, with an intuitive display of the filling process, air pressure, flow pattern and intersection position of the aluminium liquid, providing a strong basis for the improvement of the pouring method.

At the same time the numerical simulation provides a good reference direction for defect improvement and shortens the product development time.

after statistics on the types and locations of defects in die-cast motor housings, the main defects are identified and thematic improvements are made to target the main defects, allowing for focused efforts to improve process aspects.

The use of improving the manufacturing accuracy of the moulds, reducing the sharp angles at the parting positions and at the same time increasing the speed of the slow stage, can improve the product entrapment due to cooling inside the barrel and mould.

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