Introdution
With the continuous development of the casting industry, there are many advanced die-casting technology to improve the aluminum alloy process performance. Below to automobile power generator thin wall cover parts as an example, analysis of aluminum alloy throughout the die casting production process deformation thermal stress factors and share with you a set of related orthopedic process.
1. Structural analysis and dimensional requirements of thin-walled aluminum alloy castings
The part is aluminum alloy die casting, used to assemble the oil pump and engine body connection. Assembly position engine belt, suspension side, the product itself in addition to bear the engine suspension, wheel system installation, but also to ensure that the engine internal lubrication system seal, is a typical sealing thin-walled products. From the parts view in Figure 1 as a whole, the product consists of two planes, its boundary size conditions:
| Length (mm) | Width (mm) | Minimum thickness (mm) | Maximum thickness (mm) |
| 396 | 277.5 | 14 | 94 |
From the boundary dimensions, it can be seen that the projected area of the product is 109,890mm², the thickness of the thinnest part of the product is only 14mm, and the maximum thickness is located in the bolt suspension position on one side of the product, which is 6.7 times thicker than the thinnest wall thickness. Considering the strength of the suspension and the weight reduction of the product, the reinforcement on the side of the suspension was designed to be deeper and more difficult to remove from the mold later on. The reinforcement has an effect on the product deformation, and the taper of the mold pulling should be fully considered to facilitate the control of the flatness of the product in the later stage.
Because of the obvious difference in the thickness of the product, the product thickness is concentrated in the hanging side, the maximum thickness of 94mm, the other side of the uniform thin-walled products, the average wall thickness of 17mm. the weight of the product is 2.95Kg, but the weight distribution is not uniform, with the help of CAE analysis of the product’s center of gravity is located in the side of the product near the hanging side, there is a long distance from the center of the product (about 110mm), almost the main aluminum alloy is concentrated in the hanging side, which indicates that the product itself is not uniform weight. The main aluminum alloy is concentrated in the hanging side, which indicates that the weight of the product itself is not uniform.
The 14 φ7 bolt holes on the product play a role in connecting with the engine body, which requires very strict positional tolerance of the product. At the same time, the positioning point of the product is also selected in two of the pin holes, so the pin holes to ensure the position of the product directly determines the overall spatial dimensions, and to ensure that the pin holes to ensure the position of the prerequisite is the accuracy of the positioning datum is to rely on the blank flatness to ensure. This kind of aluminum alloy die casting decided in the mold manufacturing time can’t have the slightest misalignment, in the whole die casting production process parts can’t appear deformation.
2. Die casting process on the deformation of the parts of the influence of the process
Thin wall casting in the die casting process, liquid metal in the solidification and cooling process of thermal stress on the casting deformation has a great influence. Thermal stress make cooling slower thick wall by stretching, cooling faster thin wall or surface by compression, casting wall thickness difference, the greater the alloy line shrinkage or modulus of elasticity and thermal stress.
As the product has a large wall thickness at one end and a small wall thickness at the other end, the casting will be prone to deformation due to the inconsistent cooling rate of each part of the casting during directional solidification, which generates a large thermal stress.
Because the structure of the product itself has been determined, the product itself can not make major changes, so in the mold design and manufacture of the top bar position, number and balance need to be considered comprehensively. When the parts come out of the mold, the casting temperature is too high, the top bar structure is unreasonable, and the top bar ejection force is not balanced, all of which may cause deformation of the casting.
The CAE analysis resulted in the selection of the inlet sprue with the gate on one side of the product and the exhaust sprue on the other side of the product. In order to balance the center of gravity of the product, the inlet sprue and the exhaust sprue are balanced on the weight of the body. At the same time, the design of the top bar is not uniformly distributed, the diameter of the top bar in the thick part is larger and more, while the top bar in the thin part is farther away from the top bar, and the diameter of the top bar is also smaller.
3. The effect of the de-gating process on the deformation of the part
The main gate of the part is attached to one side of the part, both thick and large location night tour thin walled area. These products in the process of de-gating is very worthy of attention, if the choice of de-gating process program is not right, then may be produced parts are defective.
The direction of removing the sprue is as perpendicular as possible to the flow direction of the molded product, so as to ensure that there will be no defects in the removed sprue parts. Mature de-gating processes include: manual knockout removal, trimming mold removal, chain saw removal, and so on. When choosing a specific solution, we first confirmed the flatness of the qualified casting blanks (including the sprue system). After continuously producing 30 pieces of casting blanks, we measured the flatness of the products one by one according to the standard of 0.5mm, and got the test results as follows:
| Test quantity | Pass (0-0.5mm) | Fail (0.5mm-1mm) | Fail (> 1 mm) |
| 30 | 3 | 17 | 10 |
It can be seen that the deformation of the product still exists even under the premise of full consideration of the mold structure, top bar design and die casting process. Therefore, for the casting of the de-gating process is not only to consider the impact of deformation of the parts, but also should take into account the use of “hot calibration” techniques to ensure the flatness of the product.
Therefore, in view of the consistency of operation and the characteristics of the product, we finally adopted the edge trimming mold with the function of calibration to carry out the de-gating operation. To put it simply, the top bar and balance block are added to the non-contact position of the trimming mold and the product, so that the top bar and balance block squeeze the product while trimming action is being performed, and a correction of the flatness of the product is carried out. The operation process is briefly described as follows:
Completion of die-casting → manual pickup → put into the trimming mold → trimming mold cutting and leveling → pickup and placement
After adopting this program, the flatness of the product was tested again, this time the flatness of the product after de-gating was tested, and the test results were as follows:
| Test quantity | Pass (0-0.5mm) | Fail (0.5mm-1mm) | Fail (> 1 mm) |
| 30 | 22 | 8 | 0 |
The result of this treatment and the control of the flatness of the product is acceptable for subsequent production.
4. The influence of the shot blasting process on the deformation of parts
This part is a peripheral part of the engine assembly; the appearance of the part is required to be consistent in color and free of sharp raised burrs. The technical requirements for product development of our company require the product to be shot peened.
Shot blasting process on the deformation of the parts of the main factors are shot blasting time, the way of placement, shot blasting intensity. For the reason of the deformation of the product shot blasting, we test and analyze the process of shot blasting: taking into account the appearance of the parts required color consistency, then the parts will be hung up shot blasting, you can avoid the parts can not be thrown to the dead ends; shot blasting strength is mainly reflected in the current table parameters and join the amount of steel shot, the current parameter can be selected in the range of 8A-15A; shot blasting time is the most important shot blasting process factors in the process of mechanical energy conversion of heat caused by casting thermal expansion and contraction is not uniform, then the time of shot blasting the longer and longer. Shot blasting process due to the conversion of mechanical energy into thermal energy caused by uneven thermal expansion and contraction of castings, then the longer the shot blasting time, the greater the deformation may affect.
Therefore, the following analysis program was developed: 30 pieces of trimmed parts were selected, all 30 pieces were detected by CMM and the corresponding parts were numbered and grouped. Shot blasting method: choose the hanging type shot blasting; current parameters: 8A-15A; shot blasting time for 1 minute, 2 minutes and 3 minutes.
Finally, the grouped parts were sent to the CMM room, where 100% of each piece was inspected for position by CMM to analyze the effect of different blasting times on the deformation of the parts.
The results are as follows:
| Number of tests | Time (S) | Pass (0-0.5mm) | Fail (0.5mm-1mm) | Appearance pass |
| 10 | 60 | 10 | 0 | 3 |
| 10 | 120 | 10 | 0 | 10 |
| 10 | 180 | 9 | 1 | 7 |
From the analysis of the data to see empirically selected three time periods of the shot blasting time on the flatness of the product has little impact. However, too long or too short shot blasting time has a great impact on the appearance of the product, combined with the appearance of the product and the position of the final results of the degree of detection, the shot blasting process is selected as follows: shot blasting method: hanging type shot blasting; current parameters: 8A-15A; shot blasting time: 120S ± 20S.
5. Deformation caused by other factors
The flatness and spatial position of the product is tracked during mass production. The flatness and spatial position of the products were also tracked for a period of time before the blanks were put on line. In one batch, the flatness of the product was not good, and it was found that the height and the way the product was placed caused the product to be pressed and deformed during the storage stage.
In order to ensure the necessary product safety inventory as well as the normal flow, it is best to come up with relevant regulations that stipulate product placement and storage requirements to avoid product deformation caused by product accumulation.
6. Conclusion
Thin-walled aluminum alloy products of a large number of applications is the future of the engine overall weight reduction process in an important direction, but by the aluminum alloy casting service process characteristics brought about by the deformation of the thermal stress is objective, and therefore lead to the deformation of the casting is unavoidable. As aluminum die casting manufacturers sometimes can’t change the overall design of the product, but through effective technology and measures can fundamentally ensure that the deformation of the product thermal stress can be effectively released, and through the reasonable orthopedic process to ensure that the size of the final product qualified.
