Due to characteristics of aluminum alloy die-casting process, phenomenon of oxidation and cold material in die-cast aluminum alloy products is inevitable. Among them, condensation sheets combine the two, are a common and important defect. If improperly controlled, it will greatly affect processing performance, mechanical properties, sealing performance, etc. of product.

Oxidation process of aluminum alloy is affected by many factors. However, in die-casting process, it mainly comes from oxidation reaction of part of aluminum liquid exposed to air, forming an oxide layer on the surface. When relatively cold liquid aluminum alloy (at least partially solidified) is covered by an oxide film in some cases, and encounters a higher temperature metal solution that can flow around it, it will be covered in it. It flows into cavity with aluminum liquid, and finally distributes in solidified molded product in the form of flakes. Because of oxide layer on its surface, it cannot be completely fused with normally solidified metal melt, and there is often a certain gap.

 

 

 

As shown in Figure 1-4, condensation sheet exists inside product, in a dispersed state and of varying sizes. Because gap it produces is too small, it can generally only be found through metallographic observation, ultrasonic detection, etc. In more serious cases, it can be observed with naked eye through product fracture.

Since significant feature of this defect is that there is a gap between oxide film and normally solidified metal, this forms a channel for gas to pass through. When product wall is thick and defect is completely covered, channel is blocked and does not affect air tightness test. In some thin-walled areas or processed areas, condensation sheet is exposed, channel is open on both sides, and air tightness test often fails.

Since condensation sheet cannot be completely fused with surrounding solidified metal, metallographic state is inconsistent, there are gaps and weak bonding forces inside product, which will weaken mechanical properties of product body. Taking tensile test as an example, we sampled areas without condensation sheets and areas with condensation sheets of same product, and obtained test results in Table 1:
Table 1 Comparison of tensile data

The first group is sampling without condensation sheets, and second group is sampling with a few condensation sheets. By comparison, it can be concluded that condensation sheets will reduce tensile strength, yield strength, and elongation of product material.

Some important functional surfaces often require machining to ensure their size, flatness, roughness, appearance, etc. If there are condensation sheets in processing area and they are exposed, it will seriously affect the overall performance of product. See Figure 5.

 

Because condensation sheets are generated during injection process, we will analyze and verify according to flow path of aluminum liquid. Including: machine-side furnace, melting cup, mold.

Machine-side furnace plays a role of heat preservation to ensure that aluminum liquid has sufficient casting temperature. When aluminum liquid is in furnace, temperature is high and it is not in contact with outside world, and there is no condition for generation of condensation sheets. However, when casting begins, aluminum liquid needs to be transferred to melting cup, process will cause temperature drop and contact with air, which meets conditions for formation of condensation sheets. It should be noted that commonly used machine-side furnaces are generally divided into open type (no top cover in Figure 6, feeding by spoon) and quantitative type (feeding by pump, but drainage trough is required).

 

According to observation, since open type has no top cover, part of aluminum liquid on the surface will continue to cool down and be oxidized to form an oxide scale. At the same time, a layer of oxide scale will also be formed when spoon scoops material; although quantitative furnace internally pumps material, aluminum liquid is exposed to outside through drainage trough and is accompanied by cooling, which will also produce an oxide layer and temperature drop. Therefore, in order to reduce condensation sheets generated during material collection stage, we should take following measures:
1) Spoon should be designed with a material blocking structure to prevent oxide scale on the furnace surface from entering spoon, and furnace surface needs to be cleaned on time.
2) Feeding time is shortened to reduce exposure time of aluminum liquid.
3) Drainage trough should be shortened to reduce temperature drop and exposure time of aluminum liquid, and impurities in trough should be cleaned on time.
4) The higher initial temperature of aluminum liquid, the smaller temperature drop during process. Correspondingly, risk of scale and cold material being caught is smaller, so casting temperature of aluminum liquid should be increased as much as possible.

Main influencing factors of melting cup are analyzed as follows:
1) Temperature of melting cup (barrel). Melting cup itself is lower than room temperature, and the only heat source comes from contact with aluminum liquid. Large temperature difference between the two will cause a large temperature drop of aluminum liquid, produce more cold materials, and provide conditions for generation of condensation sheets.
2) Melting cup (barrel) filling rate. Involving diameter and length of melting cup. Amount of aluminum liquid will affect contact area with barrel. The greater contact, the more condensation sheets.
3) Residence time. Similarly, the longer aluminum liquid stays in barrel, the greater temperature drop and the more oxidation.
According to above analysis, countermeasures we should take are:
Table 2 Verification results of melting cup temperature

1) Heat melting cup front and back to reduce temperature drop of aluminum liquid.
2) Improve filling rate.
3) Shorten injection delay time and low-speed filling time.
Among them, filling degree and injection time may affect other product requirements, and it is recommended to consider them appropriately. We mainly verified influence of melting cup temperature, and results are shown in Table 2.
1) When barrel temperature increases by 15-20℃ (oil temperature machine setting increases by 20℃), probability of condensation flakes at each gate is significantly reduced, especially at gates 4/5/6, but small condensation flakes appear intermittently at gate 1;
2) When aluminum liquid temperature drops to about 650℃, temperature of rear end surface of barrel will drop (even if oil temperature machine setting remains unchanged), causing probability of condensation flakes at gates 4/5/6 to rebound and increase;
In summary, it can be concluded that barrel temperature greatly affects probability of condensation flakes.

1) Location of runner: Since condensation flakes generally exist inside material, when gate is the only damaged surface there, it means that channel formed by condensation flakes will not penetrate. If airtightness of product is more concerned, risk brought by condensation flakes is very low; on the contrary, if there is processing on opposite side of gate, a channel will be formed, affecting airtightness.
2) Structure of runner: Influence of runner itself includes generation of condensation flakes and interception of condensation flakes.
To this end, countermeasures we verified are:
1) Location of runner should avoid machining on opposite side, and it should be thick enough to avoid exposing condensation flakes.
2) Increase insulation function of runner: thicken volume of horizontal runner water distribution and reduce cold materials.
3) Intercept condensation flakes: set a higher break at inner gate to intercept condensation flakes so that they do not enter product.
After actual verification, after runner is optimized, phenomenon of gate condensation flakes is effectively improved. See Figure 6-7.

 

Oxidation of die-cast aluminum liquid and generation of cold materials are inevitable in casting process. Problem of condensation sheets cannot be completely eliminated in today's high-pressure casting process. However, we should analyze and verify characteristics of production process one by one, and improve from source aluminum liquid material, to equipment, to process, to minimize its impact on product, so that die-casting products have better quality. Technology is changing with each passing day. As we continue to research and explore, I believe that all kinds of problems of die-cast aluminum alloy products will be solved one by one, and better quality will be used to serve people's daily lives.