With continuous expansion of new energy electric vehicle market, three-electric system (battery, electronic control, motor) is core component of electric vehicles, and cleanliness control of its upstream component manufacturers plays a vital role. Particle contamination may significantly increase wear rate of system during initial trial operation and initial use stage, and even cause catastrophic failures, thereby affecting reliability and safety of the entire vehicle.
In order to ensure stability of performance of each component and system, amount of particles introduced during manufacturing process must be strictly controlled. For shell of three-electric system, core evaluation indicators of cleanliness include particle weight (mass) and particle size (size distribution). In view of direct impact of cleanliness on product quality, in-depth analysis of factors affecting cleanliness during shell processing process and formulation of targeted control measures have become key issues that industry needs to solve urgently.

 

1.1 Content of each element in aluminum alloy has a significant impact on material properties.
Specific performance is as follows: When silicon (Si) content is ≥11.5% and iron (Fe) content is ≥1.2%, aluminum alloy is prone to form excess chemical phases and flaky structures, resulting in a loose surface structure. In subsequent surface treatment process such as shot blasting, material is prone to peeling; when magnesium (Mg) content is ≥0.3%, manganese (Mn) content is ≥0.5% and zinc (Zn) content is ≥1.2%, aluminum alloy surface may form a hard and brittle phase or crack structure, and severe peeling or material loss is prone to occur during shot blasting; when copper (Cu) content is ≤1.5%, surface strength and hardness of aluminum alloy are insufficient, and it is prone to concave deformation, peeling or wrinkling during shot blasting; when proportion of recycled materials is ≥50%, difficulty of slag removal and degassing during smelting increases, resulting in a decrease in physical and mechanical properties of material. Surface of die-cast product is prone to pores and impurities, and peeling is prone to occur during shot blasting.
1.2 Impact of unstable material quality
If there are quality problems with aluminum alloy raw materials themselves (such as metallurgical defects such as impurities and pores inside), these weak areas will become stress concentration points during die-casting process due to uneven material properties. During subsequent surface treatment (such as shot blasting, sand blasting, etc.), weak parts are prone to interlayer peeling due to mechanical impact or thermal stress, resulting in surface peeling.

 

2.1 Unreasonable runner design
If cross-sectional area ratio of mold sprue and horizontal runner is inappropriate (such as the total cross-sectional area of horizontal runner is larger than straight runner), negative pressure air entrainment will occur in horizontal runner. After gas is entrained into mold cavity, it will significantly increase load of exhaust system and form pores in shallow surface layer of product. Such pores are prone to surface peeling or blistering during shot blasting. Runner spacing is too large. When spacing between multiple branch runners exceeds reasonable range, molten metal between two runners is prone to form a reflux trapped gas area; filling direction is counter-charged. If there is a counter-charge in runner filling direction, turbulence of molten metal will occur. Both of above situations will form pores (commonly known as "tumors" in industry) on shallow surface of product, and surface peeling may occur due to stress concentration during shot blasting.
2.2 Exhaust duct design defects
If exhaust duct design of mold cannot meet exhaust requirements (such as exhaust speed exceeding 350 m/s), or overflow trough is not set at filling end or defect gathering area, exhaust and slag removal will be blocked and air will be trapped. At this time, shallow pores are likely to appear at filling end of product, which may cause surface peeling during shot blasting.
2.3 Cooling system problems
If cooling water channel extends to filling end of runner, local temperature of mold will be too low, then loose defects such as cold shut, flow marks or pockmarks will be formed on shallow surface of product. Such defects are prone to cause severe peeling during shot blasting; if point cooler fails and cooling water pressure or flow is insufficient, heat concentration area of mold is prone to overheating, and corresponding product part may produce thermal shrinkage cracks or oxide skin stratification, which is also prone to peeling during shot blasting.
2.4 Mold surface condition
If surface finish of mold is not good enough (such as scratches, damage or excessive micro-roughness), it will directly affect surface quality of die casting. Such surface defects are prone to peeling due to stress concentration during demolding or subsequent processing.

 

3.1 Eccentricity of injection center
Eccentricity between injection center of die casting machine and injection hole center on fixed mold plate will cause injection punch to get stuck, pressure loss, strength and hardness of product to decrease. In addition, during shot blasting process, surface of product is prone to peeling.
3.2 Injection speed and stroke problems
When high-speed stroke is short, aluminum liquid stays near gate for a long time. Small amount of metal liquid that enters cavity first will solidify immediately after encountering low-temperature mold cavity, forming an extremely thin metal layer. Subsequent metal liquid covers it, bonding strength with substrate is low, and connection is not firm. Therefore, during shot blasting, surface layer is easy to detach and peel.
3.3 Insufficient injection pressure:
Insufficient injection pressure will make density of die castings insufficient, and defects such as pores will appear inside. In subsequent shot blasting and other treatments, die castings are prone to peeling.
3.4 Low aluminum liquid temperature
Long feeding time, long chute, low pressure chamber temperature, and mold release agent residue in pressure chamber will cause aluminum liquid temperature to drop. When two low-temperature aluminum liquids meet, molten metal liquids are not well fused, and a slight cold shut or heavy flow marks will form on the surface, resulting in peeling.
3.5 Abnormal mold temperature
Local temperature of mold is low, which will cause aluminum liquid to cool down too quickly, especially for large die castings, where aluminum liquid flow is long, temperature loss is large, and end aluminum liquid fusion effect is not good; while mold temperature is too high, it may cause problems such as heat shrinkage and oxide skinning on the surface of die casting, which is easy to peel during shot blasting.
3.6 Improper use of release agent
Excessive concentration of release agent, excessive spraying, uneven spraying, and failure to blow dry moisture in mold cavity, slider slideway and parting surface after spraying will lead to excessive water vapor content; excessive use of punch lubricating oil will also produce oil smoke. These situations will aggravate pollution of aluminum liquid. Water vapor and oil smoke will oxidize metal liquid that enters cavity first, forming an oxidation boundary layer, which will reduce its bonding strength with matrix and easily peel when subjected to external force.
3.7 Die casting cycle problem
Die casting cycle is too long or too short, which may affect quality of die castings. If cycle is too long, cooling time of die casting is too long, which may lead to uneven internal structure; if cycle is too short, die casting is not cooled sufficiently, and there is residual stress inside, which is easy to peel during subsequent processing.

4.1 Shot blasting pellet problem
Diameters of purchased shot blasting pellets are inconsistent. Some pellets are larger than diameter size set by process, which will increase impact force of shot blasting, thereby impacting surface of product and peeling.
4.2 Improper setting of shot blasting parameters
Shot blasting speed is too high, time is too long, and shot flow is too large, which will increase impact force and scraping effect, resulting in peeling of dense layer on the surface of product.
4.3 Improper grinding treatment
Flash and excess material of die-casting blank are not cleaned and polished, there will be residual flying skin or peeling after shot blasting; excessive grinding will grind off dense layer on the surface of blank, and skin will also peel due to the impact force during shot blasting.

 

1.1 Acid solution cleaning
Principle: Use acid to react chemically with peeling and delamination substances to dissolve and remove them.
Common solutions: For example, use 10%-15% acid solution. Soak aluminum alloy die casting in acid solution for a period of time, generally 10-30 minutes, specific time depends on severity of peeling and delamination. Rinse with clean water afterwards.
Notes: Acidic solutions are highly corrosive. When operating, protective measures should be taken, such as wearing gloves and goggles, and operation should be carried out in a well-ventilated environment.
1.2 Alkaline solution cleaning
Principle: For peeling and delamination caused by attachment of some pollutants such as oil, alkaline solutions can be removed by emulsification and saponification.
Common solutions: such as 5%-10% alkaline solutions. Put die casting into solution, soak it at an appropriate temperature (generally 40-60℃) for about 30 minutes, then rinse with clean water.

2.1 Grinding
Tool selection: Tools such as sandpaper or grinding wheel machines can be used. For lighter peeling and delamination, first use fine sandpaper (such as 200-400 mesh) to gently polish to remove peeling and delamination parts on the surface, then use finer sandpaper (such as 600-800 mesh) for further polishing to make surface smooth.
Subsequent treatment: After grinding, you need to wipe surface with a clean cloth to remove debris generated by grinding.
2.2 Sandblasting
Principle and operation: Use impact of high-speed sand flow to clean and roughen substrate surface. Select suitable sandblasting media (such as quartz sand, corundum, etc.) and sandblast surface of aluminum alloy die castings under a certain pressure (generally 0.3-0.8 MPa).
Advantages: It can effectively remove peeling and delamination, and at the same time improve surface roughness, which is conducive to subsequent coating or bonding processes.

3.1 Anodic oxidation method
Principle: Use aluminum alloy die castings as anodes, and energize in a specific electrolyte (such as sulfuric acid solution) to form an oxide film on the surface. This oxide film can cover defects of peeling and delamination, can improve hardness and corrosion resistance of surface.
Key points of operation: Control concentration of electrolyte (generally 15%-20%), temperature (18-22℃), current density (1-2 A/dm²) and oxidation time (30-60 minutes).

There are many cleaning agents on the market that are specially used to clean surface defects of aluminum alloys. These cleaning agents are usually developed according to characteristics of aluminum alloys and have good cleaning effects. When using, follow requirements of product manual, including control of parameters such as concentration of cleaning agent, cleaning time and cleaning temperature.