How to Prevent Black Spots on Aluminum ADC12 die-cast? (Part Two)
The composition of the white adhesion for the second defect has the main feature of having high O content. It is speculated that the main component of the white adhesion is Al oxide or hydroxide. After white adhesion is removed, the matrix of the second defect presents a small-sized porous net structure, and its O element's content is also high. Moreover, there is no steel chip adhering to the matrix of the second defect. However, the content of the Fe element in the testing area increases. It is inferred that steel chips originally adhered to the surface in this area have undergone galvanic corrosion with the substrate in the storage process, and the two went through an oxidation reaction together, so that the steel scraps are distributed on the surface of the substrate in the form of iron oxides. The alloy's matrix suffers from corrosion, showing a shape of a hole, which becomes more and more serious over time, resulting in white or gray corrosion substances on the surface of the workpiece.
In summary, the main reasons for the first and second defects are as follows:
(1) In the die casting process, plenty of non-uniform mold release agents containing silicon result in accumulation of adhesion, oxidation, discoloration and silicon spots.
(2) After shot blasting, a lot of steel shot debris adhere to the surface of the workpiece, which reacts with the aluminum substrate in a humid environment, aggravating the corrosion of the aluminum substrate.
(3) Steel shot blasting causes hitting pits, which is easy to accumulate processing media and not conducive to the drying of the workpiece, thus causing the surface of the workpiece more prone to oxidation and corrosion.
(4) The production and storage process is unreasonable, and the prevention of corrosion is not fully considered.
3. Measures to prevent black spots
In view of the characteristics and cause analysis of the different types of black spots occurred in the above-mentioned aluminum alloy cover parts of the company on site, combined with the actual situation, the main measures to prevent such black spots were proposed based on the various processes of aluminum production.
3.1 Die casting processes
After experimental analyses, the mold release agent used on site has poor insufficient inhibition corrosion for aluminum and causes severe corrosion on the surface of aluminum parts. In addition, in the process of die casting and demolding, the mold release agent containing silicon is pressed and adhered to the surface of the aluminum part, which is prone to organic black spots for large areas. We use a silicon free mold release agent that has good insufficient inhibition corrosion for aluminum by learning the market and analyzing the product's performance to reduce the corrosion caused by the mold release agent to aluminum parts, while optimizing the die casting process to improve the accumulation of mold release agents.
3.2 Shot blasting processes
Shot blasting can make the surface dense, increase the surface's strength and wear resistance and improve the existence of micropores on the surface of die castings. However, the surface of the aluminum parts after shot blasting becomes significantly rough, resulting in the residual processing fluid and cleaning fluid in the later processing and cleaning process, and it is not conducive to the drying of the aluminum parts in the storage process. Moreover, shot blasting makes a large number of steel scraps adhere to the surface of aluminum parts, forming bimetallic galvanic cells and intensifying the corrosion of aluminum parts. It is recommended that the shot's material should be changed to avoid bimetal corrosion.
3.3 Machining
Processing fluid is the main industrial medium which is in contact with aluminum parts during processing, and it may remain on the surface of the workpiece for a long time under the insufficient cleaning condition. If the pH value of the processing fluid is too high and the corrosion inhibition performance is insufficient, they will cause corrosion of aluminum parts. In addition, the processing fluid contains aluminum powder, aluminum scraps, iron filings, miscellaneous oil, bacteria, fungi and other impurities that may cause the failure and deterioration of the processing fluid and affect the corrosion inhibition performance of the processing fluid, creating conditions for the corrosion and mildew of aluminum parts and accelerating corrosion and mildew spots. The processing fluid with moderate pH value, good inhibition performance for aluminum corrosion and good biological stability should be used, and maintenance of the processing fluid on site should be done well to form a systematic real-time maintenance plan. Control the pH value and concentration of the processing fluid within a certain range, and control the content of impurities, number of colonies, hardness, etc.
3.4 Cleaning and drying processes
If water or a cleaning agent with insufficient inhibition corrosion for aluminum is used to clean the workpiece and it is not dried completely later, it's easy to form a hidden danger of corrosion and mildew in the storage process. A cleaning solution with excellent corrosion inhibition should be used, and the workpiece should be completely dried after cleaning to prevent liquid residue.
3.5 Storage processes
Die casting, processing and storage should be carried out separately to prevent dust and liquid industrial media from polluting the storage environment of the workpiece. Avoid outdoor storage. In addition, the aluminum parts should be protected against moisture in the storage process, and regular inspection and maintenance should be conducted during rainy, frosting and other seasons when corrosion is easy to occur.
4. Conclusion
The generation mechanism of the two different types of black spots on the surface of ADC12 aluminum alloy cover parts is analyzed, and control measures of two kinds of black spots are proposed in accordance with the characteristics of die casting, processing, storage and management in the processing technology of the aluminum alloy cover part, combined with the actual production experience. After taking the above measures, the incidence of the two types of black spots has been greatly reduced, which has played an important role in the quality improvement and cost control of the aluminum alloy cover part.
In summary, the main reasons for the first and second defects are as follows:
(1) In the die casting process, plenty of non-uniform mold release agents containing silicon result in accumulation of adhesion, oxidation, discoloration and silicon spots.
(2) After shot blasting, a lot of steel shot debris adhere to the surface of the workpiece, which reacts with the aluminum substrate in a humid environment, aggravating the corrosion of the aluminum substrate.
(3) Steel shot blasting causes hitting pits, which is easy to accumulate processing media and not conducive to the drying of the workpiece, thus causing the surface of the workpiece more prone to oxidation and corrosion.
(4) The production and storage process is unreasonable, and the prevention of corrosion is not fully considered.
3. Measures to prevent black spots
In view of the characteristics and cause analysis of the different types of black spots occurred in the above-mentioned aluminum alloy cover parts of the company on site, combined with the actual situation, the main measures to prevent such black spots were proposed based on the various processes of aluminum production.
3.1 Die casting processes
After experimental analyses, the mold release agent used on site has poor insufficient inhibition corrosion for aluminum and causes severe corrosion on the surface of aluminum parts. In addition, in the process of die casting and demolding, the mold release agent containing silicon is pressed and adhered to the surface of the aluminum part, which is prone to organic black spots for large areas. We use a silicon free mold release agent that has good insufficient inhibition corrosion for aluminum by learning the market and analyzing the product's performance to reduce the corrosion caused by the mold release agent to aluminum parts, while optimizing the die casting process to improve the accumulation of mold release agents.
3.2 Shot blasting processes
Shot blasting can make the surface dense, increase the surface's strength and wear resistance and improve the existence of micropores on the surface of die castings. However, the surface of the aluminum parts after shot blasting becomes significantly rough, resulting in the residual processing fluid and cleaning fluid in the later processing and cleaning process, and it is not conducive to the drying of the aluminum parts in the storage process. Moreover, shot blasting makes a large number of steel scraps adhere to the surface of aluminum parts, forming bimetallic galvanic cells and intensifying the corrosion of aluminum parts. It is recommended that the shot's material should be changed to avoid bimetal corrosion.
3.3 Machining
Processing fluid is the main industrial medium which is in contact with aluminum parts during processing, and it may remain on the surface of the workpiece for a long time under the insufficient cleaning condition. If the pH value of the processing fluid is too high and the corrosion inhibition performance is insufficient, they will cause corrosion of aluminum parts. In addition, the processing fluid contains aluminum powder, aluminum scraps, iron filings, miscellaneous oil, bacteria, fungi and other impurities that may cause the failure and deterioration of the processing fluid and affect the corrosion inhibition performance of the processing fluid, creating conditions for the corrosion and mildew of aluminum parts and accelerating corrosion and mildew spots. The processing fluid with moderate pH value, good inhibition performance for aluminum corrosion and good biological stability should be used, and maintenance of the processing fluid on site should be done well to form a systematic real-time maintenance plan. Control the pH value and concentration of the processing fluid within a certain range, and control the content of impurities, number of colonies, hardness, etc.
3.4 Cleaning and drying processes
If water or a cleaning agent with insufficient inhibition corrosion for aluminum is used to clean the workpiece and it is not dried completely later, it's easy to form a hidden danger of corrosion and mildew in the storage process. A cleaning solution with excellent corrosion inhibition should be used, and the workpiece should be completely dried after cleaning to prevent liquid residue.
3.5 Storage processes
Die casting, processing and storage should be carried out separately to prevent dust and liquid industrial media from polluting the storage environment of the workpiece. Avoid outdoor storage. In addition, the aluminum parts should be protected against moisture in the storage process, and regular inspection and maintenance should be conducted during rainy, frosting and other seasons when corrosion is easy to occur.
4. Conclusion
The generation mechanism of the two different types of black spots on the surface of ADC12 aluminum alloy cover parts is analyzed, and control measures of two kinds of black spots are proposed in accordance with the characteristics of die casting, processing, storage and management in the processing technology of the aluminum alloy cover part, combined with the actual production experience. After taking the above measures, the incidence of the two types of black spots has been greatly reduced, which has played an important role in the quality improvement and cost control of the aluminum alloy cover part.
