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ADC12 aluminum alloy surface black spot is how to form and how to prevent!

July 24, 2023
Aluminum alloy is light in weight, has good thermal conductivity and corrosion resistance, in line with the requirements of automotive lightweight, energy saving and emission reduction, therefore, the use of cast aluminum alloy to manufacture automotive transmission cover is the development trend. However, aluminum alloy workpiece is prone to various surface defects, mainly reflected as black spot defects. The black spot defect on the surface of 5A02 aluminum alloy flange forging is analyzed and the organic media pollution is determined. It is found that the black spot defects on 7A04 aluminum alloy products are caused by corrosion of corrosive substances and organic pollution. These surface defects affect the appearance and qualification rate of the product, and cause adverse effects on the image of the enterprise and the product.
A transmission manufacturer has a die casting and machining production line for automobile transmission and clutch cover. The material of transmission and clutch cover is ADC12 cast aluminum alloy. However, in the process of processing and storage of die-cast aluminum alloy shell covers, surface black spots are easy to occur.
In this paper, according to the actual production and storage conditions of the aluminum alloy workpiece processing site, combined with scanning electron microscopy and other analytical means, the surface morphology and composition of different types of black spot defects were analyzed, the influence mechanism of each link of the production process on the black spot defects was discussed, and on this basis, the targeted process optimization measures were proposed. To achieve the purpose of reducing black spot defects and improving product quality.
1. Preliminary analysis
The black spot defect morphologies generated during the processing and storage of the ADC12 die-cast aluminum alloy shell cover are mainly divided into two categories, and the macro morphologies of the two are shown in Figure 1. As can be seen from Figure 1, defect type 1 is continuous flaky black spots, which are significantly darker than normal areas. The macro appearance of defect type 2 is white or gray powdery adhesion, and after adhesive erasure, black spots with point-like distribution appear. The main production process of the aluminum alloy shell cover is: die casting → shot blasting → machining → cleaning → drying → storage. The main production processes and other conditions of the two types of black spot defects are compared in Table 1.
2. Microstructure observation and energy spectrum analysis
The two types of black spot defects occur on the unprocessed surface of aluminum alloy. The typical samples of normal unprocessed surface without defects, defect type 1 and defect type 2 are respectively taken. After ultrasonic cleaning in petroleum ether, the morphologies of the black spot parts are observed by using field emission scanning electron microscope.
As can be seen from Figure 2, the normal non-machined surface of the aluminum alloy workpiece is relatively dense under the action of steel shot blasting, but there are many pits caused by steel shot blasting, and the overall topography is not smooth. At higher magnification, it is found that flaky or granular debris is distributed on the normal surface. The microstructure of defect type 1 has no obvious difference from the normal surface, and the pits and debris caused by shot blasting process are retained. The micromorphology of the white adhesive of defect type 2 is granular debris. After the white adhesive is wiped and ultrasonic cleaned, the micromorphology of the substrate is significantly different from that of the normal surface, showing a small-size porous network structure. EDS spectral analysis was carried out on each characteristic region of the normal and defective surfaces respectively, and the analysis results were shown in Figure 3.
EDS test results show that the normal surface of the matrix is mainly composed of the original components of ADC12 aluminum alloy, in addition, it also contains a certain amount of C and O elements, which may come from the organic industrial media pollution and slight oxidation encountered by the workpiece during processing, while the composition of the normal surface debris is completely different from that of the matrix, and the main element composition is Fe. And there is a high content of Cr, it can be inferred that in the process of shot blasting, it is difficult to remove the steel shot debris adhesion.
Compared with the normal surface, the matrix and debris of defect type 1 contain higher content of C and O elements, and are more seriously polluted by organic industrial media. In addition, the content of Si element in the matrix of defect type 1 is significantly increased.
As the defect type 1 mainly occurs after the die casting process, based on its microscopic surface morphology, it can be inferred that it is caused by the organic adhesion produced during the die casting process, and the industrial medium exposed to the workpiece during the die casting process is the release agent. According to the investigation, the main component of the release agent for aluminum parts used in the die casting of this product is silicone oil, which is combined with the high silicon content in the surface matrix of the defect type 1. It can be further inferred that defect type 1 is caused by the accumulation, adhesion, oxidation and discoloration of silicon-containing release agents during die casting.
The main feature of the white adhesive of defect type 2 is the high content of O element, which suggests that the main component of the white adhesive is Al oxide or hydroxide. After the white adhesive is removed, a small porous network structure appears in the matrix of defect type 2, and the content of O element is also high. In addition, there is no adhesion of steel chips in the matrix of defect type 2, but the content of Fe element in the test area is increased. It is speculated that the steel chips originally adhering to the surface in this area have galvanic corrosion with the matrix during storage, and the oxidation reaction between the two occurs. The steel chips are distributed on the surface of the matrix in the form of iron oxides, while the aluminum alloy matrix has a porous corrosion, and becomes more and more serious with the passing of time, resulting in white or gray corrosion products floating on the surface of the workpiece.
To sum up, the main causes of defect types 1 and 2 are:
1) In the process of die casting, non-uniform enrichment of silicon-containing release agents resulted in accumulation and adhesion, and oxidation discoloration resulted in silicon spots;
2) After shot blasting, the surface of the workpiece adheres to a lot of steel pellet debris, which reacts with the aluminum matrix in the wet environment, aggravating the corrosion of the aluminum alloy matrix;
3) The blasting pit caused by the blasting of steel pellets is easy to accumulate the processing medium, which is not conducive to the drying of the workpiece and makes the surface of the workpiece more prone to oxidative corrosion;
4) The production and storage process is unreasonable, and the prevention of corrosion is not fully considered.
3. Measures to prevent black spot defects
According to the characteristics and causes of different types of black spot defects in the aluminum alloy shell cover of the enterprise, combined with the actual situation, the main measures to prevent such black spot defects are put forward from each process of aluminum production.
3.1 Die casting process
After experimental analysis, the release agent used in the field has poor effect on aluminum corrosion inhibition and will cause serious corrosion on the surface of aluminum parts. In the process of die casting and release, silicon-containing release agent is pressed into and adhered to the surface of aluminum parts, which is easy to produce large areas of organic black spots.
Based on the market understanding and product performance analysis, the non-silicon release agent with better corrosion inhibition on aluminum is used to reduce the corrosion caused by release agent on aluminum parts, and the die casting process is optimized to improve the accumulation of release agent.
3.2 Shot blasting process
Shot blasting can densification the surface, increase the surface strength and wear resistance, and improve the presence of micropores on the surface of die casting. However, the surface roughness of the aluminum parts increases significantly after shot blasting, resulting in the residue of the processing fluid and cleaning fluid easily generated during the later processing and cleaning process, and it is not conducive to the drying of the aluminum parts during the storage process. In addition, shot blasting treatment makes the surface of aluminum parts adhere to a large number of steel chips, forming a bimetallic galvanic cell, which intensifies the corrosion of aluminum parts. It is recommended to change the blasting sand material to avoid bimetal corrosion.
3.3 Machining
The processing liquid is the main industrial medium in contact with aluminum parts during the processing process, and may remain on the surface of the workpiece for a long time in the case of insufficient cleaning, and the pH value of the processing liquid is too high and the corrosion inhibition performance is insufficient, which will cause the corrosion of aluminum parts. In addition, the on-site processing liquid contains aluminum powder aluminum scraps, iron scraps, miscellaneous oil, bacteria and fungi, which may cause the failure and deterioration of the processing liquid and affect the corrosion inhibition performance of the processing liquid, creating conditions for the corrosion mildew of aluminum parts and accelerating the production of corrosion spots and mildew spots. The processing fluid with moderate pH value, good aluminum corrosion inhibition performance and good biological stability should be used, and the on-site maintenance of the processing fluid should be done to form a systematic real-time maintenance plan, and the pH value and the concentration of the processing fluid should be controlled within a certain range, and the impurity content, the number of colonies and the hardness should be controlled.
3.4 Cleaning and drying process
If water or a cleaning agent with insufficient aluminum corrosion inhibition performance is used to clean the workpiece, and the later drying is not complete, the hidden danger of corrosion mildew is easily formed during storage. A cleaning solution with excellent corrosion inhibition should be used and dried completely after cleaning to prevent liquid residue.
3.5 Storage Process
Die casting, processing and storage should be carried out in different areas to prevent dust and liquid industrial media from polluting the workpiece storage environment; Avoid outdoor storage. In addition, moisture protection should be done during the storage of aluminum parts, and regular inspection and maintenance should be carried out during the corrosion-prone season such as plum rain and frost.
4. Closing remarks
Based on the characteristics of die casting, processing, storage and management in the processing of ADC12 aluminum alloy shell cover, the main control measures to prevent the occurrence of two kinds of surface black spot defects are proposed. After taking the above measures, the incidence of two types of black spot defects is greatly reduced, which plays an important role in the quality improvement and cost control of the aluminum alloy shell cover products.
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