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How is ADC12 Aluminum Alloy Surface Black Spot Formed

The aluminum alloy is light, with good thermal conductivity and corrosion resistance. They meet the requirements of energy-saving and emission reduction. Therefore, it is a development trend to use cast aluminum alloy to manufacture automobile transmission cover. However, aluminum alloy workpieces are prone to various surface defects, mainly manifested in black dot defects. Some literature analyzed the black spot defect on the surface of 5A02 aluminum alloy flange forging and determined that it was caused by organic medium pollution. It was found that 7A04 aluminum alloy products produced black spot defects, the causes include corrosion of corrosive substances and organic pollution. These surface defects affect the appearance and qualification rate of the product and adversely affect the corporate and product image.

A gearbox manufacturer has a die-casting and machining production line for automobile transmission shell and cover, and the transmission and clutch shell and cover are made of ADC12 aluminum alloy. However, in the process of processing and storage of die-casting aluminum alloy shell cover, surface black spot defects tend to occur.

In this paper, the author according to the actual production situation of the aluminum alloy workpiece processing site and storage conditions, combined with the analysis means such as scanning electron microscopy (sem), the surface morphology and composition of different types of black spot defect was analyzed and discussed the production process each link of shading defects influence mechanism, and on this basis, put forward the corresponding process optimization measures, to achieve the purpose of reduce shading defects, improve product quality.

1. Preliminary analysis

The ADC12 die-casting aluminum alloy shell cover part has two types of black spot defect morphologies generated during processing and storage, and their macroscopic morphologies are shown in figure 1. As can be seen from figure 1, the first defect type is a continuous patchy black spot, which is significantly darker than the normal area. Defect types: the macroscopic morphology is white or gray powdery adhesion, and there are black spots with spot-like distribution after adhesive erasure. The main production process of the aluminum alloy shell cover is die-casting → 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

Two spots defect had occurred in aluminum alloy parts of processing on the surface, without defects of machining surface normal, the types of defects, defect type 2 typical sample blocks, after ultrasonic cleaning in petroleum ether, using field emission scanning electron microscope observation of shading area morphology and microstructure of the three is shown in figure 2.

According to figure 2, it can be seen that the normal non-processed surface of aluminum alloy workpiece is denser under the action of steel shot blasting due to shot blasting, but there are many pits caused by steel shot blasting, and the overall morphology is not smooth. At a higher magnification, flaky or granular debris was found on the normal surface. The microstructure of defect type I was not significantly different from that of normal surface and the pits and debris caused by shot blasting was retained. The micromorphology of the white adhesive of defect type ii was granular debris. After wiping the white adhesive and ultrasonic cleaning, the micromorphology of the substrate was significantly different from that of the normal surface, showing a small-sized porous network structure.EDS energy spectrum analysis was performed on the characteristic areas of the normal and defect surfaces, and the results were shown in figure 3.

By EDS test results as you can see, the surface of normal matrix of main ingredients of ADC12 aluminum alloy original elements, besides, also contains a certain amount of C and O elements, probably from the workpiece in machining process are exposed to the organic industry medium pollution, mild oxidation, etc., the normal surface of detrital composition is completely different from those of substrate, the main elements as Fe, and there is higher content of Cr, presumably produced in the process of blasting is difficult to remove debris adhesion of steel balls.


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