Aluminum alloy extruded materials, especially aluminum profiles, often have a kind of “pitting” defect on the surface during the extrusion process, which is called “pitting surface” in production [1]. Pockmarked surface of aluminum alloy profiles refers to the appearance of very small nodules with different densities, trailing tails on the surface of the profile. Adheres to the surface of the product. In the extrusion production of large-section profiles, this defect is more likely to occur due to the influence of ingot structure, extrusion temperature, extrusion speed, and mold complexity [2-4]. Pimple defects can be removed in the profile surface pretreatment process (especially the alkali etching process), and most of the fine particles can be removed, while a small number of particles with large particle size and strong adhesion remain on the profile surface, affecting the appearance quality of the final product. In ordinary building door and window profile products, customers generally accept minor pitting defects, but for industrial profiles that require both mechanical properties and decorative properties or more emphasis on decorative properties, customers generally do not accept this defect, especially with Pockmarked defects (black slag spots) with inconsistent background color [5-9]. Our factory has long-term occurrence of roughening of extrusion profiles and surface defects of particles, which are difficult to eliminate after flat oxidation or electrophoresis treatment, which seriously affects the surface quality of products and reduces the qualified rate of production. In order to analyze the formation mechanism of napping particles, through the morphology and composition analysis of defect positions under different alloy compositions and extrusion processes, the difference between the defect and the matrix is compared, and a reasonable solution to effectively solve napping and particles is proposed, and a trial test is carried out .

To solve the pitted defects of profiles, the formation mechanism of the pitted defects must be clear. During the extrusion process, the sticky aluminum of the working belt of the die is the main cause of pitting defects on the surface of the extruded aluminum. This is because the extrusion process of aluminum is carried out at a high temperature of about 450°C. If the effects of deformation heat and friction heat are added, the temperature of the metal will be higher when it flows out of the die hole. When the product flows out of the die hole, due to the high temperature, there is a phenomenon of aluminum sticking between the metal and the working belt of the mold [11]. The form of this kind of bonding is often: the repeated process of bonding-tearing-rebonding-re-tearing, and the product is flowing forward, so many small pits appear on the surface of the product, resulting in extrusion. Pimple defects on the surface of pressed products. This bonding phenomenon is related to factors such as the quality of the ingot, the surface condition of the working belt of the mold, the extrusion temperature, the extrusion speed, the degree of deformation, and the deformation resistance of the metal.

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Test materials and methods

Through the preliminary investigation, it is known that factors such as metallurgical purity, mold state, extrusion process, composition (insoluble impurity phase), production status and other factors may affect the roughened particles on the surface. In the test, two kinds of alloy rods, 6005A and 6060, were used to extrude the same cross-section, and the morphology and composition of nap particles were analyzed by means of direct reading spectrometer and SEM, and compared with the surrounding normal matrix.

In order to clearly distinguish the morphology of nap and particle defects, the definitions are as follows:

(1) Pulling defects are also called pockmarks or burrs. It is a kind of point defect, irregular tadpole-like and point-like scratch defects appearing on the surface of the profile, the defect starts from scratch stripes, and accumulates into metal beans at the end of the scratch until the defect falls off. The size of the nap is generally 1-5mm, and it will be dark black after oxidation treatment, which will eventually affect the appearance of the profile, as shown in the red circle in Figure 1.

(2) Surface particles are also called metal beans or adsorption particles. The surface of the aluminum alloy profile is accompanied by spherical gray-black hard particle metal, and the structure is loose. On the surface of aluminum alloy profiles, there are two types: those that can be wiped off and those that cannot be wiped off. The size is generally less than 0.5mm, and it feels rough to the touch. There is no scratch in the front section, and there is little difference from the substrate after oxidation, as shown in the yellow circle in Figure 1.