Improving the quality of high-end aluminum alloy profiles: causes and solutions to pitted defects in profiles

During the extrusion process of aluminum alloy extruded materials, especially aluminum profiles, a "pitting" defect often occurs on the surface. The specific manifestations include very small tumors with varying densities, tailing, and obvious hand feel, with a spiky feeling. After oxidation or electrophoretic surface treatment, they often appear as black granules adhering 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 the ingot structure, extrusion temperature, extrusion speed, mold complexity, etc. Most of the fine particles of pitted defects can be removed during the profile surface pretreatment process, especially the alkali etching process, while a small number of large-sized, firmly adhered particles 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 pitted defects, but for industrial profiles that require equal emphasis on mechanical properties and decorative performance or more emphasis on decorative performance, customers generally do not accept this defect, especially pitted defects that are inconsistent with the different background color.

In order to analyze the formation mechanism of rough particles, the morphology and composition of the defect locations under different alloy compositions and extrusion processes were analyzed, and the differences between the defects and the matrix were compared. A reasonable solution to effectively solve the rough particles was put forward, and a trial test was carried out.

To solve the pitting defects of profiles, it is necessary to understand the formation mechanism of pitting defects. During the extrusion process, aluminum sticking to the die working belt is the main cause of pitting defects on the surface of extruded aluminum materials. 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 mold working belt.

The form of this bonding is often: a repeated process of bonding - tearing - bonding - tearing again, and the product flows forward, resulting in many small pits on the surface of the product.

This bonding phenomenon is related to factors such as the quality of the ingot, the surface condition of the mold working belt, extrusion temperature, extrusion speed, degree of deformation, and the deformation resistance of the metal.

1 Test materials and methods

Through preliminary research, we learned that factors such as metallurgical purity, mold status, extrusion process, ingredients, and production conditions may affect the surface roughened particles. In the test, two alloy rods, 6005A and 6060, were used to extrud the same section. The morphology and composition of the roughened particle positions were analyzed through direct reading spectrometer and SEM detection methods, and compared with the surrounding normal matrix.

In order to clearly distinguish the morphology of the two defects of pitted and particles, they are defined as follows:

(1) Pitted defects or pulling defects is a kind of point defect which is an irregular tadpole-like or point-like scratch defect that appears on the surface of the profile. The defect starts from the scratch stripe and ends with the defect falling off, accumulating into metal beans at the end of the scratch line. The size of the pitted defect is generally 1-5mm, and it turns dark black after oxidation treatment, which ultimately affects 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 attached with spherical gray-black hard metal particles and has a loose structure. There are two types of aluminum alloy profiles: 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. After oxidation, it is not much different from the matrix, as shown in the yellow circle in Figure 1.

2 Test results and analysis

2.1 Surface pulling defects

Figure 2 shows the microstructural morphology of the pulling defect on the surface of the 6005A alloy. There are step-like scratches in the front part of the pulling, and they end with stacked nodules. After the nodules appear, the surface returns to normal. The location of the roughening defect is not smooth to the touch, has a sharp thorny feel, and adheres or accumulates on the surface of the profile. Through the extrusion test, it was observed that the pulling morphology of 6005A and 6060 extruded profiles is similar, and the tail end of the product is more than the head end; the difference is that the overall pulling size of 6005A is smaller and the scratch depth is weakened. This may be related to changes in alloy composition, cast rod state, and mold conditions. Observed under 100X, there are obvious scratch marks on the front end of the pulling area, which is elongated along the extrusion direction, and the shape of the final nodule particles is irregular. At 500X, the front end of the pulling surface has step-like scratches along the extrusion direction (the size of this defect is about 120 μm), and there are obvious stacking marks on the nodular particles at the tail end.

In order to analyze the causes of pulling, direct reading spectrometer and EDX were used to conduct component analysis on the defect locations and matrix of the three alloy components. Table 1 shows the test results of the 6005A profile. The EDX results show that the composition of the stacking position of the pulling particles is basically similar to that of the matrix. In addition, some fine impurity particles are accumulated in and around the pulling defect, and the impurity particles contain C, O (or Cl), or Fe, Si, and S.

Analysis of the roughening defects of 6005A fine oxidized extruded profiles shows that the pulling particles are large in size (1-5mm), the surface is mostly stacked, and there are step-like scratches on the front section; The composition is close to the Al matrix, and there will be heterogeneous phases containing Fe, Si, C, and O distributed around it. It shows that the pulling formation mechanism of the three alloys is the same.

During the extrusion process, metal flow friction will cause the temperature of the mold working belt to rise, forming a "sticky aluminum layer" at the cutting edge of the working belt entrance. At the same time, excess Si and other elements such as Mn and Cr in the aluminum alloy are easy to form replacement solid solutions with Fe, which will promote the formation of a "sticky aluminum layer" at the entrance of the mold working zone.

As the metal flows forward and rubs against the work belt, a reciprocating phenomenon of continuous bonding-tearing-bonding occurs at a certain position, causing the metal to continuously superimpose at this position. When the particles increase to a certain size, It will be pulled away by the flowing product and form scratch marks on the metal surface. It will remain on the metal surface and form pulling particles at the end of the scratch. herefore, it can be considered that the formation of roughened particles is mainly related to the aluminum sticking to the mold working belt. The heterogeneous phases distributed around it may originate from lubricating oil, oxides or dust particles, as well as impurities brought by the rough surface of the ingot.

However, the number of pulls in the 6005A test results is smaller and the degree is lighter. On the one hand, it is due to the chamfering at the exit of the mold working belt and the careful polishing of the working belt to reduce the thickness of the aluminum layer; on the other hand, it is related to the excess Si content.

According to the direct reading spectral composition results, it can be seen that in addition to Si combined with Mg Mg2Si, the remaining Si appears in the form of a simple substance.

2.2 Small particles on the surface

Under low-magnification visual inspection, the particles are small (≤0.5mm), not smooth to the touch, have a sharp feeling, and adhere to the surface of the profile. Observed under 100X, small particles on the surface are randomly distributed, and there are small-sized particles attached to the surface regardless of whether there are scratches or not;

At 500X, no matter whether there are obvious step-like scratches on the surface along the extrusion direction, many particles are still attached, and the particle sizes vary. The largest particle size is about 15 μm, and the small particles are about 5 μm.

Through the composition analysis of the 6060 alloy surface particles and the intact matrix, the particles are mainly composed of O, C, Si, and Fe elements, and the aluminum content is very low. Almost all particles contain O and C elements. The composition of each particle is slightly different. Among them, the a particles are close to 10 μm, which is significantly higher than the matrix Si, Mg, and O; In c particles, Si, O, and Cl are obviously higher; Particles d and f contain high Si, O, and Na; particles e contain Si, Fe, and O; h particles are Fe-containing compounds. The results of 6060 particles are similar to this, but because the Si and Fe content in 6060 itself is low, the corresponding Si and Fe contents in the surface particles are also low; the C content in 6060 particles is relatively low.

Surface particles may not be single small particles, but may also exist in the form of aggregations of many small particles with different shapes, and the mass percentages of different elements in different particles vary. It is believed that the particles are mainly composed of two types. One is precipitates such as AlFeSi and elemental Si, which originate from high melting point impurity phases such as FeAl3 or AlFeSi(Mn) in the ingot, or precipitate phases during the extrusion process. The other is adherent foreign matter.

2.3 Effect of surface roughness of ingot

During the test, it was found that the rear surface of the 6005A cast rod lathe was rough and stained with dust. There were two cast rods with the deepest turning tool marks at local locations, which corresponded to a significant increase in the number of pulls after extrusion, and the size of a single pull was larger, as shown in Figure 7.

The 6005A cast rod has no lathe, so the surface roughness is low and the number of pullings is reduced. In addition, since there is no excess cutting fluid attached to the lathe marks of the cast rod, the C content in the corresponding particles is reduced. It is proved that the turning marks on the surface of the cast rod will aggravate pulling and particle formation to a certain extent.

3 Discussion

(1) The components of pulling defects are basically the same as those of the matrix. It is the foreign particles, old skin on the surface of the ingot and other impurities accumulated in the extrusion barrel wall or the dead area of the mold during the extrusion process, which are brought to the metal surface or the aluminum layer of the mold working belt. As the product flows forward, surface scratches are caused, and when the product accumulates to a certain size, it is taken out by the product to form pulling. After oxidation, the pulling was corroded, and due to its large size, there were pit-like defects there.

(2) Surface particles sometimes appear as single small particles, and sometimes exist in aggregated form. Their composition is obviously different from that of the matrix, and mainly contains O, C, Fe, and Si elements. Some of the particles are dominated by O and C elements, and some particles are dominated by O, C, Fe, and Si. Therefore, it is inferred that the surface particles come from two sources: one is precipitates such as AlFeSi and elemental Si, and impurities such as O and C are adhered to the surface; The other is adherent foreign matter. The particles are corroded away after oxidation. Due to their small size, they have no or little impact on the surface.

(3) Particles rich in C and O elements mainly come from lubricating oil, dust, soil, air, etc. adhered to the surface of the ingot. The main components of lubricating oil are C, O, H, S, etc., and the main component of dust and soil is SiO2. The O content of surface particles is generally high. Because the particles are in a high temperature state immediately after leaving the working belt, and due to the large specific surface area of the particles, they easily adsorb O atoms in the air and cause oxidation after contact with the air, resulting in a higher O content than the matrix.

(4) Fe, Si, etc. mainly come from the oxides, old scale and impurity phases in the ingot (high melting point or second phase that is not fully eliminated by homogenization). The Fe element originates from Fe in aluminum ingots, forming high melting point impurity phases such as FeAl3 or AlFeSi(Mn), which cannot be dissolved in solid solution during the homogenization process, or are not fully converted; Si exists in the aluminum matrix in the form of Mg2Si or a supersaturated solid solution of Si during the casting process. During the hot extrusion process of the cast rod, excess Si may precipitate. The solubility of Si in aluminum is 0.48% at 450°C and 0.8% (wt%) at 500°C. The excess Si content in 6005 is about 0.41%, and the precipitated Si may be aggregation and precipitation caused by concentration fluctuations.

(5) Aluminum sticking to the mold working belt is the main cause of pulling. The extrusion die is a high-temperature and high-pressure environment. Metal flow friction will increase the temperature of the working belt of the mold, forming a "sticky aluminum layer" at the cutting edge of the working belt entrance.

At the same time, excess Si and other elements such as Mn and Cr in the aluminum alloy are easy to form replacement solid solutions with Fe, which will promote the formation of a "sticky aluminum layer" at the entrance of the mold working zone. The metal flowing through the "sticky aluminum layer" belongs to internal friction (sliding shear inside the metal). The metal deforms and hardens due to internal friction, which promotes the underlying metal and the mold to stick together. At the same time, the mold working belt is deformed into a trumpet shape due to the pressure, and the sticky aluminum formed by the cutting edge part of the working belt contacting the profile is similar to the cutting edge of a turning tool.

The formation of sticky aluminum is a dynamic process of growth and shedding. Particles are constantly being brought out by the profile.Adhere to the surface of the profile, forming pulling defects. If it flows directly out of the work belt and is instantly adsorbed on the surface of the profile, the small particles thermally adhered to the surface are called "adsorption particles". If some particles will be broken by the extruded aluminum alloy, some particles will stick to the surface of the work belt when passing through the work belt, causing scratches on the surface of the profile. The tail end is the stacked aluminum matrix. When there is a lot of aluminum stuck in the middle of the work belt (the bond is strong), it will aggravate surface scratches.

(6) The extrusion speed has a great influence on pulling. The influence of extrusion speed. As far as the tracked 6005 alloy is concerned, the extrusion speed increases within the test range, the outlet temperature increases, and the number of surface pulling particles increases and becomes heavier as the mechanical lines increase. The extrusion speed should be kept as stable as possible to avoid sudden changes in speed. Excessive extrusion speed and high outlet temperature will lead to increased friction and serious particle pulling. The specific mechanism of the impact of extrusion speed on the pulling phenomenon requires subsequent follow-up and verification.

(7) The surface quality of the cast rod is also an important factor affecting the pulling particles. The surface of the cast rod is rough, with sawing burrs, oil stains, dust, corrosion, etc., all of which increase the tendency of pulling particles.

4 Conclusion

(1) The composition of pulling defects is consistent with that of the matrix; the composition of the particle position is obviously different from that of the matrix, mainly containing O, C, Fe, and Si elements.

(2) Pulling particle defects are mainly caused by aluminum sticking to the mold working belt. Any factors that promote aluminum sticking to the mold working belt will cause pulling defects. On the premise of ensuring the quality of the cast rod, the generation of pulling particles has no direct impact on the alloy composition.

(3) Proper uniform fire treatment is beneficial to reducing surface pulling.