Causes of surface mottled defects of general aluminum profiles

Causes of surface mottled defects of general aluminum profiles

Aluminum profiles undergo several major processes such as melting and casting, homogenization treatment, extrusion and anodizing. Due to improper alloy composition or limited treatment process, various corrosion defects, such as white bars, spots and patches, will appear on the surface of processed profiles. In order to improve the surface quality of aluminum profiles and prevent and control surface mottled corrosion defects, It is necessary to strengthen the in-depth analysis of the causes of mottled defects. This paper will analyze the causes of spot defects in several typical steps of aluminum profile production.

1、 Spot defects in melting and casting process

During the melting and casting process of aluminum profile, the main reason for the formation of surface mottled defects is the chemical composition of the alloy. In the alloy, in addition to the main elements mg, Al and Si, there are a few elements such as Mn, Fe and Zn. Proper addition of these elements can effectively improve the mechanical and chemical properties of raw materials, but if these elements are excessive, they will also have negative effects, so they need to be strictly controlled.

A large number of experiments show that if Zn > 0.02%, on the one hand, snowflake spots may be produced if so and CI = are encountered in oxidation treatment; On the other hand, the metastable phase precipitated during aging treatment η (mgzn2) phase, in the pretreatment of anodic oxidation, η The electrochemical positions of the phase and aluminum matrix are different, forming a micro cell and speeding up the process η The dissolution rate of aluminum matrix around the phase. At this time, the phase falls off and produces snowflake spots.

Before the anodizing of aluminum profile, the aluminum solid solution Zn in alkaline washing reacts with Zn2 +, and the aluminum corrected by electrode potential is replaced by the solution. At the same time, the dissolution reaction occurs, which speeds up the dissolution rate of aluminum matrix, resulting in snowflake spots in the alloy. The higher the content of Zn, the more serious the situation will be. It should also be recognized that when the free Si in the alloy and the segregation phase of Al Fe Si compound react, the relative aluminum solid solution presents a cathode, and the surrounding aluminum dissolves first in alkali washing and forms snowflakes.

In addition, if the segregation phase presents a continuous chain distribution along the grain boundary, it is easy to produce grain boundary corrosion. Fe, Zn and other elements in the alloy are easy to form α― Fe2SiAl8、 β- FeSiAl, t-almgzn3 and other impurity phases greatly reduce the electrochemical properties, corrosion resistance, oxidation coloring uniformity and extrusion resistance of the alloy. Especially at the location where Fe is enriched, the oxide film is thin and not dense enough, and this anode area is corroded first. The intermetallic compound al6 (MnFe) formed by Mn and Fe weakens the transparency of the oxide film.

Therefore, in the actual production process, these elements should be reasonably controlled, which is of great significance to improve the efficiency of aluminum alloy. In order to minimize the harmful effects of impurities, the element content should be controlled in real life, with (FE) = 0.15 ~ 0.20 ω （Zn）＜0.02％、 ω (SI) (MN) ＜ 0.1%.

2、 Spot defects in homogenization treatment

The main goal of homogenization treatment is to refine the grains to avoid selective intergranular corrosion in the subsequent processing. And produce surface spots. During ingot casting, the strengthening phase Mg2Si and β- FeSiAl is connected in the ingot and distributed in a dendritic manner, which has a serious impact on the extrusion properties.

Although a large number of Mg2Si phases will break during extrusion, the broken phase will not disperse in the matrix. During aging, some Mg2Si phases will grow abnormally. After alkali washing, it is preferentially corroded to produce snowflake corrosion pits. because β- The electrochemical positions of FeSiAl and aluminum matrix are different, which will promote the rapid dissolution of aluminum matrix and produce corrosion pits during alkali washing.

Through scientific homogenization treatment, it can promote β- FeSiAl phase is spheroidized, while Mg2Si phase is acicular, which improves the microstructure uniformity and can effectively weaken the formation of dark spots. It is proved by practice that the standard of feasible homogenization heat treatment is: when the temperature is 560 ± 5 ℃, the holding time is more than 6 hours, and when the temperature is about 580 ℃, the holding time is about 2 hours.

3、 Spot defects during extrusion

As the extrusion performance of aluminum profiles is generally good, the extrusion speed of general solid profiles is about 20-50m / min. However, in order to improve the uniformity of aluminum profile processing structure, the speed of extrusion profile outlet should be controlled to be greater than 30m / nin. In order to strengthen the solid solution of Mg2Si phase in aluminum matrix, the outlet temperature of aluminum profile during extrusion should be greater than 500 ℃, but the surface treatment needs to be strengthened in the subsequent process; However, the outlet temperature shall not exceed 530 ℃, otherwise the surface quality of aluminum profile will be reduced and the dimensional accuracy cannot be guaranteed.

Because the pre heating temperature of ingot is about 480 ℃ before extrusion treatment, in order to increase the outlet temperature of profile at 510 ℃, the extrusion speed can be changed appropriately and the outlet temperature can be adjusted. When selecting the extrusion process, priority should be given to the mechanical properties of aluminum profiles. The process parameters adopted for products should also be different due to different requirements for materials and surface treatment quality.

After some practical test requirements, it can be found that when the outlet speed of extruded profile is controlled at 37m / min, the surface treatment of aluminum profile has strong performance, and the temperature at the outlet of profile is just 510 ℃, which meets the requirements. It can be seen that if you want to control the spot corrosion of aluminum profile, the main measures are to control the heating temperature of ingot to about 480 ℃ and work at an appropriate extrusion speed to ensure that its outlet temperature is about 510 ℃.

4、 Mottled defects in aging

In the aging process, it is mainly the precipitation process of solid solution strengthening phase. According to the properties, distribution and size of strengthening phase, the effects on aging mainly include aging time and aging temperature. The strengthening phase of Al Mg SI system is mainly Mg2Si phase, and its precipitation order is as follows: ① supersaturated solid solution is high; ② Concentration vacancy GP area; ③ Acicular coherent precipitation phase β＂； ④ Rod like semi coherent precipitate phase β'； ⑤ Plate stable precipitation phase β。

When the temperature is higher than 200 ℃ for a long time during aging, or when reheating occurs during aging, it may cause β Abnormal growth of phase. The difference of electrochemical properties is enlarged due to the uneven distribution of grains. During alkaline washing, Mg2Si phase will dissolve in advance and form corrosion pits, and snowflakes will be formed after oxidation. In order to ensure that the Mg2Si phase in the aluminum profile can be finely dispersed and distributed after aging, it is generally required to keep it at 200 ℃ for 1-2 hours, or at 170 ℃ for 7-10 hours, or the method of segmented failure can be selected, that is, the holding time at 185 ℃ is 2.5 hours, and the holding time at 205 ℃ is 1 hour, which will obtain better aging.

5、 Control of processing technology

Spot defects often occur on the surface of general aluminum profiles, and most of these defects occur in the extrusion process, especially the determination of process methods and parameters, which are very important to the quality of aluminum profiles. First, the operation of aluminum profile extrusion die is very key, and there shall be no defects. If the edge of the die is rough and there are errors in size, it will affect the quality of aluminum profile. Second, due to the thermal influence in aluminum profile welding, it will cause the problem of uneven color. In the process of weldment, a poorly colored transparent oxide film may be formed on the surface in the solid solution state, but a poorly glossy and poorly transparent colored oxide film will be formed on the surface in the precipitation state. Third, during machining such as turning, bending and milling, the cut powder is attached to the surface of aluminum profile, which is very easy to cause scars. In order to avoid such scars as much as possible, local processing with sandpaper can be used.

In addition, if local overheating occurs during sand blasting, polishing and grinding, resulting in uneven tone and spots. In the process of packaging and transportation, in order to avoid scratches and bumps of aluminum profiles, attention should be paid to the links of packaging and transportation to reduce unnecessary damage.

It can be seen from the above that there are many reasons for the surface mottled defects of aluminum profiles. This paper analyzes and expounds several typical steps in the production process of profiles. In the production process, only by finding out the causes of surface mottled defects, finding problems in time and actively solving them, can the quality of aluminum profiles entering the market be improved and the loss be reduced to a low level. In addition, the quality inspection department also needs to strictly control the product quality and strive to improve the overall quality of aluminum profile products.