Anodizing aluminum: the role of the cast structure of the billet

Microstructure as a source of defects anodized profiles

Features of the microstructure of the material aluminum profiles - aluminum alloys - very often cause a heterogeneous appearance of their anodized surface. The reasons for this heterogeneity are disorders of microstructure technology of production of aluminum profiles - from ingot casting-pillars, Sources from which are cut blanks for the extrusion press and before heat treatment technology to press quenching and artificial aging in an oven. The reasons for this lie in the manufacturing defect profile technology, but it is already apparent in the process of anodizing.

The main causes of the problems, which are related to the microstructure of aluminum profiles are:

reverse (inverse) segregation of alloying elements in the ingot;
incomplete homogenization of the ingot structure;
inhomogeneous recrystallization of the deformed grain structure along the profile section;
incomplete or non-uniform profile hardening on the press;
violations of the technology of artificial aging of profiles (under-aging or over-aging);
high content of such impurities, zinc, iron, copper.

Production technology of anodized aluminum profiles

The microstructure of aluminum profiles and the subsequent surface quality after anodizing depends on various technological parameters throughout their production:

charge quality and melt processing;
casting of ingots-pillars;
homogenization of ingots;
heating of ingots;
extrusion of profiles;
press hardening of profiles;
artificial aging of profiles;
surface preparation of profiles (degreasing, travlenye, lighting);
anodizing profiles and then filling the anodizing coating.

Scheme temperature change in the production of aluminum profiles shown in Figure 1. The magnitude of the temperature, the duration of exposure to it, as well as its rate of change during heating and cooling are key technological parameters, which affect the formation of the microstructure of the finished aluminum profiles.

Figure 1 - The change in temperature in the production process
anodised aluminum profiles

Aluminum profiles produce, usually, 6xxx series alloys, whose main alloying elements are magnesium and silicon. Intervals content of these elements in the most popular in Europe and the US aluminum alloys are shown in Figure 2.

Figure 2 - Top alloys for the production of aluminum profiles

alloys 6060 and 6063 (analogue of the domestic AD31) almost all profiles for window frames and doors are made, and curtain wall, translucent and ventilated. 6005A alloy used, if desired somewhat increased strength. alloys 6061 (AD33) i 6082 (AD35) is more often used for load-bearing building structures, which require increased strength. Comparison of different strength aluminum alloys is shown in Figure 3.

Figure 3 - Comparison strength wrought aluminum alloys

inverse segregation zone ingot

In the crystallization of aluminum ingots of aluminum alloys inevitably arises a surface layer with a high content of alloying elements and impurities. This layer is called the inverse segregation zone. When, for example, alloy ingots 6060 it has increased the magnesium content, silicon and iron (figure 4).

Figure 4 - The microstructure of the ingot-pillar made of aluminum alloy 6xxx series

The modern method of casting ingots-pillars is the so-called "Hot-Top" method (Figure 5). The figure shows a diagram of a method of molding Hot-Top for example, one of the ingot-pillar. Modern casting machines can simultaneously cast a few dozen, and even more than a hundred, bullion-pillars.

Figure 5 - Casting an ingot-pillar method Hot-Top

Features of this ingot casting method - limit heat transfer from the top of the melt, as well as the use of systems for lubricating the crystallizer with a mixture of gas and special oil provide a minimum thickness of the reverse segregation layer outside the ingots (Figure 6).

Figure 6 - Zone inverse segregation in the ingot of alloy-pillars 6060
when casting pillars method Hot-Top

In case of violations of the pressing technology, the metal from this segregated layer can fall into the finished aluminum profile (see. part 2). Because of its chemical composition is significantly different from the chemical composition of the base metal, it causes all sorts of heterogeneity profile properties, including, heterogeneity anodized surface appearance.

Figure 7 - Segregation shell blank is collected in a rear portion thereof.
With too thin press residue (less 15 % workpiece length)
material inverse segregation zone can enter the profile

homogenization of ingots

An important technological step is homogenisation of ingots. It consists of exposure of ingots at about 580 ° С for several hours and sufficiently fast forced cooling (see. Figure 1).

When homogenizing ingots, it is important to ensure:

Maximum complete conversion AlFeSi particles of beta-phase in the phase alfa-. In this case, there is a transition from large coarse particles to smaller and more rounded ones (figure 8).
The high degree of transition AlFeSi particles of beta-phase in alfa-phase provides a favorable effect on the compressibility of the workpiece, as well as the quality anodized surface due to a more uniform and homogeneous alkali etching profile surface.
The size and distribution of intermetallic particles, for example, particle Mg₂Si, affects the duration of exposure ingot at a temperature homogenization, and the cooling rate after homogenisation of ingots. for instance, for alloy ingots 6060 optimal homogenization parameters are considered: homogenization temperature 585 oC, holding at that temperature for about 5 hours and cooling after soaking at a rate of about 300 ° C per hour in a special cooling chamber.

Figure 8 - Milling and rounding irons
cast alloy structure 6060 resulting ingot homogenization

See. continued:

part 2: influence of die design and extrusion technology;
part 3: influence of the chemical composition of the workpiece material.

Source:

1. Tom Hauge, Hydro Aluminium, IHAA Symposium 2014, New York