Anodizing: aluminum composition

aluminum Microstructure

Features material microstructure aluminum profiles - aluminum alloys - very often cause inhomogeneity appearance of anodized surface. The reasons for this heterogeneity of the microstructure are usually not technology problems aluminum anodizing, disorders and production technology of aluminum profiles - the chemical composition of the charge and ingot casting-pillars before heat treatment technologies by quenching in a press furnace and artificial aging.

• In parts 1 It was considered the influence of ingot casting technology-pillars,
• c parts 2 – technology impact extrusion of profiles, as well as designs Matrices.

In this part of the - part of the 3 - it examines the effect on the quality of anodized profiles alloying elements and impurities in the applied aluminum alloy.

Temperature regimes in the manufacture of aluminum profiles

Temperature regimes in the manufacture of anodized aluminum profiles are schematically illustrated in Figure 1.

Figure 1 - The change in temperature in the production process
anodized aluminum profiles [1]

The vast majority of aluminum sections made from alloys of the 6xxx series, alloying elements of which magnesium and silicon (figure 2).

Figure 2 - base alloy for the production of aluminum profiles

To better monitor the impact of the content of alloying elements, as well as the level of undesirable impurities, Company Hydro Aluminium divides the usual alloys, for example, 6060, more narrow domestic alloys, for example, 606035 (drawings 3 and4).

Figure 3 - Internal aluminum alloys company Hydro Aluminium

Figure 4 – The content of alloying elements – magnesium and silicon –
alloys AD31, 6060, 6063 and 606035

Microstructure and alkaline aluminum pickling

• Alkaline pickling bath is the most important technological step for identifying the differences in appearance between different aluminum profiles. These differences arise from the differences in the chemical composition of the alloy or the amount, shape and distribution of intermetallic particles, for example, Mg₂Si or AlFeSi.
• Chemical composition differences lead to differences in particle types in terms of their chemical potential, These particles may be anodic or cathodic with respect to an aluminum-based alloy. This gives a different surface profiles susceptibility to alkaline etching
• Differences in susceptibility to alkali etching give rise to differences in gloss (or haze) of the surface after alkali etching and anodizing (Figure 4).

Figure 4 - Aluminum surface after alkaline etching:
Matte (left) and shiny (right)

The influence of alloying elements and impurities

• Silicon. Included in the AlFeSi particle and Mg₂Si, that affect the morphology of the aluminum surface after the alkali etching. This effect occurs through the particle size, their distribution, ratio of anode and cathode properties inclusions.
• Iron. Included in the AlFeSi particle. Low iron content (less 0,13 %) gives a shiny surface after anodizing (with an anode layer thickness of no more 10 μm) (figure 5) .
• Copper. Copper additions (0.12-0.15 %) give a shiny surface after anodizing (figure 6). The iron content thus should be reduced in order to achieve maximum brightness. Too much copper gives a matt surface after anodizing (low gloss).
• Manganese. manganese additives affect the degree of conversion of beta AlFeSi particles into particles of alpha-AlFeSi - by elongate rounded forms to forms. AlFeSi particles affect the moldability of the alloy and on its behavior in the alkaline etching. Manganese influences the size and distribution of the Mg particles₂Si. Moreover, manganese inhibits (slows down) grain growth (see. also below about chrome).
• Magnesium. Included in the particles Mg₂Si, which affect the characteristics of the surface after the alkali etching. The high content of silicon and magnesium gives a gray, non-shiny surface after etching and anodizing (picture 7).
• Chromium. When added together with the manganese inhibits grain growth.
• Zinc. Too much free zinc (Zn + 2) in the alkaline pickling bath results in a grainy anodized surface (Figures 8 and 9). It comes free zinc aluminum alloy with high zinc content, primarily, as an impurity.
• Titan. Titanium effect on grain size, together with boron. Bars ligature "titanium-boron" is introduced into the molten aluminum during casting ingots-pillars for optimum reduced grain size, as well as to prevent the formation of cracks in the ingot central.

Figure 5 - The effect of iron content on the reflectivity
anodized surface profile of an alloy 6060 [1]

Figure 6 - The effect of the copper content in alloys of the 6xxx series
to the shine of the anodized surface [1]

Figure 7 - Influence of magnesium and silicon in the alloys 6060 and 6063
to the gloss level of the anodized surface [1]

Figure 8 - "Galvanic" surface
anodized aluminum profile [1]

Figure 9 - The selective etching of the grain
with increased zinc content [1]

Source: Tom Hauge, Hydro Aluminium, IHAA Symposium, 2014, New York.