defects anodizing 

Aluminum anodizing: the impact of extrusion

Introduction

Features of the microstructure of the material extruded 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.

In the previous part - part 1 - it was considered the impact of technology ingot casting-pillars the microstructure of aluminum profiles.

In this part of the - part of the 2 - will be considered technology impact extrusion of profiles the quality of the surface of anodized aluminum profiles.

Temperature and strain

The microstructure of the aluminum profiles and the subsequent quality of the surface after the anodizing process is dependent on various parameters throughout their production. The magnitude of the temperature, the duration of exposure to it, heating and cooling rate, and the rate of deformation and the associated increase in temperature are major technological parameters, which influence the formation of the microstructure of finished aluminum profiles (figure 1).


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

Aluminum alloys

Aluminum profiles are produced mostly from alloys of the 6xxx series, in which the main alloying elements are magnesium and silicon (figure 2).

Figure 2 - base alloy for the production of aluminum profiles

alloys 6060 and 6063 (analogs of the domestic AD31) make almost all profiles for window frames and doors, and curtain wall.

Hydro Aluminium company applies its internal aluminum alloys with narrower intervals of the content of alloying elements and impurities, than conventional alloys, such as 6060, 6063, 6005, 6082. it provides, among other things, higher stability of profile properties (figure 3).


Figure 3 - Chemical composition of internal aluminum alloys
by Hydro Aluminum [2]

Pererekristallizatsiya grain structure

For many wrought aluminum is characterized by the phenomenon of recrystallization. This phenomenon is, that stretched, layered grain structure, which occurs during their plastic working, spontaneously transforms into an equiaxed recrystallized grain structure (figure 4).

and) – The grain structure of the cast ingot


b) Deformed unrecrystallized grain structure
extruded aluminum profile


c) Fully recrystallized grain structure
extruded aluminum profile

Figure 4 - recrystallization of the grain structure
profiles from aluminum alloy 606035 [1]

Depending on the doping level of the aluminum alloy and the degree of plastic deformation, which he received material during the extrusion profile , recrystallization grain structure may be complete or incomplete. Incomplete recrystallization may cause irregularities in the surface profile as spots and bands.

Dependence of the degree of recrystallization of the material of aluminum profiles on the degree of alloying and the level of strength (in order of increasing tendency to partial recrystallization) [1] (for alloys - see. drawings 2 and 3):

– low-alloyed variant aluminum alloy 6060 – alloy 606035 (F22):

  • usually complete recrystallization (see. Figure 3c);

– highly doped embodiment the aluminum alloy 6060 – alloy 606090 (F25):

  • complete recrystallization of thin-walled profiles;
  • partial recrystallization of thick-walled profiles;

– narrowed embodiment the aluminum alloy 6005 – alloy 600540 (F27):

  • usually complete recrystallization of thin-walled profiles;
  • partial recrystallization of thick-walled profiles;

– narrowed embodiment the aluminum alloy 6082 – alloy 608250 (F31) (figure 5):

  • usually complete recrystallization of the surface layer;
  • layered structure is deformed in the inner layers of profiles.


Figure 5 - Profile of aluminum alloy 608250 [1]:
fully recrystallized grain structure in the surface layers;
layered deformed unrecrystallized grain structure in the central layer

Pressing technology: billet heating

Heating the billet for subsequent pressing on an extrusion press (Figure 6) are performed for the following purposes [1]:

  • Dissolution particles Mg2Si. Insufficient particle dissolution Mg2Si has an adverse effect on the strength of the aluminum profile. Moreover, in incomplete dissolution of the particles in an aluminum matrix, different portions of the surface profile of an alkaline etching differently perceive, which is performed before anodizing. This leads to differences anodized surface gloss or mattness.
  • Softening aluminum so, to allow its compaction without breaking the extrusion die, that is, to reduce its resistance to plastic deformation.
  • Typical heating temperatures of billets before pressing from various alloys (see. Figure 3):
    – alloy 606035: 470-480 ° C
    – alloy 608250: 490-500 ° C


Figure 6 - Diagram of an extrusion press for direct extrusion of aluminum [1]

The design of extrusion dies

  • The design of the matrix has a great influence on the quality of surface profiles.
  • Matrix design affects microstructure:
    – grain size;
    – grain orientation (texture):
    – hitting the profile of the outer shell of the workpiece (reverse segregation zone) with a chemical composition, which is very different from the base metal (see. more details here). It happens, in particular, with too short press residue (figure 7).
  • Hollow matrix is ​​very important to optimize the height and width “Needle” (bridges), supporting mandrel at a predetermined position. Too long or thick “needles” can lead to significant inhomogeneity of the microstructure and the formation of stripes (figure 8).
  • The reason for the appearance of these bands is significant differences in the degree of plastic deformation in the places of welded seams and in other sections of the pipe (Figure 9).
  • Inhomogeneity of plastic deformation leads to inhomogeneous recrystallization of deformed grains (figure 10). This becomes especially evident after alkali etching surface profiles before anodizing.

Figure 7 - Scheme contact material ingot segregation zone
into the finished profile while reducing the thickness of the press residue [1]


Figure 8 - Longitudinal welds on aluminum tube,
on the molded matrix porthol, after anodizing [1]

Figure 9 - forming circuit longitudinal welds
when extruding an aluminum pipe on a matrix of the "porthole" type [1]


Figure 10 - Microstructure of the material near the longitudinal seam pipe welding,
made on a matrix of the "porthole" type [1]

See. also part 3 the impact on the quality of the surface chemical composition of anodized profile blank.

Sources

1. Tom Hauge, Hydro Aluminium, IHAA Symposium, 2014, New York.
2. Site materials https://www.hydro.com