Fig. 2.8. Grain Directionality Due to Rolling
The cold work put into the aluminum during hot rolling must be sufficient to cause recrystallization during annealing. Intermediate anneals are required after cold reductions in the range of 45-85%. Intermediate anneals are required to keep the sheet from cracking during cold rolling; however, the amount of cold work must be sufficient to cause a fine grain size during annealing. If the final product form is sheet, it is annealed and then sent to a four-high cold rolling mill for further reduction. The number of intermediate anneals required during cold rolling depends on the alloy and the final gage required.
During spray quenching from the solution heat treating temperature, the surface cools much quicker than the center resulting in residual stresses. The faster cooling surface develops compressive stresses, while the slower cooling center develops tensile stresses. This residual stress pattern with compressive
Four High Mill
Fig. 2.9. Hot Rolling Aluminum Plate stresses on the surface helps in preventing fatigue and stress corrosion cracking. However, during machining operations, if the surface material containing the compressive stresses is removed, the interior material with tensile residual stresses is exposed and the part is even more susceptible to warping, fatigue, and stress corrosion cracking. To minimize these problems, aluminum plate and extrusions are often stress relieved by stretching 1/2 to 5%; a temper designated as Tx5x or Tx5xx.
Cold working during rolling results in highly directional grain structures which can affect stress corrosion resistance. The longitudinal direction is the most resistant, followed by the long transverse direction, with the short transverse being the most susceptible. Thick 7XXX plate is therefore supplied in stress corrosion resistant tempers, such as the T73, T74, and T77 tempers, and the 2XXX alloys are given the T6 and T8 tempers. For example, 7075-T6 resists stress corrosion cracking at tensile stresses up to only 7ksi, while 7075-T73 resists stress corrosion cracking up to 44ksi when tested under similar conditions.4 For thinner sheet, which is not as affected by through-the-thickness effects, the 7XXX alloys can be used in the higher strength T6 temper and the 2XXX alloys are given the T3 or T4 tempers.
Thick Plate Product
Extruded structural sections are produced by hot extrusion in which a heated cylindrical billet is pushed under high pressure through a steel die to produce the desired structural shape. The extrusion is then fed onto a run-out table where it is straightened by stretching and cut to length. During extrusion, metal flow occurs most rapidly at the center of the ingot resulting in oxides and surface defects being left in the last 10-15% of the extrusion which is discarded.
In general, the stronger the alloy, the more difficult it is to extrude. One of the advantages of the 6XXX alloys is that they exhibit good extrudability. On the other hand, the 2XXX and 7XXX alloys are referred to as "hard" alloys because they are more difficult to extrude. A profile's shape factor (the ratio of the perimeter of the profile to its area) is an approximate indicator of its extrudability, i.e. the higher the ratio, the more difficult it is to extrude. Unsymmetric shapes, shapes with sharp corners, profiles with large thickness variations across their cross section, and those that contain fine details are all more difficult to extrude. Generous fillets and rounded corners help to reduce extrusion difficulties.
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