The term drawing, when it applies to the manufacture of tube, rod, bar, or wire, refers to the pulling of metal through a die or succession of dies to reduce its diameter, alter the cross-sectional shape, or increases its hardness. In the drawing of aluminum tubing, one end of the extruded tube is swaged to form a solid point and then passed through the die. A clamp, known as a bogie, grips the swaged end of tubing. A mandrel is inserted into the die orifice, and the tubing is pulled between the mandrel and die, reducing the outside diameter and the wall thickness of the tubing. Wire, rod, and bar drawing is accomplished in a similar manner but the aluminum is drawn through a simple die orifice without using a mandrel.
In order to ensure uniform drawing temperatures and avoid excessive wear on the dies and mandrels used, it is essential that a suitable lubricant be applied during drawing. A wide variety of lubricants are used for this purpose. Heavier draws may require oil-based lubricants, but oil-in-water emulsions are used for many applications. Soap solutions may also be used for some of the lighter draws. Drawing oils are usually recycled until their lubricating properties are exhausted.
Intermediate annealing is frequently required between draws in order to restore the ductility lost by cold working of the drawn product. Degreasing of the aluminum may be required to prevent burning of heavy lubricating oils in the annealing furnaces.
Heat treatment is an integral part of aluminum forming and is practiced at nearly every plant in the category. It is frequently used both in-process and as a final step in forming to give the aluminum alloy the desired mechanical properties. The general types of heat treatment applied are as follows:
1. Homogenizing. This increases the workability and helps control recrystallization and grain growth following casting.
2. Annealing. This softens work-hardened and heat-treated alloys, relieves stress, and stabilizes properties and dimensions.
3. Solution heat treatment. This improves mechanical properties by maximizing the concentration of hardening constituents in the solid solution.
4. Artificial aging. This provides hardening by precipitation of constituents from the solid solution.
Homogenizing, annealing, and aging are dry processes whereas solution heat treatment typically involves significant quantities of contact cooling water.
A number of chemical or electrochemical treatments may be applied after the forming of aluminum or aluminum alloy products. Solvent, acid and alkaline solutions, and detergents can be used to clean soils such as oil and grease from the aluminum surface. Acid and alkaline solutions can be used to etch the product or brighten its surface. Acid solutions are also used for deoxidizing and desmutting.
Surface treatments and their associated rinses are usually combined in a single line of successive tanks. Wastewater discharges from these lines are typically commingled before treatment or discharge. In some cases, rinsewater from one treatment is reused in the rinse of another. These treatments may be used for cleaning purposes, to provide the desired finish for an aluminum formed product, or to prepare the aluminum surface for subsequent coating by processes such as anodizing, conversion coating, electroplating, painting, and porcelain enameling.
A number of different terms are commonly used in referring to sequences of surface treatments, for example, pickling lines, cleaning lines, etch lines, preparation lines, and pretreatment lines. The terminology depends, to some degree, on the purpose of the lines, but usage varies within the industry. In addition, the characteristics of wastewater generated by surface treatment are determined by the unit components of the treatment lines rather than the specific purpose of its application. Cleaning and etch line is used in this section to refer to any surface treatment processes other than solvent cleaning.
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