Reheating Aluminum Billet for Extrusion

The Solutionizing Process

by Dr. A.J. (Bill) Bryant and Roger A.P. Fielding, BENCHMARKS

Writing for the first International Extrusion Technology Seminar in 1969, E.C. Beatty(1) described three billet reheat practices:

1) Reheat to 950°F (510°C) or higher, then extrude.

2) Reheat to 950°F (510°C) or higher, cool to a lower temperature, then extrude.

3) Reheat to lower temperatures: 750-950°F (400-510°C) and control extrusion speed to attain a temperature above 930°F (500°C) in the extrudate leaving the die.

The first two processes follow the principle of solutionizing the Mg2Si before extrusion. The latter process depends on the control of the homogenized billet structure so that any Mg2Si present will be small enough to dissolve during the reheating and extrusion operations. It depends on the mechanical work of extrusion increasing the temperature in the extruding billet to complete solutionizing the extrudate before it exits the die. Beatty emphasized that the success of using any of the processes depends, in large part, on the structure of the billet before reheating, and the controls exercised during the subsequent extrusion and cooling.

In the summary to his 1971 paper, Lynch(2) described the conditions required to achieve strength, surface finish, and anodizing appearance in AlMgSi 0.5 (AA6060) extrusions as follows: Strength and appearance are achieved when the Mg2Si is distributed throughout the structure of the extruded section as a fine precipitate of less than 0.25 microns—mostly sub-microscopic size. Surface finish, being a function of the alloy’s extrudability, depends mainly on proper homogenization of the billet.

Lynch pointed out that the relatively longer billet preheat times, which are a feature of direct impingement billet furnaces, can result in billet being held for extended periods in the range of 480-800°F (250-425°C) during which rapid precipitation of Mg2Si can occur. However, he pointed out that the billet can be “recovered” by heating to at least 930°F (500°C) before extrusion, during which the majority of the Mg2Si will be re-dissolved.

Option 1), and specifically option 2), resemble the processes illustrated by Ohmura in 1984(3), who described its application to AA6063 and AA6061 alloys, as well as the recommendations contained in Reiso’s 1996 papers.(4, 5)

Option 3) has until recently been seen by some extruders as the best way to optimize productivity, surface finish, and mechanical properties, because of the lower flow stress of the billet and consequent reduced pressure requirements.

Obviously, the thermal history of the billet prior to extrusion has a major effect on extrusion performance. The temperature distribution within the billet during homogenizing and reheating must be as uniform as possible, and taper heating or quenching systems that are employed during or immediately after reheating must be designed to ensure that the extrusion process proceeds isothermally.

The process enabled by the combination of direct impingement billet heaters and the taper quench systems has been defined in the industry literature as contributing to improved quality and increased press productivity.(6)

References

1. Beatty, E.C., “How the Properties and Microstructure of AA6063 Alloy Extrusions

Depend on Fabricating Practice.” Aluminum Association, First Extrusion Technology Seminar, 1969, Paper No. 10.

2. Lynch, C.V., “The Effect of Production Conditions on the Quality of Extruded Sections in AlMgSi0.5 Alloy”; Z. Metallkunde, 1971, vol. 62, pp. 710-715. (In German, English translation available from BENCHMARKS.)

3. Ohmura, Shinji, et al., “Microprocessor Control of Billet Shearing at the Press After Homogenization and the Effects on the Metallurgical Properties of Extruded Products,” Aluminum Association, Third Extrusion Technology Seminar, 1984, vol. II, pp. 361-363.

4. Reiso, O., et al., “The Effect of Cooling Rate after Homogenization and Billet Preheating on Extrudability and Section Properties,” part 1; Aluminum Association, Sixth Extrusion Technology Seminar, 1996, vol. I, pp. 1-10.

5. Reiso, O., et al., Ibid. (part 2); vol. I,pp. 141-147.

6. Fielding R.A.P., and A.J. Bryant., “The Quest for Optimal Extrusion Billet Heating”, Light Metal Age, vol. 59, nos 7, 8. August 2001, pp 24-36.