Quenching

Part 2

by Roger A.P. Fielding, BENCHMARKS

Dilute aluminum extrusion alloys, capable of being extruded at high speed, can be processed to achieve property levels equivalent to the higher-composition, slower-extruding alloys, provided that the process is rigorously controlled, and—specifically—that the quench rate at the exit to the extrusion press is high enough.

Writing in 1977, Barry and Hains(1) described the development of the Alcan extrusion alloy designated 65050. While acknowledging that the alloy did not meet the composition level of AA6063, they pointed out that—when processed correctly—the alloy consistently achieved AA6063-T5 mechanical property levels.

Recognizing the importance of an “architectural” extrusion alloy capable of being extruded 50 percent faster than AA6063, and the fact that most orders placed on extruders for window and door sections called for AA6063, the authors recommended that the AA6063 composition limits be extended to include the new alloy. This proposal was rejected by the relevant Aluminum Association committee, thereby delaying the introduction of AA6060 for years, and placing aluminum at another disadvantage with respect to competitive materials.

Although delayed in North America and Japan, the new alloy was quickly adopted in South America. Employing tight control over alloy composition and the extrusion process (billet temperature, container temperature, ram speed, and quenching), thin wall sections of what became AA6060 were extruded at high speed, consistently exceeding the minimum specified AA6063-T6 mechanical property levels. Production rates of 6000 pounds per hour and shipments of 2 million pounds per month were achieved.

Writing in 1984, Barry(2) compared the toughness—as measured by the Kahn tear test propagating energy—of a number of structural extrusion alloys when quenched in water and in high velocity air. And in their paper, delivered at ET 2000, Parson et al(3) compared the mechanical properties of AA6061, AA6082, and AA6005A when quenched in water or high velocity air.

Descriptions of the correct processing of the extrusion alloys referred to above are to be found in the guidelines published by the major billet producers. Referring to an Aluminium Pechiney (now part of Alcan) publication entitled Alliage de Transformation, the reference to press quenching of AA6005A is typical: “Alloy AA6005A needs to be cooled vigorously and fairly close to the point at which it leaves the press. The rate of cooling down to 250C should be in excess of 3C per second, which is generally achieved by means of (high velocity air and) a fine spray of water.”(4)

The productivity—and profitability—of an extrusion press can be increased by ensuring that the composition of the alloy being extruded is selected to maximize the extrusion speed, while meeting the mechanical properties required by the end-use application. This is achieved by managing all stages of the extrusion process and ensuring that the in-line quench press systems can cool the extrusions at the required rate.

Increasingly complex structural aluminum sections, which would distort in water quenches, while sometimes requiring the injection of a fine water spray, can often be quenched satisfactorily in high velocity air.

1. Barry, W.G., and R.W. Hains, “Current and New AA 6xxx Series Extrusion Alloys,” Proceedings of the Second International Extrusion Technology Seminar, Vol. 1, pp. 271-284

2. Barry, W.G., “Rationalization of Structural Al-Mg-Si Extrusion Alloys,” Proceedings of the Third International Extrusion Technology Seminar, ET 1984, Vol. 1, pp. 7-15

3. Parson, N., J. Hankin, K. Hicklin, and C.W. Jowett, “Comparison of the Extrusion Performance and Product Characteristics of Three Structural Extrusion Alloys: AA6061, AA6082, and AA6005A,” Proceedings of the Seventh International Extrusion Technology Seminar, ET 2000, Vol. 2, pp. 1-12

4. Marchive, D., and P. Faivre, “Medium Strength Extrusion Alloys in the AA6xxx Series,” Alliage de Transformation, Societe de Vente de l’Aluminium Pechiney, Voreppe Research Centre