A process model for on-line quenching of aluminium extrusions
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I.
INTRODUCTION
ALUMINUM alloys of the AIMgSi type obtain their good strength through a high density of fine Mg2Si precipitates. The fine precipitates are obtained by first dissolving large, nonhardening MgzSi particles at a high temperature, then allowing the fine precipitates to form through a separate aging operation at a lower temperature. During extrusion of these alloys, the hot-working temperature is normally high enough for dissolution of the Mg2Si particles. Thus, through an on-line cooling to room temperature followed by aging, the separate solution treatment becomes superfluous.Vl However, the applied cooling rate is restricted by two phenomena. On the one hand, too slow cooling leads to heterogeneous precipitation of large Mg2Si particles during cooling. This reduces the concentration o f Mg and Si in solid solution so that less of the fine precipitates can form during ageing. Significant reduction in properties can therefore be the outcome.tEl On the other hand, a too rapid cooling leads to large temperature gradients causing thermally induced deformations and high residual stresses. This makes it difficult to remain within geometrical tolerances. The optimum cooling conditions are obtained by cooling through the so-called quench window between the two limitations, as illustrated schematically in Figure I. Reduced properties are usually experienced when using air cooling, while distortions are mainly a problem with water quenching. The thermally induced distortions are strongly influenced by section shape. For instance, when the section has a difference in wall thickness, thin parts will cool faster than
NIKLAS J,~RVSTRAT, formerly with the Research and Development center, Hydro Aluminium, is with the Military Engines Division, Volvo Aero Corporation, S-461 81 Trollhiittan, Sweden. STIG TJOTTA is with the Research and Development Centre, Hydro Aluminium, N-4265 Havik, Norway. Manuscript submitted September 26, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS B
thick parts, and a banana-shaped extrusion results. The same effect can also occur if the heat transfer is uneven around a shape. This is the case when the nozzles are asymmetrically positioned. The section shape can also affect the strength. For example, an internal ligament of an extrusion cools more slowly than external walls, which may lead to a reduced strength in the ligament. In addition, the sensitivity to precipitation during cooling is highly alloy dependent. Thus, the width of the quench window depends on factors such as alloy, size and shape of the extrusion, and nozzle position. For complex extrusion shapes, optimal quenching requirements and an appropriate quenching method are not easily established under production conditions. Mainly by using empirical knowledge, the operator must choose process parameters and quench medium. The most commonly used quench media are forced air, water spray, and water. Moreover, asymmetrical shapes often require asymmetric cooling that further complicates the empirical approach. The present article d
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