(AerospaceBook)HOT WORKABILITY OF HIGH STRENGTH ALUMINUM AIRCRAFT ALLOYS

摘要

The alloy additions in 2000, 7000 and 8000 series alloys, that are able to induce (from suitablecomplex heat treatments) high strength and toughness for strenuous aircraft applications, can also producewide variations in hot workability due to different sizes and distributions of precipitates that usually differconsiderably from those that are optimum for service. Precipitation during a rolling schedule from 500 to300°C poses little problem, but in extrusion dynamic precipitation in a solution-treated billet preheated at300 C, noticeably raises the initial pressure. The use of extrusion chamber deformation heating to about500°C for solution followed by quenching of the extrudate is not usually satisfactory because of the highquench sensitivity. Characteristic flow curves are presented for selected ranges of pretreatment andstraining conditions; the dependence of peak hot strength and ductility on temperature and strain rate aretabulated along with compositions as a means of estimating hot workability for new alloys, such asemployed in the Airbus 380. Comparisons are made with high strength 6000 and 5000 series alloys. Thehot workability of precipitation hardening Al alloys is particularly challenging because the hot formingtemperature range extends down from the solvus to the traditional aging temperature with 20 C providingsolute immobility. Depending on the initial precipitation condition, there is thus the potential in anyisothermal deformation for precipitation, dissolution or coalescence (or all in sequence) to take place ataccelerated rates, although the normal duration for rolling to 90% reduction or extrusion to ratio 40 mayrange from 0.5 to 50 s at rates of 0.1 to 20 s~(-1). At mid century a start was made in understandingconstitutive relations and microstructural mechanisms. By the turn of the century, testing techniques fordetermining such behaviors were readily available so that many new alloys were successfully developedfor the Airbus 380.