Quench Sensitivity of AlMgSi-Alloys Containing Mn or Cr
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QUENCH SENSITIVITY OF AlMgSi-ALLOYS CONTAINING Mn OR Cr
A.L. Dons and 0. Lohne SINTEF, Department of Metallurgy,
N-7034 Trondheim-NTH,
Norway
ABSTRACT In AlMgSi-alloys cooled from about 773K (500 C) the strength after ageing depends on the rate of cooling. By addition of Mn or Cr dispersoids are formed which may control grain structure. The dispersoids also act as nucleation sites for coarse 3'-Mg 2 Si particles. This makes the alloys more quench sensitive, in this paper the influence of 0.56%Mn on quench sensitivity is compared to that of 0.23%Cr in Al-O.6%Mg-l.0%Si industrial alloys. The alloys were homogenized at 793K (520 C) and 853K 0 (580 C), extruded, quenched in water or cooled in air, and aged to maximum hardness. The density of dispersoids varied with homogenizing heat treatment. Quench sensitivity increased with increasing density of dispersoids. The two alloys had about egual densities of dispersoids after homogenizing at 793K (520 C), but the alloy containing Cr was found to be more quench sensitive than the alloy containing Mn. This is discussed and compared with other findings.
INTRODUCTION Aluminium-magnesium-silicon alloys have good extrudabilities and are widely used in structural engineering for applications where medium to high strength, satisfactory weldability and good resistance to corrosion are needed. The alloys are age hardenable. The strength is due to precipitation of a high density of fine 3''-Mg 2 Si precipitates. Maximum strength is obtained by quenching from the solution temperature before ageing: Too slow cooling will reduce the strength that can be obtained by ageing. This effect is called quench sensitivity, and it is more pronounced in alloys with relatively high contents of Mg and Si. Mn and/or Cr are often added to AlMgSi-alloys to avoid or reduce recrystallization and unwanted grain growth, and thereby increase toughness [1, 2]. The influence of precipitates on recrystallization behaviour of AlMgSi alloys was investigated by Scharf and Gruhl [3]. They found that the rate of recrystallization decreased with increasing degree of particle dispersion, and that it was independent of the particle composition. From an industrial point of view it is therefore of interest to know whether addition of Cr or Mn influences other properties. In industrial hot forming processes like extrusion, the forming temperature may be high enough to make a separate solution heat treatment superfluous. The extrusions are then cooled at the press by air fans or water quenching. To reduce warpage a slow cooling is advantageous. But if the cooling is too slow a loss in strength after aging is observed: the material is sensitive to the rate of quenching. It is therefore of interest
Mat. Res.
Soc.
Symp. Proc. Vol. 21 (1984) Q Elsevier Science Publishing Co., Inc.
724
to know how variations in chemical composition and microstructure influence the quench sensitivity of aluminium alloys. H. Zoller and A. Ried [4] have found that addition of 0.15%Cr increased the quench sensitivity considerably less t
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