Oct 15, 2017 - Figure 4 Concentration profiles of iron, manganese and silicon in four samples measured in a plane ... These compounds contain iron, man-.
JOURNAL
OF MATERIALS
SCIENCE
LETTERS
2 (1983)
67-70
The effect of soaking times on the mechanical properties of rapidly solidified aluminium alloys B. VAN DEN B R A N D T * , P . J . VAN DEN B R I N K S,H. F. DE J O N G * , L . K A T G E R M A N S Departments of Aerospace Engineering* and Metallurgy $, Delft University of Technology, Delft, The Netherlands
The microcrystalline state of rapidly solidified alloys has been investigated by many experimentalists in the past ten years. Much attention has been given to the process variables of the rapid solidification process, to the microstructural effects on the obtained powder, splat or ribbon material, and to the constitutional effects, e.g. extended solid solubility and the presence of non-equilibrium crystalline phases [1]. In more recent years the consolidation process from the basic material into a solid testing material has also been the object of general interest [2]. As a logical step in the development of this material the last part of the production process, the thermomechanical treatment, should be optimized and adapted to the different properties of rapidly solidified material. Here we present some preliminary results of the optimization of the modified heat treatment of the aluminium alloy 2024 in the T4 condition, I IIIII
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from which it becomes clear that a considerably shorter soaking time for solution heat treatment is desirable than in the case of the same conventional alloy. Tensile properties were used as the optimization criterion. Production of test material: smooth tensile test specimens with a reduced diameter of 5 mm were made out of bars of 8.4 mm diameter 2024 alloy, that were made by melt spinning, cold compaction and hot extrusion. The melt spinning equipment consisted of an electric resistance furnace with a capacity of 13 kg aluminium alloy and a rotating copper wheel (2000 rpm) with a diameter of 300ram. The furnace was filled with commercial 2024 alloy and the molten metal was ejected through a thermally isolated siphon with an orifice of 0.8 mm diameter onto the rotating wheel by an overpressure of argon to form rapidly solidified ribbons of 80#m thick and 1.5mm wide. The ribbons were compacted at room temperature, up to I I I IIIII
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Figure 1 The 0.2% yield strength I IIIIII
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tST (min) 0261-8028/83/020067-04502.98/0
© 1983 Chapman and Hall Ltd.
and the ultimate tensile strenth of rapidly solidified 2024 (full line) and conventional 2024 (dashed line) as a function of the soaking time tST of solution treatment. 67
Figure 2 A comparison between yield strength, ultimate tensile strength and elongation a s of the extruded ribbon (this work, shaded) and values from the literature of extruded rapidly solidified flakes ([2], shaded) and powder ([5]). The other data correspond to conventional 2024 extrusions.
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work a ribbon fraction of 62%, in the shape of extrusion billets of 50 mm diameter, the billets were preheated at 4 4 0 ° C and extruded at an area reduction rate of 38:1. The chemical composition after extrusion was A1-4.45Cu-1.34Mg-0.6Mn0 . 2 4 F e - 0 . 1 4 S i - 0 . 0 7 Z n - 0 . 0 7 C r (wt%). Results obtained with similar melt spinning equipment of smaller size have been published earlier [3] ; it may be interesting to note that recently the melt spinning of ribbons in quantities of several hundreds of kilogrammes has been successfully performed with more continuously working equipment.
Pieces of the extruded bar were brought in the T4 condition. A solution heat treatment at 495 -+ 5 ° C was given in a salt bath furnace; the soaking time for solution heat treatment tsw was chosen as a parameter and for each value at least four specimens were prepared. After quenching in cold water and subsequent naturally ageing for 7 days at room temperature the smooth tensile test specimens were machined on the lathe. Tensile tests were performed on a closed-loop 2 0 0 k N A m s l e r machine. The ultimate tensile strength and the 0.2% yield strength are shown as
Figure 3 Fracture surface of a tensile test specimen of extruded meltspun ribbon of 2024 (SEM, ×600).
68
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0 Figure 4 Concentration profiles of iron, manganese and silicon in four samples measured in a plane perpendicular on the extrusion direction. (a) Rapidly solidified material, tST = 2 min; (b) rapidly solidified, tsT = 25 min; (c) conventional, tST = 2 min; (d) conventional, tST = 55 min. The average concentrations in these 2024 samples are equal (see text). Note the differences in sensitivity for iron. a f u n c t i o n of tsw in Fig. 1. Each p o i n t represents the results o f at least four tests. For rapidly solidified material an o p t i m u m is f o u n d for tsw = 2
rain, considerably shorter than the prescribed value o f 45 to 55 min for c o n v e n t i o n a l material of the same diameter [4]. F o r c o m p a r i s o n conven69
tional material extruded with the same equipment has been tested in a similar way: it does not show an optimum (see Fig. 1). In Fig. 2 the optimum values of the tensile properties of extruded ribbon have been compared with the values of conventional bar and with data from the literature [2, 5, 6], and additionally the fracture surface is shown of a tensile specimen with these optimum values (Fig. 3). The observed dependence of the tensile properties on soaking is believed to be the result of the small grain size of the rapidly solidified material. The increasing effect on the strength may be caused by the dissolution of copper and mag- i nesium containing constituents which have precipi: rated during cooling after extrusion. The copper and magnesium in solid solution then contributes in the conventional way to the precipitation hardening effect in the T4 condition. The decreasing effect on the strength may be caused by the coarsening of non-soluble compounds at 495°C and precipitation of those compounds on grain boundaries. These compounds contain iron, manganese or silicon which elements have a smaller diffusivity as compared to copper and magnesium in aluminium. This effect is demonstrated by the concentration profiles of these elements that have been measured in a plane perpendicular to the extrusion direction. In Figs. 4a to d the concentration profiles of four samples are shown; Figs. 4a and b apply to rapidly solidified material, Figs. 4c and d to the conventional 2024 alloy. For both the rapidly solidified and the conventional material a sample with a short soaking time and a sample with a long soaking time is shown. However, the average composition of the four samples is equal; it is given above. A comparison of Figs. 4a and c shows that for short soaking times the spatial distribution of iron is much more homogeneous in rapidly solidified than in conventional 2024 (note the differences in sensitivity for iron), i.e. the influence of melt spinning on the homogeneous distribution of the elements has not been destroyed by the hot extrusion process. However, from Figs. 4a and b follows
70
that soaking for a long time leads to precipitation and therefore, to a loss of strength, as can be seen from Fig. 1. Although there is a similar influence of soaking on the precipitation of these elements in conventional 2024 this has only little effect on the strength due to the fact that it was inhomogeneous from the beginning. Because the precipitation will predominantly take place at the grain boundaries the effect on the rapidly solidified material is more drastic. A different precipitation mechanism was excluded by means of differential thermal analysis, that showed the same results for conventional and rapidly solidified material within the measurement precision. More accurate measurements are in progress using differential scanning calorimetry and transmission electron microscopy. Their purpose is also to investigate the T6 condition, since rapidly solidified material turned out to be much weaker in this temper as compared to conventional 2024, as has also been reported by others [5, 7].
Acknowledgements The authors are grateful to Mr J. Aalbers, MIFA Aluminium B.V., for provision of industrial extrusion facilities. This work was made possible by financial support from the Stichting Bevordering Industriele Research Alumininmindustrie (BIRA Industrial Aluminum Research Foundation).
References 1.
2. 3.
4. 5.
6. 7.
H. JONES, "Rapid solidification of metals and alloys" (The Institution of Metallflrgists, London 1982). M. LEBO, N.J. GRANT,Met. Trans. 5 (1974) 1547. L.M. DE GROOT, L. KATGERMAN, H. KLEINJAN, Proceedings of the 7th International Light Metals Congress, Leoben, 1981. K.R. VAN HORN, "Aluminum" Vol. 3 (American Society for Metals, Metals Park, Ohio, 1967). D. VOSS, Thesis, TH Aachen 1979 (DFVLR-FB
79-34). Aluminium Standards and Data, The Aluminium Association Inc., Washington DC 1978. D. WEBSTER,Met. Trans. 10A (1979) 1913.
Received 15 October and accepted 20 October 1982
