Metal treatment – Stock – Magnesium base
Reexamination Certificate
1997-01-31
2003-04-08
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Magnesium base
C148S666000, C148S667000
Reexamination Certificate
active
06544357
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to new magnesium and aluminum alloy articles consisting of a non-equilibrium matrix phase of essentially early, i.e. light rare earth and/or transition metals and/or metalloids made by non-equilibrium methods such as rapid solidification from the melt and from the vapor phase and by solid state synthesis with an essentially homogeneous distribution of the major part of the alloying elements on an atomic length scale of the eventually purified alloy matrix. More particularly, it relates to economically viable wrought magnesium and aluminum alloy articles made by selected processing routes and useful as extruded, forged or rolled products for space, ballistic, airframe and other aeronautical as well as for terrestrial applications such as in trains and automobiles, the products thereby achieved by novel methods to control the alloy synthesis, alloy conversion and alloy joining.
2. Description of the Related Art
Corrosion resistant commercial magnesium alloy such as the new high purity version of the Mg—Al base AZ91 alloy, i.e. AZ91E (8.3-9.7 Al, 0.35-1.0 Zn, <0.15 Mn, <0.1 Si, balance Mg) or the new Mg—Y base WE43-alloy (3.7-4.3 Y, 2.4-4.4 Nd and heavy rare earth misch-metal, 0.4-1.0 Zr, <0.2 Zn, balance Mg) are comparable with the corrosion rates of pure magnesium, of aluminum alloys A357 and A206 (all with corrosion rates of the order of 0.25-0.51 mm/year (10-20 mils per year [mpy]) in a salt fog test after ASTM B117) and they are about two orders of magnitude better then previous magnesium alloy families (cf. J. F. King, New advanced magnesium alloys,
Advanced materials technology int
., 1990, pp. 12-19). Another new magnesium alloy showing about 0.25 mm/year (10 mpy) in standardized test conditions is the rapidly solidified magnesium alloy EA55RS (5.1 Al, 4.9 Zn, 5.0 Nd, balance Mg) which has been made available quite recently as a wrought alloy product in extruded, rolled and forged form and which allows due to the fine grain structure for superplasticity and an alloy forming operation at about 150° C. lower temperatures than conventionally cast magnesium alloys so retaining the refined microstructure and the resultant improvement of engineering properties in the final product (S. K. Das, C. F. Chang and D. Raybould,
PM in Aerospace and Defense Technologies
, edt. F. H. Froes, MPIF, Princeton, N.J. 08540, 1989, pp. 63-66). On the aluminum side, many new alloy compositions with superior properties have been developed, but the methods to synthesize them from the vapor and solid state are not mature and controllable as is required by (pilot) production scale.
Aerospace applications require metallic materials with self-healing surface films to protect the interior, i.e. the bulk material when exposed to air (including rain independent on environmental particulars). None of the existing magnesium engineering alloys exhibit a surface passivation upon exposure to normal atmospheres containing saline species as it is known for titanium and aluminum alloys. For iron it is the allotropy which allows for passivation by equilibrium alloying austenitic and ferritic iron with chromium, for example. The absence of allotropy for aluminum, for example, results in deterioration of corrosion behavior of aluminum upon equilibrium alloying and this applies more seriously to magnesium alloys. Magnesium alloys yet represent the worst case among structural metals for aeronautical applications, since magnesium has not only no allotropy as titanium and iron, but Magnesium does also not develop a passive surface film on exposure to normal atmospheres as is evident for pure titanium and pure aluminum. None of the existing conventional magnesium alloys have yet shown pronounced passivation behavior by alloying as—by definition—becomes evident upon a significant decrease in corrosion rates compared to the pure metal. Hehmann et al. have shown (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys,
Conf. Proc. PM Aerospace Materials
'87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1), however, that significant passivation is possible by alloying the &agr;Mg solid solution with at least 17 wt. % Al in the supersaturated state. This type of passivation, however, was not obtainable unless very extreme conditions of rapid solidification from the melt were applied and it was therefore restricted to thin cross-sections and not obtainable by conventional ingot metallurgy. An engineering solution to this problem would provide the driving force to resolve many of the obstacles for the introduction of advanced light alloys, but the solution to this problem has not been recognized as a combined problem of the development of non-equilibrium new and/or established light alloys as well as of corresponding processes.
As long as 75 years ago, Tammann (G. Tammann,
Die chemischen und galvanischen Eigenschaften von Mischkristallen und ihre Atomverteilung
, Leipzig, 1919) and later Gerischer et al. (cf. R. P. Tischer and H. Gerischer, Z.
Electrochem
. 62, 1958, p. 50.) reported increasing pitting potentials and decreasing anodic current densities of the equilibrium Cu—Au and Ag—Au solid solutions with increasing levels of gold representing the more noble and passivating constituent. The majority of the equilibrium phase diagrams of binary Mg-alloys shows, however, a very restricted solubility range in the cph-Mg solid solution due to the formation of strong compounds suppressing equilibrium solubility in cph-Mg (L. A. Carapella, Fundamental Alloying Nature of Magnesium,
Met. Progress
48, August 1947, pp. 297-307). Only the so-called “yttrics” exhibit relatively large equilibrium solid solubilities in cph-Mg. This group consists of yttrium and the heavy rare earth metals Gd, Tb, Dy, etc. as well as scandium which, due to their physical commonalties, are found in nature as a mixture, the so-called (heavy) rare earth (HRE) misch-metals and which have led to the most heat resistant Mg-based alloys on record. Heavy rare earth metals and scandium are relatively expensive alloying additions to magnesium. Sm and Gd represent the most economically viable individual heavy rare earth alloying additions with relatively large equilibrium solid solubility in cph-Mg. If Sm and Gd are employed via a cheaper misch-metal, they may co-exist with a considerable amount of yttrium.
Yttrium, however, was reported (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Rasch Erstarrte Magnesium-Mischkristalle und Ihr Umwandlungs- und Korrosionsverhalten,
Doctoral Thesis
, University of Stuttgart, published in Fortschrittsberichte VDI', Reihe 5, No 155: Grund- und Werkstoffe', VDI-Verlag, Düsseldorf, F. R. G., January 1989) not to result in the required improvement of corrosion behavior when dissolved in cph-Mg compared to pure magnesium. Mg-HRE alloys require also relatively laborious solution and aging treatments when made by conventional casting methods (cf. M. E. Drits, L. L. Rohklin and N. P. Abrukina,
Metallovedenie i Termicheskaya Obrabotka Metallov
17, 1985, 27-28; S. Kamado, Y. Kojima, Y. Negishi and S. Iwasawa, R. Ninomiya,
Light Metals Processing and Applications
, Quebec City, Quebec Canada, Aug. 29-Sep. 1, 1993, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec H3Z 3B8, Canada, 1993, pp. 849-858).
In 1987, Hehmann and co-workers found (F. Hehmann, R. G. J. Edyvean, H. Jones and F. Sommer, Effect of Rapid Solidification Processing on Corrodability of Magnesium Alloys, Conf. Proc. PM Aerospace Materials '87, eds. B. Williams and G. Dowson, Met. Powder Report Publishing Services, Shrewsbury, England, p. 46/1; F. Hehmann, Ras
Hehmann Franz
Weidemann Michael
Hehmann Franz
Ip Sikyin
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