Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1995-10-17
1998-10-13
Wyszomierski, George
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148690, C22F 1057
Patent
active
058207082
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the extrusion of aluminium-lithium alloys, desirably in the form of relatively thin sections, which are particularly suitable for aerospace applications.
Conventionally extrusions in aerospace aluminium alloys are produced by extruding an homogenised and pre-heated billet to the required section. After cooling the extrudate is cut into appropriate lengths (in order to be accommodated in the solution treatment furnace) and heated to a suitable temperature and for a sufficient time to take the soluble alloy additions into solid solution. The section is then water quenched in order to retain the alloying additions in supersaturated solid solution at ambient temperature. This conventional treatment is known as a "formal solution treatment", and the temperature at which this treatment is carried out is referred to herein as "the solution treatment temperature".
A variety of furnaces can be used to achieve the solutionising step but, most commonly, large air furnaces are used in which the extruded sections are suspended vertically, the furnace being mounted directly over a water quench tank. When the sections have been heated for the appropriate time the bottom of the furnace is opened and the sections are rapidly lowered into the quench tank to achieve rapid and uniform quenching. After drying the sections are stretched, frequently by a controlled amount in order to give optimum strength properties in the final product and to rectify distortions arising from the solution treatment. The stretched extrusion would generally then be subjected to an ageing treatment at a relatively low temperature in order to develop the required strength properties.
In the case of commercial aluminium alloys required for less demanding applications than aerospace, the above procedure is frequently simplified. The commercial alloys tend to have much lower sensitivity to quench rate than the aerospace alloys and, in any case, the property demands are less. Thus the separate steps of cutting to length, solutionising and water quenching are omitted and the hot section is passed directly into a quench tank as soon as possible after it emerges from the extrusion die.
This so-called "press quenched" extrusion is then simply stretched and aged. These simplified procedures have, traditionally, been forbidden for structural aerospace alloys because of the requirements to achieve demanding strength property combinations and the necessity for the extrudate to possess uniformity in such properties.
In the last decade or so very great efforts have been devoted to the development of lower density aerospace aluminium alloys by the addition of lithium (in combination with a variety of other additions). To achieve worthwhile density reductions it is desirable for the lithium addition to be of the order of 2 to 21/2 wt % which equates to about 10 to 13 at %. While this produces highly desirable effects in reducing the density and increasing the elastic modulus it also has some disadvantages. In particular, because the lithium atom is very small it can, at elevated temperatures, diffuse quite rapidly from the core of the material to the surface. At the surface it will, because of its high reactivity, rapidly oxidise. If any water, or water vapour, is present the oxide will be converted into lithium hydroxide which will rapidly attack the metal surface upon which it has formed. This can, very quickly, result in serious corrosive attack of the metal substrate, frequently to the extent that the product is unsaleable. Corrosion pitting is a particular problem in relatively thin sections. The migration of the lithium to the surface also creates a lithium-depleted surface layer, and the greater the time for which the extrusion is exposed to elevated temperatures the greater the extent of lithium depletion. In addition then to the considerable potential for corrosive attack, there is added the further drawback of a loss of strength potential as a consequence of the local loss of lithium. This will have the further disadvantage of in
REFERENCES:
patent: 5413650 (1995-05-01), Jarrett et al.
patent: 5447583 (1995-09-01), Arthur et al.
patent: 5520754 (1996-05-01), Yaney et al.
Alcan International Limited
Wyszomierski George
LandOfFree
Production of extruded aluminum-lithium alloys does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Production of extruded aluminum-lithium alloys, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Production of extruded aluminum-lithium alloys will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-309703