WO1998018975A1

WO1998018975A1 - A method of working hard aluminium of standard type (us) aa 7075 t6

Info

Publication number: WO1998018975A1
Application number: PCT/DK1997/000473
Authority: WO
Inventors: Ole Frederiksen; Lars Frederiksen
Original assignee: Ole Frederiksen; Lars Frederiksen
Priority date: 1996-10-25
Filing date: 1997-10-27
Publication date: 1998-05-07
Also published as: AU4699497A

Abstract

A method of working blanks of so-called hard aluminium of the special standard type USA 7075 T6, which is otherwise difficult to work by bending, hammering out, or the like. The relevant blank is heated briefly to the so-called loosening point, which is approximately 475 °C, this heating being controlled with high precision to a blank temperature in the interval 471-475 °C, whereafter - within 60 seconds - the blank is exposed to a shock chilling by abrupt immersion in cold water, preferably with a transition time of only few theths of a second. When through-cooled, the blank is lifted off the water and is then easily workable within the next 12 hours, preferably within 2-3 hours. The metal will then regain its rigidity, even to an increased degree due to the cold working, during the following few months, but this period may be shortened by heating the product, e.g. at 120 °C for 24 hours. At the cold working the blank can be worked almost as was it tin solder, e.g. with entirely sharp foldings or bends, without these giving rise to subsequent weakness in the product.

Description

A METHOD OF WORKING HARD ALUMINIUM OF STANDARD TYPE (US) AA 7075 T6.

The present invention relates to a method of working hard-aluminium of extreme hardness, viz. in particular the type identified by US standard 7075 16 . This metal is an aluminium alloy, rich in copper and zinc, is a product of the space technology, and in more important respects it is advantageous over the otherwise highly esteemed titanium alloys, not least with respect to the material price, but also concerning e.g. strength, corrosion resistance and lifetime with respect to fatigue fractures, the material having a high degree of flexibility.

The material is purchased as a heat cured and, option- ally, pre-stretched wire or plate material, which possesses the above qualities, yet not with respect to shapeability in cold condition. In that respect it corresponds, in principle, to other hard metal types such as spring steel and various titanium alloys, although due to its pronounced hardness (Brinell 150) it is still more difficult to shape, e.g. in "sharp" bendings ; also with this material, be it wire or plate shape, it is impossible to make very sharp bends without a resulting fracture, i.e. in the working thereof it is necessary to accept the well known limitation of keeping any radius of curvature higher than the thickness of the material. The material may be heated for achieving an easy shape- ability, but only at the expense of its original strength.

Due to its superior properties the material, in the shape of wire or plate, would be highly interesting for many appli- cations, if it was better suited for cold working. As an example, reference can be made to the popular thin wire frames of eyeglasses, but also many other technical products such as special springs would be obvious candidates . It would then also be natural to take full advantage of the properties of the material, i.e. in minimising the constructions, whereby, however, it would be still more important that the material, after the working of it, could be left with its original properties . In connection with the present invention it has been found in a surprising manner that in fact it is possible to expose blanks of a standard material of the said type to a treatment that makes the material extremely willingly shape- able in cold condition, almost corresponding to the shape- ability of tin solder, without the ready worked blanks losing the original good properties of the material, and even with the possibility of the strength of the material being further increased. Thus, it is achievable not only that the material is very easy to shape, but also that it can be worked with totally sharp bends, i.e. bending radius zero, without such bends or foldings later on appearing as weak areas, as they will rather be still stronger.

The invention is based on a controlled heating/cooling treatment of a type that can be found to resemble the process disclosed in US-A-4 , 988 , 394 relating to the production of the blank material itself, only it has been found that the said special effect with respect to the shaping of the blanks is not achievable unless the degree of heating is controlled with extreme accuracy. For the said known process, a temperature interval of 500-900°F (260-482°C) is prescribed, with 650-800°F (345-425°C) as the preferred interval, but with the invention it is a condition for obtaining the advantageous result that heating is effected - even at brief duration - to a very narrow temperature interval viz. 470-475°C. This is so critical that there are even substantial measuring technical difficulties connected with an exact determination of the temperature, inter alia because it seems required to measure directly on the blanks and not on their surrounding air. Cor- respondingly, it has been found impossible to trace commercially available ovens, which can be controlled with the required precision for achieving the relevant, extremely accurate heating. Such ovens may become available, but until then it will be required to use specially produced ovens. If the temperature is not measured really accurately and is only some 10°C below the said interval, the hardness of the finished product will be reduced to such a degree, compared with the original hardness and strength, that the prod- uct must be scrapped if it is made with the desired degree of minimising. A plausible explanation is that the blank has not, then, been healed to the so-called loosening point, at which the molecules of the alloy are willingly mutually dis- placeable.

If, inversely, the blank temperature has in fact reached 480°C, the strength of the final product will also be drastically reduced, here probably with the explanation that the alloy has lost its original structure due to agglomeration of the different constituents, such that the material is no longer an alloy.

On this background, heating levels outside the interval of 460-480°C, will be entirely out of interest in connection with the invention, provided that these figures are really indicative of the blank temperature.

The first step in a process according to the invention is to effect the said heating of the blank or blanks to be worked, e.g. pieces of plate or wire material, and when this has resulted in a temperature rise to the critical interval, preferably within 60 seconds thereafter, the blanks are dumped into cold water. It could well be possible to arrange for such a sudden chilling in other ways, which would then also be comprised by the invention, but the simplest - and fully sufficient - solution will be such a dumping into cold water. It is important that this shock cooling is effected by a very rapid transfer, preferably within only few tenths of a second and thus in any way within only a few seconds, such that the blank temperature will not decrease noticeably before the dipping. As he material is well heat conducting, the blank will soon thereafter be overall cooled to the lower temperature, and after a few minutes it can be taken up for subsequent working. The blanks will then be very willing shapeable within a period of up against 12 hours, though with gradually decreasing shapeability. Within the first 2-3 hours the shapeability is so good that it is possible to form sharply bent knee bendings, including sharp foldings through 180° of both plate and wire material. Also, other deformation work- ings such as hammering, die pressing and rounding may be performed. During the first hours after the chilling the material can be worked as was it thin solder.

Thereafter the special circumstance arises that the worked blank, when stored under normal conditions, will relatively slowly - e.g. through three months - re-establish its original hardness and strength, now even further amplified as a result of the applied cold working. Even in sharply bent areas the strength will increase correspondingly without giv- ing rise to deformation stress that could later on reveal itself as a fatigue fracture. When complying with the process conditions as here disclosed, one can be sure that a minimised construction will fulfil the expectations imposed to it. It has been found that the said after-curing period can be shortened by a heating of the products, optimally to some 120°C for 2-3 days. It is important that it is the whole of the products that is heated, and not only the worked portions thereof, as otherwise the corrosion resistivity in the tran- sition areas will jeopardised. The after treatment may be further accelerated, e.g. by increasing the temperature to, for example, 150°C during some 4 hours only, but it seems as such a treatment tends to generally reduce the corrosion resistance of the products. The finished products may be surface treated just as ordinary aluminium, including the possibility of colouring the surface with any desired colour.

The invention is related specifically to the said standard material, but it will be understood that other alloys may be produced and allotted other standard designations, though being so similar to the said material that they can be treated according to the invention with the same advantages. It is assumed that the specified values may be explained in metallurgical terms with respect to the relevant limiting conditions, and it may be possible to adapt the invention correspondingly.

Claims

C A I M S :

1. A method of shaping working of blanks, e.g. of plate or wire pieces, of hard aluminium of US standard type 7075 Tβ, characterized In that the blank is subjected to a heating for briefly assuming a temperature of 460-480°C, preferably 470-476°C, whereafter the blank is abruptly subjected to a chock cooling, preferably by immersion into cold water, the blank thereafter, upon through-cooling, being subjected to cold working within the following 24 hours, preferably within the first 2-4 hours.

2. A method according to claim 1, characterized in that the blank is heated up to temperature level of 471-475°C.

3. A method according to claim 1, characterized in that the blank is transferred to the shock cooling step within 60 seconds after having reached the said temperature level.

4. A method according to claim 1, characterized in that the shock cooling is initiated less than one second after the removal of the blank from its heating surroundings.

5. A method according to claim 1, characterized in that the blank is worked so as to exhibit one or more sharp knee bendings with a radius of curvature of zero or less than the thickness of the blank.

6 . A method according to claim 1, characterized in that after the cold working the reestablishing of the hardness of the material is accelerated by a heating of the entire product, optimally at some 120°C during 2-3 days or, optionally, at about 150°C during approximately 4 hours.

7. With a method according to claim 1 the o d i f i- c a t i o n that the method is excersized with corresponding effects in connection with working of other, but related types of alloys, for each of which the said critical heating temperature and other process parametres are determined.