Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-03-17
2002-01-08
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S415000, C148S437000, C148S440000, C219S050000, C420S540000, C420S541000, C428S615000, C428S925000, C428S926000, C428S933000, C428S934000
Reexamination Certificate
active
06337147
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a weldable aluminum product comprising a structural component in the form of sheet or plate product or an extruded product, for use as a structural member in a welded structure. Further the invention relates to a welded structure comprising at least one such aluminum product. Such welded structures include marine structures e.g. fixed structures, ships, aerospace vehicles, and land vehicles such as trucks, dump trucks, cars (automobiles) and railway vehicles. While in the following description of this invention reference is mainly made to aluminum sheet or plate products, it is to be understood that products based on aluminum extrusions are also included. As is conventional, the term “aluminum product” refers to aluminum alloy products.
BACKGROUND OF THE PRESENT INVENTION
When welding aluminum plates or sheets by means of fusion welding, such as MIG, TIG or plasma welding, in particular plates or sheets made of the Aluminum Association AA5xxx-series alloys having Mg as an alloying element in the range of 2 to 6 weight %, for example welding head-to-head by using a V-bevel, a weld bead is formed having a relatively sharp angle between the base material and the weld toe. In a severe and undesirable case even an undercut may be formed. Usually fatigue cracks start at a so-called hot-spot, which is the notch where the weld bead starts. Because of this relatively sharp angle or notch several organizations setting standards, e.g. for ship-building, indicate that the fatigue behavior of the welded construction detail is essentially independent of the aluminum alloys used, but is dependent on the construction notch effect. This becomes more apparent for the AA5xxx-series alloys having a relatively high Mg content. For this reason an aluminum alloy having improved strength may still be assessed on its fatigue performance as if it were a less strong aluminum alloy, because of the notch effect. And therefore the acceptable “global stresses” in a welded construction detail are reduced to lower levels than the aluminum alloys used would withstand in themselves.
It is known to improve the conformation of the weld bead zone after welding, e.g. by hammer preening or weld re-melting, but such an extra step should be avoided if possible. There is therefore a need for a product providing an improved weld bead in the welding process.
In the context of disclosure of the invention below, some other prior art documents are here mentioned, though they are not concerned with the problem described above.
WO98/28130 describes providing a clad layer of filler material on an aluminum core sheet, for the purpose of providing a filler for a welding pool.
In light structures, not assembled by welding together of components, it is known to provide a clad layer on a core sheet, the clad layer having a corrosion potential lower than that of the core sheet. GB-A-1004868 describes materials for use in domestic and industrial water heaters having low corrosion performance, particularly illustrated by an AA6061 alloy sheet clad with an AA7072 alloy layer containing up to 0.01% Cu. Weldability is not mentioned. GB-A-1416134 describes alloys of the AA4xxx-series clad with an AA7072 layer to give corrosion resistance. Formation of tubes of this material by welding is mentioned, but weldability of the material is not discussed.
SUMMARY OF THE INVENTION
An object of the invention is to provide a weldable aluminum product for use as a structural member in a welded structure, which gives the welded structure an improved fatigue life.
To achieve this object there is provided in accordance with the invention a weldable aluminum product comprising a structural component which is a sheet, a plate or an extruded body and is made of an aluminum alloy containing not more than 1.5 weight percent (wt %) Zn and, adhered on at least one side of said structural component, a cladding layer made of an AA7xxx-series alloy having a corrosion potential lower than that of said alloy of said structural component.
With this product there is achieved the effect of obtaining after welding a much smoother transition from the parent plate or sheet to the weld metal than has been known before in welding of aluminum alloys, in particular of AA5xxx-series alloys. In the weld there is a higher barrier for the crack initiation stage with a resulting increase in fatigue life, and thus a fatigue limit can be achieved which may be considerably higher than that for welded joints known so far. Furthermore, this smoother transition overcomes the need for additional modification techniques of the weld toe profile, such as local grinding or weld toe remelting techniques. Furthermore, it has been observed that the flow behavior of the molten metal during welding is improved, resulting in a reduced amount of undesirable inclusions in the weld metal, such as oxides.
The use of the clad layer or layers in the invention can not only improve the “hot spot” of the welded joint, but may also produce compressive stress at the surface of the welded structure, because of the difference in contraction of the structural component and the clad layer. This may reduce residual tensile stress due to contraction of the weld bead and thus also influence fatigue behavior positively.
An additional advantage of welding the cladded aluminum product of the invention is that surface compressive residual stresses may be introduced in the regions where fatigue cracks are likely to initiate, which may extend fatigue life significantly. By applying a cladding on one or both sides of the product, additional laborious techniques of introducing residual stresses, e.g. peening, can be overcome.
For certain applications due to the cladding there will be advantages such as visual improvement and better forming (shaping) behavior because the relatively soft surface avoids crack initiation. Furthermore, the weldable cladding allows for higher strength tempers to be used for the alloy of the structural component because of the improved corrosion behavior due to the anodic protection given by the cladding used.
Although the invention may be applied to all kinds of aluminum alloys as the structural member having a corrosion potential higher than that of the cladding applied, the effects found are most pronounced in AA5xxx-series alloys having Mg as an alloying element in the range of 2 to 6 weight percent. The amount of Zn is preferably 0.4 to 1.5 weight percent.
Preferably the thickness of the structural component or core is in the range of up to 40 mm, more preferably in the range of 0.5 to 20 mm.
Preferably the thickness of the or each cladding layer is in the range of up to 20% of the thickness of the structural component. The minimum thickness is preferably 1% of that of the structural component, and more preferably the thickness of the or each cladding layer is 1 to 15% of the thickness of the structural component, and most preferably 1 to 10% of the thickness of the structural component, more particularly 2 to 10%.
Adhesion between the cladding layer or layers and the structural component (core sheet or core plate) is very important to obtain structural integrity after welding. Preferably the adhesion is obtained by rolling. Suitable roll-bonding processes are known. Alternatively, the adhesion is obtained by casting a composite ingot having simultaneously cast contacting portions of respectively the materials of the structural component and the cladding layer or layers, followed by rolling of the ingot obtained. This provides a mainly oxide free interface. In the case where the structural component is an extruded body, the cladding layer may be applied by any suitable method, for example roll-bonding.
In one embodiment of the aluminum product in accordance with the invention the structural component is made of an AA5083-series alloy, an AA5059-series alloy, an AA5086-series alloy, or an alloy which is a modification of one of these.
A very advantageous AA5xxx-series alloy for the structural component has the following composition, in weight
Haszler Alfred Johann Peter
Mechsner Klaus Alfons
Corus Aluminium Walzprodukte GmbH
Koehler Robert R.
Stevens Davis Miller & Mosher LLP
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