DESCRIPTION OF THE PRIOR ART In welded aluminium constructions mostly AA5xxx type Al-alloy materials are used; these alloys have the best mechanical properties of the non-heat-treatable type Al-alloys. Usually the magnesium content of the alloy is kept low to obtain good corrosion resistance. These alloys, also in the strain hardened tempers, are however prone to loss of strength in the heat effected zone.

This may be overcome by a high magnesium content in the alloy. Then, however, corrosion resistance is worse.

Heat-treatable AA6xxx type Al-alloy materials are considered use for welded constructions; they suffer however, loss of mechanical properties in the heat affected zone. AA2xxx and SA7xxx type Al-alloys have generally lack of weldability and corrosion resistance.

Particularly in the aerospace industry, it has

hitherto been found very difficult to weld aluminium alloy load-bearing structural components used in the outer skin of aircraft, without causing loss of strength Such alloy components must have resistance to impacts.

It has been common to join such components by rivetting.

There is a desire to replace rivetted joints by welded joints without loss of the integrity of the load-bearing alloy.

Proposals have been made to provide a clad layer on aluminium alloy sheets, in order to increase their weldability. FR-A-1010072 shows a method of resistance welding, in which two sheets are joined face-to-face.

Each sheet consists of three alloy layers, one of which has high strength, one is of good weldability and the third has low electrical resistance. The resistance heating effect is concentrated at the weldable layer.

EP-A-117352 discloses a welding arrangement intended to provide an air-tight seal in the packaging of semiconductor devices, using laser welding. An intermediate material, such as Al-Si alloy in the form of a clad layer, is interposed between the elements welded by laser irradiation at the weld zone. It appears that the welded elements themselves are substantially melted by the welding, since the intermediate material may be silicon, zinc or nickel metal.

EP-A-66009 describes aluminium alloy composite plates for use as armour plating. A central components of the plate has high strength and hardness, while inner and

outer components have high toughness and good weldability. When the plates are joined edge-to-edge, the respective inner components are welded together, using external filler material, as are the respective outer components.

SUMMARY OF THE INVENTION The object of the invention is to provide a weldable aluminium sheet product, particularly a weldabie product based on a type of Al-alloy generally considered to be either weldable with loss of properties or non- weldable, which product can be used in welding constructions without substantial loss of properties.

According to the invention in one aspect there is provided an aluminium sheet product for use as a structural component in a welded construction, comprising a core sheet made of heat treatable aluminium alloy and on at least one side of said core sheet a clad layer adhered to the core sheet, said clad layer being made of an aluminium alloy welding filler material intended to form the welding pool during welding without substantial melting of the core sheet.

The invention further provides a welded construction having at least two aluminium alloy structural components joined by welding, at least one of the components being the aluminium sheet product of the invention, as defined above. Particularly the invention provides such a construction as an automotive vehicle body sheet and in the aerospace and ship-building

industries, for example as a part of the outer skin sheet of an aircraft or other aerospace vehicle. In such a construction, the strength, e.g. tensile property such as ultimate tensile strength, of the core sheet can be at least SO of its strength prior to the welding operation.

In this way the structural integrity of the alloy is maintained.

For the purpose of this specification sheet product means a flat rolled product with a thickness of up to 36 mm.

In this aluminium sheet product, at welding the clad layer supplies the filler material, which therefore need not be supplied from the outside. Also only the clad layer of the aluminium sheet product is melted.

Thus there is no melting of the core sheet and consequently no cracking in a heat affected zone in the core sheet.

At welding the heat may be kept away from the core sheet as much as possible so that loss of mechanical properties in the core material is prevented.

Additionally a corrosion protection can be given by the clad layer, if the corrosion resistance of the core material is not sufficient.

In this aspect the Al-alloy of the core sheet is heat-treatable and more preferably of the AA2xxx, AA6xxx or AA7xxx type. It may be noted that AA2xxx and AA7xxx type Al-alloys are applicable due to the role of the clad layer. Without the clad layer, such alloys are regarded as difficult to weld.

Suitably the Al-alloy clad layer is made from an Al-alloy filler material known for welding processes.

Preferably the thickness of the core sheet is in the range of up to 30 mm and the thickness of the clad layer is in the range of up to 20% of the thickness c the core sheet. More preferably the thickness of the core sheet is in the range of 0.5 - 15 mm and the thickness of the clad layer is in the range of 5 - i5 o the thickness of the core sheet. The thickness of the clad layer should be sufficient to prevent loss of adhesion from the core sheet and to protect the core sheet from the heat affected zone.

Adhesion between the clad layer and the core sheet is very important to obtain structural integrity after welding. Preferably the adhesion is obtained by casting a composite ingot having simultaneously cast contacting portions of respectively the materials of the core sheet and the clad layer, followed by rolling of the ingot.

This provides a mainly oxide free interface.

Alternatively the adhesion is obtained by rolling.

According to the invention in another aspect, there is provided a method of welding structural components in which a sheet product as first structural component and an aluminium alloy second structural component are joined by welding, wherein said first structural component has a core sheet made of aluminium alloy and adhered to at last one side of said core sheet a clad layer made of

aluminium alloy welding filler material, and during welding there is formed a welding pool of molten metal at the welding location from the filler metal of said clad layer and metal of said second component, substantially without melting of said core sheet.

This method provides the advantages stated above, in particular that the strength loss of the core sheet may be minimized, e.g. at least 80D of pre-welding strength may be maintained.

Suitably the heat to form the welding pool is generated by laser or electric arc.

In this aspect the aluminium alloy of the core may be heat-treatable, e.g. of the AA2xxx, AA6xxx or AA7xxx series, or non-heat-treatable, e.g. of the AA5xxx series.

Strain hardened non-heat-treatable alloys may be employed.

The preferred thicknesses of the core and clad, and methods of adhering core and clad described above, apply in this aspect of the invention also.

The invention further provides a welded construction produced by the method of this aspect of the invention, particularly such a construction in the ship or aerospace industries, e.g. a skin sheet of an aircraft.

BRIEF DESCRIPTION OF THE DRAWING The invention will be explained by means of the drawing in which:- Fig. 1 represents the situation before welding in

one embodiment of the invention Fig. 2 represents the situation after welding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1 there is shown the aluminium sheet product l consisting of the core sheet 2 and the clad layer 3. An Al-alloy part 4 is to be welded to this aluminium sheet product. The alloy part 4 has a single homogenous composition.

In Fig. 2, which represents the situation after welding, it is shown that during welding, e.g. by laser, a pool 5 is formed mainly consisting of molten material from the clad layer 3 and the part 4, and that the core sheet 2 is substantially unaffected, i.e. substantially not melted. In this way also core sheets made from Al- alloys which are considered in the art as badly weldable can be used in welded constructions.

ExamDle An aluminium sheet product 1 in accordance with the invention consisting of a core sheet of AlMgSiCu and a clad layer of AlSil2 and a part 4 (arranged as in Fig. 1) of AlMgSiCu were welded by laser welding.

Welding configuration: core sheet : thickness 2.4 mm clad layer : thickness 0.4 mm Weld characterisation: Mechanical properties of sheet product 1: Before Welding After Welding Ultimate tensile strength Rm 400 MPa 350 M.Pa 0.2% yield strength Rp 0.2 375 MPa 340 Moa Corrosion properties: Result of the intergranular corrosion test according to MIL H 6088 after welding, relating to the whole welding construction: No JGL, only pitting.