Abstract:
Rolled or extruded products for welded constructions are made of an aluminum-magnesium-manganese type aluminum alloy, consisting essentially of, by weight: 
     3.0&lt;Mg&lt;5.0 
     0.5&lt;Mn&lt;1.0 
     Fe&lt;0.25 
     Si&lt;0.25 
     Zn&lt;0.40 
     Cr&lt;0.25 
     Cu&lt;0.20 
     Ti&lt;0.20 
     Zr&lt;0.20 
     The product has a volumetric fraction of Mn containing dispersoids grater than 1.2%.

Description:
FIELD OF THE INVENTION 
     The invention relates to the sphere of rolled or extruded products such as sheets, profiles, wires or tubes made of AlMgMn-type aluminium alloy containing more than 3% by weight of Mg, intended for welded constructions having a high yield stress, good resistance to fatigue and good toughness for structural applications such as ships, industrial vehicles or welded bicycle frames. 
     DESCRIPTION OF THE RELATED ART 
     The optimum dimensioning of welded structures made of aluminium alloy leads to the use of 5,000 series AlMg alloys according to the Aluminium Association nomenclature, in the cold-worked temper (temper H1 according to the standard NF-EN-515) or partially softened temper (temper H2), or stabilized temper (temper H3), while maintaining high resistance to corrosion (temper H116) rather than the annealed temper (temper O). However, the improvement in the mechanical characteristics relative to the temper O does not usually remain after welding, and certifying and monitoring organizations generally recommend that only the characteristics in temper O be taken into consideration for welded structures. The resistance to fatigue and the fissure propagation rate should also be taken into consideration for dimensioning. 
     In this sphere, research has concentrated mainly on the implementation of the welding operation itself. There have also been attempts to improve the corrosion resistance of the article by appropriate thermomechanical treatments. 
     Japanese patent application JP 06-212373 proposes the use of an alloy containing 1.0 to 2.0% of Mn, 3.0 to 6.0% of Mg and less than 0.15% of iron to minimize the reduction in the mechanical strength due to welding. However, the use of an alloy having such a high manganese content leads to a reduction in the resistance to fatigue and in the toughness. 
     SUMMARY OF THE INVENTION 
     The object of the invention is significantly to improve the mechanical strength and fatigue resistance of welded structures made of AlMgMn alloy, under predetermined welding conditions, without unfavourable consequences for other parameters such as toughness, corrosion resistance and cutting deformation, due to internal stresses. 
     The invention relates to products for welded constructions made of AlMgMn aluminium alloy composed of (% by weight): 
     3.0&lt;Mg&lt;5.0 
     0.5&lt;Mn&lt;1.0 
     Fe&lt;0.25 
     Si&lt;0.25 
     Zn&lt;0.40 
     optionally one or more of the elements Cr, Cu, Ti, Zr such that: 
     Cr&lt;0.25 
     Cu&lt;0.20 
     Ti&lt;0.20 
     Zr&lt;0.20 
     other elements &lt;0.05 each and &lt;0.15 in total, wherein Mn+2Zn&gt;0.75. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Contrary to earlier research which concentrated on the welding process and the thermomechanical treatments, the inventors have found a particular, range of composition for minor alloying elements, in particular iron, manganese and zinc, leading to an interesting set of properties combining static mechanical characteristics, toughness, resistance to fatigue, resistance to corrosion and cutting deformation, this set of properties being particularly well adapted to the use of these alloys for naval construction, utility vehicles or the welded frames of bicycles. 
     This set of properties is obtained by combining a low iron content, &lt;0.25%, preferably &lt;0.20%, and even 0.15%, and a manganese and zinc content such that Mn+2Zn&gt;0.75%, preferably &gt;0.8%. The Mn content should be &gt;0.5%, preferably &gt;0.8%, to have adequate mechanical characteristics, but should not exceed 1% if a deterioration in toughness and fatigue resistance are to be avoided. The addition of zinc combined with manganese has been found to have a beneficial effect on the mechanical characteristics of welded sheets and joints. However, it is better not to exceed 0.4% because problems can then be encountered in welding. 
     The magnesium is preferably kept &gt;4.3%, because it has a favourable effect on the yield stress and fatigue resistance, but beyond 5% the corrosion resistance is less good. The addition of Cu and Cr are also favourable to the yield stress, but Cr is preferably kept &lt;0.15% to maintain good resistance to fatigue. 
     The mechanical strength of the sheets depends both on the magnesium content in solid solution and on the manganese dispersoids. It has been found that the volumetric fraction of these dispersoids, which is linked to the iron and manganese contents, should preferably be kept above 1.2%. This volumetric fraction is calculated from the average of the surface fractions measured on polished cuts produced in three directions (length, width and thickness) by scanning electron microscopy and image analysis. 
     The products according to the invention can be rolled or extruded products such as hot- or cold-rolled sheets, wires, profiles or extruded and optionally drawn tubes. 
     The sheets according to the invention, which are assembled by butt welding by a MIG or TIG process and with a bevel of the order of 45° over about ⅔ of the thickness have, in the welded region, a yield stress R 0.2  which can be at least 25 MPa higher than that of a conventional alloy having the same magnesium content, that is a gain of about 20%. 
     The width of the thermally affected region is reduced by about one third relative to a conventional 5083 alloy, and the hardness of the welded joint increases from about 75 Hv to more than 80 Hv. The welded joints also have a tensile strength exceeding the minimum imposed by organizations monitoring unwelded cold-worked crude sheets. 
     The sheets according to the invention have fatigue resistance, measured by plane bending with a stress ratio wherein R=0.1 on samples taken in the cross-longitudinal direction, higher than: 
     10 5  cycles with a maximum stress &gt;280 MPa 
     10 6  cycles with a maximum stress &gt;220 MPa 
     10 7  cycles with a maximum stress &gt;200 MPa. 
     The fissure propagation rate ΔK, measured when R=0.1, is &gt;22 Mpa{square root over (m)} when da/dN=5×10 −4  mm/cycle and &gt;26 Mpa{square root over (m)} when da/dN=10 −3  mm/cycle. 
     The sheets according to the invention usually have a thickness greater than 1.5 mm. With thicknesses greater than 2.5 mm they can be obtained directly by hot rolling, without the need for subsequent cold rolling and, furthermore, these hot-rolled sheets are less distorted on cutting than cold-rolled sheets. 
     The products according to the invention have corrosion resistance which is as good as that of normal alloys having the same magnesium content, for example 5083 of common composition, widely used in naval construction. 
    
    
     EXAMPLE 
     Thirteen samples of sheets were prepared by conventional semicontinuous casting in the form of plates, were heated for 20 h at a temperature &gt;500° C. and were then hot-rolled to the final thickness of 6 mm. The reference 0 corresponds to a conventional 5083 composition and reference 1 to a composition slightly outside the invention. The 11 others (references 2 to 12) have a composition according to the invention. 
     The compositions were as follows (% by weight): 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Ref 
                 Mg 
                 Cu 
                 Mn 
                 Fe 
                 Cr 
                 Zn 
                 Ti 
                 Zr 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 4.40 
                 &lt;0.01 
                 0.50 
                 0.27 
                 0.09 
                 0.01 
                 0.01 
                   
               
               
                 1 
                 4.68 
                 &lt;0.01 
                 0.72 
                 0.12 
                 0.05 
                 &lt;0.01 
                 0.01 
               
               
                 2 
                 4.56 
                 &lt;0.01 
                 0.83 
                 0.12 
                 0.13 
                 0.01 
                 0.01 
               
               
                 3 
                 4.60 
                 &lt;0.01 
                 0.85 
                 0.17 
                 0.10 
                 0.16 
                 0.01 
               
               
                 4 
                 4.62 
                 &lt;0.01 
                 0.96 
                 0.10 
                 0.05 
                 0.02 
                 0.01 
               
               
                 5 
                 4.80 
                 0.09 
                 0.80 
                 0.11 
                 0.03 
                 0.02 
                 0.01 
               
               
                 6 
                 4.72 
                 &lt;0.01 
                 0.87 
                 0.13 
                 0.03 
                 0.02 
                 0.01 
                 0.11 
               
               
                 7 
                 4.88 
                 0.05 
                 0.78 
                 0.16 
                 0.02 
                 0.01 
                 0.09 
               
               
                 8 
                 4.92 
                 0.06 
                 0.94 
                 0.08 
                 0.02 
                 0.19 
                 0.01 
               
               
                 9 
                 4.69 
                 &lt;0.01 
                 0.72 
                 0.07 
                 0.02 
                 0.10 
                 0.01 
               
               
                 10 
                 4.71 
                 &lt;0.01 
                 0.82 
                 0.06 
                 0.02 
                 &lt;0.01 
                 0.01 
               
               
                 11 
                 4.73 
                 &lt;0.01 
                 0.95 
                 0.17 
                 0.03 
                 &lt;0.01 
                 0.01 
               
               
                 12 
                 4.70 
                 &lt;0.01 
                 0.92 
                 0.22 
                 0.03 
                 0.01 
               
               
                   
               
             
          
         
       
     
     The samples all have, after rolling, a yield stress R 0.2 &gt;220 Mpa in the L direction. 
     The mechanical strength of the joints welded from these sheets was measured under the following conditions: continuous automatic MIG butt welding with a symmetrical bevel having an inclination of 45° to the vertical over a thickness of 4 mm and filler wire of 5183 alloy. 
     The mechanical characteristics (tensile strength R m , yield stress R 0.2 ) were obtained by pulling over samples standardized by the Norwegian monitoring organization DNV for naval construction having a length of 140 mm and a width of 35 mm, the weld bead with a width of 15 mm being in the centre and the length of the narrow portion of the sample being 27 mm, that is the sum of the width of the bead and twice the thickness (15+22 mm). 
     The volumetric fractions of manganese dispersoids was also measured. 
     The results are as follows (in MPa for resistances and % for fractions): 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ref. 
                 R m   
                 R 0.2   
                 Fractions 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 285 
                 131 
                 0.62 
               
               
                   
                 1 
                 292 
                 144 
                 1.2 
               
               
                   
                 2 
                 302 
                 150 
                 1.4 
               
               
                   
                 3 
                 300 
                 146 
                 1.6 
               
               
                   
                 4 
                 310 
                 158 
                 1.7 
               
               
                   
                 5 
                 309 
                 149 
                 1.4 
               
               
                   
                 6 
                 305 
                 155 
                 1.5 
               
               
                   
                 7 
                 315 
                 166 
                 1.3 
               
               
                   
                 8 
                 318 
                 164 
                 1.9 
               
               
                   
                 9 
                 310 
                 153 
                 1.5 
               
               
                   
                 10 
                 312 
                 150 
                 1.5 
               
               
                   
                 11 
                 315 
                 153 
                 1.6 
               
               
                   
                 12 
                 315 
                 151 
                 1.5 
               
               
                   
                   
               
             
          
         
       
     
     It is found that the yield stress of samples welded according to the invention increases by between 15 and 35 MPa relative to the reference sample. 
     The resistance to fatigue of unwelded sheets subjected to plane bending wherein R=0.1 was also measured for references 0 to 5, while determining the maximum stress (in MPa) corresponding to 10 6  and 10 7  cycles respectively, as well as the fissure propagation rate ΔK measured when da/dn=5×10 −4  mm/cycle (in Mpa{square root over (m)}). 
     The results were as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ref. 
                 10 6  cycles 
                 10 7  cycles 
                 ΔK 
               
               
                   
                   
               
             
             
               
                   
                  0 
                 220 
                 200 
                 22 
               
               
                   
                 1 
                 235 
                 205 
                 22 
               
               
                   
                 2 
                 230 
                 200 
                 23 
               
               
                   
                 3 
                 225 
                 200 
                 23 
               
               
                   
                 4 
                 230 
                 205 
                 22 
               
               
                   
                 5 
                 225 
                 200 
                 22 
               
               
                   
                   
               
             
          
         
       
     
     It is found that, despite the increase in the mechanical strength, the sheets according to the invention have resistance to fatigue which is at least as good as that of conventional 5083 sheets.