EP0804626A1 - PRODUCT FOR OBTAINING WELDED AlMgMn ALLOY STRUCTURES WITH IMPROVED MECHANICAL RESISTANCE - Google Patents

Abstract

Laminated or extruded products for obtaining welded AlMgMn-type aluminium alloy structures are described, said products having the following contents in weight percent: 3.0 < Mg < 5.0; 0.75 < Mn < 1.0; Fe < 0.25; Si < 0.25; Zn < 0.40; optionally, one or more components selected from Cr, Cu, Ti, Zr, such that: Cr < 0.25; Cu < 0.20; Ti < 0.20; Zr < 0.20; other components < 0.05 each and < 0.15 in all, wherein Mn + 2Zn > 0.75. The welded products have improved mechanical and fatigue resistance, while retaining their toughness and corrosion resistance, and are particularly suitable for applications in shipbuilding, utility vehicles and bicycle frames made of welded tubes.

Description

 WELDED CONSTRUCTION PRODUCT IN ALMgMn ALLOY WITH IMPROVED MECHANICAL RESISTANCE

Technical area

The invention relates to the field of laminated or extruded products, such as sheets, profiles, wires or tubes, of aluminum alloy of the AlMgMn type at Mg> 3% by weight, intended for welded constructions having a high elastic limit, a good resistance to fatigue and good toughness for structural applications, such as, for example, boats, industrial vehicles or welded bicycle frames.

State of the art

The optimal dimensioning of welded structures in aluminum alloy leads to the use of AlMg alloys of the 5000 series according to the nomenclature of the Aluminum Association, in the hardened state (Hl state according to standard NF-EN-515), or partially softened (state H2), or stabilized (state H3), while retaining good resistance to corrosion (state H116), rather than in the annealed state (state 0).

But, most often, the increase in mechanical characteristics compared to state 0 does not persist after welding, and the recommendations of certification and inspection bodies generally advise, for welded structures, to only take account of the characteristics in the state 0. One must also take into account, for dimensioning, the resistance to fatigue and the speed of propagation of cracks. In this area, research work has mainly focused on the conduct of the welding operation itself. In addition, attempts have been made, by appropriate thermomechanical treatments, to improve the resistance to corrosion of the part.

Japanese patent application JP 06-212373 proposes, in order to minimize the reduction in mechanical resistance due to welding, to use an alloy containing from 1.0 to 2.0% of M from 3.0 to 6.0% of Mg and less than 0.15% iron. However, the use of an alloy with such a high manganese content leads to a reduction in the fatigue strength and toughness.

Subject of the invention

The object of the invention is, under determined welding conditions, to significantly improve the mechanical strength and the fatigue life of the structures welded in AlMgMn alloy, without adverse consequences on other parameters such as toughness, resistance to corrosion and cutting deformation due to internal stress.

The subject of the invention is products intended for welded constructions of AlMgMn alloy of composition (e weight%):

3.0 <Mg <5.0

0.5 <Mn <1.0

Fe <0.25 Si <0.25

Zn <0.40 optionally at least one of the elements Cr, Cu, Ti, Zr such that:

Cr <0.25 Cu <0.2

Ti <0.20 Zr <0.20 other elements <0.05 each and <0.15 in total, with the relation: Mn + 2Zn> 0.75

Description of the invention Unlike previous research focused on the welding process and thermomechanical treatments, the inventors have found a particular area of composition for minor addition elements, in particular iron, manganese and zinc, leading to a set of interesting properties. combining static mechanical characteristics, toughness, fatigue resistance, corrosion resistance and cutting deformation, this set of properties being particularly well suited to the use of these alloys for shipbuilding, commercial vehicles or welded cycle frames. This set of properties is obtained by the combination of a low iron content, <0.25%, preferably <0.20%, and even 0.15%, and a manganese and zinc content such as Mn + 2Zn> 0.75%, preferably> 0.8%. The Mn content must be> 0.5%, and preferably> 0.8%, to have sufficient mechanical characteristics, but must not exceed 1%, if we want to avoid degradation of the toughness and the resistance to fatigue. The addition of zinc in combination with manganese has been shown to have a beneficial effect on the mechanical characteristics of the sheets and welded joints. However, it is better not to exceed 0.4%, as this can cause problems with welding. Magnesium is preferably maintained> 4.3%, because it has a favorable effect on the elastic limit and the resistance to fatigue, but above 5% the corrosion resistance is less good. The addition of Cu and Cr are also favorable to the elastic limit, but Cr is preferably maintained <0.15% to maintain good resistance to fatigue. The mechanical resistance of the sheets depends on both the content of magnesium in solid solution and the manganese dispersoids. It has been found that the volume fraction of these dispersoids, which is linked to the iron and manganese contents, should preferably be maintained above 1.2%. This volume fraction is calculated from the average of the surface fractions measured on polished sections made in the 3 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 optionally re-drawn extruded tubes.

The sheets according to the invention, assembled by butt welding using a MIG or TIG process and with a chamfer of the order of 45 ^e over approximately 2/3 of the thickness, present in the welded zone an elastic limit R 2 which can be at least 25 MPa higher than that of a conventional alloy having the same magnesium content, ie a gai of around 20%.

The width of the heat affected zone is reduced by around a third compared to a usual 5083 alloy, and the hardness of the welded joint increases from around 75 Hv to more than 80 Hv. Welded joints also have a breaking strength which exceeds the minimum imposed by the inspection bodies for raw sheets of unwelded rolls. The sheets according to the invention have a resistance to fatigue, measured in planar bending with a stress ratio R * 0.1 on test pieces taken in the transverse section, greater than:

1C-5 cycles for maximum stress> 280 MPa 10 ⁶ cycles ^"""> 220 MPa 10 ⁷ cycles"^""> ₂ 00 MPa

The crack propagation speed J \., Measured for R = 0.1, is> 22 MPavTn for da / dN - 5 10 ^-4 mm / cycle and> 26 MPaV for da / dN - 10 ~ 3 mm / cycle. The sheets according to the invention are most often thicker than 1.5 mm. For thicknesses greater than 2.5 mm, they can be obtained directly by hot rolling, without the need for subsequent cold rolling, and, moreover, these hot rolled sheets exhibit less distortion on cutting than the cold rolled sheets. The products according to the invention have a corrosion resistance as good as the usual alloys with the same magnesium content, for example 5083 of common composition. widely used in shipbuilding.

Example

Thirteen sheet samples were prepared by conventional semi-continuous casting in the form of plates, reheated for 20 h at a temperature> 500 ^β C, then hot rolled to the final thickness of 6 mm. The reference 0 corresponds to a conventional composition of 5083 and the reference 1 has a composition slightly outside the invention. The other 11 (ref. 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 <0.01 0.50 0.27 0.09 0.01 0.01

1 4.68 <0.01 0.72 0.12 0.05 <0.01 0.01

2 4.56 <0.01 0.83 0.12 0.13 0.01 0.01

3 4.60 <0.01 0.85 0.17 0.10 0.16 0.01

4 4.62 <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 <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 <0.01 0.72 0.07 0.02 0.10 0.01

10 4.71 <0.01 0.82 0.06 0.02 <0.01 0.01

11 4.73 <0.01 0.95 0.17 0.03 <0.01 0.01

12 4.70 <0.01 0.92 0.22 0.03 0.01 0.01

The samples all have, after rolling, an elastic limit Ro, 2 ^> 220 MPa in the direction L.

The mechanical resistance of the welded joints was measured from these sheets under the following conditions: automatic continuous butt MIG welding, with a symmetrical chamfer of slope 45 * relative to the vertical over a thickness of 4 mm and wire 5183 alloy intake.

The mechanical characteristics (tensile strength R _m , elastic limit Q, 2) ^orvt been obtained by traction on test pieces standardized by the Norwegian inspection body DNV for shipbuilding, length 140 mm, width 35 mm, the weld bead width 15 mm being the center and the length of the narrow part of the test piece being 27 mm, the sum of the width of the bead and times the thickness (15 + 12 mm).

The volume fractions of manganese dispersoid were also measured. The results are as follows (in MPa for the resistances and% for the 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 elastic limit of the welded samples according to the invention has, compared to the reference sample, an increase of between 15 and 35 MPa.

We also measured, for the references 0 to 5, the fatigue resistance of the non-welded sheets in plane bending, with R 0.1, by determining the maximum stress (in MP corresponding respectively to 10 ^ and 10 ⁷ cycles, so that the crack propagation speed ^ K measured for da / - 5 x 10 ~ ⁴ mm / cycle (in MPaVπT) The results were as follows: Ref. 10 ^ cycles 10? Δ ^κ cycles

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 noted that, despite the increase in mechanical strength, the sheets according to the invention have a resistance to fatigue at least as good as that of conventional 5083 sheets.

Claims

1) Product for welded construction in aluminum alloy AlMgMn of composition (% by weight): 3.0 <Mg <5.0 0.5 <Mn <1.0 Fe <0.25 Si <0.25 Zn <0.40 possibly one or more of the elements Cr, Cu, T Zr such as: Cr <0.25 Cu <0.20 Ti <0.20 Zr <0.20 other elements <0.05 each and <0.15 in total, with Mn + 2Zn> 0.75 and preferably> 0.8%. 2) Product according to claim 1, characterized in that q Mg> 4.3%. 3) Product according to one of claims 1 and characterized in that Mn> 0.8%. 4) Product according to one of claims 1 to 3, characterized in that Fe <0.20%. 5) Product according to claim 4, characterized in that q Fe <0.15%. 6) Product according to one of claims 1 to 5, characterized in that the volume fraction of dispersoids e greater than 1.2%. 7) Sheet according to one of claims 1 to 6, characterized in that it is> 2.5 mm thick and is only hot rolled. 8) Unwelded sheet according to any one of claims 1 to 7, characterized in that it has a resistance to fatigue, measured in plane bending with R = 0.1 in the cross-long direction, greater than: 10 ⁵ cycles for maximum stress> 280 MPa 10 ⁶ cycles ""> 220 MPa 10 ⁷ cycles "^""> ₂ 00 MPa 9) Sheet according to any one of claims 1 to 8, characterized in that it has a crack propagation speed ^ K, measured for R = 0.1, greater than: 22 MPavîή for da / dn "5 10 ~ ⁴ mm / cycle 26 for da / dn "10 ~ 3 mm / cycle. 10) Use of a product according to one of claims 1 to 9 in shipbuilding. 11) Use of a product according to one of claims 1 to 9 for the construction of industrial vehicles. 12) Use of spun tubes according to one of claims 1 to 6 for the manufacture of welded bicycle frames. 13) Sheet according to one of claims 1 to 9 fusion welded and having in the welded area a hardness> 80 Hv.