GB2568869A

GB2568869A - Welding method and structure

Info

Publication number: GB2568869A
Application number: GB1719273.3A
Authority: GB
Inventors: John Ogborne Benjamin
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2017-11-21
Filing date: 2017-11-21
Publication date: 2019-06-05
Also published as: GB201719273D0

Abstract

A method of welding two components 5, 6 together to form a welded structure 1, which may be an aerospace structure, including the steps of; i) determining a desired joint line 27 between the two components; ii) forming the components to create an intersection between them which forms a sinuous (or curved, non-linear) interdigitated weld path 2 crossing the desired joint line and iii) welding together the components along the intersection to form the welded structure. The method may comprise determining the direction of natural crack paths in the welded structure, and constructing the weld path to pass at an angle to the expected direction of a crack path. The components to be welded may be aerospace structural components, more specifically; two portions of an aircraft wing spar, wing skin or skin reinforcing member. The welding technique used may be friction stir welding.

Description

0001. The present invention relates to a method of welding a structure, to such a welded structure and in particular to a method of friction stir welding a structure and to such a welded structure.

BACKGROUND TO THE INVENTION

0002. For welded structures, it has long been known that crack growth along the weld line or weld path is more likely to occur than in the surrounding structure. For aircraft structures, which are almost invariably constructed from aluminium alloy, fatigue cracking due to cyclic stresses imposed on the structure in flight is a considerable problem. This problem is usually solved by keeping to a minimum stress raisers within the structure and by prescribing a safe life for the structure, in terms of the number of hours flown.

0003. Welds within structures are known to be stress raisers and thus welded joints in aluminium aircraft structures are often avoided, particularly in highly stressed areas such as wing skins, spars, ribs, stringers, etc.

0004. It is often desirable to use a welded joint, for example where the size of a component is limited by the size of the billet from which it is formed and because welded joints are usually lighter than bolted joints. In particular, this can be the case for a wing spar where the length of the spar can exceed that of an available billet size.

0005. For such a joint, it would be desirable to use a low temperature welding method such as friction stir welding to weld the two spar sections together with a vertical spar joint.

0006. In the type of aluminium alloy used for aircraft structures, friction stir welded joints are currently only possible for less stressed members. However, for highly loaded structures such as spars, wing skins and the like, the possibility of crack growth in the weld has so far prevented such use of welds and heavier bolted joints have had to be used. A weld, even a friction stir weld, can be more brittle than the parent material and thus more likely to reduce the damage tolerance or increase the likelihood of crack growth within the structure to an unacceptable level.

0007. It is an object of the invention to use a welded joint in a structure in a manner which increases the damage tolerance of that structure.

SUMMARY OF THE INVENTION

0008. According to a first aspect of the invention, there is provided a method of welding two components together to form a welded structure, the method including the steps of determining for the structure to be welded a desired joint line between the two components, forming the components to create an intersection therebetween which forms a sinuous path in the region of the desired joint line and welding together the components along the intersection to form the welded structure.

0009. Forming the weld path as a sinuous path crossing the desired joint line will tend to create a weld path which does not coincide with expected crack growth paths.

0010. The welding step may be a friction stir welding step. The use of friction stir welding will be particularly useful in the welding of aluminium aircraft structures where the low temperature nature of the welding technique enables its use where other types of welding are not possible. In particular, the fatigue performance of friction stir welds can be markedly superior to gas welds.

0011. The method may include the step of determining the direction of natural crack paths for the welded structure, in use, and forming the intersection whereby the weld path will pass at an angle to the expected direction of a said natural crack path.

0012. The step of welding together the components may comprise welding together two aerospace structural components.

0013. The two aerospace structural components may comprise two portions of an aircraft wing spar.

0014. The two aerospace structural components may comprise two portions of an aircraft wing skin.

0015. The two aerospace structural components may comprise two portions of an aircraft skin reinforcing member.

0016. The step of forming the components to create an intersection therebetween which forms a sinuous path may comprise forming the sinuous path in a series of interdigitated projections.

BRIEF DESCRIPTION OF THE DRAWINGS

0017. The invention will now be described, by way of example only, with reference to the following drawings in which:Figure 1 is a schematic illustration of a portion of an aircraft wing spar, showing a weld according to the invention, and

Figure 2 shows the welded wing spar of Figure 1 with interactions of the weld with various natural crack paths shown.

DETAILED DESCRIPTION OF THE INVENTION

0018. Figure 1 shows a wing spar 1 having a “vertical” friction stir weld 2 leading a sinuous path across the spar 1, according to the invention. The spar 1 has an upper edge 3 and a lower edge 4 and the neutral axis of the spar 1 is indicated by the chain dotted line. The weld 2 joins together spar components 5 and 6. A desired joint line 27 is shown between the components 5, 6. The sinuous path of the weld 2 extends in the region of the desired joint line and crosses the desired joint line 27, forming a series of interdigitated projections 7, 8, 9, 10, 11. A heat affected zone 12 is created around the weld 2.

0019. Also shown in Figure 1 are natural crack paths 13, 14 for the welded spar 1. Crack path 13 would occur when driven by a down bend of the spar 1 and crack path 14 would occur when driven by an up bend of the spar 1. The sparl is shown loaded in bending and shear with an up bend shown.

0020. Where possible, the path of the weld 2 is designed to run at an angle to any expected natural crack path which might occur in an equivalent non-welded structure whereby a natural crack path coinciding with the weld path will be avoided.

0021. Extrapolation of the path of the weld 2 along the direction of natural cracks avoids any similar features of a continuation of the weld path.

0022. The sinuous path of the weld 2 helps to ensure that any cracks developing through the weld 2 that are in danger of becoming critical to the safety of the structure will be arrested once the crack leaves the heat affected zone 12.

0023. With reference to Figure 2, there is shown, for the same portion of spar as in Figure 1, interactions of the weld 2 with various natural crack paths 15, 16, 17, 18 which are liable to occur in the spar. Natural crack path 15 is liable to occur when the spar is subject to down bending and it will be seen that the path 15 diverges from the path of the weld 2 immediately it is formed at an upper surface 19 of the spar 1.

0024. Natural crack paths 16, 17, 18 are liable to occur when the spar 1 is subject to up bending. Similar to crack path 15, crack path 17 is formed at a surface, this time a lower surface 20, of the spar 1. And, once again, the crack path 17 immediately diverges from the weld 2.

0025. Crack paths 16, 18 are formed within the spar 1 and each originates at the weld 2. However, owing to the sinuous nature of the path of the weld 2, each crack 16, 18 immediately passes through the heat affected zone 12 into parent material of the components 5, 6, respectively, where the crack is likely to be arrested without becoming critical. In addition, extrapolation of the path of the weld 2 along the direction of natural cracks avoids any similar features of a continuation of the weld path. This is illustrated by crack paths 15, 16, 17, 18 which, if continued, would not encounter any part of the weld 2 running in the same direction as the point at which the crack started.

0026. Relatively straight portions 21, 22, 23, 24, 25, 26 of the weld 2, which run generally parallel to the interdigitated projections 7, 8, 9, 10, 11 are deliberately formed at an angle to any expected crack paths further to avoid stresses in the spar driving cracks along the weld.

0027. Thus, it will be seen that the design of a weld according to the invention will tend to discourage cracks from forming in the weld. However, if cracks are formed within the weld, the sinuous form of the weld will immediately ensure that the crack leaves the heat affected zone and enters parent material which will tend to halt the crack or prevent the crack becoming critical.

0028. Welds according to the invention are therefore particularly suitable for implementation as friction stir welds in aerospace structures where the prevention of crack propagation can be critical.

0029. The embodiments described herein are respective non-limiting examples of how the present technology, and aspects of the present technology, may be implemented. Any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined by the accompanying claims.

0030. The word “or” as used herein is to be taken to mean “and/or” unless explicitly stated otherwise.

Claims

1. A method of welding two components together to form a welded structure, the method including the steps of determining for the structure to be welded a desired joint line between the two components, forming the components to create an intersection therebetween which forms a sinuous path in the region of the desired joint line and welding together the components along the intersection to form the welded structure.

2. A method according to claim 1, in which the sinuous path is formed to cross the desired joint line.

3. A method according to claim 1 or 2, in which the welding step is a friction stir welding step.

4. A method according to claim 1, 2 or 3, including the step of determining the direction of natural crack paths for the welded structure in use and forming the intersection whereby the weld path will pass at an angle to the expected direction of a said natural crack path.

5. A method according to any of claims 1 to 4, in which the step of welding together the components comprises welding together two aerospace structural components.

6. A method according to claim 5, in which the step of welding together two aerospace structural components comprises welding together two portions of an aircraft wing spar.

7. A method according to claim 5, in which the step of welding together two aerospace structural components comprises welding together two portions of an aircraft wing skin.

8. A method according to claim 5, in which the step of welding together two aerospace structural components comprises welding together two portions of an aircraft skin reinforcing member.

9. A method according to any preceding claim, in which the step of forming the components to create an intersection therebetween which forms a sinuous path comprises forming the sinuous path in a series of interdigitated projections.

10. A method according to claim 9, when dependent upon claim 4, in which the series of interdigitated projections are formed to extend at an angle to any expected natural crack paths for the welded structure.

11. A welded aerospace structure including two components welded together along an intersection therebetween, said weld forming a sinuous path in a series of interdigitated projections.

12. A welded aerospace structure according to claim 11, in which the weld comprises a friction stir weld.

13. A welded aerospace structure according to claim 11 or 12, in which the series of interdigitated projections extend at an angle to any expected natural crack paths for the welded aerospace structure.

14. A welded aerospace structure according to claim 11 wing spar.

15. A welded aerospace structure according to claim 11 wing skin.

16. A welded aerospace structure according to claim 11

12 or 13, comprising an aircraft

12 or 13, comprising an aircraft skin reinforcing member.