Patent Description:
Composite materials enable high light-weight potential for highly loaded structures like airframes. Further, the joining of smaller subparts to large components is of high importance. In particular, joining technics are necessary requiring a low effort regarding e. preparation, joining process, post-treatment, etc. Further, additional weight shall be avoided, which is caused e. Moreover, there shall be no negative material impact on the substrates, like it is for example caused by holes or misaligned or broken fibres. Further, a failsafe option is desired, like e. a second load path or fibre reinforcement.

Several joining technics are already known. They provide individual benefits but also drawbacks. These technics comprise e. adhesive boding, riveting, welding and interdiffusion. However, adhesive bonding requires surface preparation, riveting causes added weight and requires an adapted ply stacking, and welding usually leads to matrix joining only. Interdiffusion works for thermoset/thermoplastic joining only. A polymer affinity is required, and only few combinations are feasible. Nearly all composite joints work with the overlap principle, which adds additional weight and loads. Bending usually results in out-of-plane loads.

Friction stir welding of composites is for example described in <CIT> and <CIT>.

The main load carrying component of Fibre Reinforced Polymers (FRP) are the fibres. The highest performances are achieved for continuous fibre reinforced polymers. If two parts made from FRP are joined there is a discontinuity of load carrying from one to the other part and thus a weak point. Fibres based on e. carbon, glass or aramid cannot be connected by melting as it can be done for metals during welding. In order to tackle this weak point two main strategies are currently employed. One of them is riveting which is based on bearing stress, and the other is based on adhesive or welding and transfers loads via shear stress. Both require an overlapping geometry, which in many cases lead to complex secondary loads.

Exceeding that, the solutions according to the state of the are known to be complicated - e. pre-treatment and cleaning for adhesive joining, tolerance compensation and many process steps for riveting, tooling and many process parameters for welding. Friction Stir Welding (FSW) is frequently used for metal joining. This process employs a rotating pin to generate heat to induce a welding and mixing of material between two parts. For friction stir welding of thermoplastic materials additional heat sources are frequently employed. For continuous fibre reinforced polymer parts this certainly means that in the joining area only broken-down short fibres in the welding line exist.

<CIT> and <CIT> disclose methods for joining two fiber-reinforced thermoplastic resin members.

It is the object of the invention to provide a method for joining fibre reinforced composite parts, which requires a relatively low effort, avoids additional weight, and leads to a strong joint which can resist high loads.

The object is achieved by the subject matter according to the independent claims. Advantageous embodiments are subject matter of the dependent claims.

According to a first aspect, the invention provides a method for joining fibre reinforced composite parts using friction stir welding, comprising the steps of providing at least a first and a second part, both made of fibre reinforced thermoplastic material and both having a joining surface for forming a butt joint between both parts, wherein the joining surface of the first part comprises one or more protrusions which fit into one or more grooves of the second part when forming the butt joint; positioning both parts thereby forming the butt joint; and welding the parts by friction stir welding along a welding path which follows the geometry of the one or more protrusions and/or grooves to locally melt the material of both parts in a contact area defined by that geometry.

The friction stir welding of thermoplastic composite materials according to the invention provides a beneficial result, in which pre-treatment, cleaning, drilling, additional weight and the drawbacks related thereto are omitted and a strong joint between the parts is created. In addition, using the FSW technique has many cost advantages in manufacturing, like e.g. pre-assembly spot stitching, robust geometrical tolerance, etc. Further, the Poka Yoke principle can be applied by using the method of the invention.

Preferably, the one or more protrusions and/or grooves of the joining surfaces of both parts form a puzzle-like geometry fitting into each.

Preferably, both parts are interlocking each other at the joining surfaces when forming the butt joint.

Preferably, a friction stir probe is rotating between the joining surfaces of both parts to melt the material within the contact area.

Preferably, the joining surfaces comprise a zig-zag geometry defining the contact area between both parts and the welding path.

Preferably, the joining surfaces of both parts comprise a rectangular geometry defining the contact area between both parts and the welding path.

Preferably, the joining surfaces of both parts comprise a diamond-like geometry defining the contact area between both parts and the welding path.

Preferably, the parts are configured such that their joining surfaces form the butt joint as a closed loop when the parts are positioned.

Preferably, wherein the at least one protrusion of the first part has a contour which fits into an undercut formed by the at least one groove of the second part for holding the protrusion in place.

Preferably, the at least one protrusion of the first part comprises a relatively wide head portion and a relatively narrow neck portion, and the at least one groove of the second part comprises a relatively wide inner portion and a relatively narrow front portion for holding the protrusion of the first part in place.

Preferably, the geometry of the at least one protrusion of the first part is configured such that continuous fibres within the first part, which extend into or the protrusion, are partly surrounded by the welding path without being disturbed by the welding.

Preferably, each part comprises a plurality of grooves and/or protrusions fitting into each other and defining the geometry of the contact area between both parts and the welding path. One or more protrusions and grooves can be formed in both parts.

Preferably, a bond line defined by the welding path comprises short fibres.

Preferably, the thickness of the parts in an area comprising the one or more protrusions and/or grooves is increased relative to another portion of each part.

According to a second aspect, the invention provides the use of the method according to the invention as a repair method, wherein a damaged portion of the first part is cut out along a cutting line and removed from the first part before the first part is provided, and a repair part having a shape that matches the shape of the cut portion of the first part is provided as the second part, wherein the joining surfaces of the parts are formed by the shape of the cutting line.

According to a third aspect, the invention provides an aircraft component comprising at least two composite parts made of fibre reinforced thermoplastic material which are joint by friction stir welding, wherein one or more protrusions provided in a joining surface of a first part fit into one or more grooves provided in a joining surface of a second part at a butt joint between both parts; wherein the parts are welded by friction stir welding in a contact area between the joining surfaces along a welding path which follows the geometry of the one or more protrusions and/or grooves.

Preferably, the composite parts are welded by the method according to the invention.

According to a fourth aspect, the invention provides an aircraft comprising an aircraft component according to the invention.

In particular, the invention enables carbon fibre reinforced polymers (CFRP) load carrying butt joints. As linear butt joints made from thermoplastic friction stir welding will fall back to the short fibre reinforced properties of the base material, such structures are about one order of magnitude lower in performance as to undisturbed CFRP made from continuous fibres. To enable such butt joints for an at least mild load transfer, the load case in particular changes to shear loads and/or geometric interlocking.

In particular, the joining process according to the invention can also cope with the through thickness geometry itself and the inevitable tolerances of the gap. Thermoplastic friction stir welding according to the invention, which may comprise an additional material feed, can solve this challenge.

The method may in particular comprises the following steps:.

with option to increase transferable loads by increased local laminate thickness. Bond-lines may contain short fibres. Thus, they have similar module properties compared to the base material, which leads to further benefits for bond line strength.

Embodiments of the invention are described in detail with reference to the accompanying drawings, in which.

For similar elements in the drawings or elements having the same function, the same reference numbers are used in the different drawings and embodiments described herein. Their description will only be repeated if it seems useful. Details and advantages described with reference to the method according to the invention also apply to the aircraft component according to the invention and vice versa.

A first preferred example of the method according to the invention is described in the following with reference to <FIG>.

<FIG> shows a first composite part <NUM> and a second composite part <NUM> positioned for being welded. Both parts <NUM>, <NUM> are made of fibre reinforced thermoplastic material or fibre reinforced polymers. Each composite part <NUM>, <NUM> has a joining surface <NUM>, <NUM> in order to form a butt joint <NUM> when they are connected to each other at the joining surfaces <NUM>, <NUM> (see <FIG>). The joining surface <NUM> of the first composite part <NUM> comprises a number of protrusions <NUM> designed to fit into a number of grooves <NUM> respectively, which are provided in the second part <NUM> when both parts are connected to each other at their joining surfaces <NUM>, <NUM> to form butt joint <NUM>.

Both parts <NUM>, <NUM> are reinforced by long fibres <NUM>. In the figures, only the fibres <NUM> of the first part <NUM> are shown. However, also the second part <NUM> comprises such reinforcement fibres which are not shown in the figures. Similar to the first part <NUM>, also second part <NUM> comprises protrusions, and similar to the second part <NUM> also first part <NUM> comprises grooves.

Thus, both parts <NUM>, <NUM> comprise or form a puzzle-like geometry. When both parts <NUM>, <NUM> are connected, the grooves <NUM> and recesses or grooves16 of both parts are interlocking each other at the joining surfaces <NUM>, <NUM>.

In a second step, shown in <FIG>, both parts <NUM>, <NUM> are positioned relative to each other such that their surfaces <NUM>, <NUM> get in contact at butt joint <NUM>. Then, the parts <NUM>, <NUM> are welded together by friction stir welding, wherein a rotating pin <NUM> moves along a welding path <NUM> which follows the geometry of protrusions <NUM> and grooves <NUM>. During the continuous movement of rotating pin <NUM> along welding path <NUM>, the material of both parts <NUM>, <NUM> is locally melted by the friction heat of rotating pin <NUM> in a contact area between both parts <NUM>, <NUM> which is defined by the geometry of the protrusions <NUM> and grooves <NUM>. The friction stir welding process and a friction stir welding tool comprising rotating pin <NUM> as described in more detail in <CIT> and/or <CIT> mentioned above are preferably used for welding the parts <NUM>, <NUM> to each other.

When the parts <NUM>, <NUM> are connected, variable gaps be formed between both parts <NUM>, <NUM>, i.e. between their joining surfaces <NUM>, <NUM>, due to tolerances.

The reinforcement fibres <NUM> provided with in the composite parts <NUM>, <NUM> are long fibres and extend into the protrusions <NUM>, as it is shown in the figures for the first part <NUM>. However, the same may also apply in a similar manner for reinforcement fibres provided in second part <NUM>. During welding, when pin <NUM> moves along the butt joint <NUM> between both parts <NUM>, <NUM>, the long fibres <NUM> extending within first part <NUM> and into the protrusions <NUM> are not disturbed or destroyed by the friction stir welding process, since the welding path along the geometry of the protrusions and grooves surrounds the ends of long fibres <NUM> one. The same applies for long fibres provided in second part <NUM> and extending into protrusions of second part <NUM>, which are not shown in the figures.

The protrusions <NUM> of first part <NUM> have a contour which fits into an undercut formed by the grooves of second part <NUM> in order to hold the protrusions <NUM> of the first part <NUM> with in the grooves <NUM>. The same applies vice versa for protrusions provided in the second part <NUM> and grooves provided in the first part <NUM>.

As also shown in <FIG> in more detail, the protrusions <NUM> of first part <NUM> have a relatively wide head or front portion <NUM> and a relatively narrow neck portion <NUM>. Corresponding thereto, the grooves <NUM> of second part <NUM> comprise a relatively wide inner portion <NUM> and a relatively narrow front portion <NUM> for holding the protrusion <NUM> in place within groove <NUM>.

As depicted in <FIG>, which shows a side view of welded or connected parts shown in <FIG>, both parts <NUM>, <NUM> have an increased thickness t or local laminate thickness near the meandering bond line along butt joint <NUM>, thereby increasing transferable loads.

<FIG> shows an enlarged sectional view of the butt joint <NUM> between parts <NUM>, <NUM>. Preferably, the bond lines along butt joint <NUM>, which correspond to the welding path, contain short fibres <NUM>. Thus, it has similar module properties compared to the base material, which is beneficial for the strength of the bond line. Short fibres <NUM> are distributed within the contact zone <NUM> between both parts <NUM>, <NUM>.

<FIG> shows a top view of two parts <NUM>, <NUM> prepared for welding according to a second embodiment of the invention. Different from the first example shown in <FIG>, first part <NUM> comprises only one protrusion <NUM> and the second part <NUM> comprises only one groove <NUM> in which protrusion <NUM> is placed when both parts <NUM>, <NUM> are connected and form the butt joint <NUM>. Further details and features have already been discussed above with reference to <FIG> and apply also for this example.

<FIG> depicts a top view of two parts <NUM>, <NUM> positioned and welded at butt joint <NUM> according to a third example of the invention. Here, the joining surfaces <NUM>, <NUM> of parts <NUM>, <NUM> contacting each other when forming the butt joint <NUM>, show a zig-zag geometry which defines the contact area between parts <NUM>, <NUM>, which contact area defines the welding path <NUM>, i.e. the path of the rotating pin <NUM> (see <FIG>) moving between the joining surfaces <NUM>, <NUM> of parts <NUM>, <NUM> respectively.

Rotating pin <NUM>, as visible in <FIG>, extends into the contact zone of the joining surfaces <NUM>, <NUM>, i.e. in the direction of the thickness t of both parts (see <FIG>) which is perpendicular to the image plane of <FIG>, i.e. to the upper and/or lower surfaces of parts <NUM>, <NUM>. The same applies for the other embodiments shown above.

<FIG> shows a further example, wherein the butt joint <NUM> has a diamond shaped geometry instead of a serpentine-line geometry as shown in <FIG> and <FIG> or a zigzag geometry as shown in <FIG>.

According to another preferred embodiment which is not shown here, butt joint <NUM> has a rectangular geometry. Also other geometries are possible, which can be designed or selected according to the specific requirements of the friction stir welding connection. Further details of the embodiments shown in <FIG> are discussed above with reference to <FIG> and may also apply here.

<FIG> shows another preferred embodiment of the invention, in which the method is used as a repair method. A first part <NUM> comprises e.g. a damage <NUM> (see <FIG>).

A portion <NUM> of the first part <NUM>, which portion comprises damage <NUM>, is cut out along a cutting line <NUM> forming protrusions and grooves, which cutting line <NUM> surrounds portion <NUM>, in this example as a closed line or loop (see <FIG>).

Then, portion <NUM> comprising damage <NUM> is removed from the first part <NUM> such that there is a gap or void space <NUM> in the first part <NUM>, before it is provided (see <FIG>).

A second part <NUM> is configured as a repair part to replace the portion <NUM> cut-out from the first part <NUM>. Second part <NUM> has a shape that matches the shape of the cut-out portion <NUM> of the first part <NUM>, and is inserted into gap <NUM> when both parts <NUM>, <NUM> are positioned to form butt joint <NUM>. The joining surfaces of the parts <NUM>, <NUM> forming butt joint <NUM> are defined by the shape of the cutting line <NUM> (see <FIG>).

Then, parts <NUM>, <NUM> are welded by friction stir welding along a welding path which follows the geometry of cutting line <NUM> to locally melt the material of both parts <NUM>, <NUM> in a contact area <NUM> defined by that geometry.

Further details are described above with reference to the other embodiments.

As discussed above, each composite part <NUM>, <NUM> comprises one or more protrusions <NUM> and/or grooves <NUM> which fit into one or more protrusions and/or grooves of the corresponding composite part when forming the butt joint <NUM>. Both parts <NUM>, <NUM> are configured for being used in the method according to the invention as shown in the different examples above.

Claim 1:
Method for joining fibre reinforced composite parts using friction stir welding, comprising:
providing at least a first and a second part (<NUM>, <NUM>), both made of fibre reinforced thermoplastic material and both having a joining surface (<NUM>, <NUM>) for forming a butt joint (<NUM>) between both parts (<NUM>, <NUM>),
wherein the joining surface (<NUM>) of the first part (<NUM>) comprises one or more protrusions (<NUM>) which fit into one or more grooves (<NUM>) of the second part (<NUM>) when forming the butt joint (<NUM>);
positioning both parts (<NUM>, <NUM>) thereby forming the butt joint (<NUM>); and
welding the parts (<NUM>, <NUM>) by friction stir welding along a welding path (<NUM>) which follows the geometry of the one or more protrusions (<NUM>) and/or grooves (<NUM>) to locally melt the material of both parts (<NUM>, <NUM>) in a contact area (<NUM>) defined by that geometry.