Friction stir welding apparatus

Abstract

The invention relates to a friction stir welding apparatus (1) comprising a friction stir welding tool (2) having a welding head (3) for contacting a joint zone (10) formed by two adjacent parts (11, 12), and having a shoulder (4) and a pin (5) extending axially from the shoulder, the welding apparatus comprising a motor configured to drive the pin in rotation about an axis of rotation (A), and a magnet device (7) mounted to rotate about the axis of rotation (A), the magnet device (7) comprising at least two different magnetic poles (S, N) positioned about the axis of rotation (A), and being configured such that the rotation of the magnet device relative to the parts generates an eddy current in the parts (11, 12) to preheat the joint zone (10). Fig. 1.

Description

Title of the invention: Friction-stir welding apparatus technical field

[0001] The present invention relates to a friction stir welding apparatus intended for welding, in particular, turbomachine parts, and a friction stir welding process using such a welding apparatus. Previous technique

[0002] Friction stir welding (FSW) is a solid state welding process which consists of joining two parts by bringing them locally to a paste-like state by means of a rotating pin and shoulder.

[0003] The welding process uses the heat generated by a pin and a coaxial shoulder rotating at a constant speed along a contact line formed by a parting line between two parts to be welded. This causes the materials to soften, becoming pasty. The pin then penetrates the parting line and intimately mixes the materials in an operation similar to forging or extrusion. The complete assembly of the parts is achieved by the pin progressing along the contact line.

[0004] The friction stir welding process is used to assemble parts based on aluminium, titanium, copper, nickel, magnesium alloys, steel or stainless steels.

[0005] Friction stir welding has been extended to materials with increasingly higher mechanical strength, such as high-melting-point materials or refractory metals like ultra-high-strength steels, chromium alloys, tantalum, and tungsten alloys, for example. This results in greater frictional forces on the pin and accelerates pin wear.

[0006] Furthermore, the welding time increases, and residual stresses appear in the welded area. Residual stresses and distortions of friction stir welded parts cannot be avoided due to the steep temperature gradient during welding. Indeed, welding processes result in heterogeneous heating and cooling cycles that can generate a welded joint containing significant residual stresses. These stresses shift from tension to compression along the thickness of the part. The non-homogeneous distribution of these residual stresses can, in some cases, particularly on thin parts, cause undesirable geometric and mechanical irregularities in the welded component. Concave and convex areas may appear on the parts.

[0007] To solve these problems, a known solution is to introduce an additional heating source in front of the friction stir welding tool to preheat and soften the materials before welding. This hybrid solution results in a more uniform material flow, reduced stress on the pin, and weld joints with significantly improved mechanical properties. Furthermore, preheating before welding reduces the heat generated by friction between the rotating pin and the workpieces, and consequently reduces the thermal gradient during welding. This reduces residual stress levels and associated distortion phenomena, thus improving weld quality.

[0008] The preheating source conventionally used is a plasma, a laser or an electric arc.

[0009] A process for electric assisted friction stir welding (EA-FSW) is also known from US patent 8164021, which is particularly effective for solving the aforementioned problems. The process consists of passing an electric current between the welding system pin and the workpiece to be welded to generate heat by electrical resistance.

[0010] However, hybrid friction stir welding processes necessarily require additional elements for preheating. For example, the electrically assisted process requires the use of, among other things, a power supply system, electrical circuits for the flow of electric current, and additional components for electrical insulation. The use of these auxiliary heat sources therefore increases the cost of the friction stir welding equipment as well as the risk of accidents and quality defects. Description of the invention

[0011] The invention therefore aims to resolve at least in part these drawbacks by proposing a friction stir welding apparatus comprising an auxiliary heat source simpler than those of the prior art while allowing the same welding efficiency to be obtained as with the solutions of the prior art.

[0012] The invention relates to a friction stir welding apparatus comprising a friction stir welding tool having a welding head intended to come into contact with a joint zone formed by two adjacent parts, and having a shoulder and a pin extending axially from the shoulder, the welding apparatus comprising a motor configured to drive the pin in rotation about an axis of rotation, and a magnet device mounted to rotate about the axis of rotation, the magnet device comprising at least two poles different magnetic elements positioned around the axis of rotation, and configured so that the rotation of the magnet device relative to the parts generates an eddy current in the parts to preheat the junction area.

[0013] The invention thus provides a friction stir welding apparatus comprising an auxiliary heat source simpler than those of the prior art while allowing to obtain a welding efficiency similar to that of the solutions of the prior art.

[0014] The invention reduces friction between the rotating pin and the parts to be welded, and consequently the thermal gradient generated during welding. The level of residual stresses and associated distortion phenomena are thus reduced in the welded part. Weld quality is therefore improved and pin wear is reduced.

[0015] For example, the titanium alloy Ti6A14V loses 90% of its mechanical properties between 200°C and 700°C. Heating to 700°C by eddy currents therefore makes it possible to perform friction stir welding with approximately ten times less mechanical force.

[0016] These advantages are achieved while using fewer components, unlike the electric current-assisted friction stir welding (EA-FSW) system, for example, which requires an electrical power supply system, electrical circuits for the flow of electric current and additional components for electrical insulation.

[0017] The invention therefore makes it possible to obtain a very compact friction stir welding device and to reduce the costs of the friction stir welding device.

[0018] In some embodiments, the magnet device comprises several magnets arranged around the axis of rotation so as to form an alternation of south magnetic poles and north magnetic poles around the axis of rotation.

[0019] In some embodiments, the magnet device is positioned opposite the welding head.

[0020] In some embodiments, the welding tool includes a shaft intended to be driven in rotation by the motor around the axis of rotation, the magnet device, the shoulder and the pin being supported by the shaft so as to be driven in rotation together and at the same speed.

[0021] In some embodiments, the shoulder is fixedly mounted on the welding tool, the magnet device and the pin being mounted for rotation and arranged so as to be driven in rotation by the motor and relative to the shoulder, at the same speed.

[0022] In some embodiments, the magnet device, the shoulder and the pin are arranged so that the magnet device has a different rotational speed than the rotational speed of the pin and the shoulder.

[0023] In some embodiments, the shoulder is fixedly mounted on the welding tool, the magnet device, the shoulder and the pin being arranged so that the magnet device has a different rotational speed than the rotational speed of the pin.

[0024] In cases where the shoulder does not rotate, the stationary shoulder absorbs a large amount of heat, resulting in a more uniform temperature field in the parts along their thickness. The maximum temperature reached in the junction zone (or weld zone) is reduced. Consequently, the maximum residual stress generated on the weld by the friction stir welding apparatus according to the invention is also reduced compared to the maximum residual stress generated by a conventional friction stir welding apparatus.

[0025] In some embodiments, the welding apparatus includes an arm supporting the welding tool, the magnet device being rotatably mounted on the arm around the axis of rotation, the welding apparatus comprising a first motor for driving the pin in rotation at a first speed and a second motor for driving the magnet device in rotation at a second speed.

[0026] In this configuration, the magnet device is not adjacent to the welding head but distant from it, like an additional heating source in front of the friction stir welding tool, which provides more time for the parts to heat up, like hybrid solutions of the prior art.

[0027] In some embodiments, the pin and / or the shoulder are made of ceramic material when there is a difference between the rotation speeds of the magnet device and the pin or the shoulder.

[0028] In some embodiments, the magnet device includes an electromagnetic magnet to allow the eddy current level to be adjusted according to the materials targeted for welding without changing the rotation speed.

[0029] The invention also relates to a method for welding a part by friction stir welding using a welding apparatus as defined above, comprising: • a preheating step of a junction zone formed by two adjacent parts, by rotating the magnet device around the axis of rotation and relative to the junction zone, and • a welding step of the two parts by friction-mixing the junction area using the welding head.

[0030] In some embodiments, the welding process includes a preliminary preheating step in which the preheating of the junction area is carried out before the welding head is brought into contact with the junction area, the magnet device rotating at a first rotation speed vl, the preheating of the junction area after the welding head is brought into contact with the junction area being carried out with the magnet device rotating at a second rotation speed v2 which is lower than the first rotation speed vl.

[0031] This method makes it easier to initiate welding with the welding tool.

[0032] The aforementioned features and advantages, as well as others, will become apparent from the following detailed description of examples of embodiments of the friction stir welding apparatus. This detailed description refers to the accompanying drawings. Brief description of the drawings

[0033] The attached drawings are schematic and are intended primarily to illustrate the principles of the exposition.

[0034] On these drawings, from one figure to another, identical elements (or parts of elements) are identified by the same reference signs.

[0035] [Fig-1] Fig. 1 schematically represents a friction welding apparatus mixing comprising a magnet device, according to one embodiment of the invention;

[0036] [Fig.2] Fig.2 schematically represents a bottom view of a magnet device, according to another embodiment of the invention;

[0037] [Fig.3] The [Fig.3] schematically represents a cross-sectional view of the magnet device of the [Fig.2]. Description of the implementation methods

[0038] To make the explanation more concrete, an example of a friction stir welding apparatus is described in detail below, with reference to the accompanying drawings. It should be noted that the invention is not limited to this example.

[0039] Figure 1 represents a friction stir welding apparatus 1 comprising a friction stir welding tool 2 having a welding head 3 intended to come into contact with a joint zone 10 formed by two adjacent parts 11, 12. The joint zone 10 extends perpendicularly from a joint plane formed between the respective edges of the two parts 11, 12 which are brought into contact, towards the interior of the parts 11, 12. The joint zone 10 extends from the joint plane for a few millimeters, for example.

[0040] Parts 11,12 are made of an electrically conductive but non-ferromagnetic material.

[0041] Parts 11, 12 are in the form of plates and can be based on high melting point materials or refractory metals such as ultra-high strength steels, chromium alloys, tantalum or tungsten alloys, for example.

[0042] The parts 11,12, once welded, can form an element of an aircraft turbomachine, for example.

[0043] The welding apparatus 1 includes a shaft 13 and a motor configured to drive the shaft 13 in rotation about an axis of rotation A which is substantially perpendicular to the parts 11, 12.

[0044] The welding apparatus 1 includes a housing in which the motor is housed.

[0045] The welding device 1 has a shoulder 4 and a pin 5 connected to the shaft 13. The pin 5 extends axially from an axial surface 6 of the shoulder 4, towards the parts 11, 12, when they are positioned opposite the welding device 1.

[0046] The pin 5 has a smaller diameter than that of the shoulder 4.

[0047] The welding apparatus 1 includes a magnet device 7 mounted for rotation about the axis of rotation A. The magnet device 7 includes at least two different magnetic poles S, N extending transversely with respect to the axis of rotation A.

[0048] Preferably, the two magnetic poles S, N extend perpendicularly with respect to the axis of rotation A.

[0049] When the welding device 1 is positioned opposite the parts 11, 12, the rotation of the magnet device 7 relative to the parts 11, 12 generates an eddy current in the parts 11, 12 which heats the parts 11, 12. This makes it possible to preheat a weld area which is positioned upstream of the junction area 10 relative to the direction of the weld, and to soften the materials constituting the parts 11, 12 in this weld area before and during the welding of the junction area 10 by the welding head 3.

[0050] According to the example in [Fig. 1], the magnet device 7 comprises a single annular magnet 8 having two magnetic poles S, N positioned around the axis of rotation A. The two magnetic poles S, N extend in a transverse plane X, Y which is perpendicular to the axis of rotation A. The transverse plane X, Y is formed by a first direction X and a second direction Y. The axis of rotation A extends along a third direction Z which is perpendicular to the first and second directions X and Y. The magnet 8 is fixed to and surrounds the shaft 13. The magnet 8 includes a central opening 14 through which the shaft 13 passes.

[0051] When the welding head 3 is in contact with the parts, the welding apparatus 1 advances along the joint zone 10 in the second direction Y corresponding to the direction of welding advance. The joint zone 10 comprises a portion upstream (in front of welding head 3) which is to be welded and a downstream portion (behind welding head 3) which is welded.

[0052] According to the variant shown in Figures 2 and 3, the magnet device 7 comprises several magnets 8 arranged around the axis of rotation A so as to form an alternation of opposite magnetic poles, i.e. an alternation of south magnetic poles S and north magnetic poles N around the axis of rotation A. The south magnetic poles S and north magnetic poles N extend in a transverse plane X, Y which is perpendicular to the axis of rotation A.

[0053] Each magnet 8 has two magnetic poles, a south pole S and a north pole N, which extend along the third direction Z, that is to say parallel to the axis of rotation A.

[0054] The magnet device 7 comprises a first face 19 and a second face 20, opposite the first face 19.

[0055] The second face 20 is the face which is intended to be placed opposite the parts 11, 12. The second face 20 includes one of the poles of each magnet 8. The opposite pole of each magnet 8 is positioned on the first face 19 of the magnet device 7.

[0056] The magnets 8 are in the form of sectors and each has an inner cord 15 and an outer cord 16 with a radius greater than that of the inner cord 15.

[0057] The magnets 8 are arranged in a transverse plane X, Y perpendicular to the axis of rotation A. The magnets 8 are supported by a support 17 fixed to the shaft 13.

[0058] The magnets 8 form an annular assembly having an internal radius RI formed by the succession of internal strings 15 and an external radius R2 formed by the succession of external strings 16. The external radius R2 is larger than the internal radius RL

[0059] In the embodiments of figures 1 and 2, the magnet device 7 is positioned opposite the welding head 3, a few millimeters from the pin 5.

[0060] The shaft 13 being driven in rotation by the motor around the axis of rotation A, the magnet device 7, the shoulder 4 and the pin 5 which are fixed to the shaft 13, are therefore also driven in rotation by the shaft 13 at the same speed.

[0061] In an alternative (not shown), the shoulder 4 is fixed in the welding tool 2. The magnet device 7 and the pin 5 are mounted for rotation and arranged so as to be driven in rotation by the motor at the same speed both and relative to the shoulder 4. The magnet device 7 and the pin 5 are fixed to the rotating shaft 13 but not the shoulder 4.

[0062] In an alternative (not shown) arrangement, the magnet device 7, the shoulder 4, and the pin 5 are arranged such that the magnet device 7 has a rotational speed different from the rotational speed of the pin 5 and the shoulder 4, the speeds being non-zero. The pin 5 and the shoulder are fixed to the rotating shaft 13, but not the magnet device 7. The rotation speeds of the pin 5 and the shoulder 4 are then identical.

[0063] The welding device 1 may include a first motor intended to drive the pin 5 and the shoulder 4 in rotation at a first speed and a second motor intended to drive the magnet device 7 in rotation at a second speed.

[0064] In an alternative (not shown) configuration, the shoulder 4 is fixed in the welding tool 2. The magnet device 7, the shoulder 4, and the pin 5 are arranged such that the magnet device 7 has a rotational speed different from the rotational speed of the pin 5, the speeds being non-zero. The welding apparatus 1 comprises a first motor for driving the pin 5 in rotation at a first speed and a second motor for driving the magnet device 7 in rotation at a second speed.

[0065] In an alternative (not shown) configuration, the welding apparatus 1 may include an arm supporting the welding tool 2. The magnet device 7 is rotatably mounted on the arm about the axis of rotation A. The welding apparatus 1 includes a first motor for driving the pin 5 in rotation at a first speed and a second motor for driving the magnet device 7 in rotation at a second speed. The shoulder 4 may be driven by the first motor or be fixed.

[0066] In an alternative (not shown), the magnet device 7, the shoulder 4 and the pin 5 can rotate independently of each other.

[0067] For example, the rotational speeds of the pin 5 and the shoulder 4 are both different and non-zero. The magnet device 7, the shoulder 4, and the pin 5 then rotate at three different speeds. The welding apparatus 1 comprises a first motor for driving the pin 5 in rotation at a first speed, a second motor for driving the magnet device 7 in rotation at a second speed, and a third motor for driving the shoulder 4 at a third speed.

[0068] When the rotational speed of the magnet device 7 is different from the rotational speed of the shoulder 4 and the pin 5, the speeds being non-zero, the level of preheating of the junction zone 10 formed between the two parts 11,12 can be controlled during welding.

[0069] For example, preheating can be enhanced by increasing the rotational speed of the magnet device 7 before welding begins to facilitate weld initiation. Once welding has begun, the rotational speed of the magnet device 7 can be reduced. The amount of heat supplied by eddy current preheating can be reduced due to the heat already generated by the stirring of the pin 5.

[0070] A difference between the rotational speeds of the magnet device 7 and the pin 5 or the shoulder 4 can induce eddy currents in the pin 5 or shoulder 4 and therefore heating of these elements. The use of a pin 5 or a shoulder 4 made of ceramic material eliminates this phenomenon.

[0071] In cases where shoulder 4 does not rotate, the stationary shoulder 4 absorbs a large amount of heat, resulting in a more uniform temperature field in parts 11, 12 along the thickness of parts 11, 12. The maximum temperature reached in the junction zone 10 (or weld zone) is reduced. Consequently, the maximum residual stress generated on the weld by the friction stir welding apparatus 1 according to the invention is also reduced compared to the maximum residual stress generated by a conventional friction stir welding apparatus 1.

[0072] Alternatively, the magnet device 7 includes an electromagnetic magnet to have a possibility of adjusting the level of an eddy current according to the materials targeted for welding without changing the rotation speed.

[0073] The invention also relates to a method of welding a part by friction stir using a welding apparatus 1 as described above.

[0074] The friction stir welding process includes a preheating step of the junction zone 10 by rotating the magnet device 7 around the axis of rotation A and relative to the junction zone 10, and a welding step of the two parts 11,12 by friction stir of the junction zone 10 by means of the welding head 3.

[0075] As explained previously for the embodiments of figures 1 to 3, when the shaft 13 is driven in rotation by the motor around the axis of rotation A, the magnet device 7, the shoulder 4 and the pin 5 which are fixed to the shaft 13, are therefore also driven in rotation by the shaft 13 simultaneously and at the same speed.

[0076] Preheating softens the joint area 10 before and during welding. At the beginning of the assembly operation of the two parts 11, 12, when the welding apparatus 1 approaches the joint area 10, the magnet device 7 begins to preheat the joint area 10 before the welding head 3 comes into contact with the joint area 10.

[0077] The welding head 3 then makes contact with the joining zone 10 to weld the two parts 11, 12. The welding head 3 then follows the joining zone 10 to completely weld the two parts 11, 12. The eddy current extends all around the welding head 3 and therefore upstream of the welding head 3 in the welding direction. The two parts 11, 12 are thus heated or preheated upstream, before they are welded by the welding head 3. The magnet device 7 also continues to heat the joining zone 10, which is directly above the magnet device 7, while the welding head 3 welds the two parts 11, 12 at this point. Heating and welding are carried out simultaneously on the junction area 10 at the point of contact between the parts and the welding head 3. Similarly, preheating upstream of the welding head 3 and welding at the point of contact between the parts and the welding head 3 are carried out simultaneously.

[0078] According to one variant, the welding process includes a preliminary preheating step of the junction zone 10 by the magnet device 7 which is carried out before the welding step and with a higher rotation speed of the magnet device 7 than that applied during the welding step.

[0079] This is possible when the welding device 1 includes a first motor for rotating the pin 5 and the shoulder 4 and a second motor for rotating the magnet device 7. The welding head 3, formed by the pin 5 and the shoulder 4, can then rotate independently of the magnet device 7 and at a different rotational speed.

[0080] According to this variant, the rotation speed of the magnet device 7 is different from the rotation speed of the welding head 3.

[0081] The rotation speed of the magnet device 7 and the rotation speed of the welding head 3 can be variable and adjusted by means of variable frequency motors, for example.

[0082] It is necessary to control the speed vl so as not to overheat the material, otherwise the two parts become liquid, which must be avoided.

[0083] It is possible to use an electromagnetic magnet to control the speed vl.

[0084] The preliminary preheating step therefore begins before the welding step with a first rotational speed vl of the magnet device 7 which is non-zero. The speed of the welding head 3 is zero.

[0085] After a certain preheating time of the parts, the rotation of the welding head 3 or only that of the pin 5 is carried out at a speed vt. The rotation speed of the magnet device 7 is then reduced to a second speed v2 which is lower than the first speed vl.

[0086] The second speed v2 may be lower than the speed vt of the welding head 3 or the pin 5.

[0087] This method makes it easier to initiate welding by the welding tool 2. The amount of heat supplied by preheating by eddy currents can then be reduced due to the heat generated by the mixing of the pin 5.

[0088] Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various embodiments illustrated / mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0089] It is also evident that all the characteristics described with reference to a process are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a process.

Claims

Demands

1. A friction stir welding apparatus (1) comprising a friction stir welding tool (2) having a welding head (3) for contacting a joint zone (10) formed by two adjacent parts (11, 12), and having a shoulder (4) and a pin (5) extending axially from the shoulder (4), the welding apparatus (1) comprising a motor configured to drive the pin (5) in rotation about an axis of rotation (A) and a magnet device (7) mounted to rotate about the axis of rotation (A), the magnet device (7) comprising at least two different magnetic poles (S, N) positioned about the axis of rotation (A), and being configured such that the rotation of the magnet device (7) relative to the parts (11, 12) generates an eddy current in the parts (11, 12) to preheat the zone junction (10).

2. Welding apparatus (1) according to claim 1, wherein the magnet device (7) comprises several magnets (8) arranged around the axis of rotation (A) so as to form an alternation of south magnetic poles (S) and north magnetic poles (N) around the axis of rotation (A).

3. Welding apparatus (1) according to any one of claims 1 or 2, wherein the magnet device (7) is positioned opposite the welding head (3).

4. Welding apparatus (1) according to any one of claims 1 to 3, wherein the welding tool (2) comprises a shaft (13) intended to be driven in rotation by the motor about the axis of rotation (A), the magnet device (7), the shoulder (4) and the pin (5) being supported by the shaft (13) so as to be driven in rotation together and at the same speed.

5. Welding apparatus (1) according to any one of claims 1 to 3, wherein the shoulder (4) is fixedly mounted in the welding tool (2), the magnet device (7) and the pin (5) being rotationally mounted and arranged so as to be driven in rotation by the motor and relative to the shoulder (4), at the same speed.

6. Welding apparatus (1) according to any one of claims 1 to 3, wherein the magnet device (7), the shoulder (4) and the pin (5) are arranged such that the magnet device (7) has a rotation speed different from the rotation speed of the pin (5) and the shoulder (4).

7. Welding apparatus (1) according to any one of claims 1 to 3, wherein the shoulder (4) is fixedly mounted in the welding tool (2), the magnet device (7), the shoulder (4) and the pin (5) being arranged so that the magnet device (7) has a different rotational speed than the rotational speed of the pin (5).

8. Welding apparatus (1) according to any one of claims 1 to 3, comprising an arm supporting the welding tool (2), the magnet device (7) being rotatably mounted on the arm about the axis of rotation (A), the welding apparatus (1) comprising a first motor for driving the pin (5) in rotation at a first speed and a second motor for driving the magnet device (7) in rotation at a second speed.

9. A method for welding a part by friction stir welding using a welding apparatus (1) as defined according to any one of claims 1 to 8, comprising: • a preheating step of a joint zone (10), formed by two adjacent parts (11, 12), by rotating the magnet device (7) around the axis of rotation (A) and with respect to the joint zone (10), and • a welding step of the two parts (11, 12) by friction stir welding the joint zone (10) using the welding head (3).

10. Welding method according to claim 9, comprising a preliminary preheating step in which the preheating of the junction zone (10) is carried out before the welding head (3) is brought into contact with the junction zone (10), the magnet device (7) rotating at a first rotational speed vl, the preheating of the junction zone (10) after the welding head (3) is brought into contact with the junction zone (10) being carried out with the magnet device (7) rotating at a second rotational speed v2 which is lower than the first rotational speed vl.

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