MODIFIED FRICTION STIR WELDING METHOD AND APPARATUS

Abstract: A modified friction stir welding method is used on axially rotating pin (1) moving along a predetermined circular path. The novel apparatus comprises at least one axially rotating pin (1) located eccentric in relation to the tool holder's vertical axis.

Description: "Modified friction stir welding method and apparatus" The present invention relates to a new improved method of friction stir welding of members, more particularly to a spot welding method, and furthermore to apparatuses to conduct such method. Friction stir welding is a new friction welding process representing a disruptive welding technology. The principles of the process and applied apparatus disclosed by WO93/10935 are based on a relative cyclic movement between a non- consumable probe of a harder material than the workpieces to be joined and the workpieces. Urging the rotating probe into the assembled adjacent workpieces along their joining line creates a plasticized region in the workpieces due to a generated frictional heat. Thus no heat is generated as in conventional friction stir welding due to a relative motion between the workpieces to be joined. This new welding method, having the advantage of solid state bonding, has been successfully implemented on providing plate and profile seam joints not previously feasible and gains increasing popularity for numerous joining applications. However, provision of so-called keyholes resulting from retraction of the probe by termination of the welding requiring normally removal of such inadequately filled part of the seam weld represents certain limitations to even broader applications. Thus, e.g. in spot welding processes being presently extensively applied in robotazing of assembling operations in vehicle building (welding of steel sheets to the supporting structure of the vehicle body), the use of aluminium as sheet material has so far been eliminated due to the low strength joints resulting from classical Al-welding methods (TIG, MIG). Drawbacks linked to the presently known/applied friction stir welding apparatuses - large down load forces on the welded members and presence of keyholes - have so far eliminated possible application of friction stir welding technology in this field. It is therefore an object of the present invention to provide a novel improved method of friction stir welding providing a weld avoiding the above difficulties connected to the existence of the keyholes. Another object of the present invention is to provide improved modifications of friction stir welding apparatuses ensuring a substantial keyhole filling during/after the probe's retraction from the joined members. Still another object of the invention is to provide welds having improved mechanical properties particularly for joining of thin members. These and other objects are met by provision of a novel modified friction stir welding method and novel apparatuses as defined in the attached patent claims 1 and 5, respectively. Other objects, specific features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments of the apparatuses and the mode of operation according to the invention with reference to the accompanying drawings, Figs. 1 -2, where Fig. 1 illustrates schematically in a perspective view the novel principle of the improved friction stir welding process by examples of applicable modification of the conventional apparatus, Fig. 2 shows schematically in a perspective view another modification of the friction stir welding apparatus applying two pins. The friction stir welding as described in WO93/10935 is incorporated herein by reference to the extent consistent with the present invention. Referring to Figure 1 , the rotating pin 1 is contrary to the presently known and applied friction stir welding apparatuses located eccentric with regard to the longitudinal axis, in this case the vertical axis (z-axis) of the attached tool holder 3 providing connection/transmission to the source of the rotation/cyclic movement (not shown in the Figure). The tool holder is adjustable both in the axial and tangential direction with regard to the pre-assembled, joined to be members A,B, while the pin 1 can independently rotate around its axis inside the tool holder with or without being provided with a fixed or movable shoulder 2. In the case where a traditional shoulder 2 attached to the pin 1 is applied it is as well for additional heat input to facilitate plasticized material flow and to prevent material from leaving the weld joint and thus to improve the weld surface's quality. This novel eccentric positioning of the pin 1 in the tool holder 3 ensures a circular movement (path) of the rotating pin around the tool holder's z-axis during a gradual engagement with the to be welded members' material. After reaching the full depth the pin will complete one revolution along the circular path 5 shown in the Figure having a diameter D approximately twice the diameter d of the applied pin. The rotating pin will then be gradually retracted from the now welded plasticized material flowing from the front of the pin to the rear (part) so that up to 75% filling of the actual spot weld cavity will then be accomplished, thus substantially eliminating the keyhole problem. Another embodiment of the apparatus applicable in the novel welding process according to the invention is schematically shown in Fig. 2. In this case two pins 1 located eccentrically with regard to the z-axis of the tool holder 3 are applied. Similarly to the above described welding process both pins 1 are penetrating the preassembled members A,B along a circular path 5 having a diameter D approximately four times the diameter d of the applied pins. As a result of this tool configuration the remaining keyhole volume is reduced to 1/8 of the achieved weld connection volume, or in other words up to 87.5% filling of the resulting spot weld cavity having diameter D. The obvious advantage of this novel welding process is that larger spot welds are provided simultaneously with reduced applied down load forces on the welded members resulting in improved mechanical properties (strength) of the welds, thus facilitating and simplifying also the fixation/support arrangement requested for the joined-to-be members. This applies especially for the apparatus based on two (or more) individual pins having small diameter and consequently requiring substantially less forces. Even if the obviously most favourable application of the new welding method is spot welding of thin members, e.g. sheet material, the method can be used to provide seam welds as well. In this case, after the engagement step both tool holder and pin keep rotating while the tool holder moves in a tangential direction. By balancing the tangential speed and rotational speed of the tool holder in relation to the distance between the pin axis and the tool holder axis and the pin diameter, full overlap can be ensured to achieve full welding along the seam. A larger width seam weld is then created with a smaller diameter pin exhibiting improved weld strength. Both flush and lap welds can be thus provided. The invention is not limited to the above described examples of preferred embodiments of the applicable apparatuses and the mode of their operations. Several adaptations and modifications in the apparatus' configuration and welding process parameters can be made without departing from the scope of the invention. E.g. the pin rotating independently of the tool holder can rotate in the opposite direction and with a different speed compared to the tool holder. More than two pins could also be applied. Furthermore, use of an extendible pin and shoulder combined with controlled down force (load) on the shoulder will be beneficial for the welding process, but optional.

Claims: Claims A modified friction stir welding method comprising steps of engaging at least one axially rotating pin with the pre-assembled welded to be members, welding of the members by the rotating pin, said pin moving along a predetermined circular path, and finally disengagement of the axially rotating pin by gradually retracting the pin kept in circular movement along the path. Method according to claim 1 , characterized in that the predetermined circular path has a diameter being larger than the diameter of the applied axially rotating pin. Method according to claim 1 or 2, being applied as welding method for spot joining of members. Method according to claim 1 or 2, being applied for seam welding of members. Friction stir welding apparatus applicable in the welding method according to claim 1, characterized in that the apparatus comprises at least one pin (1) and rotating tool holder (3), the pin (1) being located eccentric in relation to the tool holder's vertical axis (z). 6. Welding apparatus according to claim 5, characterized in that the pin (1) rotates independently of the tool holder's rotation both with regard to the rotation direction and speed. 7. Welding apparatus according to claim 5, characterized in that the pin (1) is further provided with an attached shoulder (2).

Inventors: BERSAAS JAN CHARLES, OOSTERKAMP ANTONIE, OOSTERKAMP LJILJANA DJAPIC, BERSAAS, JAN, CHARLES, OOSTERKAMP, ANTONIE, OOSTERKAMP, LJILJANA, DJAPIC
Applicants: BERSAAS JAN CHARLES, NORSK HYDRO AS, OOSTERKAMP ANTONIE, OOSTERKAMP LJILJANA DJAPIC, BERSAAS, JAN, CHARLES, NORSK HYDRO ASA, OOSTERKAMP, ANTONIE, OOSTERKAMP, LJILJANA, DJAPIC
Application year: 2000
Main classification: B23K20
IPC codes: B23K20