Metal fusion bonding – Process – Using dynamic frictional energy
Reexamination Certificate
2002-05-16
2004-04-20
Stoner, Kiley (Department: 1725)
Metal fusion bonding
Process
Using dynamic frictional energy
C228S002100
Reexamination Certificate
active
06722556
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for joining at least two adjoining work pieces by friction welding wherein the work pieces include an adjoining area, in which they are plasticized by a rotating pin-like projecting tool which is in contact with the adjacent surface areas of the work pieces and to an apparatus for performing such a method.
A similar method and apparatus are known from EP-B-O 615 480. The friction welding, which is also called friction stir welding (FSW), has been basically known for several years. Originally, the friction stir welding was performed in such a way that two work pieces, which were to be joined by friction welding, were moved relative to each other in the area in which they were to be joined while they were pressed against each other with a predetermined adjustable force. As a result of the heat generated by the friction, the material of the work pieces was plasticized in the area of the jointure. Upon sufficient plastification, the materials of the two workpieces intermix sufficiently so that, upon cooling, the desired weld connection between the two workpieces is formed.
In the method and the apparatus mentioned above, a pin-like projection, which is rotated by a drive or motor at a high speed, is disposed in the adjoining area between two workpieces to be joined and the pin-like projection is guided along the adjoining area for example by a special guide arrangement or by a robot. Additionally, it may be subjected to a translatory movement along the adjoining edges of the workpieces. When, after the beginning of the welding procedure, the material of the workpieces is sufficiently plasticized by the frictional heat generated by the rotation of the pin-like projection, the adjoining workpieces while the rotation of the pin-like projection, the pin-like projection is moved along the seam between the adjoining workpieces while the rotation of the pin-like projection is maintained whereby an elongated seam is formed.
It is known that two or more metallic workpieces can be joined by so-called spot welding connections which, with regard to rivet or screw connections, have the advantage that no joining elements (rivets, screws, etc.) are needed and that the joints can be established very rapidly since no holes have to be drilled through the workpieces for receiving the rivets or screws and that further no rivets or screws have to be inserted through the holes and to be compressed or tightened.
While the spot welding of two workpieces of iron or steel or their alloys to be joined can be performed with commercially available spot welding apparatus, the spot welding of two or more adjacent workpieces of aluminum or aluminum alloys as they are used in airplane or motor vehicle assembly procedures is still problematic. The reason is that aluminum and aluminum alloys form with the oxygen of the ambient air on their surfaces an oxide layer which provides for a high electrical resistance between the workpieces to be joined so that, furthermore, the molten material lens formed between the workpieces includes aluminum oxide which detrimentally affects the strength of the joints. As a result, the spot-welding joints of aluminum workpieces do not have the strength required for many applications.
Particularly in the construction of airplanes where highest qualities of the jointure between workpieces are of utmost importance, the use of spot-welding instead of rivet connections has been abandoned. The same is true in the construction of motor vehicles where, increasingly, aluminum is used for chassis and body components.
It is therefore the object of the present invention to provide a method and apparatus for providing high-strength connections between metallic and also non-metallic workpieces, which do not require additional connecting elements and which are reproducible with high precision and in a simple manner. With the method and apparatus according to the invention oxide-free connections should be established and flat continuous surface areas should be established during the welding at both sides of the jointure area. At the same time, the method and apparatus should be simple and the method should be executable by manufacturing robots.
SUMMARY OF THE INVENTION
In a method and apparatus for joining at least two adjoining workpieces by friction welding, a rotatable shaft is provided at its front end with a friction welding tool which is biased toward the workpieces to plasticize the engaged area of the workpiece into which the tool enters while the plasticized material is displaced and stored in, or around, the tool. Upon passing through the top workpiece and entering the lowermost work piece, the tool is removed and the stored plasticized material is returned to the void provided by the tool upon removal thereof from the workpieces for joining the workpieces.
The plasticized material is retained and, upon removal of the pin-like projection, is pressed back into the space voided by the removal of the tool.
The advantage of the method according to the invention resides mainly in the fact that, in contrast to the various known methods for joining two or more workpieces, it is not necessary to subject the areas to be joined to specific pre-treatments. It is not necessary to drill holes into the workpieces, no additional connecting elements are needed, no additional operating step such as the formation of rivet heads or the screwing of screws or bolts or the mounting of nuts, is needed. There is no head—contrary to the bolt friction welding, and no beads of plasticized material or holes or depressions are formed. Furthermore, with the use of workpieces of aluminum or aluminum alloys or other lightweight materials or alloys, only minor amounts of oxides will be included in the joint since the jointure area of all the workpieces being joined are plasticized and the materials are intermixed. No surface area oxide concentrations will therefore be present anywhere in the material joints. Since, with the method according to the invention all the plasticized material is pressed back into the joint area, the surface on the active side, that is, the side of the workpieces from which the pin-like projection extends into the workpieces, becomes again planar while the opposite side remains planar in any case since the pin-like projection does not extend through this side.
In order to prevent that, as a result of the insertion of the rotating pin-like projection into the workpieces that is into the lower workpiece and the displaced material flows to the surrounding area in an uncontrolled manner so that it is not available for forming the jointure, the displaced material is retained in the immediate area around the rotating pin-like projection.
Preferably, the displaced material is returned, upon removal of the pin-like projection, into the void formed thereby at the same rate at which the pin-like projection is retracted from the workpiece, whereby it is made sure that all of the plasticized material is returned and the surface of the workpiece from which the pin-like projection extends into the workpiece is again completely planar after completion of the jointure.
In this way, in contrast to the bolt friction welding, a large force transmitting area is generated which is capable of accommodating also high shear loads.
In order to provide for a rapid procedure, it is advantageous if the workpieces are preheated, in the areas in which the jointure is to be established, from the side from which the procedure is performed and also from the opposite side. This has the advantage that not the whole heat input has to be provided by friction heat. With a controlled preheating only a differential amount of heat in excess of the external heat input has to be provided by friction for the plastification of the material by the friction welding procedure.
In this way, the process according to the invention can be substantially accelerated.
With the apparatus for the jointure of at least two adjacent workpieces in accordance with the method of the invention, the
dos Santos Jorge
Schilling Christoph
Bach Klaus J.
GKSS - Forschungszentrum Geesthacht GmbH
Stoner Kiley
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