Metal fusion bonding – Process – Using dynamic frictional energy
Reexamination Certificate
1997-12-08
2001-12-04
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Using dynamic frictional energy
C228S002100
Reexamination Certificate
active
06325273
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a mobile friction welding machine. It is particularly applicable, but in no way restricted, to a mobile friction stir welding machine which is adapted for the joining of lead sheets to form a lap joint.
BACKGROUND TO THE INVENTION
The attractive properties of lead have been put to good use in the fabrication of all types of buildings from private houses to industrial complexes. Traditionally, when it is necessary to join lead the tried and trusted technique of lead burning is utilised. This joining method cannot be considered ideal because of the skill needed and the environmental and health problems that are associated with the fumes emitted. Furthermore, lead burning is a highly skilled job and there is a general shortage of skilled operators. As a consequence, the initial cost outlay for laying lead sheet as a continuous membrane is unattractive or uneconomic when compared with felt/bitumen alternatives. The initial installation cost outlay should not be the only factor used when comparing the two roofing systems. Felt roofs generally have a life expectancy of only 10-15 years whereas lead roofs last considerably longer. Over an extended period of say 20-100 years a lead roof is likely to be considerably more cost effective. However, the initial high installation costs still tend to discourage architects and builders from specifying lead as a flat roofing continuous membrane material of choice, despite its longevity and attractive physical appearance.
If a simple and cost-effective method of joining lead sheet were available, then this could change the economic equation in favour of lead sheet as a flat roof membrane material.
Possible alternatives to lead burning are friction welding and friction stir welding. The process of friction welding has been known for many years and typically involves causing relative movement between a pair of workpieces while they are urged together so as to generate a plasticised region, stopping the relative movement and allowing the plasticised region to solidify thereby joining the work pieces.
It has also been proposed in the past to join workpieces by use of a non-consumable member which does not form part of the finished joints. An example of this so-called friction stir butt welding technique can be seen in WO 93/10935 (The Welding Institute) and WO 95/26254 (NORSK HYDRO) in which two workpieces are urged together using various shaped tools which cause the region to be joined to plasticise and hence become joined. This technique allows sheet materials to be welded by a solid state process in either butt or lap geometries. The process operates by forcing a rotating tool with a specially shaped probe along the joint line, which causes intense plastic deformation of the immediate surrounding material. The tool is designed to prevent material escaping, and once the tool has passed a given point, a weld is made.
The leading edge of the rotating tool provides frictional heat and subsequent thermal softening in front of the tip, in effect preheating the area about to be bonded. This effect is especially useful in allowing the passage of the tip part of the rotating tool through the material. The greater the area of the shouldered region of the rotating tool in contact with the joint, then the greater the frictional heat available. However, increasing the diameter of the shoulder has practical limitations and tends to produce a side flash.
To date, this process has been successfully demonstrated for many aluminium alloys and also for thermoplastics. However, application of this process to lead sheet results in major difficulties because lead is relatively soft and malleable and the above techniques do not achieve satisfactory results in terms of weld acceptability.
Even if these problems were to be overcome, there is still no known portable machine which could be used on a lead roof to complete the necessary welds. By the very nature of a roof, the welding process must be carried out in situ. The precision required for this type of joint means that a hand-held tool is quite inappropriate for forming a weld of any length. The object of the present invention is therefore to provide a method and apparatus which can be used to form welded joints of the type in question in situ.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a mobile friction welding machine comprising:
(i) a wheeled body adapted to support and carry a rotatable probe;
(ii) a rotatable probe suitable for creating a friction weld joint in workpieces to be joined;
(iii) a means of rotating the probe at speeds suitable for forming a joint region in the workpieces;
(iv) a track assembly adapted to guide the friction welding machine across the workpieces;
(v) a drive means attached to or associated with the body and adapted to drive the welding machine forward in use along the track assembly;
(vi) a loading system adapted to place a downward load on the probe and to urge it into the workpieces;
the machine being adapted, in use, to move along the track assembly and to form a joint region in the workpieces as it moves.
For the first time there is provided a simple, portable machine that can be set to work on a roof comprising overlapping sheets of lead and which forms a joint as it traverses the roof on its tracks.
In a particularly preferred embodiment the welded body is fitted with endless articulated tracks.
In another embodiment the track assembly comprises a series of track sections joined together to form a track of the required length.
In this way a temporary track is laid out across the roof joint which requires welding, parallel to the desired joint line. The welding machine then moves along the joint region parallel to and guided by the track.
Preferably the drive means comprises an electric motor.
In a particularly preferred embodiment the drive means is pivotally mounted thus allowing the drive to flex with respect to changes in position of the track means, so accommodating any unevenness in the workpieces to be welded.
It is thus no longer necessary for the surface to be welded to be uniformly flat.
Preferably the loading system comprises a plurality of weights, the system thus being adapted to maintain the probe in the correct depth and orientation with respect to the workpieces.
Preferably the probe incorporates a probe head comprising a plurality of protrusions and in a particularly preferred embodiment the probe head comprises two crescent-shaped protrusions symmetrically displaced around the rotational axis of the probe.
Preferably the probe head is displaced from the plane perpendicular to the workpieces, the inclination being away from the general forward direction of travel of the machine.
Preferably the probe is displaced by 3°±2°.
The present invention also encompasses mobile welding machines using any probe suitable for use in a method of friction welding and is intended for use on any suitable workpiece material.
According to a second aspect of the present invention there is provided a method of friction welding suitable for joining workpieces and forming a joint region there between comprising the steps of:
(i) Offering a probe of material harder than the workpiece material to the workpiece surfaces in the joint region, a head region of the probe contacting one or both workpieces;
(ii) Causing a rotational movement between the probe and the workpieces while urging the probe and workpieces together whereby frictional heat is generated in the joint region so as to create a plasticised region in the workpieces around the probe;
(iii) Removing the probe or advancing the probe along the joint region and allowing the plasticised region to solidify and thereby join the workpieces together;
characterised in that the head region of the probe incorporates a plurality of protrusions adapted to penetrate the workpieces.
Moving from a single point probe to a multiprong whisk type tool causes plasticising material movement without causing melting of low-melting mate
Boon Terence
Temple-Smith Peter
Thomas Wayne M.
Dunn Tom
Knobbe Martens Olson & Bear LLP
Stoner Kiley
The Lead Sheet Association
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