US3740827A - Friction welding - Google Patents

Abstract

The disclosure provides an improved method of carrying out friction welding operations in which the workpieces are heated by friction produced by rotating one in rubbing contact with the other. The rotation of the one workpiece is effected by a drive from a power source until an equilibrium temperature condition has been reached, after which the power source is disconnected and continued rotation applied to the workpiece by energy stored in a rotating mass, the energy in said mass being dissipated in completing the weld.

Description

United States Patent [191 Hunter et al.

[ FRICTION WELDING [75] Inventors: Anthony John Hunter; Robert Graham Forbes, both of Inverness, Scotland [73] Assignee: A. I. Welders Limited, Inverness,

Scotland [22] Filed Nov. 6, 1970 [21] Appl. No.: 87,373

[30] Foreign Application Priority Data Nov. 14, 1969 Great Britain 55,895/69 [52] U.S. Cl. 29/4703, 228/2 [51] Int. Cl B231: 27/00 [58] Field of Search 228/2; 29/4703; 156/73 [5 6] References Cited UNITED STATES PATENTS 3,235,162 2/1966 Hollander 29/4703 June 26, 1973 3,417,457 12/1968 Burke et al. 29/470.3 3,455,494 7/ 1969 Stamm 29/4703 X 3,595,462 7/1971 Hirayama 29/4703 X Primary Examiner-J. Spencer Overholser Assistant Examiner-Robert J. Craig Attorney-Stevens, Davis, Miller & Mosher ABSTRACT The disclosure provides an improved method of carrying out friction welding operations in which the workpieces are heated by friction produced by rotating one in rubbing contact with the other. The rotation of the one workpiece is effected by a drive from a power source until an equilibrium temperature condition has been reached, after which the power source is disconnected and continued rotation applied to the workpiece by energy stored in a rotating mass, the energy in said mass being dissipated in completing the weld.

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LIN/TS C YCLE T/ME 1 FRICTION WELDING This invention relates to the process for bonding workpieces together which is known as friction weld- In carrying out the friction welding process, it has been proposed to employ power means directly to rotate one workpiece in rubbing contact with the other to induce the required frictional heating, the drive means beingdisconnected when the desired plastic state of the material has been attained. The relative rotation is then i either rapidly stoppedor allowed to come to rest under the natural resistance created as the interface cools .due to discontinuance of input energy.

This conventional, .or continuous drive, friction welding has been described in great detail in Czechoslovakian and Russian technical literature dating back to 1957. It has also been described,but in less detail, in German patent specifications published in 1925/26 and in a British patent specification published in 1941.

it has also been proposed to employ a rotating inertia mass which is accelerated to a predetermined speed by power means and to utilize the energy stored in the inertia mass to rotate one of the workpieces and produce the required heating thereof, the driving of the inertia mass being discontinued before the rubbing contact of the workpiece is initiated, so that the whole of the energy for the bonding process isprovided by the known energy stored in therotating mass.

It has further been proposed to obtain specific characteristics required by the materials being bonded by controlling the rotational speed on a basis of time throughout the entire weld cycle.

Each of the above-mentioned proposals has disadvantages. The first proposal requires a relatively large driving motor and calls for a relatively long weld cycle time, as well as having an energy requirement which is not precisely repeatable. In the second proposal, a shorter weld cycle time can be obtained,but the energy requirement varies greatly with the initial condition of the parts, and the final length of a component produced by welding two workpieces together varies with the initial length tolerances of the workpieces. Moreover, with this second proposal, the short cycle time may result in a heat affected zonewhich requires subsequent heat treatment, and in order to ensure a weld extending over the whole area of the mating surfaces a high inertial energy may be required. In the third proposal initial part condition and accuracy can affect repeatability of quality and final length tolerances are again dependent on the initial tolerances.

It is the object of the present invention to provide a friction welding method which avoids or greatly reduces the disadvantages of each of the previous proposals whilst providing, at least in part, the advantages of each. It is a furtherobject of the invention to provide apparatus for carrying out the said method.

According to one aspect of the invention there is provided a method of bonding workpieces across a comvmon interface in which heating of the workpieces to a plastic condition at the said interface is effected by friction produced by rubbing engagement of the workpieces at the interface, said method comprising mount ing one of the workpieces on a rotatable member capable of being rotated by power means, and of applying to the workpiece energy stored in a rotatable inertial mass mounting the other workpiece so that it is held against rotation, contacting the workpieces under pressure for an initial period during which rotation is imparted to the workpiece mounted on the rotatable member and energy is supplied to the said workpiece from the said power means, disconnecting the power means and continuing said contact under pressure for a subsequent period during which the power means remains disconnected and energy stored in the inertial mass is absorbed at the interface.

Preferably the inertial mass is driven by the said power means.

Preferably the power means are disconnected when a critical condition (as herein defined) of the workpieces is attained.

According to another aspect of the invention there is provided a machine for bonding workpieces across a common interface in which heating of the workpieces to a plastic condition at the said interface is effected by friction produced by rubbing engagement of the workpieces at the interface to develop weld heat by friction and plastic working, said machine comprising a nonrotatable workholder for one workpiece, a rotatable workholder for another workpiece, means for moving one of the workholders longitudinally to exert pressure between the workpieces, power means for rotating said rotatable workholder, an inertial mass mounted to rotate with said rotatable support, rotation transmitting means for transmitting drive from the power means to said rotatable support and to said inertial mass, and drive interrupting means to allow said rotatable workholder to be rotated by energy stored in said inertial mass unassisted by said power means, means to produce a signal during relative rotation, in rubbing contact, of the workpieces, and control means for said drive interrupting means operable at the production of said signal to disengage the said drive.

The rotation transmitting means may incorporate a hydrostatic system comprising a liquid pump and motor and control means for said hydrostatic system to provide a predetermined relative rotational speed between the workpieces at the commencement and/or during the part of the welding cycle during which the said one workpiece is driven by the power means and to provide storage of a predetermined amount of energy in the inertial mass.

It will be understood that the method and apparatus according to the invention are primarily concerned with the bonding together of metal workpieces, but their use for the bonding together of workpieces of other materials capable of being brought to a plastic condition by frictionally-produced heating is not ex? cluded.

The method according to the invention enables the contacting surfaces of the workpieces to be brought to a predetermined condition during the initial period of relative rotation so that the energy required to complete the bond, which energy is stored in the inertial mass, is independent of the initial condition of the workpieces.

Since, in the method according to the present invention, the heating phase is carried out before the stored energy begins to be utilized, and the said heating phase is arranged to establish, by choice of rotational speed and axial thrust, an equilibrium interface temperature at which the heat input is balanced by the heat dissipation, the period of the said heating phase can vary with parts in a state of equilibrium without affecting the period of the bonding phase and, provided that the change-over from one phase to another occurs within this state of equilibrium and is actuated by means, such as a limit switch, dependent for its operation on the relative positions of the workholders the final length of a component produced by the welding method is independent of variations, due to tolerances in the initial length or the prepared condition of the contacting faces, any excess length of the workpiece being burnt off before the bonding phase commences and this bonding phase, utilizing an exactly repeatable ammount of stored energy, will always commence when the parts are in'a repeatable optimum state.

If final length of'the component to be produced is not important the critical condition may be the condition when the equilibrium interface temperature is attained but if final length is important, the critical condition includes two factors, namely the attainment of the said equilibrium temperature and the reduction of length of the workpieces, by burning off or extrusion of material, to a predetermined length. I

Since the interface condition of the workpieces when the critical condition is attained is exactly repeatable, this state may be used as a weld quality monitor, the sensing device being set to indicate not only that the equilibrium state has been attained, but also to indicate the actual velocity of advancement of the axially movable workpiece whilst the equilibrium state exists. If the said velocity lies outside a predetermined range of values, the sensing device is arranged to actuate signal producing means which would indicate that the quality of the weld was suspect. Such signal producing means are in themselves known and will not be described in detail herein.

The invention will be hereinafter described with reference to the accompanying drawings in which;

FIG. 1 is a side elevation of one form of friction welding machine embodying the invention;

FIG. 2 is a side elevation of the machine shown in FIG. 1 with parts broken away;

FIG. 3 is a side elevation, similar to FIG. 1, of another form of friction welding machine embodying the invention;

FIG. 4 is a side elevation of the machine shown in FIG. 3, with parts'broken away;

FIG. 5 is a side elevation, similar to FIG. 1, of another form of friction welding machine embodying the invention;

FIG. 6 is a side elevation of the machine shown in FIG. 5, with parts broken away; and

FIGS. 7 to 12 inclusive are graphs showing typical curves of rotational shaft speed and axial velocity of workpiece movement plotted against time in friction welding cycles used when carrying out the method according to the invention. The curves of rotational shaft speed are incompletely shown, representating only the axial velocity up to the point at which rotation ceases.

In the friction welding machines shown in FIGS. 1 and 2 and in FIGS. 3 and 4, the rotating workpiece rotates about a horizontal axis, and inthe friction welding machine shown in FIGS. 5 and 6 the rotating workpiece rotates about a vertical axis.

In the machine shown in FIGS. 1 and 2 the machine comprises a hollow base 12 on which is mounted a housing 13 enclosing a slidable headstock 14v (FIG. 2)

carrying a rotatable workholder 15, the housing also enclosing other components hereinafter described. Also mounted on the base 12 is a tailstock 16 carrying a fixed workholder 17, the workholders 15 and 17 being positioned to support two workpieces such as are shown at 18 and 19 in FIG. 2, in co-axial relation one to the other. r Referring to FIG. 2, the headstock 14 is slidably guided on the base 12 by co-operating guide means 21, 22 .for movement towards and away from the tailstock 16, fluid pressure cylinders of which one is shown at 23, being provided to move the headstock in the direction in which it is guided. The cylinders 23 are anchored at 24 to the frame 12 and have slidable in them pistons connected by rods 25to the headstock 14. The workholder 15 is mounted on a shaft 26 rotatably mounted in the headstock 14 and arranged to support, at its end remote from the workholder 15, an inertial mass 27 which at all times rotates with the shaft 26. As shown, the inertial mass consists of a plurality of discs 28 clamped to a flange 2 fixed to the shaft 26, and a fixed shaft 31, co-axial 'with the shaft 26, is mounted in the housing 13, discs 28 being shiftable by sliding from one shaft to the other so that any selected number of discs can be mounted on and clamped to the shaft 26 depending onthe value of the inertial mass required, any unwanted discs being supported on thefixed shaft 31.

A hydrostatic drive is provided for the shaft 26, the said drive comprising a liquid pump 32-mounted in the machine base 12 and driven by an electric motor 33, and a liquid pressure motor 34 mounted on the headstock 14 and connected to the pump 32 by flexible conduits 35, the output shaft of the motor 34 being coupled to the shaft 26 by drive means 36 such as a toothed belt and pulleys, a chain-and-sprocket gear or spur gearing. The hydraulic motor may be of either the radial piston type or of the swashplate type with pistons parallel to its axis.

The pump/ motor system 32, 34 is controllable in known manner so that the speed of rotation of the motor output shaft can be varied over a wide range and the motor 34 can rotate idly without being driven by liquid from the pump, thus allowing the inertial mass 27 to be rotated by kinetic energy stored therein and to inpart such energy to the workpieces.

Electrical switching means of any suitable type for starting the electric motor and for initiating a welding cycle when the electric motor is already running, are operated by suitable controls on a panel 37, FIG. 1, which also carries controls for presetting for automatic operation during a welding cycle conventional speedcontrolling means for the liquid pressure motor 34 and conventional means for determining the thrust exerted on the headstock 14 by the fluid pressure cylinder 23. The electric motor is allowed to run continuously during any period for which the machine is in continuous use to carry out a plurality of welding cycles. Automatic means of any known type are provided for setting the hydrostatic drive to the condition in which the motor rotates freely. The said automatic means are controlled to determine the point in a welding cycle at which the drive to the shaft 26 and inertia mass is discontinued by electrically controlled means controlled as will now be described with reference to FIG. 2.

The said point, hereinafter referred to as the critical point is determined essentially by one criterion, namely the equilibrium condition at which the heat input to the workpieces is equal to the heat dissipated,

and preferably also by a second criterion, namely the reduction of the length of the workpieces due to burnoff and extrusion, to a predetermined length, determination by the first criterion alone ensuring uniform and repeatable weld quality and determination by both criteria together ensuring also constant length of the final product regardless of length variations in the initial workpieces.

An indication of the first-mentioned condition, namely the equality of heat dissipation with heat input, is given by the fact that when that condition is reached, the headstock 14 advances at a constant speed and there is therefore mounted to move with the tailstock 16 a sensing device 38 consisting of a speed measuring device arranged to provide a signal under constant speed conditions or an accelerometer arranged to provide a signal under conditions of no acceleration. The said signal is provided by the closing of an electric switch. The sensing device 38 is provided with an operating member 39 co-operating with an adjustable stop 41 carried by the headstock 14 the stop 41 coming into engagement with the operating member 39 substantially at the same time as the workpieces l8 and 19 make initial contact, and the subsequent movement of the operating member controlling the sensing device. The stop 41 is preferably provided with a micrometer adjustment.

When the constant speed condition is reached, the torque exerted to rotate the rotatable workpiece is also substantially constant, so a torque meter may be provided as an alternative means of indicating this condition.

An indication of the second-mentioned condition, namely the burn-off or extrusion of material to leave the workpieces at a predetermined length is given by the arrival of the headstock 14 at a predetermined distance from the tailstock, and there is therefore mounted on the headstock or tailstock a limit switch 42, shown in FIG. 2 as being mounted on the headstock 14, cooperating with an adjustable stop 43 on the tailstock, the stop 43 having a micrometer adjustment and being adapted to contact and operate the limit switch when the headstock reaches a predetermined position relative to the tailstock.

The two switches are arranged in series in an electrical circuit the closing of which operates the automatic means for setting the hydrostatic drive to the condition in which the motor 34 rotates freely.

The operation of the machine described with reference to FIGS. 1 and 2 to effect a welding operation is as follows.

The workpieces l8 and 19 to be welded together are mounted in the work-holding devices 15 and 17 respectively with their ends remote from the ends to be welded together in fixed longitudinal positions. Assuming that the electric motor 33 is running and driving the pump 32, operation of a manual control on the panel 37 actuates a valve to initiate the transfer of liquid under pressure from the pump 32 to the motor 34, driving the motor 34 to rotate the shaft 26, the workholder l5 and the inertial mass 27. The workpiece 18 is thus accelerated up to the desired speed, determined by the setting of the hydrostatic drive, and the flywheel is accelerated to the same speed at the same time by the same drive. Liquid under pressure is also admitted to the cylinder 23 to advance the headstock l4 and thus bring the workpieces into contact one with the other and bring the stop 41 into contact with the operating member 39 of the sensing device 38. The pressure acting in the cylinder 23 is preset to provide any desired pressure at the interface between the workpieces.

The peak energy requirement in a friction welding operation that must be met by input power occurs when the parts come into contact in the cold state, i.e., during initial heating, and, since the inertial mass is directly connected with both the drive motor and the rotating workpiece, the kinetic energy attained by the said inertial mass is available, as in the normal application of flywheels, for damping and stabilizing the drive. When the workpieces have been heated, by the friction between them, to the plastic state, and have reached the equilibrium condition, the energy absorbed, and therefore the speed of rotation and the interfacial condition, will have stabilized, and the inertial mass will be rotating freely with the workpiece 18, which is driven only by the motor 34. This equilibrium condition can be continued as long as is desired, the electric switch controlled by the sensing device 38 being closed when the condition is attained and remaining closed.

The said equilibrium condition will, in fact, continue until sufficient burn-off of the workpieces has taken place to allow the adjustable stop 43 to contact and close the limit switch 43, the completion of the electric circuit through the two switches actuating a valve to place the power supply to the motor 34 in a neutral condition in which the said motor rotates freely and is not driven by liquid from the pump 32. The rotating parts of the said motor, with the inertial mass 27, the shaft 26, the workholder l5 and the workpiece 18 thus continue to rotate due to the kinetic energy stored in the said parts until the said stored kinetic energy is absorbed.

Since the condition of the workpieces and the amount of stored kinetic energy at the instant at which the drive to the motor 34 is stopped are precisely repeatable for any given workpieces it follows that formation of the bond may now take place at as near optimum conditions as it is possible to attain, and welds will be repeatable in both quality and accuracy.

The value of the inertial mass can as above described be changed to meet the desired conditions for any particular welding operation, including the degree of plastic working of the material after heating and before the bond is finally formed.

In the friction welding machine shown in FIGS. 3 and 4, a rigid frame 45 supports, in a housing 46 fixed to the said frame, a fixed headstock 47, and a tailstock 48 slidable on guide rods 49 extending between the housing 46 and a rigid support 51 on the frame, is movable towards the headstock by liquid pressure acting in cylinders 52 mounted on the support 51 on pistons carried by rods 53 attached to the tailstock 48. The cylinders 52 are enclosed by a housing 54 shown in FIG. 3.

The headstock 47 supports a rotatable shaft 55 on which is mounted a workholder 56, and a second workholder 57 is non-rotatably mounted on the tailstock 48.

An electric motor 58 mounted in the frame 45 is coupled by transmission means such as a belt 59 to the driving member 61 of a clutch 62 mounted on the shaft 55, the said clutch driving member 61 being free to rotate on the shaft 55 except when engaged with a clutch driven member 63 rigidly mounted on the shaft.

The driving member 61 of the clutch 62 is provided with means for mounting thereon discs 64, similar to the discs 28 described with reference to FIGS. 1 and 2, to provide an inertial mass 65 rotating with the said clutch driving member, and a fixed shaft 66, similar to the shaft 31 in FIG. 2, is provided to receive any of the discs 64 not used as part of the inertial mass 65 in any particular welding operation.

The guide bars 49, being rigidly attached to the housing 46 and to the support 51, support the reaction to the thrust exerted by the cylinders 52 in a closed loop system formed by the housing 46, support 51 and guide bars 49, none of the forces due to axial loading being transferred to the machine frame.

A sensing device 67, corresponding to the sensing device 38 described with reference to FIGS. 1 and 2, is mounted in the housing 46, the operating member 68 of the said sensing device co-operating with an adjustable stop 69 on the tailstock, also as previously described. To carry out a welding operation with the machine shown in FIGS. 3 and 4, workpieces 70 and 71 are mounted in the workholders 56 and 57 respectively. The electric motor 58 is started up with the clutch 62 disengaged and the inertial mass is brought up to a desired speed the workholder 56 remaining stationary, so that the workpieces can be loaded whilst the motor is running if desired.

Operation of a cycle initiation control on a control panel 72 (FIG. 3) actuates suitable control means to engage the clutch 62 and thereby produce rotation of the workpiece 70, and to supply liquid under pressure to the cylinders 52 to move the tailstock 48 towards the headstock 47. The workpieces 70 and 71 are therefore brought into rubbing contact one with the other, and continue to move towards each other as they are heated and rendered plastic until the equilibrium condition described with reference to FIGS. 1 and 2 is attained.

A limit switch (not shown), similar to the switch shown in FIGS. 1 and 2, or other means responding to other chosen criteria in relation to the weld, is also provided, the said limit switch or other means acting, after the equilibrium condition has been attained, to operate means disconnecting the supply of electricity to the electric motor 58, so that the workpiece 70 continues to be rotated by the kinetic energy stored in the inertial mass 65, the rotating parts of the electric motor 58 and the transmission means 59. Thus, as in the previously described machine, the workpieces are brought to an equilibrium condition whilst rotation is being imparted to the workpiece 70 by the electric motor 58 and the inertial mass 65 is rotating freely, and a known and repeatable amount of stored energy is available for dissipation during the bonding phase of the welding operation. On completion of the bond between the workpieces, when rotation has ceased due to the stored energy being absorbed, the clutch 62 is disengaged and the electric motor 58 is re-energized, thus bringing the ,inertial mass 65 back to the desired speed whilst allowing the welded workpieces to be removed and fresh workpieces to be inserted.

In the embodiment of the invention shown in FIGS.' and 6 of the drawings, the friction welding machine is of the kind in which the rotatable workpiece holder rotates about a vertical axis, and the said rotatable workpiece holder is driven by an electric motor through a clutch instead of through a hydrostatic drive, the clutch arrangement being different from that described with reference to FIGS. 3 and 4.

The machine comprises a hollow pillar 74 from one side of which extends a bare platform 75 and at the top of which is mounted a housing 76 extending laterally over the platform 75. A headstock 77 is rigidly mounted in the housing 76 to support a rotatable spindle 78 held against axial movement, the spindle 78 having its axis vertical and carrying, at its lower end, a workholder 79. Slidably mounted on guide rods 81 extending between the platform 75 and the housing 76 is a tailstock 82 carrying a second workholder 83 co-axial with the workholder 79 and fixed against rotation. Fluid pressure cylinders 84 mounted on the platform 75 have pistons acting through rods 85 on the tailstock 82 to urge the lattertowards the headstock 77.

An electric motor 86 mounted in the housing 76 drives the spindle 78 through a'disengageable clutch 87, and a belt or equivalent drive 88.

An inertial mass 89 is mounted on the spindle 78 to rotate therewith, the said mass 89 consisting, as in the machine described with reference to FIGS. 1 and 2, of a plurality of separate discs 91, and a support 92 in the form of a rod of circular cross section, curved so that its ends are at right angles one to the other and fixed at one end-to a wall of the housing 76 has its other end co-axial with the spindle 78, so that discs 91 may be shifted from the spindle 78 to the support 92 and vice versa, enabling the value of the mass 89 to be varied.

Two workpieces between which a weld is to be formed are shown at 94 and respectively, the workpiece 94 being clamped in the workholder 79 so as to be rotatable by the spindle 78, and the workpiece 95 being clamped in the workholder 83 so as to be held against rotation.

The guide rods 81 being rigidly connected to the platform 75 and housing 76, support the reaction forces due to upward thrust exerted on the tailstock 82 by pressure in the cylinders 84 thus providing a closed loop system and avoiding the transference of forces due to axial loading to the machine frame.

It will be observed that, in the arrangement shown in FIGS. 5 and 6, the inertial mass 89 rotates at all times with the spindle 78, the clutch 87 disconnecting the electric motor from both the shaft 78 and the inertial mass 89.

A sensing device 92, corresponding to the sensing devices of the previously described embodiments of the invention is mounted on the pillar 71 and has an operating member 93 co-operating with an adjustable stop 94 carried by the tailstock 82.

The machine described with reference to FIGS. 5 and 6 operates in the same manner as that described with reference to FIGS. 1 and 2, release of the clutch 87 having the same effect as the placing of the hydrostatic drive of FIGS. 1 and 2 in the neutral condition.

A limit switch (not shown) similar to that described with reference to FIGS. 1 and 2, or other means responding to other chosen criteria in relation to the weld, is also provided to determine the actual point,

after equilibrium conditions have been reached, at which the clutch 87 is disengaged.

As already described herein, the detection of the state of equilibrium of the workpieces is effected by monitoring the velocity of the moving workpiece. As the axially moving workholder carries a part, under pressure, into engagement with a stationary part, the rate of axial advancement, being directly related to the rate of burn-off of material from these parts, will reflect the condition of the interface between theparts. When the interface is in an equilibrium condition, the rate of burn-off and therefore the rate of axial advancement, will be constant. i

The cycle of a welding operation carried out according to the method of the invention may be illustrated in simplified diagramforrn by plotting rotational speed of the rotatable workpiece, and axial velocity of the axially movable workpiece, against cycle time, and FIGS. 7 to 12 inclusive are diagrams so plotted, FIG. 7 showing the basic cycle and FIGS. 8 to 12 showing variations thereof. In these Figures, the point in time at which the equilibrium state of the workpieces is reached, is indicated at PE, and the critical point at which the power drive to the rotating workpiece is cut off, is indicated at PI. The period between the pointsPE and PI, during which the equilibrium state is maintained is shown at Referring to FIG. 7, the rotational speed is held constant, or substantially constant during the period for which the power drive is maintained in operation, the speed being affected only by the initial contact pressure being applied and by inaccuracies in the preparation of the contact surfaces of the workpieces. The axial velocity also depends on the interfacial conditions during the heating up of the material at the interfaces, and tends generally to increase up to the point PE where the equilibrium state is attained. After the point PE has been passed, the axial velocity remains constant until the sensing device operates, at the point PI, and the power drive ceases. Since the rotating workpiece is now driven only by energy stored in the inertial mass, and parts rotating therewith, the rotational speed decreases to zero during a period when plastic working of the material takes place and the axial velocity increases due to such plastic working, reaching a maximum at or about the point at which relative rotation of the workpieces ceases and then dropping to zero. The axial velocity curves in FIGS. 7 to 12 are not extended beyond the point at which rotation ceases. The shape of the curves beyond this point depends on a large number of variables.

The period SE can be varied, as already described, to allow a degree of burn-off, after the equilibrium condition is reached, to bring the workpieces to a pre-chosen length, FIG. 8 showing a cycle in which the period SE is extended as compared with that in FIG. 7.

The extent of the hot plastic working may be controlled by varying the inertial mass, for example by changing the number of discs included in said mass as hereinbefore described, FIG. 9 showing the resulting curves when the inertial mass is increased as compared with that employed to provide the curve of FIG. 7.

In the welding of some metals, it is advantageous to vary the rate of heat input, and the effect of utilizing increasing or decreasing speed during the part of the cycle when the rotating workpiece is driven by the power source and is being heated up to the equilibrium condition is shown in FIGS. 10 andll, which may be compared with FIG. 7. Starting with a highrotational speed causes faster heat build-up with higher lubricity, and a narrower heat-affected zone in the workpieces, whilst starting with a slow rotational speed causes lower temperatures resulting in lower lubricity and a wider heat-affected zone for the same axial pressure.

Speed variation can be effected by incorporating an electro-mechanical pressure control device or other pressure control device for the hydrostatic drive system when such a drive system is used, and the pressure control device may be used to provide a braking effect on the shaft which carries the rotatable workholder whilst it is being driven by the stored energy, which braking effect is accurately controllable so as to provide repeatable speed variation conditions. A cycle during which such braking is employed is shown in FIG. 12, and com parison of that Figure with FIG. 7 shows that the plastic working and bonding phase is shortened by the braking effect.

In some cases, particularly when welding non-ferrous materials, the plastic working must be limited in its duration to allow consolidation of the newly formed bond. The subtractive effect of the braking may be in discrete steps or progressive, supplying a repeatable retardation to the rotational speed, and therefore the stored energy of the inertial mass.

When a drive that does not provide variable speed is used, such as a constant speed electric motor as described with reference to FIGS. 3 and 4, the results shown in FIG. 12 may be obtained by incorporation of other devices such as mechanical or electrical braking devices.

The supply of fluid pressure to the cylinders 23 (FIG. 2), 52 (FIG. 4) or 84 (FIG. 6) may be controlled to provide a constant axial thrust throughout the welding cycle or an axial thrust which is varied in steps or progressively during the cycle, the variation of thrust being effected automatically by preset control means. Such control means are well known and will not be described herein.

We claim:

l. A method of bonding workpieces across a common interface in which heating of the workpieces to a plactic condition at the said interface is effected by friction produced by rubbing engagement of the workpieces at the interface, said method comprising the steps of: mounting a first workpiece on a rotatable member capable of being rotated by disconnectable power means to apply energy from said power means to the workpiece and when the power means is disconnected capable of applying to the workpiece energy stored in a rotatable inertial mass; mounting a second workpiece so that it is held against rotation; contacting the workpieces under pressure for an initial periodduring which rotation is imparted to the workpiece mounted on the rotatable member and energy is supplied to said first workpiece from said power means; choosing the contacting pressure and speed at which said first workpiece is driven to provide a rate of heat generation which, when a predetermined temperature is reached, is equal to the rate of heat dissipation, so that an equilibrium interface temperature condition is established; disconnecting the power means after the attainment of said predetermined interface temperature condition; and continuing said contact under similar or higher pressure for a subsequent period during which the power means remains disconnected and energy stored in the inertial mass is absorbed at the interface.

2. A method of bonding workpieces according to claim 1, wherein the inertial mass is driven by said power means.

3. A method of bonding workpieces according to claim 1, wherein the power means is disconnected said power means when the measured speed of advance is substantially constant and the predetermined spacing between the workholders is attained.

6. A method of bonding workpieces according to claim 3, including the further steps of measuring the speed of advance of one workpiece towards the other and disconnecting the power means when the speed of advance is substantially constant.

7. A method of bonding workpieces according to claim 3, wherein the power means is disconnected by the operation of a timer which is initiated when axial pressure is exerted between the workpieces at the interface.

Claims (7)

1. A method of bonding workpieces across a common interface in which heating of the workpieces to a plactic condition at the said interface is effected by friction produced by rubbing engagement of the workpieces at the interface, said method comprising the steps of: mounting a first workpiece on a rotatable member capable of being rotated by disconnectable power means to apply energy from said power means to the workpiece and when the power means is disconnected capable of applying to the workpiece energy stored in a rotatable inertial mass; mounting a second workpiece so that it is held against rotation; contacting the workpieces under pressure for an initial period during which rotation is imparted to the workpiece mounted on the rotatable member and energy is supplied to said first workpiece from said power means; choosing the contacting pressure and speed at which said first workpiece is driven to provide a rate of heat generation which, when a predetermined temperature is reached, is equal to the rate of heat dissipation, so that an equilibrium interface temperature condition is established; disconnecting the power means after the attainment of said predetermined interface temperature condition; and continuing said contact under similar or higher pressure for a subsequent period during which the power means remains disconnected and energy stored in the inertial mass is absorbed at the interface.

2. A method of bonding workpieces according to claim 1, wherein the inertial mass is driven by said power means.

3. A method of bonding workpieces according to claim 1, wherein the power means is disconnected when the equilibrium interface temperature has been reached.

4. A method of bonding workpieces according to claim 1, wherein the power means is disconnected only when both the equilibrium interface temperature of the workpieces and a predetermined spacing between workholders mounting the workpieces have been attained.

5. A method of bonding workpieces according to claim 4, wherein the step of disconnecting the power means further includes measuring the speed of advance of one workpiece towards the other and the attainment of a predetermined spacing between workholders mountiNg the workpieces, and thereafter disconnecting said power means when the measured speed of advance is substantially constant and the predetermined spacing between the workholders is attained.

6. A method of bonding workpieces according to claim 3, including the further steps of measuring the speed of advance of one workpiece towards the other and disconnecting the power means when the speed of advance is substantially constant.

7. A method of bonding workpieces according to claim 3, wherein the power means is disconnected by the operation of a timer which is initiated when axial pressure is exerted between the workpieces at the interface.

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