This invention concerns improvements in or relating to linear friction bonding.

Linear friction bonding is a technique for welding together two articles at an interface between them, the articles usually, but not necessarily, being metal and the interface planar; and comprises oscillating one article against the other in a linear direction by means of an operating mechanism which simultaneously imposes a load on the articles in a direction normal to the interface (i.e. presses them together) , until sufficient heat is generated, in the first instance to soften the interface, and secondly to assist the pressure applied to achieve a solid phase weld between the articles. The articles may, for example, be a turbine or compressor blade and a rotor disk for an aircraft gas turbine engine. "Metal", in the context of this invention, includes not only elemental metals but alloys of metals, and of a metal or metals with a non-metallic substance such as a ceramic.

Among the advantages of linear friction bonding are that it does not need relatively expensive welding materials or atmospheres, or specialist welding skills, and is quick. However, successful linear friction bonding does depend on critical parameters such as: the imposed loading, the amplitude and frequency of the oscillation, and the total period of time during which the articles are subjected to the oscillation.

Hitherto, it has been the practice to preset the amplitude of the oscillation to a predetermined value while the operating mechanism is stationary, i.e. while the articles are under zero load and the oscillation

frequency is zero. In practice, it is found that this raises a problem due to inherent elasticity of the operating mechanism and its related tooling, and backlash in the mechanism. Under operating conditions the effect of inertia and the in-plane force at the interface between the articles being bonded, in combination with the inherent elasticity and backlash of the operating mechanism will cause the amplitude of oscillation to be unpredictably different from that intended (i.e. the predetermined value) , with a resulting loss of control of process parameters. This is particularly so when the process of linear friction bonding is applied to articles having a large interface area.

Hitherto, one approach to this problem has been to set the predetermined amplitude of oscillation under zero conditions to a higher value than that required by operating conditions, in the hope that the desired result will be obtained under operating conditions. However, this can provide a useful result only for one particular set of operating parameters, and has the further disadvantage of taking up clearances which could otherwise be allocated to tooling, i.e. the tooling could be made more rigid.

It is an object of the present invention to overcome the above disadvantages and to provide a means whereby the amplitude of oscillation in a linear friction bonding process is maintained substantially at its preset value under operating conditions.

In general, the present invention senses the amplitude of oscillation in a linear fricton bonding process between the articles being bonded and automatically adjusts the amplitude so that a predetermined amplitude

is achieved under operating conditions.

According to the present invention there is provided a method of controlling a linear friction bonding process in which two articles are oscillated in a linear direction relative to one another whilst simultaneously being pressed together, so as to be welded together by the friction of the oscillation, the method comprising monitoring the value of the amplitude of the oscillation, comparing the monitored amplitude of the oscillation with a predetermined value of the amplitude, deriving a signal from any difference therebetween, and using said derived signal to change the amplitude of the oscillation so as to at least reduce said difference.

The method may include measuring the rate of change of the monitored value of the amplitude of oscillation with respect to time and using that rate of change to modify said derived signal so as to reduce any tendency of the changed amplitude of oscillation to hunt.

Preferably, the method comprises monitoring the value of the amplitude of oscillation adjacent the interface between the articles being bonded.

The invention will now be described by way of example only with reference to the accompanying drawings, in which.

Figure 1 is a schematic block diagram of a linear friction bonding machine according to the invention, and

Figure 2 is a flow chart of a program for a computer, for use in operation of the machine of Figure 1.

Referring to Figure 1 there are shown two abutting metal

components 10, 12 having a common planar interface 14, which are to be bonded together at the interface by linear friction bonding.

One component 10 is rigidly held in a vice (not shown) , and the other component 12 is moved laterally to and fro across the interface 14 in the direction of arrows 16 by means of an oscillating mechanism arranged to induce this movement, such a mechanism being well known in the art and depicted generally by box 18. The amplitude of the oscillatory movement is controlled by an amplitude control hydraulic ram, indicated schematically by box 20.

The loading applied to component 12 in a direction normal to the plane of the interface 14, so as to press the components 10, 12 together, is provided by a hydraulic system indicated generally by box 22.

The lateral motion of component 12 relative to component 10 is sensed in a plane adjacent and parallel to the interface 14 by means of a linear variable differential transformer 24 which also measures the amplitude of the oscillation and sends a signal indicative thereof to a control module 26.

The control module 26 includes a computer and a look-up table containing ideal values of the amplitude of oscillation as a function of the applied loading and the frequency of oscillation. These ideal, or required, values of the amplitude may be obtained experimentally, or may be calculated from theoretical considerations, and inserted in the look-up table; alternatively, the values may be calculated algorithmically in real time, if a suitable algorithm is available.

The operation of the invention will now be described with reference to the flow chart of Figure 2.

The machine is set up so as to bring the metal components 10, 12 that are to be bonded, into contact. The speed and amplitude of oscillation, and the loading, are set up as required by appropriate setting of the oscillating mechanism 18, the amplitude control hydraulic ram 20, and the hydraulic loading system. The bonding operation in the form of a feed-back loop is now started.

The bonding operation commences by first checking to see whether the operation is in fact complete. This may be ascertained by checking one or more of a number of parameters have reached a preset value. These parameters may include time, number of cycles round the feed-back loop, the number of oscillations, and the amount of upset produced between the bonded components. If the operation is complete then the mechanism is stopped and the bonded components removed. The parameters may also include an instruction to halt the machine in the event of safety being breached, or otherwise.

If the bonding operation is to continue, the actual frequency of the oscillation and the loading are read and fed into the computer of the control module 26. Here the ideal amplitude of oscillation required corresponding to that particular frequency and loading is calculated, either from a look-up table or algorithmically as indicated above. The actual amplitude is then measured and compared with the stored required amplitude.

If the actual amplitude is greater than the stored

required amplitude, then the difference between the two is determined and a proportional signal is sent to the amplitude control ram 20 instructing it to reduce the pressure controlling the amplitude of oscillation. The amplitude of oscillation is thereby reduced and the next cycle of the loop commences.

If the actual amplitude is the same as the stored required amplitude, then no signal is sent to the amplitude control ram 20, and the next cycle of the loop commences.

If the actual amplitude is less than the stored required amplitude, then the difference between the two is determined and a proportional signal is sent to the amplitude control ram 20 instructing it to increase the pressure controlling the amplitude of oscillation. The amplitude of oscillation is thereby increased and the next cycle of the loop commences.

In an alternative embodiment, the rate of change of the amplitude of oscillation may also be measured and used to provide second-order feed-back control of the amplitude of oscillation. This is useful if it is found that the system has a tendency to "hunt".