GB2283448A

GB2283448A - Improvements in or relating to electron beam welding

Info

Publication number: GB2283448A
Application number: GB9420374A
Authority: GB
Inventors: Thomas Kenneth Johnson; Anthony Lionel Pratt
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1993-10-27
Filing date: 1994-10-10
Publication date: 1995-05-10
Also published as: GB9420374D0; GB9322160D0; DE4438303A1

Description

IMPROVEMENTS IN OR RELATING TO ELECTRON BEAM WELDING 2283448 This

invention concerns a method and apparatus for electron beam welding with in-process heat treatment. In particular, the invention relates to an improved method of heat treating a workpiece, before, during and after the welding operation. It also relates to an apparatus for carrying out the method.

It is known to weld a workpiece using a focused electron beam. The movement of the beam over the workpiece is controlled by deflection coils of the electron beam generating apparatus so that the beam passes along that region of the workpiece to be welded. Preliminary heating of the weld region, as well as cooling of the region after welding has taken place, is controlled by rastering the electron beam; that is, the beam is moved very rapidly to and fro over the workpiece in a predetermined pattern by the deflection coils.

An arrangement of this kind in which controlled cooling of a workpiece subsequent to welding by controlling the fade-out slope of the welding region while transferring the beam to a raster pattern covering a region surrounding the weld zone is known from our published UK Patent No 2 265 103.

US Patent No 4 591 688 describes a system and method for electron beam welding in which the process is modified by modulating the weld beam along a predetermined shaped path in order to improve the quality of the weld itself to avoid internal weld defects. Impingement of the electron beam on the workpiece produces a melt zone comprising a liquid zone and directly under the beam where heat input is greatest a vapour zone. As the beam is advanced in the direction of the weld the pool of_ liquid metal flows from front to back, with respect to the directions of travel, forming a capillary tube of vaporised material. Dynamics of the characteristics of the liquid/vapour phases of the molten weld pool, their interfaces and the motions of the electron beam can produce effects which result in weld defects. These effects were found to be lessened by oscillating the beam, and the patent discloses such an arrangement for producing micromovements of the electron beam during the welding process. The electron beam is moved around the molten weld pool in a predetermined multi-lobed pattern which is found to benefit the shape and stability of the vapour capillary within the pool. The beam movements are intended to improve the structure of the weld bead and to reduce spatter. An incidental effect of the beam micro-movements may be to slightly enlarge the size of the weld pool, but the beam nevertheless apparently remains within the weld pool boundary as defined by its liquid/solid interface.

However, workpieces of some metal alloys especially nickel or titanium alloys welded in this way are found still to suffer from premature fatigue cracks in regions along the margins of the weld. It is thought that a primary cause of these cracks is the temperature regime to which the weld region is subjected during electron beam welding.

The present invention has for one of its objectives to control the temperature profile in a solid metal workpiece In regions bordering the path of an electron welding beam. Cracks and fractures can occur in the unmelted, solid parts of a workpiece adjacent an electron beam weld as a result of stresses created in the solid metal by steep temperature gradients in the X 3 - solid phase. The present invention seeks to avoid these stresses and their effects by carrying out an in-process heat treatment step simultaneously with the creation and advancement of a weld pool. Further steps of pre-heating and post-heating the workpiece may be performed in the same manner.

In accordance with the present invention there is provided a method of electron beam welding with in-process heat treatment comprising carrying out the welding and heat treatment steps in a series of repeated cycles in which the welding steps are carried out by the electron beam in a focused conditions at a first current density and the heat treatment steps are carried out by the electron beam in a defocused condition at a second current density, and wherein the second current density is less than the first current density.

Preferably in-process heat treatment is carried out with a focused electron beam at a first beam current density, includes the steps of: preheating the weld region and bordering regions ahead of the weld region by diverting the electron beam and rastering it over said regions in a defocused state and at a second beam current density, establishing a weld pool and performing the weld by moving the electron beam along an intended provided path in a focused state and at the first beam current density, controlling the temperature of regions lying laterally of the weld by diverting the electron beam and rastering it over said regions in a defocused state and at a third beam current density, and controlling cooling of the weld and the regions bordering the weld behind the weld pool by diverting the electron beam and rastering it over said regions in a defocused state and at a fourth beam current density.

The invention and the preferred mode of carrying it into practice will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic view of an electron beam welding apparatus showing a beam pattern for In-process heat treatment; Figure 2 is temperature profile through the workpiece of Figure 1 at section A-A; Figure 3 is a detail view showing an alternative beam pattern for in- process heat treatment; and Figure 4 is a temperature profile through the workpiece of Figure 3 at section B-B.

Referring now to Figure 1 there is shown an electron beam generating apparatus generally indicated at 10 including a beam generator, a set of toroidal focusing coils 14, and a set of toroidal deflection coils 16. An electron beam generated by the generator 12 is shown at 18 and is directed onto a workpiece 20. The beam 18, after passing between the focusing coils 14, is shown in two possible configurations: focused, as indicated by impingement region 22; and defocused, as indicated by impingement regions 24 and 25.

The toroidal focusing coils 14 are connected to a beam focus control subsystem 26, known in the art. The toroidal deflection coils 16 are connected to a beam deflection control subsystem 30, known in the art. The focus and deflection control subsystems 26,30 are operatively linked to a computer control system, indicated at 32. The computer control system 32 commands the deflection control subsystem 30 to move the electron beam 18 in a programmed pattern over the workpiece 20, and simultaneously commands the focus control subsystem 26 to effect focus and defocus of the electron beam in accordance with a deflection/focus program.

The supporting structure of the welding apparatus is omitted from the drawings for clarity. The features and improvements of the invention may be clearly understood without reference to said structure. Furthermore references within this description to relative movement between the electron beam and the workpiece upon which it is made to impinge may be brought about by movement of either of these items, or both. Preferably, however, the electron beam generating apparatus, and therefore its supporting structure remain stationary. The electron beam is movable within limits by the amount of deflection which may be achieved by means of the deflection coils, while large scale progression of the electron beam with respect to the workpiece in the linear direction of a weld is brought about by linear movement of the workpiece. It is to be understood, therefore, that the workpiece Is normally mounted on some sort of slide arrangement which also is not shown in the drawings. It is envisaged that such a slide would be powered and its movement controlled, or at least coordinated, by instructions generated by the deflection/focus program mentioned above. Reference to a workpiece are to be understood to include reference to two or more component parts intended to be joined together as well as to the so-joined assemblage.

The focusing coils 14 essentially comprise a ring-shaped or toroidal electro-magnet divided into four equal segments which are connected electrically in two opposite pairs. Energising current for these coils is provided through connections 34 from focus control subsystem 26, which in turn is operatively controlled by commands generated by control computer 32 and carried by connections 36.

Deflection coils 16 comprise an essentially similar ring-shaped or toroidal electro-magnet divided Into four segments to which energising current is supplied through connections 38 from deflection control subsystem 30. The individual deflection coils are arranged and electrically energised in order to produce beam movements along two orthogonal directions In the plane of the workpiece 20. The control subsystem 30 is, in turn, operatively controlled by commands generated by control computer 32 and carried by connection 40.

A third parameter controlled by the computer 32 is total beam current. A connection 42 between computer 32 and electron beam generator 12 carries beam current commands whereby during operation the beam current may be varied. The welding control program executed by computer 32 is designed to raster the beam 18 through a predetermined pattern of movements during some of which the beam current density is sufficiently high to produce, or maintain, a weld pool, and during the remainder of which the beam current density is reduced so that the regions of the workpiece onto which the beam impinges are only heated to a temperature lower than its melting point.

In general the electron beam is focused to a small spot for the welding portions of the operation which Is carried out by a first beam current density. Obviously this level of beam current density is determined by total beam current divided by the impingement area of the focused beam spot. For the in-process heat treatment portions of the operation the beam is 1 de-focused, that is the degree of focus brought about by coils 14 is altered to increase the impingement area of the beam on the workpiece. As a result of this increase the beam current density, and thus the heat input per unit area, falls and with it the temperature to which the beam is able to heat the impingement area. Clearly a number of other factors are involved in determining the required current density, including the size of the impingement area, the temperature to be reached, beam dutycycle, and conductivity of the workpiece. It has been found empirically that the width of the regions on either side of an electron beam weld which require heat treatment, or annealing, is such that to raise their temperature sufficiently total beam current has to increase during heat treatment periods.

To further illustrate one aspect of the method Figures 1 and 2 show the electron beam switched, or rastered, between three positions. In the first position the beam 18 is focused to produce a small, intense spot 22 on weld line 23. In the second and third positions the beam is switched to laterally spaced locations 24,25 equidistant from the weld line 23 in which the beam Is defocused to cover the larger impingement areas and the beam current density is switched to a second value. The beam 18 is switched or rastered between the three positions in cyclic fashion dwelling at each position for short, but not necessarily equal, periods of time. In a typical situation in order to obtain sufficient heat input into the side areas 24,25 although the second value of beam current density is lower than the first value the total beam current has to be increased in the second value to a value greater than in the first value. The pattern of beam rastering is repeated cyclically with sufficient frequency to maintain the required temperature profile.

Figure 2 shows a graph of the temperature profile through the workpiece on a section AA orthogonal to the weld line 23. The graph includes in solid line the temperature in the workpiece during in-process heat treatment according to a first aspect of the method of the invention with, for comparison, the corresponding temperature in dashed line for a simple, single spot electron beam welding operation.

The effect of the beam rastering pattern is to substantially broaden the width of the high temperature zone of the workpiece. The width of the weld pool, ie where local temperature is raised above the melting point of the workpiece material, remains substantially the same as in the basic welding operation. The temperature of the workpiece either side of the weld zone, however, is raised to its annealing temperature over a significant distance. The exact width of this heat treated region is selected to avoid stress-cracking found bordering weld beads in some alloy workpieces such as MAR M002. As is Illustrated by the dashed line temperature profile the fusion zone is narrow and deep but the surrounding material cools very quickly and this leads to problems in some materials when stresses are locked into the re-solidified material which, sometimes immediately, manifests itself as cracking. During the in- process heat treatment the focused electron beam is taken out of the weld pool for short periods of time of the order of a few milliseconds, defocused and rastered to the heat treatment zones, if necessary with raised beam current, in order to heat the zones but without producing surface marking.

The electron beam may also be rastered in axial directions to produce a temperature profile which is elongated in the direction of the weld line 23, thus 1 introducing a degree of pre-heating and controlled cooling.

According to a second aspect of the invention illustrated in Figures 3 and 4 the region s 44, 46, 48, 50 heated by a defocused beam are more closely spaced around the weld pool 22. In fact, in the arrangement depicted in Figure 3 the defocused Impingement regions abut each other. The effect on the temperature profile in the workpiece as shown in Figure 4 is to broaden the high temperature zone surrounding the welding fusion zone.

Claims

A method of electron beam welding with In-process heat treatment comprises carrying out the welding and heat treatment steps in a series of repeated cycles in which the welding steps are carried out by the electron beam in a focused condition at a first current density and the heat treatment steps are carried out by the electron beam in a defocused condition at a second current density, and wherein the second current density is less than the first current density.
2 A method as claimed in claim 1 wherein the electron beam current to achieve the second current density in the defocused condition is greater than the electron beam current to achieve the first current density in the focused condition.
A method of electron beam welding with in-process heat treatment in which welding is carried out with a focused electron beam at a first beam current density, includes the steps of:

pre-heating the weld region and bordering regions ahead of the weld region by diverting the electron beam and rastering it over said regions in a defocused state and at a second beam current density, establishing a weld pool and performing the weld by moving the electron beam along an intended path in a focused state and at the first beam current density, A 1, controlling the temperature of regions lying laterally of the weld by diverting the electron beam and rastering it over said regions in a defocused state and at a third beam current density, and controlling cooling of the weld and the regions bordering the weld behind the weld pool by diverting the electron beam and rastering it over said regions in a defocused state and at a fourth beam current density.
4 A method as claimed in claim 3 wherein the second, third and fourth beam current densities are equal.
A method as claimed in either claim 3 or claim 4 whether the beam current at the second, third and fourth beam current densities is greater than the beam current at the first beam current density.
6 A method as claimed in any of the preceding claims wherein the beam current is pulsed and the beam current density is determined by the pulse characteristics of the beam.
7 A method as claimed in any of the preceding claims wherein the beam current density is altered by increasing or decreasing beam current as appropriate.
8 Electron beam welding apparatus adapted to carry-out the method of any of the preceding claims comprising an electron beam generator, beam focusing means, beam deflection means and beam control means interconnected with the beam generator, focusing means and deflection means.

A method of electron beam welding substantially as hereinbefore described with reference to the figures of the accompanying drawings.

Electron beam welding apparatus substantially as hereinbefore described with reference to the figures of the accompanying drawings.

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