GB2286142A - Energy beam butt welding of forged and cast metal - Google Patents

Abstract

When using an energy beam, such as an electron beam, to butt weld forged metal (6) to cast metal (5), a double butt weld (X, Y, Z) is formed in order to minimize the crack length in the heat affected zone. The technique is particularly suitable for welding a replacement outer rear flange to a diffuser case of a turbine engine. <IMAGE>

Description

ENERGY BEAM BUTT WELDING OF FORGED AND CAST METAL The invention relates to using an energy beam, such as an electron beam, to butt weld a cast metal component to a forged metal component.

Figure 1 shows a diffuser case of a Pratt & Whitney PW4000 series engine. The diffuser case is interposed between the compressor and turbine sections of the engine. Figure 2 is a section along the line A-A of Figure 1. It may be seen that the diffuser case 1 has an annular inner rear flange 2 and an annular outer rear flange 3. The outer rear flange 3 contains a plurality of holes 4 which are used to fix the rear end of the diffuser case to the turbine section (not shown).

The diffuser case 1 is cast as a single component, e.g.

using HIP CAST INCO 718 or PWA 1469. There may be a casting defect in the outer rear flange 3 which requires it to be replaced before the diffuser case can be released for sale.

During the service use of the diffuser case, the presence of the holes 4 may lead to fatigue damage and thus the outer rear flange 3 may need to be replaced several times during the service life of the diffuser case.

At the present time, authorised repair facilities for the PW4000 series engine are provided with a Cleaning Inspection Repair (CIR) Manual (PN 51A357) which describes "Diffuser Case Assembly - Repair-37". This repair relates to replacing the outer rear flange 3. The reader is invited to refer to the Manual but, for the sake of convenience, the existing repair technique will be described. In order to remove the outer rear flange 3, an annular cut is made slightly in front of the outer rear flange. For example, the cut is made along the line P of Figure 2. For the sake of convenience, the portion which is cut off will be referred to as a rear flange unit.

A penetrating band (not shown) is tack welded around the inner edge of the cut surface of the main part 5 of the diffuser case 1. A replacement rear flange unit has already been manufactured out of forged metal (WROUGHT INCO 718) and has a butt weld surface which can be abutted against the cut surface of the main part 5. Part of the replacement rear flange unit which does not abut against the main part 5 is manufactured so as to be oversized in order to permit its precise dimensions to be produced by machining after the replacement rear flange unit has been electron beam welded to the main part 5.

Having obtained the replacement rear flange unit, the repair operative then proceeds to place it in position by bringing the butt weld surface of the replacement rear flange unit into abutment with the cut surface of the main part 5. The penetrating band helps to locate the replacement rear flange unit in position. The abutting surfaces of the replacement rear flange unit and main part define an annular interface which needs to be completely welded. To assist in holding the components in position, a tack weld is provide along the outer edge of the interface. Then, the main welding operation takes place, in which an electron beam is directed at the outer edge of the interface and has an intensity sufficient to produce a single butt weld whose cross-sectional profile extends radially inwards the complete radial distance of the interface. When the welding effect of the electron beam reaches the inner edge of the interface, the penetrating band absorbs the energy of the electron beam.

After the electron beam welding operation, the penetrating band is removed. The oversized replacement rear flange unit is then machined to the correct size. The use of the electron beam to produce a single butt weld has proved not to be satisfactory because the forged metal (the replacement rear flange unit) does not weld satisfactorily to the cast metal (the main part 5 of the diffuser case 1).

The problem is that unacceptable cracks of length 0.100" appear in the heat affected zone of the butt weld. The maximum permissible crack length is 0.032" (0.813 mm). Before developing this invention, we tried many different combinations of the parameters for the single butt welding. For example, we varied the speed of the electron beam over the metal, and the heat input by the electron beam, and we also tried performing, before the main full-penetration weld, a sealing run in order to produce a small sealing weld. All of the many combinations failed to produce single butt welds which consistently had cracks which did not exceed 0.032".

In the prior art, if the interface between the two butt weld surfaces is welded along both edges, this is known as a double butt weld. It is known to use an electron beam to double butt weld together similar materials. For example, a double butt weld may be used to weld cast metal to cast metal or to weld forged metal to forged metal. A double butt weld has traditionally been used when the distance between the edges of the interface is too large for a single butt weld to be used. In other words, single butt welding has been used for welding together thin items, and double butt welding has been used for welding together thick items.

The aim of the present invention is to address the problem of energy beam welding forged metal to cast metal without producing unacceptable cracks in the heat affected zone.

According to a first aspect of the present invention, there is provided a method of energy beam butt welding a cast metal component to a forged metal component, each component having an edge portion with an edge surface, the method comprising the steps of: (i) abutting the two edge surfaces to define an elongate interface having spaced apart, longitudinal first and second edges; (ii) energy beam butt welding the two edge portions to each other along the first edge of the interface by producing a first weld with a cross-sectional profile which extends only part of the way to the second edge of the interface; and (iii) energy beam butt welding the two edge portions to each other along the second edge of the interface by producing a second weld with a cross-sectional profile which extends only part of the way to the former position of the first edge of the interface and overlaps the cross-sectional profile of the first weld.

By using the energy beam to produce a double butt weld, the present invention enables forged metal to be joined to cast metal without the heat affected zone containing unacceptable cracks.

Furthermore, it is possible to dispense with the penetrating band used with the prior art single butt weld.

By choosing to use double butt welding when the thickness of the edge portions adjacent to the edge surfaces is sufficiently small (e.g. less than 0.5", 0.4" or 0.3") for single butt welding to be acceptable for most requirements, it is possible to meet the stringent crack limits required by the aerospace industry and other industries which demand high quality welds.

Along a notional line corresponding to the former position of the interface, the extent of overlap of the first and second welds may be 10 to 90% of their total length. Alternative lower limits are 20%, 30% and 40%. Alternative upper limits are 80%, 70% and 60%.

Usually, the cross-sectional profile of each weld is generally T-shaped and comprises a head portion and a shank portion. It is preferable that the double butt weld is such that the two shank portions overlap each other.

At the present time we prefer that the energy beam welding should be accomplished by using an electron beam. A laser beam may have generally the same welding characteristics as an electron beam and, as in future the cost of laser beam welding machines is likely to reduce substantially, it may be that in the future it is preferable to use a laser beam instead of an electron beam.

As applied to replacing a defective outer rear flange of a diffuser case of a gas turbine, the present invention comprises the steps of: (i) cutting off a first part of the diffuser case, the first part including the defective outer rear flange, so as to leave a second part of the diffuser case, the second part being made of cast metal; (ii) providing a third part which is made of forged metal and includes a replacement outer rear flange; (iii) abutting butt weld surfaces of the second and third parts; and (iv) energy beam butt welding the second and third parts to produce a double butt weld comprising first and second welds whose cross-sectional profiles partially overlap.

A non-limiting embodiment of the present invention will now be described with reference to the accompanyIng drawings in which: Figure 1 is perspective view of a diffuser case; Figure 2 is section along the line A-A of Figure 1; Figure 3 is a sectional view of a replacement rear flange unit and the adjacent part of the main part of the diffuser case, during the first part of the welding operation; Figure 4 is a view similar to Figure 3, but showing the second part of the welding operation; Figure 5 is similar to Figures 3 and 4 but shows the end result of the two welding operations; and Figure 6 is a diagrammatic enlargement of part of Figure 5 and shows the overlap of the two welds comprising the double butt weld.

In relation to the non-limiting embodiment of the method of the present invention, the defective rear flange unit is removed from the main part 5 of the diffuser case 1 in the same manner as previously described in relation to the prior art.

Whilst Figures 1 and 2 are accurate representations of the PW4000 series engine, Figures 3 to 6 are merely diagrammatic representations of the non-limiting embodiment of the present invention. For example, the distance from the centre line CL of the diffuser case 1 to the replacement rear flange unit 6 and the main part 5 has been shown as being smaller than it actually is.

As may be seen in Figure 3, the replacement rear flange unit is oversized with the final desired shape illustrated by a dot-dot-dash line. The excess material is removed by machining after the electron beam welding has been performed.

The replacement rear flange unit 6 (made of forged metal) is held on the cut end of the main part 5 (made of cast metal) by a fixture which is not shown. An interface 7 is formed by abutting butt weld surfaces of the main part 5 and replacement rear flange unit 6 in order to produce a double close square butt joint. The maximum permissible size of the gap between the abutted butt weld surfaces is 0.003". The butt weld surface of the main part 5 is the surface formed when cutting off the defective rear flange unit. It may be necessary to tidy up the cut surface in order to make a suitable butt weld surface. The butt weld surface of the replacement rear flange unit 6 is machined so as to be compatible with the butt weld surface of the main part 5, in order to ensure that the replacement outer rear flange 3 will be in the correct position. The interface 7 is annular and has an inner edge 8 and outer edge 9. It may be seen from Figure 3 that, adjacent to the interface 7, the main part 5 is generally tubular and the replacement rear flange unit 6 is also generally tubular. The forged and cast metal on either side of the interface 7 is cleaned.

The electron beam welding takes place in a vacuum chamber which contains a rotatable table. The front end of the main part 5 of the diffuser case 1 is placed against the rotatable table with the centre line CL of the diffuser case 1 pointing horizontally. The electron beam welding machine is stationary and projects an electron beam into the vacuum chamber. The rotatable table is rotated so as to form eight long tack welds and sixteen short tack welds around the outer edge 9 of the interface 7.

The fixture is then removed and tack welding is performed around the inner edge 8. Then a full penetration weld is performed around the outer edge 9 followed, as quickly as is reasonably possible, by a full penetration weld around the inner edge 8.

The outer edge 9 is tack welded by using eight long tack welds and sixteen short tack welds. Suitable parameters for the electron beam are as follows: Parameter Parameter Value Filament Current 5.5 MA High Voltage 150 kV Beam Current 4.0 MA Surface Current Focus Sharp Focus 660 MA Table Speed 11 inches per minute Wall Thickness 0.250" Material Type HIP Cast INCO 718 Beam Upslope 0.6 (short tack welds), 1.25 (long tack welds) Beam Downslope 0.6 (short tack welds), 1.25 (long tack welds) Gun to Work Distance 19.5" The electron beam is projected generally vertically. For the tack welding of the outer edge 9, the arrangement is as shown in Figure 3. The centre line CL is generally horizontal and is the axis of rotation of the rotatable table. Thus, as shown in Figure 3, the electron beam E is projected so as to be generally pexp.. ^ular to the centre line CL and to the rat i;; surfaces on either side of the outer edge 9.

In order to be able to tack weld the inner edge 8, it is necessary to project the electron beam over the part of the outer rear flange 3 on the opposite side to the welding location, past the centre line CL and at the part of the inner edge 8 currently being welded. The rotatable table is adjusted so that the centre line CL forms an angle of approximately 5 to the horizontal.

Thus, when tack welding the inner edge 8, the vertical electron beam E forms an included angle of 85" with the centre line CL.

The arrangement is shown in Figure 4. Thus, in relation to the surfaces on either side of inner edge 8, the electron beam is not quite perpendicular to those surfaces during the tack welding operation. The deviation from being perpendicular is 5".

When the tack welding has been performed, the full penetration welding is performed. Full penetration welding is first performed on the outer edge 9 and then on the inner edge 8.

Parameters for the full penetration welding of the outer edge 9 are as follows: Parameter Parameter Value Filament Current 5.5 MA High Voltage 150 kV Beam Current 9.0 MA Surface Current Focus Sharp Focus 660 MA Table Speed 11 inches per minute Wall Thickness 0.250" Material Type HIP Cast INCO 718 Beam Upslope 3.0 seconds Beam Downslope 8.0 seconds Gun to Work Distance 19.5" The parameters for the full penetration welding of the inner edge 8 are as follows: Parameter Parameter Value Filament Current 5.5 MA High Voltage 150 kV Beam Current 11.0 MA Surface Current Focus Sharp Focus 580 MA Table Speed 25 inches per minute Wall Thickness 0.250" Material Type HIP Cast INCO 718 Beam Upslope 3.0 seconds Beam Downslope 8.0 seconds Gun to Work Distance 40.0" The welding of the inner edge 8 is performed as soon as is practicably possible after the welding of the outer edge 9. A suitable electron beam welding machine is the Model-15000S machine manufactured by Precision Technologies Inc.

As during the tack welding of the inner edge 8, the full penetration welding of the inner edge requires the electron beam to form an inclusive angle of 85" with the centre line CL of the diffuser case (see Fig. 4).

Fig. 6 illustrates in diagrammatic form a cross-section through the double butt weld. As may be seen, a weld X is produced by the full penetration weld along the outer edge 9. A weld Y is produced by the full penetration weld along the inner edge 8. Weld Y is formed after weld X and overlaps weld X to form an overlap zone Z. It may be seen from Fig. 6 that weld X does not extend to the inner edge 8 and weld Y does not extend to the outer edge 9. Line 10 indicates the former position of the interface 7. Along line 10, weld X and weld Y each extend about 65% of the distance from their start surfaces. The thickness of the material on either side of the double butt weld is 0.250" and the extent of overlap L is 0.080 to 0.100".

With the double butt weld shown in Fig. 6, it has been found that the heat affected zone comprising weld X and weld Y and the adjacent cast and forged metal does not contain intergranular micro-cracks which exceed the maximum limit of 0.032" in length.

Claims (20)

1. A method of energy beam butt welding a cast metal component to a forged metal component, each component having an edge portion with an edge surface, the method comprising the steps of: (i) abutting the two edge surfaces to define an elongate interface having spaced apart, longitudinal first and second edges; (ii) energy beam butt welding the two edge portions to each other along the first edge of the interface by producing a first weld with a cross-sectional profile which extends only part of the way to the second edge of the interface; and (iii) energy beam butt welding the two edge portions to each other along the second edge of the interface by producing a second weld with a cross-sectional profile which extends only part of the way to the former position of the first edge of the interface and overlaps the cross-sectional profile of the first weld.

2. A method according to claim 1, wherein, at the former position of the interface, the extent of overlap of the first and second welds is 10% to 90% of their overall depth.

3. A method according to claim 2, wherein the extent of overlap is 20% to 80%.

4. A method according to claim 3, wherein the extent of overlap is 30% to 70%.

5. A method according to claim 4, wherein the extent of overlap is 40% to 60%.

6. A method according to any of claims 1 to 5, wherein the cross-sectional profile of each weld is generally T-shaped and comprises a head portion and a shank portion, and step (iii) is such that the two shank portions overlap each other.

7. A method according to claim 6, wherein step (iii) is such that the shank portion of the cross-sectional profile of each weld does not overlap the head portion of the cross-sectional profile of the other weld.

8. A method according to any of claims 1 to 7, wherein each edge portion is generally tubular and each edge surface is generally annular.

9. A method according to claim 8, wherein the first edge of the interface faces outwards and the second edge faces inwards.

10. A method according to claim 8 or 9, wherein one of the metal components has a longitudinally extending through hole and, in step (ii) or (iii), an energy beam is projected into the end of the through hole remote from the interface and travels along and across the through hole before striking the interface.

11. A method according to claim 10, wherein the metal component with the through hole is the forged metal component.

12. A method according to claim 10 or 11, wherein the energy beam forms an included angle of 70C to 89C with the central longitudinal axis of the through hole.

13. A method according to claim 12, wherein the included angle is 80" to 89".

14. A method according to claim 13, wherein the included angle is substantially 85%.

15. A method according to claims 9 and 12, wherein: the same energy beam is used in both step (ii) and step (iii), in step (ii) the components are rotated around the central longitudinal axis of the through hole and the energy beam is projected at the outwardly facing first edge of the interface and is substantially perpendicular to the central longitudinal axis of the through hole, between the steps (ii) and (iii) the energy beam is moved to adopt the included angle, and in step (iii) the components are rotated around the central longitudinal axis of the through hole and the energy beam is projected at the inwardly facing second edge of the interface.

16. A method according to any of claims 8 to 15, wherein the forged metal component comprises the generally tubular edge portion and a flange portion which is generally annular and projects outwards from the edge portion.

17. A method according to any of claims 1 to 16, wherein the energy beam butt welding of steps (ii) and (iii) is electron beam butt welding.

18. A method of replacing a defective outer rear flange of a diffuser case of a gas turbine, the method comprising the steps of: (i) cutting off a first part of the diffuser case, the first part including the defective outer rear flange, so as to leave a second part of the diffuser case, the second part being made of cast metal; (ii) providing a third part which is made of forged metal and includes a replacement outer rear flange; (iii) abutting butt weld surfaces of the second and third parts; and (iv) energy beam butt welding the second and third parts to produce a double butt weld comprising first and second welds whose cross-sectional profiles partially overlap.

19. A method according to claim 18, wherein the energy beam butt welding of step (iv) is electron beam butt welding.

20. A method of electron beam butt welding substantially as herein described with reference to Figures 1 to 6 of the accompanying drawings.