Patent Application
20070289958
Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:—
The casing has a diameter of 850 mm and a wall thickness of 12 mm. The finished casing comprises a number of features such as flange 4 and boss 6 manufactured using a TIG SMD process. The flange and the boss have a radial thickness of the order 50 mm. The inside of the casing 7 is supported by the heat sink tooling 8. The tooling comprises a first end 10 to which a linear screw thread 12 is mounted. The end 10 extends circumferentially and fits within the casing. A second end (not shown) is provided at the opposite end of the combustor casing and to which the second end of the screw thread is mounted.
Each of the ends is sized such that they just fit within the casing to support and stiffen the casing at each end. A first arm 14 is mounted to the first end by a pin joint 16 such that it pivots. The arm extends between the pivot joint at the first end and a second pivot joint 18 at a mount for a heat sink 20. The mount for the heat sink 20 is mounted to a second arm 22 in a pivoting manner by a third pivot joint 24. At the opposing end of the second arm an actuator element 26 is provided that is attached between the second arm 22 and the screw thread 12 in a pivotal manner such that rotary movement of the thread translates into linear movement of the actuator element 26 and consequently radial movement of the mount 20 for the heat sink.
The arms 14, 22 may be detached from the first end face and replaced with other arms of differing length to locate the mounting element of the heat sink the desired location relative to that of the feature 6 that is to be added. The length of the arms may be determined by simple and routine trigonometry. As an alternative, the arms may telescope to the correct length. However, such a construction adds to the complexity of the tooling, especially as sufficient stiffness is required to support the heated component during the additive process.
The heat sink 28 mounted to the heat sink mount 20 is a thermally conductive box structure of copper and contains a water cooling circuit 30 with a bifurcated feed to minimise the temperature difference across the heat sink. After flowing through the heat sink the water is passed to a chiller (not shown) where it is cooled before re-circulating back to the heat sink. The flow of water through the structure should be sufficient to absorb the heat input into the casing by the selected additive manufacture.
The heat sink has a thermally conductive coupling media 32 between the box structure 28 and the inside surface of the casing 2. The thermally conductive coupling media is a 500 micron layer of “Heat Ban”, or Magna 904 available from Magna Industrial Company, which is a jelly-like compound that conducts heat between the cylindrical casing and the heat sink whilst conforming to the surface of the casing to provide good thermal contact.
The foot print of the heat sink is greater than that of the additive boss to be formed. For a boss having a radius of 20 mm the heat sink has a radius of 30 mm. The larger footprint supports the casing during the additive process.
At the start of the additive process, the tooling is inserted into the casing and the heat sink biased against the inside surface such that it exerts a slight positive pressure sufficient to support the casing without generating unnecessary stress within the casing.
As discussed earlier, the SMD process used to deposit the boss in this embodiment is TIG (Tungsten Inert Gas) Deposition.
The casing is held within an enclosure filled with the inert gas argon. The argon prevents the casing, electrode and deposited material from reacting with gases in the atmosphere.
The TIG cathode of a tungsten matrix material is removed from the combustor casing by a short distance, typically 4-6 mm and an arc is created between the casing and the TIG electrode. The arc is of high temperature and creates a melt pool on the casing having a depth of 1-2 mm and surface diameter of a similar size. A nickel superalloy material is continuously fed into the arc and melted onto the melt pool, which has a temperature of around 1700° C.
The TIG cathode is moved relative to the substrate thereby moving the position of the melt pool and the point of deposition of the new material with a single pass of the TIG electrode a ridge of alloy is deposited that has a height of approximately 1 mm and a width of about 8 mm.
The heat from the deposition process induces thermal expansion of the casing of between 20-30 mm around the circumference of the casing. The expansion is not uniform in all directions and especially for the asymmetric feature of the boss, where the casing is unsupported or insufficiently supported, the expansion generates stresses and warping in the casing. The stresses induced by the thermal expansion are therefore mitigated by moving the heat sink radially, with the combustor casing, to maintain a relatively constant pressure against the reverse surface of the casing that is sufficient to support the expanded casing and so avoid warping of the combustor casing.
Where the additive feature is a circumferential flange it is possible to build this up in a number of passes whilst rotating the casing underneath the deposition tool. To avoid the necessity of having a heat sink extending the length of the inner circumference the heat-sink tooling remains static with respect to the deposition tooling and does not rotate with the casing.
According to a second embodiment of the invention the combustor casing 2 is supported by an external band heat sink structure 40 as depicted in
The feature to be added, in this embodiment, is an internal flange having a width of 20 mm and a height of 16 mm. The flange is sited on the reverse surface of the casing to that of the heat sink structure and immediately opposite the heat sink.
The band, depicted in
One surface of the band conforms with the surface of the casing and the conformity is improved by applying a thermally conductive gel or grease 50 to the surface of the band. In this way good thermal contact is achieved.
The support band is biased against the surface of the combustor casing by a spring element 52 that secures the two ends of the band in proximity. The spring is tensioned to allow the band to expand radially as the casing expands because of heat input during the additive process. The tension provided by the spring is selected such that the pressure exerted to the casing does not create significant distortions to the material of the casing, but sufficient pressure is applied to limit distortion in the casing once the additive process is complete. Beneficially, the same positive pressure can be applied throughout the additive process.
Various modifications may be made without departing from the scope of the invention.
Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0610966.4 | Jun 2006 | GB | national |
