Abstract:
A method of repairing a case for a gas turbine engine includes identifying areas of damage on its internal holding structures in a gas turbine case, and utilizing cold metal transfer to deposit weld material to the internal holding structures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to the use of cold metal transfer techniques to repair internal holding structures, such as the rail or seal grooves on a case for a gas turbine engine. 
         [0002]    Gas turbine engines include cases that surround the turbine sections. These cases will have internal holding structures, such as grooves to receive vanes or seals. Over time, the grooves wear, and need to be repaired. 
         [0003]    In the prior art, these grooves have been repaired by adding additional material through welding techniques, such as manual tungsten inert gas (“TIG”) welding. The cases are typically out of round and out of flatness when removed for service. Manually welding these cases is time consuming task, and is not ergonomic. Other gas turbine engine cases, such as diffuser and compressor cases, can have similar problems. 
         [0004]    A process known as cold metal transfer (“CMT”), has been incorporated into gas metal arc or metal inert gas (“MIG”) welding systems. This process reduces the spatter often associated with MIG welding. This spatter has kept MIG welding from gaining popularity in the aerospace industry. An example of the CMT process has been developed by Fronius Corporation. 
         [0005]    The term “cold” has to be understood in terms of a welding process. When compared to a conventional MIG process, CMT is a cold process. Its characteristic feature is alternating hot and relatively cold temperature. This alternating hot and cold treatment occurs by incorporating a welding wire motion into a process control. 
         [0006]    The wire moves forward and into a weld pool. A short circuit occurs. As the short circuit happens, the wire is pulled back again. In this way, a weld arc only inputs heat very briefly in the arcing period, after which the thermal input is immediately reduced. In this way, the alternating hot and cold occurs. 
         [0007]    The rearward movement of the wire assists weld material droplet detachment during the short circuit. The short circuit is thus controlled and kept small. 
         [0008]    CMT has not been utilized in repair applications for gas turbine engine cases. 
       SUMMARY OF THE INVENTION 
       [0009]    A method of repairing a case for a gas turbine engine includes identifying areas of damage on its internal holding structures in a case, and utilizing cold metal transfer to deposit weld material to the internal holding structures. A repaired case is also claimed. 
         [0010]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a turbine case. 
           [0012]      FIG. 2  is a cross-sectional view taken at line  2 - 2  of  FIG. 1 . 
           [0013]      FIG. 3A  is an enlarged view of a worn area identified in  FIG. 2 . 
           [0014]      FIG. 3B  is a flow chart of an exemplary method. 
           [0015]      FIG. 4A  shows a system for applying cold metal transfer. 
           [0016]      FIG. 4B  shows a detail of the  FIG. 4A  system. 
           [0017]      FIG. 5  shows the repaired part. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Historically, MIG welding processes have not been used to repair gas turbine engines for several reasons, including excessive porosity, spatter, and precision. With the improvements in these areas, the CMT process has been recognized as a viable alternative to TIG welding techniques on gas turbine engine components. 
         [0019]    The CMT process can perform welding with relatively low heat input into the substrate. The low heat input results in less distortion to the welded features and a smaller heat affected zone. The deposition rate of the CMT process is quite faster than conventional TIG welding processes, which can reduce repair process times from hours to minutes. 
         [0020]    A case  20  for a gas turbine engine is illustrated in  FIG. 1 . The case  20  is a low pressure turbine case. It should be appreciated that the case  20  may also be a high pressure turbine case, a compressor case or a diffuser case. Case  20  is defined by a body having a radially outer wall  21  and a radially inner wall  23 . 
         [0021]    A plurality of rails or grooves  22  are formed on the inner wall  23  of the case  20 . For purposes of this application, the rails or grooves will be referred to generically as “internal holding structures.” 
         [0022]    As shown in  FIG. 2 , the internal holding structures  22  have specific shapes to receive an engaging rail or seal. These shapes should be maintained at their desired configuration in order for the seals and rails to properly interact with the case  20 . 
         [0023]    As shown in  FIG. 3A , surfaces such as shown at  29  may wear away from the internal holding structures  22 . In fact, the worn shapes are generally less smooth than is illustrated in  FIG. 3A , and they may also include cracked areas, etc. 
         [0024]    A flow chart of a repair method is shown as  FIG. 3B . At step  100 , a case  20  is inspected to determine the extent of the damage to the grooves. The damages areas are then machined at step  102  to remove damaged and cracked areas. At step  104 , one then inspects the part to ensure that all the damaged material has been removed, and the areas are cleaned at step  106 . 
         [0025]    Cold metal transfer techniques are utilized at step  108  to repair the part. The case  20  is initially set on a rotating table  56  at step  110 , and as shown in  FIG. 4A . A system  50  includes a robotic manipulator  52  moving a cold metal transfer deposition head  54  connected to a cold metal transfer machine  58 . 
         [0026]    After the part is on the rotating table, the cold metal transfer machine is placed adjacent the table at step  112 , and is adjusted at step  114  to meet the desired welding parameters for the particular part. The cold metal transfer machine is then aligned to the case at step  116 , and the automated welding process occurs at step  118 . 
         [0027]    As can be appreciated from  FIG. 4B , the cold metal transfer deposition head  54  includes a weld wire  90 . The wire moves forward, as shown in phantom at  92  and into a weld pool. A short circuit occurs. As the short circuit happens, the wire is pulled back again. A weld arc only inputs heat very briefly in the arcing period, after which the thermal input is immediately reduced. In this way, the alternating hot and cold occurs. 
         [0028]    After the cold metal transfer welding occurs, the part is stress relieved at step  120 , and inspected for cracks in the weld at step  122 . Finish machining or blending to smooth the welded areas may occur at step  124 , and the part may then be dimensionally inspected at step  126  for return to service. 
         [0029]    As shown in  FIG. 5 , after this method, the welding material  30  has returned the surfaces of the internal holding structures  22  to their desired shape. The case  20  will now be ready for return of service, and will function properly. 
         [0030]    While a particular assembly of cold metal transfer equipment has been illustrated in  FIG. 4A , other assemblies may be utilized. As an example, the case  20  could be held stationary while the cold metal transfer deposition head could be rotated about the case  20 . Further, other robotic machines may be utilized, beyond that shown in  FIG. 4A . 
         [0031]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.