J. Allen, Medtronic Vascular, Santa Rosa, CA; P. A. Carroll, R. J. Scudamore, TWI (Yorkshire) Ltd., Rotherham, United Kingdom

Summary: In Direct Metal Laser Deposition (DMLD), which is also referred to as Laser Cladding, a laser beam is used to form a melt pool on a metallic substrate. Powder is then fed into this pool which melts to form a deposit that is fusion bonded to the substrate. Both the laser beam and nozzle from which the powder is fed, are manipulated using a robot or gantry system. DMLD is already an established technique for the repair of turbine components in the aero-engine and power generation industries. To date, applications have usually been limited to alloys that are easily welded, in simple geometries, with significant post-deposition machining. For example, repair using Inco 625 of turbine blade tips. Rolls Royce, in partnership with TWI, have developed a DMLD repair procedure for Single Crystal (SX) seal segments, fig. 1. The repair involves the build up of a diamond shaped lattice of wall thickness 0.3mm, using Relay 1, a Rolls Royce propriety Nickel superalloy developed for its high temperature oxidisation resistance, onto a CMSX-4 substrate, fig. 2. The seal segment lattice forms an abradable seal between the rotating turbine blade seal fins and the engine casing. These segments require replacement several times over the life of the engine. The technique presented here allows for the replacement of the lattice without the replacement of the CMSX-4 casting. During original component manufacture, the lattice is machined from a CMSX-4 casting by electrode discharge machining (EDM). Prior to the deposition repair, the worn lattice is machined away. Repair procedures to replace the lattice were developed at TWI using a modified Trumpf DMD505 laser deposition machine. The system includes five-axis manipulation, a powder feed system with low deposition rate capability, a 2kW CO2 laser, Siemens Sinumerik 840D NC, and a bespoke CAM software. Modifications to the standard machine include; optimisation of the laser beam profile to suit the repair application, installation of a Fraunhofer ILT co-axial powder feed nozzle, and installation of an enclosed processing chamber to ensure process cleanliness and prevent powder contamination. Experimental procedures were developed in a systematic manner, starting with straight-line deposits and determination of requirements for the controlled atmosphere. These procedures were then applied to actual segments where the NC code was optimised for minimum defect levels and processing time. Emphasis was placed on eliminating porosity and cracking within the deposit, whilst ensuring the process was industrially robust. Modifications were made to the lattice design to maximise the benefits of the flexibility of DMLD, in comparison to EDM. The deposition process concluded with the manufacture of several complete segments. The cycle time per segment repair was less than 1 hour. Following deposition, the upper surface of the lattice was finished machined to specification and an alumina powder is sintered into the structure. Testing to verify the robustness of the process has been completed, this included a complete ground engine test, during which the engine is subjected to many operational cycles over a significant period of time at temperatures in excess of normal operation. The segments performed satisfactorily. Rolls Royce are now arranging for the installation a DMLD production facility to repair Trent 500 segments. It is envisaged that this DLMD procedure will be applied to many of the Rolls Royce Trent family of engines. The paper will present general information on machine specification, process tolerances and set-up, metallurgical microstructure, and process capability whilst in production. The presentation will include a video of the actual DMLD repair.