Using the MELD process to Repair a Mg Gear Box Housing

Additive manufacturing (AM) is of high interest to manufacturers looking to “print” complex geometries in an array of metallic materials. In addition to manufacturing applications, AM technologies can also be used in repair operations of similarly geometrically complex parts. Spatially restrictive deposition chambers, slow production rates, and undesirable physical properties are the main hurdles AM technologies face as these processes look for more wide-spread adoption throughout industry. Additive Friction Stir (AFS) technology is a solid-state AM process that yields fully dense products with near wrought properties, reduced distortion, and significantly faster throughput as post deposition material-processing is not required. The process principle is similar to friction stir welding/processing, with the exception of the use of filler material. Filler material is fed through a rotating tool and deposited on a substrate. This technology is capable of depositing large complex geometries in near-net shape without the need for an inert environment.

In this work the AFS process is utilized to repair a cracked support rib of a large Bell 212 Helicopter cast magnesium AZ91 gear box housing that is intended to be used for flight operation. The gearbox features many challenging features which increase the difficulty of the repair including the location of the damaged rib, the lack of support material beneath the damaged rib, and the material from which the gear box was originally manufactured (melt based processes were not successful in repairing the AZ91 housing). Prior success of the AFS process with Mg alloys including AZ31, WE43, E675, AMX602, E21 are presented for background information. Initial process development work on analog geometries are presented including microstructural analysis and mechanical strength through the height of the deposition as well as across the stirred interfaces of the repair. The actual repair of the AZ91 gear box is presented and reviewed.