Process Quality Relationships in Material Addition Repair of High Value Aerospace Components

Abstract

With the advent of integrated components like the Blisk (blade integrated disk) in advanced aeroengines, repair of in-service damage has become an important issue. Laser Blown Powder-Directed Energy Deposition (LBP-DED) is one technology being developed by Rolls-Royce to repair such components. However, there are a number of issues that need to be addressed including; i) the means by which defects can be controlled, and, ii) how the microstructure and texture developed in the added material differs from that of the original component. Thus, this research has focused on understanding how the processing variables in LBP-DED repair of Inconel 718 Blisk aerofoils effect the quality of the repair produced, in terms of its defect distribution, grain structure and texture. High resolution XCT analysis of systematic LBP-DED aerofoils repairs has shown that the defects, such as, porosity, and shrinkage pores can be minimised by reducing the powder flow rate, and, increasing the laser power. High attenuation features have also been identified in several of the systematic and original aerofoil repairs. These features have been characterised as NbC/TiC primary carbides, using several different experimental techniques including: correlated XCT and EBSD/EDS (using PFIB), EPMA and SEM imaging. Analysis of the carbide size distribution has shown that these features are too large to have formed during solidification of the melt pool, additional analysis of these features has shown that these carbide features originate from the starting powder material EBSD examination of the systematic full builds repairs has shown that microstructure is not simply related to either laser power (at a constant heat input and powder deposition rate) or powder flow rate (at a constant laser power and laser velocity). The microstructure in the build, however, is determined by the ratio between thermal gradient and solidification rate, G/R. For instance, increasing the normalised laser power above the standard condition caused the thermal gradient in the build to drop, reducing the G/R below the CET in some areas of the melt pool, promoting the formation of equiaxed grains and the development of a mixed microstructure.1,2 Two non-typical bi-directional textures have been identified in the repair of full build aerofoils, including; a split cube texture tilted approx. 25° from the z direction, and, a //X split texture, which was consistently observed in the systematic repair samples. The formation of non-typical bi-directional textures observed in this work has been shown to be due the complex bi-directional snaking raster pattern, where, the starting hatch position is rotated between the four corners of the deposit. Additional work has been carried out to study the effect of process parameters on the melt pool size and shape, powder capture efficiency of single LBP-DED tracks. The experimental data generated has been used to interpret that behaviour in more complex full repairs and to also to fit an FE model to the melt pool profile which has subsequently been used to understand the effects process parameters, the process efficiency and cooling rate.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Prangnell (Supervisor) & Michael Preuss (Supervisor)