P. A. Carroll, R. J. Scudamore, TWI (Yorkshire) Ltd., Rotherham, United Kingdom; S. Kenny, TWI Technology Centre (Wales) Ltd., Margam, Port Talbot, United Kingdom
In DMLD a laser beam is used to form a melt pool on a metallic substrate. Powder is then fed into this pool form a deposit that is fusion bonded to the substrate. Both the laser beam and the nozzle from which the powder is fed are manipulated using a robot or gantry system. For this study, a Trumpf DMD 505 machine, equipped with a CO2 laser and a co-axial powder deposition nozzle were used. Process parameters were optimised to maximise internal quality whilst minimising process time. In addition, different powder size distributions within the range of 16 to 150 microns were used to minimise surface roughness and hence, post-deposition machining.
The blades were then examined using a 3-dimensional stylus technique to determine surface roughness. An Xtek HMX225 micro-focus x-ray system with a transmission target of 3 microns resolution was used to generate computer tomography images. This, together with optical microscopy, was used to determine the size and quantify of internal defects such as cracking or porosity. The use of high resolution micro x-ray ensures the quality of each blade is guaranteed, possibly allowing the future application of DMLD for more safety critical components. The equipment comes with software which allows accurate measurement of sub-millimetre flaws to be made when calibrated against reference targets of known dimensions.
Summary: The objective of this study was to manufacture Alloy 625 compressor blades using Direct Metal Laser Deposition (DMLD) and ensure quality using the Non-Destructive Testing (NDT) technique of micro-focus x-ray computed tomography. DMLD is an established technique for the repair of turbine components in the aero-engine and power generation industries. However, it could also be applied during the original manufacture of components. This could possibly reduce material usage, machining operations, and lead times. Furthermore, changes in component design could be verified and developed more easily.
