The surface texture of additively manufactured metallic surfaces made by powder bed methods is affected by a number of factors, including the powder's particle size distribution, the effect of the heat source, the thickness of the printed layers, the angle of the surface relative to the horizontal build bed and the effect of any post processing/finishing. The aim of the research reported here is to understand the way these surfaces should be measured in order to characterise them. In published research to date, the surface texture is generally reported as an Ra value, measured across the lay. The appropriateness of this method for such surfaces is investigated here. A preliminary investigation was carried out on two additive manufacturing processes-selective laser melting (SLM) and electron beam melting (EBM)-focusing on the effect of build angle and post processing. The surfaces were measured using both tactile and optical methods and a range of profile and areal parameters were reported. Test coupons were manufactured at four angles relative to the horizontal plane of the powder bed using both SLM and EBM. The effect of lay-caused by the layered nature of the manufacturing process-was investigated, as was the required sample area for optical measurements. The surfaces were also measured before and after grit blasting.

Additive manufacturing and in particular Selective Laser Melting (SLM) are manufacturing technologies that can become a game changer for the production of future high performance hot gas path parts. SLM radically changes the design process giving unprecedented freedom of design and enabling a step change in part performance. Benefits are manifold, such as reduced cooling air consumption through more efficient cooling schemes, reduced emissions through better mixing in the combustion process and reduced cost through integrated part design. GE is already making use of SLM for its gas turbine components based on sound experience for new part production and reconditioning. The paper focuses on: a) Generic advantages of rapid manufacturing and design considerations for hot gas path parts b) Qualification of processes and additive manufacturing of engine ready parts c) SLM material considerations and properties validation d) Installation and validation in a heavy duty GT Additive Manufacturing (AM) of hot gas path components differs significantly from known process chains. All elements of this novel manufacturing route had to be established and validated. This starts with the selection of the powder alloy used for the SLM production and the determination of essential static and cyclic material properties. SLM specific design features and built-in functionality allow to simplify part assembly and to shortcut manufacturing steps. In addition, the post-SLM machining steps for engine ready parts will be described. As SLM is a novel manufacturing route, complementary quality tools are required to ensure part integrity. Powerful nondestructive methods, like 3D scanning and X-ray computer tomography have been used for that purpose. GE’s engine validation of SLM made parts in a heavy duty GT was done with selected hot gas path components in a rainbow arrangement including turbine blades with SLM tip caps. Although SLM has major differences to conventional manufacturing the various challenges from design to engine ready parts have been successfully mastered. This has been confirmed after the completion of the test campaign in 2015. All disassembled SLM components were found in excellent condition. Subsequent assessments of the SLM parts including metallurgical investigations have confirmed the good part condition.

Fusion repair processes such as gas tungsten arc welding (GTAW) and laser welding have been introduced for repairing turbine parts made from Ni-based superalloy materials. The weld-repair of turbine parts is well established, however, the inherent susceptibility of γ' hardened superalloys to weld cracking remains an issue and has resulted in repair limitations for highly loaded areas of turbine parts. This study presents a view of the weldability of superalloys taking both, the impact of the weld process and the weld filler selection into consideration. This comprises the interpretation of specific process parameters into physical parameters controlling the weldability and cracking sensitivity such as thermal gradient in the weld pool and solidification speed. Alloy specific parameters of the weld filler material, such as melting point and solidification interval are studied and set in correlation with the solidification parameters during welding.

The re-opening of film cooling holes is currently one of the major challenges for the commercial repair of latest generation gas turbine noble parts. Protective coatings consumed in the course of a C-Interval must be replaced. An undesired side effect is that the new coating material plugs existing cooling channels. The use of reconditioned parts critically depends upon the capability to restore the original efficiency of the advanced cooling system. We present a novel process for that purpose, which has been developed at the ALSTOM Technology Center combining robotic vision and laser material processing. As a first step accurate information about cooling hole positions and orientation is obtained from a robotic vision system. This data is used to guide a short-pulse drilling laser in a subsequent machining operation. Typical overspray conditions for a great variety of film cooling geometries have been simulated and the results have been used to create a database of 3D overspray models. From these models a CAD/CAM postprocessor automatically generates individualized laser machining programs. Overspray coating material is then removed through an ablation process using a tailored 5+2 axes laser machining center equipped with an industrial q-switch Nd-YAG laser and a galvano-scanner. Airflow measurement results show that the new process is capable of meeting the cooling airflow requirements for heavily loaded hot-gas parts of the first gas turbine stage. This allows high quality reconditioning of film-cooled gas turbine components such as those in ALSTOM’s GT24/GT26 fleet.