4.1 Directed energy deposition (DED) is an additive manufacturing (AM) process, which focuses a thermal energy source to melt and fuse deposited materials. It is one of the seven main AM methods (ISO/ASTM 52900) and represents with powder bed fusion (PBF) one of the two main commercial processes for metal additive manufacture (MAM). 4.2 DED was one of the first AM processes, originating from the use of welders in the 1960s to produce three-dimensional welding, and became part of AM in the 1990s (1). Despite its long history, DED is not as well-known as many other AM processes. It was extensively developed for military equipment and consequently received little publicity. 4.3 However, it is gaining greater commercial interest as it has a number of advantages over other processes. Particularly, DED can be: 4.3.1 Attached to a multi-axis manipulator, which adds material at almost any angle and does not require a build chamber; 4.3.2 Used to repair or upgrade existing parts, remodel or replace corroded areas; and 4.3.3 Applied to very large structures. 4.4 The term DED covers a range of commercial terminologies and implementations, all of which deposit and melt a feedstock material on a build substrate where it solidifies. 4.5 However, while DED methods are commercially attractive due to the lower costs in comparison with other AM processes, there is a lack of knowledge of the mechanical and lifetime performances of the parts produced by DED. 4.6 Traditional manufacturing processes are well defined and understood. But, the increased complexity of AM processes leads to poor understanding. The relationship between process, geometry and microstructure of AM parts and the absence of predictive models make it difficult to validate the quality and integrity of the build. Accordingly, guaranteeing part mechanical performance is difficult (2). This gap in the assurance of quality of AM parts is a key technological barrier that prevents the widespread adoption of AM technologies (3). 4.7 AM is especially attractive for industries such as aerospace; with large, low volume, high-value parts in safety-critical applications. In these industries, the need for qualification and certification for requirements could not be greater. Therefore, in the absence of fully understood processes, the use of AM parts in such critical applications requires that parts are qualified by a detailed inspection (4, 5). 4.8 While industry is pushing for in-situ methods, there is a lack of in-situ monitoring capability. Current in-situ methods are only suitable for part surface defects while real-time monitoring has not been adequately demonstrated for DED (6, 7). 4.9 In the short to medium term, NDT has been repeatedly identified as critical to the success of additive manufacture by providing a means of validating the quality of the build and to gather data to increase knowledge and understanding, (3, 8, 9, 10). 4.10 Therefore, this guide focuses on the determination of the part quality through the use of NDT. It examines traditional, new and emerging techniques, identifying their suitability for use with DED. Geometric control and In-process sensing are not part of this document. 4.11 It makes recommendations for points in the process where specific NDT methods may be applied. These points in the process have been identified optimal for use of NDT, including, between deposition of layers and (intra-layer), after the build process, (post-build), and if necessary in post-processing (finished/post process) (5). 4.12 A subgroup of emerging NDT techniques which may have the greatest potential for future DED application is highlighted, with the aim that industry should adopt and accelerate their development (3, 11).
The title and scope are in draft form and are under development within this ASTM Committee.