A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 1 2 0 In recent years, additive manufacturing (AM) has begun to displace traditional manufacturing techniques for specific applications. Notable benefits of AM include reduced times from design to product, an improved buy-tofly ratio, lower waste, and the ability to produce complex geometries[1,2]. An additional benefit of additive manufacturing is the variety of manufacturing processes that span across heat source (e.g., laser, electron beam, plasma), input material type (e.g., powder, wire), atmosphere, and the number of axes of control among others[2-4]. This variability in processing route means that a process can be identified and optimized for a class of products or parts. Despite these various advantages, one of the primary drawbacks of AM processes is porosity within builds, which ultimately reduces the ability of a part to withstand tensile stresses and can lead to premature failure[4-6]. Electron beam melting (EBM) is a powder bed fusion technique that uses an electron beam as a heat source to melt powder particles that have been spread over a build plate[7]. Unlike laser-based processes, EBM requires the build chamber to be at vacuum, reducing the probability of porosity stemming from gases present within the build chamber[1]. Gas pores in EBM are thus typically caused by either gases present in the feedstock material (i.e., retained gas porosity) or vaporization of select elements (i.e., keyholing)[4,8]. Gas pores formed through either mechanism result in nearly spherical morphologies whose locations within the layer of a build and presence within a solidified part are influenced by the fluid dynamics of the melt pool[4,9,10]. The most common scan strategies of EBM are point-melting and variations on linear raster scan strategies, i.e., moving the electron beam in a linear fashion across the powder bed following a pattern[11]. Point-melting scan strategies, less commonly studied and used, involve point-by-point melting of small volumes of material of the powder bed. Research has shown that point-by-point melting strategies can be used for site-specific control of the resulting microstructure and texture by varying process parameters and the location and order of points melted, thereby leading to local and specific variations inmechanical properties[12,13]. OBSERVATIONS IN AS-BUILT SAMPLES Ti-6Al-4V specimens were produced at Oak Ridge National Laboratory Manufacturing Demonstration Facility using an ARCAM EBM Q10plus system and TEKNA Ti-6Al-4V plasma atomized powder. Each specimen had a geometry of 15 x 15 x 25 mm. Three different scan strategies were used to produce the samples: a linear raster scan (L), random point-melting (R), and what is known as the Dehoff point-melting strategy (D). The raster scan L consisted of a serpentine pattern that rotated 67.5° after the end of every layer. Each 15 x 15mmarea of R and D was segmented into coordinates. A computer-generated random DIFFERENCES IN DEFECT DISTRIBUTION ACROSS SCAN STRATEGIES IN ELECTRON BEAM AM Ti-6Al-4V The fraction and size of pores present in EBM Ti-6Al-4V specimens varies depending on the melting strategy used, whether linear raster melting or point melting. Maria J. Quintana, Iowa State University, Ames, Universidad Panamericana, Mexico, and Center for Advanced Non-Ferrous Structural Alloys, an NSF I/UCRC Katie O’Donnell, Iowa State University, Ames, and Center for Advanced Non-Ferrous Structural Alloys, an NSF I/UCRC Matthew J. Kenney, Iowa State University, Ames Peter C. Collins, Iowa State University, Ames, Center for Advanced Non-Ferrous Structural Alloys, an NSF I/UCRC, and Ames Laboratory, Iowa
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