Microstructures and Mechanical Properties of Ti6Al4V Parts Fabricated by Selective Laser Melting and Electron Beam Melting H.K. Rafi, N.V. Karthik, Haijun Gong, Thomas L. Starr, and Brent E. Stucker (Submitted March 1, 2013; in revised form May 23, 2013; published online August 6, 2013) This work compares two metal additive manufacturing processes, selective laser melting (SLM) and elec- tron beam melting (EBM), based on microstructural and mechanical property evaluation of Ti6Al4V parts produced by these two processes. Tensile and fatigue bars conforming to ASTM standards were fabricated using Ti6Al4V ELI grade material. Microstructural evolution was studied using optical and scanning electron microscopy. Tensile and fatigue tests were carried out to understand mechanical properties and to correlate them with the corresponding microstructure. The results show differences in microstructural evolution between SLM and EBM processed Ti6Al4V and their influence on mechanical properties. The microstructure of SLM processed parts were composed of an a¢ martensitic phase, whereas the EBM processed parts contain primarily a and a small amount of b phase. Consequently, there are differences in tensile and fatigue properties between SLM- and EBM-produced Ti6Al4V parts. The differences are related to the cooling rates experienced as a consequence of the processing conditions associated with SLM and EBM processes. Keywords EBM, fatigue testing, microstructure, SLM, tensile testing 1. Introduction Selective laser melting (SLM) and electron beam melting (EBM) are two powder-bed fusion-based additive manufactur- ing processes used to fabricate metallic parts (Ref 1, 2). These processes are of interest due to several advantages over conventional manufacturing methods. Freedom to fabricate intricate geometries, optimum material usage, elimination of expensive tooling etc. are some of the notable advantages of additive manufacturing processes. In these processes the CAD model of the part is fed to the machine where pre-processing software slices the model into layers of finite thickness. A powder layer is deposited on to a base plate above the build platform. A focused laser/electron beam scans the powder-bed- based on the sliced CAD data. The scanning results in localized melting and solidification of the powder to form a layer of the part. Subsequent layers are built one over the other by lowering the build platform equivalent to the layer thickness until the part is completed. Selective laser melting utilizes a fiber laser heat source. The four main parameters in SLM are laser power, scan speed, hatch spacing, and layer thickness. Generally, the process is characterized by high scanning speeds and high thermal gradi- ents, leading to high cooling rates. High cooling rates result in non-equilibrium microstructures which may require heat treat- ment for certain applications. The SLM build chamber is continuously flushed with inert gas to reduce oxygen level. Typical layer thickness lies in the range of 20-100 lm. SLM is capable of processing standard materials like Ti6Al4V, 316L, 17-4PH, 15-5PH, hot work steels, cobalt-based and nickel-based alloys (Ref 3) and more. A description of SLM processes has been detailed elsewhere (Ref 4). Arcam EBM technology uses an electron beam to melt powder layer. Electron beam-powder interactions are substantially differ- ent than laser-powder interactions. The penetration depth of an electron beam into the irradiated material is multiple times greater than it is with a laser beam (Ref 5). When the high speed electron beam interacts with the powder layer, kinetic energy is converted into thermal energy, causing the powder to melt. The build chamber is kept at an elevated temperature (approx. 700 °C) in a vacuum environment. Elevated temperatures help minimize thermally induced residual stresses and the formation of non- equilibrium microstructures. The high intensity electron beam first preheats the powder at a very high scan speed, large focal spot, and low beam current. Preheating of the powder can help lower moisture content and thus reduce the possibility of oxygen pickup. More importantly preheating can reduce residual stress buildup by bringing down the temperature-gradient between successive layers during processing. The preheating stage is followed by a melting stage where the electron beam scans the powder at a lower scan speed, smaller spot size, and higher beam current. Once the build is completed the part is allowed to cool slowly from 700 °C to room temperature. Due to the higher beam intensities and scan available with electron beams, the EBM process is much faster than the SLM process. A description of EBM processes has been detailed elsewhere (Ref 6). H.K. Rafi, N.V. Karthik, Haijun Gong, Thomas L. Starr, and Brent E. Stucker, Department of Industrial Engineering, JB Speed School of Engineering, University of Louisville, Louisville, KY 40292. Contact e-mails: khalidrafi@gmail.com and brent.stucker@ louisville.edu. JMEPEG (2013) 22:3872–3883 ÓASM International DOI: 10.1007/s11665-013-0658-0 1059-9495/$19.00 3872—Volume 22(12) December 2013 Journal of Materials Engineering and Performance