Additive Manufacturing of Ti-6Al-4V Using a Pulsed Laser Beam
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INTRODUCTION
ADDITIVE manufacturing (AM) of metal components has gained much attention in recent years, with metal-based AM machine sales increasing by 75 pct from 2012 to 2013.[1] This can, in part, be attributed to the ability to rapidly produce high-value components directly from a digital CAD model. Metal components are commonly built using one of two methods: powderbed fusion or directed-energy deposition.[2] In the former, a laser or electron beam is scanned over a bed of powder; while in the latter, powder is blown or wire is fed into the melt pool formed by a laser or electron beam. In both cases, parts are built up layer-by-layer. Though processing itself is straight-forward, the resulting internal part microstructure can be complex and non-uniform. Parts produced using AM undergo multiple, complex thermal cycles which can result in internal variations in microstructure and properties. Such intra-build variations have been observed to depend on part orientation, part size, and the scanning pattern used for part build-up. Intra-build variations have been witnessed in both powder-bed and directedenergy processes. Of particular interest is the study of intra-build variations in AM-produced titanium alloy titanium6 pct aluminum-4 pct vanadium (Ti-6Al-4V) components. This alloy, also known as Grade 5 titanium, is favored in biomedical, aerospace, and defense application, due to its non-toxicity, low weight, high strengthto-density ratio, and low creep at high temperatures.
ABDALLA R. NASSAR, Research Associate, and EDWARD W. REUTZEL, Head of Laser System Engineering & Integration Department, are with the Applied Research Laboratory, The Pennsylvania State University, University Park, PA 16804. Contact e-mail: [email protected] Manuscript submitted October 9, 2014. Article published online March 13, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
Within Ti-6Al-4V, microstructural variation is characterized by the size, shape, orientation, and ratio of the aand b-phases, prior-beta grain structure, as well as the existence of the diffusionlessly transformed a0 phase. Details of microstructural features can be correlated to the mechanical properties of Ti-6Al-4V.[3] Typically, the microstructure of additively manufactured Ti-6Al-4V components consists of Widmansta¨tten a-b, with b present between narrow a-laths, and large, columnar, prior-beta grains extending from the bottom to the top of builds.[4] A. Intra-build Variations in AM of Ti-6Al-4V Intra-build variations, such as a transition from a Widmansta¨tten to a martensitic a¢ microstructure, have been observed to occur near the top surface of parts produced using the Arcam, powder-bed, electron-beammelting (EBM) process.[4] Also using EBM, Murr et al.[5] reported a 1.5 to 2 times increase in a-lath width from the bottom to the top of a 6.8-cm tall, cylindrical build—measurements were made 1 cm from the bottom and 1 cm from the top of the build. Alpha-lath widths have also been reported to vary from interior to exterior build locations along the same build height
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