Local and Global Mechanical Behavior and Microstructure of Ti6Al4V Parts Built Using Electron Beam Melting Technology
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A critical issue in additively manufactured parts is whether consistent mechanical properties and microstructure can be achieved. Ti6Al4V is a two-phase Ti alloy which constitutes a very important class of structural metals. The mechanical behavior of these materials can be significantly varied by tailoring their microstructure.[1] Many different types of microstructures can be obtained through a controlled cooling of this material.[2] Very slow cooling rate of this material will result in a phase globular in primary b phase grain boundaries.[2] Small increase in the cooling rate results in formation of a platelets in primary b phase grains. The length of these platelets depends on the cooling rate. As cooling rate increases, at a certain level, a basketweave type of microstructure forms. Quenching the material will result in the transformation of b into martensitic type a. Mechanical behavior of this material is strongly influenced by its microstructure.[3] For example, the existence of globular a phase or platelets enhances the resistance to micro-crack development. The ductility, yield strength (YS), and crack propagation resistance are improved as the size of colonies of platelets LEILA LADANI, Associate Professor, is with the Mechanical Engineering Department, University of Connecticut, Storrs, CT. Contact e-mail: [email protected] Manuscript submitted December 29, 2014. Article published online May 20, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
decreases.[4] Plastic deformation in Ti6Al4V material at room temperature was found to be caused by planar slip in the a grains, at quasi-static strain rates, while at high strain rates deformation twinning is also observed.[5] Traditional processes such casting, rolling, or annealing have been studied vastly and their effect on the microstructural formation and mechanical behavior is well understood. Electron and laser melting processes, however, are fairly new and more complicated than traditional techniques. There is less control over the process and many process parameters can affect the outcome. The fact that each layer is built with partial re-melting and solidification of the previous layer and heat extraction occurs in different manners for different geometries adds complexity to the process. Recent investigations of electron beam melting technology have shown wide variation of microstructure depending on the geometry. Study conducted by Antonysamy[4] showed favored growth orientation along h001i b normal to the build plate. It was also found that the grain orientation depends on the sample thickness. In specimens built with larger thickness, the grains were more oriented in h001i direction, whereas in smaller thickness more random orientation was observed. Change in mechanical properties relative to distance from build plate were evaluated in an experiment conducted at NIST.[3] Even though the EBM samples showed a measurable change in the thickness of a platelets as the distance from build plate increased, the VOLUME 46A, SEPTEMBER 2015—3835
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