Microstructure of Interpass Rolled Wire + Arc Additive Manufacturing Ti-6Al-4V Components
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ADDITIVE manufacturing (AM) is a fabrication technique in which a structure is created by depositing successive layers. This approach enables substantial material savings, compared to subtractive techniques such as machining: in the aerospace sector, buy-to-fly ratios (the ratio of the weight of the initial workpiece to the one of the finished part) can be as high as 30,[1] while in AM, this can be potentially reduced to around 1.5 or lower.[2] The high cost of titanium production and machining has been the main motivation for the large number of investigations into AM methods. Wire + arc additive manufacturing (WAAM) uses metal in the form of wire in combination with an arc,[3] and can be based on either Tungsten inert gas (TIG),[4,5] plasma,[2] or metal inert gas welding.[6] The microstructures of Ti-6Al-4V produced by WAAM can be either Widmansta¨tten or martensitic which depends not on the cooling rate of the first cycle after the material is deposited, but on the peak temperature and cooling rate when the subsequent layers are deposited.[2] The prior b grains are equiaxed near the substrate; however, further away epitaxial growth results in large columnar grains that traverse the deposited layers. These grains grow opposite to the heat flow and are highly textured, leading to anisotropic mechanical properties.[4] High-pressure rolling, originally developed for welding,[7] is a local mechanical stretching method in which a FILOMENO MARTINA, Research Fellow, PAUL A. COLEGROVE, Senior Lecturer, and STEWART W. WILLIAMS, Professor, are with the Welding Engineering and Laser Processing Centre, Cranfield University, Building 46, Bedfordshire MK43 0AL, U.K. Contact e-mail: f.martina@ cranfield.ac.uk JONATHAN MEYER, Research Engineer, is with Airbus Group Innovation, 20A1 Building, New Filton House, Bristol, BS99 7AR, U.K. Manuscript submitted October 22, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
load is applied with a moving roller. If the load is sufficient to compress plastically the bead in the normal direction, a plastic stretching will occur in the rolling direction, thus decreasing the longitudinal residual stresses.[8] When applied to steel WAAM structures, rolling resulted in a reduction of the grain size, due to the enhanced recrystallization that occurred with the deposition of the subsequent layer on the plastically deformed component.[9] Rolling of Ti-6Al-4V WAAM structures was initially presented,[10,11] and will be discussed in depth in this paper.
II.
EXPERIMENTS
The experiments were performed on a custom-made rolling rig, equipped with a Lincoln Electric Invertec V310-T AC/DC TIG power supply. A schematic view of the setup is shown in Figure 1(a) (the X, Y, and Z directions are defined in this figure). The parameters for the pulsed TIG process, which are presented in Table I, produced a wall width of 6 mm. Aerospace grade 5 Ti-6Al-4V welding wire was provided by VBC Group; its chemical composition was taken from the material certificate and is shown in Table II. A. Evaluation of Strain and Microstructu
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