Abstract
Wire–arc additive manufacturing (WAAM) is an emergent method for the production and repair of high value components. Introduction of plastic strain by inter-pass rolling has been shown to produce grain refinement and improve mechanical properties, however suitable quality control techniques are required to demonstrate the refinement non-destructively. This work proposes a method for rapid microstructural assessment of Ti–6Al–4V, with limited intervention, by measuring an acoustic wave generated on the surface of the specimens. Specifically, undeformed and rolled specimens have been analysed by spatially resolved acoustic spectroscopy (SRAS), allowing the efficacy of the rolling process to be observed in velocity maps. The work has three primary outcomes (i) differentiation of texture due to rolling force, (ii) understanding the acoustic wave velocity response in the textured material including the underlying crystallography, (iii) extraction of an additional build metric such as layer height from acoustic maps and further useful material information such as minimum stiffness direction. Variations in acoustic response due to grain refinement and crystallographic orientation have been explored. It has been found that the limited α-variants which develop within prior-β grains lead to distinctive acoustic slowness surfaces. This allowed prior-β grains to be resolved. A basic algorithm has been proposed for the automated measurement, which could be used for in-line closed loop control. The practicality and challenges of applying this approach in-line with fabrication are also discussed.
Highlights
Additive manufacturing (AM) promises to herald a new age in fabrication, by improving utilisation of raw materials and removing many of the design constraints found with traditional techniques
For a single grain the velocity varies with crystallographic orientation by ± 250 ms−1 for Ti–6Al–4V; the observed variation is lower as the average velocity is measured for the grain population underneath the patch
The fine lathe size found in the α-phase prevents direct recovery of the crystalline orientation, but several conclusions may still be drawn on the crystallography of the wire–arc additive manufacturing (WAAM) depositions
Summary
Additive manufacturing (AM) promises to herald a new age in fabrication, by improving utilisation of raw materials and removing many of the design constraints found with traditional techniques. In comparison to powder-bed fusion, WAAM is able to produce larger parts at significantly faster deposition rates, up to 10 kg/h compared to 50–200 g/h [3] These techniques generally cannot produce the intricate featuring seen in powder-bed fusion, making WAAM well suited to less complex, large build-volume components [2]. Burgers orientation relationship defines the transformation between the high temperature cubic β-phase and the low temperature hexagonal α-phase, given in Eq (1) This allows 12 α orientations to form within a prior-β grain [8]. Sections of the undeformed and 75 kN rolled specimen have been left unprepared so as to allow imaging in the as-deposited state to simulate measurements performed in an industrial environment during the manufacture of a component
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