β-Grain refinement in WAAM Ti-6Al-4 V processed with inter-pass ultrasonic impact peening

As-deposited Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V parts typically contain large columnar β-grains on a centimetre scale, with a strong 〈001〉 fibre texture, leading to anisotropic mechanical properties and unacceptable scatter in damage tolerance. Inter-pass deformation, introduced by the application of Ultrasonic Impact Peening (UIP) across each added layer, has been shown to be effective in refining the β-grain structure and achieving a weaker texture. The depth of deformation and the grain refinement mechanism induced by UIP have been investigated by combining advanced electron backscatter diffraction (EBSD) characterization with a ‘stop action’ observation technique. UIP facilitates a similar refinement mechanism and nearly the same depth of deformation as conventional machine hammer peening, with the advantages of a much higher strain rate, lower peak force, and two orders of magnitude lower impact energy, making it a faster and more economical process. β recrystallization is seen within the deformation zone during re-heating through the α → β transition. Although new recrystallized β-grains formed in the UIP surface-deformed layer to a shallower depth than that of remelting, recrystallization initiated ahead of the melt pool and the recrystallized grains grew downwards to a greater depth before remelting. These refined grains were thus able to survive and act as nucleation sites at the fusion boundary for epitaxial regrowth during solidification, greatly refining the grain structure.The authors are grateful to the EPSRC programme grants NEWAM (EP/R027218/1) for funding of the research, as well as to LightForm (EP/R001715/1) and facilities provide by the Henry Royce Institute for Advanced Materials, through Royce equipment grant (EP/P025021/1), for supporting aspects of this work. P.B. Prangnell is grateful to the Royal Academy of Engineering, UK, and Airbus for financial support.Materiali

Deformation and microstructural development in a 2124Al/SiCpMMC during high strain rate superplasticity

Both superplastic deformation and the accompanying microstructural development in st 2124Al/18vol%SiCp, metal matrix composite have been investigated. Mechanical property results comparable to those found in similar materials have been achieved, the optimum superplastic elongation being 430% at a strain rate of almost 0.1s(-1) . The anomalously high activation energy which has been observed in other studies was also reproduced. Differential scanning calorimetry showed that the optimum temperature in the current material was significantly below the temperature at which melting began, however, in contrast to some previous work. Microstructural investigation showed that both the optimum elongation, and the onset of the high activation energy regime, coincided with the temperature above which the majority of the intermetallic particles present at lower temperatures started to dissolve. It is concluded that the high apparent activation energy is an artefact caused by microstructural changes. The deformation and microstructural changes observed are consistent with grain boundary sliding accommodated by slip in the aluminium matrix. It is suggested that this type of deformation may occur by groups of grains surrounding SiC particles acting as a single deformation unit