Investigation of methods to manipulate geometry, microstructure and mechanical properties in titanium large scale Wire+Arc Additive Manufacturing

摘要

Wire+arc additive manufacturing is a technique suitable for the deposition of largecomponents; a variety of materials can be processed, including titanium.For the alloy Ti{6Al{4V, an experimental model based on design of experimentand linear regression was developed to control layer geometry during deposition.The modelled variables were wall width and layer height; the former was dependenton the heat input, and the latter on the heat input as well as on the wire feedspeed to travel speed ratio. Equations enabled the automatic selection of processparameters based on geometric requirements speci c to the part being built. Thiscould ensure minimisation of production time and material waste.Additively manufactured parts are a ected by distortion and residual stress; thee ect of high pressure rolling on these two, as well as on geometry, microstructureand mechanical properties was studied. Due to plastic deformation, rolled lineardeposits were characterised by a larger width and smaller height. The variability ofthe layer height was reduced, a bene cial e ect from a production implementationviewpoint.Distortion was less than half in rolled components, a change associated with thereduction in residual stress which were still tensile in the bottom of the parts andcompressive in their top; however their overall magnitude was smaller than in theunrolled samples. The contour method showed relatively good agreement with theneutron di raction measurements, and although destructive it proved to be a fastway to characterise residual stress in additively manufactured components.Microstructurally, the columnar prior grains con guration observed in all unrolleddeposits, also a ected by a strong texture developed in the building direction,was changed to equiaxed grains due to the recrystallisation triggered by both thestrain introduced by rolling and the repeated thermal cycles induced by each layerdeposition. The microstructure was overall considerably ner and the texture randomised.A fundamental study was performed to discern the extent of the deformedzone from the one a ected thermally. While the deformed zone could not be identied precisely, the thermally in uenced zone showed a relationship between rollingload and depth of the recrystallised volume.Finally, testing of hardness and tensile strength showed superior properties ofrolled specimens than in the unrolled specimens. The mechanical performance ofrolled samples was fully isotropic too.This project was entirely sponsored by Airbus Group Innovations (formerlyEADS Innovation Works).