In-Situ Microstructure Optimisation of Wire and Arc Additive Manufactured (WAAM) Aerospace Aluminium Alloys

Abstract

High deposition rate Wire and Arc Additive Manufacture (WAAM) can provide many benefits to the aerospace industry, including reduction in the manufacturing time for large scale parts, reducing material waste and, through high solidification rates, refinement of the as-built microstructure. However, the deposition of material in layers, involving repeated moving melt pool solidification, can lead to a complex process history and non-ideal heterogeneous microstructures that exhibit microsegregation, columnar grain structures and banded heat affected zones, resulting from the overlapping thermal fields of individual melt tracks. This work initially characterises WAAM cold metal transferred (CMT) produced 2xxx series aluminium alloys to assess their metallurgical suitability for use with the WAAM process. This involved modelling their hot crack susceptibility using the Rappaz-Drezet-Gremaud (RDG) criteria, followed by EBSD to analyse the as-deposited grain structures. Their strength and precipitation response was also investigated using SEM, hardness testing and DSC. These alloys were shown to be suitable for use with the WAAM process, with low crack susceptibility and a fine equiaxed grain structure formed on solidification, however the overall strength was poor when compared to a conventionally processed alloy, due to the lack of solute in solution leading to a poor precipitation response. These high strength alloys normally require solution treatment and age hardening to achieve maximum mechanical properties. Solution treatments of large scale components can be very difficult. Therefore, exploiting the thermal conditions in the WAAM process, with each layer being subject to a complex thermal history, is of interest as a potential route to avoiding post-build solution treatment. A dissolution model for the strengthening phases of interest has been developed, based on thermodynamic principles and diffusion controlled dissolution using DICTRA, which was validated through thermal simulations combined with 2D and 3D image analysis. Parametric changes to the model were explored in order to improve the dissolution response, through investigation of a potential inter-pass deformation step and a refined microstructure on cooling. The heat source used in the WAAM process was subsequently changed to AC-Plasma, with the aim to improve the as-deposited microstructure through greater heat input and microstructure evolution. Alloys produced using this process were characterised and showed a greater precipitation response and increased strength. The thermal history of a layer in an AC-plasma WAAM part was then measured and subsequently used to calibrate the dissolution model developed and determine the optimum heating cycle required to provide maximum in-situ solutionisation. The limits to the extent of in-situ solutionisation in a 2319 alloy were explored, with re-growth of the eutectic theta phase proving a barrier to complete in-situ solutionisation.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Prangnell (Supervisor) & Joseph Robson (Supervisor)