Structural integrity assessment of cold spray additive manufactured titanium alloy Ti-6-Al-4V

  • Dibakor Boruah

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Cold spray (CS) is an increasingly popular solid-state additive material deposition technique, which has gained special attention as a potentially feasible tool for repairing high-value structural components, as protective coatings, and more recently for additive manufacturing (AM) applications. Although CS of titanium alloy Ti-6Al-4V (Ti64) alloy received significant attention by researchers globally, there are very limited or no data available in the open literature on the understanding of residual stresses, static mechanical strength, fatigue and fracture properties of CS Ti64. However, structural integrity is imperative to safely operate CS repaired/manufactured load-bearing structural components. This study aims to provide a better understanding of the structural integrity performance of CS deposited Ti64 alloy for repair and/or AM applications. The primary objectives are: (1) to understand the residual stress development mechanisms using experimental measurements and analytical modelling; (2) a parametric study on the interfacial coating-substrate adhesion strength using an adhesive-free test method to overcome the limitation of existing test methods; (3) to study the effect of different post-deposition thermal treatments (PdTTs) such as solution treatment and ageing (STA), hot isostatic pressing (HIP), and HIP+STA on the microstructure and mechanical properties of fully CS Ti64 and CS Ti64 repairs (deposit-substrate assembly); and (4) to characterise Ti64 alloy deposits produced by laser-assisted cold spray (LACS). Firstly, the through-thickness distribution of residual stresses was measured experimentally using the neutron diffraction (ENGIN-X, ISIS, and KOWARI, ANSTO) and the contour method. Residual stresses were found to be quenching dominant, high tensile residual stresses were found near the free surface of CS deposits (reaching 175±88 MPa from the contour measurements) and towards the bottom of the substrate, and compressive near the interface region. Residual stresses were found to be lower for one or more of the following cases: fewer deposited layers, lower layer thickness (i.e. at higher scanning speed), higher substrate thickness, and using the cross-hatch toolpath pattern. A good agreement has been achieved among the experimentally measured stress distributions and predictions by the proposed analytical model. No significant variation in the process-induced residual stresses was observed as a result of the machining process; the STA treatment resulted in the full relaxation of residual stresses. Secondly, an adhesive-free Collar-Pin Pull-off (CPP) test method was developed for evaluating interfacial adhesion strength. A parametric study was performed to see the influence of coating thickness, scanning speed, toolpath pattern, track spacing, and substrate surface preparation on the adhesion strength. The proposed CPP test method was found to be capable of measuring adhesion strength beyond the upper limit (70-90 MPa) of conventional adhesive based methods such as ASTM C633. A higher value of adhesion strength was measured for specimens with ground substrates, and when deposited using cross-hatch toolpath pattern. However, numerical modelling showed non-uniform stress distribution at the test area (Pin-Coating interface), which may lead to premature failure resulting in underestimation of the true adhesion strength, particularly for thinner coatings. The STA treatment significantly improved the adhesion strength from 122 MPa to >766 MPa. Thirdly, the effect of three different PdTTs (STA, HIP, and HIP+STA) was investigated in terms of microstructure, porosity, hardness, tensile properties, fracture toughness, high cycle fatigue, and fatigue crack growth rate. PdTTs significantly improved ultimate tensile strength from ~290 MPa to ~890 MPa, and elastic modulus from 81 GPa to 110 GPa for fully CS Ti64. However, it was not achievable to reduce process-induced defects/porosities close-to-zero (minimum ~1.8 % after HIP), and hence no significant improvement in elongation was observed. Therefore, fracture toughness and fatigue performance of CS Ti64 was found to be much lower when compared to mill-annealed Ti64. Finally, characterisation of LACS Ti64 deposits was carried out in terms of deposition efficiency (DE), surface profilometry, microstructure, hardness and residual stresses. DE of LACS process was found to be ~35% lower than the standard CS process. Laser-assisted regions of the CS deposits were found to be more compact, with porosity reaching ~0.37% for LACS single-layer coatings. Residual stresses in the LACS coatings were found to be tensile, maximum stresses (124-191 MPa) were measured for the specimen deposited with 3 mm track spacing and minimum values were recorded (31-40 MPa) for the specimen with lowest (1 mm) track spacing.
Date of AwardSep 2020
Original languageEnglish
Awarding Institution
  • Coventry University
SponsorsLloyd’s Register Foundation
SupervisorXiang Zhang (Supervisor), Abdul Syed (Supervisor) & Matthew Doré (Supervisor)

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