Modelling and experimental study of the spray forming of dissimilar metals

• Main Author: Lee, Tung Lik
• Other Authors: Mi, Jiawei
• Published: University of Hull 2015
• Subjects: 667; Engineering
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.691376

This research describes a systematic study to develop a strong interfacial bond in as-sprayed dissimilar metallic alloys. Non-destructive three-dimensional microstructure and residual stress characterisations via X-ray micro-computed tomography and neutron diffraction, respectively and numerical modelling of the preform shape evolution, transient heat flow and thermal residual stress developed were employed. The study revealed quantitatively the links between the key spray forming parameters, corresponding microstructures formed and the interfacial bonding characteristics in thick as-sprayed dissimilar metallic alloys. The key novelties of this research include: - The development of a preform shape and heat flow model that incorporates: (1) the use of a mesh deformation method with automated re-meshing algorithm to model the growing preforms and address the coupling of droplet mass/enthalpy input at the deposition surface and (2) a substrate induction preheating model. - Validated against experimental measurements, the preform heat flow model was used to establish the correlations between the preform thermal history and microstructures formed. The correlations established based on the tomography and diffraction measurements showed their interrelationship and agreements with one another. The correlations also provided the crucial links to take into account the effects of the microstructure formed on the corresponding material properties and stresses developed in the preform after cooling. - The combined use of thermal residual stress modelling and neutron diffraction in this research presents, for the first time, the nondestructive quantitative assessment of the interfacial bonding in thick as-sprayed dissimilar steels preforms. The effects of different substrate and spray temperatures on the microstructures, residual stress and interfacial bond developed were investigated systematically. - The dynamics of the atomised droplets rapid microstructural change during deposition were revealed when subjected to rapid Joule heating (~500 K s-1) to a range of isothermal temperatures in the vicinity of the precipitate solvus and alloy solidus temperatures with a short (~10 s) high temperature holding time. A finite element model of the transient heat flow in the powder compact was developed to provide more quantitative information of the specimen internal temperature distribution which was otherwise unavailable and the rapid microstructural change in the powders was rationalised in terms of the transient temperature conditions. These results can be useful in reconciling thermal histories and microstructures in the as-sprayed preforms, and may guide the optimisation of the spray forming process if desirable microstructural features are to be preserved into the bulk preform. - The elastic and plastic deformation behaviours of the spray formed steels containing different levels of porosity and the integrity of the bonded interface of the spray formed dissimilar steels were studied in situ via three-point bend tests with neutron diffraction. The study revealed the role of the constituent phases in the as-sprayed high speed steel when subjected to stresses above the elastic limit and the characteristics of the interfacial bond formed in comparison to the non-destructive assessment carried out in this research.