The influence of glass coating on the forging of nickel-based superalloys

Busuttil, Markus (2014). The influence of glass coating on the forging of nickel-based superalloys. University of Birmingham. Ph.D.

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Abstract

Forging of components for gas turbine application from nickel-based superalloys remains a challenge, due to the exceptional high temperature properties of these alloys. In order to enhance formability and prevent oxidation at high temperature forging, a glass coating is applied to protect the alloy. However, the contribution and significance of glass coating to the material flow during forging is not fully understood. In addition, increased competitive pressure requires a better understanding of the root cause for geometrical part variability in the forging of aerospace components. Particularly, reduction of manual rework is highly desirable, as it increases the manufacturing costs of these components significantly. The aim of this work was to identify the Key Process Variables (KPVs) of the forging of aerofoils. Therefore, numerical modelling of a full factorial design of experiment was conducted to study the significance of various process parameters. The contribution of glass coating as a process variable has been studied experimentally. The double cup extrusion test has been employed to study the influence of glass coating thickness and chemistry in hot forging condition. Ring compression tests have been conducted to investigate the contribution of graphite lubricant. It was found, that in the presence of graphite lubricant friction reduces markedly. However, in order to establish a stable lubrication system the fusion behaviour of glass coating becomes crucial. In the absence of graphite lubricant, friction increases with increased glass coating thickness. Numerical modelling revealed that the interfacial heat transfer coefficient increases with a reduction of glass coating thickness. A good agreement between experimentally and numerically derived interfacial heat transfer coefficient has been found. In contrast, in a rapid deformation process such as blade forging, the effect of heat loss becomes less important compared to the contribution of friction. The initial variability of glass coating thickness has been characterised in the industrial forging of Inconel 718 aerofoils. Glass coating thickness varies considerably across the workpiece and the batch. This variability, alongside the variability of manual process parameters such as transfer and resting time, have a notable effect on the forming temperature and thus on the resulting aerofoil geometry.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):