Eradicating hydrofluoric acid from metal etching

Lead Research Organisation: University of Nottingham
Department Name: Faculty of Engineering

Abstract

The ways in which manufacturers prepare the surfaces of metal high-value parts has changed little over the last 50 years, reliant on bath-based chemical etching in large volumes of corrosive and often toxic acids, followed by manual visual inspection for large defects. This creates large amounts of waste that is difficult to handle, while overly generous safety factors are applied to finished parts as currently not all present defects can be identified and quantified in production line processes. This represents inefficiency firstly in the generation of large volumes of waste, notwithstanding the potential for environmental and user exposure to corrosive and toxic chemicals. Secondly, inefficiency arises from the part lifetimes being significantly shorter and loading conditions lighter than possible, which increases the maintenance intervention.

We will create a new set of processes that can etch the same materials by applying targeted electrically charged fluid jets using small volumes of non-hazardous chemicals (electrolytes). These electrolytes will be recirculated and an automatic health monitoring system will be created to ensure appropriate etching response is maintained at the part surface, and ensure resource efficiency. Waste electrolyte will be filtered and metal-containing debris will be returned to the manufacturing supply chain, significantly reducing the volumes of chemicals, and ensuring that the chemicals applied present much reduced risk to the environment and the end user.

We will observe generated surfaces during and following etching, and by quantifying aspects of the surface, we will gain a valuable insight into the material and the success of the preceding manufacturing process. The information we will acquire relates to the arrangement of the material (the microstructure) and the presence of surface defects. Both of these can affect the quality of the final part and currently, parts with large defects or inappropriate microstructures are often scrapped, often after expensive manufacturing processes. We will create decision-making systems, using automation approaches with our partners to identify and quantify microstructures, and defect sizes and their occurrence over the part surface using fast techniques that can be applied in the factory. From these characteristics, machine led decision making will drive the processing strategy. Information extracted from this step will be fed back to the electrochemical jet system to enable targeted reworking. Here, surface defects can be removed by selective electrochemical etching, by supplying defect locations and sizes on a given part back to the new etching system. This will be performed using an automated etching equipped robot and will exploit the electrochemical jet technique's capability to perform different modes of material removal and create different surface responses.

By identifying, quantifying and removing defects within parts in one single manufacturing step, it is expected that the system will be capable of significantly reducing scrappage rates of high-value parts at a relatively low cost point and on timescales demanded by manufacturers. Furthermore, information pertaining to defects and microstructures will be extracted and provide information to both the manufacturer and the operator, optimising efficiency by allowing part-specific lifetimes and loading conditions. Hence this project with deliver against a sustainability objective while providing a game changing technology for high value manufacturers.

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