Developing High Performance Aluminium Alloy Tubes for Heat Exchange Applications

This dissertation presents the research work aimed at developing high performance aluminium alloy tubes for heat exchange applications. The work, in particular, focused on the manufacturing technique development and optimization, together with characterization studies of produced prototypes. The research committed to the understanding of the close relationship occurring between final properties and manufacturing route, together with finished products’ corrosion performances.

The products, centre of this entire work, were high frequency welded aluminium tubes, comprising of different embossed inner grooved pattern designs and manufactured by using precursor bi-alloy strips, designed to provide both corrosion and heat transfer performances enhancement. Such technology is of particular interest for the application in round mechanically expanded tube-fin coupled heat exchangers.

Key advantages are yielded by the employment of an all-aluminium exchanger solution. Performance improvements in terms of corrosion resistance and heat transfer, in both condensation and evaporation regimes, are given respectively by the novel material choice and advanced inner surface patterning. The industrial PhD project work thus comprised of process development work, including process parameter studies (involving i.e. on-the-field testing and finite elements simulations); tooling design, manufacturing and testing; design, manufacturing and installation of novel devices/inventions aimed at process stability and control improvement.

In parallel, research work supported the previously mentioned activities for deeper understanding of product properties to process relationship, and structure-property correlation of materials. Therefore, two parts of this PhD project were: • The development work carried out for achieving a stable manufacturing process, able to deliver desired finished tubes, within narrow window requirements in terms of final geometry, mechanical properties, inner contamination, corrosion performance; • The research carried out on the structure-property correlation of produced tubes in terms of morphology, material flow behaviour, and microstructure to manufacturing technique and characterizing corrosion performance, in different environments and of tubes’ both inner and outer surfaces.

The wide range of analysis and tests performed varied, for example light optical microscopy of samples treated with different preparation techniques (i.e. standard mechanical polishing, surface electropolishing, anodizing, etching) and with/without application of different light filters (i.e. lambda light filter), inspection modes (i.e. differential interference contrast).

More advanced nspection techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also employed. Focused ion beam (FIB) was a technique used for the preparation of both SEM and TEM samples. Elemental characterization of different samples was carried out by energy dispersive X-ray spectroscopy (EDS).

Electron back scatter diffraction (EBSD) allowed grain structure analysis. Both acidified synthetic sea water testing (SWAAT) and customized special setups were used for corrosion testing of tubes, in different laboratory simulated environmental conditions. X-ray micro computed tomography was extensively used for three-dimensional, high resolution, non-destructive characterization of specimens.

Additionally, detailed electrochemical measurements were performed (i.e. potentiodynamic anodic polarization, zero resistance ammetry) on different materials and in different electrolytes. Overall, the project yielded major key innovations, both in terms of manufacturing technology upgrade and of process - materials – performances relationship understanding.

Key achievements of the research and development work were: • Definition of embossing tooling design and manufacturing guidelines, able to optimally handle and ensure best behaviour of ma terials along the process stages, delivering final optimal tube/pattern geometry and ensuring optimal welding; • Increased understanding of the high frequency welding process dynamics and development of a new methodology (novel tooling), able to reduce inner contamination at no welding quality expense; • Design, manufacturing and installation of engineering solutions able to critically increase and/or stabilize quality of products and manufacturing process; • Increased understanding of materials behaviour, in terms of general morphology, mechanical modifications and microstructural transformations along different stages of the manufacturing route; • In depth characterization of both outer and inner surface of tubes, in terms of corrosion behaviour, highlighting key issues and interesting practical implications, respectively for resistance of tubes in contact with highly alkaline substances, compared to standard neutral/acid environmental attacks, and for what regards corrosion behaviour of the embossed inner tube surface.

An overview of the development activities carried out in this industrial PhD project is provided in chapter 5. The core research work contained in this thesis is divided into 3 main chapters (6, 7 and 8) that present the scientific results of analysis on inner grooved high frequency welded aluminium tubes.

Chapter 9 illustrates the outcome of the PhD, in terms of both scientific findings and correlated industrial development achievements.