TWI Industrial Member Report Summary 995/2011
By T Marrocco
Although well-established for a wide range of corrosion, wear and thermal barrier applications, the limitations of thermal spraying processes in terms of coating characteristics and properties are well-documented (Davis, 2004). It is the nature of processes powered by oxy-fuel combustion and electric arc that the coatings produced are characterised by voids, oxidation, weak mechanical bonding and highly stressed, non-equilibrium microstructures. As a result, coating properties are inferior to those of the bulk material of the same nominal composition as the consumable feedstock. The development of the cold spraying process, where solid particles are propelled by inert gases at temperatures below their melting points, has significantly the nature of the coating deposited, since the spraying consumable is not oxidised during spraying and is not subject to rapid changes of state, volume and phase.
The advent of cold spray technology has, therefore, facilitated the deposition of thermally sensitive materials such as titanium (Ti) for the manufacture of spray-formed products, such as seamless Ti tubing (Blose, 2005). Significant opportunities also exist in manufacturing and repair of Ti and its alloys, where current manufacturing methods such as forging and machining are expensive, time consuming and produce much waste. For these reasons, there is interest in spray-forming near net shape (NNS) Ti components. The potential for applying additive features to Ti components employing a post-deposition annealing heat treatment and a hot isostatic press (HIP) treatment to reduce the deposit porosity has also been demonstrated (Blose, 2006).
A further reason for the considerable interest in cold spray technology is the inability of the thermal spraying processes to produce as-deposited coatings of noble materials such as Ti with the corrosion resistance of the parent material (Ishikawa, 1999). This gap in technology has been addressed by finishing techniques such as machining, peening, sealing and laser surface treatment. It is clear that a wider industrial opportunity exists if corrosion resistant coatings can be produced without post-deposition operations. To date, with the possible exception of cold sprayed tantalum (Zimmermann, 2006), fully dense coatings have not been achievable with either thermal or cold spray processes.
The aim of this programme of work was to advance the state-of-the-art by investigating the potential of the cold spray process to produce very thick Ti and Ti alloy deposits, appropriate to generic applications such as repair and spray-forming, and its potential for producing corrosion resistant Ti coatings, with and without a range of post-deposition treatment.
The main objectives of this programme of work were:
- To evaluate the characteristics and properties of cold spray formed Ti and Ti-6Al-4V.
- To evaluate the effect of post-deposition treatment on the mechanical properties of cold sprayed Ti and Ti-6Al-4V, and the corrosion behaviour of Ti cold spray coatings.