Cold Spray Deposition of Precipitation-Hardenable Aluminium Alloys for Structural Repairs

Magnesium alloys are widely used in the aerospace industry for the good strength to weight ratio; however, the Mg components can suffer from significant damage in service due to corrosion. Over the past ten years, significant research investment has led to the cold spray technology being approved for use as a repair technology for the dimensional restoration of components suffering manufacturing defects, machining error, wear and corrosion. This enables cost savings on complex and expensive Mg components, allowing the aerospace industry and defence force to operate more efficiently. It also has an environmental benefit by reducing the use of energy and raw materials to manufacture replacement parts. The current repair technology using cold spray is, however, mainly cosmetic, as the cold sprayed deposits are only load-bearing in a very limited sense and consist of pure Al and non-hardenable Al alloys.

The challenge now is to develop a similar repair technology using precipitation-hardenable (6xxx and 7xxx series) Al alloys. These alloys are much more challenging to deposit by cold spray (due to their lower ductility) and they are likely to require some form of post-deposition treatment to improve their density, adhesion and mechanical strength. This is a new area of study with little previous research and a considerable lack of fundamental understanding. Examples of possible post-deposition treatments that could be considered include heat treatment, electron beam melting, laser surface melting and friction stir processing. Cold gas dynamic spray is an emerging coating deposition technology in which a fine metallic powder (typically 5-25 pm) is injected into a specially designed convergent-divergent nozzle where it is accelerated to 300 to 1200 m/s towards the substrate to be coated. A high pressure helium or heated nitrogen gas is used as an accelerating gas and the particles upon impact with the substrate deform plastically. The bonding occurs at solid-state and below the melting temperature of the material. This is a promising technology for depositing oxygen sensitive materials with very high deposition efficiency (> 95%). A coating is built up by scanning the gun across the surface in a number of passes allowing the powder particles to impact a prepared surface and create a coating ca. 100 p.m to several tens of millimetres thick. As well as surface engineering cold spray is a promising new technique for near net shape manufacturing.

Thus, the overall aim of this project is to use cold spray to develop precipitation hardenable Al alloys for structural repair applications with low porosity, good yield strength, high fatigue life and coatings which are well bonded to the Al Mg substrate. The proposed research has five main objectives:

i. To investigate the fundamental characteristics of the Al 6xxx and Al 7xxx feedstock powder and optimise the particle size range distributions for the cold spray process.
ii. To manufacture Al alloy deposits with the new state-of-the-art cold spray system onto All Mg alloys (using design of experiment studies) to establish the spray parameter window.
iii. To select a small number of samples for detailed investigation of how cold spray process parameters affect coating microstructure (scanning electron microscopy), porosity (mercury intrusion porosimetry) and adhesion (pull-off bond strength test).
iv. To investigate the mechanical properties of the coating using tubular coating tensile tests (for ultimate tensile strength of the deposits) and uniaxial fatigue tests.
v. To enhance the mechanical properties of the deposits by post-deposition heat treatment route to consolidate porosity and improve inter-particle bonding. It is envisaged that the electron beam and laser surface melting at UoN and friction stir processing at TWI will be used as routes to improve mechanical properties of the deposits.