Image icon  Cold Gas Dynamic Spray (CGDS): formation of Ti-Al intermetallics

 
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Description: Cold gas dynamic spray, CGDS (or kinetic spray) is a rapidly developing coating and manufacturing technology in which metal particles are deposited at velocities of over 700 m/s without significant melting. A new method for fabricating intermetallics is being developed, where the mixture of powders is compacted using CGDS technique, before it is transformed into a intermetallic by a controlled thermal treatment using a conventional furnace. This image is a SEM (back scattered electron) micrograph showing the set of Ti-Al intermetallic phases after two-stage annealing of a Ti/Al deposit at 650°C (below the melting point of Al) for 6 hours and following holding at 850°C for 3 hours. The purpose of this two stage treatment was to avoid the formation of molten aluminium as this could lead to the component changing shape. The analysis shows that after this two-stage annealing all the phases (Ti3A, TiAl, r-TiAl2 and TiAl3) in the Ti-Al system are present. It also shows that compared to after the low-temperature stage, there was much less pure Ti and more r-TiAl2. There was also an increase in the desirable TiAl and Ti3Al phases after the high temperature stage, and this increased with time at temperature. The micrograph also shows quite an increased porosity, if to copmare to as-sprayed and low-temperature alloyed samples. A significant cause of porosity is the Kirkendall effect where voids are formed because of a large difference in the diffusivities of the diffusing species. In this system, where TiAl3 grows at the expense of Ti by Al diffusing through the TiAl3 to form new intermetallic at the TiAl3/Ti interface, then void formation due to the Kirkendall effect would be expected and may be a significant problem. The Ti/Al deposites are produced using a CGDS system, build in the Institute for Manufacturing, Department of Engineering, University of Cambridge.
Keywords: cold spray • CGDS • cold gas spray • coating • deposition • sintering • intermetallics • aluminium • titanium • powder metallurgy • micrsocopy • porosity • Kirkendall effect • SEM • image
Categories: Materials > Metals & alloys > Aluminium & alloys
Materials > Metals & alloys > Titanium & alloys
Processes > Shaping > Spraying
Processes > Shaping > Powder methods
Processes > Surface treatments > Surface coatings
Processes > Thermal treatments > Sintering
Testing, analysis & experimentation > Microscopy
Created by: Dr. Tatiana Novoselova, The University of Liverpool
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Date created: 30 June 2005
Date added: 17 May 2011
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Resource ID: 3643