Non-symmetrical design of coaxial nozzle for minimal gas consumption on L-DED process for Ti6Al4V reactive alloy

Abstract

Laser Directed Energy Deposition (L-DED) provides the possibility to manufacture and repair high-added-value components for sectors such as the aeronautical or medical. In these sectors, reactive materials, among which Titanium alloys excel, are widely employed. These materials pose a challenge to their manufacture via L-DED due to the need for a protective atmosphere to minimize the risk of oxidation and to ensure the metallurgical quality of the deposited material. The generation of an appropriate protective atmosphere, especially when big parts are to be manufactured or repaired, involves either high gas consumption or long processing times to guarantee a vacuum atmosphere. Alternatively, local protective atmospheres, which focus only on the working area, can be implemented. However, these systems do not always ensure the required protection level or present a high gas consumption. Therefore, in the present research, a non-symmetrical protective nozzle has been developed and validated, to provide a more efficient method for the generation of a local protective atmosphere for the L-DED of reactive materials. This configuration enables the concentration of the protective gas flow directly where required, and it behaves similarly to full protective and symmetrical nozzles but reduces the gas consumption by 75%. Based on CFD simulations, the influence of employing a non-symmetrical protective atmosphere on the oxygen level at the nozzle exit is evaluated and the impact of the proposed configuration in the powder distribution is also studied. In addition, the two main factors limiting the feed rate during the L-DED of reactive materials have been studied: the process mass efficiency and the oxidation of the component. The obtained results for the proposed non-symmetrical protective nozzle are compared with the full protective symmetrical and the non-protective nozzle designs, and the three configurations are experimentally tested to guarantee the validity of the CFD simulations. The non-symmetrical nozzle offers the possibility of increasing the process velocity by 62% with regard to the non-protective situation; similarly to the full protective, but at a quarter gas consumption.