WP4: Optimisation of powder process manufacturing

Lead by: LPW
Participating partners: VN, TWI, NAU KHAI, AVIO, EADS
Duration: October 2013 – February 2016 (M05-M33)

Overview: Challenges and current SoA

Powder requirements for CS are not currently well defined or optimized. WP4 will produce a set of specifications which will accurately define the requirements of CS and this will be used to optimize the production route to maximize performance and minimize environmental impact by reducing waste from production.

Main Objectives

WP4 will define a characterisation protocol for the feedstock material to be used in Cold Spray repair techniques. The powder characteristics requirements will be examined and an evaluation of current powder atomisation/spheridisation and post-processing techniques will be realized. The powder atomisation/spheridisation production process is expected to be optimized by taking into account the powder characteristics and the deposited coatings properties. The same target is posed on the powder post-processing techniques, taking into account the powder characteristics and the deposited coatings. The handling requirements of the powders will be tested in order to ensure personal safety, environmental, and explositivity safety legislation are all satisfied. The final results will be compared against commercially available solutions on repair applications, in order to validate the beneficial effects of the CORSAIR research.

Related Activities

Task 1: Definition of powder characteristics and supply specification

Cold Spray is very sensitive to powders characteristics and one of the most important activities is related to defining the optimised properties of powder feedstock. These activities will be realized with the support and guidelines of partners already performing cold spray techniques. Aeronautical companies will also provide guidelines in material selection.

A detailed description of powders specifications and the procedure to allow the proper characterisation beyond the state of art will be defined. In particular:

  • Powder composition (in particular oxygen content) and respect of base alloy composition requirements
  • Powder shape factor, morphology and roughness
  • Powder size distribution
  • Powder mechanical properties
  • Requirements for the safe handling of powders (explosivity, personal safety, transportation, disposal)

The characterisation setup and procedures will be defined based on the experimental validation and laboratories tests on different feedstock as well with the support of the aeronautic companies concerning the current requirements and necessary specifications for aeronautical applications. These guidelines will be applied both on commercially available powders and on specific feedstock developed during the project.

This procedure will be fundamental to identify and define the powders characteristics responsible for the macroscopic properties of deposited materials such as adhesion, composition, mechanical properties as well as process reliability. These topics are actually in discussion by many companies and research institutes worldwide but not yet well understood.

Task 2: Preparation and validation of optimised feedstock materials

Subtask 2.1. Powder preparation

The most common method for good quality metal powder atomisation is gas atomisation. However, powder can also be manufactured by water atomisation and mechanical crushing. Gas atomisation gives the most spherical outcomes but it is also the most expensive manufacturing method. Furthermore, the current level of spheridicity for Cold Spray applications is not optimized. This task will examine and optimize the powder atomization process.

Subtask 2.2 Powder characterization

Currently commercially available Cold Spray feedstock powders will be characterised following the procedures defined in Task 1. The powder specifications as well as the metallurgical and mechanical properties of the deposited coatings will be examined after production as well as in laboratory before spraying. The aeronautic companies will provide feedback considering the requirements of final applications.

Subtask 2.3: Put on trial of the systems with the new nozzle and production of a preliminary set of coatings (on specimens)

Powders will be subjected to spheridisation in order to enable the use of water atomised (Aluminium alloys) and mechanically crushed (Magnesium & Titanium alloys) feedstock, while not failing to enhance the capability of gas atomised powders (Aluminium, Magnesium, and Titanium Alloys). The process of spheridisation involves passing the powder through a plasma field within an inert gas atmosphere. The potential advantages are:

  • Processing under an inert atmosphere enables processing of reactive Magnesium and Titanium alloys
  • Morphology will be improved and the powder will be made more spherical. A spherical powder is potentially more suitable for use in the CS process
  • Residual levels of contaminates will be reduced
  • Take a low-value feedstock and transfer it into a high-value material by enhancing the processing capabilities
  • If mechanically crushed feedstock is used then the number of processing steps is reduced, reducing costs and environmental footprint
  • Allowing the evaluation of a wider range of alloys by CS by providing a cost-effective of optimised powders based on low-value feedstock

LPW will perform and optimise powder spheridisation processes. LPW has already undertaken some spheridisation of Ti-6Al-4V mechanically milled flake feedstock with very promising results that suggest this route as both cost-saving and high quality solution. The initial trials will be undertaken using external resource and LPW collaborators, and if successful, a tailored and completely devoted to CORSAIR development cell will be built and installed within LPW.

Subtask 2.4 Optimisation of powder post-processing requirements.

Once a powder has been manufactured, it must sized and handled according to the alloy and application. Sizing is the single most critical processing step to ensure that a powder is suitable for application. CS trials will be undertaken with several different size ranges and distributions to determine the optimised size range for each alloy. Sizing must be undertaken within an inert atmosphere in order to reduce risk of explosion. Therefore, the explostivity of the alloys must be determined and the alloys rated against international standards. This information will also be used to generate guidelines for the handing during shipping and CS processing of these powders. Current equipment will be modified to ensure the safe sizing and handling of these powders based on recommendations from the results of the explostivity testing, so as to in turn allow powder post-processes.

Expected Results beyond the SoA

WP4 involves a thorough examination of feedstock materials. The final results will suggest methods for the preparation and characterization of optimized feedstock material. One of the main outcomes of WP4 will be a Characterisation Protocol regarding feedstock materials with evaluation of current methods of powder manufacture and post processing techniques.

The research will also output deposition tests describing the experimental testing of optimised feedstock materials (by spraying with cold spray) compared against commercially available powders. Finally, WP4 will deliver a Production Protocol. This will be a report containing the standards and recommendations for powder manufacturing for Cold Spray based on the results obtained.

Progress

June 2013-November 2014 (M01-M18)

LPW has modified and optimised Aluminium powder production (gas atomization) towards eliminating the risk of contamination from previously atomized powder in a closed system. A powder contamination screening methodology has been developed using EDX and Elemental Mapping, in order to ensure that the highest quality powder is supplied.

Benchmarking of test methods for powder characterization has concluded that traditional methods for sizing and flow are not sufficient in accuracy or resolution in order to effectively characterize Cold Spray powders. Advanced characterization such as Rheometry, shape characterization and image analysis have been successfully employed towards overcoming this deficiency; these are required to inform tight powder specifications suitable to demanding applications, such as the repair of aerospace components.

Comparison of two alloy types (Ti vs. Al) has shown that powder properties are not transferable in different materials, as improved quality coatings were obtained with fine Ti6Al4V powders and coarser Aluminium powders.

A comparison of Al powders with different shape (highly spherical and irregular-spheroidal) has shown variability in coating quality. Techniques to describe these shape differences quantitatively based on circularity, convexity and elongation have been successfully demonstrated.


 

The activities related to the CORSAIR research are being organized in the following Work Packages (WPs):