WP5: Coldspray to repair Al and Mg alloy components

Lead by: VN
Participating partners: POLIMI, TWI, NAU KHAI, URJC, METALOGIC, AVIO, EADS, EADS-CASA, IMPACT, IBERIA
Duration: August 2013 – January 2016 (M03-M32)

Overview: Challenges and current SoA

Currently several experiments and procedures have been developed concerning MRO in aeronautics, especially on & with Aluminum Alloys. However, the current SoA consist of preliminary and promising results of some isolated tests. CORSAIR challenge is to explode these results testing more alloys and applications; optimizing process characteristics, efficiency and reliability; defining procedures according to aeronautic companies guidelines and finally improving the whole Cold Spray Repair Chain including new and optimized raw materials.

Main Objectives

WP5 aims to explore and optimize the deposition process with & on Aluminum and Magnesium Alloys. In order to realize this, the research will deeply characterize the deposited coatings and use these results as feedback for process optimization. Then, the deposition protocols for repaired parts will be defined, including feedstock material selection and post-deposition treatment if necessary. The final results will be compared against commercially available solutions in order to justify their beneficial effect on repair applications.

Related Activities

The WP5 activities are subdivided in 3 Tasks: “Deposition Process Optimization” ; “Characterization of Deposited Coatings” and “Deposition Process Characteristics for application on Aeronautical Components”.

The first task is devoted to prepare specimens with artificial defects taking into account accurately the aeronautical enterprise needs, prepare optimized feedstock powders, and optimize the deposition process both with mechanized and portable units. Post-deposition treatments will be also considered.

The second task is devoted to the realization of an extended Characterization Plan: metallurgical characterization with observation of coating structure and morphology both at micro & nano-scale. Corrosion and stress corrosion performances. Mechanical characterization in terms of hardness and surface properties, tribological behavior, coating adhesion and cohesion (YS, UTS). Residual stress evaluation and fatigue characterization both at room and high temperature. Custom characterization to depict specific performances (wear resistance, surface properties, morphology, etc.) depending on the required application needs.

Experimental results will be also matched with theoretical values predicted by mathematical model developed within WP2 and still used to validate the model. Finally the deposition process will be preliminary evaluated on industrial specimens in view of assessing the expected properties of aeronautical components repaired with CS. This represent the first step to the subsequent demonstration processes planned in WP7.

Expected Results beyond the SoA

The outputs of WP5 research will include guidelines for the preparation of the necessary lab specimens, as well as protocols with definitions of the optimized deposition process for each material being studied. These will cover the areas of spray conditions, process preparation and realization.

Additionally, a characterization report will be produced, containing all coating characteristics required to validate the repair applications and correlation with process parameters. Finally, a selection of results and process/materials depositions will be assorted in order to devote to dissemination activities.

Progress

June 2013-May 2014 (M01-M12)

The aim of WP5 is the investigation and optimization of cold spray processes on Al and Mg alloy substrates. The activities are carried out in the laboratory, especially on Lab specimens even if the realization of some preliminary deposition on components and prototype can be done.

WP5 has started on M3 with the realization of the deposition process, by using the state-of-art conditions employed in stationary high pressure cold spray system. These are limited in particular to the use of a carrier gas temperature 350°C due to the mandatory use of a polymeric PBI nozzle. The deposition has been carried out with two representative alloys strongly employed in aeronautics: A2024 and AlSi10Mg. The coating has been realized on flat specimens in order to investigate microstructure, microhardness, residual stress and adhesion. Further specimens to deeply investigate the mechanical behavior and corrosion resistance are in preparation.

The second part of WP5 has been devoted (and it is already in progress) to guide CORSAIR beyond the state of art. Firstly, the steady system has been equipped with innovative accessories developed within WP3 in order to test and use them for the Al alloy coating deposition. For instance, specific water cooling of the nozzle has been adapted and installed in the cold spray facility at VN and the gun equipped with a SiC nozzle in order to allow the achievement of gas temperature up to 600°C, avoiding nozzle clogging.

Deposition process investigations with spray conditions beyond state of art are currently in progress. The next steps will be the identification of a new set of optimized deposition parameters and perform a full investigation of coating properties profiting of all knowledge and availability of the consortium and finally the realization of some preliminary prototypes that are planned (and expected) before M18.

The activity of this WP is mainly a research activity so the impact is focused on scientific excellence, however, the process guidelines obtained in Laboratory will represent the tool to start the realization of deposition processes on real components.

June 2014-November 2014 (M13-M18)

During the third semester of CORSAIR the WP5 activities progress with the optimization of cold spray deposition processes of Al alloys with the newly assembled deposition layout that allow to achieve deposition conditions beyond the state of art. A set of coatings have been deposited increasing the carrier gas temperature up to 600°C and preliminary evaluating the influence in terms of coating microstructure and deposition efficiency. First of all, thanks to the new cooling device no clogging of the nozzle has been observed after several hours of operations enabling the realization of a fully reliable process. A2024 powders have been used to firstly explore the process conditions and a set of cohesion tests have been implemented to define optimized conditions to obtain the maximum coating strength. The deposition efficiency increases with gas temperature as expected even if for spray temperature higher than 500°C several drawbacks related to the excessive heating and quenching during deposition promote coating delamination and residual stress development. In this sense some deeper investigations are in progress to emphasize this effect. On the other side, it has been studied the possibility to increase temperature and reduce the heat input by increasing the standoff distance up to 200 mm as in thermal spray process.

After this first round robin the selected conditions to define the optimized protocol are SOD 20 mm, gas temperature 500°C. the comparison between the cohesion strength of state of art process (350°C PBI nozzle, SOD 20mm) and some selected processes at 500°C ranging the SOD are reported and emphasize as the strength can be more than doubled.

Deeper coatings investigation in terms of micro and nanostructure and residual stresses are currently in progress at URJC and POLIMI to depict a complete picture of the coating properties of the CORSAIR optimized coatings.

Furthermore, the study of the deposition process of other alloy (A357 in particular) is currently in progress to widen the benefits of the CORSAIR optimized process to other materials and components.

Finally some prototypes and parts are currently in realization thanks to the supply of scrap parts from IBERIA and GE AVIO in order to preliminary demonstrate the feasibility of the repair process.

December 2015-February 2016 (M19-M33)

Two Al alloy C355 powder feedstocks, manufactured at LPW in -60+45µm and -45+20µm particle size ranges, have been utilised for Cold Spray deposition trials at TWI Ltd. The powder morphology was highly irregular, with a large number of elongated particles, in contrast with a spherical morphology suitable for the Cold Spray process. The Cold Spray trials were carried out using a range of process gas temperatures (350 – 500°C) and pressure conditions (35 – 60bar) using the CGT Kinetiks 4000 and Impact Innovations 5/11 systems. The metallographic examinations of coatings using optical microscopy indicate that increasing gas temperature from 350°C to 500°C reduces the porosity level by up to 40% for both feedstocks trialed. Also, increasing gas pressure above 40bar and traverse speed to 100mm/s further enhances coating density. The coarser feedstock produces slightly denser coatings. Deposition efficiency is rather low at 28-50%, as expected for Al alloys, for both feedstocks, with no clear correlations observed. The initial set of data suggests that process conditions of 500°C and 60bar result in ultra-high density coatings (<0.4% porosity) for this highly non spherical powder feedstock. The next set of trials is planned using a spherical feedstock powder. It is envisaged that the newly developed high density Al alloy C355 coatings would enable successful repair of aircraft gearboxes, thus reducing material waste, increasing component lifespan and minimising the environmental impact. High density Al alloy coatings also offer the opportunity for corrosion protection of thermally sensitive devices.

March 2016 – May 2016 (M34-M36)

A different Al alloy C355 powder feedstock in the -45+20µm size range and spherical particle morphology was manufactured and utilised for Cold Spray trials at TWI Ltd. The trials concentrated on maximum process parameters (60bar gas pressure, 450-500°C gas temperature), optimized robot manipulation and surface preparation, to obtain higher density Al alloy coatings with improved mechanical properties. The metallographic examination of coatings using this powder confirmed previous findings that increasing gas temperature and pressure towards 500°C and 60bar respectively, significantly reduce the coating porosity levels to approx. 2.5%. However, the initial results suggest that higher coating density is obtained using the previously tested, coarser and non-spherical Al alloy powder feedstock. Further testing is underway to clearly identify the benefits of the two different powder feedstocks for repair applications of aerospace components. As part of the testing schedule, a variety of specimens were cold sprayed for microstructural analysis, mechanical and tribological testing, corrosion resistance and fatigue testing. The Cold Spray coating development activity and the extensive testing using selected powder feedstocks will help the decision making process towards achieving successful repair procedures of aircraft components.


 

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