WP6: Coldspray to repair Ti alloy components

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

 

Overview: Challenges and current SoA

Repair of Ti components using existing fusion-based methods (thermal spray, laser-DMD) presents problems including oxidation, hot cracking, distortion and loss of properties. Cold spray, being a solid state process, could overcome these difficulties but, to date, these necessary repairs cannot be carried out either cost effectively enough, or to a high enough fitness for purpose. Due to the limited deformability of Ti, so far costly helium had to be used to obtain sufficiently high impact velocities and plastic deformation. Only then, coatings with the requested low porosities were obtained. Using nitrogen as process gas, mostly low deposition efficiency, quite high porosity and non-sufficient coating strength were achieved.

The progress beyond the state-of-art provided by CORSAIR in WP6 is to provide a full-process-investigation and optimisation of the repair procedure of Ti components via cold spray, using optimised CORSAIR powder feedstock developed in WP4 and portable system developed in WP3.

Main Objectives

WP6 works in parallel with WP5 to study the deposition process on Ti alloy components and shares the same primary goals regarding characterisation of the deposited coatings properties, definition of protocols for repaired parts, optimisation and benchmarking of the results with commercially available solutions.

Related Activities

Task 1 – Deposition process optimisation

 

Subtask 1.1: Preparation of specimens with artificial defects

Artificially induced defects on specimens to reproduce similarity with aeronautical components.

  • Material properties
  • Surface roughness and properties;
  • Defect geometry and dimensions;
  • Specimen geometry and dimensions.

Subtask 1.2: Powders preparation & characterisation

In accordance with procedures defined in WP4. Commercially available powders against CORSAIR powders

Subtask 1.3: Repair of selected aeronautical components

Use of STATIONARY system (e.g. Kinetiks® 4000) to repair specimens prepared in Task 1.1. Use results to tailor the design of CORSAIR PORTABLE unit and to evaluate how different deposition strategies affect the geometry and shape of the coating layers. CORSAIR PORTABLE vs. STATIONARY systems. Process optimisation and definition of deposition protocol for each material  procedures transferred to the real aircraft components.

Subtask 1.4: Deposition of other materials

Specific substrates and components – e.g. repair of steel tools used to manufacture aeronautic components. Cold spray vs. traditional repair technologies (e.g., thermal spray, welding).

Subtask 1.5: Post-deposition treatments

Thermal treatment in order to reduce residual stresses and enhance mechanical properties (e.g., adhesion and cohesion). Laser post-deposition treatments so as to consolidate the deposit structure.

Task 2: Characterisation of the specimens and deposited coatings

 

Subtask 2.1: Metallurgical characterisation

(Quick feedback for process optimisation) Coating morphology and microstructure via OM and SEM. (In depth) Microstructure characterisation via TEM, SAED, NBED, HREM and XRD on selected specimens to correlate the coatings performance with process parameters and post-deposition treatments.

Subtask 2.2: Corrosion performance characterisation

(Short-term) Open circuit potential, passivation behaviour, pitting potential and polarisation resistance via DC/AC electrochemical techniques. (Long-term) corrosion performance of machined surfaces via salt spray and climate chamber.

Subtask 2.3: Mechanical characterisation

(Quick feedback for process optimisation) Coating static mechanical properties by measuring hardness, microhardness, adhesion and cohesion. (In depth) Mechanical performance by depth sensing indentation. Wear behaviour at room and elevated temperature. Use of test results to validate the developed numerical models.

Subtask 2.4: Residual stress evaluation

Hole-drilling method, XRD, modified layer removal, curvature method. Residual stresses developed in the substrate as a result of deposition compared with numerical predictions.

Subtask 2.5: Fatigue characterisation

S-N curves, -2N curves, fatigue crack propagation curves, at room and at high temperature (when of interest).

Subtask 2.6: Stress corrosion cracking characterisation

Wet and dry cycles will be applied to the coatings to simulate in service conditions.

Subtask 2.7: Mathematical model correction & validation

Mathematical model vs. experimental data. Application of coatings with pre-determined mechanical and maintenance properties.

Task 3: Deposition on aeronautical components

 

Final evaluation of the properties shown by specimens with artificial defects and repaired by cold spray  to assess the expected properties of aeronautical components to be repaired by cold spray. Mechanised and/or optimised portable cold spray systems vs. traditional repair technologies. TRL of repair by cold spray vs. target TRL (MCRL) for this WP.

Expected Results beyond the SoA
  • Guidelines for the preparation of lab specimens;
  • Definition of optimised deposition protocol for each material being studied;
  • Definition of optimised post-deposition treatments;
  • Characterisation reports to validate repair applications and correlation of coating characteristics with process parameters.
  • Representative examples devoted to dissemination activities.
Progress

June 2013-May 2014 (M01-M12)

The effects of Ti-6Al-4V (Ti64) powder size distribution and deposition strategy (i.e., gun traverse speed, number of passes and spray angle) were evaluated in terms of process deposition efficiency (DE), coating microstructure and porosity content and microhardness. Some additional assessment was carried out by focus variation microscopy to determine surface roughness (Ra and Rz values).

The aim was to achieve a Ti64 layer ~700µm thick. The coating deposited using a single pass at a slower gun traverse speed (GTS) of 230mm.s-1 was denser and harder than coatings deposited using multiple passes at a higher traverse speed of 700mm.s-1. The scattering of data was also lower with lower gun traverse speed due to repeated peening of particles. Cold spraying of Ti64 was carried out at angles of 90° and 60°; while both produced a coating, the coatings deposited using a spray angle of 90° were both denser and harder.

Of the five Ti64 powders (namely, -45+20µm, -45+32µm, -32+15µm, -32µm and -45µm), the powder with the highest content of fine particles (-32µm) generally gave a denser and harder coating with higher DE.

The above spray trials facilitated down selection of the best set of spray parameters for attempted repair and dimensional build-up of Ti64 aircraft components as defined in the CORSAIR deliverable D1.1.

June 2014-November 2014 (M13-M18)

A series of Ti-6Al-4V (Ti64) powders were supplied to TWI by LPW. These were characterised by laser particle size analysis, and a long term trial into oxygen uptake into powder (while stored) is under way, with regular measurements of oxygen content of a stored powder. A series of Ti64 coupons were machined for TWI with various geometries (as agreed in WP1) for the purposes of down selection of cold spraying parameters and for defining a characterisation protocol. These included small flats, adhesion test samples, tensile test samples and TCT samples amongst others.

Cold spray trials were carried out on flat coupons, depositing cold sprayed Ti64 layers approximately 700µm thick. The effects of powder size distribution and deposition strategy were investigated through a series of 27 cold spray runs. This included variation of gun traverse speed (GTS), number of passes, spray angle, spray direction and powder feedstock. The deposition efficiency (DE) was also determined (by considering feedstock mass vs deposited coating mass) and taken into consideration. The coating quality was evaluated by cross-sectioning and examination of coating microstructure, apparent porosity, and microhardness. Several of these coupons were supplied to other project partners for further testing ahead of development of a full characterisation protocol for these specimens.

It was found that the coatings deposited using a single pass and GTS 230mm.s-1 were generally denser and harder than coatings deposited using multiple passes but with a higher traverse speed. The scattering of data was also lower for the coatings deposited using the slower GTS, due to repeated peening of particles. It was also observed that a 90° angle was the best for deposition of a cold sprayed coating (as expected) but that coating deposition was still viable at an angle of 60° provided that the coating was always built up in the advancing direction. Building up the coating in the retreating direction led to a lower quality coating.

Of the five powders investigated, with particle size distributions of -45+20µm, -45+32µm, -32+15µm, -32µm and -45µm, the best quality coatings were those formed using the -32µm feedstock. This is likely due to the higher content of fine particles, leading to a denser and harder coating with higher deposition efficiency. These coatings were scanned in 3D using focus variation microscopy and their roughness values (Ra, Rz) evaluated optically.

Based on the above, a set of cold spraying parameters were down selected to produce the best quality Ti64 coating when applied to actual components (in WP7) and plans have been drawn up to deposit several examples of this coating onto appropriate substrate geometries (tensile, TCT, adhesion, flats etc.) in order to formulate a characterisation protocol and determine whether any post-processing improves the coating quality or is indeed required (this work will be presented in deliverable D6.3-6.5).

December 2014 – May 2015 (M19-M24)

Progress on WP6 has been very positive. As of M24, WP6 is nearly complete with a few remaining tasks. D6.1 and D6.2 were completed at the start of the period. The majority of WP6 Ti-6Al-4V cold spraying has been successfully completed at TWI using LPW Technology’s feedstocks. A variety of geometries were sprayed including notched tensile specimens, notched fatigue specimens, adhesion specimens, tubular coating tensile specimens, tribological testing specimens, stress-corrosion cracking specimens (two different geometries) and many other samples. This represents completion of a large project milestone.

Impact Innovations have carried out several spraying trials on a higher temperature/pressure system to optimize/improve cold sprayed Ti-6Al-4V coating properties, leading to reduced coating porosity. This also represents strong technical progress in the area.

In-depth coating characterisation is underway at VN, URJC, POLIMI, METALOGIC and TWI (D6.4, D6.5) with several results already available to contribute towards the understanding the properties of these Ti-6Al-4V coatings prior to application onto actual aeronautical components. This will be a step forward in available knowledge in this field.
Post-processing trials (D6.3) are also in progress on Ti-6Al-4V coatings by means of a) laser surface modification and b) heat treatment. This will ideally lead to improvement of coating properties beyond that possible by cold spraying alone.

June 2015 – February 2016 (M25-M33)

Cold Spray trials were carried out at TWI using the maximum available gas temperature and pressure of the CGT Kinetiks 4000/47 system (800°C and 40bar), and -32µm LPW powder. Under these conditions, the coating porosity is relatively high at ≈5%, while coating adhesion is low at <20MPa. A detailed investigation of thermal treatment and laser post-processing effects on Ti-6Al-4V alloy coatings was carried out with the aim of enhancing coating density and adhesion. This work is described in D6.3 (December 2015). Heat treatment at 950°C for 1h in argon resulted in a substantial increase in coating adhesion (from 20MPa to 55MPa), reduction of coating porosity and a decrease in microhardness. The deposition of coatings at higher process temperature (1100°C) and pressure (50bar) is currently in progress using the newly installed Impact Innovations 5/11 system. Trials are being carried out using several powder feedstocks manufactured at both AP&C and LPW. The metallographic analysis indicates a 50% reduction in coating porosity compared to previous best coatings deposited using the CGT Kinetiks 4000/47 system. Further process optimisation is currently ongoing to maximise coating adhesion and density for this challenging material. It is envisaged that the newly developed high density Ti-6Al-4V alloy coatings would enable successful repair of non-structural aircraft components, thus increasing component lifespan.

March 2016 – May 2016 (M34-M36)

Cold Spray trials were carried out at TWI using the maximum range of available gas temperature and pressure of the Impact Innovations 5/11 system (1000-1100°C, 50-60bar) and two Ti6Al4V powder feedstocks with spherical morphology, in a similar particle size range (-25+15µm) from two different suppliers (AP&C and LPW). Under selected conditions (1100°C, 50bar and optimised robot manipulation), the coating porosity is <2.0% for both powder feedstocks. The coating adhesion is >70MPa for the Ti6Al4V coatings sprayed on matching substrates with minimal surface preparation (ground surface using 120 SiC grit paper, followed by solvent cleaning). The cohesive strength of the as sprayed Ti6Al4V coatings is 225MPa and increases to 819 MPa following heat treatment, close to the typical Ti6Al4V bulk value of 825MPa. A variety of specimens have been cold sprayed for detailed characterisation by CORSAIR partners, such as microstructural analysis, mechanical and tribological testing, corrosion resistance and fatigue testing.


 

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