WP2: Specific process simulation and nozzle design

Lead by: POLIMI
Participating partners: VN, TWI, NAU KHAI, IMPACT
Duration: August 2013 – October 2015 (M03-M29)

Overview: Challenges and current SoA

Many models have been proposed to simulate cold spray, both as regards the fluid dynamics of the process and as concern the adhesion condition. However they are based on limiting assumption and/or have been used and tested only for a few sets of materials. CORSAIR challenge is to remove the limiting hypotheses of existing models and to develop a multiscale approach able to lead to a complete understanding of the process.

Main Objectives

WP2 is aimed at obtaining an accurate and reliable numerical tool for assessing how the different process parameters affects the final result of cold spray and to identify their best combination in view of optimizing the process with respect of the combination of materials of interest and in view of developing new customized nozzles able to meet the optimized process parameters.

Related Activities

The fluid-dynamics of the Cold Spray process will be studied and investigated so as to explore the optimized process parameters to be used with a portable unit. Dynamic simulation Numerical simulations of the to determine the critical velocity.

The critical velocity and its relation with the coating, target material and main process parameters (temperature, pressure, powder dimension, etc.) will be explored with a multiscale approach. In particular, the effect of the CS process on the state of the coated surfaces in terms of residual stresses, roughness and plastic deformation and hardness will be studied. In the end, the involved partners will research the optimal cold spray parameters of the deposition process by means of the developed numerical tools. The following tasks are planned to this aim:

Task 1: Modelling fluid-dynamic process to optimize the deposition process with portable unit

Multi-physics simulations will be used to simulate and optimise the deposition process. Experimental validation will be performed by using PIV diagnostic during deposition by cold spray performers. The research will involve mathematical modeling of fluid-dynamic processes in Cold Spraying Nozzles with fluid dynamics equations and by the Method of Characteristics to evaluate the set of energetic parameters, (gas temperature, pressure and velocity) for all cross sections of the Nozzle.

Task 2: Determination of the critical velocity and of its relation with the coating and target material and with the main process parameters (temperature, pressure, powder dimension, etc)

Advanced dynamics finite-element analysis will be used to determine the critical velocity of the CGDS process for specific combinations of powder and substrate materials: material properties, obtained from experiments, will be used to develop the material models that will be in turn implemented in a Lagrangian Finite-Element Explicit code to model the deposition process. These simulations will allow to characterized the surface state after cold spray in terms of residual stresses, roughness, plastic deformation, hardening.

Task 3: Development of multi-scale finite element methods for the assessment of properties and performance of repaired components

In order to accurately assess and understand the behavior of the materials under high-velocity impact, a multi-scale approach will be developed by implementing layers of complexity into standard models of a plastic material response. In order to model the elastic behavior of the material during impact the well-known Mie–Grüneisen equations of state will be implemented while the starting point to model the plastic flow of the material is the Johnson–Cook material model. The development of this methodology for the assessment of the surface state of the coated surfaces will allow to determine the expected in terms of adhesion, residual stresses, roughness and plastic deformation and hardness, that are key parameters for the mechanical behaviour form the repaired materials.

Task 4: Numerical determination of the optimal parameters of deposition treatment

On the basis of the models and of the results obtained in the previous tasks the optimal process parameters, namely the optimal nozzle geometry, to attain the critical velocity with minimal imposed pressure in the low-pressure regimes able to produce repaired components with an adequate mechanical behaviour and strength, will be investigated and will guide the final design of the new nozzle to be used in a portable unit.

Expected Results beyond the SoA

Essentially WP2 activities will lead to the definition of a theoretical/numerical model describing the cold spray fluid-dynamics and suitable for determining the critical velocity as a function of different powders and target materials. Additionally, the collection and analysis of the various parameters will allow the development of a multi-scale simulation in order to predict the properties and performances of repaired parts. Further to that, research will lead to the development of numerical tools able to estimate the optimal choice of parameters during the deposition process, e.g. including nozzle geometry and powders characteristics that will be used in the design of the new portable cold spray equipment.

Progress

June 2013-May 2014 (M01-M12)

WP2 utilizes finite element and computational fluid dynamics to simulate cold spray deposition with the aim of optimizing processing parameter and nozzle design. WP2 deals with two important parameters in cold spray deposition. The first one is the particle’s impact velocity, which is related to cold spray apparatus, deposition parameters and nozzle type. The second is the critical velocity, which is an intrinsic property of feedstock materials.

No particle adhesion to the substrate occurs if the impact velocity is below the critical velocity. Moreover, the main coating characteristics can be described as a unique function of the ratio of the particle impact velocity to the critical velocity. Therefore, correct estimation of these two values is a central point in order to determine the optimized spraying parameters and to reduce the manufacturing cost, by increasing the deposition efficiency.

Two main tasks have been included in WP2. In the first task, a computational fluid dynamic model was used to estimate the particle impact velocity. The model was also utilized as a tool for a new nozzle design based on optimum powder acceleration. In the second task the critical velocity for Al2024 and Ti6Al4V was calculated using finite element simulation. Since high strain rates and large deformations are involved, the appropriate powder properties, enabling to capture strain, strain rate and temperature effects, are needed. An experimental procedure was designed to obtain material properties for the feedstock to be employed in the finite element simulation.

June 2014-November 2014 (M13-M18)

The previous finite element simulation of high velocity impact of micro-sized particles to the substrate (M12 activities) showed that material jet formation in the particle acts a crucial precursor of adhesion. Having an accurate knowledge of what occurs in the material jet could be an effective pathway to understand adhesion in cold spray. The current work proposes an effective and novel simulation of material jet formation and growth during cold spray of Al2024 and Ti6Al4V. General deformation, induced plastic strain, temperature rise in the particle and in particular in the material jet and eventually a critical discussion on critical velocity of the two materials is given.

December 2014-May 2015 (M19-M24)

The ratio of particle impact velocity to particle’s critical velocity defines coating properties and is the most important parameter in cold spray. For particle impact velocity, fluid dynamic calculations have been adopted to optimize particle velocity at the outlet of nozzle. For critical velocity, finite element simulation is used to study material jet formation at the periphery of particles during high velocity impact. Primary investigations were performed on Al-2024 and Ti6Al4V which are the main materials for the project. As the impact velocity increases, the maximum temperature induced in the powder increases up to the melting point of the material where a plateau behavior appears. The range of impact velocity leading to the plateau behavior at the melting point is in the 500-600 m/s and 650-850 for Al2024 and Ti6Al4V respectively. Estimations of critical velocity for Al2024 and Ti6Al4V are 575 and 675 m/s respectively. Localized melting and balanced hardening/softening simultaneously contribute in coating build up. The model was then extended from a single impact to multiple impact simulation and eventually to a full consolidated coating. The particles’ boundary and their degree of adhesion to one another play a major role in coatings mechanical properties. Damage model was incorporated in the coating simulation to account for inter-particle imperfect bonding. The model can be used to assess mechanical behavior of cold spray coating under different loading conditions.


 

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