Abstract
In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness.
Highlights
Cold gas spraying has become an important thermal spray process with numerous applications (Ref 1–3)
INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas
A powder based on the INCONEL 718 composition (Oerlikon-Metco, Troy, MI, USA) with spherical morphology, and a mean particle size of 14 lm was used as feedstock powder [more details are found in Ref 19]
Summary
Cold gas spraying has become an important thermal spray process with numerous applications (Ref 1–3). The basic deposition mechanisms have been identified as a confined deformation, the so-called adiabatic shear instability, which leads to a removal of oxide scales, good intermixing and mechanical bonding of the deposited particles even without melting (Ref 5) This bonding mechanism implies that only in a certain process window, the deposition is efficient. Further recent work shows that the manufacture of highly dense INCONEL 718 coatings by the cold spray process is possible especially by using high pressures up to 7 MPa and high propellant gas temperatures of 1000 °C (Ref 16). A very recent publication (Ref 17) on cold gas sprayed INCONEL 718 focuses on the optimization of the spray process. It covers residual stress profiles in these coatings, and the results will be compared to our results. In order to better understand the origin of the drop of strength with coating thickness, coatings with three different thickness values have been prepared in the present investigation and characterized in depth with respect to microstructure and residual stress state
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