Experimental and Numerical Study of Al2219 Powders Deposition on Al2219-T6 Substrate by Cold Spray: Effects of Spray Angle, Traverse Speed, and Standoff Distance

Abstract

Cold spray (CS) is an emerging technology for repairing and 3D additive manufacturing of a variety of metallic components using deformable metal powders. In CS deposition, gas type, gas pressure, gas temperature, and powder feed rate are the four key process parameters that have been intensively studied. Spray angle, spray gun traverse speed, and standoff distance (SoD) are the other three process parameters that have been less investigated but are also important, especially when depositing on uneven substrates or building up 3D freeform structures. Herein, the effects of spray angle, traverse speed, and SoD during CS deposition have been investigated holistically on a single material system (i.e., Al2219 powders on Al2219-T6 substrate). The coatings' mass gain, thickness, porosity, and residual stress have been characterized, and the results show that spray angle and traverse speed exercise much more effects than SoD in determining coatings' buildup. Finite element method (FEM) modeling and computational fluid dynamic (CFD) simulation have been carried out to understand the effects of these three parameters for implementing CS as repairing and additive manufacturing using aluminum-based alloy powders.

Keywords: CFD; cold spray; finite element modeling; spray angle; standoff distance; traverse speed.

Figure 1

(a) Picture of three stainless steel fixtures with seven tilting angles from 90° to 30°. There were 3 cm and 6 cm thick aluminum spacers below the second fixture (F2) and the third fixture (F3), respectively. (b) Picture of 21 pieces of Al2219-T6 coupons clamped in the three fixtures after spraying five passes at a traverse speed of 500 mm/s. The bottoms of three fixtures were aligned while there was about 4 cm gap between each fixture.

Figure 2

(a) SEM surface morphology as-received Al2219 powders. (b) Powder size distribution (PSD) after sonification in deionized water. (c) SEM image of the cross-section of the Al2219 powder and EDS elemental mapping of copper (Cu) and aluminum (Al) among the powder. (d) XRD analysis of Al2219 powders with Rietveld phase analysis results. (e) Inverse pole figure in z direction of Al2219 powders with inset at the top right corner showing phase map of one single powder with 0.2 µm step size. The scale bars in (c,e) represent 25 µm. (f) Hardness and Young’s modulus profiles vs. displacement depth into surface probed from the cross-section of Al2219 powder.

Figure 3

Pictures of two sets of Al2219 coupons deposited using (a) T1F3 and (b) T3F1 combination of parameters. Each set had seven coupons representing seven spray angles from 90 (far left) to 30° (far right). The respective contours of deposits measured by surface profilers are shown in (a′) and (b′), respectively. The dotted box in (a) represents a small piece cut from each coupon for subsequent tests.

Figure 4

SEM surface morphology of the seven deposits in (a–g) T1F3 set and (a′–g′) T3F1 set, with each of them representing a spray angle from 90 (far left) to 30° (far right). The scale bar in each figure represents 40 μm.

Figure 5

Optical microscope images of the cross-sections from the seven deposits in (a–g) T1F3 set and (a′–g′) T3F1 set of Al2219 deposits. Each picture represents a spray angle from 90 (far left) to 30° (far right). The scale bar in each figure corresponds to 50 μm. The occasional cracks between the coating and substrate serve as an indication of the interface.

Figure 6

SEM images of the cross-sections from the five deposits representing a spray angle from 90 (far left) to 50° (far right) in (a–e) T1F3 set and (a′–e′) T3F1 set of Al2219 deposits. The scale bar in each figure represents 30 μm, and their lengths differ due to the different magnifications of each image. The images from 40 and 30° were similar to those at 50° and hence were not shown here. The blue arrows represent the Al2219-T9 substrate.

Figure 7

(a,c) SEM images overlay with phase maps and (b,d) inverse pole figures (IPF) in z-direction from electron backscattering diffraction (EBSD) analysis of the cross-section of Al2219 deposits sprayed at (a,b) 90 and (c,d) 70° spray angle in T1F3 set of samples. The white dotted boxes in (a,c) represented the areas for EBSD analysis in (b,d) with a step size of 0.1 µm. The scale bars in the four images represent 50 µm.

Figure 8

(a) Mass change, (b) coating thickness, (c) coating porosity, (d) residual stress of nine sets of Al2219 deposits along spray directions with respect to nine spray angles at the combination of three traverse speeds and three standoff distances. Only 15 Al2219 deposits with continuous coatings were selected to measure their thickness in (b) and porosity in (c). In (d), solid symbols represent continuous deposits, while open symbols represent scattered Al2219 deposits or shot-peened Al2219-T6 substrates. (e) shows the schematic decomposition of a powder’s velocity (ν) when it hits the substrate at an angle θ with respect to the surface plane.

Figure 8

(a) Mass change, (b) coating thickness, (c) coating porosity, (d) residual stress of nine sets of Al2219 deposits along spray directions with respect to nine spray angles at the combination of three traverse speeds and three standoff distances. Only 15 Al2219 deposits with continuous coatings were selected to measure their thickness in (b) and porosity in (c). In (d), solid symbols represent continuous deposits, while open symbols represent scattered Al2219 deposits or shot-peened Al2219-T6 substrates. (e) shows the schematic decomposition of a powder’s velocity (ν) when it hits the substrate at an angle θ with respect to the surface plane.

Figure 9

FEM simulation of single Al2219 powder’s velocity profile (in m/s) after impacting on Al2219-T6 substrate at an angle from 70 to 65, 60, 55, 50, and 45°.

Figure 10

(a) Schematic diagram of multiple particle FEM computational model used for estimation of residual stresses in CS coating. (b) Simulation results of equivalent plastic strain (PEEQ) and von Mises stress after the deposition in 90° impact angle. (c) Simulation results of averaged in-plane residual stresses distribution (along spray direction) through the Al2219 coating thickness direction after the Al2219 coatings were deposited on Al2219-T6 substrate at four different spray angles. (d) Comparison of averaged residual stress results from (c) with reference experimental data based on Figure 8d by X-ray stress measurement.

Figure 11

CFD simulation of Al2219 powders’ spatial distribution on Al2219-T6 substrate at 3 cm standoff distance with impact angles from 90 to 30°.

Figure 12

CFD simulation about the effects of three traverse speeds (500, 350, and 200 mm/s represented by blue, red and black dots, respectively) on powder spatial distribution at a SoD of 3 cm.

Figure 13

CFD simulation about the spot dimension (left vertical axis) and velocity distribution (right vertical axis) at three stand-off distances (SoD) of (a) 3, (b) 6, and (c) 9 cm.