July/August_AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 8 4 5 iTSSe TSS need to account for the layer-by-layer relaxation and gradient for x-ray penetration; these studies make use of neutron dif- fraction as more representative to determine residual stress in cold spray coatings. Other methods include hole drilling and the Barkhausen noise method for ferromagnetic materials. Typically, all cold spray coatings exhibit high compressive residual stress. In some cases, the compressive stresses are known to inhibit formation of through-thickness cracks and improve bonding, as in aluminum and magnesium alloys. In contrast, for harder metals (e.g., nickel and cobalt), they have been known to have a reverse effect by inducing a debit to the fatigue strength of the coated component. Figure 2 (a-d) shows an example of the residual stresses in alumi- num (Fig. 2a), aluminum alloy (Fig. 2b), copper on copper alloy (Fig. 2c), and magnesium alloy (Fig. 2d), as deter- mined by the XRD technique. The effect of helium versus nitrogen gas for imparting coating residual stress and the effect of heat treatment or stress relaxation are determined for the case of IN625 with a NiCr coating, as shown in Figs. 3a and b. Choi et al. com- pared the effect of grit blasting on the residual stress of an aluminum coating on an aluminum substrate. It appears that the coating has a compressive stress after grit blasting, which is balanced after the application of a cold spray coat- ing. The average through-thickness stress in their coatings is hardlywithin the standarddeviationof the XRDmeasure- ment system, ranging between 50 and 80MPa (7 and 12 ksi) in compression. Some reports have recorded the beneficial effect of these compressive stresses in improving the bond- ing. Other reports—especially for hardmaterials such as In- conels and MCrAlY—have reported the contrary with debits FEATURE Fig. 1 — Schematic of through-thickness residual stress measurement in a cold spray coating by electropolishing and removal of layers. Fig. 2 — Residual stress measured by x-ray diffraction sin 2 Y method on various cold spray coatings: (a) aluminum, (b) aluminum alloy, (c) copper alloy, and (d) magnesium alloy. (a) (b) (c) (d) 11