Drop impingement erosion of metals

Abstract

The object of the work has been to investigate experimentally the mechanisms of erosion in metals and alloys under drop impingement attack. For this purpose an apparatus of the wheel and jet type has been used to erode aluminium, copper, iron, cobalt and alloys of these metals. The various stages in the process from the first detectable microplastic deformation to the eventual pitting and removal of material from the surface have been investigated. In addition, experiments were carried out with the purpose of examining the effects of the normal impact pressure of a liquid on a surface in the absence of shear forces associated with liquid flow. This was achieved by propagating impact-generated compression waves through a liquid column in a filled and sealed cylinder onto a specimen surface inside the cylinder. With this arrangement the initial damage—small shallow depressions in the specimen surface— was identical with that produced under standard drop impact conditions in the wheel and jet apparatus. In either case the calculated values of the maximum impact pressure were lower than the average yield strength of each metal investigated. A complementary series of experiments was carried out in order to examine the erosive effects of liquid flow over the surface in the absence of high impact pressures. The technique used here involved a continuous high-speed water jet impinging against a solid surface at glancing incidence. This study showed that while flat well polished surfaces were apparently unaffected by the flow, lightly roughened surfaces or surfaces which contained the shallow impact depressions were severely eroded in regions adjacent to discontinuities. These various experiments suggest that the initial yielding which gives rise to the depression is associated with non-uniformity in the strength, structure and shape of the solid surface rather than with local variations in the impact pressure over the surface. The subsequent acceleration in the erosion rate is linked with the increased roughening of the surface and with an increase in the shear damage. When the surface becomes very rough and pitted, the impinging drop is deflected into less damaging streams by surface projections. This effect would account for the eventual decrease observed in the erosion rate. Further studies of the structure of the eroded surfaces have shown that the fractures have a number of features which are characteristic of metal fatigue failure. The connexion between erosion and fatigue is illustrated by similarities between the endurance curves for erosion and for the same metal in a standard fatigue test. As in the case of fatigue failure, strain energy to fracture appears to be one of the most important mechanical properties determining the erosive behaviour of a ductile metal.

Effects of pulsating water jet impact on aluminium surface

“…For water impacting the solid surface at velocities up to v = 1000 m s −1 , the constant k = 2 (Heymann, 1969), and the acoustic velocity, v s , is about 1480 m s −1 (Foldyna et al, 2006). Thomas and Brunton (1970) have shown that the magnitude of the impact pressure is independent of the geometry of the drop but the duration of the pressure is affected by the size and shape of the drop. It lasts for the short period, , that it takes a release wave, generated at the contact edge of the jet, to reach the impact axes.…”

“…2. When the liquid begins to flow away from the point of impact, there is evidence that the velocity of this tangential flow may be as much as five times the impact velocity (Thomas and Brunton, 1970). Therefore, there are additional shear forces associated with the high-speed flow across the surface acting on the surface in addition to the normal forces.…”

Water Impingement Erosion of Deep-Rolled Ti64

“…The failure type of the eroded parts was referred to as a fatigue mechanism due to repeated impacts of the water droplets [8]. Thus, previous studies [9][10][11][12][13] have introduced several mechanical surface treatments as potential solutions to improve the fatigue life of specimens.…”

Use of Acoustic Waves for Pulsating Water Jet Generation

“…When the liquid begins to flow away from the point of impact, there is evidence that the velocity of this tangential flow may be as much as five times the impact velocity (Thomas & Brunton, 1970). The velocity increase is thought to be connected with the shape of the head of the jet.…”

“…The magnitude of the impact pressure is independent of the geometry of the drop (Thomas & Brunton, 1970), but the duration of the pressure is affected by the size and shape of the drop. For a sphere or cylinder the corresponding radius or half-width of the contact area R exposed to this pressure is given by…”