AN EXPERIMENTAL INVESTIGATION INTO THE RELATIONSHIP BETWEEN TEMPERATURE-TIME HISTORY AND SURFACE-ROUGHNESS IN THE SPRAY QUENCHING OF ALUMINUM PARTS

Repeated hear-quench cycles of Al-1100 samples resulted in increased s urface roughness and corresponding shifts in the temperature-time cool ing curve towards shouter overall quench periods. Three different type s of initial surface roughness were applied to the test samples: polis hed, particle blasted and milled finishes. For each of the three test surfaces, cooling curve shifts during repeated heat-quench cycles were accompanied by surface roughening, the shift was smallest with the mi lled sample. The surface roughness was examined with the aid of scanni ng electron microscopy, surface contact profilometry, and X-ray photoe lectron spectroscopy. Surface profiles obtained via the profilometer r evealed on a relative basis, significant changes in surface roughness on the polished and particle blasted surfaces but not on the milled (r oughest) surface. The roughening was the result of (a) hydrogen diffus ion associated with oxidation, (b) oxidation buildup, and, to a lesser extent, (c) expulsion of impurities along dendrite boundaries. The hy drogen diffusion caused localized pressure buildup within the surface and along grain boundaries resulting in the formation of both microsco pic (1 to 10 mu m) features on the polished and particle blasted surfa ces and relatively large (20 to 1000 mu m) bumps and blisters on the p article blasted surface. It is shown how these wide spectrum surface r oughness features affect cooling rate by (a) raising the Leidenfrost t emperature separating the film and transition boiling regimes, (b) inc reasing the number of boiling sites on the quenched surface, and (c) a ltering the impact dynamics of the spray drops.