Tensile and fatigue properties of 6063-T6 aluminium alloy coated with electroless Ni–P deposit

Abstract

The tensile and fatigue properties of a 6063-T6 aluminium alloy coated with an electroless Ni–P (EN) deposit have been investigated. The EN plating was in the as deposited condition, that is to say, without any post-heat treatment. The thickness of the coating was of about 40–50 μm and it had a P content of ∼9·5 wt-%. The results obtained indicate that such a plating gives rise to a decrease in the number of cycles to failure N_f, when the substrate coating system is tested both in air and in a 3 wt-%NaCl solution. When the coated system is tested in air the decrease in N_f varies from about 56–45%, whereas during testing in the NaCl solution such a decrease ranges between about 65–44%. In both cases, larger decrements in N_f are observed to be associated with smaller maximum alternating stresses. It is shown that the EN deposit employed in the present investigation has a very good adhesion to the substrate, since no delamination is observed even when the coated system is subjected to tensile stresses above the yield strength. Extensive cracking in the form of rings normal to the tensile axis were observed to form during tensile testing and therefore the coating is unable to contribute to the yield strength of the coated system. The comparison of the fatigue curves of the uncoated and coated specimens indicates that the presence of the coating would be equivalent, as far as the fatigue behaviour of the material is concerned, to the superposition of a mean tensile stress in the range of ∼23 MPa. Fatigue cracks have been observed to nucleate at the surface of the EN plating and to propagate from there towards the underlying substrate. Also, it has been concluded that, although the EN plating could provide wear and corrosion resistance to the substrate, such an improvement in surface properties takes place at the expense of its fatigue properties, a fact that should be taken into account for design purposes.