Ion nitriding of explosively-clad titanium/steel tandems

Abstract

Ti-6Al-4V, that was explosively welded to a 304 stainless steel plate, was ion nitrided in a d.c. plasma. An effective nitriding was achieved due to the high dislocation density and vacancy concentration that emaneted from the shock wave exposure of explosive welding which, in turn, accelerated diffusion of nitrogen into the titanium matrix. Processed Ti-6Al-4V developed a surface layer of TiN (delta) followed by a Ti2N (epsilon) and interstitial nitrogen containing diffusion layer of alpha-titanium. The growth rate of compound layer (delta + epsilon) and case depth were found to be controlled by the diffusion of nitrogen. Depending on the temperature of ion nitriding, high Knoop hardness values, between 800 and 2520, were obtained and these values show two to sixfold increase in the hardness when compared with unprocessed samples. XRD results showed VN formation also, together with TiN and Ti2N nitrides and a preferred TiN growth in (002) orientation. Interfaces between Ti-6Al-4V and 304 stainless steel showed higher hardnesses as was seen following explosive welding than associated cores and a small amount of recrystallization was observed in the Ti-6Al-4V layer at the higher temperature processing. Nitriding of clad-Ti-6Al-4V thus provides an excellent opportunity of cladding surfaces with titanium alloys, in advanced structural applications without the expense of their monolithic counterparts. (C) 1999 Kluwer Academic Publishers.