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Microstructure and mechanical properties, with emphasis in impact toughness behaviour, of two multilayer composites have been investigated. The multilayer materials are constituted by thirteen layers of high strength Ti-6Al-4V alloy. Stacked layers have been successfully joined by diffusion bonding considering two processing temperatures, 750 and 900ºC. The proportion of α/β phase in the microstructure during diffusion bonding process determines the presence of pores at the interface, and therefore the mechanical properties of the composite material. Higher bcc β phase proportion during diffusion bonding process at the highest temperature of 900 ºC leads to high bonding degree, without noticeable toughness increase of the multilayer composite. In contrast, higher proportion of harder hcp α phase at 750ºC makes difficult the healing of interfacial pores, decreasing interfacial toughness and leading to a toughness value of the multilayer composite seven times higher than the reported value for the as-received Ti-6Al-4V alloy.

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This paper reports on investigations of the solid state formability and weldability of Ti-6Al-4V titanium alloy sheet with submicrocrystalline structure (grain size 0?5 and 0?8 mm) under superplastic conditions at temperatures of 700 and 800uC. It has been established that the submicrocrystalline condition of the sheet material reduces the temperature of superplastic forming (SPF) by 200uC and of diffusion bonding (DB) by 100uC, compared to the conventional superplastic forming Ti-6Al-4V material with microcrystalline structure (grain size 3 mm). Unlike commercially available sheet, the submicron grained sheet is characterised by isotropic formability. Submicrocrystalline processed Ti-6Al-4V sheet can be successfully used in SPF/ DB technology. The temperature range of superplastic forming can be between 700 and 800uC, and diffusion bonding at 800uC.