Mechanical testing of titanium/aluminium–silicon interface: Effect of T6 heat treatment

Abstract

A previous paper reported on the mechanical behaviour of insert-moulded Ti/Al–7Si bimetallic test pieces as studied by a classical push-out test as well as a variant: the circular bending test. When a chemical bond was formed at the Ti/Al–7Si interface, promising results were obtained in terms of joint strength and damage mechanism (Dezellus et al., 2008 [1]). As a continuation, the aim of the present work was to examine the influence on this mechanical behaviour of a T6 heat treatment (re-heating for 10h at 540°C, quenching in cold water and ageing for 6h at 170°C) applied to the as-moulded Ti/Al–7Si test pieces. For that purpose, push-out and circular bending tests were performed on heat-treated samples, and the results were correlated with a characterization of the morphology, the constitution and composition of both transverse sections through the metal/metal reaction zone and fracture surfaces, as revealed after removal of the Ti insert. As expected, applying the T6 heat treatment to chemically bonded Ti/Al–7Si bimetallic assemblies resulted in an improvement of the mechanical properties of the Al–7Si matrix itself. Moreover, a significant increase of the load level for the onset of joint damage in push-out mode was observed. Concerning the damage mechanism, the presence of angular Si particles in the eutectic region of the Al–7Si matrix near the interface had a weakening effect. After T6 solution heat-treatment, the shape of the Si particles changed from angular to globular. Moreover, due to the formation of Si-rich compounds at the Al–7Si/Ti interface, Si diffuses from the alloy towards the Ti rod and the size and number of Si particles became significantly decreased near the insert/alloy interface. These two features explained the favourable influence of the T6 heat-treatment on the mechanical properties of the Ti/Al–7Si assemblies.