Characterisation of y' Across Inertia Friction Welded Alloy 720Li

Abstract

A comprehensive characterization of γ′ in inertia friction welded Alloy 720Li has been undertaken to quantify the dramatic variation of the γ′ microstructure in the heat affected zone (HAZ) and the effect of the post weld heat treatment (PWHT). Experiments were performed on samples in the as-welded and two different PWHT conditions. High energy synchrotron diffraction was used to investigate the overall variation of γ′ volume fraction across the weld line with a high spatial resolution. During this experiment it was also possible to determine the misfit between γ and γ′ as a function of position in the HAZ and the base material. In addition high and ultra high resolution field emission gun scanning electron microscopy (FEGSEM) studies were carried out in order to individually characterize primary, secondary, tertiary γ′ and γ′ that had reprecipitated upon cooling at the end of the friction welding process. To study the relation between γ′ variation and any changes of strength in the welds, microhardness tests were performed on the three samples. The high energy synchrotron diffraction measurements revealed that in the as-welded condition the HAZ was depleted in γ′ with a trough observed at about 1.5mm from the weld line. No variation in γ′ volume fraction was observed in the two PWHT conditions. FEGSEM studies revealed that at the weld line primary, secondary and tertiary γ′ had been dissolved during the welding process and that reprecipitated γ′ could be observed, even in the as-welded condition. The reprecipitated γ′ appeared to be smallest not at the weld line but at around 1mm axially away from it. Between 1 and 4mm from the weld line, significant coarsening of tertiary was observed even though the welding process had taken only a few seconds to complete. The recorded microhardness of the as-welded sample can be understood in terms of γ′ depletion and the non-optimal particle size of the fine tertiary and reprecipitated γ′. Both PWHT conditions displayed a pronounced microhardness increase towards the weld line that can be explained by the large volume fraction of reprecipitated and coarsened γ′ in this region. Measurements of the lattice misfit between γ and γ′ showed a variation in the HAZ of the welds. A γ/γ′ misfit between 0.11% to 0.15% was observed in the base material of the three samples studied and a minimum of about 0.05% misfit was measured at about 1mm from the weld line. In all cases the lattice spacing of the matrix (γ) was smaller than the lattice spacing of the coherent γ′ precipitates, i.e. representing a positive γ/γ′ misfit. At the moment the nature of the γ/γ′ misfit variation is not clear but it can be assumed that it is related to the temperature history that the material experienced in this region.