The evolution of residual stress and microstructure has been investigated in electron-beam welded Ti-6Al-4V alloy rings in order to develop an understanding of how the distribution of through-thickness residual stress correlates with microstructural evolution. A multiple technique approach to residual stress measurement was employed using a combination of different measurement techniques including X-ray diffraction (XRD), hole drilling method based on electronic speckle pattern interferometry (ESPI), and the contour method. It was found that there is a strong correlation between the change in residual stress and alpha phase morphology across the weld. The fusion zone exhibited highly tensile residual stress which was typified by an entirely acicular α′ microstructure formed by a displacive transformation within prior β grains on cooling. The tensile residual stress in the centre of the weld reduced towards the heat affected zone, transitioning to a compressive residual stress upon increasing distance from the weld centre. The transition from tensile to compressive residual stress correlates with a significant decrease in the volume fraction of α′ and an increase in the bimodal morphology of equiaxed primary alpha in a diffusional transformed beta matrix leading to elongated alpha in the base material

Residual stress fields dynamically fluctuate throughout the manufacturing process of metallic components and are caused by local misfit of a thermal, mechanical or metallurgical nature. Recent advances have been made in the area of microstructure and residual stress prediction; yet few have considered dual-phase titanium alloys. The aim of the work presented was to carry out a review of the existing state-of-the-art in residual stress modelling with an intended application to industrial heat treatment of Ti–6Al–4V alloy. Four areas were evaluated: thermal, mechanical and metallurgical sub-models, and model validation via residual stress measurement. Recommendations for future research include further investigation of transformation induced plasticity and stress relaxation behaviour in Ti–6Al–4V. This review was submitted as part of the 2019 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.

Titanium alloys, such as Ti-6Al-4V, are extensively used in critical aerospace applications. Heat treatments are often conducted during forging processes to produce final microstructures which exhibit advantageous mechanical properties. However, high thermal gradients present during processing may lead to the generation of undesirable levels of residual stress. Stress relief can be achieved by conducting aging or annealing treatments at elevated temperatures, yet there is limited quantitative understanding of how holding temperature affects the evolution of residual stress. Stress relaxation testing was conducted between 500-750°C and the resulting response was modified to describe creep strain. This was implemented in DEFORM™ finite element analysis software to model the evolution of residual stress during solution treatment followed by aging between 500-750°C, with comparison against solutions which did not consider creep strain. Stress relaxation phenomena was found to have an important impact on the reduction of residual stress and needs to be considered when carrying out thermo-mechanical processing at elevated temperatures.

In the initial stages of the reaction at the rubber-brass interface, formation of cuprous sulfide takes place. After this compound is formed, three possible reactions can occur. (1) The cuprous sulfide can unite with a “free” sulfur atom to form cupric sulfide. (2) The cuprous sulfide can attach itself to a sulfur atom already combined with the rubber. (3) The cuprous sulfide can attach itself directly to the rubber at points of unsaturation. In all cases of bad bonding brass examined by physical means, failure appeared to be due to nonuniformity in the structure of the plating. This lack of uniformity gave rise to a greater degree of corrosion and to conversion of the cuprous sulfide formed in the first place to cupric sulfide. Secondly, bad bonding stocks were characterized by two things : (1) The rate of reaction of the sulfur with rubber proceeded at a greater pace. (2) A greater degree of sulfide formation occurred at the brass rubber interface. This implied that reaction (2) was taking place, with the added complication of chains of sulfur atoms between rubber and copper. In other words it is necessary that a proper balance be maintained between the velocities of combination of sulfur with rubber and with brass. Thus, when the balance is disturbed, for example, by addition of more active accelerators, or, if the brass is in a more reactive state, then failure in adhesion results.

Microscopic examination of average brass deposits revealed a fairly uniform deposit of thickness varying from 0.0006 to 0.00004 inch, the brass deposit being thinner at the centre of the plate than at the edges. β-brass was obtained at points where porosity occurred in the base metal. The deposit was collected in aggregates or hillocks dispersed fairly evenly over the plate, a condition which was confirmed by examination under the electron microscope. With this instrument it was demonstrated that bad bonding brass was nonuniform over the surface, a fact further substantiated by subsequent x-ray analysis. Differences in crystallite size and in degree of crystalline orientation appeared to have no consistent effect on the bonding properties of the brass. The brass deposit showed, however, a high degree of preferred orientation, as is usual with electrodeposits. As regards the preparation of the base metal, the use of too much oil and too high a speed of machining appeared to have a detrimental effect. This appeared to be due to the fact that a nonuniform state of strain was produced over the surface as samples of mild steel of various degrees of strain, but uniform over the surface, gave consistently good adhesion.