Inertia friction welding has been used across the aerospace, automotive, and power-generation industries for the fabrication of complex axisymmetric components for over forty years. The process involves one axisymmetric piece being held stationary and another piece being brought into contact set to rotate about its axis of symmetry by a flywheel with the system under an applied load across the joint. Plasticization at the joint interface through the frictional heating sees the two pieces bond together. The titanium alloy Ti-6Al-4V has been widely studied for inertia welding applications. A successful selection of processing parameters (flywheel energy and mass, applied load) allows an inertia welding process which produces a very high-integrity weld, with a minimal heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ), formed as a narrow band at the interface and extending further into the material. The width of this narrow band of heated material is dependent upon the process parameters used. A series of experimental inertia friction welds were performed using Ti-6Al-4V, and a finite element (FE) modeling framework was developed using the FE code Deform in order to predict the widths of the HAZ and TMAZ at the weld interface. The experimentally observed HAZ boundaries were correlated with the thermal fields from the FE model, while TMAZ boundaries were correlated with the Von Mises plastic strain fields.

A supply of medium carbon boron steel rod has been used industrially to produce the "rib-like" rod structures for mechanical conveyor systems, used across a number of non-safety critical industries, such as agricultural harvesting. The steel rod is resistive-heated and subsequently mechanically deformed such to produce a small region of flattened proportions, to allow for easier mechanical attachment to a belt system to attach all rods to the conveyor system. It has been noted industrially that after the flattening operations have taken place, a region at the shoulder of the flattened section is susceptible to cracking problems. The root cause of this cracking was desired to be understood, hence three likely causations for the cracking were explored, namely (i) mechanical stresses at the region, (ii) micro-segregation of the alloying elements at the location, and (iii) overheating. A 2D axi-symmetric finite element framework was developed to predict the stresses generated in the flattened section. This model showed that there were some areas of concern regarding the predicted effective stress and strain distributions, compared to the material flow stresses, thus potentially a mechanical reason for the cracking to occur. Microscopy methods were considered to understand the microstructure of the surrounding material and the nature of the cracks. However, these suggested that there was no likely element segregation to cause a significant variation in material property. Finally, temperatures generated by the resistive heating procedure were measured, and this does suggest that the material may have been overheated, thus producing coarser austenite grains whilst the material is held at elevated temperatures for a short time, and so producing inferior mechanical properties in this small region of heated material. The effects of overheating are impossible to eliminate without a complete re-melt of the steel. Thus, the research has demonstrated that a combination of overheating, and in-situ stress and strain distributions, could be the root cause of the cracking. (C) 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

The mechanical behaviour of welded components is dependent on the microstructures produced during the joining process. This is particularly true for alpha-beta titanium alloy welds where the structure and properties are dominated by the phase transformation that occurs during the thermal cycle. In this work the mechanical properties of the weld metal, heat affected zone and parent material have been determined for the Titanium-6%Aluminium-4%Vanadium alloy (Ti64) in sheet form and these are related to the microstructural changes that were observed. The flow stress behaviour is a complex function of temperature and microstructural condition with the parent material exhibiting higher strength than weld metal at low temperatures while the converse was observed at high temperature. A modelling approach has been developed to allow these effects to be incorporated into finite element based analyses of the residual stresses and distortions produced during welding.

Weld simulation methods have often employed mathematical functions to describe the size and shape of the molten pool of material transiently present in a weld. However, while these functions can sometimes accurately capture the fusion boundary for certain welding parameters in certain materials, they do not necessarily offer a robust methodology for the more intricate weld pool shapes that can be produced in materials with a very low thermal conductivity, such as the titanium alloy Ti-6Al-4V. Cross-sections of steady-state welds can be observed which contain a dramatic narrowing of the pool width at roughly half way in to the depth of the plate of material, and a significant widening again at the base. These effects on the weld pool are likely to do with beam focusing height. However, the resultant intricacy of the pool means that standard formulaic methods to capture the shape may prove relatively unsuccessful. Given how critical the accuracy of pool shape is in determining the mechanical response to the heating, an alternative method is presented. By entering weld pool width measurements for a series of depths in a Cartesian co-ordinate system using FE weld simulation software Sysweld, a more representative weld pool size and shape can be predicted, compared to the standard double ellipsoid method. Results have demonstrated that significant variations in the mid-depth thermal profile are observed between the two, even though the same values for top and bottom pool-widths are entered. Finally, once the benefits of the Cartesian co-ordinate method are demonstrated, the robustness of this approach to predict a variety of weld pool shapes has been demonstrated upon a series of nine weld simulations, where the two key process parameters (welding laser power and travel speed) are explored over a design space ranging from 1.5 to 3 kW and 50 to 200 mm/s. Results suggest that for the faster travel speeds, the more detailed Cartesian co-ordinate method is better, whereas for slower welds, the traditional double ellipsoid function captures the fusion boundary as successfully as the Cartesian method, and in faster computation times.

The deformation behaviour and associated microstructural evolution during hot forging of Ti-6Al-4V with a range of initial alpha lamellar microstructures has been examined. Uniaxial compression tests were carried out at temperatures both low and high in the alpha+beta stability field, 880 degrees C and 950 degrees C respectively, using strain rates (0. 01,0. 1 and 1/s) relevant to industrial press forging. Orientation image mapping and texture analysis of the alpha phase was used to examine the progressive break-up of the initial structure during the early stages of spherodisation (epsilon <= 1). Results indicate that, for the microstructures examined, there is no influence of alpha lath thickness on overall stress-strain response. Flow softening and the attainment of a steady state flow stress appears to be associated with progressive realignment of alpha laths with their long axes perpendicular to the compressive loading direction, rather than with spherodisation of the alpha structure. At 950 degrees C the development of a single-pole basal texture indicates that the alpha laths become oriented predominantly in a single transverse orientation. At 880 degrees C deformation also leads to a transverse basal texture, but with the laths distributed radially about the axi-symmetric loading direction.