Microstructure and texture evolution during thermo-mechanical processing of two phase titanium alloy Ti-6Al-4V

Understanding the relationship between high-temperature deformation and microstructure evolution during hot forging of aero-engine alloys is important in ensuring optimum material properties in the final component. Of particular interest, with respect to two-phase titanium alloys is the break-up, below the beta-transus temperature, of an initial transformed alpha-lamellar microstructure during thermo-mechanical processing; this plays a key role in the development of the equiaxed alpha microstructure desired for the final product. Significant research effort has been put into understanding the mechanism for dynamic globularization, formation of kinking and shear banding within the alpha lamellae that can lead to break-up, but no complete quantitative analysis of the evolution of microstructural parameters and crystallographic texture with deformation currently exists. Therefore, reliable quantitative microstructural data associated with this process is important for both informing and validating models describing high-temperature metal-forming. To investigate the influence of hot working parameters, strain, strain rate and temperature, on microstructure evolution in Ti-6Al-4V a series of hot isothermal axis-symmetric compression tests have been carried out at temperatures both low and high in the alpha + beta stability field (880°C and 950°C, respectively), using strain rates (0.01, 0.1 and 1/s) relevant to industrial press forging. The experimental results showed that the morphology of the secondary a phase transforms gradually from lamellar to equiaxed under the influence of the deformation parameters and that the a lath thickness appears to have little influence on flow behaviour. It was also found that, at lower strains, the alpha laths appeared to be undeformed or only partially distorted. As strain progressed the laths were further broken up by distortion, bending and kinking. The mean aspect ratio of the alpha laths was found to exhibit a gradual reduction with increased strain. The lath area, length and perimeter showed a tendency to decrease with increasing strain. Furthermore, orientation image mapping (by EBSD) and texture analysis (by neutron diffraction) of the alpha phase were used to study the textural evolution during the hot deformation of the specimens and the mechanism involved on development and evolution of crystallographic texture during the a-β-a phase transformation. The strengthening of the ~-phase texture is observed on heating the sample during a-β phase transformation, where it was observed that Burgers relationship was followed but no evidence of preferential transformation was detected. In contrast a definite deviation from the Burgers relationship was observed during hot-compression. During β-a phase transformation upon cooling, however, the Burgers relationship was followed, which is evidence for a texture memory effect due to the growth of the primary alpha phase present at high temperatures

Texture Evolution during Thermal Processing of Ti-6Al-4V: A Neutron Diffraction Study

This paper reports results of phase transformation experiments in the α/β alloy Ti-6Al-4V from α→β→α on heating below and near the β-transus using in situ neutron diffraction. The development and evolution of crystallographic texture during the α→β→α phase transformation was determined. Annealing at a sub-transus temperature 1562K (850°C) shows similar α-phase and β-phase pole figures from room temperature up to 1562K (850°C), with a slight increase in the intensity of the β-phase pole figures, showing that the β-phase texture strengthens during heating. During cooling, the β→α transformation occurs very quickly and the initial alpha-texture obtained at room temperature is significantly weakened by the heat-treatment. Strengthening of the β-phase texture is observed on heating the sample from [1163K to 1253K (890°C to 980°C), respectively] and a Burgers relationship ((0002) α/(110) β and {11-20} α/{111} β) is seen with the α-phase texture at room temperature; but no evidence of variant selection was observed during α→β transformation. During the β→α phase transformation on cooling, the Burgers relationship holds. This indicates a texture memory effect due to the growth of the primary alpha phase present at high temperature

The Effect of Hot Deformation Parameters on Microstructure Evolution of the α-Phase in Ti-6Al-4V

The effect of high-temperature deformation and the influence of hot working parameters on microstructure evolution during isothermal hot forging of Ti-6Al-4V in the alpha phase field were investigated. A series of hot isothermal axis-symmetric compression tests were carried out at temperatures both low and high in the alpha stability field [(1153 K and 1223 K (880 °C and 950 °C), respectively], using three strain rates (0.01, 0.1 and 1.0/s) relevant to industrial press forging. The microstructures and orientation of the alpha laths were determined using optical microscopy and electron backscatter diffraction techniques. The experimental results show that there is a change in lath morphology of the secondary α phase under the influence of the deformation parameters, and that α lath thickness appears to have little influence on flow behavior

An Integrated Modeling Approach for Predicting Process Maps of Residual Stress and Distortion in a Laser Weld: A Combined CFD–FE Methodology

Laser welding has become an important joining methodology within a number of industries for the structural joining of metallic parts. It offers a high power density welding capability which is desirable for deep weld sections, but is equally suited to performing thinner welded joints with sensible amendments to key process variables. However, as with any welding process, the introduction of severe thermal gradients at the weld line will inevitably lead to process-induced residual stress formation and distortions. Finite element (FE) predictions for weld simulation have been made within academia and industrial research for a number of years, although given the fluid nature of the molten weld pool, FE methodologies have limited capabilities. An improvement upon this established method would be to incorporate a computational fluid dynamics (CFD) model formulation prior to the FE model, to predict the weld pool shape and fluid flow, such that details can be fed into FE from CFD as a starting condition. The key outputs of residual stress and distortions predicted by the FE model can then be monitored against the process variables input to the model. Further, a link between the thermal results and the microstructural properties is of interest. Therefore, an empirical relationship between lamellar spacing and the cooling rate was developed and used to make predictions about the lamellar spacing for welds of different process parameters. Processing parameter combinations that lead to regions of high residual stress formation and high distortion have been determined, and the impact of processing parameters upon the predicted lamellar spacing has been presented

Keyhole formation and thermal fluid flow-induced porosity during laser fusion welding in titanium alloys: Experimental and modelling

High energy-density beam welding, such as electron beam or laser welding, has found a number of industrial applications for clean, high-integrity welds. The deeply penetrating nature of the joints is enabled by the formation of metal vapour which creates a narrow fusion zone known as a “keyhole”. However the formation of the keyhole and the associated keyhole dynamics, when using a moving laser heat source, requires further research as they are not fully understood. Porosity, which is one of a number of process induced phenomena related to the thermal fluid dynamics, can form during beam welding processes. The presence of porosity within a welded structure, inherited from the fusion welding operation, degrades the mechanical properties of components during service such as fatigue life. In this study, a physics-based model for keyhole welding including heat transfer, fluid flow and interfacial interactions has been used to simulate keyhole and porosity formation during laser welding of Ti-6Al-4V titanium alloy. The modelling suggests that keyhole formation and the time taken to achieve keyhole penetration can be predicted, and it is important to consider the thermal fluid flow at the melting front as this dictates the evolution of the fusion zone. Processing induced porosity is significant when the fusion zone is only partially penetrating through the thickness of the material. The modelling results are compared with high speed camera imaging and measurements of porosity from welded samples using X-ray computed tomography, radiography and optical micrographs. These are used to provide a better understanding of the relationship between process parameters, component microstructure and weld integrity