M.J. Bermingham

Grain-refinement mechanisms in titanium alloys

Despite the importance of the prior-beta grain structure in determining the properties of titanium-based alloys, there are few published studies on methods of controlling the size of these grains in commercial alloys. The existing research raises questions about the relative importance of solute elements in grain-refining mechanisms, particularly the common alloying elements of aluminum and vanadium. The effect of these elements was investigated by producing a series of castings in a nonconsumable arc-melting furnace, and the results were interpreted with the aid of available phase-diagram information and solute-based models of grain refinement. A small reduction in grain size was obtained with increasing solute additions; however, this was not expected from the theoretical analysis. Possible reasons for this discrepancy are discussed.

Finite Element Modeling of Cutting Force and Chip Formation During Thermally Assisted Machining of Ti6Al4V Alloy

The improvement in machinability during thermally assisted turning of the Ti-6Al-4V alloy has been investigated using finite element modeling. A 2D thermally assisted turning model was developed and validated by comparing the simulation results with experimental results. The effect of workpiece temperature on the cutting force and chip formation process was examined. The predicted cutting forces and chip morphologies from the simulation strongly correlated with the experimental results. It was observed from the simulation that the chip forms after the coalescence of two deformed regions in the shear band and that the cyclic cutting forces are strongly related to this chip formation process.

Effects of boron on microstructure in cast zirconium alloys

Trace additions of boron to cast zirconium result in significant microstructural changes similar to those observed with additions of boron to titanium alloys. These changes include the promotion of dendritic growth and a refinement in both the prior beta and alpha grain size. The refinement of the prior beta grain size is explained using a model of grain refinement in association with values calculated from the binary Zr-B phase diagram. It is proposed that the refinement of the alpha phase occurs through a combination of increased nucleation and altered diffusion mechanisms during cooling through the beta transus.

Mechanical properties and biocompatibility of porous titanium scaffolds for bone tissue engineering

Synthetic scaffolds are a highly promising new approach to replace both autografts and allografts to repair and remodel damaged bone tissue. Biocompatible porous titanium scaffold was manufactured through a powder metallurgy approach. Magnesium powder was used as space holder material which was compacted with titanium powder and removed during sintering. Evaluation of the porosity and mechanical properties showed a high level of compatibility with human cortical bone. Interconnectivity between pores is higher than 95% for porosity as low as 30%. The elastic moduli are 44.2GPa, 24.7GPa and 15.4GPa for 30%, 40% and 50% porosity samples which match well to that of natural bone (4-30GPa). The yield strengths for 30% and 40% porosity samples of 221.7MPa and 117MPa are superior to that of human cortical bone (130-180MPa). In-vitro cell culture tests on the scaffold samples using Human Mesenchymal Stem Cells (hMSCs) demonstrated their biocompatibility and indicated osseointegration potential. The scaffolds allowed cells to adhere and spread both on the surface and inside the pore structures. With increasing levels of porosity/interconnectivity, improved cell proliferation is obtained within the pores. It is concluded that samples with 30% porosity exhibit the best biocompatibility. The results suggest that porous titanium scaffolds generated using this manufacturing route have excellent potential for hard tissue engineering applications.

Titanium as an endogenous grain-refining nucleus

Several studies have confirmed the applicability of the inverse growth restriction theory for predicting grain size in titanium alloy systems. However, until now, no work has identified nuclei particles that could be used to refine the β-grain size of titanium alloys during solidification. This work investigated whether titanium powder can be used to nucleate β-grains during solidification. A novel technique was used to introduce titanium powder to a series of titanium alloys, which results in significant grain refinement with an order of magnitude increase in grain density. Electron back-scattered diffraction (EBSD) was used to prove that titanium substrates can epitaxially nucleate titanium during solidification, and although a number of other potential mechanisms were investigated, it was concluded that the titanium particles heterogeneously nucleate β-grains.

FEA Modelling of Cutting Force and Chip Formation in Thermally Assisted Machining of Ti6Al4V Alloy

The improvement of machinability during thermally assisted turning of Ti-6Al-4V alloy was investigated by finite element modelling. A 2D thermally assisted turning model was developed and validated by comparing the simulation results with experimental results. Detailed analyses were carried out on the simulations in terms of the influence of the initial work-piece temperature on cutting forces and chip formation in the TAM process. The predicted cutting forces showed a very good correlation to the experimental results, and both the simulation and experiments have proved that the initial work-piece temperature plays an important role in determining the cutting force, with increasing initial temperature reducing the cutting force.

Latest developments in understanding the grain refinement of cast titanium

Grain refinement of cast titanium alloys is believed to have many benefits. However, literature on how to control and manipulate β-grain size during the solidification of cast components is scarce. This paper discusses the current state of research in grain refining practices in cast titanium alloys.

Introduction to the interdependence theory of grain formation and its application to aluminium, magnesium and titanium alloys

The Interdependence Theory is a theoretical description of grain formation that links heterogeneous nucleation to grain growth early in the initial transient of a previously nucleated grain. Thus nucleation is the result of a repeating cycle of growth and nucleation events moving towards the thermal centre of a casting. The principles of this theory are introduced and then the Interdependence equation that embodies the Interdependence Theory, is applied to the prediction of experimental grain size data for aluminium, magnesium and titanium-based alloy systems.

Effect of Oxygen on the β-Grain Size of Cast Titanium

Grain refinement of titanium alloys during solidification is believed to have many benefits for processing and properties. Recent work has emphasized the importance of solute elements in grain refining cast titanium and it was demonstrated that the growth restriction factor is useful for predicting the grain refining effectiveness of solute elements in titanium. Despite oxygen being the major impurity element present in titanium alloys and having been previously identified as a theoretical growth restricting solute, its effect as a β-grain refiner is still unexplored. This paper investigates the effect of oxygen on the grain size in cast titanium alloys.

Biocompatible porous titanium scaffolds produced using a novel space holder technique

We describe a new fabrication strategy for production of porous titanium scaffolds for skeletal implants which provides a promising new approach to repair and remodel damaged bone tissue. The new strategy involves powder sintering of titanium powder, employing pharmaceutical sugar pellets as temporary space holders, to facilitate production of porous scaffolds with structures optimized for mechanical performance and osseointegration of implants. The spherical sugar pellets, with controlled size fractions and excellent biocompatibility, are easily removed by dissolution prior to sintering providing an ideal space holder material for controlled synthesis of titanium scaffolds with desired porosities and pore sizes. The scaffolds contain pores with high degrees of sphericity and interconnectivity which impart excellent mechanical properties and superior biocompatibility to the structures. Scaffolds with 40% porosity and a pore size range of 300-425 µm exhibited an effective Youngs modulus of 16.4 ± 3.5 GPa and strength of 176 ± 6 MPa, which closely mimics the properties of human bone, and were also able to support cell adhesion, viability and spreading in cell culture tests. Porous titanium scaffolds manufactured by this approach have excellent potential for hard tissue engineering applications.