David St John

SOLIDIFICATION OF CAST MAGNESIUM ALLOYS

A description of the key solidification steps in the formation of the as-cast micro structure of magnesium alloys is presented. The focus is on the two common magnesium alloy groups: Mg-Al alloys and Mg-Zn-rare earth alloys. The key elements described are: nucleation (including grain refinement), growth of the primary phase and the formation of the eutectic phases. In addition the effect of casting process (e.g. high-pressure die-casting and sand casting) on the outcomes from solidification are discussed. This includes consideration of the formation of banded defects during solidification in the dynamic environment of high-pressure die casting.

Evolution of the microstructure and mechanical properties during fabrication of mini-tubes from a biomedical β-titanium alloy

The processing of Ti-25Nb-3Mo-3Zr-2Sn tubes with outside diameters of 5.6-8.0 mm and wall-thicknesses of 0.7-1.0 mm were investigated in order to study the evolution of microstructure and mechanical properties and their impact on the processing of the tubes. The annealed small tubes with single β phase microstructures exhibit double yielding during tensile tests. The onset of martensitic phase transformation was observed to occur after the lowest point of the strain hardening. Cold rolling also activates the formation of the stress induced martensitic α″ phase. Its volume fraction increased with increasing ε. The rate of strain hardening and the modulus of the tubes are related to the stress induced transformation of the β phase to the α″ phase. The stress induced α″ slightly improves the yield strength of the tubes at low levels of strain. However, larger strains result in grain growth during annealing, which diminishes the mechanical properties.

The grain refinement of Al-Si alloys and the cause of Si poisoning: Insights revealed by the interdependence model

Recent work has extended the Interdependence Model to the prediction of grain size of aluminium alloys when refined by Al-Ti-B master alloys. The relative contribution of Ti solute and TiB2 particles to the as-cast grain size was determined indicating the optimum level of addition required to provide cost effective use of master alloy. This paper quantifies the effect of Al5Ti1B master alloy additions on the grain size of Al-Si alloys. The Al-Si system is a special case where additions of Si above a few percent poison grain refinement resulting in larger grain sizes. Consideration of the complicating effect of Si poisoning on the prediction of the grain size of these alloys and possible approaches to dealing with these complications are presented.

Freezing diagrams: Part I. Development and Implications for glass formability

Prediction of equilibrium and metastable phase formation for a particular system, after cooling from the liquid state, is possible by the development of afreezing diagram. the freezing diagram is constructed by consideration of kinetic as well as thermodynamic factors. The kinetic factors divide the diagram into three zones. These are: (1) where long-range diffusion can occur; (2) where short-range diffusion only is possible; and (3) where no significant diffusion is possible in the liquid ahead of the solid-liquid interface. The solid-liquid interface temperature,T1, determines which diffusion process occurs.T1 is the vertical axis of the diagram plotted against composition. the microstructure that results is predicted by the thermodynamic possibilities in each kinetic zone. In the first zone, the equilibrium phase diagram is applicable. The second zone only allows massive transformations to single-phase structures, and their limits are defined byTo lines. The third zone is below the glass transition temperature, and any liquid retained to this zone is quenched-in and becomes a glass. The development, significance, and limitations of the freezing diagram are discussed.

Improvement of the interdependence analytical model through selection of interfacial growth rates during the initial transient

The Interdependence Theory is based on the concept that grain refinement of an alloy during the initial transient of solidification is governed by the simultaneous occurrence of growth of an already nucleated grain and the associated Constitutional Supercooling (CS) developing ahead of the growing grain. Nucleation of a new grain occurs when the maximum CS attained at the end of the diffusion field of the solute rejected in front of the growing grain, is larger than the nucleation undercooling DTn of an inoculant particle present ahead of this point. The amount of grain growth plus the length of the diffusion field constitute the Nucleation-Free Zone (NFZ). The mathematical form of this theory is an analytical model which has been shown to be sensitive to the value of the growth rate v of the solid-liquid interface chosen to predict the as-cast grain size. This work presents the development and application of a varying growth rate expression rather than a constant growth rate used in the analytical model to improve its agreement with mMatIC, a numerical model of solidification which has been shown to effectively model grain growth and the evolution of the diffusion field. The possible range of values of growth rate that occurs during initial growth of a grain is obtained from numerical simulations. The resulting modified analytical model is compared against the numerical model for assessing the extent of NFZ formed.

Freezing diagrams: Part II. Comparison with experimental observation and relevance to crystallization of metallic glass

The significance and limitations of the freezing diagram are discussed in terms of experimental observations reported in the literature. A freezing diagram constructed for the Cu−Ti system is shown to be an accurate predictor of metastable phase formation when the position of theT0 lines are known. Freezing diagrams also enable the construction of qualitative continuous cooling transformation diagrams for all alloy compositions, and these help in understanding the microstructure produced after cooling at a range of rates. The diagram further enables a prediction to be made of the subsequent crystallization of quenched-in glass or the decomposition of metastable phases, and this is verified by experimental observation. Liquid phase separation also is discussed in terms of freezing diagrams, and it is found that a reported case of liquidphase separation is more accurately described as primary crystallization resulting in liquid stabilization.

The Challenges Associated with the Formation of Equiaxed Grains during Additive Manufacturing of Titanium Alloys

It is well established that columnar grain structures usually form when metal alloys are used to additively manufacture components. A challenging goal is to produce an equiaxed grain structure throughout the component to remove anisotropy and refine the grain size in order to improve its mechanical performance. The high cooling rates and associated high temperature gradients are the main reasons for the formation of columnar grains via epitaxial growth of each added layer of material. There appear to be limited strategies for promoting equiaxed nucleation of grains. In addition to cooling rate and temperature gradient, we explore other variables such as the potency of natural or added inoculant particles and the composition of the alloy, and their possible impact on the nucleation of equiaxed grains. Although changing the composition can help, finding a suitably potent nucleant particle is a major challenge. Operating parameters can also influence the microstructure and optimization to produce equiaxed grains may be possible. The limitations of these strategies and possible ways to overcome them are evaluated.

Practical considerations on the application of ultrasonic treatment to Mg-Al shape castings

This paper investigates the use of ultrasonic treatment on the grain refinement of Mg-Al alloys in castings similar to commercial gravity castings. It shows that it is a very effective form of grain refinement but it is limited spatially and by the advancing solidification front if partially-solidified product is washed into the die. It was found that the best way to obtain a homogeneous fine grain size through-out the casting was to combine a grain refining addition, SiC, with ultrasonic treatment during the initial stages of solidification.

Machining of titanium alloys with and without coolant

Machining titanium is challenging due to its low thermal conductivity which results in very high temperatures at the tool/workpiece interface and in addition there is a tendency for titanium to react with most cutting materials, resulting in surface and subsurface deformation in the workpiece. This paper investigates the relationship between vibration and surface deformation that occurs while machining commercially pure titanium and Ti6Al4V alloy materials under both wet and dry machining conditions. The results have demonstrated that vibration monitoring (normalised peak frequency amplitude) can be used as a predictive tool for optimising the surface quality of the machined workpiece. Twinning plays a prominent role in the subsurface of the machined Grade 2 material.