Abstract: Precursor phenomena of melting in pure metals and alloys have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). The examined materials were the pure metals In, Sn, Pb and Bi, and some alloys of the systems In-Sn and Pb-Bi with different compositions.MS tests have been carried out by means of a novel method developed by us that permits to operate in resonance conditions and employs hollow reeds of stainless steel containing the liquid metal. In all the metals a sharp drop of dynamic modulus and a Q-1 maximum were observed in a temperature range ΔT before melting that depends on the specific metal and its structure. Such anelastic behaviour is consistent with an increase of the interstitialcies concentration as predicted by the Granato’s theory.Moreover, HT-XRD evidenced that sudden grain re-orientation, shift and broadening of diffraction peaks occur just before the formation of the first liquid, therefore self-interstitials and vacancies seem to play a synergic role in melting. The increase of self-interstitials over ΔT has the effect of weakening interatomic bonds that favours the successive vacancy avalanche leading to the collapse of crystal lattice (melting).
Abstract: Developments in synchrotron and home laboratory X-ray sources and fast low noise X-ray imaging detectors over the last 15-20 years has enabled real time X-radiography of alloy solidification from the melt. These investigations have been an important tool for in–situ investigations of dendrite-, eutectic and monotectic growth, dendrite fragmentation etc. At the same time, the techniques have allowed studies of phenomena in the melt such as convection, formation of solute boundary layers and minority phase droplet interactions. The article will review the X-radiography techniques and some of the results with emphasis on studies of phenomena in alloy melts.
Abstract: The metallurgical and mechanical properties of fusion welded joints are influenced, among others phenomena, by the weld pool dimension and shape. Weld pool shape is important in the development of grain structure and dendrite growth selection process as well as in the development of residual stresses. For these reasons, significant advances have been made in recent years to understand, in greater detail, the dynamics of the heat and fluid flow in the weld and the subsequent development of the pool shape. In numerical simulation of welding processes, there are two different approaches used to model the fusion zone. If the prediction of distortions and residual stresses is the primary objective of the simulation (computational weld mechanics simulation), the phenomenological approach is the most suitable method used to model the fusion zone. Otherwise, when the weld pool shape has to be predicted, the fluid-dynamic equations must be solved at the expense of a significant ‘computational load’ increase. In this work, after a brief description of weld pool characteristics, such two different approaches are described and compared.
Abstract: The liquid lead-bismuth eutectic (LBE) alloy is of great interest for applications in future nuclear reactors. The structure and clustering of alloying elements have been investigated on an extended range of temperature (125-720 °C) by high temperature X-ray diffraction (HT-XRD), XPS and scanning photoemission microscopy (SPEM) at the ELETTRA synchrotron in Trieste. After melting the short-range order in liquid metal corresponds to a cuboctahedral arrangement of atoms that progressively evolves towards an icosahedral one as temperature increases. Such process, which is completed around 720 °C, is accompanied by a micro-chemical re-arrangement of atoms with changes of cluster size and composition. At high temperature the atom distribution results to be more homogeneous and the average size of clusters noticeably smaller.
Abstract: The physics of the interaction between a liquid metals and an externally applied magnetic field is analysed starting from the basic equations of the phenomena involved: electromagnetic fields, fluid dynamics, viscosity and surface tension. A set of general equations is deduced to describe a levitation melting system. The equations are progressively simplified in order to highlight the most important contributions to the phenomena. The basis criteria for the design and operation of a cold crucible system are then described and together with some of the experimental results obtained.
Abstract: The effect of melt superheat and oxide inclusions on the fluidity of a commercial A356 alloy has been investigated. Fluidity measurements have been performed by means of Archimedean spiral in sand moulds. The specific testing method and the experimental apparatus show a good reproducibility. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes and the amount of defects occurring at the tip of the spirals. The results reveal that oxide films increase the variability in the fluidity results obtained at the same apparent experimental conditions. A long permanence in the holding furnace and the introduction of some turbulence during sampling increase the oxide formation and entrapment in the molten bath, thus decreasing the repeatability of the fluidity results. The fluidity increases linearly with superheat and it extrapolates to zero at the temperature corresponding to a fraction solid of about 23%. The initial Ti content in the alloy produces an independent crystallization during freezing of the fluidity spirals.
Abstract: Al-Si-Mg alloys are commonly employed for the production of automotive castings. In view of the recent stringent emissions standards and consequent engine downsizing, these components must withstand higher temperatures and stresses than in the past. In this regard, the heat treatable quaternary Al-Si-Cu-Mg alloys gained particular interest in recent years, due to their superior mechanical properties and higher thermal stability. The present research activity was addressed to evaluate the influence of cooling rate on microstructure and consequently on room temperature tensile and fatigue behaviour of the A354 and C355 alloys. Samples for mechanical tests were produced under controlled cooling rates, in order to induce different secondary dendrite arm spacing (SDAS) values, classified as fine (20-25μm) and coarse (50-70μm). The experimental results showed that the cooling rate strongly influences the type, size and morphology of intermetallic particles. The presence of coarse intermetallic phases, mostly Fe-based, observed in coarse SDAS specimens, was reported to strongly affect ultimate tensile strength (UTS), elongation to failure and fatigue strength of both the investigated alloys. A correlation between UTS and fatigue resistance was found, independent of microstructural coarseness.
Abstract: The present chapter reports a short history and a state-of-the art of semi-solid techniques, with particular emphasis to the role of the liquid-solid fraction, being the rheological behaviour of the semi-solid slurry crucial and fundamental. The general description, with the related theoretical fundaments, is based on the data available in the recent and the past literature and finally it is integrated with some experimental results of the research carried out by the authors.
Abstract: This paper focuses on the role played by the liquid metal management on the solidification microstructure in industrial solidification processes. In particular attention is paid to the elimination of solidification defects by governing the microstructure evolution through fluid-dynamics and heat and mass transport in the liquid. The formation of hot tearing and gas porosities as well as columnar and equiaxed microstructures and micro and macro segregation are analyzed to explain how the liquid management is used to avoid defects. Examples on continuous casting and welding are also included.A very powerful tool for dealing with the complex phenomena associated with the solidification process is numerical modeling. Its increasingly growing use contemplates fluid-dynamics of the liquid phase, mass transport of solutes and solid-liquid interface evolution. Models using phase field and volume-averaging techniques, as well as models integrating multi-physics and multi-scale phenomena, are described as their use is taking on increasing importance in the design of solidification processes.