Uptake of iron and its effect on grain refinement of pure magnesium by zirconium

The uptake of iron by molten magnesium from uncoated new mild steel crucibles at temperatures 680°C, 730°C, and 780°C has been investigated. It was shown that the uptake of iron was sluggish at 680°C and the use of 0.05% zirconium addition could effectively suppress the increase in iron content within the first 2 h of holding at temperature. Rapid and severe uptake of iron was observed at 780°C. As a consequence, it was found that the grain refinement of pure magnesium achieved by 1% zirconium addition nearly vanished after 60 min hold at 780°C due to the depletion of soluble zirconium. The uptake of iron at 730°C was conspicuous but it was still controllable by use of 0.05% zirconium addition within the first 60 min of holding at temperature. The work conducted using an aluminium titanite crucible and a boron nitride coated mild steel crucible at 730°C further confirmed the highly detrimental influence of the uptake of iron on the grain refinement of pure magnesium by zirconium. The characteristic zirconium rich coring structures developed from circular to rosette like when the melt was held at 730°C in an uncoated mild steel crucible, while no such evolution was observed when held in an aluminium titanite crucible at the same temperature. Recommendations to minimise the consumption of zirconium by the uptake of iron were made based on the results obtained from this investigation. The mechanism of grain refinement of magnesium by a low concentration of zirconium is discussed

Grain refinement of stainless steel in ultrasound-assisted additive manufacturing

Metals and alloys fabricated by fusion-based additive manufacturing (AM), or 3D printing, undergo complex dynamics of melting and solidification, presenting challenges to the effective control of grain structure. Herein, we report on the use of high-intensity ultrasound that controls the process of solidification during AM of 316L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, promoting the formation of fine equiaxed grains with random crystallographic texture. Moreover, the use of ultrasound increases the number density of grains from 305 mm-2 to 2748 mm-2 despite an associated decrease in cooling rate and temperature gradient in the melt pool during AM. Our assessment of the relationship between grain size and cooling rate indicates that the formation of crystallites during AM is enhanced by ultrasound. Furthermore, the use of ultrasound increases the amount of constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool, thus creating an environment that favours nucleation, growth, and survival of grains. This new understanding provides opportunities to better exploit ultrasound to control grain structure in AM-fabricated metal products.Comment: Paper published in Additive Manufacturin

Grain refinement of commercial pure al treated by Pulsed Magneto-Oscillation on the top surface of melt

Commercial pure Al can be refined by Pulsed Magneto-Oscillation (PMO) treatment applied via a plate induction coil above the top surface of the melt. The proportion of the equiaxed zone area increases with decreasing Height to Diameter (H/D) ratios from 3.5 to1.8 and further to 1.0. Meanwhile, it increases and then decreases with increasing peak current for the three kinds of ingots with H/D ratios of 3.5, 1.8 and 1.0, respectively. However, when the H/D ratio decreases to 0.44, the area proportion of equiaxed zone can reach the maximum value with a lower peak current. FEA software simulation indicates that smaller H/D ratio results in larger current density, electromagnetic force and convection on the top surface of the melt, favoring nucleation and subsequent grain formation. Through evaluating Joule heating effect by PMO, it was found that the proper amount of Joule heating benefits grain refinement. Excessive Joule heating can reduce the size of the equiaxed zone and change the growth morphology of the grains

The influence of ternary Cu additions on the nucleation of eutectic grains in a hypoeutectic Al-10 wt.%Si alloy

The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.% Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al-Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that constitutional supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al-Si alloys. (C) 2015 Elsevier B.V. All rights reserved

Recent developments in the application of the interdependence model of grain formation and refinement

The Interdependence model will be briefly reviewed and then applied to two different casting situations. One is the solidification of Mg–Al–Sm alloys to determine the optimum composition for achieving a fine as-cast grain size. Because the size range of the nucleant particles can be measured, the key factors describing the potency of the particle can be calculated providing a more complete description of the grain formation mechanisms operating for this alloy. This approach should be relevant for other Mg–Al–RE alloys. The other casting situation is where the melt of an AM60-AlN nanoparticle composite was treated ultrasonically producing a fine grain size on solidification. The limitations to grain size reduction by nanoparticles are discussed in terms of the Interdependence and Free Growth models

The effect of ultrasonic treatment on the mechanisms of grain formation of as-cast high purity zinc

The potential for producing a large refined equiaxed zone by ultrasonic treatment (UST) of high purity zinc was investigated in order to improve the mechanical performance and formability. The macrostructure of cast ingots changed from large columnar grains without UST to three zones of fine columnar grains adjacent to the mould walls of the ingot, a refined equiaxed zone and a zone of a mixture of coarse equiaxed and columnar grains. A small zone of equiaxed grains was obtained when UST was applied during cooling from 440 °C to 419 °C for 2 min. The size of the equiaxed zone increased from about 20% of the casting's cross section to 50% when UST was applied for 3 or 4 min. In contrast, the application of UST for a longer time from a higher temperature (450–419 °C for 4 min) resulted in a smaller equiaxed zone of 18% indicating that a specific combination of UST time and temperature is required for the formation of a large equiaxed zone. The factors affecting the formation of the equiaxed zone throughout the solidification cycle are described

The Influence of the effect of solute on the thermodynamic driving force on grain refinement of Al alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.The Australian Research Council (ARC DP10955737)

Grain refinement of wire arc additively manufactured titanium by the addition of silicon

This study demonstrates that silicon additions are effective in refining the microstructure of additive layer manufactured (ALM) titanium components. The addition of up to 0.75 wt% silicon to commercially pure titanium manufactured by wire arc ALM results in a significant reduction of the prior-β grain size. It is observed that silicon also reduces the width of the columnar grains and allows for the nucleation of some equiaxed grains through the development of constitutional supercooling and growth restriction. The grain size of the ALM components is compared to a casting process and it is found that the as-deposited microstructure produced during ALM exhibits larger average grain sizes. Using the Interdependence model for predicting grain size, it was determined that the population of nucleant particles that naturally occur in titanium, has comparable potency (i.e. ability to activate at a similar undercooling) regardless of the processing method, however, the ALM process contains fewer, sufficiently potent, nucleant particles than for the casting process due to the effect of subsequent cycles of remelting and heating

An epitaxial model for heterogeneous nucleation on potent substrates

© The Minerals, Metals & Materials Society and ASM International 2012In this article, we present an epitaxial model for heterogeneous nucleation on potent substrates. It is proposed that heterogeneous nucleation of the solid phase (S) on a potent substrate (N) occurs by epitaxial growth of a pseudomorphic solid (PS) layer on the substrate surface under a critical undercooling (ΔT ). The PS layer with a coherent PS/N interface mimics the atomic arrangement of the substrate, giving rise to a linear increase of misfit strain energy with layer thickness. At a critical thickness (h ), elastic strain energy reaches a critical level, at which point, misfit dislocations are created to release the elastic strain energy in the PS layer. This converts the strained PS layer to a strainless solid (S), and changes the initial coherent PS/N interface into a semicoherent S/N interface. Beyond this critical thickness, further growth will be strainless, and solidification enters the growth stage. It is shown analytically that the lattice misfit (f) between the solid and the substrate has a strong influence on both h and ΔT ; h decreases; and ΔT increases with increasing lattice misfit. This epitaxial nucleation model will be used to explain qualitatively the generally accepted experimental findings on grain refinement in the literature and to analyze the general approaches to effective grain refinement.EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineerin

Risk assessment for the spread of Serratia marcescens within dental-unit waterline systems using Vermamoeba vermiformis

Vermamoeba vermiformis is associated with the biofilm ecology of dental-unit waterlines (DUWLs). This study investigated whether V. vermiformis is able to act as a vector for potentially pathogenic bacteria and so aid their dispersal within DUWL systems. Clinical dental water was initially examined for Legionella species by inoculating it onto Legionella selective-medium plates. The molecular identity/profile of the glassy colonies obtained indicated none of these isolates were Legionella species. During this work bacterial colonies were identified as a non-pigmented Serratia marcescens. As the water was from a clinical DUWL which had been treated with Alpron™ this prompted the question as to whether S. marcescens had developed resistance to the biocide. Exposure to Alpron™ indicated that this dental biocide was effective, under laboratory conditions, against S. marcescens at up to 1x108 colony forming units/millilitre (cfu/ml). V. vermiformis was cultured for eight weeks on cells of S. marcescens and Escherichia coli. Subsequent electron microscopy showed that V. vermiformis grew equally well on S. marcescens and E. coli (p = 0.0001). Failure to detect the presence of S. marcescens within the encysted amoebae suggests that V. vermiformis is unlikely to act as a vector supporting the growth of this newly isolated, nosocomial bacterium