C. J. Davidson

Effects of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy

Sand-cast plates of a commercial AZ91C alloy have been used for the study. Varying the solidification rate by placing large cast-iron chills in the mould produced a range of secondary dendrite arm spacing (SDAS) within the cast plates. The plates were solution heat-treated. quenched and aged at 165 degreesC for up to 350 h. The SDAS (mum) varied with the solidification time, t(f) (s), as SDAS = 5.3 t(f)(0.43). The tensile ductility in the as-quenched (T4) condition did not depend on the solidification rate whilst in the T6 condition it tended to decrease for slowly solidified material (SDAS > 50 mum). The yield strength and hardness increased and the ductility decreased with ageing. The fracture mode changed from predominantly transgranular in the T4 condition to predominantly intergranular in the T6 condition. The properties of the sand-castings are compared with those of high-pressure diecastings and the possible strengthening mechanisms are discussed. A number of areas that require more research are pointed out

Solidification and precipitation behaviour of Al-Si-Mg casting alloys

The effect of Mg content on the solidification and precipitation behaviour of both unmodified and Sr-modified Al-7Si-Mg casting alloys has been investigated at various solidification rates using cooling curve analysis, differential scanning calorimetry (DSC) and optical and electron microscopy. The Mg concentrations covered the range from 0.3 wt% to 0.7 wt%. The results indicate that increasing Mg content or cooling rate lowers the liquidus and binary Al-Si eutectic transformation temperatures. The latent heat of fusion of these alloys is strongly dependent on the level of Si present, but there is no observed dependence on Mg content. The solidification reactions observed under DSC are identified and it is noticed that the ternary eutectic solidification reaction L → Al + Si + Mg2Si is only observed at Mg levels of 0.6% and higher. The minor phases formed on solidification are identified and their response to solution heat treatment is examined. Increasing Mg content usually enhances precipitate hardening. However when Mg levels are increased above 0.5wt%, no apparent increase of yield strength with Mg is observed. This is correlated with dissolved Mg levels and energy released during reprecipitation.

Oxide films, pores and the fatigue lives of cast aluminum alloys

In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles, and nonmetallic inclusions are insignificant. The authors review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze casting, and semisolid casting, and compare A356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects that are characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations. No single process is inherently superior. For defects of the same size (in terms of projected area normal to the loading direction), oxide films are less detrimental to fatigue life than pores. Areas of current controversy are highlighted and suggestions for further work are made.

Observation of crack initiation during hot tearing

Abstract While the general mechanisms of hot tearing are understood, i.e. the inability of liquid to feed imposed strain on the mushy material, work continues on improving the understanding of the mechanisms at play. A hot tear test rig that measures the temperature and load imposed on the mushy zone during solidification has been successfully used to study hot tearing. The mould has now been modified to incorporate a window above the hot spot region to allow observation of hot tear formation and growth. Combining information from visual observation with load and temperature data has led to a better understanding of the mechanism of hot tearing. Tests were carried out on an Al–0·5 wt-%Cu alloy. It was found that load development began at about 90% solid and a hot tear formed a short time later, at between 93% and 96% solid. Hot tearing started at a very low load.

Observation and Prediction of the Hot Tear Susceptibility of Ternary Al-Si-Mg Alloys

An investigation into the hot tear susceptibility of ternary Al-Si-Mg alloys has been made using direct crack observation, measurement of load response, and predictions made by a modified Rappaz-Drezet-Gremaud (RDG) hot tearing model. A peak in both the hot tear susceptibility and the load at solidus occurred at approximately 0.2Si and 0.15Mg, and then the hot tear susceptibility decreased as the total solute content increased. In general, a good correlation was found among the observation of cracks, the load at solidus, and the predictions of the RDG hot tearing model, although it was shown that correlation with the RDG model depended critically on the fraction solid at which solid coalescence was assumed to occur. A combination of these approaches indicated that when the total Si+Mg content and the Si:Mg ratio increased toward four, a decrease occurred in the hot tear susceptibility because of an increase in the amount of final eutectic formed. At the lowest Si:Mg ratio of 0.25, the RDG model also predicted a lower relative hot tear susceptibility than that measured by the load at solidus. In these alloys, the final stages of solidification are predicted to occur over a large temperature range, and hence, both the predictions of the RDG model and the measurement of the load were dependent on which fraction solid was chosen for grain coalescence. In the alloys studied in this article, the formation of small amounts of the ternary eutectic Al+Mg2Si+Si caused the highest hot tear susceptibility.

Control and Removal of Impurities from Al Melts: A Review

Control of impurity elements in Al-based alloys is of increasing technological importance, both in primary and secondary alloy production. In primary alloy production, Ni and V concentrations in the coke are rising and this is increasing the level of impurities in the final products to the extent that they are out of specification. Impurity control is also of concern in recycling due to the pickup of elements such as Fe from contaminants in the scrap, which can detrimentally affect the alloy properties. Dissolved elemental impurities can be removed by a number of different processes including boron treatment for some of the transition metals. Other processes in the cast shop for removing impurities and inclusions from Al melts include fluxing, floatation, and filtration. Gas purging, vacuum treatment, filtration, the use of salts, and combinations of these presently find commercial applications. Ultra purity Al and its alloys can be obtained using zone refining and three-layer electrolysis methods. The demand for higher purity Al has been largely met by additional electrolytic refining processes. This paper reviews the tools that are currently available to the casthouse for removing impurities and inclusions from Al melts and suggests approaches that may be useful to meet future challenges.

Hyperspectral cathodoluminescence imaging and analysis extending from ultraviolet to near infrared.

The measurement of near-infrared (NIR) cathodoluminescence (CL) with sufficient sensitivity to allow full spectral mapping has been investigated through the application of optimized grating spectrometers that allow the ultraviolet (UV), visible, and NIR CL spectra to be measured simultaneously. Two optical spectrometers have been integrated into an electron microprobe, allowing simultaneous collection of hyperspectral CL (UV-NIR), characteristic X-rays, and electron signals. Combined hyperspectral CL spectra collected from two natural apatite (Ca5[PO4]3[OH,F]) samples from Wilberforce (Ontario, Canada) and Durango (Mexico) were qualitatively analyzed to identify the emission centers and then deconvoluted pixel-by-pixel using least-squares fitting to produce a series of ion-resolved CL intensity maps. Preliminary investigation of apatite has shown strong NIR emissions associated primarily with the rare-earth element Nd. Details of growth and alteration were revealed in the NIR that were not discernable with electron-induced X-ray mapping. Intense emission centers from Nd3+ and Sm3+ were observed in the spectra from both apatites, along with minor emissions from other 3+ rare-earth elements. Quantitative electron probe microanalysis was performed on points within the mapped area of the Durango apatite to produce a calibration line relating cathodoluminescent intensity of the fitted peak centered at 1,073 nm (1.156 eV) to the Nd concentration.

Modelling of microstructure formation and evolution during solidification

A comprehensive probabilistic model for simulating microstructure formation and evolution during solidification has been developed, based on coupling a Finite Differential Method (FDM) for macroscopic modelling of heat diffusion to a modified Cellular Automaton (mCA) for microscopic modelling of nucleation, growth of microstructures and solute diffusion. The mCA model is similar to Nastacs model for handling solute redistribution in the liquid and solid phases, curvature and growth anisotropy, but differs in the treatment of nucleation and growth. The aim is to improve understanding of the relationship between the solidification conditions and microstructure formation and evolution. A numerical algorithm used for FDM and mCA was developed. At each coarse scale, temperatures at FDM nodes were calculated while nucleation-growth simulation was done at a finer scale, with the temperature at the cell locations being interpolated from those at the coarser volumes. This model takes account of thermal, curvature and solute diffusion effects. Therefore, it can not only simulate microstructures of alloys both on the scale of grain size (macroscopic level) and the dendrite tip length (mesoscopic level), but also investigate nucleation mechanisms and growth kinetics of alloys solidified with various solute concentrations and solidification morphologies. The calculated results are compared with values of grain sizes and solidification morphologies of microstructures obtained from a set of casting experiments of Al-Si alloys in graphite crucibles.

Hyperspectral cathodoluminescence examination of defects in a carbonado diamond.

Hyperspectral cathodoluminescence mapping is used to examine a carbonado diamond. The hyperspectral dataset is examined using a data clustering algorithm to interpret the range of spectral shapes present within the dataset, which are related to defects within the structure of the diamond. The cathodoluminescence response from this particular carbonado diamond can be attributed to a small number of defect types: N-V0, N2V, N3V, a 3.188 eV line, which is attributed to radiation damage, and two broad luminescence bands. Both the N2V and 3.188 eV defects require high-temperature annealing, which has implications for interpreting the thermal history of the diamond. In addition, bright halos observed within the diamond cathodoluminescence, from alpha decay radiation damage, can be attributed to the decay of 238U.

Modeling of grain refinement: Part I. Effect of the solute titanium for aluminum

Over the past few decades, the grain refinement of Al alloys has been extensively investigated theoretically and experimentally. However, the relative importance of the parameters that contributes to grain refinement still remains unclear and is likely to be dependent on specific solidification conditions. This paper aims to investigate the mechanisms by which Ti, a common grain-refining addition in commercial-purity aluminum (CP), contributes to grain refinement using a cellular automaton-finite control volume method (CAFVM). CAFVM is used to model the grain formation and microstructure morphology under different conditions, e.g., with and without refiners, for Al alloys. In this Part I, the effect of adding solute of Ti on grain formation through its effect on growth restriction, constitutional undercooling, and the formation of extra-potential particles are taken into account in the calculations. It is shown that the calculated results are in reasonable agreement with the experimental data.