Treatment of the slurry is important during RheoMetalTM casting. In this work, semi-solid slurries were prepared under different stirring intensities, using two types of stirrers: a naked rod (for regular stirring) and a rod with two blades (for intensive stirring). Tensile tests were performed, investigating fracture surfaces, as well as metallographic samples. The results show that intensive stirring produces castings with finer primary particles and a more homogeneous microstructure. On the other hand, more faceted Fe-rich phases are found along the α-Al grains boundary, due to the dissolution of Fe from the stirrers. Moreover, for intensive stirring castings, the porosity found on the fracture surfaces are smaller, while more second (intermetallic) phases, especially Fe-rich phases, are observed. Consequently, the castings with intensive stirring show worse ductility. Finally, a quantitative analysis was made regarding ductility, affected both by porosity and the presence of Fe-rich phases.

In this study, a new tag-and-trace method of α-Al crystals was developed and used to study the dissolution of a rotating Al-3Si-0.15Ti cylinder immersed into a superheated commercial purity aluminum melt. The developed tag-and-trace method consists of tagging the primary crystals of an alloy with the microsegregation of a peritectic forming solute element, e.g., titanium in aluminum alloys. During solidification, the primary crystals form with a high concentration of the peritectic forming solute, decreasing in the adjacent growth regions of the same crystal. After solidification, the solute microsegregation tag in the interior of the primary crystals can be revealed by color etching. In this work, an Al-3Si-0.15Ti cylinder with all the primary α-Al crystals tagged with titanium was immersed into a superheated titanium-free aluminum alloy. The superheat was varied, and all samples were quenched 10s after immersion of the cylinder. The tagged α-Al crystals from the original cylinder could be distinguished from the non-tagged α-Al crystals formed in the thermally undercooled region surrounding the cylinder and during quenching. Indications of liquid penetration were observed in some α-Al crystals, which indicates that disintegration of α-Al crystals may occur during stirring of the alloy cylinder into a superheated alloy.

Al–Cu–Si ternary eutectic alloys with a bimodal eutectic structure exhibit a good combination of strength and plasticity owing to the multiscale hierarchical microstructure. Previous studies have reported that bimodal eutectic structures can be successfully prepared by rapid solidification at a cooling rate of greater than 102 °C/s. In the present study, a bimodal eutectic structure was successfully prepared at a cooling rate of less than 10 °C/s by adding a small amount of Sr to Al–Cu–Si ternary eutectic alloys. Additions of Sr led to a notable modification of the microstructure from a mixed eutectic structure to a bimodal eutectic structure. Samples with the bimodal eutectic structure showed greater hardness, strength, and elongation than samples with mixed eutectic structure. In particular, the elongation increased significantly from 6% to 28%. The additions of Sr changed the microstructure of the Al–Si phase from needles to globular shapes, which are finely dispersed all through the microstructure. These results indicate that the improved mechanical properties with the addition of Sr can mainly be attributed to the development of a multiscale hierarchical microstructure with fine eutectic cells.

Cast Al–Mg alloys with high Mg content have attracted considerable interest as a result of exceptionally low mass density and enhanced Mg solid-solution strengthening effect. However, poor ductility in this class of cast Al–Mg alloys originating from the increased amount of brittle Al3Mg2 phase presents a challenge. This work aims to improve ductility of a cast Al-10 wt%Mg alloy by prolonging time and/or increasing temperature during solid-solution treatment, hence minimizing the amount of the Al3Mg2 phase. The amounts of Al3Mg2 decrease and the concentrations of solute Mg increase with prolonging time and/or increasing temperature, improving both ductility and strength. Solid-solution treatment at 425 °C for 9 h or at 438 °C for 3 h induces complete dissolution of the Al3Mg2 phase, achieving an average uniform elongation of ~27% accompanied by an average 0.2% offset yield strength of ~186 MPa. The mechanisms improving both ductility and strength are discussed. 

Primary type polyhedral silicon crystals were observed in the vicinity of oxides located at or near the surface of strontium modified semi-solid Al–7Si–Mg castings. The coarse silicon polyhedrons were observed mainly on the outer side of oxides surrounding the macrosegregation regions. The silicon crystals most likely nucleated on AlP-particles located near the oxides. These AlP particles may nucleate on the wetted surface of oxides, or form at different locations in the interdendritic regions.

Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process. 

This work investigates the role of primary austenite morphology on the eutectic and eutectoid microstructures and the ultimate tensile strength (UTS) in a hypoeutectic compacted graphite iron (CGI) alloy. The morphology of primary austenite is modified by isothermal coarsening experiments in which holding times up to 60 min are applied to the solid–liquid region after coherency. The cooling conditions for the subsequent eutectic and eutectoid reactions are similar. Miniaturized tensile tests are performed to evaluate the UTS. The morphological characteristics related to the surface area of primary austenite, the modulus of primary austenite, ^Mγ, and the hydraulic diameter of the interdendritic region, D^Hyd_ID, increase with the cube root of coarsening time. The eutectic and eutectoid microstructures are not significantly affected by the morphology of primary austenite, thus indicating that the morphology of the interdendritic regions does not control the nucleation frequency and growth of eutectic cells or graphite. UTS decreases linearly with the increasing coarseness of primary austenite for similar eutectic and eutectoid microstructures, demonstrating the strong influence of primary austenite morphology on the UTS in hypoeutectic CGI alloys.

The evolution of primary austenite morphology during isothermal coarsening has been studied in the three main Fe–C–Si alloys used in industry, LGI, CGI, and SGI. The dendritic microstructure increases length scale during coarsening accompanied by fragmentation and coalescence of austenite crystals. The morphological parameters, SDAS, Mγ, D^hyd_ID, and Dγ show a linear relation with the cube root of coarsening time, t1/3, with similar rates for the three different Fe–C–Si alloys. The eutectic microstructures after coarsening of primary austenite in CGI and SGI alloys are not significantly affected by the surface area of primary austenite and the size of the interdendritic regions. Fraction, nodularity, shape distribution of graphite particles and the number of nodules and eutectic cells are similar when studied as a function of coarsening time. These results suggest that the nucleation frequency in CGI and SGI, and the growth of eutectic microstructures in CGI, are not significantly influenced by the morphology of primary austenite.

The influence of cooling rate on the formation of iron-rich intermetallic phases during solidification of unmodified and strontium-modified Al-7Si-0.3Mg alloys has been investigated. The effect of strontium on the intermetallic phases was evaluated in unquenched and quenched samples. Samples were quenched before the start of the Al-Si eutectic reaction, along the Al-Si eutectic reaction and just after the end of solidification. The results show that the addition of strontium increased the size of both β-Al5FeSi and π-Al8FeMg3Si6 at low cooling rates. For unmodified and strontium-modified alloys, an increase of cooling rate resulted in a decrease in size of the intermetallic phases, particularly in the strontium modified alloy. In the strontium modified alloy quenched before the start of the Al-Si eutectic reaction, π-Al8FeMg3Si6 appeared as thin platelets at the eutectic cell boundaries. Chinese script-like π-Al8FeMg3Si6 and platelet-like β-Al5FeSi intermetallic phases were observed uniformly distributed in the eutectic regions in the unmodified alloy quenched before the start of the eutectic reaction. Strontium modified semi-solid Al-7Si-0.3Mg castings were produced and the type of intermetallic phase, morphology, size, area fraction and distribution were similar to that observed in the strontium modified alloy quenched before the start of the Al-Si eutectic reaction.

The effect of primary austenite morphology on the ultimate tensile strength (UTS) of hypoeutectic compacted graphite Fe-C-Si alloys (CGI) is studied by isothermal coarsening experiments. Secondary dendrite arm spacing (SDAS) and the morphological characteristics related to the surface area of primary austenite, M γ and D ID Hyd , increase with the cube root of coarsening time. UTS decreases linearly with increasing coarseness of primary austenite. The eutectic and eutectoid microstructures are unaffected by the primary austenite morphology. These observations demonstrate the strong influence of primary austenite morphology on the mechanical properties of hypoeutectic CGI alloys.