Effect of Inclusion and Filtration on Grain Refinement Efficiency of Aluminum Alloy

It is well known that the filtration efficiency of ceramic foam filters (CFF) on aluminum melt can be significantly reduced by the addition of grain refiner particles under a high inclusion load. Also, it is usually considered that the filtration process has little impact on grain refinement efficiency. In this work, the influence of inclusions and filtration on the grain refinement effect of AA 6060 alloy has been studied. This was done through TP-1 type solidification experiments where the aluminum melt prior to and after the filter during a pilot-scale filtration test was investigated. In the experiments, 80 PPi CFFs were used to filtrate aluminum melt with an ultra-high inclusion load and two addition levels of Al–3Ti–1B master alloys. It is found that both inclusions and filtration significantly reduce the grain refinement efficiency of the grain refiner master alloys. A detailed characterization of the used filters shows that the reduction of grain refinement efficiency is due to the strong adherence of TiB2 particles to the oxide films, which are blocked by the CFF during filtration. A grain size prediction model based on deterministic nucleation mechanisms and dendritic growth kinetics has been applied to calculate the solidification grain size and estimate the loss of effective grain refiner particles during filtration. It is shown that due to the strong adherence between TiB2 particles and oxide films in the melt, the high addition level of aluminum chips also has an influence on reducing the grain refinement efficiency of aluminum melt without filtration. The results of this study extended our understanding of the behavior and performance of inoculant particles in CFF and their interactions with the inclusions.

A study on grain refinement of AZ91E magnesium alloy with Al-5TiB₂, AI-Al₄C₃ and ZnO additions

There is great interest in increasing the use of magnesium (Mg) alloys in transportation applications to reduce weight. The use of these alloys would increase if their strength and castability were improved. Through grain refinement, it is possible to achieve significant improvement in specific mechanical properties such as strength and hardness. For aluminum (A1)-containing Mg alloys, a commonly used grain refiner is hexachloroethane (C₂Cl₆). Though effective, C₂Cl₆ use releases harmful chlorinated hydrocarbons. It is therefore desired to find novel grain refiners that are effective and environmentally safe. This thesis focused on the grain refinement of AZ9lE alloy with three refiners: Al-5TiB₂, Al-A1₄C₃ and ZnO. The refiners were chosen due to their known grain refinement efficiency in low-Al Mg or Mg-Zn alloys. Castings with each refiner were made in graphite molds to establish i) the optimum addition levels to achieve the smallest average grain size and ii) the effect of holding time on fading of grain refinement efficiency. These castings ere used to collect thermal data and sectioned for microscopy and hardness testing. Castings were also made with the optimum parameters in a permanent mold specifically designed to investigate hot tearing susceptibility. The results indicated that all three additions enabled grain refinement of the base alloy, and no fading of grain refiner efficiency was observed. These refiners transformed the coarse dendritic microstructure in AZ9lE to one that was equiaxed and globular. At optimal levels, the refinement mechanism was heterogeneous nucleation. Also, hot tearing was significantly decreased with all refiners except for ZnO. The excess Zn from ZnO addition led to an increase in the freezing range, thus increasing the hot tear severity. The hardness of AZ9lE did not increase with ZnO addition as it did with the other two refiners.

A study on grain refinement of AZ91E magnesium alloy with Al-5TiB₂, AI-Al₄C₃ and ZnO additions

There is great interest in increasing the use of magnesium (Mg) alloys in transportation applications to reduce weight. The use of these alloys would increase if their strength and castability were improved. Through grain refinement, it is possible to achieve significant improvement in specific mechanical properties such as strength and hardness. For aluminum (A1)-containing Mg alloys, a commonly used grain refiner is hexachloroethane (C₂Cl₆). Though effective, C₂Cl₆ use releases harmful chlorinated hydrocarbons. It is therefore desired to find novel grain refiners that are effective and environmentally safe. This thesis focused on the grain refinement of AZ9lE alloy with three refiners: Al-5TiB₂, Al-A1₄C₃ and ZnO. The refiners were chosen due to their known grain refinement efficiency in low-Al Mg or Mg-Zn alloys. Castings with each refiner were made in graphite molds to establish i) the optimum addition levels to achieve the smallest average grain size and ii) the effect of holding time on fading of grain refinement efficiency. These castings ere used to collect thermal data and sectioned for microscopy and hardness testing. Castings were also made with the optimum parameters in a permanent mold specifically designed to investigate hot tearing susceptibility. The results indicated that all three additions enabled grain refinement of the base alloy, and no fading of grain refiner efficiency was observed. These refiners transformed the coarse dendritic microstructure in AZ9lE to one that was equiaxed and globular. At optimal levels, the refinement mechanism was heterogeneous nucleation. Also, hot tearing was significantly decreased with all refiners except for ZnO. The excess Zn from ZnO addition led to an increase in the freezing range, thus increasing the hot tear severity. The hardness of AZ9lE did not increase with ZnO addition as it did with the other two refiners.

The Interactions Between Oxide Film Inclusions and Inoculation Particles TiB2 in Aluminum Melt

In this work, a systematic study on the interactions between aluminum oxide films and TiB2 grain refiner particles and their effect on grain refinement behavior have been conducted. Oxide films were introduced into a commercial purity aluminum melt by adding AA 6061 alloy chips while the grain refiner particles were introduced by adding Al-3T-1B master alloy. Strong sedimentation of TiB2 grain refiner particles was observed in aluminum melt without chip addition during long-time settling. Most of the TiB2 particles were settled and accumulated at the bottom of crucible. In contrast, the sedimentation of TiB2 particles is much less in the melt with the addition of oxide films. A large fraction of TiB2 particles were found to be adhered to the oxide films located at the top part of the crucible, which inhibited the sedimentation of grain refiner particles. TP-1 type tests were also done to study the grain refinement efficiency of Al-3Ti-1B master alloy under different melt cleanliness and settling time. It is found that sedimentation of TiB2 particles greatly reduces the grain refinement efficiency. The introduction of oxide films seems to slightly alleviate the fading effect. This is owing to the strong adherence between the oxide films and TiB2 particles, which leads to a retardation of particle sedimentation.

Influence of Coarse Mg3Bi2 Particles on Deformation Behaviors of Mg-Bi Alloys

Mg-Bi binary alloys with concentrations of 1, 3, 6, and 9 wt% Bi and ternary Mg-9Bi-2.5Zn alloy were prepared by casting and hot extrusion. The results show that alloying with Bi refined as-cast grains of Mg alloys and the refinement efficiency of Bi is in between Al and Zr. It was also found that a critical value for the area fraction of constituent Mg3Bi2 particles seems to appear, which influences the dynamic recrystallization mechanism during extrusion. This influence results in either uniform or heterogenetic distribution of grain size and different extrusion texture intensities. Furthermore, the addition of Zn affected both the grain size and the area fraction/size of Mg3Bi2. Mechanical results and microstructure/fractography observation suggest that Mg3Bi2 particles do not impact the tensile nor compression yielding stress but act as fracture sources.

Different Influences of Rare Earth Eu Addition on Primary Si Refinement in Hypereutectic Al–Si Alloys with Varied Purity

The effect of alloying the Eu element on primary Si refinement in varied purity Al–16Si alloys was studied by scanning electron microscopy (SEM), thermal analysis, micro x–ray diffraction (μ–XRD), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The results indicate that the P impurity element in hypereutectic Al–Si alloys has a great influence on the rare earths’ refinement efficiency of primary Si. Coinstantaneous primary Si refinement and eutectic Si modification by Eu was obtained in high purity (HP) Al–16Si and commercial purity (CP) Al–16Si–0.06P alloys, but the primary Si was gradually coarsened in CP Al–16Si alloys. An excellent integration of ultimate tensile strength (144.8 MPa) and elongation (9.8%) of CP hypereutectic Al–16Si–0.06P alloy was obtained by adding 0.15% Eu. The refinement of primary Si in Eu–modified HP Al–16Si alloys was related to the constitutional undercooling of Eu. There was no sufficient Eu element partition into the primary Si particles, and fewer parallel twins, rather than multiple twins, were observed within them. The refinement of primary Si in CP Al–16Si–0.06P alloys was caused by the overlay of two kinds of mechanisms including the heterogeneous nucleation mechanism of AlP and the constitutional supercooling mechanism of Eu. However, in order to refine the primary Si in CP hypereutectic Al–16Si alloys, the Eu:P weight ratio should not exceed 3.33, otherwise the refinement efficiency of primary Si will be reduced due to mutual poisoning between Eu and P. This work can be used to interpret the controversy concerning the influence of rare earths on the primary Si in hypereutectic Al–Si alloys, thereby elucidating the importance of alloy purity to primary Si refinement by rare earths.