Fig. 1 and Fig. 2 are SEM photos of microstructure of as cast samples of A1 and A2 alloys by ultrasonic rheological squeeze casting. It can be found that the white massive iron rich phase appears in the microstructure of A1 alloy with only ultrasonic treatment and no extrusion force. The EDX analysis results show that the bulk phase is δ phase with X (AL) = 52.08%, X (SI) = 31.9%, X (FE) = 16.02%.
During the equilibrium solidification process of al-17si-1fe alloy, Si precipitates at 650 ℃, then δ phase precipitates at 625 ℃, and peritectic reaction (L + δ = = β) occurs at 597 ℃ to form β phase. When there is no ultrasonic treatment and the extrusion pressure is 0, the δ phase is precipitated and fully reacts with the liquid phase to form β phase, so A1 phase is formed The Fe rich phase in the alloy is mainly acicular β phase. When ultrasonic treatment is introduced (the temperature range of ultrasonic treatment is 650 ~ 625 ℃), the cavitation effect and acoustic flow effect of ultrasonic will promote the nucleation of δ phase, and promote the formation and refinement of a large number of δ phase. Therefore, in the solidification process after ultrasonic treatment, part of δ phase reacts with liquid phase and transforms into β phase, and the other part comes from δ phase As shown in Fig. 1 (a), small δ phase and short needle like β phase coexist.
The size of Fe rich phase in Al alloy by ultrasonic rheological squeeze casting can be seen that the size of massive δ phase and acicular β phase in Al alloy decreases gradually with the increase of extrusion pressure to 300 MPa.
It can be found that after ultrasonic treatment, the iron rich phase in A2 alloy is obviously refined, that is, the δ phase transformation is small, and the length of needle like β phase becomes shorter. It can be seen from Fig. 6 that with the increase of extrusion pressure, the size of massive δ phase gradually decreases. Under the extrusion pressure of 0, 100, 200 and 300 MPa, the length of δ phase in A2 alloy is 41 ± 7, 34 ± 5, 29 ± 4, 23 ± 2 μ M in turn, and the number of acicular β phase gradually decreases.
Similar to the case of al-17si-1fe alloy, during the equilibrium solidification of al-17si-2fe alloy, Si phase first precipitates, then δ phase (al4fesi2), and then precipitates at 597 ℃ The β phase was formed by perieutectic reaction between δ phase and residual liquid phase, which lasted until ternary eutectic reaction occurred at 576 ℃. However, due to the relatively high content of Fe in al-17si-2fe alloy, the precipitation temperature of δ phase is higher than that of al-17si-1fe alloy, so there are more and coarser δ phases in al-17si-2fe alloy In the peritectic reaction at ℃, a part of coarse δ phase is retained without reaction. Therefore, in the microstructure of A2 alloy, the iron rich phase consists of coarse lath like δ phase and acicular β phase. After ultrasonic treatment, the cavitation and acoustic flow effect of ultrasound make the δ phase significantly refined and become short strip shape, and the needle like β phase also changes into short needle shape, as shown in Fig. 2 (a).
After ultrasonic treatment, the alloy melt is poured into the die cavity of the extruder and solidified under pressure, which further refines the iron rich phase. This is because, on the one hand, with the increase of solidification pressure, the heat exchange coefficient between the alloy and the mold also increases, the cooling rate of the alloy increases and the grain size decreases; on the other hand, according to Clausius Clapeyron equation: Δ TM / Δ p = TM (vl-vs) / Δ H (where Δ TM is the melting point change of molten metal under high pressure; TM is the melting point under equilibrium condition of liquid metal, TM = 933.3k; VL is the specific volume of liquid metal, VL = 0.42m3/kg; vs is the specific volume of metal in solid state, vs = 0.37m3/kg; Δ H Is the enthalpy change of metal solidification process, Δ H = 0.397 × 106j / kg; Δ P is the pressure change value, Δ P = 106pa). It is calculated that the melting point of pure aluminum is increased by 11.7k at 100MPa. It can be seen that the melting point of the alloy melt increases significantly under high pressure, resulting in the increase of melt undercooling and nucleation rate at the front of the solid-liquid interface, and the solidification structure of the alloy is refined. In addition, the diffusion of atoms is inhibited under high pressure, and the diffusion activation energy is increased, which reduces the crystal growth rate. Therefore, the crystal growth rate will decrease under high pressure solidification conditions, which will also lead to the decrease of the size of the second phase. Therefore, when the extrusion pressure is gradually increased to 300 MPa, the sizes of iron rich massive δ phase and acicular β phase in Fig. 1 and Fig. 2 gradually decrease. The results show that the liquid structure of the alloy increases with the extrusion pressure.
