A. Effect of the ultrasonic vibration process

Figure 1 shows the microstructures of the semisolid Al₂O_3np/Al7075 composite slurry prepared without and with the ultrasonic vibration process at the cooling rate of 10 °C/min, and then quenched in water at 625 °C. It can be seen that the primary α-Al grains (light in the image) are distributed in the liquid matrix (gray in the image). The formation of the liquid matrix is due to the solidification of remaining liquid in the semisolid slurry during water quenching. Without the ultrasonic vibration process, the primary α-Al grains with coarse rosette-like and dendritic shapes were found in the semisolid slurry [Fig. 1(a)]. After SSUV in the range of 700 °C–625 °C, the primary α-Al grains became nearly globular and were uniformly distributed in the liquid matrix [Fig. 1(b)]. Thus it can be seen that SSUV influences significantly the size and morphology of primary α-Al grains.

FIG. 1.

Metallographs of the semisolid Al₂O_3np/7075 composite slurry prepared by different processes [(a) stir casting and (b) ultrasonic vibration] at the cooling rate of 10 °C/min and then quenched in water at 625 °C.

The refinement of primary α-Al grains depends mainly on the cavitation and acoustic streaming induced by ultrasonic vibration. Application of ultrasound leads to oscillation of liquid molecules about their mean position, i.e., mechanically vibrating wave. Periodic tension-pressure forces are created in the liquid medium by passage of these waves.^{Reference Liu, Zhai, Qi and Zhang13,Reference Taghavi, Saghafian and Kharrazi16} During the half cycle of tension, the tensile stresses above the cavitation threshold create numerous small cavitation bubbles at weak regions of the melt.^{Reference Sauter, Emin, Schuchmann and Tavman17} However, if there is in the half cycle of pressure during passage of these waves, the bubbles are forced to collapse rapidly. The energy of the collapse is transformed into high pressure pulses. Increased pressure causes an increase in melting point in the local zone of the melt and can be described as^{Reference Yan, Rao and He12}

(2)$${{\Delta {T_{\rm{m}}}} \over {\Delta P}} = {{{T_{\rm{m}}}\left( {{V_{\rm{L}}} - {V_{\rm{S}}}} \right)} \over {\Delta H}}\quad ,$$

where T _m is the melting point of the alloy at ordinary pressure, P is the pressure, V _L and V _S are the specific volume of liquid and solid phases, respectively, and ΔH is the latent heat of fusion. On the other hand, during the growth of the bubbles, they will absorb the energy in the melt and lead to decreasing of their surface temperature.^{Reference Taghavi, Saghafian and Kharrazi16} The coupled effect of ultrasonic vibration finally makes supercooling to occur in the local zone. The violent reduction of temperature induces a mass of nucleation sites, accordingly refining the primary α-Al grains. In addition, the dissolved gases (mainly hydrogen) have a strong tendency to adsorb on the surface of Al₂O_3np,^{Reference Liu, Han, Li and Huang18} hence reducing the wettability between the Al₂O_3np and matrix alloy. And yet, the impulse force and cumulative jets induced by ultrasonic vibration can remove the gases from the particle surface, thereby accelerating heterogeneous nucleation.

Grain refinement is also related to the showering and propagation of fragmented dendrite arms. The equivalent impulse force, which is produced during the cavitation bubble collapsing, can be expressed by the following expression^{Reference Liu, Zhai, Qi and Zhang13}:

(3)$$F = {I_{{\mathop{\rm int}} }}S\exp \left( { - 2\alpha x} \right){{{e_1}} \over c}\quad ,$$

where I _int is the incident intensity, s is the area on which ultrasonic vibration is imposed, α is the attenuation coefficient, x is the transmitted distance, e ₁ is a unit vector in the propagation direction of ultrasonic sound, and c is the speed of ultrasonic sound in the melt. The impulse force is exerted toward dendrite arms, finally leading to shearing or bending of dendrite arms and to fragmentation. Under the role of acoustic streaming, the fragmented dendrite arms are uniformly distributed into the whole melt and act as new sources of nucleation. Meanwhile, when a dendrite is fragmented, as a result of impulse force, fragmented dendrite arms encounter with the adjacent arms and cause more fragmentation of dendrite arms. In addition, dendrite arms have lower melting point because of solute concentration, which suggests that dendrite arms break off at the roots preferentially, resulting from the overheating temperature generated from bubble collapse. Thus, with the increasing of fragmented dendrite arms, the nucleation rate increases and more grain refinement is achieved, which is evident from Fig. 1(a).

B. Effect of the cooling rate

Figures 2(a)–2(d) show the quenched microstructures of the semisolid Al₂O_3np/Al7075 composite slurry treated by applying an SSUV of 1 kW from 700 °C to 620 °C at various cooling rates. In this experiment, different cooling rates were obtained by controlling the temperature of the slurry holder around. For a given temperature range, different cooling rates means different ultrasonic vibration times. For higher cooling rate (117 °C/min), the semisolid microstructure consists of liquid matrix and fine rosette-like primary α-Al grains [Fig. 2(a)]. When the cooling rate was 33 °C/min, a few coarse rosette-like grains and nearly globular grains were present in the semisolid slurry, and fragmented dendrite arms could be found [as indicated with red box in Fig. 2(b)]. When the cooling rate was decreased to 10 °C/min, the primary α-Al grains became more round and the liquid phases were uniformly distributed along the grain boundaries [Fig. 2(c)]. Further decreasing the cooling rate to 4 °C/min, the grains coarsened obviously [Fig. 2(d)]. Figure 2(e) depicts the effect of the cooling rate on the average size and shape factor of primary α-Al grains. It can be found from Fig. 2(e) that the good semisolid slurry with an average size of 73 μm and a shape factor of 0.84 could be obtained by applying SSUV under the cooling rate of 10 °C/min, and further increasing the cooling rate led to decrease in average size and shape factor of the primary α-Al grains. When the cooling rate was 117 °C/min, the average size of grains was 60.75 μm, but its shape factor was only 0.59.

FIG. 2.

Microstructure evolution of the semisolid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of 1 kW from 700 to 620 °C with various cooling rates: (a) 117 °C/min, (b) 33 °C/min, (c) 10 °C/min and (d) 4 °C/min, and (e) average grain size and shape factor versus cooling rate in the temperature range.

A higher cooling rate means a shorter ultrasonic vibration time, which influences the growth of primary α-Al grains. When the cooling rate is high, the ultrasonic vibration time carried out on the semisolid Al₂O_3np/Al7075 composite slurry reduces so that the grains can grow freely. On the other hand, the shorter ultrasonic vibration time is not enough to distribute fragmented dendrite arms into the liquid matrix uniformly [Fig. 2(b)]. The presence of the fine globular grains, as shown in Fig. 2(c), can be mainly attributed to grain boundary wetting and SSUV. If the double solid–liquid interface energy (2γ_SL) becomes less than the summation of the grain boundary surface energy at the roots of dendrite arms (γ_gb) and the component of impulse force parallel to the γ_gb direction, liquid penetrates the grain boundary until dendrite arms are detached and then the grain boundaries of solid grains are completely wetted.^{Reference Doherty, Lee and Feest19} To minimize the solid grain/liquid interface energy, the primary α-Al grains become more round. In addition, under acoustic streaming, the solute and temperature distributions tend to uniformity ahead of solidification, accordingly hindering preferred growth of the primary α-Al grains, contributing to growth of globular grains. Thus, completely wetted grain boundary and uniformity depend on the using of low cooling rate.

Lower cooling rate also causes the primary α-Al grain coarsening. Coalescence phenomena were found in the semisolid microstructure [as indicated with the red arrow in Figs. 2(c) and 2(d)]. This may be because two grains share a common crystallographic orientation by grain rotating to obtain a low energy of grain boundary, when the liquid films present between the grains are relatively narrow.^{Reference Chaim20} In addition, Ostwald ripening also occurred in the semisolid microstructure with large grains growing at the expense of smaller ones [marked as ‘×’ in Fig. 2(c)]. When the cooling rate was further decreased, most of the coarse grains were observed in the microstructure [marked as ‘A’ in Fig. 2(d)]. Given grain shape, there are likely to have resulted from coalescence.

C. Effect of the ultrasonic vibration temperature range

Figures 3(a)–3(e) show the microstructure evolution in the semisolid state for the Al₂O_3np/Al7075 composite treated by applying SSUV of 1 kW in different temperature ranges at the cooling rate of 10 °C/min. To avoid re-aggregation of the dispersed Al₂O_3np, the starting temperature of SSUV was selected at 700 °C. And the temperature range of SSUV is from 700 to 630 °C, from 700 to 625 °C, from 700 to 620 °C, or from 700 to 615 °C. Figure 3(f) depicts the effect of the SSUV temperature range on the average size and shape factor of primary α-Al grains. It can be seen from Fig. 3 that the solid fraction increases with the reduction of the ending temperature of SSUV. In addition, it is also clear that all the primary α-Al grains are relatively small in the semisolid microstructure treated by SSUV in the temperature ranges of 700–630 °C, 700–625 °C, and 700–620 °C (65–73 μm) and its shape factors are above 0.8 [Figs. 3(a)–3(c), and 3(f)]. When the ending temperature of SSUV was decreased to 615 °C, the grains became coarse (164.6 μm) and obviously irregular (0.63), which would have an adverse influence on the thixotropic behavior and the final mechanical properties [Figs. 3(d)–3(f)].

FIG. 3.

Microstructure evolution of the semisolid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of 1 kW in different SSUV temperature ranges at the cooling rate of 10 °C/min: (a) 700–630 °C, (b) 700–625 °C, (c) 700–620 °C, and (d and e) 700–615 °C, and (f) average grain size and shape factor versus ending temperature of SSUV at the cooling rate.

The lower the ending temperature of SSUV is, the more the solid fraction is in the semisolid slurry, thereby weakening the effect of SSUV on the microstructure. In Figs. 3(a)–3(c), the average sizes and shape factors of primary α-Al grains in the three semisolid slurries are nearly the same, and the morphology of the grains becomes fine globular shape. The formation of globular grains can be mainly attributed to grain boundary wetting and SSUV. Higher ending temperature of SSUV indicates that there are more liquid fractions in the microstructure, contributing to grain boundary wetting of solid grains and giving full play to the SSUV effect. Meanwhile, it is noted that agglomerates of fine globular grains can be observed in Fig. 3(a). This is mainly related to shorter ultrasonic vibration time resulting from the selection of higher ending temperature of SSUV. The grains, thus, could not be dispersed fully. The higher solid fraction makes the intensity of forced convections induced by SSUV in the semisolid slurry weaker, so it is difficult to obtain the solute concentration and temperature uniformities in the residual liquid if the ending temperature of SSUV is much lower than the liquidus, accordingly causing the occurrence of the irregular grains resulting from preferred orientation growth. Grain coalescence and Ostwald ripening were also found in the microstructure.

Figure 3(e) shows the SEM micrograph of the semisolid slurry treated by applying SSUV from 700 to 615 °C at the cooling rate of 10 °C/min. Large pools of liquid were found in the coarse irregular grains [marked as ‘B’ and ‘C’ in Fig. 3(e)]. They may originate from melting of eutectic phases in the grains, which are caused by chemical segregation.^{Reference Bolouri, Shahmiri and Cheshemeh21} In addition, during spheroidization of rosette-like grains, the liquid may be wrapped in grains to become liquid pools when the migration velocity of liquid phases is less than that of spheroidization [marked as ‘C’ in Fig. 3(e)].^{Reference Manson-Whitton, Stone, Jones, Grant and Cantor22} The presence of intragranular liquid pools makes the actual liquid fraction become less compared to that predicted from the DSC curve. Meanwhile, when the ending temperature of SSUV is lower, the semisolid microstructure contains relatively low liquid phases. Thus, in the semisolid slurry, the liquid films present between the grains are narrow and then it is difficult to carry out successful thixoforming under this condition.

D. Effect of ultrasonic power

Figure 4 shows the semisolid microstructures prepared by applying SSUV with different ultrasonic powers from 700 to 620 °C at the cooling rate of 10 °C/min. For comparison, the ultrasonic powers of 500 W and 1.5 kW were carried out, respectively. It can be seen from Fig. 4(a) that aggregated nearly globular grains and liquid segregation are present in the microstructure because the forced convections caused by SSUV with low ultrasonic power are not enough to make the semisolid composite slurry uniform. Fine globular grains, large grains with a dendrite growth feature, and gas cavities can be observed in Fig. 4(b). When the ultrasonic power was high (1.5 kW), there were strong acoustic streaming and impulse force in the slurry. This contributes to fragmenting of dendrite arms and uniformly dispersing of solid grains in the residual liquid, which indicates that fine globular grains can be achieved in a short period of time. Nevertheless, it is difficult to control the processing because the time is too short. Meanwhile, SSUV with high ultrasonic power also triggers adverse effects. The high temperature induced by cavitation in the local zone causes remelting of the fine globular primary α-Al grains which makes large grains grow further. The SSUV of high power generates strong turbulent flow, finally causing fluctuation of the slurry, indicating capture of extra gas in the semisolid slurry, as shown in Fig. 4(b). The temperature field and solute distribution, which are in the equilibrium state, become unstable after a prolonged period of SSUV with high power,^{Reference Schultz, Ferguson and Rohatgi23} indicating preferred growth of the primary α-Al grains (marked as ‘D’ in Fig. 4). Thus, the ultrasonic power could not be too low or too high, and the appropriate ultrasonic power in this study is 1 kW.

FIG. 4.

Microstructures of the semisolid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of (a) 500 W and (b) 1.5 kW, respectively, from 700 to 620 °C at the cooling rate of 10 °C/min.

In summary, the good semisolid Al₂O_3np/Al7075 composite slurry with an average grain size of 73 μm and a shape factor of 0.84 can be obtained by applying an SSUV of 1 kW from 700 to 620 °C at the cooling rate of 10 °C/min.

Size and shape factor of primary α-Al grains have great influence on semisolid processing^{Reference Bolouri, Shahmiri and Kang7,Reference Rokni, Zarei-Hanzaki, Abedi and Haghdadi8,Reference Dong, Cui, Le and Lu24–Reference Qin, Zhao, Cong, Zhou and Xu27} and final mechanical properties of products. Kirkwood^{Reference Kirkwood28} reported that the semisolid slurry with fine grains was suitable for processing and its samples possessed improved mechanical properties over those with coarse microstructure. During semisolid forming processes, samples with different grain sizes and shape factors present different semisolid behaviors. The shape factor of primary α-Al grains, however, has a large influence than grain size in semisolid behavior of the material.^{Reference Ashouri, Nili-Ahmadabadi, Moradi and Iranpour29} It is generally reported that if the shape factor is less than 0.5, the semisolid formability of materials is decreased.^{Reference Jiang, Wang and Atkinson30} If the grains are not sufficiently round, the grains interfere with each other when sheared, leading to a disappearance of thixotropic behavior.^{Reference Ji, Fan and Bevis31} Thus, the semisolid microstructures in the experimental conditions of this study (D _eq = 73, S _F = 0.84) is suitable for thixoforming compared to those obtained via the conventional process.

E. Second phases

Figure 5 shows DTA curves of the Al7075 alloy and Al₂O_3np/Al7075 composites in different conditions with a heating rate of 10 °C/min. For the as-cast Al7075 alloy, it can be seen that two endothermic peaks are located at about 471 and 530 °C, which was due to the melting of multicomponent eutectic phases and binary precipitates, as shown by the dotted line in Fig. 5. The eutectic phases were first melted due to their lower melting point. However, for the semisolid Al7075 alloy, there are no obvious endothermic peaks induced by the eutectic melting. This may be attributed to dissolving of many alloying elements into the solid grains. The endothermic peaks of second phases cannot be clearly observed on the DTA curve of the as-cast composites, suggesting that the addition of Al₂O_3np may have affected the diffusion behavior of alloying elements in the melts and then lead to the change in the number and type of precipitates, for which the reason is not completely clear and further investigation is needed. Nevertheless, there are less obvious endothermic peaks for the semisolid composites compared to the as-cast composites. The reduction of coarse and continuously distributed eutectic phases along the grain boundaries contributes to achieving the composites with high performance. The changes in DTA curves have been attributed to the chemical homogeneity induced by applying SSUV in the semisolid temperature range. The occurrence of an exothermic peak at about 664 °C on the DTA curve of the semisolid composites may result from the interface reaction between Al₂O_3np and liquid Al.

FIG. 5.

DTA curves (10 °C/min) of different materials.

Figure 6 shows SEM images of the microstructure in the as-cast Al7075 alloy and EDS results of principal phases in the alloy. As indicated in Figs. 6(a) and 6(b), it can be seen that there are most of coarse Chinese script-type eutectic phases continuously distributed along the grain boundaries. The EDS result of the eutectic phase (69.30 at.% Al, 9.19 at.% Zn, 15.28 at.% Mg, 6.23 at.% Cu) is shown in Fig. 6(c). The result indicates that Cu and Al contents are higher than the average level of chemical composition in the Al7075 alloy. It is obvious that MgZn₂ dissolved the Cu and Al elements and formed the Sigma-Mg(Zn, Cu, Al)₂ phase.^{Reference Fan, Jiang, Meng and Zhong32} Meanwhile, square-like phases were found in the eutectic structure [indicated with the point 2 in Fig. 6(b)]. The EDS result (79.27 at.% Al, 3.86 at.% Zn, 3.01 at.% Mg, 7.54 at.% Cu, 4.18 at.% Fe, 2.13 at.% C) is shown in Fig. 6(d), which is close to the stoichiometric Al₇Cu₂Fe phase in composition. The results are in accordance with a study by Fan et al.^{Reference Fan, Jiang, Meng and Zhong32} In addition, low melting point T-Al₂Mg₃Zn₃ phases appeared between the Chinese script-type eutectic phases, as arrowed in Fig. 6(b) (70.99 at.% Al, 11.45 at.% Zn, 13.89 at.% Mg, 3.66 at.% Cu).

FIG. 6.

(a and b) SEM images of the primary eutectic structure and coarse phase in as-cast Al7075 and (c and d) EDS results of point 1 and 2.

Figure 7 shows SEM images of the semisolid composite slurry treated by applying an SSUV of 1 kW from 700 to 630 °C at the cooling rate of 10 °C/min. It is noted that there are a mass of coarse second phases distributing at the primary α-Al grain/liquid interface [as indicated with red box in Fig. 7(a)]. From high magnification image of a red box, it can be found that the morphology of the coarse second phases is fishbone-like shape. According to the EDS result (93.36 at.% Al, 3.74 at.% Fe, 2.90 at.% Zn), the fishbone-like second phase is Fe-rich phase (Al₁₃Fe₄). The presence of the phases will be detrimental to final mechanical properties of materials. When the temperature was 630 °C, SSUV could be effectively applied due to higher liquid fraction existing in the semisolid slurry. Higher ending temperature of SSUV, however, indicates the decrease in ultrasonic vibration time so that alloying elements accumulated at the solid–liquid interface during solidification are uniformly distributed throughout the liquid Al. According to Du et al.^{Reference Du, Chang, Huang, Gong, Jin, Xu, Yuan, Liu, He and Xie33} and the Arrhenius equation, the diffusion coefficients of Zn, Mg, Cu, and Fe elements in liquid Al are 2.65 × 10⁻⁹ m²/s, 7.13 × 10⁻⁹ m²/s, 4.30 × 10⁻⁹ m²/s, and 2.20 × 10⁻⁹ m²/s, respectively. It clearly shows that the Fe element has the slowest rate of diffusion and Zn element comes second, indicating that Fe and Zn elements are aggregated at the solid–liquid interface, in accordance with the EDS results. This is in favor of formation of fishbone-like Al₁₃Fe₄ phases.

FIG. 7.

(a) SEM image of the semi-solid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of 1 kW from 700 to 630 °C at the cooling rate of 10 °C/min and (b) a high magnification SEM image of (a).

Figure 8 shows SEM images and EDS results of semisolid composite slurry treated by applying an SSUV of 1 kW from 700 to 620 °C at the cooling rate of 10 °C/min. It can be observed from Fig. 8(a) that coarse eutectic phases do not appear either at the primary α-Al grain/liquid interface, or inside the liquid matrix, which suggests that SSUV has enough time for uniformly dispersing alloying elements into the liquid Al, finally avoiding the formation of coarse eutectic phases. Meanwhile, it can also be found that “small grains” in the liquid matrix are much smaller than those in Fig. 7, which further proves that appropriate ultrasonic treatment accelerated nucleation of the α-Al phase. It is of interest to note that the microstructure seems to consist of cellular interface, as arrowed in Fig. 8(a). The pushed particles disturbed the temperature and solute gradient ahead of solidification front, leading to growth velocity difference between the interfaces with and without the particles, and finally promoting the formation of a cellular interface.^{Reference Chen and Yan15,Reference Dong, Cui, Le and Lu24} The formation of the cellular structure may also be the consequence of solidification of the liquid on the surface of globular primary α-Al grains during quenching.^{Reference Birol6,Reference Qin, Zhao, Cong, Zhou and Xu27} It also can be seen from the high magnification image of the liquid matrix [Fig. 8(b)] that the phases with an average particle size of 1–2 μm are distributed uniformly and discontinuously along the grain boundaries of the “small grains”. The EDS results in Figs. 8(c) and 8(d) reveal that the granular phases distributed in the liquid matrix mainly consist of Fe-rich intermetallic, AlZnMgCu eutectic, and Al₂O₃ particles (at point 1: 68.96 at.% Al, 2.47 at.% Zn, 1.99 at.% Mg, 7.17 at.% Cu, 3.72 at.% Fe, 10.64 at.% O, 5.05 at.% C; at point 2: 68.63 at.% Al, 5.94 at.% Zn, 4.21 at.% Mg, 2.48 at.% Cu, 8.71 at.% O, 10.03 at.% C). The carbon element in the clay-graphite crucible might diffuse into the melts during fabrication. Under applied stress, the granular phases discontinuously distributed in the liquid matrix act as an obstacle to dislocation movement, leading to dislocation pileups and enhancing the mechanical properties of thixoformed products of the composites.

FIG. 8.

SEM–EDS images of the semisolid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of 1 kW from 700 °C to 620 °C at the cooling rate of 10 °C/min: (a) SEM image of the composite, (b) high magnification image of a liquid matrix region, and (c and d) EDS result of point 1 to 2, respectively.

Figure 9 shows TEM images of the semisolid composite slurry treated by applying an SSUV of 1 kW form 700 to 620 °C at the cooling rate of 10 °C/min. Al₂O_3np and nanosized MgAl₂O₄ and Al₇Cu₂Fe phases were found to be uniformly distributed in the matrix, as shown in Fig. 9(a). It can be seen from the HRTEM image [Fig. 9(b)] that clean interfaces between the matrix and nanoparticles appear and there is no transition layer, indicating that no injurious interfacial reaction occurs. Al₂O_3np had been mixed with Mg powders before they were added into the melt. According to Ref. Reference Chen and Yan15, introducing Mg powders would result in the weakening and early rupturing of the Al₂O_3np surface, thus promoting wettability, or reacted with alumina directly, resulting in the formation of a spinel (MgAl₂O₄). SAD patterns in Figs. 9(d)–9(f) further verify the presence of Al₂O_3np and formation of nanosized MgAl₂O₄ and Al₇Cu₂Fe particles. According to the crystal structure of α-Al, Al₂O₃, MgAl₂O₄, and Al₇Cu₂Fe phases, it was confirmed that there are the well-defined crystallographic orientation relationships among the phases: $\left( {200} \right){\left[ {010} \right]_{{\rm{Al}}}}//\left( {10\bar{1}4} \right){\left[ {40\bar{4}1} \right]_{{\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}}}$, $\left( {111} \right){\left[ {110} \right]_{{\rm{Al}}}}//\left( {111} \right){\left[ {110} \right]_{{\rm{MgA}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}}}$, and $\left( {100} \right){\left[ {100} \right]_{{\rm{Al}}}}//\left( {10\bar{1}} \right){\left[ {10\bar{1}} \right]_{{\rm{A}}{{\rm{l}}_{\rm{7}}}{\rm{C}}{{\rm{u}}_{\rm{2}}}{\rm{Fe}}}}$, indicating that there is a good interfacial integrity between the particles and the matrix. The lattice misfits between Al₂O₃, MgAl₂O₄, and Al₇Cu₂Fe, and α-Al are listed in Table I. It is clear that the lattice misfits between the hard phases and α-Al are low. Bramfitt^{Reference Bramfitt37} reported that the nucleation catalysts were effective in promoting nucleation when the lattice misfits between the lattice parameters of the substrate and the nucleating phase were less than 6%. The results as above, thus, justify the fact in which Al₂O₃, MgAl₂O₄, and Al₇Cu₂Fe particles can effectively act as potent sites for the nucleation of a-Al grains, finally improving the nucleation rate of primary α-Al grains.

FIG. 9.

TEM micrographs of the semisolid Al₂O_3np/7075 composite slurry prepared by applying an SSUV of 1 kW from 700 to 620 °C at the cooling rate of 10 °C/min: (a) bright field TEM image, (b) HRTEM image showing Al₂O_3np interface and MgAl₂O₄ formation at the interface of the Al7075 alloy with 1.5 wt% Al₂O_3np, (c) HRTEM image of the Al₇Cu₂Fe phase, and (d–f) SAD pattern of Al₂O₃, MgAl₂O₄, and Al₇Cu₂Fe phases, respectively.

TABLE I.

Lattice misfits between Al₂O₃, MgAl₂O₄ and Al₇Cu₂Fe, and α-Al, where “S” stands for “substrate” and “N” for “nucleated phase”.

By comparing the microstructure of semisolid slurry with the DSC curve, it is found that the liquid fraction apparent in the micrographs is much less than the quantity derived from the DSC result. Again, it is found that the solid fraction in the semisolid slurry, treated by applying an SSUV of 1 kW from 700 to 620 °C at the cooling rate of 10 °C/min, is about 0.715 via image analysis software. Nucleation of primary α-Al grains was enhanced by means of the cavitation and acoustic streaming induced by SSUV, finally causing increased solid fraction. On the other hand, the enhanced heterogeneous nucleation in the Al7075 alloy due to the presence of Al₂O₃, MgAl₂O₄, and Al₇Cu₂Fe particles also helped in premature precipitation of primary α-Al grains and increasing of solid fraction.

For thixoforming, grain size and shape factor have to be balanced with liquid fraction. If the liquid fraction is high, the formability cannot be developed profitably because of the substantial amount of casting defects. If the liquid fraction is low, the deformation force increases and the slurry is not flow adequately to fill the die. Thus, applying an SSUV of 1 kW from 700 to 620 °C at the cooling rate of 10 °C/min can prepare a fine globular microstructure with high solid fraction [Figs. 2(c) and 3(c)], which is satisfactory for meeting the microstructural requirement of thixoforming. This microstructure is reasonably consistent with that successfully used to forge Al7075 alloy in the semisolid state.^{Reference Rokni, Zarei-Hanzaki, Abedi and Haghdadi8,Reference Wu, Lü, An and Nakae10}