Introduction

In the industrial practices of aluminum–silicon casting alloys, grain refinement and modification are usually employed to obtain fine α-Al grains together with fibrous eutectic Si phases of cast microstructures and to reach the good comprehensive mechanical properties of castings (Ref 1,2,3,4,5,6,7). In the previous works, the ternary Al-Ti-B refiners, e.g., Al-5Ti-1B refiner, have been extensively utilized to refine α-Al dendrites in the wrought aluminum alloys, since the heterogeneous nucleation effect with TiB2 and a high growth restriction factor (GRF) of titanium (Ref 8,9,10). On the other hand, in the casting practices of Al-Si casting alloys, strontium was usually used as modifier, which transforms the eutectic Si phases from a platelike to a fiber-like feature with less fading phenomenon (Ref 11,12,13,14,15). However, the grain refiners with high ratios of Ti: B usually work in a unfavorable refining efficiency in Al-Si casting alloys due to the interaction of Ti with Si, forming titanium silicides, which depletes the effective quantity of Ti elements in the melt of alloys (Ref 16). In addition, a mutual poisoning effect between B and Sr elements, forming SrB6 particles (Ref 17, 18), has been suggested to be responsible for impairing the grain refinement efficacy and modification effect. To eliminate aforementioned mutual poisoning in the Al-Si casting alloys, Bolzoni et al. (Ref 19,20,21) explored that, with the substitution of niobium for titanium in the ternary Al-Ti-B refiners, an excellent grain refining efficiency can be obtained in the commercial Al-Si casting alloys; considering crystal lattice mismatch between LaB6 and Al less than 5% (Ref 22), Pan et al. (Ref 23) reported that co-alloying of La and B elements can suppress the mutual poisoning effect of Sr and B in both binary and ternary Al-Si casting alloys, resulting in the finer equiaxed α-Al grains and fibrous eutectic Si phases in the as-cast microstructures; Liu et al. (Ref 22, 24) also developed a novel Al-La-B refiner via aluminum melt reaction method using pure La, Al-3B alloy and commercial pure Al as raw materials, but adding pure La into a high-temperature melt will inevitably encounter massive burning loss of La.

In the present work, an improved method to prepare a novel Al-2La-1B refiner, utilizing Al-10La and Al-3B master alloys as raw materials, was developed. The grain refining efficacy of as-synthesized Al-2La-1B refiner in Al-10Si-0.3Mg alloy was investigated. The effects of Al-2La-1B refiner, in Al-10Si-0.3Mg casting alloy, on the microstructures, solidification process and tensile properties were also investigated.

Experimental

Synthesis of Alloys

Al-2La-1B refiner and Al-10Si-0.3Mg alloy were both prepared in a graphite crucible. Commercial pure aluminum (99.7 wt.% Al), Al-10La and Al-3B master alloys were used to prepare the Al-2La-B refiner. Firstly, the pure aluminum was melted at 760 ± 5 °C, and then, the Al-10La and Al-3B master alloys were separately added into the melts at the temperature of 800 ± 5 °C. After holding the melts at 900 ± 20 °C for 30 min, the melts were poured into a permanent waffle mold to obtain the Al-2La-1B refiner ingots. When preparing Al-10Si-0.3Mg alloy, grain refiners and Al-10%Sr master alloy were added into the melt when the temperature was brought to 720 ± 5 °C (for comparing refining efficiency, the addition of Al-5Ti-1B and Al-2La-1B was both 1 wt.%). Most of the melts (720 ± 5 °C) were poured into ASTM: B-108-type permanent mold preheated to 250 ± 5 °C to produce tensile test bars. Each composition was cast at least 6 tensile bars.

Characterizations

The grain refining performance of grain refiners was evaluated by a cylindrical graphite mold (Φ25 mm × 100 mm) surrounded by refractory brick. The samples for refining efficacy evaluation were sectioned 25 mm from the bottom surface. The samples were anodized in Barker’s solution with the voltage of 15 V and time of 60 s. The pictures were taken with an optical microscope under polarized light. The grain sizes were measured with the linear intercept method (ten pictures for each specimen, three lines for each picture).

The chemical compositions of the samples were analyzed with optical emission spectroscopy (OES) (Table 1). An optical microscope (OM, SODTOP) was employed to analyze the microstructure of metallographic specimens. The specimens for OM were polished with metallographic abrasive paper and polishing cloth and then etched in a 0.5 vol.% HF reagent. The phase composition was characterized by means of x-ray diffraction (D8-Discover, Bruker, Germany). The morphology of the particles in different states was observed by scanning electron microscope (SEM, Sirion, FEI) equipped with energy-dispersive x-ray spectrometer (EDX). Transmission electron microscopy (TEM) analysis was used to discover the crystalline orientation relationship between LaB6 and α-Al, which was performed on a Tecnai G2 instrument. The foil sample for TEM analysis was fabricated by focused ion beam (FIB, Helios nanolab 600, FEI) machine. The as-cast tensile test bars were conducted on a mechanical testing machine (CMT5105, SANS). All 6 tensile bars were tested for each composition, and testing data of them were adopted to reach the average tensile properties.

Table 1 Chemical composition (wt.%) of Al-10Si-0.3Mg alloy in the present study
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Results and Discussion

Figure 1 shows the XRD pattern of as-synthesized Al-2La-1B refiner. Except a group of diffraction peaks referring to Al matrix (JSPDF No. 65-2869), only LaB6 phase (JSPDF No. 65-1831) can be detected. This result suggests a sufficient reaction between La and B elements to produce LaB6 phase under our experimental condition.

Fig. 1
figure 1

The XRD spectrum of Al-2La-1B master alloy

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Figure 2 indicates the morphology of Al-2La-1B refiner. Obviously, it confirms dual-phase feature in the microstructures of as-synthesized Al-2La-1B refiner and particles distribute homogenously in the Al matrix. Furthermore, the specific morphology of LaB6 was examined to be cubic shape (Fig. 2b). The EDS spectrum further verifies the particles are consisted of La and B with an atomic ratio of ~ 1/6, suggesting the formation of LaB6 (Fig. 2d)

Fig. 2
figure 2

SEM analysis (a) the Al-2La-1B master alloy; (b) extracted LaB6 particles; (c) FESEM of cubic LaB6 particle; (d) EDS spectrum of LaB6 particle

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Figure 3 illustrates the microstructures of Al-10Si-0.3Mg alloy with 1 wt.% of Al-2La-1B refiner. For comparison, the Al-5Ti-1B refiner was also utilized to modification of Al-10Si-0.3Mg alloy. Before adding grain refiners, the grain size of Al-10Si-0.3Mg is about 1670 ± 310 μm. With 1 wt.% Al-5Ti-1B, the grain size is decreased to 800 ± 150 μm. Interestingly, the grain size is dramatically reduced to 190 ± 15 μm with 1 wt.% Al-2La-1B addition. Evidently, to the Al-10Si-0.3Mg alloy, the Al-2La-1B refiner on grain refining has a much better efficacy in comparison with the Al-5Ti-1B refiner. Furthermore, considering the chemical stability of LaB6 particles, this Al-2La-1B refiner is more suitable for near-eutectic Al-Si alloys.

Fig. 3
figure 3

Grain structures of Al-10Si-0.3Mg alloy before and after grain refinement with different master alloys: (a) without master alloy; (b) 1 wt.% Al-5Ti-1B; (c) 1 wt.% Al-2La-1B

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Figure 4 demonstrates that a LaB6 particle is in the center of a α-Al grain. It suggests that LaB6 particles in this Al-Si alloy can be as heterogeneous nucleation sites. Previous works had investigated the lattice matching between the grain refiner particles and aluminum by means of TEM analysis, such as Al3Ti and TiB2, and discovered that some crystalline orientation relationships existed between Al3Ti/TiB2 and Al (Ref 25,26,27,28,29). In the present work, TEM analysis was also employed to investigate the lattice matching between LaB6 and α-Al. Figure 5 shows the TEM analysis of a LaB6 particle in the center of α-Al. The selected-area electron diffraction (SAED) of the particle (Fig. 5c) and matrix (Fig. 5d) can be assigned to a simple cubic lattice (CsCl structure type, Pm-3 m) with a parameter of a = 4.156 Å referring to LaB6 and a simple FCC lattice (Fm-3 m) with a = 4.049 Å referring to Al matrix, respectively (Ref 24). According to the electron diffraction, (100)Al is nearly parallel to (100)LaB6 and (111)Al is also parallel to (111)LaB6. As a result, the specific crystalline orientation relationships do exist between LaB6 and Al with the zone axis of [110], i.e., (100)Al || (100)LaB6, [110]Al || [110]LaB6 and (111)Al || (111)LaB6, [110]Al || [110]LaB6. Therefore, it is reasonable to suppose that the refining efficacy of Al-2La-1B refiner can be attributed to the supply of heterogeneous nucleation sites of α-Al phases with LaB6 particles.

Fig. 4
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(a) SEM microstructures and (b) EDS analysis of the central region of a α-Al in the Al-10Si-0.3Mg alloy with 1 wt.% Al-2La-1B master alloy

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Fig. 5
figure 5

TEM analysis of LaB6 located in the center of α-Al: TEM images of (a) the whole sample and (b) the region around the interface between LaB6 and Al, the SAED pattern of (c) LaB6 particle, (d) Al matrix and (e) the interface between LaB6 and Al

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The tensile properties of as-cast Al-10Si-0.3Mg alloys before and after the inoculation of Al-2La-1B refiner were investigated at room temperature. The engineering stress–strain curves of Al-10Si-0.3Mg alloy before and after grain refinement are charted in Fig. 6. It can be seen that grain refinement with LaB6 leads to higher strength and ductility. The specific value of ultimate tensile strength (UTS), yield strength (YS, i.e., σ0.2) and elongation investigated in the present work is summarized in Table 2. After the inoculation of Al-2La-1B refiner, the UTS can be improved from 219 ± 4 to 238 ± 4 MPa, the YS is enhanced from 105 ± 3 to 118 ± 2 MPa, and the elongation is increased from 3.9 ± 0.2 to 5.6 ± 0.3%, respectively. Figure 7 shows fractographs of each fractured sample. Before inoculation, the fracture surface displays a mixed quasi-cleavage and dimple morphology. With LaB6 inoculation, however, the smaller and deeper dimples uniformly distribute in the fracture surface, leading to the enhancement of tensile properties.

Fig. 6
figure 6

Typical engineering stress–strain curves of Al-10Si-0.3Mg alloys before and after grain refinement

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Table 2 Grain sizes and tensile properties of Al-10Si-0.3Mg alloy before and after grain refinement
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Fig. 7
figure 7

SEM fractographs of Al-10Si-0.3Mg alloys (a) before grain refinement and (b) after grain refinement

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Conclusions

In this work, Al-2La-1B refiner was successfully prepared, and utilizing this refiner in an Al-Si alloy, a good grain refinement was achieved and the mechanical properties were also investigated. This work can be concluded as follows,

  1. 1.

    An Al-La-B refiner was successfully prepared by using the Al-10La alloy and Al-3B alloy as the lanthanum and boron sources, respectively. There is no other phases except LaB6 existing in Al matrix. Adding this novel grain refiner, the grain size of Al-10Si-0.3Mg is decreased to 190 ± 15 μm.

  2. 2.

    The crystalline orientation relationships between LaB6 and Al were explored, that is, (100)Al || (100)LaB6, [011]Al || [011]LaB6 and (111)Al || (111)LaB6, [011]Al || [011]LaB6. This further confirms that LaB6 particles are the potent heterogeneous nuclei for α-Al grains.

  3. 3.

    After the inoculation of LaB6 particles, the UTS can be improved from 219 ± 4 to 238 ± 4 MPa; the YS is enhanced from 105 ± 3 to 118 ± 2 MPa, and the elongation is increased from 3.9 ± 0.2 to 5.6 ± 0.3%, respectively. With LaB6 inoculation, the smaller and deeper dimples uniformly distribute in the fracture surface, leading to the enhancement of tensile properties.