Effect of Al-3Ti-4.35La Master Alloy on the Aggregation and Sedimentation Characteristics of TiC in Al-7Si Alloys

Abstract

The Al-Ti-C alloy is the most widely used grain refiner for Al-7Si alloys. However, TiC particles in Al-7Si alloys tend to aggregate and settle, thereby reducing their refinement efficiency. In the present paper, a novel Al-3Ti-4.35La master alloy was developed, and its influence on the stability of TiC particles in Al-7Si alloys was investigated by XRD, SEM, and TEM. The results show that when the Al-TiC alloy is added to the Al-7Si alloy, TiC will accumulate and settle obviously after holding the alloy for a certain time (15 min, 30 min, and 60 min). When the Al-TiC alloy and the Al-3Ti-4.35La master alloy are added to the Al-7Si alloy, the aggregation and settlement of TiC are weakened under the same holding time. Additionally, the refinement effect of TiC is enhanced. The Ti and La elements dissolved by Ti₂Al₂₀La in the Al-3Ti-4.35La master alloy are absorbed on the surface of the TiC particles, which improves the wettability between the TiC particles and the aluminum melt, reduces the agglomeration and sedimentation of TiC particles in the aluminum melt, and improves its stability.

1. Introduction

Al-Si casting alloys have garnered extensive application in lightweight automotive structural materials due to their high specific strength, excellent mechanical properties, low manufacturing cost, and easy recycling [1,2,3,4]. However, the large α-Al grains in the microstructure of unrefined Al-Si alloys can significantly reduce the mechanical properties of the alloys. Therefore, adding refiners to Al-Si alloys during casting has become the main means to improve their mechanical properties [5,6,7,8].

Currently, Al-Ti-B is the commonly used refiner [9,10]. However, the Al-Ti-B grain refiner loses its refining ability when Si and Zr elements are added at about 0.2 wt.% [11]. Additionally, the Al-Ti-C grain refiner resists Si and Zr “poisoning” and uses TiC as its primary nucleating particle to refine α-Al [12,13]. TiC particles are about one-third the size of TiB₂, which reduces their settling in the aluminum melt and enhances grain refinement [14].

However, the stoichiometric ratio between Ti and C in the TiC crystal structure is not fixed, and a large number of vacancy defects are prone to occur in the lattice or atomic gap, therefore decreasing the stability of TiC [15]. In addition to the structural instability of TiC, studies have shown that Si also significantly impacts TiC particles. When the Si content exceeds 3.5%, the stability of TiC particles will be reduced, and the decomposition of TiC particles will be accelerated [16]. To improve the stability of TiC particles in Al-Si alloys, Nie et al. [17] added B and N elements to TiC particles to form TiC_x by regulating the chemical properties of TiC particles. Eskin et al. [18] improved the agglomeration of TiC particles and promoted the nucleation efficiency of TiC by ultrasonic melt treatment technology. Recent research [19,20] has shown that by combining rare earth elements with Al-Ti-B/C, Al-Ti-B/C-RE-refining modifiers can effectively change the coarse grain of Al-Si alloys. Wu et al. [21] proposed that the wettability of liquid aluminum and graphite can be improved by adding a small amount of Ce into molten aluminum. Zhao et al. [22] showed that Ti₂Al₂₀RE will release rare earth elements and Ti to the A356 alloy, enriching the surface of TiC particles and improving the stability of TiC particles. Furthermore, the incorporation of the rare earth element La into the Al-Ti-C alloy not only facilitated further grain refinement but also significantly bolstered the material’s mechanical properties [23,24,25]. Despite those findings, the direct evidence regarding the effect of rare earth elements on TiC’s nucleation ability and stability remains insufficient. Furthermore, how to improve the stability of TiC particles by adding rare earth elements, reduce the aggregation and sedimentation of TiC particles, and ensure their even dispersion in Al-Si alloys requires further study.

Based on the aforementioned findings, the Al-3Ti-4.35La master alloy was combined with the Al-7Si and Al-TiC master alloys to investigate the influence of the Al-3Ti-4.35La master alloy on the stability of TiC particles in hypoeutectic Al-7Si alloys. This study analyzed the mechanistic influence of the Al-3Ti-4.35La master alloy on the aggregation and sedimentation behavior of TiC particles in Al-7Si alloys. The current research provides a theoretical basis for the industrial application of Al-TiC master alloys.

2. Experiment

The Al-3Ti-4.35La master alloy and the Al-TiC alloy were prepared by the in situ reaction synthesis method of aluminum melt. For the specific preparation process, see references [11,26]. The detailed compositions of the Al-3Ti-4.35La master alloy, the Al-TiC alloy, and the Al-7Si alloy are shown in

Table 1. Alloy composition (wt%).

Alloy	Al	Si	Mg	Ti	Mn	La	C	Zn
Al-3Ti-4.35La	Bal.	-	-	3.4	-	4.24	-	-
Al-TiC	Bal.	-	-	4.7	-	-	1.1	-
Al-7Si	Bal.	6.78	0.315	0.18	0.039	-	-	0.037

, which were measured by an X-ray spectrometer(Panako Zetium, Breda, The Netherlands).

To study the effect of the Al-3Ti-4.35La master alloy on the aggregation of TiC particles, the Al-7Si alloy, Al-3Ti-4.35La master alloy, and Al-TiC alloy (mass ratio, Al-3Ti-4.35La:Al-TiC:Al-7Si = 1:1:2) were put into a graphite crucible at 750 °C, holding for 15 min. A total of 1wt% C₂Cl₆ was added for refining and degassing. The alloy was cast into a steel mold (a diameter of 10 mm) with a preheating temperature of 200 °C, as shown in Process Ⅰ of Figure 1. The Al-7Si alloy and Al-TiC alloy (mass ratio, Al-TiC:Al-7Si = 1:2) were prepared as a control group. To further investigate the sedimentation behavior of TiC particles in the hypoeutectic Al-7Si alloy, the alloy according to the above mass ratio is held at 750 °C for 30 min and 60 min, respectively. After refining and degassing, the alloy was cast into a square mold with a preheating temperature of 200 °C, as shown in Process Ⅱ of Figure 1. The sample numbers are shown in

Table 2. Sample number and mass ratio of different alloys.

Samples	Mass Ratio			Holding Time (min)	Mixing Method
	Al-Si Alloy	Al-TiC	Al-Ti-La
A0	2	1	-	15	stirring
A1	2	1	1	15	stirring
A2	2	1	-	30	stirring
A3	2	1	1	30	stirring
A4	2	1	-	60	stirring
A5	2	1	1	60	stirring

.

The Al-3Ti-4.35La master alloy, Al-TiC alloy, and Al-7Si alloy by the addition of the Al-3Ti-4.35La master alloy and the Al-TiC alloy were characterized by using a Thermo ARL X-ray diffractometer (XRD, Thermo Fisher Scientific, Waltham, MA, USA) and monochromatic Cu Kα radiation at 2θ angles ranging from 20 to 90°. The microstructure of the alloy was analyzed by a Tescan Mira4 SEM and Xplore 30(Tescan, Brno, Czech Republic). Talos F200X G2 transmission electron microscopy (TEM, Thermo Fisher Scientific, Waltham, MA, USA) with selected electron diffraction (SAED, Thermo Fisher Scientific, Waltham, MA, USA) was used to confirm the crystal structure, and the TEM samples were thinned by double-jet electrodeposition using a mixture of nitric acid.

3. Results and Discussion

3.1. Analysis of Al-TiC and Al-3Ti-4.35La Master Alloys

Figure 2a shows the microstructure of the Al-TiC master alloy, which has a large number of granular particles dispersed in the matrix. Figure 2b shows that the main phases of the Al-TiC master alloy prepared by the in situ reaction method are α-Al and TiC. Figure 2c,d is a larger version of Figure 2a and an EDS analysis. The Ti and C elements are evenly distributed in the granular phase. The ratio of Ti:C in the granular phase is close to 1:1. Combined with an XRD analysis in Figure 2b, TiC particles are abundant in the Al-TiC master alloy.

Figure 3a shows the microstructure of the Al-3Ti-4.35La master alloy. A large number of white bulk second phases are distributed in the matrix, and some of the white phases are mixed with gray phases. Additionally, fibrous white structures exist at the grain boundaries. Figure 3b shows the XRD pattern of the Al-3Ti-4.35La master alloy. The main phases of the Al-3Ti-4.35La master alloy are α-Al and Ti₂Al₂₀La, which also have a small amount of the Al₃Ti and Al₁₁La₃ phases. Figure 3c,d is a larger version of Figure 3a and an EDS analysis. Combined with an XRD analysis in Figure 3b, the white blocky, fibrous, and gray phases of the Al-3Ti-4.35La master alloy are Ti₂Al₂₀La, Al₁₁La₃, and Al₃Ti, respectively.

3.2. Influence of Al-3Ti-4.35La Master Alloy on the Aggregation Property of TiC

Figure 4 shows the XRD pattern after adding the Al-TiC master alloy (A0) and adding the Al-TiC and the Al-3Ti-4.35La master alloy (A1) to the Al-7Si alloy at 750 °C for 15 min. The intensity of TiC corresponding to the diffraction peak in the A1 sample is significantly higher than that in the A0 sample. The number of TiC particles in the Al-7Si alloy increases after adding the Al-3Ti-4.35La master alloy.

Figure 5 shows the SEM and EDS analyses of the Al-7Si alloy with the addition of the Al-TiC master alloy (A0). The TiC particles tend to aggregate. This is mainly due to the affinity of the TiC particles themselves and their poor wettability with other particles in the melt [27]. The aggregation of TiC particles will reduce the effective heterogeneous nucleation sites of α-Al, thereby decreasing the refining effect.

Figure 6 shows the SEM and EDS analysis of the A1 sample. Figure 6a shows the SEM image of TiC in the Al-7Si alloy after composite addition, and the degree of TiC aggregation is significantly lower than that of the A0 sample. Figure 6b is the enlarged view of Figure 6a, and the EDS analysis is shown in Figure 6c–f. Meanwhile, the EDS results of each point in Figure 6b are shown in

Table 3. Analysis results of EDS points (d–f) in Figure 6.

Point	Elements At.%
	Al	Ti	C	La	Si
1	21.34	34.02	38.34	2.02	4.28
2	32.80	0.00	0.00	4.68	66.20
3	0.00	0.00	0.00	40.40	59.60

. The results show that the distribution of TiC particles in the A1 sample is strongly correlated with the distribution of free La, and La elements are distributed on the surface of TiC. At the same time, some of the La will combine with Si in the melt to form La-Si intermetallic compounds.

3.3. Influence of Al-3Ti-4.35La Master Alloy on the Settling Property of TiC

Figure 7 shows the macroscopic structure of A2~A5 samples. In sample A2, which was heated for 30 min, the grain size at the bottom of the sample was significantly smaller than that in the upper middle part of the sample. In sample A3, which is heated for 30 min after adding the Al-3Ti-4.35La master alloy, the grain size at the bottom and the upper middle part tended to be consistent, as shown in Figure 7a,b. The grain distribution of the A4 and A5 samples also has the same rule. When TiC particles exist alone, they are easy to settle to the bottom of the melt. TiC can be used as an effective heterogeneous nucleation site of α-Al, promote α-Al nucleation, and reduce the grain size of α-Al. Due to its sedimentation characteristics, the grain size at the bottom of the sample is smaller than the middle and upper parts. When Ti₂Al₂₀La and TiC particles co-exist, the sedimentation of the TiC particles can be effectively reduced so that the grain size distribution in the sample is consistent.

Figure 8 shows the XRD analysis results of the bottom region of the A2~A5 samples. After holding for 30min, the diffraction peak (θ = 42.10°) corresponding to TiC particles at the bottom of the sample with the Al-3Ti-4.35La master alloy added is significantly lower than that without the addition. After holding for 60 min, the diffraction peak intensity corresponding to TiC particles at the bottom of the sample with the Al-3Ti-4.35La master alloy is also lower than that without the addition. That is, the Al-3Ti-4.35La master alloy can weaken the sedimentation of TiC particles in Al-Si alloys.

Figure 9 shows the SEM and EDS images of the A2~A5 samples. There is an obvious black precipitate at the bottom of the samples, and the corresponding EDS surface scanning analysis reveals the enrichment of Ti and C elements. Combined with the XRD analysis results in Figure 8, the black precipitate is TiC particles. The precipitation of obvious TiC particles at the bottom of the sample without adding the Al-3Ti-4.35La master alloy is obvious, as shown in Figure 9a,b. However, TiC precipitation in the bottom of the alloy sample significantly decreases after adding the Al-3Ti-4.35La master alloy, as shown in Figure 9c,d.

3.4. Effect of Al-3Ti-4.35La Master Alloy on the Stability of TiC in Al-Si Alloy

From the above analysis, it can be seen that the addition of the Al-3Ti-4.35La master alloy reduces the agglomeration and settlement characteristics of TiC in Al-Si alloys, thus improving the refinement effect of TiC in Al-Si alloys. Studies have shown that the main reason for the decrease in TiC stability in Al-Si alloys [28] is that TiC decomposition occurs, as shown in the following equation:

3TiC + 4Al ⇄ Al₄C₃ + 3Ti

(1)

In this study, Al₄C₃ was not detected. This is mainly because the increase in Ti concentration in melt could promote the reaction to reverse. TiC particles would not be “poisoned” into Al₄C₃.

At the same time, the long-term static insulation of the Al-TiC master alloy in the Al-7Si alloy will attenuate the refinement effect [29], which is mainly due to the sedimentation of TiC particles, thereby decreasing the effective nucleation site of α-Al. The sedimentation characteristics of TiC particles in the melt can be described by the Stokes formula [30]:

υ = 2r²(ρ₁ − ρ₂)/9μ

(2)

where υ is the particle sedimentation rate, r is the particle radius, ρ₁ is the particle density, ρ₂ is the melt density, and μ is the melt viscosity. The density of TiC in the aluminum melt is 4.93 g/cm³, and the density of aluminum melt is about 2.4g/cm³ [11]. Due to the density difference, TiC sedimentation will occur in the aluminum melt. At the same time, TiC particles tend to aggregate into large particles and settle. However, the sedimentation characteristics of TiC particles can be improved when the Al-3Ti-4.35La master alloy is added.

Figure 10 presents the microstructure analysis of TiC in the A1 sample. Figure 10a shows the TEM bright-field image of TiC. The high-resolution image at the interface between TiC and α-Al is shown in Figure 10b, and the inverse Fourier transform image corresponding to region 2 is shown in Figure 10c. The results show that the TiC atoms at the interface between TiC and α-Al are arranged in an orderly manner, with no evidence of atomic distortion. Additionally, the interface between TiC and α-Al exhibits good lattice matching, with a TiC lattice spacing of 0.21 nm, demonstrating that TiC can act as an effective heterogeneous nucleation substrate for α-Al.

Furthermore, there is a rich area of the La element around the TiC particles, and the concentration of Ti around and on the surface of TiC is higher, as shown in Figure 10e,f. The Ti₂Al₂₀La phase, which is the main particle in the Al-3Ti-4.35La master alloy, will dissolve in the Al-7Si alloy and release Ti and La elements. The poor wettability between the Ti/Al and TiC particles leads to the formation of a Ti-rich zone around or on the TiC surface, reducing the density difference between the TiC particles and the aluminum melt (Ti density: ~4.51 g/cm³ [11]). Simultaneously, free La elements accumulate on the TiC surface, which can promote the wettability between the TiC particles and the matrix Al. Therefore, the sedimentation phenomenon of the TiC particles is significantly reduced due to the combined effects of the Ti and La elements released by Ti₂Al₂₀La.

Ti₂Al₂₀La is the primary phase in the Al-3Ti-4.35La master alloy. Although previous studies [23,24,25] have shown that the addition of La improves the wettability between TiC and Al, the specific mechanism remains unclear, particularly whether Ti₂Al₂₀La directly influences the stability of TiC particles in the Al-7Si alloys. Figure 11 presents a microstructural analysis of Ti₂Al₂₀La in the aluminum melt, with EDS point scanning, area scanning, and line scanning analyses. The results indicate that TiC particles are not observed around Ti₂Al₂₀La in the hypoeutectic Al-7Si alloy, suggesting that Ti₂Al₂₀La does not directly interact with TiC. This further confirms that the improvement in TiC stability is primarily due to the release of La and Ti elements during the dissolution of Ti₂Al₂₀La. Additionally, Si tends to accumulate around Ti₂Al₂₀La. The interfacial relationship between Si and Ti₂Al₂₀La was analyzed through high-resolution imaging and inverse Fourier transform mapping, as shown in Figure 11b,c. The analysis reveals that the lattice spacings of eutectic Si and Ti₂Al₂₀La are 0.17 nm and 0.39 nm, respectively, with well-aligned lattices and no significant atomic distortion regions. This indicates that eutectic Si merely adheres to the periphery of the Ti₂Al₂₀La phase without affecting its structural integrity. This discovery provides a valuable reference for further investigation into the modification mechanism of eutectic Si by Ti₂Al₂₀La in Al-Si alloys.

Previous studies have shown [31] that Si can accelerate the decomposition of TiC particles at 800 °C. To study the effect of master Si on the stability of TiC in Al-7Si alloys, a detailed structural analysis of the Si-TiC interface was conducted, as depicted in Figure 12. Figure 12a shows the TEM bright field phase of TiC particles and eutectic Si. The interface between Si and TiC is analyzed by high resolution and inverse Fourier transform, as shown in Figure 12b,c. The results show that the atoms are arranged neatly at the interface of Si and TiC with no evidence of atomic distortion. Meanwhile, the lattice fringe spacing is uniform, and the lattice spacing is 0.19 nm. Figure 12e,f show the surface scanning and line scanning results in Figure 12a. Si is only distributed on the surface of TiC, and the Si element distribution is not found inside Ti. Ding et al. [32] calculated the energy required for Si diffusion into the TiC lattice, indicating that Si can spontaneously diffuse into the TiC lattice, thereby enhancing the wettability between TiC and the aluminum matrix. Additionally, Xia et al. [33] reported that the presence of Si particles near TiC particles is a common phenomenon, consistent with our experimental observations. Furthermore, the proximity of Si particles to TiC accelerates the decomposition of TiC, reducing its effectiveness as a grain refiner in Al-7Si alloys. In summary, the presence of Si does not improve the wettability of TiC particles within Al alloys.

4. Conclusions

The effect of the Al-3Ti-4.35La master alloy on the agglomeration and sedimentation of TiC in Al-7Si alloys was studied. The main conclusions are as follows:

(1).

The dissolution of the Ti₂Al₂₀La phase releases La elements into the melt, which accumulate on the surface of TiC particles. This accumulation enhances the interfacial wettability between TiC particles and the Al matrix, thereby reducing the agglomeration tendency of TiC particles. When the Al-3Ti-4.35La master alloy was incorporated into the Al-7Si alloy and held for 15 min, the distribution of TiC particles became more uniform.

(2).

The Ti elements released by the dissolution of the Ti₂Al₂₀La phase will form a Ti-rich zone around or on the surface of TiC particles. This Ti-rich zone reduces the density difference between the TiC particles and the aluminum melt, thereby decreasing their sedimentation behavior. After holding periods of 30 min and 60 min, the amount of TiC precipitation at the bottom of the sample was notably reduced.

(3).

The atomic arrangement at the Si-TiC interface exhibits an ordered structure without any detectable distortion, which demonstrates that Si does not exert a direct influence on the agglomeration and sedimentation behavior of TiC particles in the Al-7Si alloy.