Microstructure of in situ Al₃Ti/6351Al composites fabricated

with electromagnetic stirring and fluxes

LI Gui-rong(李桂荣), WANG Hong-ming(王宏明), ZHAO Yu-tao(赵玉涛),

CHEN Deng-bin(陈登斌), CHEN Gang(陈 刚), CHENG Xiao-nong(程晓农)

Institute of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China

Received 27 February 2009; accepted 25 August 2009

Abstract: The 6351 wrought aluminum alloy and K₂TiF₆-CaF₂-LiCl components were selected as raw materials to fabricate in situ Al₃Ti particulate reinforced aluminum alloy at 720 ℃ via direct melt reaction method with electromagnetic stirring (EMS). CaF₂ and LiCl acted as fluxes to lower the reaction temperature of the system. It is shown that the electromagnetic stirring and fluxes accelerate the emulsion process of K₂TiF₆. Optical microscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive spectrum were utilized to analyze the microstructure and components of composites. Compared to composites fabricated without EMS and fluxes, the sizes of endogenetic Al₃Ti are refined from 10-15 μm to 2-4 μm, which are often accompanied with silicon element. The morphology of Al₃Ti or Al₃TiSi_0.22 exhibits triangle, quadrilateral and other clumpy patterns. Because of the Ca elements from CaF₂, the sizes of Mg₂Si decrease from 8-10 μm to 1-2 μm due to the formation of Ca₂Si.

Key words: 6351 Al alloy; microstructure; in situ particle reinforced aluminum composites; electromagnetic stirring; fluxes

1 Introduction

In recent years, the demands for new materials with excellent comprehensive properties become stronger and stronger[1-3]. The necessary characteristics of advanced functional and structural materials include high specific modulus, strength, hardness, ductility, corrosion resistance, low heat expansion coefficient, abradability and so on[4-5]. Over the past twenty years, considerable attentions have been paid to the particles reinforced metal matrix composites (abbreviated PRMMCs), especially the aluminum matrix ones which were developed rapidly due to their good performances and competitive cost[6-7]. The aerospace, automotive industries and other structural applications promote their developments, too.

Among all the reinforced particles, transition-metal trialuminide intermetallics such as Al₃Zr and Al₃Ti are good candidates for the in situ reinforcements of light metal matrix[8-9], which have low densities and high elastic modulus. YIN et al[10] selected Al-TiO₂ to fabricate Al₃Ti/Al at the reaction temperature of 1 000-1 200 ℃. Later, more attentions were paid to K₂TiF₆ as the source of Ti element in Al₃Ti. Not only its low cost but also the lower reaction temperature than that of Al-TiO₂ system attributes to the decrease of the sizes of Al₃Ti particles. 6351 wrought aluminum has been utilized in the architecture, automobile, pipes and so on due to its middle strength and high elongation (T6 condition, tensile strength σ_b=310 MPa, yield strength σ_s=283 MPa, δ=14.2%)[11]. However, the strength is not strong enough to act as some important structures. It is necessary to explore new ways to fabricate new materials using the wrought alloy as matrix.

In this work, 6351 wrought aluminum alloy is selected as matrix and K₂TiF₆ salt is utilized to fabricate in situ Al₃Ti reinforced 6351 aluminum wrought alloy composites with electromagnetic stirring. The main aim is to lower the temperature of reaction system and the particle size of Al₃Ti. The thermodynamic process and microstructure of Al₃Ti/6351 composites are studied in detail.

2 Experimental

2.1 Materials and sample preparation

The compositions of 6351 wrought aluminum alloy were (mass fraction): 0.7%-1.3% Si, 0.4%-0.8% Mg, 0.4%-0.8% Mn, ＜0.5% Fe, ＜0.2% Zn, ＜0.2% Ti, ＜0.1% Cu.

In order to reduce the initial melt temperature, CaF₂ and LiCl were added as fluxes. The mass ratio of mixture K₂TiF₆, CaF₂, LiCl is 80:15:5. The melting points of pure K₂TiF₆, CaF₂ and LiCl are 682, 1418 and 605 ℃, separately. The mass ratios of mixture were optimized via testing melting points through semi sphere method. The melting point of batched mixture was 486 ℃, which was decreased by 29% compared to that of pure K₂TiF₆.

Firstly, the K₂TiF₆ salt (industry reagents, ＞99.5%) was pre-heated to dehydrate the bounded water in electric oven at 250 ℃ for 3 h. Then, it was cooled, ground and screened together with CaF₂ and LiCl. At the same time, the Al alloy ingot was melted in an electric furnace under an argon atmosphere and held at 720 ℃(while without using fluxes the furnace temperature should be controlled at 800℃). Certain amount of dehydrated reactants powder covered by aluminum foils was added into the melt with graphite bell, so the in situ reaction between the aluminum alloy melt and added salts took place instantly. An electromagnetic stirrer (DJMR-1616W) was utilized to increase the mass transfer during the whole in situ reaction. The electric furnace was placed at the cavity center of stirrer, where the average magnetic induced intensity of electromagnetic stirring apparatus was 0.025 T. The reaction time was controlled as 3 min, then the subsidiary products of in situ reaction were removed and the composite melt was refined by CCl₆ and stewed for several minutes. The composite melt was cast by semi continuous casting at 710-720 ℃ and cooled at 40-60 ℃/s. The diameter of the billet was 100 mm.

2.2 Characterization

The sample was ground, polished and tectorial membraned so as to observe the microstructure clearly. LEICA DM 2500M optical microscope (OM), JSM-7001F scanning electronic microscope (SEM), Inca Energy 350 energy dispersive spectrum (EDS), JEOL-JEM-2100-HR transmission electronic microscope (TEM) and D/max-2500PC X-ray diffractometer (XRD) were used to observe the microstructure and analyze the phases in composites. STA449C DSC (differential scanning calorimetry) was utilized to analyze the thermal effect of the chemical reaction between Al powder and K₂TiF₆, at a heating rate of 5 ℃/min without protective atmosphere.

The theoretical volume fraction of Al₃Ti particles was set as 3% (volume fraction). The recovery rate of K₂TiF₄ was presumed as 90%. The mass fraction of added K₂TiF₆ to the matrix is 7.68%. The elementary components in composites were nominated as: 1.1%Si- 0.67%Mg-0.4%Mn-1.2%Ti-0.7%Ca-0.2%Cu-0.02%Cr- 0.01%Fe-Al (Bal.). The mass ratio of Mg to Si was 0.61. It was less than 1.73, which was the mass ratio of Mg to Si in Mg₂Si particle. The mass fraction and volume fraction of Mg₂Si reinforced particles in the matrix were 1.11% and 1.69%, respectively.

3 Results and discussion

3.1 Thermodynamics and kinetics of in situ reaction

The integral chemical reaction between K₂TiF₆ and molten Al is deduced as

13Al+3K₂TiF₆=3Al₃Ti+K₃AlF₆+3KAlF₄                (1)

According to the second law of thermodynamics, the Gibbs free energy ΔG of the reaction system can be expressed as:

?G=?H-T?S                                (2)

where H, S and T stand for the enthalpy, entropy and thermodynamic temperature respectively. At certain temperature, the ΔG_T can be expressed as:

where G^Θ is the standard Gibbs free energy, c_{p, r} and  c_{p, P} are the specific heat capacity at constant pressure of reactants and products, respectively; V_r and V_P are the mole volume fraction of reactants and products, respectively. Some thermodynamic parameters of involved substances in Eq.(1) are listed in Table 1.

Table 1 Some thermodynamic parameters of reactants and products in Eq.(1)

Accordingly, the ΔG_T of in situ reaction of Al-K₂TiF₆ components can be calculated as

ΔG_T =-15489.45+12.87T+0.13×10^-3T2-1.94×10⁵T^{-1                   (4)}

When the system temperature is 993 K (720 ℃), the corresponding ΔG_T is -186 kJ/mol, so the in situ chemical reaction between Al and K₂TiF₆ salt will take place spontaneously.

Fig.1 shows the DSC curve of the thermo process of Al-10%K₂TiF₆ (mass fraction). At 452.4 ℃ and 606.3 ℃, there exists an apparent exothermal peak respectively. While from 796.3 ℃ the slope coefficient is larger than the heating-up velocity, which demonstrates that there is an exothermal reaction. Connecting the ΔG^Θ values the three exothermal reactions are deduced as

2Al+3K₂TiF₆=2K₃AlF₆+6[F]+3[Ti] (at 452.4 ℃)    (5)

2Al+K₂TiF₆+2[F]=2KAlF₄+[Ti] (at 606.3℃)          (6)

3Al+[Ti]=Al₃Ti                             (7)

Fig.1 DSC curve of Al-10%K₂TiF₆

Among the products, intermetallic compound Al₃Ti is the reinforced particle. Some properties of Al₃Ti and other pure substances in the composites are listed in Table 2.

Table 2 Some properties of phases in composites

Al₃Ti in the as-cast composites comes from two ways. One is the resultant of chemical reaction as Eq.(1), the other is the precipitate of molten Ti and Al atoms during solidification. The process can be illustrated in Fig.2.

Fig.2 Phase diagram of Al-Ti binary alloy: (a) Al-Ti binary phase; (b) Partial magnification diagram of Fig.2(a)

When the volume fraction of Al₃Ti in melt is 3%, the mole ratio of Ti element to Al element is about 2:98.

The component is shown as point A in Fig.2(a). The corresponding liquidus temperature is about 900 ℃. In the in situ process, the melt is stable at 720 ℃ (point B) when the solid Al₃Ti precipitates. If the system is at equilibrium state, the relationship between precipitated Al₃Ti and alloy melt can be expressed as

Most titanium stays in the melt, and the precipitation process can be explained as

With the decrease of melt temperature, the precipitated amount of solid Al₃Ti increases, which can be calculated along the ANP curve in Fig.2(b). The process proceeds till to 664 ℃.

At 664 ℃, the peritectic reaction takes place as

The aluminum melt solidifies the surrounding solid Al₃Ti. In the other words, the precipitated Al₃Ti becomes the nucleus of primary aluminum. The whole process can be illustrated by Fig.3.

Fig.3 Evolution process of Al₃Ti particles from 720 ℃ to 664 ℃: (a) Melt at 720 ℃; (b) Cooling from 720 ℃ to 664 ℃;  (c) Peritectic reaction from 664 ℃

The behavior of Al₃Ti formation can be disintegrated into two steps. The first is the nucleation. Electromagnetic stirring and high inner energy lead to apparent component fluctuation in melt. So in some certain position the amount of Ti is higher than the average level, which satisfies the nucleus condition of Al₃Ti. The second is the growing up of Al₃Ti nucleus, whether the Al₃Ti phases grow up is subject to the diffusion velocity of Ti element.

3.2 Microstructure of Al₃Ti reinforced phases

In Al₃Ti/6351 composites, Al₃Ti, β-Mg₂Si, Si are all the reinforced phases, among which the volume fraction of Al₃Ti is the highest. Fig.4 shows the morphology of Al₃Ti particles synthesized without and with EMS and fluxes.

In Fig.4(a) the morphology of Al₃Ti exhibits triangle, quadrilateral and other clumpy shapes, and the sizes of the blocks are in the range of 10-15 μm. In Fig.4(b), the morphology of Al₃Ti does not change obviously, but the sizes of Al₃Ti are refined from 10-15 μm to 2-4 μm. When doped K₂TiF₆ salt with CaF₂ and LiCl, the system temperature decreases from 800 ℃ to 720 ℃. Consequently, the diffusion velocity of Ti element is restricted, which prevents the Al₃Ti from growing[12]. The most important reason is that the electromagnetic stirring force and fluxes addition accelerate the emulsion process of K₂TiF₆. As a matter of fact, the K₂TiF₆ powder enters into the melt via emulsion process. The melting point (T_m) of the mixture K₂TiF₆-15%CaF₂-5%LiCl is 486 ℃, which is much lower than that of K₂TiF₆ salt (the T_m of pure K₂TiF₆ is 682 ℃[13]). At the initial stage of the in situ reaction, the fluxes help the K₂TiF₆ emulsify and react with the molten Al, which enhances the recovery rate of K₂TiF₆ salt. The solid–liquid reaction has been partly changed into a liquid-liquid one. The electromagnetic stirring forces further improve the entering condition of emulsified salt into melt and the dynamics condition of in situ reaction between Al and K₂TiF₆ components. So the nucleation rate of Al₃Ti particles is accelerated and the sizes of particles are refined. The rapid emulsion can be illustrated in Fig.5.

Fig.4 Morphology of Al₃Ti phases in as-cast composites fabricated without (a) and with (b) EMS and fluxes

Fig.5 Schematic diagram of emulsion process of K₂TiF₆ salt when using EMS and fluxes (○ means K₂TiF₆ particle): (a) No slat; (b) Adding salt without fluxes; (c) Adding salt with fluxes; (d) With EMS and fluxes

Based on the EDS analysis, it is found that the extra silicon element gathers around the nuclei of Al₃Ti. Fig.6 shows the EDS spectra of some phases in Fig.4.

It can be seen that the components of clumpy phases are different. The components of comparatively bigger ones include Al, Ti and Si elements. From the element ratio listed in Fig.6(a), it is inferred that the phases are Al₃TiSi_x, while the components of comparatively smaller phases consist of Al and Ti elements. From the element ratio listed in Fig.6(b), it is deduced that the phases are Al₃Ti. Considering the relationship between the components and the sizes of clumpy phases, it is presumed that the extra silicon elements in the melt are likely to adhere to the formed Al₃Ti and promote the growth of Al₃Ti.

Fig.6 EDS spectra of different phases in Al₃Ti/6351 composites shown in Fig.4: (a) For A, C areas: AlTiSi eutectic phases;   (b) For B, D areas: AlTi phases

The reaction between Al₃Ti and Si can be expressed as

Al₃Ti+3xSi→TiAl₃Si_3x                       (11)

Form EDS result the value of x is deduced as 0.073. Eq.(11) can be turned into

Al₃Ti+0.22Si→TiAl₃Si_0.22                    (12)

Fig.7 shows the XRD pattern of Al₃Ti/6351 composites. It demonstrates that the phases in the as-cast composites are α(Al), Al₃Ti and Mg₂Si. The amount of extra silicon is not large enough to be detected.

Fig.7 XRD pattern of Al₃Ti/6351 composites

Fig.8 indicates the TEM morphology of Al₃Ti phase in the as-cast composites. The interface between Al matrix and Al₃Ti particles is clear, where is no unexpected subsidiary product. The bonding state is helpful to increase the mechanical properties of composites.

Fig.8 TEM morphology of Al₃Ti phase in as-cast composites

3.3 Microstructure of Mg₂Si eutectic phases

Fig.9 presents the morphologies of Mg₂Si particles before and after using fluxes. It is shown that without fluxes the Mg₂Si phase exhibits as Chinese character or dendrite[14]. The size of Mg₂Si phase is 8-10 μm. Nevertheless, the patterns of Mg₂Si particles change into polygonal shapes in composites fabricated with fluxes [15]. Meanwhile, the Mg₂Si particles often combine oxygen or ferrum elements together. The average size of Mg₂Si particles is 1-2 μm.

Fig.9 Morphologies of Mg₂Si phases in Al₃Ti/6351composites: (a) Without fluxes; (b) With fluxes

The refinement effects on Mg₂Si can be explained from two views. One is due to the modification effects of_. K₂TiF₆. The anisotropic growth of Mg₂Si during solidification is suppressed by K₂TiF₆ as modifier[16]. The effect is caused either by poisoning the surface of Mg₂Si nuclei through potassium (K) segregation at the liquid–solid interface or by changing the surface energy of Mg₂Si crystals via lattice distortion due to the insertion of potassium (K) in Mg₂Si lattice[17].

The other reason is due to the formation of Ca₂Si. The Ca atom is released because the CaF₂ reacts with molten Al，which can be illustrated as:

CaF₂=Ca²⁺+F^-                             (13)

2F^-+(2/3)Al= (2/3)AlF₃+2e^-                  (14)

Ca²⁺+2e^-=Ca                              (15)

The Ca₂Si phases generate through the combination reaction between calcium and silicon atoms as:

2Ca+Si=Ca₂Si                             (16)

The melting point of Ca₂Si substance which becomes the nuclei of primary Mg₂Si particles is above 900 ℃[18-19]. In the crystal cell of Mg₂Si, the bonding force of Mg-Si is stronger than that of Si-Si, which is beneficial for Mg₂Si particles to precipitate[20].

4 Conclusions

1) The Al₃Ti particles reinforced 6351 aluminum matrix composites were fabricated by K₂TiF₆-CaF₂-LiCl components at 720 ℃ via direct melt reaction method with electromagnetic stirring. The CaF₂ and LiCl acted as fluxes to accelerate the emulsion process of K₂TiF₆ and decrease the in situ reaction temperature from 800 ℃ to 720 ℃.

2) The reinforced phases in the composites are Al₃Ti, Al₃TiSi_0.22 and Mg₂Si. Compared to composites fabricated without EMS and fluxes, the sizes of endogenetic Al₃Ti or Al₃TiSi_0.22 particles are refined from 10-15 μm to 2-4 μm. The morphology of Al₃Ti particles exhibits triangle, quadrilateral and other clumpy patterns.

3) The size of Mg₂Si phases is decreased from 8-10 μm to 1-2 μm due to the modification effect of K₂TiF₆ salt and the formation of Ca₂Si.

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Foundation item: Project(2007AA03Z548) supported by the National High-Tech Research and Development Program of China; Project(207038) supported by the Key Program of Ministry of Education of China; Project(06-D-021) supported by the Talents Peak in Six Key Fields of Jiangsu Province in China; Project(07JDG084) supported by the Technical Enablement Foundation for the Super Special Talents of Jiangsu University; Project(20071108) supported by the Technical Enablement Foundation of Ministry of Education for the Returned Scholars; Project(20060299006) supported by the PhD Programs Foundation of Ministry of Education of China

Corresponding authors: ZHAO Yu-tao; Tel: +86-511-88797658; E-mail: zhaoyt@ujs.edu.cn; LI Gui-rong; Tel: +86-13951408072; E-mail: whmlgr@ujs. edu.cn

DOI: 10.1016/S1003-6326(09)60181-3

(Edited by FANG Jing-hua)