Al/TiB2 metal matrix composites obtained through in-situ technique.

Usurelu, Emilia-Maria ; Butu, Mihai ; Moldovan, Petru 等

1. INTRODUCTION

In the last decades the aluminum matrix composites reinforced with discontinues particles represent a priority field in structural composite researches.

Metal matrix composites have been manufactured by various techniques such as powder metallurgy, spray deposition and several casting methods such as rheocasting, squeeze-casting, stir-casting and compo-casting, but there exist some challenges in manufacturing the discontinuously reinforced metal matrix composites. One of the main problems lies in the thermodynamic instability of reinforcing ceramic phases in the matrix [1, 2, 6, 8, 9].

In-situ method for manufacturing of these composites, comes into sight in the middle of '80 years, offers a thermodynamic stability of fine and disperses particles into the aluminum matrix due to the fact that germination and reinforcement particles growth take place exact in the alloy matrix, ensureing a strong connection at the interface between particles and matrix [5].

In-situ fabrication of the metal matrix composites is a process, in which dispersed (reinforcing) phase is formed in the matrix as a result of precipitation from the melt during its cooling and solidification.

In-situ technique involves a chemical reaction resulting in the formation of a very fine and thermodynamically stable reinforcing phase within a metal matrix [10].

Prasad et al. studied KB[F.sub.4] and [K.sub.2]Ti[F.sub.6] salts, using differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermo-gravimetry (TG) up to 1073 [degrees]C. Prasad et al. also examined the reactions between two salts and aluminum [7].

Donaldson et al. examined the reaction between salts and aluminum by XRD analysis of the slag, as well as the transfer efficiency of the Ti and B from salt to aluminum by chemical analysis of the alloy [4].

The purpose of this paper is to present the results of fabrication of in-situ Al/Ti[B.sub.2] composites using the exothermic reaction between metallic melt and salts containing boron (from KBF4) and titanium (from [K.sub.2]Ti[F.sub.6]).

2. EXPERIMENTAL PROCEDURE

In-situ Al/Ti[B.sub.2] composites were manufactured with an exothermic reaction process via KB[F.sub.4] and [K.sub.2]Ti[F.sub.6] salts. As matrix was used 6xxx series aluminum alloys (Al-Cu-Mg-Si), the chemical composition being presented in Table 1.

The aluminum alloys 6xxx series were selected as matrix because it possesses medium strength, good formability, machinability, weldability and corrosion resistance compared to other grades of aluminum alloys.

The composites elaboration was realized in an electric furnace with KANTHAL resistance and graphite crucible, equipped with control devices and temperature control (nickel chromium-nickel aluminum thermocouple and temperature regulator with a [+ or -]5 [degrees]C deviation).

The fabrication of in-situ Al/TiB2 metal matrix composites is based on a process in which boron (from KBF4) and titanium (from [K.sub.2]Ti[F.sub.6]) containing salts react with molten aluminum to generate in-situ Ti[B.sub.2] particles, at temperatures between 750-950 [degrees]C.

Also, was added cryolite ([Na.sub.3]Al[F.sub.6]) which has the role to eliminate aluminum oxide in dros. The other role of cryolite is taking as activator, decreasing activated energy and accelerating the in-situ reaction.

A pre-weighted mixture of [K.sub.2]Ti[F.sub.6] and KBF4 salts was mixed and preheated at 300 [degrees]C for 2 hours in order to eliminate humidity.

The exothermic reaction between the salts yield in-situ formed Ti[B.sub.2] particles is:

3[K.sub.2]Ti[F.sub.6] + 6KB[F.sub.4] + 10Al=4[K.sub.3]Al[F.sub.3] + 6Al[F.sub.3] + 3Ti[B.sub.2] (1)

The salts react with the molten aluminum alloy such that the boron and titanium enter the aluminum and combine to form in-situ Ti[B.sub.2] particles. The remaining cryolite slag (dross) is removed from the surface and the molten composites were cast into a mold.

The reactions that occur in the simultaneous introduction of fluorides [K.sub.2]Ti[F.sub.6] and KB[F.sub.4] in liquid aluminum alloy are very complex and involves reaction in melt, diffusion processes through the boundary layer between melt salt and melt metal, reactions inside of the salt particle and gas formation.

Since the chemical reaction took place entirely in the aluminum molten, no oxidation layer on the surfaces of the Ti[B.sub.2] particles could be formed.

In-situ Ti[B.sub.2] particles are formed in one of two ways. The first is that Ti[B.sub.2] forms directly at the interface between aluminum and the flux. The formation and dispersion of this kind of boride particles is strongly influenced by the alloy elements. The second is that titanium and boron disperse into the aluminum melt when [K.sub.2]Ti[F.sub.6] and KB[F.sub.4] are reduced by aluminum and subsequently react to form Ti[B.sub.2] particles.

3. CHARACTERIZATION OF COMPOSITES

The composites obtained have been characterization by electron microscopy (with EDS) and also through X-ray diffraction (XRD).

From the figure 1 results that the reaction between liquid aluminum and complex mixture fluorides ([K.sub.2]Ti[F.sub.6] and KB[F.sub.4]) led to obtaining a composite material with a Ti[B.sub.2] particles fine array.

Titanium boride particles are hexagonal shape, with almost echiaxa shape, and are situated at the grain boundaries. Average size of particles is 1-1,20 [micro]m.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Figure 3 shows an X-ray diffraction (XRD) pattern of the Al/Ti[B.sub.2] composite. The pattern reveals the presence of aluminum and Ti[B.sub.2] peaks, indicating that Ti[B.sub.2] particles are formed in the composite, only.

4. CONCLUSIONS

From the results obtained, the following may be concluded: In-situ composites Al/Ti[B.sub.2] were produced with an exothermic reaction in the Al-[K.sub.2]Ti[F.sub.6]-KB[F.sub.4] system, at 750-950 [degrees]C.

Cryolite salts was added as activator and reacting with [Al.sub.2][O.sub.3] to form drosses.

The reaction mixture of salts ([K.sub.2]Ti[F.sub.6] and KB[F.sub.4]) with aluminum alloy is highly exothermic.

X-ray diffraction analysis (XRD) and EDS microanalysis confirmed the presence of Ti[B.sub.2] particles.

5. REFERENCES

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Tab. 1. Chemical composition of aluminum alloys, wt.%

Alloy Si Fe Cu Mn

6060 0.47 0.21 0.02 0.015
6063 0.43 0.22 0.01 0.014

Alloy Mg Zn Ti Al

6060 0.61 0.029 0.010 Bal.
6063 0.43 0.017 0.013 Bal.