Trans. Nonferrous Met. Soc. China 23(2013) 1543-1548

Evolution behavior of TiB₂ particles during laser welding on aluminum metal matrix composites reinforced with particles

Chao MENG, Hai-chao CUI, Feng-gui LU, Xin-hua TANG

Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Received 2 May 2012; accepted 18 June 2012

Abstract: The evolution behavior of TiB₂ particles was studied during laser welding on aluminum metal matrix composites (MMCs) reinforced with TiB₂ particles by a high power laser. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy spectrum analysis (EDS) were used to analyze the phase, thermodynamics and morphology characteristics of the particles in the weld seam, and the interface reactions between TiB₂ particles and aluminum matrix were also discussed. The results show that partial TiB₂ particles melt and merge into larger ones in the middle of the weld seam and that fracture can be found in larger particles when the scale of TiB₂ cluster is larger than the spot size of laser. Al₃Ti and AlB₁₂ are produced by the interface reaction between TiB₂ particles and Al matrix. The interface reaction of broken TiB₂ particles in the middle of the weld seam is stronger than that at the edge of the weld seam.

Key words: metal matrix composites; TiB₂ particles; laser welding; interface reaction

1 Introduction

With the development of industry and technology, the special materials are required in the fields such as aerospace, aviation and automobile. Aluminum metal matrix composites (MMCs) reinforced with particles for their low density, high strength and high modulus, good wearing resistance become one of the most promising materials. However, there are a lot of difficulties in welding Al MMCs, such as brittle phase Al₄C₃ or brittle spinel laser MgAl₂O₄ produced in welding Al MMCs reinforced with particles like SiC or Al₂O₃, which can reduce the mechanical properties of the welding joint [1-3]. Many methods were used to prevent the formation of a brittle phase or layer. Elements Si and Ti could be added into SiC-reinforced Al composites to inhibit the interface reaction between SiC and Al to ensure the weld joint quality [4-6]. TiB₂-reinforced particulate Al MMCs have higher strength, better wearing resistance and compatibility between the reinforced phase and the matrix compared with Al matrix composites reinforced with other particulates. Welding plays an important role in enlarging the applications, but the weldability of TiB₂-reinforced Al MMCs was rarely reported. VIJAY and MURUGAN [7] studied the mechanical properties of friction stir welded Al-10%TiB₂ (mass fraction) metal matrix composites. It was found that TiB₂ particulates homogeneously distributed both in the weld zone and Al matrix. NAHED et al [8] found that Al₃Ti and AlB₁₂ were produced in the process of in-situ preparation of TiB₂-reinforced Al matrix composites through the reaction between molten Al and K₂TiF₆ and KBF₄, which caused the decrease of the reinforcement effect of TiB₂ particles. In addition, TiB₂ will be easily oxidized into TiO₂ and B₂O₃ in the air at elevated temperature [9]. Compared with traditional welding methods, laser welding becomes an increasingly significant joint technique for its higher power density, deeper penetration and higher efficiency. Laser welding on particles reinforced Al composites is becoming a new research focus. CUI et al [10] studied TiB₂ particle distribution and the reaction possibility between the reinforced phase and the matrix in the weld seam by laser full-penetration welding on in-situ TiB₂-reinforced Al composites. It was found that there is no obvious evidence for the reaction between TiB₂ particles and Al matrix and also TiB₂ particles distribute more uniformly in the seam compared with the base metal.

It is known that the reinforced particles as ceramic phase possess higher electric resistance which is proportional to the absorption rate to laser. So, when the cluster size of reinforcement is larger than the laser spot size, which is possible for the composites reinforced by high volume fraction particles, the temperature of particles increases promptly due to the high absorption and the low thermal conductivity. But there are few researches reported about TiB₂ cluster’s evolution behavior in the laser welding process of Al MMCs. In order to study the behavior of TiB₂ particles with larger cluster size than the laser spot size, an Al matrix composite with 1-5 mm TiB₂ particles and ZL101 alloy was particularly designed in this work to explore the evolution of TiB₂ particles. And also the behavior of TiB₂ particles with a high volume fraction in the laser welding process was studied, and the interface reaction between TiB₂ particles and Al matrix was discussed to obtain a high quality weld joint of Al MMCs reinforced with higher volume fraction of TiB₂ particles.

2 Experimental

15 kW CO₂ laser welding system with a spot diameter of 0.8 mm was used to weld the TiB₂-ZL101 composite. Pure helium as shielding gas with a flow rate of 28 L/min was ejected by a forward nozzle inclining to the beam axis for 45°. Self-manufactured TiB₂-ZL101 composites were employed. Small holes were made inside the specimens with the dimensions of d2 mm× 2 mm in the middle of the up face to remove the internal gas in materials in vacuum. The surface of specimens was machined to remove the oxide film surface and cleaned with acetone. After desiccating for 2 h, the specimens were compressed tightly with smooth ZL101 plates to make sure of no gap between up and down plates in the welding process. The welding specimen filled with TiB₂ particles in the middle of plates is shown in Fig. 1. The chemical composition of ZL101 matrix is shown in Table 1.

Fig. 1 Model of welding specimen

Table 1 Composition of ZL101 matrix (mass fraction, %)

The microstructure and EDS analysis of TiB₂ particles are shown in Fig. 2. It is found from Fig. 2 that the size range of TiB₂ particles is from 1 to 5 mm, and no other impurity elements are detected.

Fig. 2 Microstructure (a) and EDS analysis (b) of TiB₂ particles

Fig. 3 XRD pattern of weld seam

3 Results and discussion

3.1 Phase and thermodynamics analysis of weld seam

The XRD pattern (Fig. 3) shows the possibility of the presence of Al₃Ti, B₂O₃, AlB₁₂ and TiO₂ in the weld seam. It illustrates that TiB₂ particles are evolved into TiO₂ and B₂O₃ through the reaction with oxygen molecules in the laser welding process, as shown in Reaction (1). With increasing the temperature, Ti and B ions are released from the broken Ti—B covalent bonds and diffuse into liquid Al, which causes the formation of AlB₁₂ and Al₃Ti in the reaction with Al matrix, as shown in Reactions (2) and (3).

TiB₂(s)+(5/2)O₂(g)→TiO₂(s)+B₂O₃(g)            (1)

Al+12[B]→AlB₁₂                            (2)

3Al+[Ti]→Al₃Ti                             (3)

In order to evaluate the stability of the precipitated phases, it is crucial to have a reliable calculation for the Gibbs free energy change △G in Reactions (1) to (3). In these reactions, △G is not only dependent on temperature but also on the concentration of reactants and products. Therefore, the Gibbs free energy change of Reactions (1) to (3) can be expressed as:

                    (4)

        (5)

         (6)

where △G^Θ is the standard Gibbs free energy change, for the standard Gibbs free energy change of different substances, the △G^Θ values are listed in Table 2 [11,12]; and α_i represents the activity of component i in the melting composite. The activities of products TiO₂, B₂O₃, AlB₁₂ and Al₃Ti in molten Al can be approximate to unity. So, the Gibbs free energy change of Reactions (1) to (3) approximately equals the standard Gibbs free energy change of different substances.

Table 2 △G^Θ values of TiO₂, Al₃Ti, B₂O₃ and AlB₁₂ [11,12]

The curves between the Gibbs free energy change of reactions and temperature are shown in Fig. 4. It can be seen that the Gibbs free energies change of all reactions are negative below 3000 K, so these reactions are possible to occur with the negative Gibbs free energy in the laser welding process. Although the above results are obtained under the circumstance of equilibrium thermodynamics, they can also theoretically illustrate the possibility and the trend of oxidation reaction between TiB₂ particles and O₂ existing in air or dissolving in the molten pool and interface reaction between TiB₂ and molten Al under the circumstance of nonequilibrium thermodynamics in this work. The products are TiO₂ and B₂O₃ in oxidation reaction and AlB₁₂ and Al₃Ti in interface reaction, respectively.

Fig. 4 Curves of △G versus temperature

According to the reaction kinetics, chemical reactions rely on the diffusion of atoms. The above reactions illustrate that TiB₂ particles melt during the welding process, which promotes the diffusion of Ti, B, Al and O atoms. As the maximum temperature of the molten pool except for the keyhole during the welding is under the boiling point of Al (2740 K), the negative Gibbs free energy change promotes the molten TiB₂ to react with Al and O atoms.

3.2 Microstructure and interface reaction of weld seam

Figure 5 shows the microstructures of the weld seam observed by a JSM-6460 SEM. The TiB₂ particles blocked together inside the weld seam have a distinct boundary with Al matrix due to more concentrated energy in the center of the welding section, as shown in Fig. 5(a). There are no distinct boundaries between large scale TiB₂ blocky structures. From Fig. 5(b), it can be seen that the fracture occurs on TiB₂ particles with larger radius to some extent. In this situation, some Al can be found among TiB₂ particles. TiB₂ would be irradiated by laser when the volume fraction of TiB₂ clusters in the weld seam is large enough, which causes the temperature of TiB₂ clusters to increase sharply and transfer energy to surrounding Al matrix, leading to the melting of TiB₂ clusters and the breakage of covalent bonds between Ti and B.

3.2.1 Melting of TiB₂ particles

EDS analysis was performed to analyze the molten TiB₂ in the weld seam, as shown in Fig. 6. It can be seen that Ti and B elements are accounted for the vast majority of mass. The laser beam irradiates TiB₂ powder band completely because its width (2 mm) is far larger than the diameter of the laser spot (0.8 mm). So it is reasonable that TiB₂ particles in melting merge together in high density energy. As the thermal conductivity of the TiB₂ particles (25 W/(m·K)) is far less than that of Al (151 W/(m·K)) and the accumulation of TiB₂ particles is porous, which makes that energy cannot be transferred timely from TiB₂ particles to Al matrix in the laser welding process of TiB₂/ZL101 MMCs. The surrounding Al matrix around molten TiB₂ particles vaporizes or reacts with TiB₂ due to its drastic flow in the molten pool. Therefore, TiB₂ particles are pushed away from the keyhole region by the reaction force of Al evaporation before Al liquid enters the keyhole region and re-solidifies to large scale TiB₂ blocky structures. The results show that TiB₂ particles will melt and re-solidify when the size of TiB₂ particles cluster is larger than that of laser beam in the laser welding process.

Although TiB₂ particles are not directly irradiated by the laser in this work, the keyhole is complete within the area of TiB₂ particles in the welding process, which leads to sharp temperature elevation and weak convection, further to the melting of TiB₂ particles and a series of evolution of TiB₂.

3.2.2 Fracture of TiB₂ particles

As the laser energy absorption rate of TiB₂ particles is far greater than that of Al matrix, the evolution of TiB₂ will be influenced dramatically if TiB₂ particles are exposed directly to the laser energy in the welding process. According to Ref. [13], the bond energies of Ti—Ti, Ti—B and B—B are 0.162, 0.13 and 0.967 eV, respectively. So, the covalent bond of Ti—B will be the most likely to crack with the decrease of stability of TiB₂. Some broken TiB₂ particles are found in the weld seam in Fig. 5(b). The microstructure and EDS analysis of the broken TiB₂ particles are shown in Fig. 7. A small quantity of Al element is found on the surface of TiB₂ particles, which indicates that new phase is probably generated from the interface reaction between TiB₂ particles and Al matrix in the laser welding process.

Fig. 5 Microstructures of weld seam

Fig. 6 Microstructure (a) and EDS analysis (b) of molten TiB₂ in weld seam

TiB₂ particles located in the keyhole wall were heated to high temperature rapidly because of direct irradiation of laser, which moved into liquid Al at the evaporation force and cooled rapidly with laser welding process going on. Different local temperatures of TiB₂ particles due to rapid heating and cooling induced thermal stress within unevenly expanded TiB₂ particles, which made these particles break when the stress reached a critical value. The thermal stress (p) [14] in materials can be expressed as

                         (7)

where △α is the difference of thermal expansion coefficients due to different temperatures, △T is the change of the temperature, ν is the Poisson ratio, and E is the elastic modulus of TiB₂ particles.

So, the thermal stress becomes increasingly greater with larger temperature difference within the particles, which leads to the breakage of Ti—B covalent bonds and the fragmentation of TiB₂ particles finally.

3.2.3 Interface reaction in weld seam

As shown in Fig. 8, the microstructure of some TiB₂ particles is distinct from the others, indicating that the TiB₂ particles semi-fuse with Al matrix. According to the EDS analysis and also combined with the XRD analysis above, Al₃Ti is preliminarily judged as the produced phase in the welding process, which illustrates that the molten TiB₂ reacts with Al matrix. Based on the Gibbs free energy change curve in Fig. 4, the reaction is feasible. In addition, the product of the reaction might consist of the mixture of AlB₁₂ and Al₃Ti, as AlB₁₂ is confirmed by XRD analysis and three elements of Al, Ti and B are also found in EDS spectra of some particles.

Fig. 7 Microstructure (a) and EDS analysis (b) of cracked TiB₂ particles

Fig. 8 Microstructure (a) and EDS analysis (b) of product of TiB₂ particles and Al matrix

Fig. 9 Microstructure (a) and EDS analysis (b) of TiB₂ particles at edge of weld seam

The microstructure at the edge of the weld seam is shown in Fig. 9. Although TiB₂ particles located at the edge of the weld seam experience the interface reaction based on the EDS spectrum, the mass fraction of Al element is far less than that of TiB₂ particles in the middle of the weld seam. For the high energy density of the laser, the interface reaction between TiB₂ particles and Al matrix is sufficient. As the laser energy subjects to a Gaussian distribution, the interface reaction of the broken TiB₂ particles in the middle of the weld seam is stronger than that at the edge of the weld seam.

4 Conclusions

1) TiB₂ particles merge into large blocks when rare Al liquid moves into the molten TiB₂ clusters. Small Al liquid can move into the molten TiB₂ clusters. TiB₂ particles will not merge into large ones but fracture in larger ones.

2) When TiB₂ particles melt and encounter the Al liquid, the interface reaction occurs and AlB₁₂ and Al₃Ti are produced. For the high energy density of the laser beam that subjects to a Gaussian distribution, the interface reaction of the broken TiB₂ particles in the middle of the weld seam is stronger than that at the edge of weld seam.

3) For the Al matrix composites with high volume reinforcement, the laser welding technology probably produces the poor welding properties.

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激光焊接颗粒增强铝基复合材料中TiB₂颗粒的演变行为

孟 超，崔海超，芦凤桂，唐新华

上海交通大学 材料科学与工程学院，上海市激光制造与材料改性重点实验室，上海 200240

摘  要：研究大功率激光器焊接TiB₂颗粒增强铝基复合材料时TiB₂粒子的演变行为。采用X射线衍射(XRD)、扫描电镜(SEM)及能谱(EDS)分析焊缝内粒子的物相、热力学过程及形貌特征；同时对TiB₂和铝基体的界面反应进行讨论。结果表明：当TiB₂团簇尺寸大于激光光斑直径时，焊缝中部的TiB₂粒子会熔融在一起，较大尺寸的TiB₂会发生断裂；当与铝熔体接触后，熔化后的TiB₂粒子会与Al发生反应生成Al₃Ti和AlB₁₂，并且焊缝中部的界面反应比焊缝边缘的剧烈。

关键词：金属基复合材料；TiB₂粒子；激光焊接；界面反应

(Edited by Wei-ping CHEN)

Foundation item: Project (11ZR1417500) supported by the Natural Science Foundation of Shanghai, China; Project(51035004) supported by the National Natural Science Foundation of China

Corresponding author: Feng-gui LU; Tel: +86-21-54748940-8025; E-mail: lfg119@sjtu.edu.cn

DOI: 10.1016/S1003-6326(13)62628-X