Laser Welding of Dissimilar Metal Joint of 6061 Al Alloy and Al Matrix Composite

Abstract

In this paper, dissimilar metal joints of 6061 aluminum alloy and aluminum matrix composite material are investigated by laser welding. TiB₂ particles were added into the lap joint. The welding process, microstructure, and the corrosion properties of welding joints are examined. The results demonstrate that the selected optimization process parameters are laser power 6 kW, welding speed 0.6 mm/s, pulse width 11.5 ms, and laser frequency 4.5 Hz. There are a few obvious pores in the molten pool. Al₂Ti, Fe₂Si, and Al_0.5Fe₃Si_0.5 are present in the microstructure. During the welding process, some TiB₂ particles are decomposed and reacted with molten Al. Other TiB₂ particles are nucleated and solidified, and the excess TiB₂ particles are pushed to the grain boundaries by molten Al. TiB₂ particles are wetted well by molten matrix metal. The corrosion resistance of alloys in different conditions decreased in the following order: the weld beam >6061 Al > AMC.

1. Introduction

Aluminum alloys and aluminum matrix composites (AMCs) are two highly important lightweight metals currently used in many fields such as automotive, electronics, and aerospace industries because of their good formability and lightweight. So the issues of joining Al alloy and AMC cannot be avoided. Up to now, there are numerous reinforcing phases for AMC, most commonly TiC, ZrB₂, nanotube, and so on [1, 2]. At present, there are many researches on the welding of AMC enhanced by SiC, Al₂O₃, and B₄C [3–8], but there are few reports on pulsed laser welding of TiB₂-reinforced AMC [9–11]. According to the research of constantly high-power laser welding of TiB₂-reinforced AMC [10], it is found that TiB₂ particles have a great influence on the flowability of molten pool and there is a certain trend in the interface reaction between TiB₂ particles and matrix metal.

Up to now, there are some friction welding processes of dissimilar Al/AMC alloys [12, 13]. But research on Al/AMC laser-welded joints and the corrosion properties of welding joints is little. This study is intended to investigate Nd-YAG laser welding of dissimilar Al/AMC joints with TiB₂ particles. Welding process is discussed using the orthogonal test. The microstructure and the corrosion properties of laser-welded Al/AMC joints were also discussed.

2. Experimental

AMCs were prepared by using the in situ autogenous method. In the reaction, K₂TiF₆ and KBF₄ salts were added in proper Ti : B ratios to the molten ZL101 aluminum alloy liquid at 850°C, stirred for 30 min at regular intervals (LHS-RLL), and cast in a constrained rod casting (CRC) mold (KTZL-1) at 750°C. The formula of exothermic reaction between mixed salt and metal is as follows:

The size of the test specimens of 6061 Al and AMC material is 100 mm × 50 mm × 1 mm with 99.9% purity TiB₂ particles. The chemical compositions of 6061 Al and AMC are given in Table 1.

Table 1 

The chemical composition of two alloys (wt.%).

The Nd-YAG-pulsed laser source (WF-300) was utilized. Before welding, the oxide film on the surface of the specimen was removed and then cleaned using acetone. After welding, the metallurgical sample was prepared and etched with HF (HF : H₂SO₄ : H₂O = 5% : 10% : 85%) solution. The microstructures were observed by using the Olympus GX51 optical microscope (OM) and scanning electron microscopy (SEM) Zeiss EVO 18. The chemical compositions were analyzed and identified by using energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD) Bruker APEX II DUO.

The longitudinal section of welding joints is used for the corrosion test. 5% NaCl solution is accompanied by a CS350H electrochemical system. The specimens were treated with metallographic polishing, followed by washing with distilled water and alcohol, and finally dried in warm air before experiment. Open circuit potential measurement immediately began after the specimens were immersed into the solution. For polarization curves, after the immersion of the electrode into the corrosive solution, the working electrode was abandoned at open circuit potential for more than 10 min in order to stabilize the corrosion potential. The polarization started from a cathodic potential of −250 mV relative to the open circuit potential and stopped at an anodic potential where the anodic current increased significantly. The scanning rate was 1 mV/s. 6061 Al, and AMC base metal are also tested as contrast.

3. Results and Discussion

3.1. Welding Process

The samples were lap welded using the Nd-YAG laser welding technique. A schematic diagram of the process is shown in Figure 1. Welding process is discussed using the orthogonal test. In this experiment, the parameters of power (P), laser frequency (f), and velocity (V) were selected. The parameters are given in Table 2.

Figure 1 

Schematic diagram of the laser welding process.

Table 2 

Parameters of the orthogonal test.

Nine welding joints can be deserved utilizing the parameters. Welding penetration and welding width of every welding joint are tested using OM. The results are presented in Table 3. Scores of every welding joint were calculated in accordance with four aspects of weld penetration, weld width, weld evenness and uniformity, and weld defects. The score of each aspect was 10 points, with a total of 40 points. K₁, K₂, and K₃ are the total scores of each parameter. k₁, k₂, and k₃ are the average values of each parameter. The consequences of the orthogonal test are displayed in Table 4.

Table 3 

The results of welding penetration and width (µm).

Table 4 

The results of the orthogonal test.

Through the results and range analysis, the laser power difference reaches 1.67; the laser frequency range is 1.51, and the velocity range is 1.33. The laser power is the most significant influence on weld formation, followed by laser frequency and welding speed. According to the previous experimental results, the selected optimization process parameters are laser power is 6 kW, laser frequency is 4.5 Hz, welding speed is 0.6 mm/s, the protection of argon gas flow 15 L/min, laser defocus is 0, and the laser pulse width is 11.5 ms.

3.2. Microstructure

The microstructure of welding joints is illustrated in Figure 2. It can be seen from Figure 2(a) that the dissimilar metal joint is good. There is no obvious defect in the joint. The microstructure of the fusion zone is shown in Figure 2(b). Many equiaxed grains are there in the fusion zone.

Figure 2 

Microstructure of welding joint. (a) Cross section. (b) Microstructure of the fusion zone.

The map of element is presented in Figure 3. It can be seen that Al element is the main element in the welding joint. Ti and Si elements are mostly distributed along the grain boundary. The consequences of EDS of different locations (Figure 3) are shown in Table 5. The content of Ti element in the grain boundary is about 2 times of that in the crystal. Si is mainly distributed at the grain boundary and contains about three times as much as in the crystal.

Figure 3 

Map of the element.

Table 5 

EDS results at different locations (at.%).

It can be observed in the reaction formula (2) that some TiB₂ particles are irradiated by laser radiation and oxidation, Ti and B atoms react with O, and Ti-B covalent bond breaks and released. There are accompanied by a reaction with liquid Al to generate AlB₁₂ and Al₂Ti with higher temperature, as shown in reactions (3) and (4):

In order to assess the stability of the precipitated phases, it is crucial to have a reliable calculation for the Gibbs free energy ΔG of the reaction formulas (2) to (4). In these reactions, ΔG is not only a function of temperature but also dependent on the concentration of reactants and products. Thus, the Gibbs free energy ΔG of the reaction formulas (2) to (4) can be expressed as follows:where is the standard Gibbs free energy, and the standard Gibbs free energy for the different substances is listed in Table 6, where α_i represents the activity of the element i in the melting composite. The activity of TiO₂, B₂O₃, AlB_{12 ,} and Al₂Ti in molten Al is approximately 1, so the Gibbs free energy of the reaction formulas (2) to (4) is approximately equal to the standard Gibbs formation free energy of unlike substances.

Table 6 

ΔG values of formation to TiO₂, Al₂Ti, B₂O₃, and AlB₁₂.

It can be indicated in Table 6 that the Gibbs free energies of all products are less than zero [14], so these reactions are likely to occur during laser welding of TiB₂ enhanced AMC. The consequences can theoretically explain that TiB₂ particles react with O₂ present in the air or dissolve in the molten pool to produce TiO₂ and B₂O₃. The results can also illustrate that TiB₂ will react with the Al matrix to produce AlB₁₂ and Al₂Ti when it is resurrounded by molten Al. Since the maximum temperature of the molten pool is lower than the boiling point of Al (2740 K), that is, less than 3000 K, negative Gibbs free energy promotes the reaction of molten TiB₂ with Al and O atoms.

The results of XRD of the weld beam are presented in Figure 4. It can be assumed that mainly Al, Al₂Ti, Fe₂Si, and Al_0.5Fe₃Si_0.5 are present. During the welding process, some TiB₂ particles are decomposed and reacted with molten Al. Some complex products are generated during the welding process. Other TiB₂ particles are nucleated and solidified, and the excess TiB₂ are pushed to the grain boundaries by molten Al.

Figure 4 

XRD results of the weld beam.

In the process of welding solidification, the interfacial interaction model is proposed by combining the mutual wettability between the solid phase and liquid phase/particle three phases, as shown in Figure 5. Assuming that the contact angle θ between the particles and the liquid/solid interface during the solidification process is determined by the interfacial energy between the three phases [15], and there are two states:(1) θ < 90°, as shown in Figure 5(a), the particles are captured because rPL > rSP, and wetting between particles and solid phase is more likely to occur and (2) θ > 90°, as shown in Figure 5(b), where the particles are repelled by the liquid/solid interface.

(a)

(b)

Figure 5 

Reaction model of interface between solid phase and liquid phase. (a) (b) .

For high-interface energy systems such as AMC, the most important thing in the welding process is to change the interface energy in order to encourage the combination of particles and solid phase and form a uniform distribution. In this paper, the interfacial energy of each phase in AMC is modified by TiB₂ particles and introducing the pulsed laser during the welding process. The interaction between the particles and the liquid/solid interface is controlled to a certain extent. Some particles are well wetted by molten matrix metal, and the distribution of the particles is improved in the matrix metal.

3.3. Corrosion Properties

The potentiodynamic polarization curves for different samples in 5% NaCl solution are presented in Figure 6. The curves of the cathodic region to some extent represent the polarization behaviour of the noncorroded surface of the specimen and the reaction of hydrogen evolution. As the potential increases gradually to a relatively high (anodic) potential, corrosion starts on the surface and becomes more severe with the potential increase. The corrosion resistance of alloys in different conditions decreased in the following order: the weld beam >6061 Al > AMC.

Figure 6 

The potentiodynamic polarization curves for different samples in 5% NaCl solution.

The TiB₂ phase is dispersed in the weld beam, which will obviously reduce the corrosion rate of the weld beam. The schematic diagram of TiB₂ phase retarding corrosion is displayed in Figure 7. Therefore, the corrosion resistance of the weld is the best. The content of TiB₂ in AMC is 5%, which is not enough to improve the corrosion rate of aluminum alloy. Therefore, the corrosion rate of AMC is the fastest.

Figure 7 

Schematic diagram of TiB₂ phase retarding corrosion.

4. Conclusions

(1)According to the results of the orthogonal test, the laser power is the most important influence on weld formation of dissimilar 6061 Al/AMC joints with TiB2, followed by laser frequency and welding speed. The selected optimization process parameters of 1 mm 6061 Al/AMC lap welding are laser power is 6 kW, laser frequency is 4.5 Hz, the velocity is 0.6 mm/s, laser pulse width is 11.5 ms, the protection of argon gas flow is 15 L/min, and laser defocus is 0.(2)Mostly Al, Al₂Ti, Fe₂Si, and Al_0.5Fe₃Si_0.5 are present in the microstructure. During the welding process, some TiB₂ particles are decomposed and reacted with molten Al. Other TiB₂ particles are nucleated and solidified, and the excess TiB₂ particles are pushed to the grain boundaries by molten Al. TiB₂ particles are wetted well by molten Al.(3)The corrosion resistance of alloys in different conditions decreased in the following order: the weld beam >6061 Al > AMC.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (Grant no. 51505040), Qing Lan Project of Jiangsu Province, and Jiangsu Key Laboratory of Large Engineering Equipment Detection and Control under Grant no. JSKLEDC201507. The project was supported by Open Fund of Shanghai Key Laboratory of Materials Laser Processing and Modification (Grant no. MLPM2017-1). This project was also supported by Postdoctoral Research Funding Plan in Jiangsu Province (Grant no. 2018K055C).