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 Al0.5Fe3Si0.5 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.
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 [
Up to now, there are some friction welding processes of dissimilar Al/AMC alloys [
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
The chemical composition of two alloys (wt.%).
Alloy | Al | Zn | Mn | Si | Fe | Cu | TiB2 | Ti | Mg |
---|---|---|---|---|---|---|---|---|---|
AMC | Bal. | 0.01 | 0.04 | 4 | 0.05 | 0.05 | 5 | 7 | |
6061 Al | Bal. | 0.25 | 0.15 | 0.4 | 0.7 | 0.16 | 0.1 | 1 |
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.
The samples were lap welded using the Nd-YAG laser welding technique. A schematic diagram of the process is shown in Figure
Schematic diagram of the laser welding process.
Parameters of the orthogonal test.
Level | Power (kW) | Laser frequency (Hz) | Velocity (mm/s) |
---|---|---|---|
1 | 5 | 4 | 0.6 |
2 | 5.5 | 4.5 | 0.8 |
3 | 6 | 5 | 1 |
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
The results of welding penetration and width (
Number | Width | Penetration |
---|---|---|
1 | 968.19 | 437.92 |
2 | 938.56 | 488.14 |
3 | 1010.40 | 506.24 |
4 | 1058.81 | 490.47 |
5 | 1149.63 | 547.38 |
6 | 1158.64 | 567.10 |
7 | 1200.25 | 683.38 |
8 | 1331.10 | 828.14 |
9 | 1357.30 | 834.95 |
The results of the orthogonal test.
Number |
|
|
|
Score |
---|---|---|---|---|
1 | 5 | 4 | 0.6 | 26 |
2 | 5 | 4.5 | 0.8 | 27 |
3 | 5 | 5 | 1 | 26 |
4 | 5.5 | 4 | 0.8 | 28 |
5 | 5.5 | 4.5 | 1 | 28 |
6 | 5.5 | 5 | 0.6 | 27 |
7 | 6 | 4 | 1 | 28 |
8 | 6 | 4.5 | 0.6 | 27 |
9 | 6 | 5 | 0.8 | 29 |
|
79 | 79 | 80 | |
|
83 | 82 | 84 | |
|
84 | 85 | 82 | |
|
26.33 | 26.33 | 26.67 | |
|
26.67 | 27.33 | 28 | |
|
28 | 29.52 | 27.33 | |
Range | 1.67 | 1.51 | 1.33 |
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.
The microstructure of welding joints is illustrated in Figure
Microstructure of welding joint. (a) Cross section. (b) Microstructure of the fusion zone.
The map of element is presented in Figure
Map of the element.
EDS results at different locations (at.%).
Location | Al | Mg | Ti | Si |
---|---|---|---|---|
1 | 95.18 | 2.76 | 1.54 | 0.52 |
2 | 90.96 | 3.67 | 3.73 | 1.64 |
It can be observed in the reaction formula (
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 (
ΔG values of formation to TiO2, Al2Ti, B2O3, and AlB12.
Reaction | ΔG |
---|---|
TiB2(s) + 5/2O2(g) ⟶ TiO2(s) + B2O3(g) | −1470544.4 + 124.2 T |
Al + 12[B] ⟶ AlB12 | −220000 + 7.5 T |
2Al + [Ti] ⟶ Al2Ti | −144242 + 21 T |
It can be indicated in Table
The results of XRD of the weld beam are presented in Figure
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
Reaction model of interface between solid phase and liquid phase. (a)
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.
The potentiodynamic polarization curves for different samples in 5% NaCl solution are presented in Figure
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
Schematic diagram of TiB2 phase retarding corrosion.
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. Mostly Al, Al2Ti, Fe2Si, and Al0.5Fe3Si0.5 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. The corrosion resistance of alloys in different conditions decreased in the following order: the weld beam >6061 Al > AMC.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
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).