Objective Light metal and thermoplastic polymer have excellent material properties, and the mixed structure formed by the combination of light metal and thermoplastic polymer can meet the requirements of structural performance and lightweight design in automotive industry and aerospace field. The combined application of these two materials is an important way to improve structural performances and reduce energy consumption, so it can be seen that it is very important to realize the connection between them. Laser direct jointing technology has attracted extensive attention in the industrial sector due to the advantages of high welding efficiency, less defects, and flexibility. In existing studies, there are still some shortcomings of laser direct connecting of metal and polymer. On the one hand, when welding aluminum alloy materials with a high reflection property, most of laser energy is reflected which leads to low energy efficiency, and the reflected light is easy to damage laser optical elements. On the other hand, in order to improve the bonding strength, some scholars have proposed to pretreat the metal material surfaces, but the process is complex, the cost is high, and there is still the problem of chemical reagent pollution. More importantly, the treatment area of the material surface is not easy to control, which influences the appearance and performance of the unconnected area. Laser direct jointing technology has a broad application prospect. We hope that through the research in this paper, one can explore a more efficient, high-quality, low-cost laser direct connection and pretreatment process.
Methods The research objects are A5052 aluminum alloy and PBT, which are widely used. Firstly, the upper surface of aluminum alloy is blackened with a black marker, and then the joint of the lower surface of aluminum alloy is oxidized locally by a nanosecond pulse laser. Then, the effect of oxidation power on the surface morphology of aluminum alloy is observed by the scanning electron microscope, and the oxygen content on the surface of aluminum alloy is analyzed by the EDS system. The surface roughnesses of aluminum alloys after oxidation are detected by laser confocal microscope under different laser oxidation powers, and the changes in the surface contact angles of aluminum alloys under different oxidation powers are measured by the contact angle measuring instrument, so as to further analyze the effect of laser oxidation treatment on the surface wettability of aluminum alloy. The effect of oxidation treatment on the laser bonding strength of aluminum alloy and PBT is tested by the tensile test, and the changes in the surface chemical compositions of aluminum alloys before and after oxidation are analyzed by X-ray photoelectron spectroscopy (XPS) to explore the effect of oxidation treatment on the surface chemical composition of aluminum alloy and whether a new chemical bond is formed at the bonding interface. Through these means, one can reveal the mechanism of connection.
Results and Discussions A micro-nano structure is formed on the surface of aluminum alloy oxidized by laser, and with the increase of laser oxidation power, the more attachments appear on the surface (Fig. 3), the roughness gradually increases (Fig.6), and the oxygen content on the surface also gradually increases (Fig. 4). When the laser oxidation power is 19 W, the oxygen content (mass fraction) is 13.64% and the oxygen content on the untreated aluminum alloy surface is only 0.9%, which is increased by about 14 times. The surface energy of untreated aluminum alloy is 74.61 mN/m. With the increase of laser oxidation power,the surface energy of aluminum alloy first decreases to 44.68 mN/m, and then increases rapidly to 83.13 mN/m (Fig. 10). Laser surface oxidation treatment obviously improves the surface energy of aluminum alloy, which greatly improves the surface wettability of aluminum alloy. Through the XPS analysis, it is found that a large amount of Al2O3 is formed on the surface of aluminum alloy after laser oxidation, which is also the reason for the increase of oxygen content on the surface of aluminum alloy, and the thickness of the new Al2O3 layer is obviously thicker than that of the naturally formed Al2O3 film (Fig. 11 and Fig. 12). Based on the analysis of the interface of the stripped joint, it is found that the Al2O3 layer formed on the surface of aluminum alloy by laser surface oxidation treatment promotes the chemical reaction between aluminum alloy and PBT during welding, resulting in new bonds, which are Al—O—C and Al—C (Fig. 14). Chemical bonding is one of the key factors to improve the strength of the joint.
Conclusions After laser surface oxidation treatment, the micro-nano structure is formed on the surface of aluminum alloy, and the surface roughness and the oxygen content increase obviously, which increase with the increase of laser oxidation power. During welding, the melted PBT material flows into the micro-nano structure of the surface, forming a strong anchoring effect. When the welding power is too large, the PBT material near the weld decomposes and produces large bubbles, and the existence of air bubbles adversely affects the strength of the joint. Laser surface oxidation treatment can effectively improve the surface wettability of aluminum alloy, which is conducive to the wetting and spreading of molten PBT material on the aluminum alloy surface, promote the anchoring connection between aluminum alloy and PBT, and effectively improve the strength of the two welded joints. Through the XPS analysis, it is found that a large amount of Al2O3 is formed on the surface of aluminum alloy after the laser oxidation treatment, which promotes the chemical reaction between aluminum alloy and PBT at the interface during welding, resulting in new bonds, namely Al—O—C and Al—C. Chemical bonding effectively increases the strength of the welded joint. Laser oxidation treatment of aluminum alloy and PBT in the laser direct connection results in mechanical connection, physical connection, and chemical connection, so it can effectively improve the strength of the joint.
Objective At present, laser-induced optical breakdown has been widely used in biological sample detection, manipulation, laser-induced breakdown spectra and laser processing of transparent media (glass, etc.). Using the local plasma resonance effect of gold nanoparticles, the laser-induced optical breakdown effect can be enhanced by gold nanoparticles. Under the laser irradiation with strong pulse energy, the morphology of gold nanoparticles may gradually change into irregular spheres with sharp angles and convex edges, leading to significant changes in their photothermal conversion abilities. From the microscopic point of view, it is of great guiding significance to reveal the photothermal conversion rule inside gold nanoparticles in the process of nanosecond or femtosecond laser irradiation and to explore the mechanism of the morphology change of gold nanoparticles under the action of two kinds of lasers. In this paper, a theoretical model of high intensity pulsed laser irradiation of gold nanorods is constructed to study the effects of laser energy density and pulse duration on the photothermal conversion process. Combined with the experimental study of laser irradiation of gold nanorods, the difference in the microscopic melting characteristics of gold nanorods under nanosecond or femtosecond laser irradiation is analyzed.
Methods In this paper, the electron-phonon dual temperature model under the action of laser is used to simulate the heating process of gold nanorods in water by intense laser pulses. Firstly, the basic properties of each domain are strictly defined, including the initial temperatures of gold nanorods and surrounding environment and the selection of boundary conditions for the surrounding water. The electron and lattice temperature variations are obtained by solving the governing equations based on the two-temperature model. According to the solved values of electron and lattice temperatures, we can use the energy conservation equation of water to obtain the transient changes of water temperature along the R and Z axes. By changing the pulse duration time and energy density of the laser, we can calculate the changes in lattice temperature and water temperature of the gold nanorods to compare with the experimental results.
Results and Discussions After the femtosecond laser irradiation, the free electrons in the gold nano-rods first absorb the laser energy, leading to the temperature rise of electrons. After electron-lattice relaxation, the electrons transfer heat to the lattice. Because the laser action time is short, the lattice and electrons do not reach a thermal balance, so the electron temperature is much higher than the lattice temperature. In addition, the energy has not been transferred to the surrounding environment, so the surrounding water temperature is significantly lower than that of the gold nano-rods. The pulse duration is significantly prolonged under the nanosecond laser irradiation, the temperature difference between the lattices and the free electrons in gold nanorods is significantly reduced compared with that under femtosecond laser irradiation, and the surrounding water temperature is significantly increased.
Comparing gold nanorods irradiated by nanosecond laser and those irradiated by femtosecond laser, we can find that when the gold nanorods are irradiated by 0.001 J/cm2 femtosecond laser, the gold nanorods will be melt, but at the moment, the temperature of gold nanorods around the water is far lower than the melt temperature of the gold nanorods, and the temperature difference between gold nanorods and water is as high as 1100 K. Such a large temperature difference results in that the surface and interior of the gold nanorods cannot change in thermal expansion volume at the same time. The huge internal stress is formed due to the different volume change of each part. As a result, the gold nanorod is prone to produce point defects and line defects from the inside, and these defects subsequently evolve into plane defects and then nanorods fracture. When the 0.1 J/cm2 nanosecond laser is applied to the gold nanorod and makes it melt above threshold, the gold nanorod melt temperature, the water temperature around the interface of gold nanorods and the water temperature are 550 K, far below the femtosecond laser heating threshold. The gold nanorod surface thermal stress is significantly reduced and the gold nanorods are not easily broken.
Conclusions In this paper, the theoretical and experimental studies are carried out to analyze the photothermal conversion inside gold nanoparticles and the influence on environmental media during laser irradiation from the microscopic point of view. The results show that the changes in electron and lattice temperatures under nanosecond laser irradiation are basically the same as those under femtosecond laser irradiation. However, compared with those under femtosecond laser irradiation, the temperature difference between the lattices and the free electrons in the gold nanorods is significantly reduced and the surrounding water temperature is significantly increased due to the significantly long pulse duration under nanosecond laser irradiation. Since the high peak power is favorable for the formation of defects on the crystal surface, the melting threshold of gold nanorods under femtosecond laser irradiation (about 0.001 J/cm2) is 99% lower than that under nanosecond laser irradiation (about 0.1 J/cm2). By comparing the experimental results under femtosecond and nanosecond laser irradiations on gold nanoparticles, the difference in the photothermal conversion characteristics of gold nanoparticles under different pulsed laser irradiations are further analyzed. The results show that when the temperature of the gold nanorods reaches the threshold, the shape of the gold nanorods changes under femtosecond laser irradiation, while the morphology of the gold nanorods changes under nanosecond laser irradiation. The results here have important guiding significance for the future experiments of high intensity pulsed laser ablation of metal materials.
Objective Pure aluminum and pure copper have excellent electrical and thermal conductivities and corrosion resistance, and are widely used in aerospace, heat exchange equipment, electronic products and batteries. The density and cost of copper and aluminum are quite different. In order to give full play to the physical characteristics of these two materials and reduce the cost of components, the aluminum/copper composite structure has emerged as a great application requirement in the industrial field. How to achieve reliable welding of aluminum and copper is the key to the reliable application of aluminum and copper composite structures. This paper proposes a new method for laser brazing-fusion process based on the wire deep penetration mode, and conducts the welding process test and the analysis of the joint structure and performance.
Methods Laser wire filler brazing-fusion was performed for 3 mm thick aluminum/copper dissimilar metals, and the influences of groove size, process parameters and the height of the intersection of laser beam and wire from the base material were studied. Cleaning and mechanical grinding are needed to remove the oxide film before welding. Before welding, the 1060 aluminum alloy was first cleaned with the 20%(mass fraction) NaOH solution at 50--60 ℃ for 5 min to remove the surface oxide film. Then it was soaked in the 30% (volume fraction)HNO3 solution for 5 min to neutralize the residual alkali. Finally, the aluminum alloy surface was rinsed with clean water, dried with compressed air, and stored in a sealed bag to avoid contact with air. T2 copper was pickled with the 30% (volume fraction) H2SO4 solution for 5 min, and the surface was washed with clean water to ensure that there was no acid residue on the surface. It was dried with compressed air and stored in a sealed bag. After chemical cleaning, the welding test shall be conducted within 12 h, otherwise the chemical cleaning of oxide film shall be conducted again. The aluminum alloy and pure copper plates with preset groove shall be assembled according to the zero gap butt joint method, horizontally placed on the surface of the worktable, and fixed with clamping tooling, so as to ensure that the assembly gap does not change during welding. High speed camera was used to observe and record the welding process. The high-speed camera was positioned at the welding pool, and the auxiliary light source was placed on the other side to irradiate the welding wire and pool. The CCD imaging system of the camera was used to transmit the image signal to the connected computer for real-time observation and recording. After the welding was completed, metallographic sample was prepared, and the sample was ground and polished. Then, the microstructure of the weld was observed with optical microscope and scanning electron microscope, and the joint performance was tested with universal tensile testing machine and microhardness tester.
Results and Discussions Choosing a reasonable groove form can help reduce welding defects such as undercuts and sidewalls that are not fused, and improve the quality of weld formation. The shape and size of the groove are optimized, and the position of the laser in the groove is changed. When the laser beam is biased to the copper side, the laser energy will melt the welding wire and at the same time act on the copper side base material, which has a stronger preheating effect (Table 3). When the height of the intersection of laser beam and wire from the base material is 0, the top view of the weld is continuous and uniform, and the back view of the weld is completely penetrated. According to the observation of high-speed camera, the droplet transition is in the form of liquid bridge. And there is a bolling front in the front of the welding wire. This shows that when the laser beam with an enough high power density irradiates on the welding wire, the melting front will be formed. At this time, the welding wire absorbs the laser energy in a deep penetration mode (Table 4). Scanning electron microscope observation results show that the joint can be divided into aluminum side melting zone, weld zone, and copper side brazing zone. The brazing area at the copper side can be finely divided into the interface layer, the eutectic zone, and the weld zone (Fig. 2). The interface layer is mainly composed of Al2Cu intermetallic compounds, and a few Al4Cu9 phases are also generated (Fig. 4). The hardness distribution in the center of the weld is uniform, and the hardness of the interface layer is up to 296 HV (Fig.5). The average tensile strength of the joint with grinding is about 80 MPa. In contrast, the tensile strength of the joint without grinding is about 60 MPa (Fig. 6). The fracture surfaces of the two kinds of fractures are flat with tearing edges and typical river-like patterns, which can be judged as brittle fractures (Fig. 7).
Conclusions The aluminum/copper laser brazing-fusion process based on the wire deep penetration mode was used to obtain a well-formed joint, the tensile strength can reach 80% of that of the aluminum base metal, the fracture of the joint occurs in the interface layer which can be judged as brittle fractures. The groove form and size have significant influence on the weld formation and joint quality. When the laser is biased to the copper side, it can produce a better preheating effect on the copper matrix, which is conducive to the infiltration and spreading of the weld pool, improve the weld forming, and inhibit the undercut defect of the base metal on the copper side. The brazing area on the copper side is subdivided into three areas: the interface layer, the eutectic area, and the weld area. The thickness of the upper interface layer is about 50 μm, and the thicknesses of the middle and lower interface layers are about 20 μm. The interface layer is mainly composed of Al2Cu and Al4Cu9 phases.
Submission Open:1 October 2021; Submission Deadline: 31 January 2022
Editor (s): John Kline, Jianqiang Zhu, Leonida Gizzi, Robbie Scott
Editor (s): Zongfu Yu, Yang Chai, Li Gao, Darko Zibar
Submission Open:25 February 2021; Submission Deadline: 31 July 2021
Editor (s): Colin Danson, Jianqiang Zhu