Hybrid joints between polypropylene (PP) and 316L stainless steel (316L) have been widely used in various industrial fields, such as automotive, medical equipment, and electronic devices. The difficulty in the joining PP to 316L is that the former is a non-polar polymer, which makes it difficult to initiate a chemical reaction at the interface between the two materials. This ultimately results in a joint with low strength. Additionally, the melting point and thermal conductivity of the two materials differ significantly, so it is necessary to properly control the laser heat input. To tackle this challenge, the PP surface was pre-treated by plasma and then joined to 316L by laser transmission welding technology. The polar groups introduced by the plasma-treated PP form new chemical bonds with the metal and metal oxides of 316L, resulting in high quality dissimilar joints. The macromorphology and microstructure of the interface were investigated comparatively with different scanning speeds. The results showed that the optimal scanning speed was 10 mm/s at a laser power of 60 W and a defocusing distance of 0 mm, resulting in a maximum lap shear force of 149.18 N and an optimal macroscopic morphology. Furthermore, the correlation between the change in weld morphology and the mechanical properties was investigated, and the morphological and chemical bonding of the fracture were analyzed to elucidate the joint connection mechanisms.

This investigation employed different laser powers to conduct the laser welding–brazing process of 5052 aluminum alloy to both Al-Si coated and uncoated 22MnB5 steel. The flux-cored Zn-Al22 filler metal was employed during the procedure. The influence of Al-Si coatings on the microstructure and corrosion resistance of Al/Steel welded joints was investigated using microstructural characterization and electrochemical tests. It was noted that the interfacial microstructure of the laser Al/steel joints was significantly altered by the Al-Si coating. Moreover, the Al-Si coating suppressed the formation and growth of the interfacial reaction layer. Electrochemical corrosion tests showed that the impact of Al-Si coating on the corrosion resistance of laser joints depended on the laser powers and thickness of the interfacial intermetallic compound (IMC) layer. The research suggests that galvanic corrosion occurs due to the differences in corrosion potential between fusion zone (FZ), steel, and Fe-Al-Zn IMCs, which accelerate the corrosion of the joint. The IMC layer acts as a cathode to accelerate the corrosion of the FZ and as an anode to protect the steel from corrosion.

Diode laser with a flat-top beam was very suitable for the joining of plastic and metals as its uniform energy distribution to fine tuning the temperature, which facilitates the melting of plastic achieving a sound bonding and prevents the decomposition of plastic forming undesirable bubbles. Therefore, the control and study on the temperature field during diode laser welding of plastic to metals are of importance. In this paper, a flat-top thermal source model was established to assist the simulation of the transient temperature field distributions of laser conduction welding of 304SS and PET. With the increase in welding speed from 10 to 30 mm/s, the peak temperature at the PET side decreased from 702.14 to 591.9 K, which was even below the PET decomposition temperature 659 K thus eliminating the formation of bubble within PET. Two regions were observed in the laser joints which corresponded to the PET melting region and PET glass transition region base on the comparatively study between numerical simulation and experimental investigation.

In this study, copper foil and poly-ether-ether-ketone (PEEK) film were directly bonded using nanosecond lasers. To strengthen the interfacial bonding between the copper foil and PEEK, ultraviolet/ozone (UV/ozone) and silane coupling treatment (APTMS) methods were proposed to modify the surface of the copper foil. The UV/ ozone treatment led to the oxidation of the surface of the copper foil to Cu2O. The bonding interface and fracture were characterized and analyzed via optical digital microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The Cu/PEEK joints demonstrated a 95 % improvement in peel strength following UV/ ozone + silane coupling treatments. Additionally, the red shift of the typical functional group wavenumbers demonstrated a strong hydrogen bond interaction between the copper foil treated with UV/ozone + silane coupling and the PEEK film. The mechanism behind the superior peel strength of UV/ozone + ATPMS-Cu/PEEK joints was subsequently validated through the density functional theory (DFT) analysis. Moreover, DFT was utilized to calculate the differential charge transfer and binding energies of Cu/silanol and Cu2O/silanol. As per the results, the bonding of Cu2O to silanol was more stable than that of Cu to silanol.

Metal-polymer hybrid joints can contribute to weight reduction and have potential use in transportation industries. In this work, laser texturing was used to enhance bonding strength of laser-welded 304 stainless steel/carbon fiber reinforced thermoplastics (CFRTP) joint. The tight relationship between surface texture spacing and porosity characteristic as well as tensile property of joint was systematically investigated. The temperature field distribution during welding process was simulated and the mechanism of porosity suppression depending on grid spacing was discussed. The result showed that the number of pores at cross-section decreased first and then rose with the increase of micro-structure spacing, which corresponded to the variation trend of bonding strength and was inconsistent with the evolution of connecting area. The joint achieved the minimum porosity defect and reached maximum fracture load of 2594 N when the grid spacing was 1.2 mm, while the maximum joint width of 9.28 mm was obtained when 0.4 mm spacing was adopted, indicating that the joint quality rather than mechanical interlocking governed the mechanical property of joint. Appropriate interface contact conductance (TCC) related to texture spacing could optimize the interface heat dissipation rate, and was the key factor to efficiently control the shrinkage porosity. The TCC of small or large grid spacing was too high or too small, resulting in failure of the heat transfer path through the interface to metal during cooling process. Moreover, the proper texture spacing improved the uniformity of joint strain distribution in tensile-shear testing, which benefited joint strength.

For lightweight applications, the interfacial bonding reliability determines the service life of Ti-6Al-4V titanium alloy (TC4)/carbon fiber reinforced thermoplastic (CFRTP) joints. Traditional joining methods limit their applications. In this study, a functional Schiff base -contained polymer was successfully synthesized to modify the TC4 surface. The results indicated the directionally induced high -density complexation between the synthesized Schiff base and the titanium atom from the TC4 sheet and the secondary interaction between the synthesized Schiff base -contained polymer and CFRTP matrix occurred, which improved the bonding reliability of the TC4/ CFRTP joint. The combination of the high -density secondary interaction and synergistic effect between the functional groups enhanced the interfacial bonding strength by 287 % to 38.73 MPa. This was remarkably higher than the bonding strength improved by 46 % to 14.64 MPa via the introduction of micromolecule Schiff base that consists of same functional groups. The significant enhancement of the interfacial bonding reliability was thus achieved. These results were beneficial to future research to improve the reliability of metal-CFRTP hybrid joints.

Joining carbon fiber reinforced thermoplastic composites (CFRTP) with metals is a significant challenge for lightweighting in the automotive sector. The lower strength of hybrid joints limits their applications in the relevant fields. In this study, three distinct surface textures were applied to 6061 aluminum alloy (6061Al), including a traditional groove texture and two novel bionic textures (shark skin and fish scale), to enhance the strength of the hybrid joint between the materials. The impact of these textures on the mechanical properties of 6061Al/CFRTP hybrid joints was investigated through experimental methods and finite element simulations. The results indicated that the stress distribution at the interface of the bionic textured samples was more uniform, reducing stress concentration at the interface. Furthermore, the hybrid joint strength was improved as the bionic texture hindered crack initiation and propagation and facilitated crack deflection. Compared to the untextured samples, the fish scale textured samples exhibited the highest strength, which increased by 431.3 % relative to the untextured samples.

The present study investigated the inhomogeneity of microstructure and properties of hybrid laser-MIG welded 10-mm-thick 316L/AH36 joints. The study focused on the inhomogeneity of the laser zone, hybrid zone, and transition zone. The temperature field was simulated using ABAQUS and showed a higher cooling rate in the laser zone. As a result, the microstructure in the laser zone was dominated by martensite while that in the hybrid zone was composed of 84 % austenite and 16 % ferrite. The laser zone showed poorer impact toughness and corrosion resistance than the hybrid zone. However, the corrosion resistance of the whole weld was the worst. Because the inhomogeneous microstructure formed a galvanic couple that accelerated the corrosion process. This work indicated that the energy ratio (the ratio of laser to arc energy) should be considered in the welding of dissimilar thick steel to reduce the inhomogeneity in the two zones and obtain a desirable property balance among hardness, toughness, and corrosion resistance.

A thermomechanical coupling model for the solid-state flashing process of high nitrogen steel was established, based on finite element simulations and experiments. The effect of flash current on the microstructure and mechanical properties of the welded joint was investigated, and the temperature field of the flash-butt welding (FBW) process was simulated. The phase composition of the joint was determined according to the phase diagram and cooling curve. In addition, the joint with optimal parameters was subjected to full immersion corrosion tests. The results demonstrated that the interface structure was composed of austenite and δ-ferrite with a thyristor angle (flash current) of 45°. The microstructure of the overheated zone (OZ) was composed of austenite, ferrite and a small amount of the M2 phase, in which the heat-affected zone exhibited a single-phase austenite microstructure. The joint hardness displayed a “V” shaped distribution with the lowest interface hardness. As the flash current increased, the hardness and tensile strength of the interface area of the joint first increased and then decreased, with a maximum tensile strength of 902 MPa at 45°. During the full immersion corrosion tests, the joint exhibited the most serious corrosion in the interface center and gradually reduced corrosion on both sides.

Laser welding-brazing of 5052 aluminum alloys and Al-Si coated 22MnB5 steel was carried out with a Zn-22Al filler metal. The influence of laser power on weld appearance, interfacial microstructure and mechanical properties were investigated. The weld morphology and the interfacial microstructure changed significantly with laser power. When the laser power was 1500 W, the intermetallic compounds (IMCs) at the FZ/steel interface consisted of Fe2Al7Si which was pre-existed in the Al-Si coating. As the laser power increased from 1700 W to 2100 W, the interfacial IMCs changed to Fe2Al5-xZnx and FeZn10. With the increase of the laser power, the tensile strength raised first and then decreased, the maximum value of 135.0 N/mm was obtained at 1900 W laser power. Differently, the bending angle first decreased and then increased with the increase of laser power. Nu-merical simulations showed that the maximum interfacial temperature increased from 835.7 degrees C to 1424.6 degrees C as the laser power increased from 1500 W to 2100 W. It resulted in the total dissolution of Fe2Al7Si and the newly generation of Fe2Al5-xZnx and FeZn10 therefore inducing the change of joint mechanical properties.

304 stainless steel (304SS) and polyethylene terephthalate (PET) were lapped joined by laser conduction welding using a semiconductor laser with a flat-top thermal distribution. Experimental investigation and numerical simulation were conducted to analyze the weld morphology, pores distribution, interfacial microstructure, temperature field, joint strength and fracture behavior of the laser dissimilar joints. Because of the uniform thermal distribution of the laser, the decomposition of the PET base material was well controlled with pore-free joints obtaining at the welding speed over 25 mm/s. Besides, a compound layer was generated at the interface between 304SS and PET with its thickness decreased when the welding speed increased. Chemical reactions and mechanical anchoring were both observed at the interface suggesting a dual joining mechanism. The joint fracture load first increased then decreased with the increased welding speed. Besides, a ductile to brittle failure transition was clearly seen. The underlying mechanism was also discussed.

Transparent hard and brittle (THB) materials have generated significant interest due to their excellent properties, such as wide spectral transmittance, heat resistance, chemical inactivity and high mechanical strength. To further explore the application of THB materials, it is inevitable to be confronted with a range of joining THB materials and THB material–metals. Ultrafast (UF) laser microwelding enables a new means of joining THB materials and THB material–metals, due to a localized energy deposition method, which is dominated by nonlinear absorption. This process can realize high-quality micro-zone direct joining of THB materials or THB material–metals without the assistance of a light-absorbing intermediate layer. In this paper, we review the advances in UF laser microwelding of THB materials and THB material–metals considering the last two decades, from the analysis of the interaction mechanism between UF laser and matter to the key influencing factors and practical applications of this technology. Finally, the existing problems and the future research focus of UF laser microwelding technology of THB materials and THB material–metals are discussed.

The bonding of copper foil to Poly Ether Ether Ketone (PEEK) film has become a hotspot in research of the 5G RF (Radio Frequency) antenna substrate. In order to make the flexible copper clad laminates(FCCL) in the RF substrate thinner and more resistant to bending, there is an urgent need to replace the use of adhesive to join the copper foil to the PEEK film with a direct joining method. In this paper, a nanosecond laser was used to join the copper foil to the PEEK film effectively and the comprehensive property of joint was enhanced by the laser-textured grid on the surface of copper foil. The influence of textured grid on the adhesion work between copper foil and PEEK film was investigated. The interfacial morphology was observed and the result showed that molten PEEK completely filled the textured grid and formed a mechanical interlocking with copper. The complex reaction between Cu and PEEK was indicated by detecting the red-shift for the wave number of functional groups and the weak chemical bond between Cu and PEEK. Besides, the bonding mechanism was proved by density functional theory (DFT) calculation. The tensile shear strength of Cu/PEEK joint was significantly enhanced by laser texturing treatment, and the maximum force of joint with 0.4 mm-textured grid was increased by approximately 1.29 times compared to the untreated condition. This study proposes the use of nanosecond laser joining to replace the traditional use of adhesives to join copper foils to PEEK films.

A novel composite filler was prepared by introducing CoCrNi medium-entropy alloy (MEA) into the AgCuTi filler and it was then used to enhance the Sapphire/4 J33-Kovar alloy brazed joints. The effects of CoCrNi MEA on the interfacial microstructure and mechanical properties of brazed joints were discussed by comparatively studying the brazed joints with single AgCuTi and composite filler. The results demonstrated that the porous CoCr2 phase surrounded by the Ni3Ti layer was in-situ synthesized in the brazing seam with the addition of CoCrNi MEA, which relieved the local residual stress during the cooling process. Moreover, the effect of porous structure on retarding the diffusion of metal atoms limited the reaction of Ti with Fe, which promoted the sufficient interfacial reaction between brazing filler and sapphire. The maximum shear strength of 95 MPa was achieved when 3 wt% CoCrNi MEA was added into the AgCuTi filler, which was similar to 57.5% higher than that of the joint brazed with the single AgCuTi filler. The reaction mechanism at the brazing seam with different CoCrNi MEA amounts was also illustrated.

The heterostructure joint of sapphire/Invar alloy was prepared using femtosecond laser selective welding tech-nology,and the effects of laser power on the sealing perform-ance,macro,micro-morphology,and shear strength of the joint were investigated.The interface welding defects,elemental distribution,and fracture behavior of the joint were character-ized using scanning electron microscopy,energy dispersive spectroscopy,and laser scanning confocal microscope.The results showed that under ultrafast laser irradiation,sapphire and Invar alloy underwent melting,mixing,and diffusion pro-cesses through nonlinear absorption and linear absorption,re-spectively.This resulted in the formation of interlocking inter-faces with intermingled features,indicating the presence of both metallurgical bonding and mechanical interlocking.The shear strength of the joint increases monotonically with the laser power,and the maximum shear strength is 145.3 MPa at a laser power of 10.19 W.The fracture of the sapphire/Invar alloy joint is mainly in the form of cleavage fracture,and both sides of the fracture contain the elements of Fe,Ni,Al,and O,which further suggests that the ultra-fast laser promotes metal-lurgical reactions of the two materials at the interface.

CMT cycle-step (CMT-CS) is a novel mode in CMT welding. It has the characteristics of minimal heat input, droplet size control and a high degree of automation. This work proposed a torch-offset CMT-CS welding method to join 5052 Al alloy to T2 copper with Al-12Si filler in different joint configurations. The influence of joint configuration was investigated, in terms of weld heat input, weld appearance, weld defect, interfacial microstructure, joint mechanical properties and fracture mode. The results indicated that offset distance to copper increased the weld heat input and promoted the wettability of Al-Si liquid filler wire on the substrate. The change of offset distance and joint configuration will change the melting amount and heat transfer form of copper, thus changing the composition of the reaction layer. The average joint strength of the Al/Cu (Aluminum alloy on top) joint was slightly higher than that of the Cu/Al (pure copper on top) joint. The maximum values were 129 N/mm and 127 N/mm, respectively. As the offset increases, the joint failure shifts from the fusion zone to the interface; moreover, the fracture mode was changed from brittle fracture to ductile fracture.

Severe porosity and coarse columnar grain are prone to be formed in the laser-MIG welded joint of aluminum alloy, deteriorating the strength and ductility seriously. In this study, the ultrasound was designed to assist the laser-MIG hybrid welding of aluminum alloy, and the influence of ultrasonic vibration on weld formation, porosity, and microstructure was investigated. The weld depth was increased from 3.6 to 4.2 mm when external ultrasound with the pressure of amplitude transformer (PAT) of 132 N was used, indicating that the penetration ability of hybrid heat sources to the welded plate could be improved. It was attributed to the dispersion effect of ultrasound on arc plasma in the laser channel. The porosity rate was reduced from 5.66 to 1.05% under PAT of 132 N, because of the increase in escape velocity of bubbles. Moreover, the columnar to equiaxed transformation (CET) of grain in the weld was promoted and the width of columnar grain zone was gradually reduced with the increasing ultrasonic energy, owing to the breaking effect of cavitation and the stirring effect of acoustic stream. As a result, lower porosity rate and finer grain size led to improvement of the microhardness and the strength of weld by ultrasound. The study provides more guidance on employing ultrasound to improve the quality of welded joints.

A porous FeCoNiCr high entropy alloy (HEA) coating was prepared on steel substrate via vacuum sintering. Wetting behavior of liquid Al-12Si alloy over coated and uncoated steel substrate was comparatively ana-lyzed. The results show that the liquid Al-12Si alloy droplets were quickly spread and infiltrated into the porous structures under the enhanced capillary force by the micro-channels in the porous coating. A complete wetting similar to 0 degrees contact angle (CA) was thus achieved; however, the CA was up to 41.2 degrees in the case of uncoated steel substrate. In the case of uncoated steel substrate, the interfacial microstructure consisted of theta-Al13Fe4, eta-Al5Fe2 and tau(1)-Al2Fe3Si3, while it changed to Cr-enriched FCC, AlFe-enriched BCC and AlNi-enriched B2 +Al-enriched BCC eutectic-like structure embedding in the HEA skeleton. The suppression of the formation of intermetallic compounds (IMCs) at the interface was mainly attributed to the sluggish diffusion and HEA effects of the porous coating. The altered interfacial metallurgical reactions and the enhanced capillary force were contributed to the improvement of wettability. This research provides a new method for improving the wettability and suppressing the interfacial IMCs in reactive metallic wetting systems. (c) 2022 Published by Elsevier B.V.

Surface functionalization of metallic micro-nanoscale system is an emerging strategy for the realization of multifunctional materials. As a facile one-step process, ultrafast laser micromachining has emerged in recent years as a new technique for micro-nanostructure fabrication. In the past, lots of investigations on ultrafast laser micromachining were focused to understand the complex ablation mechanism, whereas recent works are mostly concerned with the fabrication of various metallic surface structures owing to their numerous potential functions, such as wetting, metallurgical and optical properties. This paper provides a short overview of advances in fabrication of functionalized metallic surfaces by ultrafast laser micromachining. The principles of interaction between ultrafast laser and metallic materials are provided. According to the surface topography, state-of-the-art knowledge on the fabrication of surface functionalization using ultrafast laser are presented. Functionalized properties of laser micro-machined metals are given. In addition, the challenges and outlooks in surface functionalized metals are presented.

The use of multi-materials structures is nowadays one of the most sought solutions to decrease weight and reduce both emission of greenhouse gases and fuel consumption in the automotive industry. Dissimilar joining of aluminum (Al) alloys to steels by fusion-based welding technologies is often difficult to achieve as a result of the significant mismatch in these materials’ physical and chemical properties. Moreover, when mixed in the liquid state, hard and brittle intermetallic compounds are easily formed. Due to characteristics that include high processing speed, flexibility and energy density, multiple attempts have been made to join Al to steel using laser-based processes. This thorough review article provides a comprehensive and exhausting analysis of the recent achievements and progress on joining of Al alloys to steel by various laser-based joining processes, including laser keyhole welding, laser welding-brazing, laser-arc welding, laser-assisted friction stir welding, laser roll pressure welding and joining based on laser additive manufacturing. This paper also evaluates the joining conditions, filler materials, phase constitution, microstructure, mechanical properties and joining mechanisms associated to each process. Furthermore, special emphasis is given to factors affecting the joint strength such as welding defects, joint geometry, intermetallic compounds formation and interfacial strength. The review is then concluded with an outlook providing the summary and future trends of this field.