Laser Joining Research Articles

Femtosecond laser joining of Stellite and stainless steel

This research explores the practicality of fusing Stellite 6, a cobalt-chromium alloy known for its high performance, with stainless steel, utilizing various laser welding approaches. The primary challenge addressed is the joining of dissimilar materials, which presents obstacles such as divergent melting points and disparate coefficients of thermal expansion. The aim is to achieve a metallurgical bond between Stellite and stainless steel that retains desirable properties. The study employs both continuous wave and femtosecond laser welding techniques, subjecting the resultant joints to rigorous analysis to assess their impact on the properties of the bond. Initial tensile testing delineated the intrinsic mechanical characteristics of the materials, revealing that while Stellite exhibits a lower ultimate tensile strength, it compensates with greater elongation compared to stainless steel. The use of continuous wave laser welding proved to be capable of creating the bond; however, it also precipitated a considerable decline in the tensile strength of the Stellite component as a result of the thermal processing involved. In contrast, femtosecond laser welding emerged as a more effective method, enhancing the joint’s overall strength and ductility. This improvement is attributed to the femtosecond laser’s precise control over thermal exposure, which confines the heat to the intended weld zone, thereby safeguarding the adjacent material from damage. Further insights were gleaned from Scanning Electron Microscopy, which showed a preferable intergranular fracture in samples welded with the femtosecond laser—a feature typically associated with ductile failure modes. The femtosecond laser welding approach culminated in a joint efficiency of 53.7%, mirroring the innate yield strength of the Stellite wire. This outcome suggests that such welded joints possess the requisite robustness for practical deployment, thus underscoring the potential of femtosecond laser welding in applications requiring the joining of Stellite to stainless steel.

Effect of laser speed on neck formation in SS316L powder joining for LPBF

The objective of this study is to comprehend how changes in laser speed affect the behaviour and features of neck formation in SS316L during powder bed fusion. This study examines the impact of various laser speeds on the neck formation in the commonly used SS316L. Uniweld Laser 3000 was used for laser joining. A constant laser power of 15 W and laser scan speed of 25–150 mm/s were implemented, and powder joining within a single track was observed. Results show that neck size decreases with increasing laser scanning speed and no particle joining occurs at 150 mm/s scan speed, as it is considered too fast for sufficient heat absorption and melting. As laser speed decreases to 125 mm/s, neck starts to form because the particles absorb more heat, which facilitates melting and bonding. Surface diffusion occurs early at this stage because it requires only a small amount of energy to break atomic bonds on the surfaces of particles compared to those in the internal part. This process involves atoms moving along the surfaces of the particles, facilitating the initial stages of sintering by reducing surface energy and causing neck formation between particles. At 125 mm/s, the particle shape remains spherical, and the average neck size is 49 μm. At 100 μm, the powder average neck size is 62 μm. Moreover, the powder completely merges at 50 mm/s and 25 mm/s. The neck formed between particles considerably improves the product's mechanical strength by strengthening bonds between particles. Material density increases with neck quality, and stable neck growth can reduce the porosity of the final parts. Ensuring thermal stability in the laser powder bed fusion allows consistent heat to particles, ensuring even temperature in materials and ultimately facilitating stable neck growth. Thermal stability enables atoms in particles to migrate efficiently and prevents defects, such as bubbling, which increases porosity in printed parts. Therefore, a slow scanning speed allows particles to absorb more heat for melting, increasing the neck size of particles. This knowledge is useful for optimizing laser processing parameters in various industrial applications.

Ultrastrong heterojunctions without bubble defect in laser joining metal to polymer via two-step strategy

A two-step joining approach was proposed to minimize or eliminate decomposition bubbles in laser joining metal to polymer. Finite element simulation was utilized to analyze the temperature distribution and incorporated the bubble shrinkage mechanism and thermal capillary effect, providing the theoretical basis for explaining the reduction in bubbles and their movement. Two experimental setups were employed: with and without laser beam offset. In experiments without offset, the bubbles after the second joining process underwent significant shrinkage, resulting in a notable reduction in volume. In offset experiments, the bubbles not only reduced in size but also exhibited movement and elimination. This was induced by the laser beam offset, which created a temperature gradient perpendicular to the joining direction, triggering thermal capillary effects that caused the bubbles to migrate towards the hotter side and eventually escape from the joint edges. Consequently, the joints obtained through double welding exhibited higher fracture stress as compared to those obtained through single welding. The theoretical analysis of the bubble reduction mechanism aligned with experimental observations, further validated by camera images.

Effect of anodizing pretreatment on laser joining stainless steel to CFRTP

Metal-carbon fiber reinforced thermoplastic (CFRTP) hybrid joints have demonstrated its effectiveness in achieving lightweight design concept in engineering application. However, it is difficult to produce high-reliability joint due to differences in physical as well as chemical properties between metal and CFRTP. In this study, porous oxide film was prepared by anodizing treatments with various anodic oxidation times on steel surface to enhance joint performance of laser-welded steel/CFRTP. The maximum steel/CFRTP joint fracture load of 1960 N was reached, which was three times as much as that obtained with pretreated steel. The results indicated that porous structure was introduced into steel surface by anodization process, which resulted in better wettability of molten CFRTP and higher surface roughness, thus contributing to the enhancement of joint area as well as mechanical interlocking at the interface, further improving joint performance. Meanwhile, more Cr-O-C chemical bonds were produced because Cr-rich oxide film formed after anodizing contributed to interfacial chemical reaction. The chemical bond between oxide film and CFRTP was proved through density functional theory (DFT) calculation. Additionally, with the increase of anodic oxidation time, chemical bond content and surface roughness first enhanced and then reduced, which suggested that the porous structure prepared with 20 min anodizing time could best improve the interfacial mechanical interlocking, and promote the chemical bonding as well as steel/CFRTP joint interfacial bonding.

Influence of Polymer Surface Roughness on the Fractions of Transmitted, Reflected and Absorbed Energy in Operation of Laser Transmission Welding

The study of energy fractions plays a fundamental role in laser joining operations: from their knowledge, it is possible to calculate the amount of laser beam energy that is effectively available during the formation of chemical and physical bonds, and how much energy is dissipated. This study examines semi-crystalline polymers of polyamide 6.6 (PA), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and polypropylene (PP), semitransparent to light radiation, with the aim of studying the influence of surface roughness on the distribution of energy fractions, and in particular on the reflection portion. For this purpose, polymeric samples with different surface finishing were prepared and characterized by profilometric analysis. Subsequently, an experimental setup was implemented to directly measure the transmitted ratio, obtaining the reflected energy fraction from the Beer-Lambert law, and the absorbed ratio by energy balance. The results showed a decrease in the power transmitted by polymers subjected to surface treatment, due to an increase in the reflection fraction, a phenomenon particularly evident for PET, for which the reflection share increased from ~ 0.5% to ~ 15.3%, following P240 treatment. A lower influence was verified for PA and especially PTFE, due to a lower influence of the treatment on surface morphology. On the basis of the experimental results, it is hypothesised that roughening the lower section of the irradiated polymer could allow an increase in the total internal reflection fraction, favouring the joint at the interface point.Graphical

Elucidation of role of carbon fibers on joining of metal to composite

Weight reduction in the various application fields has propelled to the forefront in investigations of combining metal with polymer. Different from other studies, this work focused on exploring the effect of carbon fibers on the thermal joining of metal to composite. Three plastics with different fiber contents including pure polyether-ether-ketone (PEEK), short carbon fibers reinforced PEEK (SCF/PEEK), and continuous reinforced PEEK (CF/PEEK) were chosen to join to the 6061aluminum alloy using a fiber laser. Results showed the thermal transfer was fostered along the carbon fiber orientation due to a higher thermal conductivity, decreasing the heat accumulation at the interface. However, the existence of carbon fibers also hindered the spreading of plastics on the aluminum alloy surface. This led to a smaller adhesion width and lower tensile-shear force of joints. In addition, the tensile-shear strength of joints prepared with carbon fibers was enhanced owing to lower thermal expansion coefficient, which improved the thermal shrinkage and reduced the stress concentration at the interface. Ultimately, the role of carbon fibers during laser joining of aluminum alloy to plastics was explained, clarifying the bonding mechanism of two materials.

Thermocapillarity transition-induced unconventional thermal behavior and dilution in laser welding of metals with dissimilar sulfur content

An enhanced three-dimensional transient Thermal-Fluid-Dilution model, integrating a sub-model that accounts for surface tension, is proposed to investigate the unconventional thermal behavior and dilution induced by the surface-active element (sulfur) during the laser joining process of 304SS with higher sulfur content and pure nickel with lower sulfur content. Surface tension is modeled as a function dependent on local temperature and the concentration of the surface-active element (sulfur) at the free surface. The temperature and sulfur-dependent surface tension initially generate an unconventional driving force, subsequently leading to the transitions of heat transfer, flow pattern, and species dilution. Specifically, the inclusion of the sulfur effect results in a redistribution of larger surface shear stress along the transition boundary, causing unconventional flow behavior. The temperature coefficient of surface tension (TCST) is the coexistence of TCST (-) and TCST (+), leading to a novel flow pattern with combined inward-outward flows. Moreover, the anomalous flow exhibits observed phenomena such as the rotation of the major axis (RA) and center line shift (CLS) within the melt pool, showing more reasonable simulated melt pool dimensions when accounting for the sulfur effect. Additionally, the dilution model integrated with the unconventional flow pattern presents a more uniform concentration distribution compared to the conventional dilution. Finally, the simulated distribution of alloy elements is validated effectively by experimental data obtained from Energy Dispersive Spectrometer (EDS) analysis.

Enhancement mechanism of ultrasonic vibration on interface bonding strength in laser joining of polymer to ceramic

The current challenge in laser joining polymer and ceramic heterogeneous structures lies in their relatively low mechanical strength, primarily attributed to weak interface bonding. To address this issue, this paper proposes the application of ultrasonic vibration during the laser joining process of PET with both Si3N4 and Ti-coated Si3N4 sheets to enhance the interface bonding strength. Investigating the mechanisms underlying the enhancement of interface bonding strength due to ultrasonic vibration is crucial for improving the overall mechanical strength of the joints. It is found that the physical wetting thermodynamic conditions and chemical bond thermodynamic conditions for laser joining of PET to ceramics were the surface tension of ceramics higher than that of the molten PET and the variation in the Gibbs function less than −24.3 kcal/mol, respectively. Experimental results demonstrated that the bonding interface between Si3N4 sheet and PET exhibits only physical and mechanical bonding, with no evidence of new chemical bonds. However, XPS analysis detected new TiC bonds at the bonding interface of Ti-coated Si3N4 and PET. Furthermore, experimental results substantiated that ultrasonic vibration could improve the spreading wetting of PET melt on the Si3N4 surface, resulting in a 17 % increase in the bonded area of the weld seam. Additionally, ultrasonic vibration enhanced the molten PET filling into the blind hole structure on the Si3N4 surface, resulting in an approximately 73 % increase in the filling depth of the blind holes. Lastly, ultrasonic vibration facilitated the chemical reaction between the molten PET and Ti atoms, resulting in an approximately 57 % increase in the thickness of the interface containing TiC bonds. It is evident that ultrasonic vibration can bolster the interfacial bonding strength of PET/Si3N4 and PET/Ti-coated Si3N4 in laser joining. Moreover, in comparison to physical bonding, its more pronounced impact on mechanical and chemical bonding is noteworthy.

Version 2.0 — LaserbeamFoam: Laser ray-tracing and thermally induced state transition simulation toolkit

In this update to the laserbeamFoam toolbox, a multi-component version of the solver is included with pair-wise Fickian Diffusion between selected component pairs. This new solver, multiComponentlaserbeamFoam, captures the multi-component nature of mixing between alloys, and as-such could be used to simulate dissimilar laser joining processes, and dissimilar laser-powder-bed additive manufacturing, among others. Additionally a multi-laser version of the original laserbeamFoam solver is included (arraylaserbeamFoam) that can simulate N incident laser sources on a metallic substrate, where the direction, velocity, and other laser characteristics can be specified independently. In both these solvers, as in the original laserbeamFoam solver, the phenomenological recoil pressure treatment is used, as opposed to the full volumetric dilation treatment, for computational tractability.

The Influence of Low-Pressure Plasma Treatments on the Lap Shear Strength of Laser-Joined AISI 304 Hybrids with Polypropylene and Polyamide 6.6

This paper investigates if the polar groups induced by a plasma treatment can increase the lap shear strength of laser-joined metal and plastic hybrids. Optimal laser joining parameters for cold-rolled AISI304–polyamide 6.6 and sandblasted AISI304–polypropylene hybrids were developed at 2.85 MPa and 4.22 MPa, respectively. The surface free energy was doubled for all used plasma gases to a value of ca. 80 mN m−1 at 180 s. The plasma-treated samples were joined and tested. The arithmetic means of the plasma-treated hybrids’ lap shear strength with polyamide 6.6 varied slightly, but all measured values were within the range of the untreated samples. Residue on the sheared metal samples indicated covalent bonds between AISI304 and polyamide 6.6. The lap shear strengths of the plasma-treated polypropylene hybrids were significantly reduced between −30.8% and −53.3%, depending on the used plasma gas. This was attributed to the over-aging and development of low-molecular-weight oxidized materials, which led to a weak boundary layer. No residue of polypropylene was found on treated or untreated lap shear samples. No correlation between the surface free energy and lap shear strength could be found.

The effect of groove texturing direction on laser joining of titanium alloy to CFRTP

Hybrid structure of titanium alloy and carbon fiber reinforced thermoplastic composite (CFRTP) provided a new direction for lightweight and laser had a great prospect for application in joining metal/plastic. In this work, before laser joining TC4 (Ti6Al4V) and CFRTP (PA66), three groove micro-structures in different texturing direction (0°, 45°, 90°) were prepared on the TC4 surface respectively by nanosecond laser. The effect of different groove texturing was studied. Research results showed that the existence of three groove texturing increased the affinity between TC4 and molten CFRTP to the same extent. The resin-carbon fiber mixture adhered to the surface of TC4 after laser texturing increased obviously. However, the groove micro-structure had different impact on the tensile performance of the joint. The maximum tensile shear strength could reach 10.9 MPa, which was increased by 111 % compared with the untreated when the groove micro-texturing direction was perpendicular to the tensile direction (0° texturing) while the 90° textured joint performance was only increased by 42.9 %. The FEA results revealed that micro-texturing provided mechanical resistance for the joint during the tensile process and 0° micro-texturing provided the largest mechanical resistance followed by 45° and 90° micro-texturing.

Laser joining of CFRTP to metal: A review on welding parameters, joint enhancement, and numerical simulation

AbstractAs a lightweight material with high performance, carbon fiber‐reinforced thermoplastic plastic (CFRTP) is being implemented increasingly in aerospace, automotive, new energy, and so forth. The connection between CFRTP and metals, which are still used in these areas, is an inevitable problem. Among the various connection techniques, laser welding of CFRTP to metal is attractive due to high efficiency, excellent strength, and no contact. This paper presents the most recent research on laser welding CFRTP and metals in three aspects: (a) the principle of laser welding CFRTP to metals and the influence of process parameters on joint quality including laser power, defocus distances, scanning speed, clamping pressure, and beam shape, (b) the joint enhancement mechanisms and methods, and (c) numerical simulations of temperature and stress fields of laser‐connected CFRTP to metals. The findings show that joint strength is significantly influenced by process parameters and that the maximum joint strength can be achieved by choosing the right parameters. Apart from the conventional techniques of texturing the metal surface, applying a suitable layer of resin, and applying chemical treatment, other novel joint improvement measures have also been used, including the use of blue laser and CFRTP embedded in the metal. The temperature field and residual stresses can be successfully simulated by finite elements, which can be useful when studying new processes and choosing parameters for metal laser welding and CFRTP.Highlights We review the progress from parameters, simulations, and enhancements. The main parameters have essential effects on the dimensions and strength of joints. FEM (Finite Element Method) is applied to simulate the temperature and stress of the laser welding. Joints can be reinforced by surface pre‐texturing, adding resin, and grafting.

Analysis of energy release rate of laser-induced cohesive bonding of carbon fiber-reinforced polyetherketoneketone thermoplastic composites

In this study, laser joining of carbon fiber/polyetherketoneketone composites with several patterns was investigated to achieve weight reduction without any mechanical fasteners. The laser beam concentrated the energy, and heat conduction in the through-thickness direction of heterogeneous carbon fiber-reinforced thermoplastics caused molecular diffusion at the joining interface. According to the double-beam cantilever test results, the maximum energy release rate (ERR) was 2.01 J/mm2, which was 900 % higher than that of epoxy adhesive joining using laser power densities in the range of 1.05–1.34 W/mm2. A higher ERR was achieved because the optimized laser joining pattern secured repetitive cohesive joining at the interface. Thus, a complicated and step-wise crack opening was observed, which interfered with the propagation.

Bonding mechanism of laser joining of Poly-Ether-Ether-Ketone film which used for 5G antenna substrate to copper foil

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.

Effect of linear heat input on the interface and mechanical properties of steel/CFRP laser welding joint

An investigation on laser joining of PA66 carbon fiber reinforced plastic (CFRP) to DP780 dual-phase steel was presented. Effect of linear heat input on the interface and mechanical properties of steel/CFRP joint was performed. When linear heat input increases, the softening phenomenon is not observed in heat-affected region of dual-phase steel. Meanwhile, the melting point and decomposition temperature of resin are less impacted. The maximum tensile-shear force of steel/CFRP joint reaches 1959.4 N. The interface temperature is contained between melting and decomposition temperature of resin, the spreading of resin on steel surface is promoted, and a favorable steel/CFRP bonding interface is obtained. In addition, the new chemical bonds such as M−C and M−O are formed, which is conducive to the chemical connection of steel/CFRP interface. However, as excessive linear heat input is applied, the interface temperature will exceed the decomposition temperature of resin, resulting the generation of decomposing pores. Moreover, due to excessive heat on resin, its crystallinity decreases, the interface between resin and fiber is weakened, so that the fracture behaves as a mixed fracture mode of interface and cohesion fractures, which deteriorates steel/CFRP joint properties.

Enhancing the properties of metal-composite interface by a nano-TiO2 coating

The application of metal-polymer structures has been restricted by challenges in achieving effective bonding between the two materials. To address this, nano-TiO2 coatings were introduced to improve the interfacial bonding between the treated 6061 Al alloy and carbon fiber-reinforced thermoplastic composite (CFRTP) during the laser joining process. The results highlighted that the joints created with nano-TiO2 coatings achieved significantly higher tensile shear force and strength, reaching 1.43 and 1.37 times the values of those without particles, respectively. This improvement can be attributed to the following factors: firstly, the incorporation of coatings led to an increase in the absorption of hydroxy groups, thereby improving the wettability of the Al alloy, which in turn promoted the formation of hydrogen bonds at the interface. Secondly, the increased surface roughness favored the chemical interaction between the Al alloy and CFRTP, ascribed to the enlarged contact area. Moreover, this change influenced the thermal contact conductance (TCC), thereby changing the temperature at the interface and joining areas of the two materials. Finally, a notable transformation in the failure mode was observed, transitioning from adhesive failure to cohesive failure when compared to joints without nano-TiO2 coatings. This underscored the reinforcing role played by nano-TiO2 particles, providing an innovative solution for enhancing the interface bonding between dissimilar materials.

Moisture-induced defects produced by direct laser joining of AA7075 aluminum and PEEK

The present investigation aims to determine how polymer moisture and volatile compounds influence the mechanical behavior of hybrid metal-polymer joints made by Laser-Assisted Joining. Experimental laser joining tests were performed on AA7075 aluminum alloy and Polyetheretherketone (PEEK). Some polymer samples were pre-treated through drying to remove the moisture stored in the PEEK before joining. Laser spot joints were produced with dried and as-received samples at different heating times. Different characterization techniques were adopted to determine the influence of moisture on the performance of the joints. Single lap shear tests were performed to assess the mechanical behavior of the joints. Fracture surface analysis was carried out through Optical Microscopy to determine the influence of the moisture content on the mechanical behavior. The results indicated a significant influence of moisture on the mechanical behavior of the joints. This led to the formation and coalescence of voids at the metal-polymer interface that affected the mechanical behavior of the joints. Dried samples were characterized by higher shear force (up to 33 %) and higher toughness (up to 130 %) compared to as-received samples.

Optimization on the diode laser joining process of Al2O3 liners

The effects of fillers width and energy density of laser on weld morphology were studied. The mullite filler with the quality ratio of 70wt.% Al2O3-30wt.% SiO2 was determined as the initial fillers for the joining of Al2O3 liners according to the crystalline phase of Al2O3 liners. The transient multiple physical model of diode laser joining Al2O3 ceramic liners was built by Multiphysics COMSOL, and the coupling of temperature and stress field was carried out. The thermodynamic properties of the Al2O3 liners and fillers were analyzed and tested. Numerical results showed that the increase of fillers width would effectively reduce the thermal stress of the joint. With the increase of the energy density, the temperature and thermal stress of the seam zone increased gradually. With the increase of laser joining heat input, weld penetration increased slowly and then sharply. The experimental results indicated that the weld zone free from pores and macro cracks can be formed with the fillers width of 3mm, laser power of 700W and welding speed of 1mm/s.

Modelling and Optimization of Continual Laser Joining Processes for Silicon Aluminum Alloy in Microwave Devices

Due to its higher energy and smaller heating area, laser joining technology is widely used in aluminum alloy welding and other industrial fields, which meets the solder sealing requirements for electronic packaging. According to experiments, cracks were prone to occur at the corners and spot-welding positions near the weld. In this paper, the depth and width of the melt pool were measured experimentally, and the results were used to calibrate and validate the heat source model. An empirical relationship between heat source parameters and melt pool morphology is presented. The heat source model of laser deep penetration welding was established under the same experimental conditions. And the results were in agreement with the experimental results. The finite element method was used to numerically simulate the welding process of a 50%SiAl shell and a 27%SiAl cover plate. The effects of different spot-welding sequences and numbers on the residual stress and cracking possibility of laser welded samples were analyzed. The results show that under sequential spot-welding, when the amount of spot-welding is increased, the stress peak value decreases. Compared with sequential spot welding and side-by-side spot welding, the spot-welding sequence of diagonal points first, and then side-by-side spot welding, can effectively reduce the residual stress. This research enables us to provide some guidelines in terms of studying the reliability issues of microwave devices.

Optimization of Laser-Assisted Polypropylene Aluminum Joining

Laser joining of polymers to metals is a rising research subject due to the potential of considerably reducing the weight of structures. This article deals with the laser joining process between polypropylene and aluminum. Without pre-treatment, laser joining of these materials is not feasible, and the method applied in this study to circumvent this issue is a surface modification of aluminum with a pulsed laser to create mechanical interlocking for the heat conduction laser joining technique. Different patterns and various laser parameters are analyzed with the design of experiments to best understand the effects of each parameter along with microscopic observations. It is found that engraving weakens the mechanical properties of the aluminum samples. The compromise between the engraving depth and the mechanical properties of the samples is optimized, and the engraving process with a 0.28 mm line width, 27.3% density and 150 mm/s speed provides the highest mechanical performance of the assembly with minimum degradation of aluminum samples. Moreover, by adjusting the laser power and using power modulation below 300 W, the decomposition of polypropylene occurring at high temperatures is reduced to a minimum. After the final optimization, the joined samples reliably withstand a maximum force of 1500 N, which is, approximately, a shear strength of 20 MPa.