Laser Welding Technology

1. Introduction

Laser welding technology, recognized for its advantages such as its fast welding speed, high productivity, and energy concentration, is widely used in the industrial manufacturing field. Laser-arc hybrid welding comprises characteristics of strong gap-bridging ability, deep penetration, and high welding efficiency. In practical applications, the joining of the same or dissimilar metals or polymers through laser welding technology is an extremely complicated process involving many physical and metallurgical effects.

The improvement of laser welding product quality requires the monitoring and control of the laser welding process, which involves seam tracking based on optical and acoustic sensors, welding status monitoring and online diagnosis, and dynamic modeling of the laser welding process. The following Special Issue (“Laser Welding Technology”) covers a wide range of topics in the field of laser welding. Research on advanced control, the numerical analysis of the laser welding process and structure, the optimal design of laser welding systems, and weldment mechanical properties fall within the scope of this Special Issue.

2. Contributions

This Special Issue brings together contributions from different research groups, focusing on the effects of different welding control parameters, the effects of different detection methods on the welding process, and the characteristics of different metallic and non-metallic materials using laser welding technology. The included studies involve methods for improving welding quality and efficiency. Following review and acceptance, twelve papers were included in this Special Issue, including one review article and eleven research articles.

The authors of the review article provide an extensive analytical overview of the characteristics of laser welding for metal and polymers Contribution 1. In order to solve issues relating to the physical and chemical differences between metals and polymers in addition to the problems of low bonding strength and poor morphology, the authors of this article review and analyze the process characteristics of metal–polymer hybrid structures for lightweight automobiles and discuss the advantages of laser joining technology. Existing problems, the characterization index, and control measures for laser joint forming quality are summarized.

The authors of article Contribution 2 focus on improving welding quality by analyzing different process parameters and methods in the laser-MIG (metal inert gas) hybrid welding process. The stiffness of the arc in the laser-MIG hybrid welding process significantly affects welding quality. Using high-speed cameras and spectral diagnostic technology, the characteristics of arc plasma and droplet transfer, electron temperature, and density are analyzed, and AC magnetic field-assisted narrow gap laser-MIG welding is investigated. The morphological characteristics of arc plasma and droplet transfer in the hybrid welding process explicitly demonstrate that the high-speed rotation of the arc still maintains a relatively stable status, with the stiffness of the arc significantly improved under the action of an alternating magnetic field.

In contrast with the above laser welding method, the authors of article Contribution 3 studied the underwater surfacing and air laser welding of 10 mm thick 304 stainless steel plates under different laser powers, providing evidence of the relationship between the microstructure and corrosion resistance of 304 stainless steel in underwater laser welding. The results showed that the high-angle grain boundary content of the sample prepared via underwater laser welding increased by about 1.5 times, with the fraction of the coincidence site lattice (CSL) boundary increasing remarkably. The authors of article Contribution 4 describe 27 experiments involving the use of three different welding speeds and three types of protective gases, quantitatively and qualitatively studying the laser (light amplification by stimulated emissions of radiation) welding process parameters and the relationship between the differences in the microchemical composition of thin-walled AISI316 industrial stainless steel castings. The research methodology involved the use of an optical microscope to analyze the geometry of the welding area and determine the weld morphology (depth, weld aspect ratio, and weld area) relationship with welding parameters. The authors of article Contribution 5 examine the effect of electron beam oscillation with a circular trajectory on weld structure and mechanical properties. A small spot (0.1 mm) swing welding method is used for butt joint weld QP980 steel, which differs from general laser welding; beam oscillation can reduce the decomposition of pre-existing massive martensite, leading to a narrower width and higher hardness soften zone and improving the welding quality.

Various welding defects such as cracks, pores, welding instability, and insufficient penetration will inevitably occur in the laser welding process; therefore, the automatic detection of laser welding defects is an important technique to ensure welding quality. Different methods have been developed to detect defects during the welding process. Among these detection methods, visual sensing has the advantages of non-contact, high detection accuracy, and intuitive imaging, with it attracting significant attention in the welding field. A high-speed imaging sensor can be used to record and analyze the droplet transition and keyhole fluctuation behavior during aluminum alloy laser arc hybrid welding and establish and verify the heat transfer and flow model of laser arc hybrid welding Contribution 6, with experimental results showing that the keyhole fluctuations and weld pores mainly originate from the dynamic process of globular transition in the melting process. The droplet transfer frequency, keyhole fluctuations, and porosity increase with the increase in welding current, with porosity having an almost positive correlation with the standard deviation of keyhole fluctuations.

The most notable feature of laser welding is that it is suitable for the welding of a variety of materials. The authors of article Contribution 7 analyze the laser welding characteristics of different materials to solve current welding problems. Laser welding for amorphous thermoplastic polymer (PMMA) and 304 austenitic stainless steel plates is investigated in this article. To explore the influence of laser welding process parameters on the performance of plastic–metal joints, a high-speed camera was used to record the dynamic process of a molten pool, and the formation process of bubbles was measured using thermocouples. The impact of the process parameters on the joints was analyzed, and the microstructural morphology of the joints was observed using a scanning electron microscope. The results showed that when the laser line energy was 20.16 J/mm², the temperature was 305 °C, and the bubbles were small, with a stable hybrid joint with optimal shear stress being obtained. Under the maximum mechanical resistance, the shear stress of the effective joint was found to be 4.17 Mpa. Similarly, the authors of article Contribution 8 investigated the laser welding process of 316L stainless steel and polylactic acid dissimilar materials and analyzed the process parameters that have the greatest impact on joint quality. The authors used orthogonal experiments, single-factor experiments, response surface methods (RSMs), and Box–Behnken design (BBD) to optimize the experimental design.

One promising manufacturing process is the laser welding of a fiber-reinforced polyether ether ketone resin matrix composite (PEEK-CFRP) and Ti-6Al-4V titanium alloy Contribution 9. In their study, the authors found that the structural characteristics of the composite material change with the welding speed; the tensile shear increases first and then decreases. When the welding speed is 10 mm/s, the shear force reaches the maximum value of 36.8 N/mm. The results showed that the welding speed significantly affects the quality of the joint. During laser welding, Ti at the interface reacts with oxygen and carbon in the CFRP to generate TiO₂, TiO, and TiC and form a stable joint structure. The bubbles, cracks, and anchor effect at the interface are the main factors that affect the mechanical properties of the joint.

The task of welding metal and non-metallic materials presents considerable difficulties. In addition, there exists a considerable number of problems in the welding of aluminum–copper dissimilar metals; for instance, the inclusion of intermetallic compounds (IMCs) could reduce welding strength. To increase the laser absorption rate, the authors of article Contribution 10 used a high-density, high-quality 5 kW single-mode laser source to weld aluminum and copper plates with a thickness of 0.2 mm at a welding speed of 200–1000 mm/s. The tensile shear strength measurement results showed that most aluminum areas are fractured in the range of 118–151 N, and the strength is highest at 154 N when fractured in copper.

Lastly, the authors of article Contribution 11 propose a method for the analysis of weld metals containing unconventional alloy compositions. As a result of the incomplete mixing between the filler wire and base metal in a molten pool during laser welding, the mechanical properties of the joints were significantly reduced. Therefore, the nickel-based welding wire Inconel82 was used to weld a high-silicon steel plate with a chemical composition in wt% of 2.6 Si, 0.5 Al, and Fe balance. The technique used is the same as that used in another article that discusses techniques for reducing the formation of brittle phases during the laser welding of dissimilar titanium–aluminum metals Contribution 12. The Nb foil was used as an interlayer to effectively minimize the formation of brittle intermetallic phases during dissimilar laser welding. The mechanical properties of welded joints were tested through tensile tests. A comprehensive analysis of the microstructure of the transition layer was conducted using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. Performance was evaluated, and the results showed that the effective welding width and joint penetration depth at the joint interface were reduced in Ti/Al dissimilar metals when Nb was added as an intermediate layer, improving the mechanical properties of the joint.