Significance The sustained demand for the joining techniques with high quality, high efficiency, high flexibility, and environmental protection, as well as the emerging demand for joining various novel materials, jointly promote the research and applications of laser-welding techniques. Benefited from the rapid development of laser components and supporting equipment, as well as the understanding of the interaction mechanism between the laser and materials, laser welding has achieved the transformation from laboratory development to large-scale industrial applications in the past 30 years. The laser energy can be precisely controlled in time and space. High energy density and low heat input enable laser welding to control the weld shape accurately and restrain the adverse effect of the welding thermal cycle on the base metal. On the one hand, high energy density causes metal evaporation, thereby forming keyhole, which is a common defect in laser welding. On the other hand, low heat input leads to a fast cooling rate, affecting the microstructure of the weld and heat-affected zone. The control of penetration, porosity, and microstructure is closely related to the laser energy. By analyzing four typical cases, welding of steel, magnesium alloy, titanium alloy, and dissimilar material, this study introduces the influence of laser energy on the welding penetration, porosity, and microstructure and its control methods. Moreover, it reflects the characteristics and advantages of laser welding and provides a useful reference for solving the welding problems of new materials and structures using laser-welding techniques.

Progress Laser welding can reduce the heat input, obtain a high aspect ratio, and improve the welding efficiency and accuracy. It can also accurately control the temperature change of the base metal and obtain the desired melting shape and temperature distribution. The semi penetration laser welding of stainless-steel car body can avoid the burning traces of the outer plate and ensure joint strength ( Fig. 1). Using double beams, the hump of thin-walled stainless-steel welded by laser can be avoided, and the welding speed can be greatly improved ( Fig. 2). By adjusting the laser incidence position and heat input, the fusion ratio or heating temperature of dissimilar materials can be controlled, and the bad microstructure, holes, and cracks can be avoided ( Fig. 3).

The metallurgical pores of laser welding are mainly affected by the gas source. The impurity on the surface of a workpiece is removed using the method of laser surface cleaning before welding. This suppresses the pores of laser welding, improves welding efficiency and flexibility, and reduces environmental pollution. For materials with high gas content in the base metal, such as die-casting magnesium alloy, with high energy density and appropriate heat input, the precipitation of solid solution gas can be avoided, reducing the porosity (Fig. 5). The porosity process of laser welding is affected by the stability of the keyhole. Appropriate laser energy is essential to improve the stability of the keyhole and reduce the tendency of porosity. Due to the influence of gravity, the influence of the laser energy on the pore tendency under different welding positions differs; thus, it is necessary to select appropriate laser energy according to specific welding positions (Fig. 6).

Laser welded joints undergo heating, cooling, melting, and solidification processes, which lead to changes in structures, deteriorating performance. Laser welding is the ideal-welding method for ultrafine grain steel due to the low heat input, which has little influence on the entire welded joint. The rapid cooling rate of laser-welding results in a great difference between the microstructure of the welded joint and base metal. The laser surface heat treatment after welding can control the microstructure of the weld and heat-affected zone properly, providing more possibility for controlling the microstructure of the joints by adjusting the laser energy (Fig. 9).

Conclusions and Prospect The essence of laser welding is the interaction between laser energy and material. Penetration, porosity, and microstructure are key factors affecting the properties of laser welded joints. In this study, typical examples of laser welding of steel, magnesium alloy, titanium alloy, and dissimilar materials are selected. The influence of laser energy on the welding penetration, porosity, and microstructure and their control methods are summarized. For future development, the laser energy modulated by wavelength, pulse, and compound heat source, and other laser-processing methods combined with the pretreatment or posttreatment of welding provide more possibilities for controlling the melting shape, porosity, and microstructure of laser welded joints, improving the quality and efficiency of laser welding and expanding the applications of laser welding. From the application perspective, laser-welding techniques may have great development and breakthrough in microjoining, nanojoining, and joining materials with special properties or functions. Moreover, due to the rapid development of artificial intelligence, more techniques, such as big data and machine learning, will be used in laser welding in the future.