With wide applicability and low-cost processing advantages, laser processing, as a mature and versatile tool, is forming an alternative to conventional processing technologies. In recent years, the global laser market ushered in a broad new demand since the gradual increase in the penetration rate of laser processing in consumer electronics, automotive processing, aerospace, medical and other fields. Ultrafast laser and continuous wave (CW) laser are the representatives of high-quality and high-efficiency in laser processing, respectively, but it is a huge challenge to achieve high-quality and high-efficiency laser processing at the same time in the true sense, which has been the goal of the joint efforts of scientific researchers in recent decades. In this context, combined pulse laser (CPL), one of the hybrid laser processing technologies, has proven to be a reliable tool for the high-quality and high-efficiency processing through the processing advantages of different types of lasers and controlling the laser-matter interaction. In this review, the basic principles and characteristics of the CPL processing method are systematically introduced. Also, how the CPL method is for precisely and efficiently ablating, drilling, welding and structuring and their recent advancements are analyzed. Finally, a summary and outlook of the CPL technology is highlighted. 

Combined pulse laser: Reliable tool for high-quality, high-efficiency material processing

This review discusses the main factors of the formation mechanism of the laser fabricated nano-materials, focusing on the precision microstructures, mechanical properties, biocompatibility, also the research status. Laser nano-technology (LNT), that is, the precision nano-particle/structure/texture can be produced in the light alloys under an action of the laser beam, which can effectively improve the properties of the light alloys, mainly including the laser cladding (LC), selective laser melting (SLM) and laser shock peening (LSP) nano-technology. The trend of development of LNT is discussed, which includes: the preparation of the nano-gradient, the amorphous, the quasicrystalline light materials, the smart nano-materials, the connection of LNT and other techniques. Taking full use of the opportunities for the nano-materials produced by LNT to enlarge the applications of the light alloys also overcome the series of the scientific/technological issues in the coming future.

Laser nano-technology of light materials: Precision and opportunity

Laser cladding assisted by an in-situ laser shock wave was proposed in this study to obtain a refined grain structure, and NiCrBSi coatings -with and without laser shock waves, (LSW) -were prepared on the surface of a 16Mn steel sample. The phases, microstructure, grain size, texture orientation, and microhardness of coatings were investigated. The results showed that the phases for both coatings were γ-Ni, M7C3 and that CrB is formed in the coating with LSW, while the content of the ceramic phases was increased after applying the LSW. The grain size of the γ-Ni in the coating with LSW was significantly refined and tended to be equiaxed. The texture orientation of γ-Ni was also weakened and the microhardness of the coating increased by 14.45% after the application of in-situ laser shock wave because the LSW propagates to the bottom of the molten pool, breaking the growing dendrites and resulting in grain refinement. The refined grains shortened the distance between the atoms and grain boundaries, promoting the formation of ceramic phases. Thus, this technique can successfully control microstructure in metallic materials during laser cladding. 

Microstructure evolution of laser cladded NiCrBSi coating assisted by an in-situ laser shock wave

Alumina ceramic substrate is an ideal material for the new generation of microelectronic systems and devices, which is widely used in aerospace, 5G communication, high-power semiconductors, and high-power LED lighting, etc. The high-quality micro-hole processing of the alumina ceramic is the basis for system interconnection and device packaging. The existing laser processing methods, such as the single millisecond laser drilling and nanosecond laser drilling, et al., are difficult to achieve high processing efficiency and quality at the same time. So, hybrid laser processing technologies, such as combined pulse laser (CPL) and liquid-assisted laser drilling, have been widely studied. In this review, we detail the current research status of laser drilling theory of alumina ceramic and its development trend. Also, the current status of the drilling research of long-pulsed laser, short-pulsed laser, and ultrafast laser is discussed, and their advantageous points and respective limitations are compared. In addition, the research hotspot of hybrid laser processing technology is highlighted, specifically from the CPL drilling and liquid-assisted laser drilling. Finally, the future key research directions that need to be conducted in the field of laser drilling of alumina ceramic substrates are discussed to further promote the applicability of the laser drilling technology. 

Laser drilling of alumina ceramic substrates: A review

The effective activation of chemically inert nitrogen under room temperature and normal pressure conditions has positive effects on the ammonia synthesis process. Herein, we propose a laser activated nitrogen to assist ammonia synthesis for the first time, which effectively activates nitrogen and facilitates the subsequent ammonia synthesis process. The nitrogen gas with a high bond dissociation enthalpy is ionized into nitrogen plasma on the local central region of catalyst tablet by pulsed laser, which can be controlled by the focused laser with high pulse energy but not be affected by the unfocus laser induced high temperature. Meanwhile, the ammonia synthesis is proceeded in the intermediate temperature annular region with Fe or iron nitride as catalyst, which is accelerated by pulsed laser produced nitrogen plasma. The laser-induced ammonia synthesis (LIAS) achieves the original ammonia yield of 70.8 μmol g−1 min−1 (28.32 μmol min−1) with a high hydrogen converting ratio of 9.5% by Fe as catalyst and a stable yield of 23.63 μmol g−1 min−1 (9.45 μmol min−1) with a hydrogen converting ratio of 3.2% by iron nitride as catalyst, which are 14.9 and 49.7 times than those of thermocatalysis using the same catalysts, respectively. As a precedent for laser-induced catalytic reactions, this work provides a perspective for nitrogen activation in the ammonia synthesis process. 

Laser-induced Plasma and Local Temperature Field for High-efficiency Ammonia Synthesis

The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment. Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm−2 and 12 J cm−2, respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm−2, the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 ms > Δt > 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.

The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

Experimental study of the damage morphology induced by the millisecond–nanosecond combined-pulse laser with different pulse delay on silicon

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The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

Experimental study of the damage morphology induced by the millisecond–nanosecond combined-pulse laser with different pulse delay on silicon