Results and Discussions By geometric beam conversion, this paper obtained a mathematical model of the energy intensity of the annular laser beam in the negative defocus region (Fig. 3). Laser irradiation (Fig. 4) and the mathematical model of irradiation situation (Fig. 5) were illustrated. The substrate irradiation proportion was defined and calculated. The energy distribution (Fig. 6) in the irradiated area was obtained by combining the mathematical model and the shape form. Two types of transition processes in wire-melting deposition were photographed by a high-speed camera, and the irradiation proportion curves (Fig. 8) were drawn to characterize the stability condition of wire-melting deposition.

Laser wire-melting deposition is a directed energy deposition technology that uses the laser as a heat source to melt the wire materials. During the wire feeding and melting processes, the annular laser beam is blocked and separated by the central wire material. As a result, only part of the beam is irradiated onto the substrate. Therefore, research needs to be conducted to investigate the influences of the beam irradiation proportions of the wire and the substrate on the stability of laser coaxial wire-melting deposition. The mechanisms and effects of two different transition processes in wire-melting deposition, namely, "droplet" transition and "bead" transition, are analyzed and explained through the relationship between the laser irradiation proportions of the wire and the substrate.

To study and analyze the mechanism and effect of the laser coaxial wire-melting process, this paper explored the influence of the proportion of the laser energy absorbed by the substrate with a self-developed inside-laser coaxial wire-feeding processing head. The mechanism of the melting transition process was analyzed with a high-speed camera at 500 frame/s. In addition, the deposition process and the relationship between the dynamic process and the experimental parameters in the wire-melting deposition technology were studied by mathematical model calculation and experiment verification.

According to the laser melting deposition experiment using the coaxial wire feeding technology, the substrate irradiation proportion, which is between 36% and 73%, increases as the defocusing amount increases. The wire-melting deposition process is closely related to the substrate irradiation proportion. Specifically, a small proportion will cause the "droplet" transition behavior that is in a critical state between stability and instability. The surface morphology of the melting track is discontinuous and droplet-like, and the intervals among the droplets increase as the proportion decreases. In contrast, a large proportion will lead to the "bead" transition behavior that is in a relatively stable state during the whole wire-melting process. The aspect ratio of the melting track is between 3.39 to 4.87. In the following laser wire-melting deposition experiment, a better melting track shape is achieved when the laser power is 3700 W, the wire-feeding speed is 25 mm·s^-1, the scanning speed is 3 mm·s^-1, and the defocusing amount is 4.5 mm. In this case, the substrate irradiation proportion is 71.8%.