Powder-feeding laser additive manufacturing is a technology using laser beam as the heat source, and adding powder particle simultaneously. The part is deposited after the molten pool solidified. This technology is widely used in aerospace, medical, automobile and other fields. Laser beam and powder particle are the boundary condition of heat input and mass input respectively. The energy density distribution of laser beam, material properties and process parameters are the most important factors for powder-feeding laser additive manufacturing. They not only determine the characteristics and evolution of the molten pool but also affect the morphology and microstructure of deposited layer. In this paper, laser beam with Gaussian distribution and strip distribution were selected as the heat source. Meanwhile, the property of powder particles and base metal, the concentration distribution of powder flow and process parameters was analyzed. The formation, evolution and solidification of the molten pool caused by the interaction between laser beam, powder particle and base metal were studied. It has important scientific significance and engineering value to understand the physical mechanism for laser additive manufacturing and control the quality of deposited layer
The main research contents and conclusions of this paper are as follows:
1. Different structures of powder feeding nozzle were selected for laser beam with Gaussian distribution and strip distribution. For Gaussian laser beam, coaxial powder feeding was used. The characteristics of concentration distribution of powder flow were studied under different process parameters, and the optimal convergence status and position of focus area for powder flow were obtained. For laser beam with strip distribution, the off-axial powder feeding was applied. According to the size of the line spot, a novel wide-band powder feeding nozzle structure was designed which could generate wide-band powder flow with uniform distribution.
2. A three-dimensional transient model studying the heat transfer and fluid flow in molten pool was established, taking into account powder addition, latent heat, convective heat transfer and the track of gas-liquid interface. The dimensionless parameters related to the driving force, heat transfer, fluid flow and mass transfer of the molten pool were quantitatively analyzed. The characteristics of the molten pool under different heat input and mass input were studied. The differences of molten pool under Gaussian spot and line spot was analyzed in geometric features, heat transfer, fluid flow.
3. A cladding coatings was deposited with line spot laser beam. The coating is about 5 mm wide and has a uniform thickness of 50μm. Equiaxed crystal structure was observed in all cross-section. Because the value of undercooling was huge in the molten pool and the rate of crystal nucleation was high for laser beam with line distribution.In addition, a base metal containing Sulfur was selected to investigate the effect of surface active element in substrate on the morphology and microstructure of laser cladding layer. The Sulfur content plays a primary role in temperature coefficient of surface tension, the sign of which determined the direction of Marangoni convection.