Empirical and theoretical approach to optimize the gas-powder flow in laser additive technology
Laser powder cladding is widely used for protective and functional coatings of metal surfaces, recovery of broken shapes, as well as for layer-by-layer additive synthesizing of metal products. Multi-jet nozzles are typically used for powder deposition. These nozzles differ by functionality and mutual orientation of the powder jet and laser beam. Dynamics of protective and transport gas at the nozzle’s outlet and in vicinity of the substrate plays an important role in the formation of a powder jet. Supersonic laser cladding is a rapidly developing method of additive laser technology. It combines the advantages of a classical subsonic laser cladding and a cold gas spraying. Gas flow dynamics between the nozzle and the substrate as well as temperature distribution in area of powder coating take a significant effect on cladding quality and metallurgical contact between coating and the substrate.
That is why experimental and numerical study of gas-powder flow dynamics in subsonic and supersonic nozzles for laser powder cladding seem to be relevant.
Visualization of transport gas and shielding gas flow was performed by shadow Schlieren method on original experimental equipment. High-speed imaging with the “laser knife” lighting system and a laser Doppler anemometer are used to study the flow of powder jets. Spatial distribution of temperature on the substrate was measured by means of FLIR SC7700 BB thermal imaging camera. Thermal vision experiments and gas flow visualization were done simultaneously.
Modeling of heated supersonic gas flow leaving de Laval nozzle is based on transient numerical solution of full set of the Navier-Stokes equations using the control volume method with the SIMPLE pressure correction technique (semi-implicit method for pressure-linked equations). In addition, the energy equation and the gas state equation were solved simultaneously. Consequent turbulent effects are simulated using the large eddy simulation model (LES).
Visualization of gas-powder flow in an off-axis nozzle showed that diameter of orifice in cyclone’s cap plays an important role in formation of powder flow. This orifice resets the atmosphere of excessive transport gas. Diameter of each type of nozzle has a corresponding optimal diameter of cyclone’s orifice to produce the most stable and focused flow of powder particles.
Figure 1. Off-axis nozzle for laser cladding:
a - shadow image of the transport gas stream; b - a frame from a high-speed video of the powder particles flow
Spatial structure of supersonic gas flow and transient temperature fields on the substrate in supersonic laser cladding system are investigated via Schlieren shadow imaging and the thermal vision system. Smoothness of inner surface of the de Laval nozzle is crucial for the stability of outer gas flow. Welded powder particles occasionally precipitate inside the nozzle and make the supersonic flow unstable. Heated supersonic gas stream becomes much less stable than the cold one. However, the heating area of the substrate has a symmetrical shape within the diameter of supersonic gas jet. It means that the shape of the gas jet stays constant before the collision with the substrate.
(a) (b) (c)
Figure 2. Shadow image of nitrogen stream for the supersonic nozzle for laser cladding
a - the flow in the free space, the room temperature of the gas;
b - flow with a flat obstacle, room temperature of the gas;
c - flow with a cylindrical obstacle, gas temperature 500 ° C
Figure 3. Thermal image of the cylinder treated by supersonic flow. The gas temperature 500 ° C
Numerical experiments revealed that the gas flow at the end of the nozzle has much lower velocities than the ones within the nozzle. This effect is observed in a wide range of boundary conditions on temperature and pressure of the gas entering de Laval nozzle. Average length of the gas jet decreases with temperature according to a quadratic law. This effect has the following explanation: the viscosity of a heated gas is greater than the one for cold gas. The results of the modeling are in good agreement with the data obtained via high speed Schlieren imaging.
Figure 4. The modeling results. Inner flow pattern: acoustic shockwaves.