Aiming at the problem of powder transmission under the special working conditions of inner hole laser cladding, a new type of air-vented inner hole laser cladding powder feeding nozzle was designed. The discrete phase model was used to simulate and select the appropriate air-vent size, so that the nozzle can output gas-powder flow with better quality. The cladding test was compared under different carrier gas flow conditions, and it was found that the cladding layer quality using the air-vented powder feeding nozzle was better than that of the non-air-vented nozzle. The results show that the use of the air-vented powder feeding nozzle can improve the powder utilization rate and the cladding forming quality.

Laser cladding technology is an important method of material surface modification technology. This technology uses laser processing to quickly heat and fuse the alloy powder with the thin layer on the surface of the substrate to form a metallurgical bond, so that the surface of the workpiece has the characteristics of wear resistance, heat resistance and corrosion resistance.

With the rapid development of laser cladding technology, in order to meet the needs of inner hole cladding for the manufacture of hole parts, its powder transportation is characterized by gas as a driving force to transport powder particles horizontally. When the carrier gas flow rate is small, the airflow power is not enough to overcome the resistance and transport the powder evenly to the bottom of the nozzle; when the carrier gas flow rate is large, on the one hand, the strong airflow acts on the molten pool, which is not conducive to the formation of the molten pool. On the other hand, the speed of the powder particles reaching the bottom of the nozzle is too large, and the coupling time with the laser is short. Some powders are not completely melted and bounce off the substrate, which reduces the powder utilization rate. At the same time, it may cause the rebounded powder to stick to the powder feeding nozzle, affecting the service life of the nozzle. This paper proposes a gas-release powder feeding nozzle for inner hole cladding, the purpose of which is to divide the air flow into two parts: one part of the gas is used to carry the powder, and the other part of the gas is discharged from the gas-release port. The nozzle is divided into a gas-release section, a transition section, and a convergence section. This paper mainly studies the influence of the gas-release port diameter of the gas-release section on the inner hole laser cladding.

1 Gas-release powder feeding nozzle structure The inner hole laser cladding powder feeding nozzle adopts a three-section structure design
(as shown in Figure 1), including: gas-release section, transition section and convergence section. The air release section component contains a small cavity with an air release port at the top and connected to the transition section component at the bottom. The flow channels of the transition section and the convergence section are cylindrical, and a conical smooth transition is used at the change of flow channel size. The end of the convergence section component is the gas-powder flow outlet. The air release section component is made of 6061 aluminum alloy, and its inner wall roughness must be guaranteed to reduce the impact on the gas-powder flow state. The transition section and the convergence section components are made of copper, which has good thermal conductivity and avoids the high heat generated by the light-powder coupling to damage the nozzle and the internal powder blockage. The following explores the influence of the air release powder feeding nozzle on the powder feeding effect and cladding quality of the inner hole laser cladding.

The simulation model selects the standard k-ε turbulence model. See formulas (1) (2) (3) in the figure.
Where: k is turbulent kinetic energy, ε is dissipation rate, ρ is fluid density, μt is turbulent viscosity, μ is dynamic viscosity of fluid, Gk and Gb are turbulent kinetic energy caused by average velocity gradient and buoyancy respectively, xi is coordinate direction, μi is time-averaged velocity, σk and σε are Prandtl numbers corresponding to k and ε, Cμ=0.09, C1ε=1.44, C2ε=1.92, σε=1.3, σk=1.0 are model constants obtained by experimental verification; ui is the component of velocity in the i-th direction.

2 Structural simulation and optimization of air-release powder feeding nozzle

2.1 Meshing and condition setting
The nozzle fluid domain is meshed in ANSYS ICEM software. The tetrahedral meshing structure is adopted, the meshing size of the convergence segment is 0.1 mm, and the meshing size of the remaining fluid domains is 0.6 mm. Among them, the inlet type is velocity inlet, the outlet type is pressure outlet, and the boundary conditions of the remaining surfaces are walls. The effect of the diameter of the air vent on the powder properties is simulated by the overall structure. As shown in the grid in Figure 2, the side of the air vent section is the inlet, and the calculation domain and the air vent below are the outlets. The gas-powder flow is simulated under the condition of normal pressure 1.013 MPa, and argon is used as the conveying gas, with a viscosity μ of 2.125×10-5 kg/m·s and a density ρ of 1.622 8 kg/m3. The powder is iron-based powder Fe316, and the powder particle size is Rossin-Rammler distribution with a particle size of 50-100 μm.
2.2 Nozzle structure optimization
The diameter of the air vent plays a key role in the powder feeding effect of the air vent type inner hole laser cladding powder feeding nozzle. The opening of the air vent can change the size of the powder-carrying gas flow and divert the air flow that completes the horizontal transportation of powder. The simulation model in this section uses the structural parameters of the convergence section as 55° powder feeding angle, 45 mm convergence section length, and 1.2 mm powder outlet diameter. Four parameter values ​​of 0, 0.8, 1.2, and 1.6 mm are selected for simulation analysis. Figure 3 shows the powder concentration distribution in the XY plane with different vent diameters. It can be seen from the figure that as the vent diameter gradually increases, the powder convergence gradually deteriorates. When the vent diameter is 0 mm and 0.8 mm, the powder convergence and stiffness are good, and a powder beam with good quality can be formed; when the vent diameter is 1.2 and 1.6 mm, the powder divergence is serious, and a more regular powder beam is not formed. The powder utilization rate is low when coupled with the laser beam.
Under the condition of other constant parameters, as the vent diameter increases, the carrier gas flow rate at the powder outlet decreases, the air flow reaching the bottom of the nozzle decreases, and the air flow velocity of the powder is reduced, which will lead to a decrease in the powder particle velocity and a deterioration in the powder convergence and stiffness. As shown in Figure 4, when the diameter of the powder outlet is 1.2 mm and other conditions remain unchanged, when the diameter of the air vent is smaller than the diameter of the powder outlet, the powder convergence and stiffness can be guaranteed, a more regular powder bundle can be formed, and the powder feeding effect is good; when the diameter of the air vent is larger than the diameter of the powder outlet, most of the powder-carrying airflow is discharged from the air vent, and the powder feeding effect of the remaining airflow transporting the powder is poor, the powder diverges seriously, and the formed powder bundle cannot be used for laser cladding experiments.

In summary, when selecting the diameter of the air vent, a structure with a smaller diameter should be selected to ensure that the purpose of air venting is achieved under the premise of ensuring that the powder feeding effect is affected to a small extent. For the above four structural parameters, it is more appropriate to use a powder feeding nozzle with an air vent diameter of 0.8 mm.

3 Laser cladding experiment
In order to solve the powder feeding problem under the special working conditions of inner hole laser cladding, the air vent inner hole cladding powder feeding nozzle uses the flow of carrier gas to divert and improve the cladding forming effect. The parameters such as powder feeding amount of 1.5 r/min, laser power of 1800 W, and scanning speed of 10 mm/s are adopted. The material is iron-based powder. The cladding effects of the powder feeding nozzle without air vent and the air vent of 0.8 mm are compared, and the parameter range is selected from the 9 groups of experiments with carrier gas flow of 2, 4, 6, 8, 10, 12, 14, 16, and 18 L/min.
According to the results in Table 1, the cladding layer was prepared at different carrier gas flow rates using a non-deflating powder delivery nozzle. When the carrier gas flow rate was 4 L/min, the morphology of the cladding layer was slightly improved compared with other parameters, but the surface was still uneven. When the carrier gas flow rate was 2 L/min, the uneven powder delivery caused ripples on the surface. As the carrier gas flow rate gradually increased from 6 L/min, the surface of the cladding layer became more and more uneven, which was caused by severe powder rebound. After using deflating, when the carrier gas flow rate was 8, 10, and 12 L/min, the surface gloss of the cladding layer was better than that of other parameters. Comparing the cladding effect of the powder delivery nozzle without deflating and after deflating, the deflating nozzle can improve the cladding quality and reduce the roughness of the cladding surface.
At a carrier gas flow rate of 8 L/min, the cladding state of the non-vented and vented powder feeding nozzles during laser cladding is compared (as shown in Figure 5). Due to the serious powder rebound of the non-vented nozzle, more powder rebounds to the outside of the molten pool and is oxidized, while the vented powder feeding nozzle has almost no powder rebound, which is beneficial to improve the powder utilization rate.
The above test results show that the cladding quality is significantly improved after the vent is opened, and the powder utilization rate is also relatively improved. When the carrier gas flow rate is 8~12 L/min, the surface quality of the cladding layer is better.

4 Conclusions
This paper proposes a new type of vented inner hole laser cladding powder feeding nozzle structure, which can improve the problems of poor uniformity of inner hole cladding powder feeding, low powder utilization rate, and poor cladding quality, and the following conclusions are drawn.
1) The vented nozzle will divert the carrier gas flow that completes the lateral transportation of powder, reduce the speed of the powder particles and the carrier gas flow rate reaching the bottom of the nozzle, make the powder particles completely melted, reduce the number of particle rebounds, and improve the powder utilization rate.
2) The diameter of the air vent should be smaller than the powder outlet. Under the premise of ensuring that the powder feeding effect is not affected to a small extent, the powder speed is reduced to achieve the purpose of air venting.
3) The experiment shows that the powder feeding nozzle after air venting effectively improves the surface quality of the cladding layer and increases the powder utilization rate. When the carrier gas flow rate is selected to be
8~12 L/min, the surface quality of the cladding layer is better.