Numerical simulation of interaction between laser

2022-08-09
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Numerical simulation of interaction between laser, powder and molten pool in laser rapid prototyping

Abstract: Aiming at the interaction between laser, powder and molten pool in the process of laser rapid prototyping, the models of gas/powder two-phase flow field, laser rapid prototyping parts and molten pool temperature field are established in this paper. The growth of laser heating and cladding layer as well as the shape of molten pool and free interface are simulated by using ANSYS element birth and death technology. The particle tracking model based on Lagrangian method of CFX is used to track powder particles, and the particle flatness and vertical momentum loss are set according to the interface temperature to simulate the capture of particles by molten pool and the reflection of powder by workpiece. The interaction between 316L stainless steel powder, laser and molten pool is calculated, and the calculated results are in good agreement with the experiment

key words: laser rapid prototyping; Gas/powder two-phase flow field; Molten pool; Interaction

1 preface

laser rapid prototyping technology is a new advanced manufacturing technology, which can realize the rapid, die free, near net shape manufacturing of high-performance dense metal parts with complex structures, and has a bright application prospect in high-tech fields such as aviation, aerospace, automobile []. With the in-depth research of laser rapid prototyping technology, it is urgent to develop a theoretical model that can accurately describe the laser cladding process to accurately grasp its internal mechanism

the core of laser rapid prototyping is laser cladding - the process of laser melting powder and stacking layer by layer (as shown in Figure 1). In this process, the free surface of the molten pool is the free interface for laser energy and powder quality to enter the molten pool, and it is also the dynamic boundary for the growth of the cladding layer. Therefore, the interaction between powder and molten pool is an inevitable basic problem in the process of laser rapid prototyping, In order to realize the overall accurate manufacturing of dense metal parts with high performance and complex structure, it is necessary to establish a reliable interaction between laser, powder and molten pool, so as to realize the simulation of laser rapid prototyping process on this basis

Figure 1 process of laser cladding powder melting and layer by layer stacking

2 interaction between laser, powder and molten pool

in the process of laser rapid prototyping, the growth of cladding layer is caused by laser heating and the input of powder flow, and the continuous growth and layer by layer stacking of cladding layer lead to the continuous change of the continuous body region of formed parts, which will affect the formation and development of dynamic temperature field and flow field, In turn, the dynamic temperature field and flow field will affect the absorption of laser energy and powder by the molten pool, resulting in the continuous body domain of the formed part changing constantly. The above process is shown in Figure 2

Figure 2 interaction process between laser, powder and molten pool

it can be seen from the figure that the growth of cladding layer is the result of the interaction between powder and free surface of molten pool, the mixed flow of powder and molten liquid in molten pool, and the final solidification and accumulation. The reason why powder can enter the molten pool is that the laser heats the surface of formed parts to form a temperature field distribution, and the temperature at the molten pool exceeds the melting point, It forms a window for powder to enter the molten pool - the free surface of the molten pool in liquid state. The free surface of the molten pool is the dynamic boundary for the growth of the cladding layer of formed parts, and it is the free interface for energy and mass to enter. Therefore, the interaction between laser, powder and the molten pool, accompanied by the movement of the free surface interface of the molten pool and the growth of the cladding layer, is inseparable from the formation and development of the temperature, flow field and gas/powder two-phase flow field of the molten pool. In general, the interaction between laser, powder and molten pool mainly includes: a. the interaction between laser and powder, including the shielding of laser by powder, the rise of powder temperature and the spatial distribution of laser energy; b. The formation of gas powder two-phase flow and the spatial mass distribution of powder on the surface of workpiece and molten pool; c. The interaction between powder and molten pool surface and workpiece surface, including the capture and rebound of powder; d. The interface movement of the free surface of the molten pool and the growth of the cladding layer, including temperature and flow field and the evolution of the free surface interface, if the instrument firmware is loose, and so on. The above problems are closely related to each other, and it is expected that it is quite difficult to solve them at the same time with a unified model

2.1 basic assumptions and method steps:

in view of the complexity of the interaction between laser, powder and molten pool, this paper makes the following assumptions based on the experiments and theoretical calculations of other scholars:

1) the laser energy obeys the Gaussian distribution on the beam cross-section, and the shielding of powder to laser is a linear function of the volume fraction of powder [3]

2) the volume fraction of powder in gas is very low. Only the influence of gas flow and gravity on powder is considered, and the effect between powder particles is not considered [4]

3) solid particles will rebound when they hit the solid surface, and will be absorbed when they hit the liquid molten pool [5]

4) after entering the molten pool, the powder has been completely mixed or completely distributed in the liquid metal before melting [6][7]

5) for the convection heat transfer of liquid metal, it is assumed that the thermal conductivity of the material above the melting point is 2.5 times the average [8]

in this way, the two-phase flow field of gas and powder, the spatial mass distribution of powder on the surface of workpiece and molten pool, the temperature field of molten pool and the shape of molten free surface, the capture and rebound of powder and the growth of cladding layer are mainly considered. ANSYS element birth and death technology [9] is used to simulate the growth of laser heating and cladding layer, the shape of molten pool and free interface, and CFX particle tracking model based on Lagrangian method is used to track powder particles. The specific method steps are as follows:

1) establish the geometric model of nozzle and workpiece in ANSYS and divide the lattice

2) import the ANSYS geometric model into CFX, set the interaction rules between the gas/powder two-phase flow field and the workpiece surface, set the particle flatness and vertical momentum loss according to the interface temperature, simulate the capture of particles by the molten pool and the reflection of the workpiece on the powder, and calculate the distribution of the gas/powder two-phase flow field inside and outside the nozzle by CFX, and get the size of the powder spot by coupling with the workpiece surface The distribution of solid fraction of powder and the surface temperature of workpiece cooled by gas/powder two-phase flow

3) the powder spot distribution, powder yield and workpiece surface temperature calculated by CFX are imported into ANSYS, coupled with the laser energy distribution to calculate the temperature field of laser rapid prototyping, and the growth rules are set: if the powder yield of adjacent units on the molten surface is greater than the unit volume, the growth unit is activated by ANSYS unit birth and death, and the growth interface is reconstructed to obtain a new geometric model of the formed part

4) the new geometric model of the formed part reconstructed from the interface of ANSYS is introduced into CFX together with the temperature field of laser rapid prototyping after lattice division. The interaction rules between the gas/powder two-phase flow field and the surface of the workpiece and the molten pool are set. The particle flatness and vertical momentum loss are set according to the interface temperature to simulate the capture of particles in the molten pool and the reflection of the workpiece on the powder. The distribution of solid phase fraction of the powder spot powder of the model after growth is calculated by CFX Powder yield of molten pool and surface temperature of workpiece and molten pool under gas/powder two-phase flow cooling

5) repeat 3 and 4 steps as the laser moves until the laser forming enters the quasi steady state

6) the powder spot distribution and powder yield are calculated by quasi steady state CFX, and the laser forming temperature field is calculated in ANSYS until the end

2.2 calculation and result analysis:

2.2.1 physical model:

316L stainless steel powder is used on lrf855 laser rapid prototyping machine at 30 mm × 10 mm × 7 mm (length × wide × High) 45 steel block samples are laser rapid formed from one end to the other along the width centerline. The forming parameters are shown in table 1:

to simplify the calculation, take the symmetrical 1/2 model, the laser irradiates the workpiece surface vertically, and the inclination angle between the nozzle and the workpiece Φ= 46 °, its axis intersects with the laser optical axis on the workpiece surface, as shown in figure 3:

Figure 3 calculation model

the whole calculation domain includes the gas powder two-phase external flow field of 100mm × 25 mm × 50 mm (length × wide × High) and the solid two domains of the formed sample, which are automatically connected through the interface. The boundary conditions of the external flow field are shown in Figure 4 A, in which the inlet gas flow rate is 0.01m/s, the outlet relative average static pressure is 0 PA, the opening (opening boundary) relative opening pressure is 0 PA, and the constrained gas flow rate is 12m/s. The powder is spherical powder, with a minimum diameter of 0.01~0.1mm, which is normally distributed by mass, and the initial velocity is 0.01m/s; The whole calculation domain is divided by unstructured tetrahedral physique, as shown in figure 4b:

Figure 4 lattice model and boundary conditions

laser rapid prototyping temperature field and cladding layer growth are simulated by element birth and death in ANSYS, and the whole calculation domain is divided by structural hexahedral solid70 cells, as shown in Figure 5a, killing all forming units first, And load the heat exchange boundary conditions as shown in Figure 5 b:

Figure 5 ANSYS solid70 cell model and boundary conditions

2.2.2 calculation results:

the workpiece surface was not melted at the beginning, The gas/powder two-phase flow field and the average volume fraction distribution on the workpiece surface are shown in figure 6:

Figure 6 the gas/powder two-phase flow field and the average volume fraction distribution on the unmelted surface

it can be seen from the figure that: first, the powder arrives at the nozzle with the powder carrying gas and falls from the powder outlet under the action of gravity. Driven by gravity and the restriction of the gas flow, they named their company "matrix new material", and accelerate the fall to form the gas/powder two-phase flow, And an approximate elliptical mass distribution is formed on the workpiece surface. Because the workpiece surface is not melted, at this time, all powder particles rebound, and no cladding layer is formed

with the heating of the laser and the evolution of the temperature field, the molten surface is formed, and the molten pool begins to capture powder particles. After mixing and melting, a new shape of the clad surface is formed, as shown in figure 7:

Figure 7 powder clad surface shape and interface reconstruction

based on the assumption that solid particles will rebound when they hit the solid surface and will be absorbed when they hit the liquid molten pool and the volume fraction distribution of the powder there, It can be obtained whether the element is activated to become a cladding layer in the time step. Figure 7a shows the element life and death simulation cladding surface, and figure 7b shows the transformation of ANSYS model into CFX through interface reconstruction and re temperature interpolation

calculate the external flow field of gas powder two-phase flow in CFX. The gas/powder two-phase flow field and average volume fraction distribution on the surface of the cladding layer are shown in Figure 8:

figure 8 gas/powder two-phase flow field and average volume fraction distribution on the surface of the cladding layer

comparing figure 8 A, B and Figure 6 A, B, it can be seen that after the formation of the cladding layer, the rebound particles of the powder are greatly reduced due to the capture of the molten free surface, The distribution of the maximum average volume fraction of powder is also concentrated on the small inclined plane in front of the surface of the cladding layer. Increase the number of tracked particle samples to 200, and the powder enters the molten pool and bounces as shown in Figure 9:

Figure 9 powder enters the molten pool and bounces

it can be seen from the figure that 166 of the 200 samples enter the molten pool and the rest bounce. According to the calculation, the powder utilization rate is 83%

according to the temperature field calculation of ANSYS: the free surface temperature of the molten pool is as high as about 1800 ℃, while the constrained gas temperature is only 25 ℃, so there is heat exchange on the surface of the molten pool. The free surface temperature and surface gas temperature of the molten pool are shown in figure 10:

Figure 10 molten pool surface heat exchange

Figure 10 A shows the free surface temperature of the molten pool, and figure 10 B shows the surface gas temperature of the molten pool. It can be seen from the figure that due to the cooling of the constrained gas, The maximum temperature of the free surface of the molten pool is 1781 ℃ (FIG. 10 A). At the same time, the gas temperature on the surface of the molten pool also rises from 25 ℃ to 159 ℃, but it is limited to the gas on the surface of the molten pool, and the temperature of the gas slightly far away from the surface of the molten pool is not affected (FIG. 10 b)

according to the powder spot distribution, powder yield and heat transfer boundary conditions when laser forming enters the quasi steady state, the laser forming temperature field and cladding layer growth are calculated in ANSYS as shown in Figure 11:

Figure 11 laser forming temperature field and cladding layer growth

Figure 11 A shows the temperature field and cladding layer when entering the quasi steady state, and Figure 11 b~c shows the temperature field distribution and cladding layer at the end of cladding and cladding

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