The external 3D software is used to draw the geometric model and import it into Ansys. The Transient Thermal and Static Structural modules in Ansys software are used for Thermal indirect coupling analysis. The substrate size is 45 mm × 45 mm × 5 mm, and the material is 45# steel. The inner diameter of thin-walled ring parts is 18 mm, the outer diameter is 20 mm, the height is 6 mm, each layer is 1 mm, the material is 316L stainless steel. The fine degree of mesh division will directly affect the calculation accuracy of simulation analysis. To improve the calculation efficiency under the premise of ensuring the accuracy of the calculation results [31], the mesh in the forming layer area is refined, while the mesh in the substrate area is sparse. The grid division is shown in Fig. 1.

Finite Element Simulation is one of the most accurate and popular techniques [32], the initial conditions and boundary conditions of the simulation process need to be set according to the actual working conditions. The boundary conditions of this simulation are set as follows:

(1) Initial conditions

During the laser additive manufacturing process, the initial temperature of the metal powder and substrate is assumed to be 22°C, and the ambient air temperature is also assumed to be 22°C.

(2) Boundary conditions

The heat exchange in the laser additive process is mainly based on thermal convection and thermal radiation. Therefore, the thermal boundary conditions in the calculation process are the third type of boundary conditions. When the boundary conditions are set, the effect of thermal radiation is small, so it can be ignored. The air convection coefficient in the laser additive manufacturing process is selected according to the empirical value, h = 10 W/(m²⋅°C).

LAM is a transient thermal analysis process that includes a complex series of changes in rapid melting, phase change, cooling, and solidification, and the model can become extremely complex if all these factors are taken into account. To simplify the operation, the following assumptions are made for the model [33]:

The influence of liquid flow on temperature field and stress field distribution in the molten pool is ignored.

In this paper, a Gaussian heat source is used to simulate laser forming. The equation is as follows [34,35]:

(1) q(r)=3Qπra2exp⁡[−3(rra)2]=3ηPπra2exp⁡[−3(rra)2]=qmexp⁡[−3(rra)2]

where ra is the radius of the heat source area of action (m); r is the distance from the center of the heat source to any point in the heat source area of action (m); Q is the thermal input of the laser beam ( W ); η is the laser thermal efficiency; P is the laser power ( W ).

In this paper, the thermal coupling method is used to solve the temperature field and stress field, and the APDL language is used to realize the loading of the continuous Gaussian distribution heat source. Then, according to the results of the temperature field, the node temperature is applied as the ‘body force’ load in the subsequent stress analysis to realize the thermal coupling.

In the process of material processing, the physical parameters of the powder and the forming entity differ greatly, and in the process of conducting simulation calculations, the parameters need to be set according to the actual change of material physical quantities with temperature. The metal powder used in this paper is 316L stainless steel, and its thermal physical parameters are shown in Table 1 [36].

Meier et al. [17,18,21,29] studied 316L stainless steel with different process parameters. To systematically study the influence of different process parameters on the forming of thin-walled ring parts, combined with the existing research results of many scholars, the following simulation scheme is designed: The scheme is shown in Table 2, and the influence of process parameters on temperature field and stress field is observed by controlling a single variable. In the LAM simulation experiment, the laser power, scanning speed, and scanning mode are selected for the simulation experiment. Other parameters such as the spot diameter is 2 mm, the laser absorption rate is 0.35.

Scheme 2 is to fix the laser power P = 800 W and the scanning speed V = 6 mm/s. The effects of different scanning methods on the temperature field and stress field of 316L stainless steel thin-walled ring parts fabricated by LAM were studied by three different scanning methods. The scanning mode is shown in Fig. 2. The first scanning method is shown in Fig. 2a, starting from point A, forming a circle counterclockwise, and returning to point A, and the subsequent forming layers are scanned in this way, hereinafter referred to as normal scanning. The second scanning method is shown in Fig. 2b, starting from B, forming a circle in a counterclockwise direction and returning to point B, when the next layer is still starting from point B, forming a circle in a clockwise direction and returning to point B, the cycle repeats, the following is called alternate scanning. The third scanning method is shown in Fig. 2c, starting from C, forming a circle in a counterclockwise direction and returning to point C, and starting from D for the next layer, forming a circle in a counterclockwise direction and returning to point D point, and then the next layer takes E as the starting point, and so on, every 4 layers have a cycle, hereinafter referred to as periodic scanning.

Experiments were conducted to verify the effectiveness of the model. The surface temperature was recorded by a handheld infrared thermal imager in the process of LAM and compared with the simulation results. The experimental parameters are laser power 800 W, scanning speed 6 mm/s, spot diameter 2 mm. By comparison, it is found that the surface temperature of the actual machining process is very similar to the simulation temperature results, and from Fig. 3a, it can be seen that the peak temperature is 1774.4°C, and the peak temperature of Fig. 3b simulation is 1758.6°C. Within the allowable error range, it is proved that the simulation model in this paper is accurate and effective to a certain extent, and can be used for the analysis of LAM thin-walled ring parts.

Fig. 8 shows the weld pool sizes of specimens 1, 2, and 3 at 9.948 s with only increasing laser power. The weld pool size increases with the increase of laser power. This is because the increase in laser power leads to an increase in heat input and heat input. The more fully the powder is melted, the larger the weld pool structure will be.

Under the condition of changing scanning velocity, the weld pool sizes of specimens 3, 4, and 5 are shown in Fig. 9. It can be seen that the size of the molten pool is largest when the scanning speed is 4 mm/s. under the condition of constant laser power, the decrease of scanning speed makes the forming layer have more time to contact the laser heat source and obtain a large amount of heat input, thereby increasing the peak temperature of the molten pool and causing the size of the molten pool to increase.

Through the above analysis, the larger the laser power, the larger the weld pool size and temperature peak, and the laser power have a greater influence on the weld pool structure than the scanning speed.

Fig. 10 is the deformation under different laser power. With the increase of laser power, the deformation increases first and then decreases. This is because when the laser power increases from 600 W to 800 W, the processing temperature is increased, resulting in the thermal expansion of the material. Then the stress generated during this process will be compressed during cooling, so the residual stress becomes larger and the deformation becomes larger and larger. When the laser power increases from 800 W to 1000 W, due to the increase of laser power, the laser energy is not all used to melt the powder, and part of the energy is transferred to heat, which reduces the temperature gradient so that the residual stress becomes smaller and the deformation is reduced.

Fig. 11 is the deformation under different scanning speeds. With the increase of scanning speed, the deformation decreases gradually. Due to the decrease of scanning speed, the time of additive manufacturing is prolonged, and the contact time between the high-energy laser beam and the powder and substrate is increased, that is, the utilization rate of laser energy is increased, the heat input is increased, and the deformation is increased.

Fig. 12 shows the maximum deformation position and the maximum deformation angle. Fig. 13 shows the maximum deformation angle under different laser power and scanning speed. With the increase of laser power from 600 W to 1000 W, the deformation trend of the forming layer to the center of the ring also gradually increases. With the decrease of scanning speed, the deformation trend of forming layer to the center gradually increases. The stress distribution is dominated by Y direction, the outside is tensile stress, and the inside is compressive stress, which can cause the trend of centripetal deformation.

Fig. 14 shows the deformation cloud map of different scanning methods. When the laser power and scanning speed are fixed, the deformation increases or decreases with different scanning methods. The minimum deformation is 0.141 mm with normal scanning, and the maximum deformation is located at the bottom of the ring. When alternating scanning is adopted, the deformation increases to 0.212 mm, and the maximum deformation is located at the bottom of the ring. However, the deformation decreases to 0.176 mm when periodic scanning is adopted, and the maximum deformation is located at the top of the ring. Therefore, it is better to choose the normal scanning method from the perspective of deformation.

Through the above analysis, no matter increasing the laser power, reducing the scanning speed, or changing the scanning mode, the maximum deformation of the specimen is located near the overlap position of the starting point and the endpoint of the forming. This is because when the specimen is initially formed, the scanning speed gradually increases from 0 to the set parameter so that the height of the forming layer near the starting point is relatively high. As the forming process proceeds, each layer is affected by the starting position of the first layer, so the maximum deformation is located near the overlap position of the starting point and the endpoint.

Fig. 15a shows the variation of residual stress with time under the condition of changing the laser power when the scanning speed is fixed at V = 6 mm/s. The residual stress curves of laser power P = 600 W, 800 W, and 1000 W have the same distribution trend, showing a trend of increasing at the initial stage, then decreasing, and finally tending to be stable at the cooling stage. Fig. 15b shows the residual stresses of the samples at t = 0 s~120 s under different laser powers. It can be seen from the figure that the residual stresses of the six forming layers change obviously. When the laser power P = 1000 W, the stress increases significantly in the second band, and the distribution trends of the stress bands are roughly the same.

Fig. 16a shows the residual stress curve at the midpoint of the formed third layer with time for different laser powers. It is observed that when cooling to room temperature, the laser power P = 800 W has the highest stress, followed by P = 600 W and P = 1000 W has the lowest stress. This is because the increase of laser power will lead to the increase of heat input per unit time, making the temperature peak larger, but the relative temperature gradient will become smaller, and the stress will become smaller accordingly. In the band t = 50 s~70 s, the residual stress at laser power P = 1000 W is much larger than the remaining two.

To investigate the correlation between the scanning speed and the residual stress, the variation of the residual stress at the midpoint of the formed third layer was obtained for scanning speeds of 4 mm/s, 6 mm/s, and 8 mm/s, respectively, as shown in Fig. 16b. Although the scanning speeds were different, the stress curve distribution trends were the same. The overall residual stress decreases with increasing scan speed. As the scanning speed increases, the laser beam action time becomes shorter, the overall heat absorbed by the substrate decreases, and the temperature decreases, resulting in a decrease in the final temperature difference, so the residual stress decreases with the increase in the scanning speed.

Fig. 17 shows the stress of the middle node of the second layer in three different scanning methods: normal scanning, alternate scanning, and periodic scanning. It can be seen from Fig. 17a that after cooling to room temperature, the periodic scanning method has the minimum stress, and the normal scanning method has the maximum stress. According to Fig. 17b, it can be seen that the stress distribution of the specimens prepared by three different scanning methods is completely different at this node, and the stress distribution of alternating scanning and periodic scanning is relatively flat and uniform of normal scanning.

Figs. 17c–17e show the stress changes in the X, Y, and Z directions with different scanning methods. It can be seen from the figure that the stress in the X direction, the normal scan is much higher than the other two scan methods. Fig. 17d shows that the stress distribution trends of these three different scanning methods in the Y direction are very similar, and there is no significant difference in the peak value.

In the Z direction, as shown in Fig. 17e, the stress fluctuations of the three are large, but the peak difference is not large. This is because compared with alternate scanning and periodic scanning, the end-point and starting point of normal scanning are separated by a long time, the temperature gradient is large, and the material shrinkage is greatly inhibited increasing residual stress.

The chemical composition of 316L stainless steel powder is shown in Table 3.

The experiments of cycle scan were abandoned because of their poor forming quality during the preparation of cycle scan specimens. The processing experiments were carried out according to some of the process parameters used in the simulation and the optimized parameters, which are shown in Table 4, and the prepared specimens were analyzed for macroscopic morphology and metallographic organization.

Fig. 18a for laser power P = 600 W, scanning speed V = 6 mm/s, sample A1 prepared by normal scanning method, can be seen that the edge is uneven, there are some deformations, and the outer wall of the sample has more tumor. Fig. 18b shows sample A2 prepared by normal scanning method with laser power P = 800 W and v = 6 mm/s. The edge of sample A2 is relatively smooth and neat, and the boundary between layers is clear. The thickness of the layer is almost the same, and there is no obvious defect. Fig. 18c shows the sample A3 prepared by the alternate scanning method with laser power P = 800 W and V = 6 mm/s. It can be seen that the edge is smooth and flat, but the boundary between layers is not clear and the thickness of layers is different.

The cross-section of thin-walled ring parts is shown in Fig. 19. It can be seen from the figure that the clear boundary between the forming layer and the layer is closely combined. In Fig. 20a, it can be seen that there are obvious cracks at the junction of the forming layer and the substrate, and the upper layer is accompanied by the tumor. In Fig. 20b, the sample is prepared by using the optimized process parameters. It can be seen that the forming layer is closely combined with the substrate, without obvious defects, and the thickness and width of the layer are uniforms. Fig. 20c is a cross-sectional view of sample A3, which is prepared by alternate scanning with laser power P = 800 W, V = 6 mm/s, and the thickness and width of the layer were not uniform.

From Fig. 20, it can also be found that (a), (b) the width of the forming layer near the substrate is larger, and the width of the upper layer is smaller and relatively uniform, while (c) the sample is opposite, showing the bottom layer width is small, and the upper layer width is large and uniform. This is because (a), (b) sample in the initial processing, the scanning speed is gradually increased from 0 mm/s to 6 mm/s, the initial stage of scanning speed is slow, resulting in powder and laser at the bottom of the combination more, layer width becomes larger, then the scanning speed to maintain 6 mm/s uniform speed, layer width becomes smaller, the overall forming layer becomes uniform. Fig. 20c shows the opposite situation because when using alternate scanning mode, the starting point of the latter layer is close to the end of the previous layer, the overall temperature is high, the temperature gradient is large, and the over-melting phenomenon occurs, which makes the upper layer width larger.

The SEM images of the joint between the forming layer and the substrate of three samples with different parameters are shown in Fig. 21. This area formed by the fusion and diffusion between the powder material and the substrate, and the following is mostly planar crystal. This is mainly because the temperature of the substrate is almost the same as the ambient temperature in the initial stage of forming thin-walled circular parts, while the temperature of the molten pool is relatively high. A large temperature gradient is formed between the substrate and the molten pool, and it is not easy to appear a constitutional supercooling zone, which is conducive to the growth of planar crystals. Most of the above are epitaxially grown columnar crystals, and there are cellular crystals around them. This is because the thin-walled ring parts are stacked layer by layer from the bottom during the forming process. The closer to the high-energy laser beam, the higher the temperature, that is, the temperature gradient in the Z direction is the largest. Therefore, the grain growth rate in the Z direction is greater than that in other directions. Therefore, the columnar crystal exhibits epitaxial growth, which has obvious directivity and decreases along the direction of the temperature gradient. The lower columnar crystals are relatively coarse, while the upper columnar crystals are relatively small, mainly because the bottom is continuously heated, and the energy accumulated during the forming process promotes grain growth, resulting in coarse columnar crystals [37].

The thin-walled ring part layer structure SEM diagram is shown in Fig. 22. The microstructure of the junction between the layer and the interlayer is dense, and the grains are uniform and fine. The interlayer structure is filled with columnar crystals and cellular crystals. Since the heat dissipation is fast and the temperature gradient is large perpendicular to the substrate direction, the grains grown perpendicular to the substrate direction grow preferentially, and the growth direction is shown in Fig. 22.

The top SEM image of the thin-walled ring parts is shown in Fig. 23. The middle area still has columnar crystals and cellular crystals coexisting, and the area near the top layer is equiaxed crystals. This is because the LAM process is very cold and hot, and the area near the top layer dissipates heat and cools quickly. The finer the equiaxed crystal structure, the better the mechanical properties of thin-walled parts.

In the process of thin-walled ring parts forming, according to the simulation analysis results and experimental results, the defects such as over-melting and crack appeared in the sample with alternate scanning, while the sample prepared by normal scanning had good forming quality and no obvious defects. Combined with finite element simulation analysis, the optimal parameters were obtained as follows: laser power P = 800 W, scanning speed V = 6 mm/s, and scanning mode was normal scanning.