*2.2. Forming Experiment*

Firstly, 316L stainless steel powder was used for 50 μm layer thickness experiment, and the forming quality under processing parameters such as different angles, different scanning strategies, support addition, different boundary scanning schemes, boundary scanning times, different upper surface remelting schemes, and upper surface remelting times were compared. The processing parameters and the combination of exposure time, point distance, hatch space, and scanning strategy are shown in Table 1. The laser scanning surface can be divided into upper surface, inner filling surface, and outer boundary scanning surface (as shown in Figure 4a). In the forming experiment, the main body was the noumenon, and three scanning strategies—meander, stripe, and chessboard—were adopted, and the boundary scan was solidified once. The boundary solidification scan occurred after the noumenon scan. The specific parameters were as follows: Table 1 (No. 1–3). There were nine kinds of optimization processes, which were divided into three categories: support addition (Table 1, No. 4), optimal matching boundary remelting processes with different power boundaries (Table 1, No. 5–7), and optimal matching remelting times of surface layers with different power levels (Table 1, No. 8–12). A total of 12 groups of experiments were carried out on noumenon formed by different scanning strategies, supported noumenon 1, noumenon 1 with boundary optimization, and noumenon 1 with upper surface optimization. The scheme of the block-forming experiment is shown in Table 2. Each block was inclined by 15◦, 30◦, 45◦, 60◦, 75◦ (as shown in Figure 4). The bottom of all samples was 5 mm \* 5 mm and the length of the inclined plane was 10 mm. The experiment was carried out in terms of the presence or absence of support, the number of boundary remelting times, and the number of remelting of the upper surface layer. According to the 316L forming experiment, the chrome–nickel alloy powder was studied in the vicinity of the limit forming angle with the layer thickness of 35 μm, and the overhanging structures of 20◦, 25◦, 30◦, and 40◦ were formed. The forming parameter tables are shown in Table 3. The overhanging portion of the formed overhanging structure is determined by the formula (1) [12]:

$$a = h \cdot \cot \theta \tag{1}$$

where *a* is the length of the overhanging portion, *h* is the layer thickness, and *θ* is the angle between the contour of the layer and the overhanging surface. In this experiment, the 316L stainless steel had a layer thickness of 50 μm and a spot diameter of 70 μm. The stable boundary forming can be obtained

when the diameter of the spot is larger than the length of the overhanging portion. The forming limit inclination angle was 35.5◦ from the formula (1). Considering the formation of stainless steel, the thickness of the formed chrome–nickel alloy steel layer was 35 μm, and the limit forming angle was 26.6◦.


**Table 1.** Block-forming test data sheet of 316L stainless steel.

**Figure 4.** Schematic diagram and physical drawing of the 316L stainless steel: (**a**) 15◦; (**b**) 30◦; (**c**, **f**) 45◦; (**d**)60◦;(**e**)75◦.

**Table 2.** Block-forming scheme of 316L stainless steel.


Three scanning strategies, namely meander, chessboard, and stripe scanning, are shown in Figure 5. The angle between the layers was 67◦ counterclockwise. In the selective laser melting technology, in order to ensure the boundary quality during the melt-solidification forming, a boundary scan curing can be performed after each layer of scanning is completed. The boundary remelting is a boundary solidification operation after scanning the boundary. It is not a remelting operation according to the original boundary, but a solidification of the boundary inside the original boundary. Boundary counting includes boundary scan and boundary remelting. With the increase of counting times, boundary scanning and boundary remelting will be carried out separately (that is, boundary remelting will be carried out in the middle of two boundary scans). Different counting methods were used for forming the overhanging surface, as shown in Figure 6, and counting was performed multiple times to ensure the forming effect of the boundary.


**Table 3.** The parameters of chromium–nickel alloy steel block-forming experiment.

**Figure 5.** Schematic diagram of scanning strategy.

The measurement of the surface roughness was carried out using laser scanning confocal microscopy (VK-X100, Osaka, Japan) to obtain surface roughness values, and the roughness of the final surface was measured three times and averaged as the surface roughness of the final surface. The optical microscope (OM) (DM4000M; Leica, Wetzlar, Germany) was used to measure the forming precision of the sample with a measuring accuracy of 1 μm. The measuring method was to fit the microprofile of the part to an ideal straight line to measure the accurate dimension value, and the calculated average value was the accurate dimension value of the part.

**Figure 6.** Boundary count (Corresponding Table 3, No. 1, 4, 5) (red line: scanning boundary; pink line: boundary remelting): (**a**) 1 time; (**b**) 4 times; (**c**) 10 times.

## **3. Results and Discussion**

#### *3.1. 316L Forming Surface Quality Analysis*

The upper surface topography of different scanning strategies (a–o) and overhanging structures with additional support (p–t) is shown in Figure 7. Due to the lower forming angle of 15◦–30◦, other scanning strategies cannot be formed except for the additional support strategy. The chessboard scanning strategy can ensure the upper surface forming quality of 30◦, and the missing of materials in the stripe strategy is the most serious.

**Figure 7.** Different scanning strategies and support for adding surface features of the formed overhang structure (corresponding Table 2, No. 1–4): (**<sup>a</sup>**–**<sup>e</sup>**) meander; (**f**–**j**) chessboard; (**k**–**<sup>o</sup>**) stripe; (**p**–**<sup>t</sup>**) support.

As shown in Figure 8c, in the stripe scanning process, some scanning lines are not supported and rely entirely on the transverse bonding force of melted materials. In the case of insufficient bonding force, materials are prone to be missing, which results in the process defects of collapse. Due to the other scanning method, after lamination (as shown in Figure 8b), even if some areas are overhanging, the remaining areas can be supported by not only the transverse bonding force but also the materials of the bottom in the structure to ensure the lap joint effect, so the forming effect is

excellent. The forming quality can be guaranteed by the additional support component from 15◦ to 30◦. There are overhanging defect marks on the forming upper surface (red circle of Figure 7) because of the unreasonable additional support, and the defect can be eliminated by adjusting the position of the additional support.

**Figure 8.** Effect diagram of different scanning strategies after lamination: (**a**) meander; (**b**) chessboard; (**c**) stripe.

The forming quality with the angle 45◦–75◦ above the limit forming angle is excellent, and the meander scan strategy is better than the chessboard scan and stripe scan in the upper surface forming quality. Although the additional support can ensure good forming effect, the surface spheroidization is still serious. As shown in Figure 9, the spheroidizing defect happened by spatter. The irregular spheres spattered by excessive laser power during scanning forming solidify and are prone to forming the upper surface spheroidizing phenomenon.

**Figure 9.** Surface spheroidization defect formation mechanism.

Warpage and slag hanging easily happen when SLM is used to form suspension structures. The reason for these defects lies in the vast quantities of heat generated by the interaction between the radiated laser and the powder material in the forming process. The heat conductivity of metal powders is not as good as that of solidified metals, so heat easily accumulates to form larger melting pools, resulting in slag hanging and overburning defects. So it is necessary to control the energy density of the laser when forming the surface of the hanging structure.

The low energy density is used from the suspension scanning laser, and the forming effect will be improved. In addition, rapid heat dissipation can prompt liquid metal to solidify in time, thus avoiding suspension defects and improving the forming effect of the suspension structure. Therefore, the control of laser scanning process parameters of the overhanging surface is very important to the forming quality of the suspension structure. Since the underside overhanging surface is directly in contact with the powder, a smaller energy density should be used to prevent burn-through, thereby a better quality of the underside overhanging surface can be obtained.

The topography of the overhanging structure is shown in Figure 10 for different scanning strategies. The quality of the side surface corresponds to the upper surface forming effect; with the inclination angle increased, the forming quality becomes better than before. The forming quality is poor under the minimum forming angle, in which the chessboard scan is best and the stripe scan is the worst. The forming effects of various scanning strategies above the limit forming angle are favorable, and the best forming quality can be obtained, wherein the meander scan strategy is optimal. Different scanning strategies for forming a 30◦ overhanging structure are shown in Figure 10b,g,l, and the chessboard scan forming effect is relatively favorable, followed by the meander scan, and the stripe scan forming effect is the worst.

**Figure 10.** Side profile of shaped suspension structures with different scanning strategies (corresponding Table 2, No. 1–3): (**<sup>a</sup>**–**<sup>e</sup>**) meander; (**f**–**j**) chessboard; (**k**–**<sup>o</sup>**) stripe.

## *3.2. 316L Surface Roughness*

The surface roughness of the upper overhanging surface formed by different optimization processes is shown in Figure 11. During the forming process with the fabrication angle from 15◦ to 75◦, as the angle increases, the surface roughness decreases, and the surface forming quality is favorable. The forming quality and surface roughness is better when the forming angle of the overhanging surface is larger and the forming quality of the upper surface layer is also better. The values of upper surface roughness after optimization with the fabrication angle of 45◦–75◦ are all below 30 μm in the experiment. With the increasing of the remelting power of the upper surface, the upper surface morphology deteriorates (Figure 12c–e), and the value of upper surface roughness increases. Due to the thickness of the forming layer being 50 μm, the higher laser power increases the splash phenomenon, resulting in an increase of surface spheroidization (Figure 12d), and the surface quality deteriorates. The number of laser scans also has an effect on the upper surface formation. The higher power scanning twice results in a poor surface topography, as shown in Figure 12f. The smaller power scanning twice has some improvement on the surface. As shown in Figure 13, the spheroidization phenomenon almost disappears with respect to the primary remelting surface. However, due to the instability of the process and the difference in the scanning directions, the surface is uneven, as shown in Figure 12e. The result shows that the best remelting technology is using 110 W power laser, and the upper surface roughness value can be as low as 11.9 μm.

**Figure 11.** Different optimization schemes to form the surface roughness of the overhanging structure (corresponding Table 2, No. 1, 8–12).

**Figure 12.** 75◦ upper surface topography (corresponding Table 2, No. 1, 8–12): (**a**) noumenon1; (**b**) surface1; (**c**) surface2; (**d**) surface3; (**e**) surface4; (**f**) surface5.

**Figure 13.** Remelted surface topography (corresponding Table 2, No. 8, 11): (**a**) 110 W power laser surface remelting once; (**b**) 110 W power laser surface remelting twice.

The surface roughness of the forming underside overhanging surface with different scanning strategies is shown in Figure 14. The overhanging structure is formed below the limit forming angle, and the chessboard scanning strategy is best for forming. The meander scanning above the extreme forming angle is more favorable for forming. Due to the unique scanning method of meander scanning, partition scanning, the board has a poor lap joint effect between the overhanging area and the other area when forming the overhanging structure. When the overhanging structure is formed below the forming angle using meander scanning or stripe scanning, the single line directly acts on the powder, and the chessboard scan has corresponding support and can obtain the better forming effect. The number of boundary scanning has a grea<sup>t</sup> influence on the underside overhanging surface of the shaped structure. The underside surface roughness value of the suspension structure with twice scanning 75◦ is reduced from 94.1 μm to 31.1 μm. The number of boundary scanning can improve the forming quality of the overhanging structure. The multiple scanning of the boundary is similar to the remelting of the narrow boundary by the laser and is beneficial to improve the forming quality. Thus multiple boundary scanning is beneficial to consolidate the forming quality of the underside overhanging surface. The lower laser power also has a good effect on the improvement of the underside overhanging surface. The surface roughness value of the underside overhanging surface increases with the increase of the laser power, and the boundary scanning with the 110 W power laser has the best underside surface roughness quality.

**Figure 14.** Different optimization schemes for forming overhanging surface roughness of overhanging structures (corresponding Table 2, No.1, 5–7).

The profile of the upper and underside overhanging surfaces of the overhanging structure is observed under the SEM. As shown in Figure 15, the defect of the sticky powdery phenomenon is more serious both in the upper and underside overhanging surfaces. Current research on the phenomenon of sticky powder shows that there is no better solution than using postprocessing to improve the quality of the overhanging surface. After sandblasting in the later process, it was found that the amount of powder adhering to the overhanging surface can be effectively reduced, and the forming quality of the overhanging surface greatly improved. The forming precision of the 45◦ suspension block formed in different directions of the x, y, and z (Figure 4) is shown in Table 4. The forming effect is good in this experiment, and the forming accuracy error (x1, y1, z1) is within 0.4 mm. The scanning with additional support has the best forming precision, and the remelting of boundary is beneficial to improve the forming precision. The effect of the meander scanning forming the overhang structure is better than the chessboard and the stripe scanning, and the forming precision is worse with the increasing of the remelting power.

**Figure 15.** Different optimization schemes for forming overhanging surface roughness of overhanging structures: (a–b) upper overhanging surface; (c–d) underside overhanging surface.

**Table 4.** Block forming accuracy table.


#### *3.3. Chrome–Nickel Alloy Steel Forming Surface Quality Analysis*

Considering the result of the 316L stainless steel powder forming experiment, the chrome–nickel alloy steel powder is used to form 20◦, 25◦, 30◦, 40◦ overhanging structure with a layer thickness of 35 μm. The forming effect at different angles as shown in Figure 16. Compared with 316L stainless steel, the minimum forming angle (20◦, 25◦) can ensure good forming, and no slag, warp, and so forth in the forming experiment of chrome–nickel alloy steel powder. The overhanging structure below the limit forming angle can also achieve a good forming effect. The warping deformation is caused by joint action of the thermal stress, the tissue stress, and the residual stress existing in the rapid laser forming process. Due to the force exceeding the strength of the material, the plastic deformation happened. The overhanging structure has a large warpage deformation due to the lack of support [7], so that it is difficult to avoid the defect of overhanging in the overhang structure experiment without the support. The single layer with the overhanging portion shrinks when the scanning is completed, and the volume of the molten powder shrinks during the process of solidification, causing the overhang portion to warp upward [13]. Due to the difference of temperature between the top and bottom of the scanning layer and the uneven distribution of thermal conductivity, the upper portion of the forming layer shrinks faster than the bottom, and is prone to forming the defect of overhanging [14]. As shown in Figure 8, in the red circle is a warp phenomenon, and the warped portion is scattered around each of the suspension members. In contrast, the overhanging structure of the chrome–nickel alloy steel powder has almost no warpage, and it can be seen that the chrome–nickel alloy steel powder has better forming effect than the 316L stainless steel powder. It can be seen that the adhesion of chrome–nickel alloy is better than that of 316L stainless steel.

**Figure 16.** Side profile of the overhanging structure at different angles (corresponding Table 3, No. 1): (**a**) 20◦; (**b**) 25◦; (**c**) 30◦; (**d**) 40◦.

#### *3.4. Chrome–Nickel Alloy Steel Surface Roughness*

The trend curve of the influence of boundary counting strategy on overhanging surface roughness is shown in Figure 17. The different trends of the upper and underside overhanging surfaces are contrasted in the diagram. At the same time, it can be clearly found that with the increase of counting times, the surface roughness of both the upper and underside overhanging surfaces shows a downward trend. When the counting times are 10, the surface roughness value is the lowest and the forming effect is the best. The upper overhanging surface roughness value of the suspension surface on the 20◦ suspension structure is the lowest, which can reach 15.716 μm, and the underside surface roughness value of the suspension surface is the lowest, which is 30.716 μm.

**Figure 17.** Boundary scanning strategy affects surface roughness trend (corresponding Table 3, No. 2–5).
