## 3 Results and Discussion  
### Microstructure Analysis of NiTiNb Prepared by LSF  
#### 3.1.1 Macroscopic Morphology  
It can be seen from Figure 3 that the L1-L7 samples are well bonded to the substrate, while the L8 samples are separated from the substrate, and it can be seen from Figure 3 that spheroidization has occurred. Although the actual size accuracy of the samples is not high, the size accuracy of the samples prepared with the laser power of 1500 W and 2000 W in the small size samples is relatively good, and the samples do not appear serious oxidation. However, the actual size of samples L3 and L6 with a laser power of 2500 W deviate too much from the design size, and severe oxidation appears on the top of the sample, which  
Figure 4: (**a**) partition of samples L7, (**b**) partition of sample L8 (unit: mm).  
is related to the excessively high power. When the scanning speed is 900 mm/min, the sample L7 with a laser power of 2000 W is close to the design size, and it is well combined with the substrate. The problem is that one side of the sample is warped, which indicates that internal stress has been generated during the preparation process. The sample L8 with a laser power of 1500 W is separated from the substrate and the bottom is uneven due to spheroidization, and the sample has a large degree of curvature. This is due to the low laser power and the difficulty to mix the powder completely evenly. When the size of the prepared sample is large, it is difficult for the laser to melt all the powder, so it is difficult to form a good combination between powder and powder. The composition of the powder is different from that of TC4 substrate, which may lead to poor metallurgical bonding due to unsatisfactory wetting. These will cause the samples to separate from the substrate or large warpage.  
Under the same volumetric energy density, the line energy density [45; 46] can also be used as a reference standard for energy input:  
\[E_{\text{L}}=\frac{P}{v} \tag{2}\]  
Under the same scanning speed, the line energy density of the high laser power is large. The formula calculation shows that the line energy density of sample L7 is \(E_{\text{L7}}=133.33\) J/m, and the line energy density of sample L8 is \(E_{\text{L8}}=100\) J/m. As the preparation process progresses, large residual stress will accumulate on the subsequent tracks, leading to the formation of cracks at the end of the part, so when processing long specimens, the end of the specimen will often break away from the substrate [47]. But increasing the input of line energy by increasing the laser power can increase the working temperature to completely melt the powder to form a dense deposit layer [8]. When the linear energy density is high, the high processing temperature can make the powder melt more fully. In the layer-by-layer process of LSF, the melting and solidification of the powder and the remelting and solidification of the previous layer occur in cycles. Therefore, the high linear energy density can form a tight combination between the layers of the sample. Therefore, L7 samples with high linear energy density can form close metallurgical bonding between layers during the melting-solidification-remelting-solidification process, so we can obtain a complete sample L7.  
When LSF uses an energy density of more than 50 J/mm\({}^{3}\) to prepare NiTi, a sample with a density of more than 90% can be obtained [8]. Compared with the powder bed system, the building volume of the powder feeding system is usually larger, so the LSF technology is easier to expand the building volume [48]. Observed from the morphology of the samples, it can be found that the combination of LSF process parameters with 2000 W laser power and 900 mm/min scanning speed can fully melt the Ni + Ti + Nb mixed powder, and the sample size will not deviate too much, and there is no serious oxidation on the surface, the powder, and the substrate can also form a good metallurgical bond when preparing larger size samples, so LSF has advantages when using Ni + Ti + Nb mixed powders.