2.3.2　Distribution of the temperature field

The laser scans the area above the workbench and heats the metallic powders to very high temperature to melt them.As the diameter of the laser spot is only 200µm, only the area around the laser scanning path(Zone 1) can be analyzed in Fig.2-7 and Fig.2-8.Considering that large temperature fluctuations may exist at the beginning and end of the scanning process, the temperature fields at 0.045 s, 0.145 s, 0.245 s, 0.345 s, and 0.445 s are analyzed.As shown in Fig.2-7, the point with the highest temperature is always close to the head region of the heating zone, and there is a long tail created by the residual heat.In Fig.2-7(a), under a laser power of 100 W, the highest temperatures in the heating areas at the five time points are 2738 K, 2790 K, 2938 K, 2831 K, and 2706 K, respectively.The deviation is only 232 K, which is less than 8% of the average observation temperature, indicating that the temperature is almost stable during the scanning process.Temperature fluctuation may occur because of a small degree of non-uniformity in the distribution of the flow field above the workbench(Fig.2-5 and Fig.2-6).Furthermore, although there is a significant increase in temperature in the laser heating regions, most areas along the scanning path remain at the initial environmental temperature after the laser moves away because metal workbench conducts heat rapidly.Owing to the rapid loss of heat caused by heat transfer, radiation, and convection, the high temperature is confined to a small area.Similar characteristics in the temperature distributions for a laser power of 200W are presented in Fig.2-7(b).

Fig.2-7　Time evolution of temperature distribution on the workbench as the laser moving along a single scanning path:(a) PLaser = 100 W, (b) PLaser = 200 W

Fig.2-8 shows the laser induced velocity disturbance inside the chamber.The height of the observation window is 1.6 mm, and the velocity disturbance is minimal, although the degree of velocity disturbance increases as the laser power is increased from 100 W to 200 W.The reason can be that the laser spot is too small, in the order of micrometers.Therefore, the laser induced velocity disturbance inside the L-PBF printer can be ignored.

Fig.2-8　Laser induced velocity disturbance above the workbench(height < 1.6 mm) at different laser powers of 100 W and 200 W