# Research on Microstructure and Mechanical Properties of Laser Welding of BR1500 HS Joints
## 3 Results and Discussion
### Effect of Process Parameters on Weld Formation  
When the laser power is 2600 W, the macro morphology of the joint under different welding speeds is shown in Figure 10; when the welding speed is 75 cm/min, the macro morphology of the joint under different laser power is shown in Figure 11. It can be seen  
Figure 11: Macro morphology of the joint under different power: (**a**) 1800 W; (**b**) 2200 W; (**c**) 2600 W; (**d**) 3000 W; (**e**) 3400 W.  
Figure 10: Microhardness of joint under different laser power: (**a**) before quenching; (**b**) after quenching. from Figures 10 and 11 that all the samples are deep penetration. As can be seen from Figure 10, when the laser power is 2600 W, the cross-sectional shape of the weld joint is the \"wine glass\". With the increase in welding speed, each part of the \"wine glass\" shrinks in almost the same proportion. As is shown in Figure 11, as laser power increases, the foot of the \"wine glass\" gradually becomes thicker and transits to an \"hourglass\" shape. As the laser power increases to 3000 W, the cross-section becomes a typical \"hourglass\" shape. For in-depth analysis, taking the connecting line where the curvature of the fusion line profile changes the most as the boundary, the cross-section of the weld is divided into two parts, the upper part is the \"smooth segment\", and the lower part is the \"deep segment\", as shown in Figure 12. The parameters of the weld shape are as follows: front-bead width A, back-bead width B, and the height of smooth segment H. Figure 13 shows the changes in the parameters of the weld shape with the welding speed and laser power. It can be seen from Figure 13a that when the laser power is constant, the front-bead width A, back-bead width B, and the height of smooth segment H all decrease as the welding speed increases. Figure 13b shows that when the welding speed is constant, the shape of the weld is affected by the laser power more complicatedly. The back-bead width B increases as the laser power increases, while the front-bead width A and the height of the smooth segment H increase first and then decrease. The front-bead width A reaches the maximum when the laser power is 3000 W, and the height of the smooth segment H does not change much. Figure 12: Schematic diagram of the parameters of weld shape. Figure 13: Changes in the parameters of the weld shape with: (**a**) welding speed; (**b**) power. All samples are deep penetration; that is, a stable \"keyhole\" is formed during the welding process, and the keyhole is the heat source that forms the molten pool [29]. After the keyhole penetrates, the plasma in the hole will be ejected from the opening at the bottom of the keyhole, taking away a part of the heat. And some of this energy will conduct and radiate to the bottom of the molten pool on the back of the workpiece, which will increase the back-bead width. As the plasma is ejected out of the hole, it promotes the energy to transfer from the upper part of the keyhole downward so that the strength of each part of the cone is uniform. The Marangoni flow in molten pool results in the basic shape of the \"hourglass\" of the penetration weld, and the difference in the strength of each part of the combined heat source causes the size of each part of the \"hourglass\" to change. The higher the laser power is, the more plasma is generated by the ionization of the metal vapor in the hole. Then the temperature and density increase. When the pressure inside the keyhole increases, the plasma erupts at the bottom of the hole more intensely, and the heat source outside the hole increases. At the same time, the intensity of the heat source inside the hole transfers downward. In short, as laser power increases, the energy obtained at the bottom of the molten pool increases, but the energy obtained at the top of the molten pool does not necessarily or even reduces. Moreover, the energy obtained by the entire molten pool is not necessarily positively correlated with the laser power as the energy is taken away by the plasma ejection [30; 31].