4.2.1. Analysis of Microstructure for the Weld Beads

The grain size is very important for the material characteristics, especially for the metals. The influence of grain size on the materials characteristics actually resulted from the influence of the grain boundary surface area. A smaller grain size denotes a bigger grain boundary surface area, and larger influence on the materials characteristics. In general, smaller grain size means higher strength and stiffness, as well as higher toughness and ductility for the metals in room temperature. This is because a plastic deformation can occur in more grains when the grain sizes are smaller, which can make the deformation more equal. Moreover, smaller grain size means more overall grain

boundary surfaces, and boundaries are more complex, which can make the combination of different grains may more compacted. Under this circumstance, the propagation and growth of the cracks may be effectively prevented, which means that the metal has good strength and toughness.

To explore the materials characteristics, above nine weld beads can be observed by Phenom pro-electron microscopy (Thermo Fisher Scientific Inc, Waltham, MA, USA), which has 2000 multiplication times. The observations can be seen in Figure 4. In addition, to clearly present the grain sizes of the weld beads, the grain length was introduced to describe the grain size. In general, smaller grain length meant smaller grain size. For these nine weld beads, the corresponding grain length were 10.8, 10.38, 11.74, 9.64, 7.94, 9.00, 8.44, 6.53 and 6.75 μm from the weld beads from L1 to L9. It can be noticed that all the microstructures were cellular grain structure. The grain sizes of the weld bead of L1–L3 were larger, while the grain sizes of weld beads of L4–L6 were relatively smaller. In addition, the grain size of weld bead of L4 was a little larger than that of L5 and L6, and the weld beads of L7–L9 have the smallest grain sizes.

**Figure 4.** Microstructures of the weld bead observed by electron microscopy. (**a**) The weld bead of L1, (**b**) the weld bead of L2, (**c**) the weld bead of L3, (**d**) the weld bead of L4, (**e**) the weld bead of L5, (**f**). the weld bead of L6, (**g**) the weld bead of L7, (**h**) the weld bead of L8, (**i**) the weld bead of L9.
