*3.3. Flow Behavior of Weld Pool*

To study the weld pool flow behavior, positions of the camera and backlight source need to be changed, as shown in position two of Figure 2. 650 nm near infrared filter were installed to obtain metal flow information of the weld pool. When active gas served as a shielding gas, alloying elements like silicon and manganese, which were present in the base metal and the wire, had a high affinity to react with oxygen and form silicon oxide and manganese oxide. These oxides accumulate on the surface of the weld pool and form slag [18]. the slags have a lower density than the molten metal and follow the flow pattern of the weld pool. Hence, slag flow pattern and accumulation location can disclose the weld pool flow behavior [19].

GMAW weld pool consists of the hot part of the weld pool and the cold part of the weld pool [19,20]. The hot part of the weld pool consisted of the area directly under the arc and the surrounding region, and the cold part of the weld pool is located behind the hot part of weld pool. According to Grong et al., the metal oxides in the high temperature zone of the weld pool exists in the form of metal oxide powder, which cannot aggregate into slags [20,21]. The metal oxides in the cold part of weld pool accumulate into blocks to form slags. Slag is a poor conductor of heat and prevents the red glow of the weld pool, which can block the light at a wavelength of 650 nm. The slag flow pattern can be clearly observed by using 650 nm polaroid as filter.

Figure 15 represented the frames from the high speed video to show the partly penetrated weld pool flow pattern and the slag accumulation location for conventional process, LSC, and Cold Arc, respectively. The white powder in the front and middle of the weld pool was the silicon oxide and manganese oxide particles, which are separated from the weld metal due to the strong turbulence in the weld pool in this part and pushed to the low temperature area of the weld pool under the action of the pool flow. The slags flow patterns behaviors of different processes showed significant difference.

**Figure 15.** The partly penetrated weld pool flow pattern and the slag accumulation location: (**a**) Conventional; (**b**) LSC; (**c**) Cold Arc (the slag islands were outlined by white dotted lines).

The weld pool flow in partly the penetrated weld pool is explained with the assistance of Figure 16. When the weld pool is forced to flow downwards (by the external force), it is blocked by the base metal and is forced to flow to the back of the weld pool. At the back of the weld pool, the metal liquid flow will rebound off the solid metal interface and flow to the front of the pool. By the present experiment results in Section 3.1, the impact of the traditional S-GMAW process on the weld pool is the strongest of the three, and the bounced metal flow was the strongest which led to the formation of two spinning large slag islands. The impact of the LSC on the weld pool was smaller than the conventional process. The dumbbell shaped slag island in LSC was formed with co-extrusion of the bounced metal flow and the backward flow of metal on the surface of weld pool. Cold Arc process has little impact on the weld

pool, because the bounced metal strength was negligible, so the metal oxides gathered into a single round island of slag.

**Figure 16.** Schematic diagram of flow of partial penetrated weld pool: (**a**) inside. (**b**) surface.

As shown in Figure 17, compared with partial penetrated weld pool, the characteristics of the full penetrated weld pool flow pattern have the obvious differentiation, the full penetrated weld pools also have obvious low temperature zone and high temperature zone, but the metal oxides in the cold part of the weld pool did not gather and form huge slag islands, instead distributed at the back of the weld pool evenly. The bottom surface of the full penetrated weld pool can expand and contract with impact, and the energy was absorbed due to the existence of the free surface at the bottom, as shown in Figure 18. The liquid metal in the hot part of weld pool was not pressed, so the volume and length of the fully penetrated weld pool was larger than that of partly penetrated weld pool. For the fully penetrated weld pool, the slag at the end of the pool formed discrete scattered islands and did not gather into a large slag island, as was observed for the partial penetration case. This change was simply dependent on the surface wave of the weld pool.

**Figure 17.** The fully penetrated weld pool flow pattern and the slag accumulation location: (**a**) Conventional; (**b**) LSC; (**c**) Cold Arc (the slags were outlined by dotted lines).

**Figure 18.** Schematic diagram of flow of fully penetrated weld pool: (**a**) inside. (**b**) surface.

Since the composition of shielding gas was consistent, the influence of the Marangoni flow can be excluded. And for the cold part of the weld pool lie on the further from the center of the arc, the influence of plasma flow force and electromagnetic force can be ignored. The difference observed in weld pool flow pattern was attributed to the varied degrees-of-freedom of weld pool.
