*3.3. E*ff*ect of Process Parameters on Microstructures*

The microstructure of fusion weld metal is controlled by the solidification behavior of weld pool [21]. Metallographic observation of all weld cross-sections with different welding parameters was conducted, and no defects such as incomplete fusion and porosity can be found. As mentioned above, the thermal pulse of DP-GMAW caused significant fluctuations in weld pool size and oscillation amplitude during welding process. It resulted in significant differences in welding microstructure between P-GMAW and DP-GMAW. The weld cross-sections, typical microstructures and heat affected zone (HAZ) of different weld processes (No.3 and No.12) are shown in Figures 16 and 17.

The microstructure of the P-GMAW weld was composed of coarse austenite (γ) columnar structures and small part of equiaxed austenite grain. The equiaxed crystal regions are distributed in the gaps between the ends of the columnar crystal regions, there is no obvious boundary between the equiaxed crystal regions and the columnar crystal regions, as shown in Figure 16b. The ferrite (δ) morphology was skeleton-shaped, distributed in the columnar austenite grain gap.

Compared to P-GMAW, the microstructure of DP-GMAW weld exhibited an obvious refinement of microstructure along with large distribution of equiaxed crystal regions. A clear boundary emerged between the equiaxed crystal area and columnar crystal area, as shown in Figure 17b. Although the structure of austenite columnar crystals was significantly refined, it can be found that the ferrite size increased significantly by comparing the ferrite morphology in austenite gap between P-GMAW and DP-GMAW, as shown in Figures 16d and 17d.

**Figure 16.** Optical micrographs of microstructure and heat affected zone (HAZ) of P-GMAW (No.12): (**a**) weld cross-section; (**b**) typical microstructure of P-GMAW weld, (**c**) coarse austenite columnar structures, (**d**) ferrite morphology and (**e**) HAZ.

**Figure 17.** Optical micrographs of microstructure and HAZ of DP-GMAW (No.3): (**a**) weld cross-section; (**b**) typical microstructure of DP-GMAW weld, (**c**) austenite equiaxed dendrites structures, (**d**) ferrite morphology and (**e**) HAZ.

Figure 18 is a schematic sketch explaining how the thermal pulse of DP-GMAW helps grain refining. The dynamic behavior of the weld pool and the fluctuation of solidification parameters are the main factors that trigger the refinement of structure of DP-GMAW. The thermal pulse of DP-GMAW caused synchronized periodical fluctuations in heat input and pool amplitude, as shown in Figure 18a. Low heat input led to the large temperature gradient G of the weld pool during Tb. Ferrite was the primary grain in weld pool, ferrite columnar dendrites dominate with less constitutional supercooling (the area surrounded by TL and Tactual), as shown in Figure 18b.

**Figure 18.** Thermal pulse helping grain refining: (**a**) process of the heat input and pool oscillation amplitude in DP-GMAW; (**b**) constitution supercooling in thermal base period; (**c**) constitution supercooling in thermal peak period.

The high heat input during Tp can reduce the temperature gradient G in weld pool. The oscillation amplitude of weld pool was greatly increased, resulting in the enhancement of convection, which is expected to reduce the temperature gradient G in weld pool, leading to more constitutional supercooling (the area surrounded by TL and Tactual) in the weld pool, as shown in Figure 18c. The peak thermal period can cause reheating and melting of dendrite arms, thus hindering the further crystal growth and causing dendrite fragmentation [22]. At the same time, the shear stress produced by the enhanced convection in weld pool aggravates the dendrite fragmentation during Tp. The broken dendrite particles provide the necessary crystal nucleus for the liquid metal crystallization. Excessively constitutional supercooling was beneficial to the dendrite fragments survival and grow into equiaxed grains.

The equiaxed ferrite grains in the freshly solidified weld transforms into austenite by diffusion transformation in subsequent Tb of thermal pulse. While the fast weld cooling speed reduced transition time of "δ→γ", both diffusion of ferrite-forming elements and austenite-forming elements were suppressed. It is the main factor leading to increasing the size and the content of ferrite structure in DP-GMAW weld microstructure compared to P-GMAW, as shown in Figure 16c,d and Figure 17c,d.

The microstructure of HAZ of weld joints prepared by P-GMAW and DP-GMAW are shown in Figures 16e and 17e. There is no obvious difference in the grain size of HAZ between the weld joints prepared by P-GMAW and DP-GMAW respectively. Similar heat input rate between welding parameters of No.11 and No.3 may be the main reason. The average size of weld microstructure and HAZ with different welding parameters were measured using the intercept method (as per ASTM E112-10), the values are statistic presented in Figure 19a. When the base metal of HAZ is heated, the microstructure undergoes a process of recrystallization, new undistorted equiaxed grains appear in the microstructure and gradually replace distorted grains. After the recrystallization is completed, continue to heat up or prolong the elevated temperature holding time could make the grain continue to grow. The size of HAZ in weld joint mainly depends on the heat input rate, Large heat input rate leads to longer elevated temperature holding time of HAZ, which is conducive to the diffusion and recrystallization process of tissue, thus leading to serious growth of grains. The fitting curve between grain size of HAZ and heat input rate is shown in Figure 19b. It can be found that the heat input rate has a good linear relationship with the grain size of HAZ, while no obvious correlation could be found between the grain size of HAZ and the thermal pulse of DP-GMAW.

**Figure 19.** (**a**) Average grain size of weld metal and HAZ; (**b**) grain size of HAZ with different heat input; (**c**) grain size of weld metal with different heat input; (**d**) tensile strength with different heat input.

The increase of heat input rate could also coarsen the weld metal grain. The weld grain size has a good linear relationship with the heat input rate in P-GMAW [6,23], as shown in Figure 19c. In order to evaluate the beneficial effect of thermal pulse of DP-GMAW on grain refinement of weld metal microstructure with different welding parameters. "ΔSize" is used to characterize the degree of grain refinement, "ΔSize" is the difference between the size of DP-GMAW welding grain and P-GMAW welding grain under the same heat input. The grain size of P-GMAW welding was estimated by the linear fitting equation between the heat input and weld metal grain size of P-GAMW, as shown in Figure 19c. A larger "ΔSize" represents the more significant effect of thermal pulses on grain refinement. The transverse tensile test of the weld joints prepared by P-GMAW and DP-GMAW with different heat input is given in Figure 19d. It is observed that the tensile test of DP-GMA weld joint is higher than those of P-GMA weld joint due to the thermal pulse. As for the hardness of the weld joints, Ping Yao found that the variation characteristic of the hardness was approximately the same as that of grain size of weld joints of the 304 stainless steel prepared by P-GMAW and DP-GMAW [12].

Figure 20a shows "ΔSize" with different thermal pulse frequencies. In contrast, the average grain size of double pulse weld metal microstructure at thermal frequency = 2 was the smallest. Increasing the frequency of thermal pulses can effectively refine the grains of the weld metal with the given heat input rate. Figure 20b shows "ΔSize" with a different duty cycle of thermal pulse. A low duty cycle leads to the short duration of large constitutional supercooling of the weld pool, and the dendrite arms could not be melted off fully. Insufficient dendritic fragments can be the equiaxed nuclei in the weld pool. A large duty cycle leads high heat input which promote coarse-grain. Figure 20c shows "ΔSize" with different thermal pulse current change. Under the premise of maintaining the stability of the arc, the larger the current difference, the more effective the grain refinement of the thermal pulse. However, large current difference could result in poor weld formation. Therefore, increasing the current difference of thermal pulse cannot simultaneously obtain the fine grain and the good weld formation. It is necessary to analyze the actual situation in the application.

**Figure 20.** Variation of ΔSize with (**a**) heat thermal frequency; (**b**) duty cycle of the thermal pulse; (**c**) current amplitude of thermal pulse.
