# Effect of Heat Input on LMHMW Joint of Carbon Steel
## 3 Experimental Results and Discussions  
### Joint Macroscopic Morphology and Interfacial Microstructure  
The cross-section of the LMHMW welded joint with a typical goblet fusion zone morphology is shown in Figure 3. There were no hot or cold cracks observed anywhere in the fusion or HAZ, and interpass fusion between the adjacent layers of the weld bead was fairly good. However, there exists some process and metallurgical porosities randomly distributed in different regions of the fusion zone. By observing the macro-morphology of the welded joints, it is found that the weld width increases with the increase of heat input in samples 4 and 5. In addition, obvious root concavity and a lack of penetration were observed in sample 1, which is attributed to relatively higher welding speed. As it is evident from Figure 3c, sample 3 had maximum weld joint soundness without any indication of undercut, excessive root or face reinforcement. Hence, the welding parameters of sample 3 are the most optimized in terms of macromorphology. The microstructure of the weld seam (WS) under different magnifications is shown in Figure 4. The microstructure of the weld is mainly eutectoid ferrite precipitated along the grain boundary of the original austenite and a small amount of ferrite and pearlite with needle and plate-like morphology between the grains. Microstructure morphology in the vicinity of the fusion line is mainly coarse columnar dendrites of pro  
Figure 3: Weld appearance and macro morphology of joint cross-section: (**a**) specimen No.1; (**b**) specimen No.2; (**c**) specimen No.3; (**d**) specimen No.4; (**e**) specimen No.5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Weld Layers** & **Laser Power/W** & **Welding Current/A** & **Welding Speed/cm\(\cdot\)s\({}^{-1}\)** & **Heat Input/kj\(\cdot\)cm\({}^{-1}\)** \\ \hline \multirow{4}{*}{1} & Backing weld & 5000 & 216 & 1.667 & 6.66 \\  & Filling weld & 2000 & 207 & 0.583 & 13.48 \\  & Cosmetic weld & 2000 & 225 & 0.583 & 14.57 \\  & Backing weld & 5500 & 216 & 1.667 & 7.02 \\
2 & Filling weld & 2000 & 225 & 0.583 & 13.93 \\  & Cosmetic weld & 2000 & 234 & 0.583 & 15.09 \\  & Backing weld & 6000 & 216 & 1.333 & 9.16 \\
3 & Filling weld & 2000 & 207 & 0.583 & 14.18 \\  & Cosmetic weld & 2000 & 245 & 0.583 & 15.97 \\  & Backing weld & 5000 & 250 & 1.167 & 10.76 \\
4 & Filling weld & 3000 & 270 & 0.583 & 19.44 \\  & Cosmetic weld & 2500 & 300 & 0.583 & 20.82 \\  & Backing weld & 4000 & 250 & 1.667 & 6.89 \\
5 & Filling weld & 4500 & 230 & 0.583 & 19.17 \\  & Cosmetic weld & 3000 & 270 & 0.583 & 19.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Detailed parameters for each specimen. grow normal to the fusion line and are aligned in the direction of the maximum temperature gradient. At the weld centre line, a relatively small proportion of equiaxed grains form, which hinders the further growth of columnar crystals. This pattern of microstructural morphology is attributed to constitutional supercooling owing to a difference in the G/R ratio at various positions along the fusion boundary. The weld centre line experiences the least temperature gradient and maximum growth rate, so it is possible that nucleation starts ahead of the columnar dendrites because of a very high amount of constitutional supercooling. However, the orientation of columnar crystals in the vicinity of the base metal (BM) region is not entirely consistent with the orientation of the base metal microstructure, which is caused by the coupling of laser and arc. Figure 5 exhibit the microstructure of HAZ of welded joints. Depending upon the proximity to the fusion line, HAZ has been categorized into a superheated zone, normalized zone and incomplete recrystallization zone. As can be seen from Figure 5a, very narrow HAZ was observed because of the synergic effects of the laser and arc.