# Stability and Heat Input Controllability of Two Different Modulations for Double-Pulse MIG Welding
## 4 Process Control Experiment on Reduction of Heat Input in TP-MIG Welding and DP-MIG Welding
### Effects of Welding Speed on Input Energy  
The welding arc generates heat, and a heat-affected zone is created close to the joint. An increase in the welding speed leads to a proportionate decrease in heat transfer and a significant decrease in input energy if the effective power is unchanged. This concentrates the energy of the arc and reduces the total energy fed into the weld pool, which further reduces heat input to the sides of the welding track. As a result, the heat-affected zone in the base metal becomes narrow. Hence, the welding speed negatively correlates with the input heat. It should be noted that the heat-affected zone significantly affects the mechanical properties of the joint, and a large heat-affected zone could lead to joint failure or fractures. The heat-affected zones were analyzed by carrying out hardness tests on the welded joint. The results of these tests are shown in Figure 19.  
Figure 18: TP-MIGW at different frequencies; (**a**) Average current of 90 A, frequency of 3 Hz; (**b**) Average current of 80 A, frequency of 2.4 Hz. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \(I_{ps}\), \(I_{psav}\) & \(t_{psav}\) & \(t_{psav}\) & \(I_{bs}\) & \(t_{bs}\) & \(n\) & \(I_{bsav}\) & \(t_{bsav}\) & \(m\) & \(n_{raw}\) & \(n_{raw}\) & \(n_{raw}\) & **Average** & **Input** \\ \(I_{psav}\) & \(I_{psav}\) & \(t_{psav}\) & (**A**) & (**ms**) & & (**A**) & (**ms**) & & & **(A)** & **(V)** & **(D)** \\ \hline
210 & 2 & 77 & 6 & 15 & 50 & 20 & 7 & 4 & 4 & 90 & 22 & 138,600 \\
210 & 2 & 77 & 6 & 15 & 50 & 30 & 7 & 4 & 4 & 80 & 22 & 123,200 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of TP-MIGW test data of low-energy pulse group at different frequencies. DP-MIG welding and TP-MIG welding were performed at a constant current of 88 A, and the welding speed was gradually increased from 0.6 m/min to 1.0 m/min. The resulting joints are shown in Figures 20 and 21. The Vickers hardnesses of the welded joints formed at 0.6 m/min and 1.0 m/min were determined with a load of 0.98 N and loading time of 15 s. Hardness tests were first conducted on the base metal to perform detailed analysis of changes in the welded joint hardness; it was found that the hardness of the base metal was 38 HV. The hardness of each zone of the weld is shown in Figure 22.  
The hardness of the weld at its center is closely related to that of the welding wire. Figure 23 indicates that welded joints formed at 1.0 m/min are generally stronger than joints formed at 0.6 m/min. In DP-MIG welding, an increase in the welding speed increases the hardness by 19% and decreases the area that is softer than the base metal (i.e., softened zone) by 21%. In TP-MIG welding, an increase in the welding speed increases the hardness by an average of 17% and decreases the softened zone by 27%. The change in hardness with welding speed is shown in Figure 23. The figure shows that an appropriate increase in welding speed helps control energy input in thin-plate welding and is beneficial for reducing the spread of the heat-affected zone. By comparing the results of DP-MIG welding and TP-MIG welding, it was found that the latter approach resulted in harder joints. In addition, it was shown that the input energy can be reduced in both approaches. However, the heat-affected zone in TP-MIG welding was slightly smaller, and the decrease in input energy due to the increase in the welding speed was more pronounced in TP-MIG welding. Figure 19: Sample extraction of welded joint. Figure 21: TP-MIGW welding joint at 1.0 m/min. Figure 22: Hardness distribution of welded joints at two speeds: (**a**) Comparison of DP-MIGW hardness of welded joints; (**b**) Comparison of TP-MIGW hardness of welded joints. Figure 24 shows the microstructure of welded joints welded by DP-MIG and TP-MIG at welding speeds of 0.6 m/min and 1.0 m/min. It is observed that the grain size of the fusion zone and the heat-affected zone at the welding speed of 1.0 m/min is less coarsening than that at the welding speed of 0.6 m/min.