4.5.1. The Influence Analysis of Appearance on the Weld Bead

The first analysis target was the appearance, and the analysis was based on evaluation results in Table 9 and mapping between grades and mathematical values shown in Table 8. The analyses parameters selected the four key operational parameters, which were robot welding speed *VR*, twin pulse frequency TPF, twin pulse relation *DT* and twin pulse current change in percent *I*Δ. Each parameter had three different levels. For each parameter in each level, the corresponding *Scoreavg* can be calculated following Equation (8):

$$Score\_{avg}(i,j) = \frac{\sum\_{k=1}^{3} Score(i,j,k)}{3} \tag{8}$$

where *i* was the sequence of the operational parameter, *i* = 1, 2, 3 and 4, and *j* was the sequence of the level, *j* = 1, 2 and 3. For each parameter with one level, there were 3 scores in the experiment, these different scores were marked as *k*. The values of *i*, *j* and *k* were different under different situations. The value of *Score* can be obtained combing Tables 8 and 9. Then, the influence analysis of four operational parameters on appearance is shown in Table 10.


**Table 10.** Influence analysis of four operational parameters on appearance.

Where the range value *D* can be calculated based on Equation (9):

$$D(i) = \max\_{j=1}^{3} \left( score\_{\text{avg}}(i, j) \right) - \min\_{j=1}^{3} \left( score\_{\text{avg}}(i, j) \right) \tag{9}$$

where the meanings of *i* and *j* were the same as those in Equation (8). It can be seen that the robot welding speed is the most influential factor on the appearance; the corresponding range value was 21.667, and the following influential factors were twin pulse current change in percent, twin pulse frequency and twin pulse relation. Hence, to obtain satisfactory appearance of weld bead, a proper combination of the robot welding speed and twin pulse current change in percent was very important. It was because the robot welding speed and twin pulse current change in percent can determine the amount of energy delivery into the base plate in one-unit length, and this energy delivery was a key factor which can determine the final appearance.

According to above analyses results, to obtain the weld bead with optimal appearance, the optimal level for *VR* should be Level2, the optimal level for TPF should be Level2, the optimal level for *DT* should be Level1, and the optimal level for *I*<sup>Δ</sup> should be Level1. Those corresponding values were respectively 30 cm/min, 2 Hz, 30% and 30%. Using these optimal parameters, an experiment was conducted and the result can be seen in Figure 11. It can be seen that the weld bead was straight and regular, and the fish scale ripples were so compact. The bead width and bead height were also proper, and the forming was satisfactory, which was the same as that in former analyses.

**Figure 11.** The appearance of the weld bead using optimal parameters combination.

4.5.2. The Influence Analysis of Grain Size on the Weld Bead

Grain size is another important element affecting the quality of weld bead, corresponding analyses are shown in Table 11, based on the same calculation methods as that in the preceding part. It can be seen that the robot welding speed was still the most influential factor to determine the grain size. Higher robot welding speed denoted smaller grain size. When the robot welding speed was 40 cm/min, which corresponded to Level3 in the robot welding speed, the smallest grain size was obtained. It may be due to two reasons. The first was that when high robot welding speed was employed, the weld bead could be fast cooling, which induces the number of grains increasing and the size being reduced. The second reason was that the weld pool was stirred because a series of regular pulses with high and low frequencies were alternatively used during the DP-GMAW operational process, and then a high robot welding speed denoted more frequent stirring in the weld pool, which could deliver more extern energy into the liquid metal and energy fluctuations were more severe, which can also make the grain size smaller. Hence, to obtain a satisfactory microstructure of weld bead, the robot welding speed should be properly increased.


**Table 11.** Orthogonal analyses of the grain size of the weld bead.

According to above analyses results, to obtain the weld bead with the most proper grain size, the optimal level for *VR* should be Level3, the optimal level for TPF should be Level2 or Level3, the optimal level for *DT* should be Level1, and the optimal level for *I*<sup>Δ</sup> should be Level1. Those corresponding values were, respectively, 40 cm/min, 2 Hz or 1 Hz, 30% and 30%.
