**2. Operational Characteristics of the DP-GMAW Process**

The DP-GMAW process involves a lot of typical operational parameters. The current pulses in TPP and TBP have different frequencies. The thermal period (TP) denotes the duration of two phases. Another important operational parameter, twin pulse frequency (TPF), which is a reciprocal of the TP, can describe the speed of current waveform adjustment between the two double pulses. Also, the TPF can determine the number of fish scale ripples. It can be noticed that this process is a low frequency modulation based on high frequency phases (TPP and TBP), in other words, the low frequency current pulsation or the thermal pulse is superimposed on a pulsed current for active metal transfer control and weld pool stirring [26]. During the process, TPF can reflect the varying speed of the strong set and weak pulses set, and each set may include up to 10–20 high frequency pulses, the maximum frequencies of the pulses may achieve is 100 Hz. Hence, in a general case, the value of TPF is below 5 Hz. The welding currents were switched between two high frequencies with TPF of the switch frequency. The frequency in TPP is higher than that in TBP, and the pulse set in TPP is called a strong pulse set, which is signed as *PulseS*, and the pulse currents are switched between base current *Ibs* and peak current *Ips* while the pulse set in TBP is called as weak pulse set, which is signed as *PulseW*, the corresponding base current and peak current are respectively *Ibw* and *Ipw*. The durations of TPP and TBP were, respectively, *Ts* and *Tw*. In addition, the proportion of the time of thermal pulse

phase, which is *Ts*, in one thermal period TP, is called twin pulse relation, which can be mathematically described in Equation (1):

$$D\_T = T\_s / TP \tag{1}$$

where this parameter is denoted as *DT* in this work.

The strong pulse set is to control the droplet transfer for obtaining enough welding penetration, in a general case, one pulse corresponds to one droplet, while the function of the weak pulse set is obtaining a series of regular pulses to stir the weld pool [27], and one TBP corresponds to one weld pool. In addition, the average current in TPP is *Iavs*, while the average current in TBP is *Iavw*, and the average current in one thermal period is marked as *Iav*. In general, the *Iav* is a preliminary arc welding setting value during the process. Under this circumstance, the regular fish scale grain can form if all the parameters are properly set. Figure 1 shows the schematic and main operational parameters of the DP-GMAW process.

**Figure 1.** Schematic presentation of the double-pulsed gas metal arc welding (DP-GMAW) process.

In this figure, TP is the sum of *Tw* and *Ts*. Apart from the relations shown in the Figure 1, twin pulse current change, whose value is half of the subtraction between mean value of currents in TPP and TBP, is also an important parameter during the process because it can reflect the current variation in between two phases. In this work, to clearly reflect the current adjustment, this parameter can be described combining the *Iav* and in percent format as in the following equation:

$$I\_{\Delta} = \frac{I\_{\text{avs}} - I\_{\text{avw}}}{2I\_{\text{av}}} \times 100\% \tag{2}$$

where *I*Δ is called twin pulse current change in percent. Moreover, the relations between *I*Δ, and *Iavs* as well as *Iavw*, can be also be derived as follows:

$$\begin{cases} I\_{\text{avs}} = I\_{\text{av}} \times \left(1 + I\_{\Delta}/100\right) \\\ I\_{\text{avw}} = I\_{\text{av}} \times \left(1 - I\_{\Delta}/100\right) \end{cases} \tag{3}$$

During the welding process, proper combination of the operational parameters of the pulse sets in TPP and TBP, and other process parameters, are the utmost important for improving the welding quality and obtaining weld bead with satisfactory fish scale ripples. During the traditional DP-GMAW operation process, various relative operational parameters cannot be accurately adjusted and matched one by one. In this work, the latest *twinpulse* XT DP control technology developed by the LORCH Company was employed. This process has two significant features:


According to the above introduction about the DP-GMAW process, it can be noticed that there are various operational parameters included in the DP-GMAW process, and to obtain the weld bead with satisfactory quality, all the operational parameters should be carefully and seriously considered. Employing *twinpulse* XT DP control technology can decrease the setting complexity and improve the control performance, because this new control technology can use a few key operational parameters to control two waveforms, some important process parameters, such as peak current, base current, frequency, duty cycle, peak current, base current, peak time, base time, did not individually set. Therefore, realizing the proper control of energy delivery into the base plate was so convenient. Then in the next section, corresponding experiments can be conducted to produce weld beads with different qualities, and then the influential levels of different operational parameters on the selected performance of weld bead can be seriously explored after combining selected quality evaluation method.
