# Stability and Heat Input Controllability of Two Different Modulations for Double-Pulse MIG Welding
## 2 Design of Current Waveform Parameters for Double-Pulse MIG Welding with Two Different Modulations
Owing to welding behaviors and environmental effects, the arc length during double-pulse MIG welding is always in a varying state. Thus, double-pulse MIG welding current has a rectangular waveform. When the pulse train undergoes a transition from high energy to low energy, the arc length and wire extension continue to increase until they reach their maximum values, whereas the current parameter decreases. This results in considerable change in energy. The reduced current eventually becomes insufficient to sustain the arc, which causes the arc to breakdown. The effects of this type of damage on the base metal are particularly significant in low-temperature environments. In this work, trapezoidal waveforms were investigated to address this issue. Our new approach is referred to as TP-MIG welding; the current waveform of this scheme is shown in Figure 4.  
In TP-MIG welding, the transition of a high-energy pulse train to a low-energy pulse train in DP-MIG welding is supplemented by a transitional high- to low-energy pulse train, whereas the opposite transition (low-energy pulse train to high-energy pulse train) is supplemented by a transitional low- to high-energy pulse train. The rectangular waveform exhibits trapezoidal characteristics owing to the transitional pulse trains. When the arc extends to its maximum length during the high-energy to low-energy transition of the pulse train, the current gradually decreases because of the pulse train, which is conducive for controlling arc breakdown. This subsequently improves the efficacy of aluminum alloy welding and ensures the construction of an energy gradient that improves the controllability and stability of the welding process. Figure 4: TP-MIG low-frequency modulation waveform. Figure 3: DP-MIG current waveform diagram. The analysis of low-frequency modulation signals includes two aspects: peak values and base values. If the peak values are stable, low-frequency modulation may be performed using \(I_{p}\). The change in base current along the four aforementioned modes of transfer is shown in Figure 4. \(T_{d}\) and \(T_{u}\) are the low- to high-energy and high- to low-energy pulse transition times, respectively, and \(T_{d}\) = \(T_{u}\). In Figure 4, \(T_{l}\) is the time required to complete the processes of low-frequency modulation, and the corresponding frequency is \(f_{l}=\frac{1}{T_{l}}\). Hence, the time, \(t\), at which the interval [0, \(T_{l}\)] lies in the low-frequency period is as follows:  
\[\widetilde{t}_{1}=t-INT\bigg{(}\frac{t}{T}\bigg{)}\times T_{1}. \tag{1}\]  
Therefore, the base current can be expressed as follows for all intervals of the low-frequency signal:  
\[I_{bl}=\left\{ \begin{array}{ll}I_{bs}&0\leq\widetilde{t}_{l}\prec T_{s}\\ I_{bs}-\frac{I_{bs}-I_{bw}}{T_{d}}(\widetilde{t}_{l}-T_{s})&T_{s}\leq \widetilde{t}_{l}\prec T_{s}+T_{d}\\ I_{bw}&T_{s}+T_{d}\leq\widetilde{t}_{l}\prec T_{s}+T_{d}+T_{w}\\ I_{bw}+\frac{I_{bs}-I_{bw}}{T_{d}}(\widetilde{t}_{l}-T_{s}-T_{d}-T_{w})&T_{s}+T _{d}+T_{w}\leq\widetilde{t}_{l}\prec T\end{array} \right. \tag{2}\]  
The analysis of high-frequency modulation signals involves four different stages. The first stage is the high-energy pulse train. In this stage, the energy is highly concentrated, and the melting depth of the weld pool is the most important regulatory factor. The next stage is the transitional high- to low-energy pulse, followed by the low-energy pulse train. The behaviors in this stage include preheating of the material and stabilization of the weld pool. The final stage is the transitional low- to high-energy pulse. During this stage, the peak currents of the high-energy and low-energy pulse trains are always the same, but there are marked differences in their base currents (Figure 5). Suppose that high-frequency modulation occurs over a period of \(T_{h}\), and the period of the high-energy pulse train is \(T_{hs}\), corresponding to a frequency of \(f_{hs}\) = 1/\(T_{hs}\).