## 1 Introduction  
Gas metal arc welding (GMAW) is one of the most commonly used welding processes to join a wide range of materials. It is an important constituent for the fabrication of pressure vessels, power plants, ships structures, aero-engine components and steam pipes. The further development of GMAW processes focuses on the possibilities of higher material deposition with the reduced heat input to the base metal (BM) and to raise the weldability of the materials with less heat affected zone (HAZ) and spatters. In recent years, pulsed alternating current gas metal arc welding (AC-GMAW), an advanced variant of conventional GMAW, has emerged, a relatively new welding process owing to its distinct characteristics [1; 2]. The pulsed AC-GMAW process contains two distinct portions of the pulse. The electrode positive (EP) portion of the pulse provides better arc stability similar to conventional GMAW process, whereas the electrode negative (EN) portion of the pulse helps in reducing the problems such as burn-through, gap-bridging, and distortion because of low heat input to the BM [2]. Being the low heat input process, heat sensitive materials like ultra-high strength steels (UHSS) would get better mechanical properties in the HAZ [3]. However, the successful application of the AC-GMAW process involves the better understanding of its arc behavior and heat distribution in the weldment. In the case of the conventional direct current gas metal arc welding (DC-GMAW) process, significant efforts have been made to study the process behavior in terms of arc stability and weldment, but very few studies have been reported and found in the technical and scientific literature in the case of the pulsed AC-GMAW process. Moreover, most of the studies on pulsed AC-GMAW have been performed on aluminum welding due to its gap bridging capability as discussed by [4; 5; 6]. [PERSON] et al. [7] investigated the effect of the EN ratio on the wire melting rate, arc melting phenomenon and drop size in the pulsed AC-GMAW process in aluminum welding. They found that, with the increase in EN ratio, the drop sizes and wire melting rate increased, whereas the heat supplied to the BM decreased. [PERSON] et al. [8] investigated the microstructure and mechanical properties of butt and overlap welding joints of 1 mm thin 6082 aluminum sheets. They reported that the depth of penetration can be controlled by adjusting the EN ratio. Moreover, shallow penetration was observed with an increase in EN ratio, which resulted in excellent welding of thin sheets. For the application of the AC-GMAW process on steel plate welding, initial efforts were put by [PERSON] et al. [9], who studied the effect of different waveforms and EN ratios on the arc and metal transfer instabilities. They carried out bead on plate (BOP) welding on mild steel using high speed imaging. They observed sputter generation and arc instabilities at high EN ratios. [PERSON] et al. [10] performed experiments on steel with high EN ratios. They reported that with the increase in EN ratio, the penetration depth decreased while the reinforcement height increased. Later, [PERSON] and [PERSON] [11] reported that using long EP durations in steel welding, heat was concentrated on the base material, which may have resulted in reasonable penetration depths. However, they found a decrease in penetration depth with an increase in EN duration. [PERSON] and [PERSON] [12] investigated the metal transfer behavior in the AC-GMAW process in order to predict the drop size. An analytical simulation based on force displacement model was used to predict the drop size in AC-GMAW and the results were validated with experiments. A reasonable agreement was found. [PERSON] and [PERSON] [13] analyzed the wire melting phenomenon in the pulsed AC-GMAW process using BOP welding on steel plates. A new form of the wire melting rate equation for the AC-GMAW process is proposed based on energy conservation theory and arc physics. [PERSON] et al.