**2. The GMAW process and its variants**

The industry's anxiety to promote systems with high levels of productivity generated the need for adaptations in consolidated techniques such as welding with the GMAW process (also known as MIG—Metal Inert Gas and MAG—Metal Active Gas), which ended up being an important source from adaptations to consolidating alternatives that may well be better developed by companies and entering the market as proposals to increase melting and metal deposition rates. Therefore, there are many innovations for the advancement of welding in the industry, with a large number of different variations of the more traditional processes, such as the MIG/MAG process, where it can be said that the GMAW with double wire is the technical variant that most manufacturers have developed commercially. The authors [12] cite more variant techniques using this same conventional process (GMAW) as a basis, thus generating hybrid combinations such as MIG-Laser and Plasma-MIG systems. Thus, the use of GMAW welding is also being extensively studied in manufacturing by additive manufacturing, always aiming at the same objectives.

### **2.1 Cold wire (CW-GMAW) and double cold wire (DCW-GMAW)**

The first variation of GMAW welding to be mentioned is the CW-GMAW, where the term CW – cold wire, consists of the addition of a non-energized wire that is inserted directly into the arc, the weld pool, or the transition zone, to increase the

*A Brief Study of Unconventional Variants of GMAW Welding: Parameters, Weld Bead… DOI: http://dx.doi.org/10.5772/intechopen.104525*

#### **Figure 1.**

*Wire feeding scheme of the GMAW and CW-GMW processes.*

rate of molten metal and subsequently the rate of material deposition, using only an electric arc (**Figure 1**). This helps to reduce the energy imposed on the part, in addition to the possibility of reducing the number of passes for filling the chamfer, decreasing dilution, decreasing the Heat Affected Zone (HAZ), and the application of coatings on surfaces, being able to be used in manual, semi, and fully modes automated. Thus, due to the similarity of the GMAW process with the FCAW (Flux Cored Arc Welding), the CW-FCAW (Cold Wire–Flux Cored Arc Welding) variant has already been tested for joining sheets in the naval industry and has been shown to have high potential for use in the assembly and manufacturing industry [13, 14]. Both were tested when welding parts in the flat position.

Preliminary work using CW-GMAW has been developed since the early 2000s. But the variant has been consolidated in the past decade with several applications that will be dealt with below. Primarily used to fill V-chamfers (with different opening angles), but with low cold wire feed rates. It should be noted that the feed rate of the cold wire is based on the feed rate of the electrode wire responsible for the electric arc. That is, the cold wire feed rate ratio is a percentage of the electrode feed rate and is called the electrodeless feed rate ratio (%), defined as follows:

$$R\_{\mathcal{W}} = \frac{\mathcal{W}\_{\text{s}}}{E} \tag{1}$$

where Ws is the cold wire feed rate in m/min, and E is the electrode feed rate in m/ min. This parameter is used to decide the quantity of all the variants mentioned in this chapter, whether the variants with cold wires or with hot wires. Thus, the initial rates corresponded to low values from 10–60%, but currently some works such as [4] have already demonstrated the possibility of using rates of up to 140% and the use of CW-GMAW with pulsed current. Another important factor is the diameter of the cold wire, which also bases your choice on the electrode wire. In addition, it should be noted that it is possible to work with the possibility of mixtures of wires (electrode + cold) causing the formation of weld metals with the most varied chemical compositions and applications for joining and coating dissimilar materials, more precisely for wear-resistant coatings.

Otherwise, the equipment for the application of the CW-GMAW technique also has a relatively low cost in terms of the necessary adaptations to carry out the welding. Bearing in mind that you only need an extra power head and a torch adapted to inject the cold wire at the desired location with coupling to the electrode wire welding torch.

Relevant works have been produced over the last few years, such as [15, 16], which studied several factors such as wire feed rate, pulsed current, energy efficiency, and their influence on electric arc stability, metallic transfer modes, bead geometry, as well as the possibility of applying this type of welding in narrow gap of 4 mm [8, 16]. Another study [17] evaluated the possibility of using the CW-GMAW by varying the electrode polarity in negative (DCEN—direct current electrode negative) and positive (DCEP—direct current electrode positive). In this way, these works help to consolidate the CW-GMAW process as a suitable process as an alternative for the implementation of high productivity with less energy to melt more metal.

This is confirmed by the various applications that have already been carried out, such as: [18] CW-GMAW welding was used to weld high-strength armor steel plates used for vehicle armoring; [19] performed steel welding for oil and oil pipelines (API X80) and [20] tested the process in automotive applications. On the other hand, the decrease in the penetration of the molten metal and the dilution that occur end up favoring the use of CW-GMAW in coatings, which is what was done by [7], who used this type of welding covering austenitic stainless steel plates for cobalt-chromium alloys (Stellite 21) and [21], which coated AISI-SAE 1020 carbon steel sheets with a nickel superalloy ER NiCrMo-4 (Hastelloy). On the other hand, other works compared the welded joint by GMAW and the CW-GMAW variant, the authors [10] joined sheets of naval steel and subjected the welded joint to fatigue cycles, noting the excellent resistance of the material, and the results of the variants are better than the original process, and also [9] performed similar work welding marine steel and observed that the CW-GMAW process helps to reduce residual stress peaks by up to 100 MPa, after measuring the sheets before and after welding, using X-ray diffraction and acoustic birefringence methods. Finally, many of the works mentioned also confirmed that due to the lower imposed heat transferred to the base metal, there is a decrease in the HAZ.

However, the DCW-GMAW variant uses the same idea as the CW-GMAW variant, using an energized electrode wire, but with the insertion of two non-energized wires (colds), hence the term DCW–double cold wire. The original idea started with [22], where he tested percentages of, up to 100%, of addition of cold wire in relation to the electrode wire, obtaining good results in terms of bead geometry (width and reinforcement), low dilution, and absence of discontinuities. The choice of insertion angle of the cold wires of the DCW-GMAW was based on the same proposal of the CW-GMAW; however, there is another option for the injection of these wires; this position was called coplanar, since they are inserted in the same plane as the welding torch [23] (**Figure 2**). In this same work, the authors concluded that there is a 15% loss in the hardness properties of the weld metal compared with the same GMAW metal using coplanar feeding. However, the author [24] verified that the DCW-GMAW process with angular feed and the CW-GMAW have better mechanical properties of hardness than the normal GMAW process.

#### **2.2 Hot wire (HW-GMAW)**

The HW-GMAW variant was designed using the idea of the CW-GMAW process; however, the additional wire that was cold and served only to add mass to the molten pool became a conductor of electric current, but that has no enough energy to strike an electric arc, and this function belongs only to the electrode wire. But now, in

*A Brief Study of Unconventional Variants of GMAW Welding: Parameters, Weld Bead… DOI: http://dx.doi.org/10.5772/intechopen.104525*

#### **Figure 2.**

*DCW-GMAW variant wire feed scheme: (a) angular (b) coplanar.*

addition to the extra power head, an auxiliary power supply is needed. One of the main motivations for hot wire (HW) is the preheating of additional wire, which reduces the amount of energy required to melt it, thus increasing the efficiency of the process.

**Figure 3** schematically shows the operation of the variant. Thus the HW-GMAW was largely designed to increase the melt rate by Joule preheating the extra wire

without significantly increasing the total heat input to the substrate. The low dilution between the weld metal and the base metal makes this process suitable for the application of hardfacing using dissimilar materials [25–28].

The HW-GMAW process presents as advantages higher productivity due to its high melting and deposition rates, versatility in joint construction, together with low operating costs, due to the low system power required for melting the preheated wire when introduced into the weld pool.

According to studies by [11], the HW-GMAW process has shown promise for welding in narrow gap or in the deposit of corrosion-resistant coatings, and in this process, the preheated filler material reduces the main arc energy required for the electrode melting and test piece melting, in turn creating a cooler weld pool. These authors consider that the advance in the development of inverter welding sources, with advanced manipulation of waveforms, allows the increasing use of this variant.

It is worth mentioning that the intrinsic parameters of the welding processes each have their importance, and if changed together or individually, they promote different results, so that the effect of the parameters on the morphology of the weld beads can generate different results according to the methodology used for each work performed.
