A Brief Study of Unconventional Variants of GMAW Welding: Parameters, Weld Bead, and Microstructures

Abstract

The GMAW (Gas Metal Arc Welding) process is an electric arc welding technique widely used around the world due to its ease of use, low equipment cost and, mainly, due to the high deposition rate, the quality of the metal of solder, which makes it versatile and susceptible to modification. Thus, variants such as CW-GMAW (Cold Wire–Gas Metal Arc Welding), DCW-GMAW (Double Cold Wire–Gas Metal Arc Welding), and HW-GMAW (Hot Wire–Gas Metal Arc Welding) emerged from the conception of small adaptations to the original process that ended up generating better and more adjusted results than GMAW. Thus, variations of some parameters will be shown and their respective effects on the weld bead geometry, dilution, penetration, deposition rate, in addition to the effects on macro and microstructure. This provides the possibility of using the variants in different types of applications in the industry in general. Where the application in narrow 4 mm chamfer has already been observed, reduction of residual stresses, increase in fatigue resistance and coatings with special alloys.

1. Introduction

In a general context, the welding area currently develops on the conceptual and technological foundations of Industry 4.0, as well as the latter actively collaborates to advance the former. Welding joining processes have never presented so many changes in their techniques as in recent years. This is mainly due to the insertion of arc welding in additive manufacturing [1, 2], as robotic systems have sought to improve the manufacture of parts and components by deposition or coating of flat and tubular surfaces for any type of materials. Whether carbon steel [3] or special alloys [4, 5]. However, the difficulty of implementation and the cost of additive manufacturing favor more traditional processes such as the electric arc to remain for longer acting as the front line of the metalworking industry in the welding segment.

Thus, processes such as GMAW (Gas Metal Arc Welding) and FCAW-G (Flux Cored Welding with Gas Protection), among others, still remain firm, being used in the manufacturing and heavy assembly industries such as shipbuilding, oil industry, in addition to construction of structures and pipelines for the power generation industry, for example. Thus, it is known that the mentioned processes have high deposition rates, weld metal quality, in addition to versatility in their applications, which consolidates them in the market. However, due to the excessive need to increase productivity, many variants have emerged in order to assist in this procedure.

Currently, there are some aspects of the GMAW process, with the Cold Wire–Gas Metal Arc Welding (CW-GMAW), the Double Cold Wire–Gas Metal Arc Welding (DCW-GMAW), and the Hot Wire–Gas Metal Arc Welding (HW-GMAW). CW-GMAW and DCW-GMAW welding contributed significantly to increased productivity [6, 7]. CW-GMAW welding allowed for narrow bevel welding with 4 mm gap [8]. Furthermore, it is responsible for reducing the level of residual welding stresses [9] and increasing fatigue strength [10]. On the other hand, the HW variant presents as its main characteristic the increase in productivity, with the possibility of variation in penetration [11]. These variants have the possibility to achieve deposition rates ranging from small percentages from 10% to more than 100% of extra molten metal, also influencing the formation of microstructures such as acicular ferrite, which contribute to increase the properties, mainly mechanical, of the weld metal. In this context, some variants of the GMAW process will be presented below, some of which have already been tested, and others are still being tested for possible field applications, thus showing the importance of these welds for the academy and the manufacturing industry, assembly, and maintenance.

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 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.

Figure 1.

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

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:

E1

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.

Figure 2.

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

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 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, 26, 27, 28].

Figure 3.

HW-GMAW variant wire feed scheme.

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.

3. The geometry of weld bead and the metallurgical changes in the weld metal of the GMAW process variants

3.1 CW-GMAW

First, before dealing directly with metallurgy, there is a need to evaluate the geometry of the weld beads, considering that the first step to estimate whether a given process worked or not is to evaluate the geometric characteristics of the weld, and furthermore, to observe whether there is some apparent discontinuity. Thus, it will be possible to understand whether, in fact, the variant has validity and prospects for future development.

Several studies prove that the weld bead geometry is dependent on several factors such as current, voltage, polarity, welding speed, shielding gas, metal transfer mode, torch position, welding position, etc., that act directly on the weld pool and, consequently, provide a specific profile and dimensions of reinforcement, width, penetration, and dilution. This applies to the most varied types of arc welding, including GMAW, and it is still possible to predict this geometry according to the parameters used. Thus, the GMAW variants mentioned in this chapter also present their reinforcement convexity patterns and their respective characteristic dimensions of the cross sections, since the search for optimized welding parameters is always prioritized to propose a process stability standard used. Therefore, the authors [15] state that cold wires or hot wires must have smaller diameters than the electrode wire, since much larger cold wires can cause lack of fusion in the weld pool. Also, there is the possibility of simulating the bead profile using mathematical modeling of the dimensions in mm of penetration, dilution, height, and width of the bead, proposed by the same authors.

As an application of the CW-GMAW variant in welds for marine steel (ASTM A 131—different grades), the parameters of Table 1 are generally used, all using wires of the AWS ER70S-6 class. The weld beads produced, most of the time, have a good surface finish and absence of discontinuities. Thus, the consolidation of the CW-GMAW variant provides a geometric pattern of the beads (Figure 4) when welded in the flat position in situations of simple deposition and V-bevels. GMAW process weld and the CW-GMAW variant for three electrode wire feed values and the percentages of cold wire added. It can be seen that the gradual increase in the melting rate and the deposition rate with the insertion of the cold wire provides an increase in convexity and a decrease in dilution, since the energy supplied to the part was converted to the melting of the wire additionally.

Parameter	Range
Current	280–380 A
Voltage	35–38 V
Shielded gas	Ar + 25% CO2
Gas flow rate	1 a 1.5 m^3/h
Welding speed	50–70 mm/s
Wire feed speed	10–20 m/min
Wire diameter	0.8–1.2 mm
Contact-tip-to-workpiece distance (CTWD)	17 mm–22 mm

Table 1.

Standard welding parameter range for CW-GMAW.

Figure 4.

Standard profile of welded beads with the GMAW and CW-GMAW processes with percentages of 20%, 40%, 60%, 80%, and 100% of cold wire. Based on the work of [22].

In this sense, the works of [9, 10, 22, 24] showed significant results regarding the CW-GMAW variants, the bead profiles are shown in Figure 4. All these authors welded steel sheets naval ASTM A 131 of different grades and dimensions of 9.5 mm X 150 mm X 300 V chamfer, 45° chamfer angle (bisel 22.5°), and the other parameters identical to Table 1, modifying the percentages used. Thus, using percentages of 20% and 40% with CW-GMAW, the authors [14] measured residual stress levels through X-ray diffraction and acoustic birefringence methods and using comparative analysis concluded that the variant helps to decrease the level of these stresses with the percentage increase of cold wire incorporated into the weld metal. Figure 5 shows a specimen welded with CW-GMAW 40%, where it is observed that the sheet was clamped during welding to avoid distortion and measure residual stresses with restrictions. The geometric dimensions of the beads (in mm) were measured by determining the values of the width (w), penetration (p), height of the reinforcement (h), and the angle of wettability (α) of the weld metal in Figure 6. When comparing the measures presented, the conclusions can be reached: increase of the width of the bead and height of the reinforcement and the reduction of the penetration and of the HAZ.

Figure 5.

Specimen welded with CW-GMAW 40% with clampers for measure residual stresses.

Figure 6.

The geometric dimensions of the beads of specimens welded with GMAW and CW-GMAW with clampers, 20% and 40% of cold wire.

Likewise, the results of [10] previously compared the fatigue strength of joints welded with GMAW and CW-GMAW, both in semiautomatic mode, showing the versatility in welding ASTM A131 grade A naval steel sheets when using wires with diameters varying from: 1.2 mm as electrode wire and cold wires of 0.8 mm and 1.0 mm, with cold wire feed rates of 50%. The dimensions of the sample body were thesame (9.5 mm X 150 mm X 300 mm), requiring two passes for the total filling of the chamfer, the first pass was the one from scratch applied with GMAW to all parts, the second finishing pass, this being the comparative parameter between the GMAW process and the CW-GMAW. It was concluded that the fatigue behavior of the joints welded by the GMAW-CW process in both conditions is practically the same when compared with the conventional GMAW process. In addition, some metallurgical considerations were observed, such as the decrease of HAZ in the coarse-grained region and the formation of the fraction of primary ferrite and Widmansttäten, which influences the increase in hardness in the region, this occurs proportional to the diameter of the cold wire added, as the more cold wire, the greater the change. This demonstrates that the addition of cold wire may be affecting the cooling rate due to the lower energy imposed on the weld pool.

However, works [22, 23] comparatively studied the GMAW process with the two variants, CW-GMAW and DCW-GMAW, with 03 wire feed speeds of 10 m/min, 12 m/min, and 14 m/min varying the percentages of 20%, 40%, 60%, 80%, and 100%. Thus, [22] studied the stability of both processes capturing the oscillograms, melting rates, deposition rates, and the geometry of the weld beads (Figure 4), where the following results were highlighted comparing the GMAW and the CW-GMAW:

1. Firstly, as a parameter and some possible analyses, the w/h ratio can be used, which helps to establish the ideal relationship for obtaining very convex beads that help to concentrate stresses and can be discarded, so it is estimated that values must be below 0.30 in order not to exceed this principle, studied by [29]. Thus, welds with percentages of cold wire ranging from 20–60% achieve ideal relationships with values, on average, of 0.25. Showing that in these cold wires, percentage ranges and all levels of electrode wire feed speeds, weld beads are the most suitable.

2. The linear penetration, on average, decreases by 31%, considering the 03 levels of feed speed and all percentages of cold wire, reaching 58%, with the level of 14 m/min and 100% of cold wire.

3. The dilution decreases, on average, by 32% for all percentages of cold wire, with the biggest drop being 48%, also with the level of 14 m/min and 100% of cold wire.

4. The highest deposition rate is the 14 m/min level and 100% cold wire with a value of 11.48 kg/h, corresponding to 61% higher than the GMAW deposition rate for the same level of wire feed speed.

5. While studying various metallurgical parameters exclusively for the wire feed speed of 12 m/min, [24] observed that there is an increase of up to 29% in the percentage of the average silicon content in the weld metal. On the other hand, the grain size decreases with the increase of the percentage of cold wire by up to 32%, which favors the mechanical properties by the Hall–Petch principle. This grain refinement can be seen in Figure 7, where images of weld metals etched with Nital solution (2%) from the GMAW, CW-GMAW, and DCW-GMAW processes are analyzed under an optical microscope.

Figure 7.

Weld metal grain size: (a) GMAW, (b) CW-GMAW, and (c)DCW-GMAW. Nital solution (2%), optical microscope [17].

Continuing, using the IIW (International Institute of Welding) C-Mn metal microstructure classification scheme [30], together with the measurement of the volumetric fraction of phases using images obtained by optical microscopy, both works mentioned above studied the influence of cold wire on the formation of the main microconstituents of the weld, mainly acicular ferrite (AF), since this microstructure has a desirable presence in the weld metal, due to its excellent mechanical properties. However, in addition to this, the presence of allotriomorphic ferrite and Widmansttäten ferrite is also part of the predominant phases in carbon steel welds as deposited [31]. Thus, [22, 24] found that in all parts the weld metal is formed by ferrite, in several different forms: primary ferrite (PF), grain boundary ferrite—PF(G), acicular ferrite (AF), intragranular polygonal ferrite—PF(I), non-aligned second-phase ferrite—FS(NA), and aligned second-phase ferrite—FS(A). PF, PF(G), and FS(NA) ferrites predominate in the composition of the microstructures present with almost 100% of the composition for the highest wire feed speeds.

However, for the three wire feed speeds, with up to 60% cold wire, there is an average acicular ferrite increase of around 24% compared with GMAW. For percentages of cold wire of 80% and 100%, an inverse behavior is observed with a decrease in the amount of AF, on average, of 36%. What can possibly be observed is that the microstructures are benefited or inhibited by the presence of certain chemical elements. As, for example, the presence of low and medium percentages of aluminum (Al) forms a TiO layer around the inclusions, where the circular ferrite is nucleated, thus favoring its growth. However, for high Al content, such formation does not occur [32]. Overall, the gradual increase in the insertion of cold wire improved the mechanical properties of hardness.

3.2 DCW-GMAW

The DCW-GMAW came from the idea of the other variant CW-GMAW to evaluate the ability of how much the GMAW process was able to increase the insertion of “cold” mass using only one electrode wire. One of the great challenges of this variant is the placement of the cold wires. Therefore, the profiles of the evaluated beads are based considering the entry of the cold wires in the angular position in relation to the welding torch, as shown in Figure 2a. Still not having significant results, however, [23] concluded that high percentages of cold wire, from 60%, cause a reduction of approximately 15% in the hardness properties of the weld metal. However, before that, it is necessary to deal with the geometry of the weld beads.

Based only on practical works, the DCW-GMAW variant was first tested and patented by [22] and soon after, also analyzed by [24], where in general it was observed that this process was capable of being applied in the industry in fact using the data obtained for this conclusion. Working with carbon steel, parameters similar to Table 1, percentages from 20–100%, with a variation of 20% and with wire feed speeds of 10 m/min, 12 m/min, and 14 m/min. Figure 9 presents the standard profiles found for the weld beads based on [22], the summary below describes some significant results such as:

1. The w/h ratio, in general, was well below the failure limit, on average, of 0.23. Where for the lowest wire feed speeds and low cold wire contents (20%), we have the lowest values with 0.19. With the gradual increase in the rate of cold wire, the w/h ratio increases, with values of 0.28 for a rate of 100% of cold wire. But the value is still below the allowed 0.30.

2. Linear penetration decays similarly for both electrode wire feed speeds. For small cold wire rates, there is a drop from 10%. However, for rates of 80% and 100% of cold wire, this drop reaches values of 50%. That is, generating a weld of very low penetration. This is visible in Figure 9.

3. In the case of dilution, normal values of GMAW are around 50%; however, previously following the results of linear penetration of DCW-GMAW, there is a decrease in dilution up to 20.44% for cold wire rate of 100% at the lowest wire feed speed (10 m/min). This dilution represents an approximate 60% drop compared with normal values. As a rule, the dilution attenuates as the percentage of cold wire increases.

4. The deposition rate depends directly on the wire feed speed, so each of these speeds has a proportional characteristic value, so the lowest percentages start with a speed of 10 m/min with 20% cold wire with a slight increase of 17% until reaching 79% for addition of 100% cold wire. Overall, the value of 102% more stands out for the speed of 14 m/min with a rate of 14.41 kg/h compared with the value of 7.13 kg/h for the normal GMAW.

5. Considering only the wire feed speed of 12 m/min, in the work of [24], it was found that the silicon content for samples from 20–80% of cold wire has almost the same values, an average of 1.3%. Only, for the 100% cold wire sample, this increase is 0.1% over the average content. As for the grain size, the samples of 20% and 40% have, respectively, 74.78 μm and 73.06 μm, values ​​close to the comparative GMAW (76.40 μm). However, considering the DCW-GMAW-100%, there is a significant decrease in grain size by 34%, an image of the grains can be seen in Figure 7c. This effect could probably have been caused by the increase of chemical elements refining the austenitic grains of the steel or by the disturbance of the weld pool by the volume of added metal.

In the case of the predominant microstructures, the phases in the forms of PF, PF(G) and FS(NA) ferrites constitute 98% of the composition of the microstructures present in the weld metal. What changes are the amount of each phase in each image analyzed? For low amounts of cold wire (20% and 40%) the percentages of AF and FS(NA) have an average increase of 47% and 28%, respectively. However, for percentages of cold wire from DCW-GMAW-60%, there is a decrease in the amount of these phases, while the FS(A) increases, even tripling its composition in the case of low wire feed with 100% of cold wire. An image of the phases present in a sample of DCW-GMAW-60% for the feed speed of 12 m/min can be seen in Figure 8c. The work of [24] still shows that the silicon levels are drastically high, in the CW-GMAW and DCW-GMAW variants, thus also increasing the weld metal hardness levels. From average values of 155 HV to peaks of up to 190 HV.

Figure 8.

Microstructures present in the weld metal: (a) GMAW, (b) CW-GMAW, and (c) DCW-GMAW [17].

Figure 9.

Standard profile of welded beads with the GMAW and DCW-GMAW processes with percentages of 20%, 40%, 60%, 80%, and 100% of cold wire. Based on the work of [22].

Figure 10.

Standard profile of welded beads with the HW-GMAW processes with percentages of 20% and 100% of hot wire in the torch movements: (a) pull and (b) push. Based on the work of [11].

In the case of the predominant microstructures, the phases in the forms of PF, PF(G), and FS(NA) ferrites constitute 98% of the composition of the microstructures present in the weld metal. What changes are the amount of each phase in each image analyzed? For low amounts of cold wire (20% and 40%), the percentages of AF and FS(NA) have an average increase of 47% and 28%, respectively. However, for percentages of cold wire from DCW-GMAW-60%, there is a decrease in the amount of these phases, while the FS(A) increases, even tripling its composition in the case of low wire feed with 100% of cold wire. An image of the phases present in a sample of DCW-GMAW-60% for the feed speed of 12 m/min can be seen in Figure 8c. The work of [24] still shows that the silicon levels are drastically high, in the CW-GMAW and DCW-GMAW variants, thus also increasing the weld metal hardness levels. From average values of 155 HV to peaks of up to 190 HV.

3.3 HW-GMAW

Thinking about increasing the melting rate and the deposition rate of the GMAW process, making an adaptation in the CW-GMAW variant, the design of the HW-GMAW was arrived at. That is, the additional wire, which was previously free of energy, now has a low direct current to assist in the fusion of the filler metal, using the Joule effect as a basic principle, which, through the resistance of the metal, converts electrical energy into thermal energy. Bearing in mind that when introducing hot wire into the process, it is not intended to significantly increase the heat imposed on the part, but only to increase productivity with the design of this variant of GMAW.

The proposition of the HW-GMAW variant is relatively new, despite the similarity with more consolidated and widely used processes in the industry in general. Currently, his studies focus on the application of hard coatings on surfaces to increase wear resistance [27, 28, 33].

Firstly, in terms of welding itself, the works by [11, 34] studied the influence of generic parameters such as: welding direction, hot wire feed rates. However, remembering that the extra wire feed rates obey the ratio given according to Eq. (1), both of which are related having as a reference point a percentage of the electrode wire feed speed (m/min). Thus, [27] using 5 m/min electrode wire speed with a percentage of 140% hot wire, casing welds were performed on flat bars (9.5 mm X 56 mm X 225 mm) of AISI/SAE 1020 carbon steel, both wires used were of the AWS ER70S-6 class, the electrode wire having a diameter of 1.2 mm and the energized wire having a diameter of 1.0 mm. The welding parameters were a voltage of 23.6 V, current of 180 A and a contact tip-to-work distance of 15 mm, in the pulling welding technique. The variable parameter used was the direct polarity current of the hot wire at levels of 40 A, 80 A, 120 A, and 150 A. The solder used as a comparison was the CW-GMAW with 50% cold wire and parameters almost identical to those mentioned previously. The results obtained suggest that the w/h ratio has values above the previously established limit, greater than 0.3, with an average of 0.35. They may not be ideal for chamfering, but excellent for application as a coating. Penetration is slightly higher with values of up to 30% higher and the HAZ practically remains very similar. In general, the bead profiles are similar to those in Figure 10, in which they are based on the work of [11], in which the interference of other parameters such as the stability of the process through cyclograms, the polarity of the hot wire (on both poles: positive and negative), the welding direction (pull or push) varying wire feed rates at 20% and 100% hot wire. Emphasizing that the same material of low carbon steel and electrode wire of the same AWS class were used.

Still in the work of [11], the authors concluded that, for high wire feed rates, the penetration can drop by up to 45% and the dilution by up to 25%, when compared with the original GMAW. It has also been shown that hot wire polarity can attract or repel the arc and, together with the HW feed rate, can change bead geometry through changes in penetration depth and bead height. On the other hand, welding directions and wire feed rates are the parameters that most affect arc stability. And finally, in most weld beads the penetration is lower than the same weld in the conventional process.

However, regarding the metallurgical issues of grain size and the formation of microstructures from HW-GMAW welding in carbon steel materials, it will be necessary to continue the research, since it has not yet been published. Noting that there are already many works that show the structures of alloys based on Ni and FeCrC, which will not be addressed in this chapter.

4. Conclusions

Modifications of the GMAW process giving rise to the CW-GMAW, DCW-GMAW, and HW-GMAW variants provide very significant results, despite a very similar trend when the percentages of extra wire become very high from 80%, regardless of whether this extra wire is energized or not. However, some observations must be addressed:

1. The CW-GMAW variant in several works presents greater versatility of implementation and use in different sectors of the industry, both for straight and narrow gap and for angled chamfers, in addition to the possibility of application for coatings, whether special or not. In general, low electrode wire feed speeds and low percentages of extra wire favor the variant in use for splices, as linear penetration remains almost unchanged. However, high wire feed speeds and high percentages of cold wire allow for a decrease in penetration, dilution, and w/h ratio, increasing the height of the reinforcement, favoring its use for coatings. This feasibility is further increased when the combination of wires used can be manipulated, generating different chemical combinations for the deposited metal.

2. In the case of DCW-GMAW, the behavior changes in relation to the chances of application for certain types of industry, being more limited for use in coating situations, since the cold wire mass drastically reduces metal penetration of weld, the dilution, favoring for low values ​​of w/h, increasing the profile of the reinforcement. Also, there is a potential chance of decreasing residual stresses when welding with this variant, and this has already been observed in the CW-GMAW variant, but not yet proven.

3. For the application of the variant HW-GMAW, similar to the DCW-GMAW, its application is already well established for use in hard coatings, with several works already published, where this variant is used in coatings of special alloys to increase the resistance to wear. The weld bead profile of this variant is characterized by low dilution and small w/h values, being strongly influenced by the polarity of the current applied to the hot wire.

4. For all abovementioned variants:

   1. There is a significant decrease in the HAZ, which favors the implication that there is a decrease in defects in this region, which has been extensively studied and confirmed to be problematic in several situations.

   2. In the case of application in low carbon steel, it is concluded that the grain size is slightly reduced, favoring the increase of the mechanical strength of the weld metal. Where several works observed this increase in the hardness of the weld metal. In addition, it was found that there are few changes in the microstructure, where in certain cases, the formation of acicular ferrite, which is very desired in weld metals, is increased.

Acknowledgments

To Federal University of Pará, Postgraduate Program in Natural Resources Engineering in the Amazon (PRODERNA/UFPA) and to the Metallic Materials Characterization Laboratory (LCAM) for all support in carrying out the tests and coworker who ceded their scientific work.