**2. Stability control in the GMAW process**

#### **2.1 GMAW process operation**

GMAW process is characterized by producing an arc between a consumable electrode that is constantly fed, a protective gas, and the piece to be welded, as represented in **Figure 1**.

Conductor tube: It is a welding torch component device and fulfills the function of guiding the gas flow in the welding process.

**Figure 1.** *Basic diagram of the MIG/MAG process (modified from [4]).*

consist of taking samples of weldments to evaluate the metallic continuity,

sonic sensing methods have been implemented.

**2. Stability control in the GMAW process**

of guiding the gas flow in the welding process.

*Basic diagram of the MIG/MAG process (modified from [4]).*

**2.1 GMAW process operation**

represented in **Figure 1**.

of spatter.

*Welding - Modern Topics*

directions.

**Figure 1.**

**4**

mechanical strength, and other determining factors for the correct performance in service. Sometimes these tests lead to the destruction of the body tested. On the other hand, Wu et al. [1] affirm that online quality control allows the saving of financial resources through the reduction of defects in the production line. For this purpose, sensors for visual imaging, sound acquisition, infrared cameras, and ultra-

One concept that is strongly correlated to the online quality is the control of the process stability. According to Ponomarev [2], the stability of the GMAW process is evaluated online by three factors: metallic transfer regularity, arc stability, and the operational behavior of the welding process. Meneses [3] also ensures that the higher the transfer stability, the higher the penetration and the lesser the amount

The objective of this work is to present a bibliographical review of the scientific literature related to weld quality evaluation, focused mainly on those studies that present qualitative and quantitative indexes to evaluate the stability of the GMAW process. The chapter is structured as follows: Section 2 discusses Stability Control in the GMAW process; Section 2.1 discusses GMAW process operation; Section 2.2 discusses factors that affect stability; Section 2.3 presents a Summary of Stability index; and finally, Section 3 reveals a synthesis of the study and future research

GMAW process is characterized by producing an arc between a consumable electrode that is constantly fed, a protective gas, and the piece to be welded, as

Conductor tube: It is a welding torch component device and fulfills the function

Contact tip: It is a torch device that has the function of guiding and supplying voltage to the wire.

Electrode: It is the consumable copper-coated steel electrode that melts with the electric arc and transfers to the melting pool.

Workpiece: composed of the metal bodies to be joined by the weld.

CTWD (contact tip to work distance): It is often confused with the distance between the contact tip and the work piece, which coincides when the nozzle front cut is also the same as the contact tip front cut.

Stick out: It is the length of free wire after it has passed through the contact tip. The gas composition aims to stabilize the arc and protect the welding material from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. Depending whether the gas is inert (Ar or He) or active (CO2, or mixtures including N2 or O2), it can sbe classified as metal active gas (MAG) or metal inert gas (MIG).

The weld bead geometry depends directly to the parameters that govern the process. **Figure 2** outlines these geometric parameters in the cross section of a weld bead. The most important parameters affecting penetration and geometry in the GMAW process are welding current, arc voltage, torch travel speed or welding speed, stick out, torch tilt, and the diameter of the electrode.

According to [6] the process parameters of GMAW can be divided into five basic groups (as shown in **Figure 3**):


**Figure 2.** *Weld bead geometric characteristics [5].*


to work the piece length) will lead to a destabilization of the process, producing variations in the intensity of the welding current and the arc voltage. At the same time, when the voltage is too small, the arc length is short, so the droplet does not

The parameter's wire feed speed also has an influence. By increasing the feed rate of the wire, the diameter of the drop decreases; very high or lower values coincide with the most unstable conditions. But the degree of this influence depends

Furthermore, the variation of the current affects the metallic transfer regularity, and furthermore the transfer regularity reflects the stability of the process. Then it can be said that these factors are going to be influenced by the dynamic behavior of the GMAW welding process, particularly by the physical variations during the different transfer modes. Consequently, to understand how these factors influenced the stability, it is necessary to delve into the characteristics of the metal transfer. The metal transfer has a direct influence on the stability of the arc and final geometry of the weld bead. The metal transfer is controlled by several parameters such as current, voltage, electrode diameter, and shielding gas composition. It directly influences the way that metal droplets are transferred; the uniformity and

The three first transfer modes are short circuit, globular spray, and pulsed GMAW. In addition to these modes of transfer, there are others classified as freeflight transfer modes which happen when the arc voltage is high and includes repelled globular, projected spray, streaming spray, and rotating spray. The present

Spray transfer is characterized by small, uniform drops with diameters close to the size of the electrode. This transfer is obtained with high intensities and high voltages; its current intensities are from 150 to 500 A and its voltages from 24 to 40 ⱱ. Inert shielding gas favors this type of transfer. The process is presented with high arc stability, with high currents and deep penetration in the workpiece, and a high frequency of detachment. It allows high penetration to be achieved. Voltage and welding current oscillograms do not differ significantly, as shown in **Figure 4**.

In the globular transfer, the drop grows until exceeding the size of the electrode, and the detachment occurs by the action of the gravitational force. Typical parameters in globular transfer are voltage 20–36 ⱱ, current intensity 70–255 A. It has been unwanted in the industry for its instability and high grade of spatter. During this transfer mode, the output currents are kept oscillating depending on the detach-

Pulsed transfer is considered a particular case of spray transfer but is character-

ized by great stability that is achieved by controlling the process variables, in

fully grow and then contacts with the molten pool.

*Stability on the GMAW Process*

*DOI: http://dx.doi.org/10.5772/intechopen.90386*

on the shielding gas used and the welding voltage.

the volume of the drop; and variations in arc length.

ment of the drop, as shown in **Figure 5**.

*Waveform factors spray transfer mode [7].*

**Figure 4.**

**7**

study focuses on the three first natural modes of transference.






#### **Figure 3.**

*Classifications of GMAW parameters [6].*

## **2.2 Factors that affect the stability**

The operational behavior has a big influence on stability. If the gas is not supplied accurately, the arc may not initialize, there would be no stable or continuous plasma ionization, and the protective effect will be affected; nitrogen, oxygen, and water vapor enter the welding region and directly contact with the arc and melting metals, reducing the arc stability and forming a variety of welding defects. In the same way presence of grease, paint, dust, humidity, and extreme temperature produce a variation on the welding voltage.

But arc stability is directly influenced by the parameters of the process. It is possible to mention that a relationship exists between the arc length and process stability. Increasing the length of the arc (due to the increase of the contact nozzle

#### *Stability on the GMAW Process DOI: http://dx.doi.org/10.5772/intechopen.90386*

to work the piece length) will lead to a destabilization of the process, producing variations in the intensity of the welding current and the arc voltage. At the same time, when the voltage is too small, the arc length is short, so the droplet does not fully grow and then contacts with the molten pool.

The parameter's wire feed speed also has an influence. By increasing the feed rate of the wire, the diameter of the drop decreases; very high or lower values coincide with the most unstable conditions. But the degree of this influence depends on the shielding gas used and the welding voltage.

Furthermore, the variation of the current affects the metallic transfer regularity, and furthermore the transfer regularity reflects the stability of the process. Then it can be said that these factors are going to be influenced by the dynamic behavior of the GMAW welding process, particularly by the physical variations during the different transfer modes. Consequently, to understand how these factors influenced the stability, it is necessary to delve into the characteristics of the metal transfer.

The metal transfer has a direct influence on the stability of the arc and final geometry of the weld bead. The metal transfer is controlled by several parameters such as current, voltage, electrode diameter, and shielding gas composition. It directly influences the way that metal droplets are transferred; the uniformity and the volume of the drop; and variations in arc length.

The three first transfer modes are short circuit, globular spray, and pulsed GMAW. In addition to these modes of transfer, there are others classified as freeflight transfer modes which happen when the arc voltage is high and includes repelled globular, projected spray, streaming spray, and rotating spray. The present study focuses on the three first natural modes of transference.

Spray transfer is characterized by small, uniform drops with diameters close to the size of the electrode. This transfer is obtained with high intensities and high voltages; its current intensities are from 150 to 500 A and its voltages from 24 to 40 ⱱ. Inert shielding gas favors this type of transfer. The process is presented with high arc stability, with high currents and deep penetration in the workpiece, and a high frequency of detachment. It allows high penetration to be achieved. Voltage and welding current oscillograms do not differ significantly, as shown in **Figure 4**.

In the globular transfer, the drop grows until exceeding the size of the electrode, and the detachment occurs by the action of the gravitational force. Typical parameters in globular transfer are voltage 20–36 ⱱ, current intensity 70–255 A. It has been unwanted in the industry for its instability and high grade of spatter. During this transfer mode, the output currents are kept oscillating depending on the detachment of the drop, as shown in **Figure 5**.

Pulsed transfer is considered a particular case of spray transfer but is characterized by great stability that is achieved by controlling the process variables, in

**Figure 4.** *Waveform factors spray transfer mode [7].*

**2.2 Factors that affect the stability**

*Classifications of GMAW parameters [6].*

*Welding - Modern Topics*

**Figure 3.**

**6**

produce a variation on the welding voltage.

The operational behavior has a big influence on stability. If the gas is not supplied accurately, the arc may not initialize, there would be no stable or continuous plasma ionization, and the protective effect will be affected; nitrogen, oxygen, and water vapor enter the welding region and directly contact with the arc and melting metals, reducing the arc stability and forming a variety of welding defects. In the same way presence of grease, paint, dust, humidity, and extreme temperature

But arc stability is directly influenced by the parameters of the process. It is possible to mention that a relationship exists between the arc length and process stability. Increasing the length of the arc (due to the increase of the contact nozzle

**Figure 5.**

*Waveform factors of globular transfer mode [7].*

#### **Figure 6.**

*Waveform factors (modified from [8]).*

particular the current. The welding equipment generates two levels of current. In the first, the base current (Ib) is kept low so that there is no transfer, but only the onset of wire fusion; in the second, the peak current (Ip) is higher than the globular transition current causing the transfer, under optimal operating conditions, of a single drop. Typical parameters in pulsed transfer are voltage 20–30 ⱱ and current intensity 100–300 A, as shown in **Figure 6**.

Furthermore, a relationship between the waveform factor of the short circuit and the arc stability exists. Some parameters (relating to time and current) used to quantify stability are easy to calculate from the waveform factor, as the short-circuit time, the arcing time, the transfer period, and the short-circuit frequency. Mita et al. [9] also affirm that the correlation between those parameters and the stability

Using the abovementioned concepts, several indexes have been proposed to infer the stability and quality of the welding process. They were calculated using image processing techniques, acoustic monitoring, and analysis of the electrical signals. **Figure 8** shows the percentage of papers classified by transfer modes, and it was found that the highest percentage of indexes focused on the short-circuit

Knowing that a signal behaves according to a stochastic process, it is possible to determine a probabilistic model and apply some algorithms to process this signal. Hence, several works have focused on the study of the electrical signals at the

Adolfsson and Bahrami [10] calculate the variance of weld voltage (every 1024

signals). The study validates the hypothesis that the instability of the process

becomes weaker with increasing current.

*2.3.1 Statistical analysis to identify disturbances*

moment of disturbance, using a statistical treatment.

**2.3 Summary of stability indexes**

*Papers classified by transfer modes.*

transfer mode.

**9**

**Figure 7.**

**Figure 8.**

*Waveform factors (modified from [9]).*

*Stability on the GMAW Process*

*DOI: http://dx.doi.org/10.5772/intechopen.90386*

Another parameter that influences the stability of the process is the transition current, which changes the frequency and diameter of the transferred drops.

In case of a given current of short-circuiting transition, the droplet transfer exists in the form of short-circuiting, and the welding is stable. When the welding current increases, the droplet transition changes from the short-circuiting mode to the mixed mode, so the welding process and electric signal become unstable.

On the other hand, the globular-spray transition current also presents instability; a big number of spatters but the arc is no longer extinguished. Studies show that with the increase of CO2 in the gas mixture, an increase of the transition current is produced.

Finally, a peculiarity of the short-circuit transfer mode is the existence of regular contact between the electrode and the workpiece. Typical short-circuit parameters are voltage 16–22 ⱱ and current intensity 50–150 A. When the short circuit occurs, the arc is extinguished establishing two characteristic phases: the arcing period and the short-circuit period. Droplet growth occurs in the arcing period, whereas during the contact period, the metal is transferred. Also, the voltage and current oscillate to high and low at the same frequency of the metal transfer (**Figure 7**).

**Figure 7.** *Waveform factors (modified from [9]).*

#### **Figure 8.** *Papers classified by transfer modes.*

Furthermore, a relationship between the waveform factor of the short circuit and the arc stability exists. Some parameters (relating to time and current) used to quantify stability are easy to calculate from the waveform factor, as the short-circuit time, the arcing time, the transfer period, and the short-circuit frequency. Mita et al. [9] also affirm that the correlation between those parameters and the stability becomes weaker with increasing current.
