**3. Welding fundamentals and equipment**

There are three primary welding processes—gas metal arc welding (GMAW/ MIG), gas tungsten arc welding (GTAW/TIG), and shielded metal arc welding (SMAW/Stick/Arc). This section will describe the characteristics and advantages of each process. Various welding positions, the different weld joint types, weldment preparations to the base materials, the calculation of weld fillet size, and weld strength will additionally be reviewed.

### **3.1 Arc welding**

Shielded metal arc welding (SMAW/Stick/Arc) is chosen for a variety of applications due to its uniquely robust properties, allowing it to be used in many environments. Stick welding is heavily used in structural and industrial heavy metal applications when less than ideal conditions are present. Common SMAW/Stick/Arc welding joints include the assembly of structural frameworks for buildings and the joining of pipe segments together in pipeline applications. The connections of highpressure pipeline segments are generally made by SMAW/Stick/Arc welding. The pipe ends are typically beveled, mated, and welded with an E6010 electrode. This is called a root pass. There can be several root passes depending on the application. Root passes are then overlaid and capped by several additional weld passes, building additional filler material with each pass. This kind of weld is durable and self-healing around small weld defects. It is the oldest form of electrode welding and is still preferred when deep penetration is required.

There is an assortment of consumable electrodes available to the repairman. These are also referred to as "rods." Electrodes are 30 cm long sticks of filler metal, wrapped with a flux coating. As shown in **Figure 1**, they are referenced with a four-digit numerical code with each number referencing a specification for the electrode. The first two numbers reference the tensile strength of the weld material that the rod will produce. The third number refers to the position(s) in the electrode that can be used for welding. One means that it can be used in all positions, and two means it can only be used in the flat and horizontal positions. The fourth number refers to the type of flux coating, the hydrogen content of the electrode, and if it can be used with alternating current, direct current, or both. An advantage of the stick welding process is its ability to make high penetrating welds on imperfectly prepared material with oxidation and mill scale, the oxidized scaly substance on the surface of most hot-rolled material.

This makes SMAW/Stick/Arc welding an excellent choice for field repairs. The stick welding process is commonly used by repair shops for agricultural, off-road, and construction equipment repairs. With the correct rod, rusty and dirty metal can be welded effectively with minimal preparation. This process produces a very strong weldment with high material penetration and can produce an esthetically pleasing weld bead using certain electrodes. Unfortunately, as shown in **Figure 2**, it generates far more splatter and slag peel than other welding processes. However, those other processes require clean metal with minimal mill scale to make solid weldments. Stick welding can operate with either direct current or alternating current. Stick welding machines use a ground clamp and an electrode holder, commonly referred to as a "stinger" to make the necessary electric circuit.

SMAW/Stick/Arc welding is the generally preferred choice of the process by many repairmen for field repairs. As previously mentioned, field repairs often require the use of mobile welding equipment. Notable welding equipment manufacturers, such as Lincoln Electric and Miller Electric, manufacture gasoline and diesel-powered welders for this purpose. Test engineers, pipefitters, construction crews, and other field repairmen will generally have their service vehicles equipped with machines of this category. Mobile welding rigs typically have electrode holders and ground leads in lengths of 15–30 m for repairs and jobs, where only limited vehicle access is available. If the stick welding process is utilized for repair work to an appearance-critical piece of equipment, extreme care must be taken to effectively tarp and shield the balance of the unit from the inevitable splatter of the process.

## **3.2 MIG welding**

Gas metal arc welding (GMAW/MIG) is the predominate joining method in manufacturing and industrial settings. This process is preferred, because of its weld

**Figure 2.** *Stick welded components showing the splatter form the process.*

quality and speed, making it less expensive to implement in production situations. GMAW/MIG welding produces very little slag and creates a very clean welded connection when welding clean material. MIG welding is often referred to as "wire feed welding." As shown in **Figure 3**, its nickname is due to the spool of wire fed through the MIG gun by the machine. When the trigger on the gun is actuated by the operator, electrified wire is fed through the end and shrouded by a gas cloud. A typical combination of shielding gas for welding mild steel is 75% argon and 25% CO2. As the welding wire is pushed toward the base material, it strikes an electric arc and melts the target material and filler materials together. Filler metals are rated by their tensile strength. A tensile strength rating of 482 mPa is common for most spools of filler wire. MIG welding is not as effective as SMAW/Stick/Arc for use on oxidized material, without significant clean-up and material preparation. However, it is more timeefficient than other processes for clean assemblies, making it an excellent process in production environments. As shown in **Figure 4**, it makes a beautiful, clean weld on clean material. Materials such as steel, aluminum, magnesium, carbon steel, nickel, and other alloy metals can be welded with the GMAW/MIG process, making this a

**Figure 3.** *Spool of 0.58 mm filler wire mounted on MIG welding machine.*

versatile industrial tool, when multiple materials are in use. The ability to weld a variety of materials makes it a top choice in high production manufacturing [11].

### **3.3 TIG welding**

Gas tungsten arc welding (GTAW/TIG) is a process commonly used for specialty welding repairs and applications, where additional craftsmanship is necessary. The GTAW/TIG welding process is commonly used on components that are made of stainless steel and other non-ferrous metals. TIG welding is expensive and timeconsuming, due to its precision, but as shown in **Figure 5**, the quality of the GTAW/ TIG weld is unsurpassed. Consequently, it is not often used for facilitating field repairs, but may be used for smaller repairs, especially for stainless-steel repairs. Stainless steel is an alloy with unique properties that do not allow it to be welded easily with other welding processes or without specific equipment configurations. Stainless steel material is subject to cracking at high welding temperatures from other welding processes, causing additional weld defects, which result in weld quality issues. However, stainless can be welded nicely with the use of a TIG welder, due to its low heat application from the tungsten rod in tandem with an argon shielding gas. The user has the ability to control the welding heat with great precision. TIG welding uses a foot pedal to control the machine amperage. It can operate on direct current or alternating current, but the tungsten electrode rod is not actually consumed very rapidly. The process requires filler metal to be fed manually by the welder into the arc that is struck by the tungsten rod attached to what is referred to as a "TIG torch." TIG welding is commonly used in applications where bead esthetics are preferred because it produces no weld slag. These applications include stainless-steel exhaust headers, pipe/tubing, and roll cages. TIG welding produces a high tensile strength weld and is a very good choice for a variety of applications requiring clean-finished beads or the repair of exotic materials. It is a well-suited process for applications requiring clean, goodlooking welds, and it is a highly regarded choice for critical stainless-steel joints and connections.

#### **Figure 4.**

*Horizontal fillet weld on clean material done by GMAW/MIG welding.*

**Figure 5.** *GTAW/TIG fillet weld on aluminum material.*
