Welding Engineering: An Introduction

1

What is Welding Engineering?

Welding Engineering is a complex field that requires proficiency in a broad range of engineering disciplines. Students who pursue a degree in Welding Engineering engage in a curriculum that is more diverse than other engineering disciplines (Figure 1.1). They take advanced courses in welding metallurgy and materials science that cover materials ranging from steels and stainless steels, to nonferrous alloys such as nickel, aluminum, and titanium, as well as polymers. Welding process courses emphasize theory, principles, and fundamental concepts pertaining to the multitude of important industrial welding processes. While many associate welding with arc welding processes, a Welding Engineer may be responsible for many other processes. Therefore, in addition to arc welding, the Welding Engineering curriculum includes coverage of processes such as Laser and Electron Beam Welding, solid-state welding processes such as Friction Welding and Explosion Welding, and resistance welding processes including Spot and Projection Welding. Students are trained in many important electrical concepts associated with welding such as process control and transformer theory and operation. Welding design courses cover the principles of important subjects such as heat flow, residual stress, fatigue and fracture, and weld design for various loading conditions. Analysis through numerical modeling is included in many of the courses. Nondestructive testing techniques including x-ray, ultrasonics, eddy current, magnetic particle, and dye penetrant are emphasized as well. The diverse Welding Engineering curriculum prepares its graduates for a wide range of possible career paths and industrial fields. Working environments include automation and high speed production, fabrication, manufacturing, and research. Welding Engineering graduates are typically in high demand, and choose jobs from a variety of industry sectors including nuclear, petrochemical, automotive, medical, shipbuilding, aerospace, power generation, and heavy equipment sectors.

1.1 Introduction to Welding Processes

Taking into account the recent developments in hybrid approaches to welding, there are now over 75 types of welding processes available for the manufacturer or fabricator to choose from. The reason that there are so many processes is that each process has its own unique advantages and disadvantages that make it more or less applicable to a given application. Arc welding processes offer advantages such as portability and low cost, but are relatively slow and rely on a considerable amount of heating to produce the weld. High-energy density processes such as Laser Welding produce low heat inputs and fast welding speeds, but the equipment is very expensive and joint fit-up needs to be nearly perfect. Solid-state welding processes avoid many of the weld discontinuities associated with melting and solidification, but are also very expensive and often are restricted to limited joint designs. Resistance welding processes are typically very fast and require no additional filler materials, but are often limited to thin sheet applications or very high production applications such as the seams in welded pipe.

Each of these processes produces a weld (metallic bond) using some combination of heat, time, and/or pressure. Those that rely on extreme heat at the source such as arc and high-energy density processes generally need no pressure. A process such as Diffusion Welding relies on some heating and some pressure, but a considerable amount of time. Explosion Welding relies on a tremendous amount of pressure, with minimal heating and time to produce the weld.

When choosing an optimum process for a given application, the Welding Engineer must consider all of the above, including much more that will be covered in the next few chapters.

Figure 1.1 A sampling of Welding Engineering topics—design (top left), processes (top right) and welding metallurgy (bottom)

2

Arc Welding Processes

2.1 Fundamentals and Principles of Arc Welding

This section serves as a general introduction to all of the arc welding processes. The common features, important concepts, and terminology of this family of welding processes are reviewed, with more process-specific details provided in the sections that follow. Arc welding refers to a family of processes that rely on the extreme heat of an electric arc to create a weld. They usually, but not always, involve the use of additional filler metal to complete the weld. As one of the first welding processes, arc welding continues to be very popular primarily due to its low equipment cost, portability, and flexibility. Some of the key developments that led to modern arc welding include the discovery of the electric arc in the 1820s (Davies), the first welding patent using a carbon electrode in 1886, the first covered electrode in 1900 (Kjellberg), and the first process using a continuously fed electrode in the 1940s.

The most common arc welding processes today are charted in Figure 2.1. The abbreviations refer to the American Welding Society (AWS) terminology as follows: SMAW—Shielded Metal Arc Welding, GMAW—Gas Metal Arc Welding, GTAW—Gas Tungsten Arc Welding, PAW—Plasma Arc Welding, SAW—Submerged Arc Welding, FCAW—Flux Cored Arc Welding, SW—Arc Stud Welding, and EGW—Electrogas Welding. Although technically not an arc welding process, ESW—Electroslag Welding is very similar to EGW and as such, is often included with arc welding. In practice, older designations and trade names of processes are often used, some of which are given in italics in the figure. Examples are Stick or Covered Electrode welding for SMAW, MIG meaning Metal Inert Gas for GMAW, and TIG meaning Tungsten Inert Gas for GTAW. One key generalization in the modern terms is the substitution of G for IG denoting inert gas, since these processes no longer rely solely on inert gasses for shielding.

Figure 2.1 Common arc welding processes

With all arc welding processes, the initiation of an arc basically completes (or closes) an electrical circuit. As shown in Figure 2.2, the most basic arc welding arrangement consists of an arc welding power supply, electrode and work cables (or leads), means to connect to the electrode (electrode holder with SMAW as shown), and the work piece or parts to be welded. A range of typical currents and voltages are shown. Voltages provided by the power supply are commonly a maximum of 60–80 V with no arc, referred to as the open circuit voltage of the power supply. Such voltages are high enough to establish and maintain an arc, but low enough to minimize the risk of electrical shock. Arc voltages range between 10 and 40 V once the arc is established.

Figure 2.2 Arc welding circuit depicting SMAW

Welding power supplies are usually designed to deliver direct current electricity referred to as DC. A pulsing output called pulsed direct current has become a prominent feature in many advanced welding power supplies. Programmable pulsing parameters or preprogrammed pulsing schedules can be used to optimize welding performance, primarily for GMAW. Alternating current (AC) is sometimes used. One benefit is that AC machines are simple and inexpensive. Welding with AC is also a very effective way to weld aluminum, which will be discussed later in the section on GTAW. A form of pulsing known as variable polarity is another advanced capability of many modern power supplies. Variable polarity capability allows for the customization of pulsing frequency and waveform to optimize welding performance.

Processes such as GMAW, FCAW, and SAW use a continuous motorized wire feed mechanism. With these processes, the power supply controls the arc length by a concept known as self-regulation to be discussed later. Since the welder only needs to control the position and movement of the gun, these processes are relatively easy to learn, and are referred to as semiautomatic processes. Manual processes such as SMAW and GTAW require the welder to control the delivery of the filler metal while maintaining the arc length. As a result, the manual processes require considerably more welder skill than the semiautomatic processes. Arc welding processes are referred to as mechanized or automated if they are attached to a travel mechanism or robotic arm.

2.1.1 Fundamentals of an Electric Arc

An electric arc is a type of electrical discharge that occurs between electrodes when a sufficient voltage is applied across a gap causing the gas to break down, or ionize (Figure 2.3). Gas is normally an insulator, but once ionized it becomes a conductor of electricity. Ionization occurs when the gas atoms lose bound electrons that are then free to travel independently in the gas to produce an electric current. These free electrons pick up energy from the electric field produced by the applied voltage and collide with other gas atoms. This allows the ionization process to grow resulting in an avalanche effect. Once the gas is highly ionized, it becomes relatively easy for electrons to flow, and under the right conditions a stable electric arc can be formed.

Figure 2.3 Ionization of a gas and current flow in an arc

An ionized gas consists of free electrons that flow in one direction and positive ions that flow in the other direction. Collisions with mostly neutral atoms produce a tremendous resistive heating of the gas, so, in a sense, the arc is a large resistor. The extreme heat also maintains the ionization process. Electromagnetic radiation is given off due to the high temperatures resulting in the characteristic glow of the arc. In addition to the observable visible wavelengths, large amounts of invisible infrared and ultraviolet wavelengths are emitted. The ionized glowing gas that makes up the arc is often referred to as plasma. In order for the arc to be maintained, the power supply must be able to supply the high current and low voltage demanded by the arc.

The utility of the electric arc to welding is the extreme heat that is produced under stable arc conditions, which is able to melt most metals and form what is known as a weld pool or puddle. Arc temperatures are known to range from 5 000 up to 30 000 K. As Figure 2.4 indicates, the temperature of an arc is hottest at its center since the outer portions of the arc lose heat to the surroundings due to convection, conduction, and radiation. The major contribution of heat to the welding and work electrodes is actually not due to the extremely high arc temperatures, but instead due to the intense energy dissipative processes at the arc attachment points to the welding and work electrodes. This will be discussed later.

Figure 2.4 Thermal diagram of gas tungsten arc

(Source: Reproduced by permission of American Welding Society, ©Welding Handbook)

For consumable electrode processes, the arc contains molten droplets of filler metal, which melt from the electrode and travel through the arc to the weld pool. As will be discussed later, the size, shape, and manner in which the molten metal drops travel through the arc are known as the modes of metal transfer. This is of particular interest with the GMAW process, and is not considered to be important to the other arc welding processes. Filler metal transfer through the arc inevitably results in some molten drops being ejected from the arc or weld pool that may stick to the part. This is called spatter and is often a quality concern. GTAW and PAW processes that involve the delivery of filler metal directly to the weld puddle (not through the arc) are not susceptible to spatter.

2.1.2 Arc Voltage

It was mentioned previously that operating arc voltages typically fall in the range of 10–40 V. Arc voltages are primarily related to arc lengths. Longer arc lengths produce higher arc voltages and shorter arcs produce lower voltages. Figure 2.5 shows how voltage (potential) varies through the arc. As the figure indicates, a significant amount of the voltage distribution or drop across the arc is close to the anode and the cathode. These regions are known as the anode drop or fall at the positive electrode (the work piece in the figure) and the cathode drop or fall at the negative electrode (the welding electrode in the figure). The primary change in arc voltage as a function of arc length is known to be associated with the region between the anode and cathode drops called the plasma column. The anode and cathode drops are known to be insignificantly affected by arc length. As a result, even extremely short arc lengths will exhibit voltages much greater than zero. This provides evidence that the majority of the arc voltage exists at the two voltage drops at the electrodes. For a typical welding arc length, these voltages may represent as much as 80–90% of the total arc voltage. Since heat generation and power dissipation are functions of voltage and current, and the level of current is uniform through the arc, the amount of power dissipation must therefore be greatest at the electrode drop regions and not in the plasma column. These anode and cathode drop regions are extremely narrow, and, therefore, their effect is not revealed on thermal diagrams of arcs such as that shown in Figure 2.4. Nevertheless, they play a critical role in the melting at the anode and cathode, which is why the arc temperature alone is not the key to explaining the arc as an effective heat source for welding.

Figure 2.5 Voltage across a welding arc

(Source: Reproduced by permission of American Welding Society, ©Welding Handbook)

Higher voltages are required to ionize a gas across a given gap and gas pressure. The latter is usually atmospheric pressure for a welding arc: however, arc welding can be conducted at other pressure conditions as well such as under water or in a chamber. Since the open circuit voltage typical of power supplies (60–80 V) is relatively low, it is not sufficient to simply break down a gap. With manual arc welding processes, it is usually necessary to touch, or so-called scratch or drag the electrode on the work piece. This produces an instantaneous short circuit current from the power supply, and is referred to as striking or drawing an arc. Once the ionization process has been initiated, the gap can be increased to achieve a stable arc. With semiautomatic processes, the wire is driven into the work piece by the wire feed mechanism producing a short circuit. The power supply reacts by producing a very high short circuit current that rapidly melts the wire forming a gap. The differences between arc welding power supplies for manual and semiautomatic processes will be explained later in this chapter. With GTAW, special arc starting systems that impose high voltage at a high frequency are included with the machine so that the welder can initiate the arc without touching the tungsten electrode to the part. Touching the tungsten electrode might produce contamination in the weld or on the electrode tip altering its performance.

2.1.3 Polarity

The electrical polarity applied to the arc via the power supply is very important for the operation of an arc process. The direction of current flow in the arc produces the main effects of polarity on welding. There is potential for confusion with the direction of current flow since in welding literature, it is common to see current described as being in the direction of electron flow, from the negative to the positive electrode. However, according to standard electrical convention, current is described as flowing from the positive to the negative electrode, or in the direction of the positively charged ions. In any case, for a welding arc, electrons flow from the negative electrode (cathode) to the positive electrode (anode), but this has different effects with different processes. In arc welding, when the electrode is negative and the work is positive, this is referred to as DCEN (DC electrode negative) or historically DCSP (DC straight polarity). The electrons flow out of the welding electrode, through the arc and into the work. When the electrode is positive relative to the work it is called DCEP (DC electrode positive) or historically DCRP (DC reverse polarity). Other polarity options are AC (simple alternating current) and VP (variable polarity) referring to voltage waveforms that are more complex and variable than simple AC. In both cases, the polarity and direction of electron flow alternate.

The effect of polarity on heat input and arc behavior differs with the process and the characteristics of the material being welded. For GTAW, DCEN produces the predominance of heat into the work, and is the most common polarity (Figure 2.6). This is because the tungsten electrode can be heated to extremely high temperatures without melting. At the extremely high temperatures, electrons are easily emitted or boiled off from the tungsten electrode (cathode) by a process known as thermionic emission. This produces a stable arc with the majority of the arc heat deposition at the work piece where the electrons are deposited. When operating with DCEP polarity, the tungsten arc is erratic due to the difficulty of electron emission at the lower temperature of the work. But DCEP can be beneficial when welding aluminum since the electron emission process can help remove the tenacious aluminum oxide from the surface, a process known as cleaning action. This is where AC current can be advantageous since it delivers a half cycle of DCEN, which heats the work piece, and a half cycle of DCEP, which removes the oxide.

Figure 2.6 DCEN—common with GTAW

With GMAW, DCEN is not usable since the lower temperature of the melting bare electrode wire cannot achieve thermionic emission. Thus, DCEN produces an arc that is very erratic and difficult to control. With DCEP, the work is negative and the greatest amount of heat goes into the part where the electrons can be more stably emitted. This is primarily due to oxides on the work surface, which facilitate the electron emission process. The end result is a much more stable arc, which is why DCEP polarity is used almost exclusively with GMAW (Figure 2.7).

Figure 2.7 DCEP—common with GMAW

Processes where fluxes are used such as SMAW, FCAW, and SAW can use DCEP, DCEN, or AC polarities, depending on the type of flux and the application. Flux additions that are in contact with the welding electrode can promote electron emission when the electrode is the cathode (DCEN). This allows the DCEN polarity to be an effective process choice. In some cases, DCEN may be selected in order to produce higher deposition rates due to greater electrode heating, with less heat input to the work. It can also be used for welding thinner materials.

2.1.4 Heat Input

The energy or heat input that occurs in the making of an arc weld is an important consideration. It is expressed as energy per unit length, and is primarily a function of voltage, current, and weld travel speed as indicated in Figure 2.8. Although voltage plays a prominent role in the equation, it is a variable that is chosen primarily to create a stable arc, and does not vary much as a parameter for heat input. Arc efficiency, f1, refers to what percentage of the total heat produced by the arc is delivered to the weld. Weld heat input is important because it affects the amount of distortion and residual stress in the part, and the mechanical properties of the welded part that are a function of the metallurgical transformations that take place during welding.

Figure 2.8 Heat input during welding

Figure 2.9 compares some measured efficiencies (f1) for different processes. Less efficient processes such as GTAW might lose 50% or more of the arc heat to the surrounding atmosphere. On the other extreme, efficiencies of 90% or greater can be achieved with SAW because the flux and molten slag blanket act as an insulator around the electrode and the arc. Arc efficiencies for other processes lie between. Since precise arc efficiencies are difficult to know, they are typically not considered to be an important practical factor. This is also due to the fact that there are usually other more important reasons driving the selection of the proper welding process for a given application.

Figure 2.9 Arc efficiency comparisons

(Source: Reproduced by permission of American Welding Society, ©Welding