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"The second edition, a valuable resource for practitioners,researchers and students, contains more exercises and offers asolution manual upon request..." (Materials Evaluation,February 2003)
"For many years this review has been looking, without success,for a book on the metallurgy of welding. This...second editionfulfills all those needs and expectations...all those who need abasic understanding of...welds will greatly benefit...a valuableacquisition…highly recommended." (Choice, Vol. 40, No.7, March 2003)
Fusion welding processes will be described in this chapter, including gas welding, arc welding, and high-energy beam welding. The advantages and disadvantages of each process will be discussed.
1.1.1 Fusion Welding Processes
Fusion welding is a joining process that uses fusion of the base metal to make the weld. The three major types of fusion welding processes are as follows:
1. Gas welding:
Oxyacetylene welding (OAW)
2. Arc welding:
Shielded metal arc welding (SMAW)
Gas-tungsten arc welding (GTAW)
Plasma arc welding (PAW)
Gas-metal arc welding (GMAW)
Flux-cored arc welding (FCAW)
Submerged arc welding (SAW)
Electroslag welding (ESW)
3. High-energy beam welding:
Electron beam welding (EBW)
Laser beam welding (LBW)
Since there is no arc involved in the electroslag welding process, it is not exactly an arc welding process. For convenience of discussion, it is grouped with arc welding processes.
1.1.2 Power Density of Heat Source
Consider directing a 1.5-kW hair drier very closely to a 304 stainless steel sheet 1.6mm (1/16 in.) thick. Obviously, the power spreads out over an area of roughly 50mm (2 in.) diameter, and the sheet just heats up gradually but will not melt. With GTAW at 1.5kW, however, the arc concentrates on asmall area of about 6mm (1/4 in.) diameter and can easily produce a weld pool. This example clearly demonstrates the importance of the power density of the heat source in welding.
The heat sources for the gas, arc, and high-energy beam welding processes are a gas flame, an electric arc, and a high-energy beam, respectively. The power density increases from a gas flame to an electric arc and a high-energy beam. As shown in Figure 1.1, as the power density of the heat source increases, the heat input to the workpiece that is required for welding decreases. The portion of the workpiece material exposed to a gas flame heats up so slowly that, before any melting occurs, a large amount of heat is already conducted away into the bulk of the workpiece. Excessive heating can cause damage to the workpiece, including weakening and distortion. On the contrary, the same material exposed to a sharply focused electron or laser beam can melt or even vaporize to form a deep keyhole instantaneously, and before much heat is conducted away into the bulk of the workpiece, welding is completed.
Therefore, the advantages of increasing the power density of the heat source are deeper weld penetration, higher welding speeds, and better weld quality with less damage to the workpiece, as indicated in Figure 1.1. Figure 1.2 shows that the weld strength (of aluminum alloys) increases as the heat input per unit length of the weld per unit thickness of the workpiece decreases. Figure 1.3a shows that angular distortion is much smaller in EBW than in GTAW. Unfortunately, as shown in Figure 1.3b, the costs of laser and electron beam welding machines are very high.
1.1.3 Welding Processes and Materials
Table 1.1 summarizes the fusion welding processes recommended for carbon steels, low-alloy steels, stainless steels, cast irons, nickel-base alloys, and aluminum alloys. For one example, GMAW can be used for all the materials of almost all thickness ranges while GTAW is mostly for thinner workpieces. For another example, any arc welding process that requires the use of a flux, such as SMAW, SAW, FCAW, and ESW, is not applicable to aluminum alloys.
1.1.4 Types of Joints and Welding Positions
Figure 1.4 shows the basic weld joint designs in fusion welding: the butt, lap, T-, edge, and corner joints. Figure 1.5 shows the transverse cross section of some typical weld joint variations. The surface of the weld is called the face, the two junctions between the face and the workpiece surface are called the toes, and the portion of the weld beyond the workpiece surface is called the reinforcement. Figure 1.6 shows four welding positions.
1.2 OXYACETYLENE WELDING
1.2.1 The Process
Gas welding is a welding process that melts and joins metals by heating them with a flame caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown in Figure 1.7, is the most commonly used gas welding process because of its high flame temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld metal. Figure 1.8 shows three different types of flames in oxyacetylene welding: neutral, reducing, and oxidizing, which are described next.
1.2.2 Three Types of Flames
A. Neutral Flame This refers to the case where oxygen ([O.sub.2]) and acetylene ([C.sub.2][H.sub.2]) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame (Figure 1.8a). The inner cone is the area where the primary combustion takes place through the chemical reaction between [O.sub.2] and [C.sub.2][H.sub.2], as shown in Figure 1.9. The heat of this reaction accounts for about two-thirds of the total heat generated. The products of the primary combustion, CO and [H.sub.2], react with [O.sub.2] from the surrounding air and form C[O.sub.2] and [H.sub.2]O. This is the secondary combustion, which accounts for about one-third of the total heat generated. The area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and [H.sub.2] here consume the [O.sub.2] entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used.
B. Reducing Flame When excess acetylene is used, the resulting flame is called a reducing flame. The combustion of acetylene is incomplete. As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing flame (Figure 1.8b). This flame is reducing in nature and is desirable for welding aluminum alloys because aluminum oxidizes easily. It is also good for welding high-carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal.
C. Oxidizing Flame When excess oxygen is used, the flame becomes oxidizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame (Figure 1.8c). This flame is preferred when welding brass because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool.
1.2.3 Advantages and Disadvantages
The main advantage of the oxyacetylene welding process is that the equipment is simple, portable, and inexpensive. Therefore, it is convenient for maintenance and repair applications. However, due to its limited power density, the welding speed is very low and the total heat input per unit length of the weld is rather high, resulting in large heat-affected zones and severe distortion. The oxyacetylene welding process is not recommended for welding reactive metals such as titanium and zirconium because of its limited protection power.
1.3 SHIELDED METAL ARC WELDING
1.3.1 The Process
Shielded metal arc welding (SMAW) is a process that melts and joins metals by heating them with an arc established between a sticklike covered electrode and the metals, as shown in Figure 1.10. It is often called stick welding. The electrode holder is connected through a welding cable to one terminal of the power source and the workpiece is connected through a second cable to the other terminal of the power source (Figure 1.10a).
The core of the covered electrode, the core wire, conducts the electric current to the arc and provides filler metal for the joint. For electrical contact, the top 1.5 cm of the core wire is bare and held by the electrode holder. The electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely.
The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt off as droplets (Figure 1.10b). The molten metal collects in the weld pool and solidifies into the weld metal. The lighter molten flux, on the other hand, floats on the pool surface and solidifies into a slag layer at the top of the weld metal.
1.3.2 Functions of Electrode Covering
The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below.
A. Protection It provides a gaseous shield to protect the molten metal from air. For a cellulose-type electrode, the covering contains cellulose, [([C.sub.6][H.sub.10][O.sub.5]).sub.x]. A large volume of gas mixture of [H.sub.2], CO, [H.sub.2]O, and C[O.sub.2] is produced when cellulose in the electrode covering is heated and decomposes. For a limestone-(CaC[O.sub.3-]) type electrode, on the other hand, C[O.sub.2] gas and CaO slag form when the limestone decomposes. The limestone-type electrode is a low-hydrogen-type electrode because it produces a gaseous shield low in hydrogen. It is often used for welding metals that are susceptible to hydrogen cracking, such as high-strength steels.
B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation.
C. Arc Stabilization It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly.
D. Metal Addition It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate.
1.3.3 Advantages and Disadvantages
The welding equipment is relatively simple, portable, and inexpensive as compared to other arc welding processes. For this reason, SMAW is often used for maintenance, repair, and field construction. However, the gas shield in SMAW is not clean enough for reactive metals such as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering tends to over-heat and fall off when excessively high welding currents are used. The limited length of the electrode (about 35 cm) requires electrode changing, and this further reduces the overall production rate.
1.4 GAS-TUNGSTEN ARC WELDING
1.4.1 The Process
Gas-tungsten arc welding (GTAW) is a process that melts and joins metals by heating them with an arc established between a nonconsumable tungsten electrode and the metals, as shown in Figure 1.11. The torch holding the tungsten electrode is connected to a shielding gas cylinder as well as one terminal of the power source, as shown in Figure 1.11a. The tungsten electrode is usually in contact with a water-cooled copper tube, called the contact tube, as shown in Figure 1.11b, which is connected to the welding cable (cable 1) from the terminal. This allows both the welding current from the power source to enter the electrode and the electrode to be cooled to prevent overheating. The workpiece is connected to the other terminal of the power source through a different cable (cable 2). The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is much better in GTAW than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. For this reason, GTAW is also called tungsten-inert gas (TIG) welding. However, in special occasions a noninert gas (Chapter 3) can be added in a small quantity to the shielding gas. Therefore, GTAW seems a more appropriate name for this welding process. When a filler rod is needed, for instance, for joining thicker materials, it can be fed either manually or automatically into the arc.
Figure 1.12 shows three different polarities in GTAW, which are described next.
A. Direct-Current Electrode Negative (DCEN) This, also called the straight polarity, is the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. As shown in Figure 1.12a, electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced.
B. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The electrode is connected to the positive terminal of the power source. As shown in Figure 1.12b, the heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, a large-diameter, water-cooled electrodes must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, as shown in Figure 1.13, knocking off oxide films and producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong oxide-forming materials such as aluminum and magnesium, where deep penetration is not required.
C. Alternating Current (AC) Reasonably good penetration and oxide cleaning action can both be obtained, as illustrated in Figure 1.12c.This is often used for welding aluminum alloys.
Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current-carrying capacity, and resistance to contamination than pure tungsten electrodes. As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature required to emit electrons and hence a greater risk of melting the tip.
1.4.4 Shielding Gases
Both argon and helium can be used. Table 1.2
Excerpted from Welding Metallurgy by Sindo Kou Excerpted by permission.
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|1||Fusion Welding Processes||3|
|2||Heat Flow in Welding||37|
|3||Chemical Reactions in Welding||65|
|4||Fluid Flow and Metal Evaporation in Welding||97|
|5||Residual Stresses, Distortion, and Fatigue||122|
|II||The Fusion Zone||143|
|6||Basic Solidification Concepts||145|
|7||Weld Metal Solidification I: Grain Structure||170|
|8||Weld Metal Solidification II: Microstructure within Grains||199|
|9||Post-Solidification Phase Transformations||216|
|10||Weld Metal Chemical Inhomogeneities||243|
|11||Weld Metal Solidification Cracking||263|
|III||The Partially Melted Zone||301|
|12||Formation of the Partially Melted Zone||303|
|13||Difficulties Associated with the Partially Melted Zone||321|
|IV||The Heat-Affected Zone||341|
|15||Precipitation-Hardening Materials I: Aluminum Alloys||353|
|16||Precipitation-Hardening Materials II: Nickel-Base Alloys||375|
|17||Transformation-Hardening Materials: Carbon and Alloy Steels||393|
|18||Corrosion-Resistant Materials: Stainless Steels||431|