1、SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSES Prepared by the Welding Handbook Chapter Committee on Joining and Cutting Processes: W. H. Kielhorn, Chair LeTourneau University Y. Adonyi LeTourneau University R. L. Holdren Edison Welding Institute R. C. Horrocks, Sr. Springeld however, as codes and
2、 stan- dards undergo frequent revision, the reader is encouraged to consult the most recent edition. 3. American National Standards Institute (ANSI) Committee Z49 on Safety in Welding and Cutting, Safety in Welding, Cutting, and Allied Processes, ANSI Z49.1, Miami: American Welding Society. 4. OBrie
3、n, R. L., ed., 1991, Welding Processes, Vol. 2 of Welding Handbook, 8th ed., Miami: American Welding Society.SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSES SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSES3depth coverage of each of the welding, cutting, and allied processes.JOINING PROCESSESThe goa
4、l of the joining processes is to cause diverse pieces of material to become a unied whole. In the case of two pieces of metal, when the atoms at the edge of one piece come close enough to the atoms at the edge of another piece for interatomic attraction to develop, the two pieces become one. Althoug
5、h this concept is easy to describe, it is not simple to effect. Surface roughness, impurities, tting imperfections, and the varied proper- ties of the materials being joined complicate the joining process. Welding processes and procedures have been developed to overcome these difculties by incorpora
6、t- ing the use of heat or pressure, or both. Though por- tions of this description do not apply to brazing, soldering, and adhesive bonding, an explanation will be given when these processes are described later in the chapter. Barring a few exceptions, most welding processes apply signicant heat to
7、the base material. This heat is only a means to bring the atoms at the edge of one piece of material close enough to the atoms of another piece for interatomic attraction. However, this heat is detri- mental to the microstructure of the materials being joined. As hot metal tends to oxidize, sufcient
8、 protec- tion from oxidation must be provided by the welding process to prevent this detrimental reaction with ambi- ent oxygen. Some metals are far more sensitive than others, in which case protection from oxidation becomes more demanding. Thus, while examining each welding process, the reader shou
9、ld consider whether heat is produced by the process and, if so, the manner in which it is produced. The means by which sufcient protection against oxidation is provided by the process should then be identied. The selection of an appropriate joining and cutting process for a given task involves a num
10、ber of consider- ations. These include the following: 1. Availability and tness for service; 2. Skill requirements; 3. Weldability of the base metal alloy with respect to type and thickness; 4. Availability of suitable welding consumables; 5. Weld joint design; 6. Heat input requirements; 7. Demands
11、 of the welding position; 8. Cost of the process, including capital expendi- tures, materials, and labor; 9. Number of components being fabricated; 10. Applicable code requirements; and 11. Safety concerns. The overview of the joining processes featured in Table 1.1 presents an initial reference gui
12、de to the capa- bilities of various joining processes with respect to a variety of ferrous and nonferrous metals. This table indicates the processes, materials, and material thick- ness combinations that are usually compatible. The col- umns on the left list various engineering materials and four ar
13、bitrary thickness ranges. The processes most commonly used in industry are listed across the top. It should be noted that additional information such as the considerations listed above must be taken into account before process selections are nalized. None- theless, Table 1.1 serves as a useful tool
14、in providing general guidelines for the screening and selection process.ARC WELDINGThe term arc weldingapplies to a large, diversied group of welding processes that use an electric arc as the source of heat. The creation of a weld between met- als using these processes does not usually involve pres-
15、 sure but may utilize a ller metal. The arc is struck between the workpiece and the tip of the electrode. The intense heat produced by the arc quickly melts a por- tion of the base metal, resulting in the formation of a weld. The arc welding processes may be moved along the joint to produce the weld
16、 or held stationary while the workpiece is moved under the process. Arc welding operations are performed by conducting the welding current through consumable electrodes, which take the form of a wire or rod, or nonconsum- able electrodes, consisting of carbon or tungsten rods. Metal arc processes ut
17、ilize consumable electrodes that combine electrode ller metal with the molten base metal to create the weld. They may also produce a slag covering to protect the molten metal from oxidation. The nonconsumable arc processes can generate a weld by melting the base metal only, resulting in what is term
18、ed an autogenous weld. If ller metal is required in a nonconsumable process, it may be fed either manually or mechanically into the molten weld pool. In this case, the nonconsumable electrode serves only to sustain the arc.Shielded Metal Arc WeldingIllustrated in Figure 1.1, shielded metal arc weldi
19、ng (SMAW) is a basic, versatile process used to weld fer- rous and some nonferrous metals. The most widely known of the arc welding processes, shielded metal arc welding is sometimes referred to colloquially as stick 4SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSESTable 1.1 Capabilities of the Comm
20、only Used Joining ProcessesMaterial ThicknessProcesses*S M A W S A W G M A W F C A W G T A W P A W E S W E G W R W F W O F W D F W F R W E B W L B W T B F B R B I B D B I R B D F B SCarbon steel S xx x x xxx x xxxxxxxxx I xxxxx xxx xxxxxxxxxxx M xxxx xxx xxxxxx x T xxxx xx x x xx x x Low-alloy steel
21、 S xxx x xxxx x xxxxxxxxx I xxxxx xx x xxxxxx x x M xxxx xx x xxxxxx x T xxxx x x x xx x x Stainless steel S xxx xx xxxx x xxxxxxxxx I xxxxxx xx x xxxxxx x x M xxxx x x x xxxxxx x T xxxx x x x xx x x Cast iron I x x x xx x x M xxxx x x xx x x T xxxx x x x Nickel and alloys S x x xx xxx x xxxxxxxxx I
22、 xxx xx xx xxxxxx x x M xxx x x xxxxx x T x x x x xx x x Aluminum and alloys S x x xx xxxxxxxxxxxxxxx I x x x xx x xxxxx x x x M x x x x xx x x x x T x x xx x x x x Titanium and alloys S x xx xx x x x xx xx I x xx x x x x x x M x xx x x x x x x T x x x x x x x x Copper and alloys S x xx xx x x xxx x
23、 x I x x x x x x x M x x x x x T x x x x Magnesium and alloys S x x x x xxx x x I x x x xxxxx x x M x xxx x x T x x Refractory alloys S x xx xx x x xxx xx I x x xx x x x x M x T* SMAW = shielded metal arc welding; SAW = submerged arc welding; GMAW = gas metal arc welding; FCAW = ux cored arc welding
24、; GTAW = gas tungsten arc welding; PAW = plasma arc welding; ESW = electroslag welding; EGW = electrogas welding; RW = resistance welding; FW = ash welding; OFW = oxyfuel gas welding; DFW = diffusion welding; FRW = friction welding; EBW = electron beam welding; LBW = laser beam welding; TB = torch b
25、razing; FB = furnace brazing; RB = resistance brazing; IB = induction brazing; DB = dip brazing; IRB = infrared brazing; DB = diffusion brazing; and S = soldering.S = sheet (up to 1/8 inch in. 3 millimeters mm); I = intermediate (1/8 in. to 1/4 in. 3 mm to 6 mm); M = medium (1/4 in. to 3/4 in. 6 mm
26、to 19 mm); T = thick (3/4 in. 19 mm and up).Commercial process.Copper requires molybdenum-coated tips. SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSES5weldingor simply arc welding. This process, which is applied without pressure, incorporates the use of a metal arc (an arc that transfers metal) whi
27、ch is formed between a covered electrode and the weld pool. The electrode consists of a wire core around which a concentric mixture of silicate binders and powdered materials such as uorides, carbonates, oxides, metal alloys, and cellulose is extruded. This covering serves as a source of arc stabili
28、zers and vapors to displace air as well as metal and slag to protect, support, and insulate the hot weld metal.55. An excellent guide to the classication of shielded metal arc weld- ing electrodes is provided in the appendices of the applicable ller metal specications. See, for example, American Wel
29、ding Society (AWS) Committee on Filler Metals, Specication for Covered Carbon Steel Electrodes for Shielded Metal Arc Welding,ANSI/AWS A5.1, Miami: American Welding Society. Generically, the qualier shielded metal arccould be used to describe a number of arc processes. However, this term is unique t
30、o the process in which shielding is achieved by means of the decomposition of the coating on the electrode as it is consumed by the heat of the arc. The bare section of the electrode is clamped into an electrode holder, which, in turn, is connected to the power source by a cable. The workpiece is co
31、nnected to the other power source terminal. The arc is initiated by touching the tip of the electrode on the workpiece and then withdrawing it slightly. The heat of the arc melts the base metal in the immediate area along with the electrodes metal core and covering. The molten base metal, the wire c
32、ore, and any metal powders in the cov- ering coalesce to form the weld. When arc welding came into existence during the lat- ter part of the nineteenth century, welding was hindered by the less-than-ideal electrical power sources availableSource:Adapted from Linnert, G. E., 1994, Welding Metallurgy,
33、 4th ed., Miami: American Welding Society, Figure 6.8.Figure 1.1Schematic Representation of Shielded Metal Arc Welding 6SURVEY OF JOINING, CUTTING, AND ALLIED PROCESSESand the fact that a bare metal rod served as the consum- able electrode. This bare metal electrode made it very difcult to initiate
34、and maintain an arc. Moreover, because the bare electrode provided no protection from the atmosphere, the resulting weld had poor properties. Coverings for welding electrodes were developed in the 1920s and have undergone improvements in terms of formulation and production ever since. These improvem
35、ents have resulted in ease of arc initiation and operation as well as excellent weld properties. The ingredients in the coatings perform a number of func- tions. They stabilize the arc, thereby rendering excellent operation performance. They produce (1) shielding vapors, which displace air; (2) deox
36、idizers and other scavengers that purify the weld; and (3) slag, which provides a physical protection or “lid” over the hot weld metal. Electrode coatings that incorporate pow- dered metal increase the deposition rate potential and enable the operator to drag the electrode lightly instead of having
37、to maintain a proper arc length manually. This reduces operator fatigue, minimizes skill require- ments, and often increases productivity. Covered welding electrodes are available in diame- ters ranging from 1/16 inch (in.) to 5/16 in. (2 milli- meters mm to 8 mm). The smaller diameters are used wit
38、h low currents for the joining of thin sections, lim- ited-access work, and welding in the vertical and over- head welding positions. The larger-diameter electrodes require higher currents, which produce higher weld deposition rates. With respect to economy, the goal is to use the electrode with the
39、 highest deposition rate and the highest current practical for the application. In order to realize the economic potential of the larger electrodes coated with metal powder, the amper- age per square inch (in.2) (square millimeter mm2) of the electrode cross-sectional area, termedcurrent density, mu
40、st be optimized. Although electrodes can function at a signicantly lower current density than the optimum setting, a low current density decreases productivity. Weld deposition rates vary directly in relation to the current density used. While smaller electrodes cannot successfully conduct high curr
41、ents, they are able, at a certain point, to main- tain high current density. A point is reached at which small electrodes sustaining high current densities yield higher deposition rates than larger electrodes with lower current density. For this reason, other welding processes that utilize a continu
42、ous wire feeder often render greater productivity. They use an electrode that is much smaller in diameter, which can sustain much higher current density than shielded metal arc welding electrodes. The application of the shielded metal arc welding process involves relatively high labor costs. This pr
43、ocess yields a deposition efciency of less than 60% based on a comparison of the weight of electrodes purchased to the weld weight obtained. This relatively low efciency is due to a number of factors. These include the discard- ing of electrode stubs when the electrode is consumed to within 2 in. to
44、 3 in. (50 mm to 75 mm) of the elec- trode holder or when a portion of the covering is knocked off. In addition, slag must be chipped from the completed weld. Compared to the wire-feed arc pro- cesses, the labor costs of the shielded metal arc process are high due to its slower deposition rates and
45、the inter- ruptions of the work required to change electrodes and remove the slag from the weld after each pass. The equipment used in shielded metal arc welding is the simplest and least expensive of that used for the electric welding processes. The necessary components are a power source of adequa
46、te current rating and duty cycle, suitably sized electrical cables, an electrode holder, and a workpiece-lead clamp. Utility-duty, single- phase alternating-current (ac) welding machines are the least expensive and can be used with small electrodes designed for ac current. Industrial-duty alternatin
47、g cur- rent/direct current (ac/dc) or dc power sources can be used with the greatest variety of electrodes. Engine- driven portable machines with a wide range of capabili- ties are marketed at various prices, depending on the polarity and power output options desired. With respect to personal protec
48、tive equipment, shielded metal arc welding operators must wear sturdy dry clothing and leather gloves for protection against spatter and electric shock. A helmet equipped with a dark lens shields the eyes from the brilliance of the arc, electromagnetic radiation, and ying slag particles. The shielde
49、d metal arc process offers several other advantages in addition to the simplicity, low cost, and portability of SMAW equipment. With this process, shops can handle many welding applications using a wide variety of electrodes. Moreover, this process per- mits welds to be made in conned spaces. For these rea- sons, shielded metal arc welding has wide application in the construction, pipeline, and maintenance industries, among others.Submerged Arc WeldingA versatile production welding process, submerged arc welding (SAW) effects the joining of me
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1