1、Metal fabrication processes（金屬制造工藝）（金屬制造工藝）Lesson 13Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication ProcessesJoining two or more elements to make a single part is termed as a fabrication process. A fairly large number of industrial components are made by fabrication process :aircraft（飛機(jī)），（

2、飛機(jī)），ship bodies（船（船體）體）,bridges（橋梁）（橋梁）,building trusses（建筑桁架）（建筑桁架）,welded machine frames（焊機(jī)架），（焊機(jī)架），sheet metal parts（沖件，鈑金件）（沖件，鈑金件）(secondary manufacturing process)1. Classification mechanical joining by means of bolts（螺栓）（螺栓） ,screws（螺釘）（螺釘） and rivets（鉚釘）（鉚釘）.Adhesive bonding by employing synt

3、hetic glues（合成膠）（合成膠） such as epoxy resins（環(huán)氧樹(shù)脂）（環(huán)氧樹(shù)脂）. welding ,brazing（硬釬焊）（硬釬焊） and soldering（軟釬焊）（軟釬焊）Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication ProcessesThe choice of a particular fabrication method depends on a number of factors Type of assembly（連接的類型）（連接的類型）: permanent（永久）（永久）:

4、welding semi-permanent（半永久）：（半永久）： rivetstemporary（臨時(shí)）（臨時(shí)）: bolts 、screws Materials being joined（連接材料）（連接材料）: steel cast irons aluminum dissimilar metals（異種金屬）（異種金屬） Economy achieved and the type of service required（經(jīng)濟(jì)性及服役要求）：（經(jīng)濟(jì)性及服役要求）：Heavy loading（重載）（重載） impact loading（沖擊載荷）（沖擊載荷） high temperatu

5、re（高溫）（高溫）Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication Processes2. Welding processes Welding processes Arc welding Oxyfuel gas welding Resistance weldingSoldering brazing Solid state welding ClassificationOther weldingElectric arc welding（電弧焊）（電弧焊）Introduction to Fabrication Processes T

6、ypes of joints 對(duì)接對(duì)接搭接搭接角接角接T形接頭形接頭邊緣連接邊緣連接Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication Processes welding positions （焊接位置）（焊接位置）平焊平焊立焊立焊橫焊橫焊仰焊仰焊Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication Processes Joint bevel preparation（坡口準(zhǔn)備）（坡口準(zhǔn)備）Electric arc welding（電弧焊）（電弧焊）Introductio

7、n to Fabrication Processes welding terms Backing :It is the material support provided at the root side of a weld to aid in the control of penetration. Base metal:The metal to be joined or cut is termed as the base metal. Bead or weld bead :The bead appears as a separate material from the base metal.

8、 Crater ：In arc welding, a crater is the depression in the weld metal pool at the point where the arc strikes the base metal plate. Fillet weld：The metal fused into the corner of a joint made of two pieces placed at approximately 90 to each other. （襯墊）（襯墊）（母材）（母材）（焊縫）（焊縫）（弧坑）（弧坑）（角焊縫）（角焊縫）Electric a

9、rc welding（電弧焊）（電弧焊）Introduction to Fabrication ProcessesPenetration： It is the depth up to which the weld metal combines with the base metal as measured from the top surface of the joint. Puddle： The portion of the weld joint that melted by the heat of welding . Root：it is the point at which the tw

10、o pieces to be joined by welding are nearest .Tack weld：A small weld, generally used to temporarily hold the two pieces together during actual welding.Toe of weld：It is the junction between the weld face and the base metal. Torch ：In gas welding, the torch mixes the fuel and oxygen and controls its

11、delivery to get the desire flame. （熔深）（熔深） （熔池）（熔池）（焊跟）（焊跟）（定位焊）（定位焊）（焊趾）（焊趾）（焊炬（焊炬)Electric arc welding（電弧焊）（電弧焊）Introduction to Fabrication ProcessesElectric arc welding（電弧焊）（電弧焊）What is electric arc welding?The electric arc welding process makes use of the heat produced by the electric arc to fus

12、ion weld metallic pieces.1. Principle of arcAn arc is generated between two conductors of electricity, cathode （陰極）（陰極）and anode （陽(yáng)極）（陽(yáng)極）(considering direct current. DC).An arc is a sustained electric discharge through the ionised gas column called plasma（等離子體）（等離子體） between the two electrodes.（arc

13、column 弧柱）弧柱） In order to produce the arc, the potential difference between the two electrodes (voltage) should be sufficient to allow them to move across the air gap. The larger air gap requires higher potential differ. Electric arc welding（電弧焊）（電弧焊）2. Arc welding equipmentAlternating current (AC)

14、machines（交流焊機(jī)）（交流焊機(jī)）Direct current (DC) machines （直流焊機(jī)）（直流焊機(jī)） source of electric power Though DC arc welding is more expensive than AC welding, it is generally preferred because of the control heat input（熱輸入）（熱輸入） offered by it .about 70% of the heat is liberated near the anode in DC arc welding.If

15、more heat is required at the workpiece side, such as for thicker sheets or for the work materials which have higher thermal conductivity such as aluminium and copper. the workpiece can be made as anode, liberating large heat near it. Electric arc welding（電弧焊）（電弧焊）直流正接直流正接Straight polarity gives rise

16、 to a higher penetration .直流反接直流反接The polarity could he reversed by making the workpiece as negative. In reversed polarity ,the penetration is small. In the case of AC welding, the bead obtained is some- where in between the above two types. It can maintain a stable arc. Electric arc welding（電弧焊）（電弧

17、焊）based on the characteristics The constant current welding machines （恒電流）（恒電流）The constant voltage welding machines （恒電壓）（恒電壓）open circuit voltage is set at 50 volts, it can be seen that for a large change in output voltage, the corresponding change in current is so small any small change in voltag

18、e makes for an extremely large change in the output currents. Electric arc welding（電弧焊）（電弧焊）3. Electrodes（電極）（電極）The electrodes used for providing heat input in arc welding are of two types, the consumable（自耗電極）（自耗電極） and the non-consumable electrodes（非自耗電極）（非自耗電極）.Consumable electrodes are made of

19、various materials depending on the purpose and chemical composition of the metals to be welded. Thus, they may he made of steel, cast iron. copper. brass, bronze or aluminium. Non-consumable electrodes made of carbon, graphite or tungsten. The carbon and graphite electrodes are used only in DC weldi

20、ng, whereas tungsten electrodes are used for both AC and DC welding. Electric arc welding（電弧焊）（電弧焊）Consumable electrodes, used in welding, can be either bare or coated. Electric arc welding（電弧焊）（電弧焊）1. shields the molten metal pool and protects it from the atmospheric oxygen，hydrogen and nitrogen.2.

21、 The coatings forms a slag. The slag being lighter, floats on the top of the puddle and protects it against the surrounding air . 3. Some elements that arc required for stabilisation of the arc are also added in these coatings. 4. Special alloying elements can be introduced through these coatings to

22、 improve the strength and physical properties of the weld metal. 5. Since the electrode coating is consumed at a slower rate compared to the core , this would increase the available heat for melting the electrode and thus help in improving the metal deposition rate. The coating on the electrodes ser

23、ve a number of purposes ：金屬熔池金屬熔池熔渣熔渣合金化合金化元素元素熔敷率熔敷率Electric arc welding（電弧焊）（電弧焊）6. The coatings are normally insulators of electricity and thus, permit the electrode to be used in narrow grooves. 7. It is possible to include iron powder in the coating in large amounts so that the electrode can be

24、 kept in contact with the workpiece. which may he necessary for welding in overhead and other positions. Also this increase the penetration and metal deposition rate. 8. The coatings also contain materials which can control the slag to be viscous （粘性）（粘性）The stick electrodes are normally available i

25、n diameters 3.2, 4, 5, 6, 8 and 9 mm and the length is 350 or 450 mm .絕緣體絕緣體坡口坡口鐵粉鐵粉Electric arc welding（電弧焊）（電弧焊）4. Manual Metal Arc Welding （手工電弧焊）（手工電弧焊）The manual metal arc welding also called the shielded metal arc welding (SMAW) is the most extensively used manual welding process. Advantages:

26、the equipment is least expensive Welds by this process can be made in any position Any thickness can be welded Disadvantages : slow speed AC or DC power source, current usage may vary from 50 to 500 A with voltages from 20 to 40 V. Electric arc welding（電弧焊）（電弧焊）The size of the electrode to be chosen

27、 (based on the thickness of the plate and the weld position)= flat welding （平焊）（平焊）立焊立焊橫焊橫焊Down handVerticalHorizontalElectric arc welding（電弧焊）（電弧焊） Electrodes types High cellulose type（纖維素型）（纖維素型） of electrode coating gives rise to a gas shield but also burns off quickly. (deep penetration weld fin

28、ish coarser ) ； When the content of titania （二氧化鈦）（二氧化鈦）(as rutile（金紅石）（金紅石） titanium-white （鈦白粉）（鈦白粉）or ilmenite（鈦（鈦鐵礦）鐵礦）) is high. (smooth arc, used for the flat position , weld finish is smooth ). Iron oxide when present in the coatings gives rise to higher metal deposition rates. Other material

29、s: manganese oxide, calcium carbonate （碳酸鈣）（碳酸鈣）and fluoride（氟化物）（氟化物）.Bare electrodesThe covered electrodesElectric arc welding（電弧焊）（電弧焊） The parameters of the welding Electrode diameter(mm)Current(A)Voltage(V)3.24568100 to150140 to 200200 to 220275 to 350375 to 47518 to 2220 to 2421 to 2523 to 272

30、4 to 28Electric arc welding（電弧焊）（電弧焊）The type of edge preparations required for butt weldsElectric arc welding（電弧焊）（電弧焊） Start the arc（引弧）（引?。㏕he scratching method: The electrode is moved in an arc so that it will scratch the work metal and thus establish the current flow. Tapping start: The electro

31、de is held vertically above the point where the welding is to start.Electric arc welding（電弧焊）（電弧焊）Movement of the electrode:After establishing the correct arc length, the welder should move the electrode along the length of the joint maintaining the arc, at the same time it should be moved side ways

32、 in a weaving motion to maintain the bead width（焊縫寬度）（焊縫寬度）. Electric arc welding（電弧焊）（電弧焊）Multipass welding The brittle slag coating present on the head after the root pass（根部焊道）（根部焊道） is made, is chipped off and then the area is cleaned with a wire brush, before the second pass is commenced. Same

33、procedure is followed for all the subsequent passes. Electric arc welding（電弧焊）（電弧焊）Horizontal positionThe welding becomes somewhat difficult because the molten metal cannot he held in position against gravity. Short arc length is maintained so that filler metal will he properly deposited in the groove. Electric arc welding（電弧焊）（電弧焊）The major problems in the horizontal welding are the undercutting（下陷）（下陷） and sagging（下垂）（下垂） of the weld bead. Electric arc welding（電弧焊）（電弧焊）There are two positions in the vertical vertically upward vertically d