热加工：27 fusion welding process

1、Joining Process and Equipment,Introduction,The principal characteristics of joining is 1. For joining process, it is easy to assemble various components into an integral structure which is relatively light and the cost of producing it is lower than other processes. 2. Some large parts can be manufac

2、tured by this process relatively easily but difficult even impossible for such some other processes as casting and forging. 3. Some joining processes are widely used for various modern industries as shown in following figures,Joining is an all-inclusive term, covering processes such as welding(焊接）,

3、Brazing（铜焊）, soldering（钎焊）, adhesive bonding（粘接）, and mechanical fastening,Introduction,Plane spherical vessel,Containers for chemical industry satellite,Why are the joining processes important and necessary ? 1. Some products are impossible to manufacture as a single piece. 2. more economical 3. ea

4、sy to maintain and repair 4. different properties may be desirable for functional purpose. 5. Transporting the product in individual components and assembling them,Introduction,Joining processes can be classified as following outline,Bolt joining,Joining,Rivet joining,Welding,Fusion welding,Solid-st

5、ate welding,Adhesive bonding,Electrode arc welding,High-energy -beam welding,consumable,nonconsumable,ultrasonic,friction,resistance,explosive,Electro-beam,Laser-beam,Mechanical fastening,brazing and soldering,凡钎铁之法-小钎用铜末，大钎则竭力挥铁锤而融合,Introduction,Fig.V.4 Example of joints that can be made by various

6、 joining processes described in following chapters,27 Fusion Welding Processes,Definition: Fusion welding is defined as melting together and coalescing(接合) materials by means of heat. The heat required for melting is usually supplied by chemical and electrical means. Filler metals may or may not be

7、used,The welded joint undergoes important metallurgical(冶金的) and physical(物理的) changes which, in turn, have a major effect on the properties(性能) and performance(特性) of the welded component or structure,Oxyfuel,Arc,High-energy-beam,Content,27.1 Oxyfuel Gas Welding (气焊) 27.2 Arc-Welding Processes (1)

8、(电弧焊): Consumable Electrode 27.3 electrodes 27.4 Arc-Welding Processes (2): Nonconsumable electrode 27.5 Thermit Welding (铝热焊) 27.6 Electro-Beam Welding (电子束焊) 27.7 Leaser-Beam Welding (激光焊) 27.8 Welding Safety,Oxyfuel,Arc,High-energy -beam,27.1 OxyFuel Gas Welding (OFW,OFW is a welding method utili

9、zing the flame of gases to melt basic and filler metals, as shown in Fig.27-1. Video 27.1 shows the process of OFW,The fuel gases used are usually oxygen(氧气) and acetylene(乙炔),although other gas are also used, such as hydrogen and methylacetylene propadiene (丙二烯,Two reactions with combustion process

10、es,This reaction occurs in the inner core of the flame and supplies about one-third of the total heat,This reaction occurs in the outer envelope of the flame and products two-third of the total heat,1. The temperature in this process can reach 3300. 2. The secondary reaction products water vapor,Not

11、ice,1. Neutral flame(中性焰): Ratio of oxygen to acetylene: 1:1-1.2:1, that is , when there is no excess oxygen. 3050 3150 :低合金、低碳钢、 不锈钢、紫铜、铝及铝合金,3. Reducing(还原性的) If the ratio is below 1,the temperature is lower (2700 3000 )，So it is proper for brazing and soldering,2.Oxidizing flame(氧化焰): If the rati

12、o is up to1.2, it is harmful for steels because of its oxidizing the steel. It is only proper to weld brass and zinc plate because of its oxidation. 3100 3300,27.1 Oxyfuel Gas Welding,The filler metals(填充金属) are used to supply additional material to the weld zone during welding. They are available a

13、s rods or wire metals or chip metals compatible(相容的) with those to be welded. Those consumable filler rods may be bare, or they may be coated with flux(焊剂). The purposes of the flux: (1) Retard (避免)oxidation of the surfaces of the part; (2) Help to dissolve and remove oxides and other substances; (3

14、) The slag（熔渣） protects the molten puddle of metal against oxidation as it cools,27.1 OFWThe equipment for OFW,The oxygen cylinder, which is painted blue or green, is used to reserve the high-pressure oxyfuel, the pressure of which is up to 15 Mpa. The acetylene cylinder is filled with sponge reserv

15、ing acetylene, and it is white or red,Fig.27-3 The two containers for reserving fuel gases To ensure correct connecting, all threads on acetylene fittings are left-handed, whereas those oxygen are right-handed,27.1 OFWThe equipment for OFW,27.1 OFW-the backfire(回火） phenomenon,Fig.27.4 The principle

16、graph of water-proof backfire safety,27.1 OFW-the backfire problem,What is the backfire phenomenon? The flame will burn backward acetylene bottle along the acetylene pipe ,and resulting in exploding because the spraying speed of mixed gas is lower than that of its burning. The reasons which cause th

17、is phenomenon are mainly following two aspects : (1) The acetylene following in the pipe is blocked because it is too thin, too long and too much twist; because the solid carbon grain is adhered on the inner surfaces of pipe and welding tube tools and the welding tip is blocked resulting in the bloc

18、king following of gas. (2) the working time or the distance between the welding pipe and workpiece is too long,27.1 OFW-welding torch,Fig.27-5 The schematic illustration of welding torch The welding torch is a tool that mix oxygen and acetylene at a ratio, such as 1:1 and so on, and spouts the mixtu

19、re and ignites it,The basic steps,Prepare the edges to be joined, and established and maintain their proper position,Open the acetylene valve, and ignite the gas,Hold the torch at about 45 , and the filler rod at about 30-40,Open the oxygen valve, and adjust the flame,Touch the filler rod to the joi

20、nt and control its movement along the joint length,27.1 OFW-advantages and disadvantages,Advantages of OFW: 1. In his process, the heat for melting metal is generated by chemical reaction, so no electricity is required and it is a desirable choice at the workplace where there is no electricity accom

21、modated. 2. The cost of the equipment is low, less than 500. 3. The equipment is portable, versatile, and economical for simple and low-quantity work,27.1 OFW-advantages and disadvantages,Disadvantages: 1.The productivity is slow because of manual operation. 2. The skills of operator are required hi

22、gher relatively. 3. The temperature of flame is low relatively, and the quantity of heat disperses. 4. The range heated of materials welded is wide, and so the distortion of materials are severe. 5. The relatively low heat input limits the process in practice to thickness of less than 6 mm,27.2 Arc-

23、Welding Processes,INSTRUCTION The arc-welding（电弧焊） processes : consumable/ nonconsumable- electrode. In arc welding processes, the heat required is obtained through electrical energy. The arc occurs between the tip of the electrode and the workpiece to be welded. The temperature of the arc is in the

24、 range of 30,000 , which is much higher than those developed in OFW,27.2 Arc-Welding Processes:Consumable-Electrode,Consumable-Electrode arc-welding processes : 1. Shielded Metal Arc-Welding (SMAW, 手工电弧焊, 金属保护电弧焊) 2. Submerged Arc Welding( SAW, 埋弧焊) 3. Gas Metal Arc Welding (GMAW, 气体保护焊) 4. Flux-Cor

25、ed Arc Welding (FCAW, 药心焊条电弧焊) 5. ElectroGas Welding (EGW, 气电焊) 6. ElectroSlag Welding (ESW, 电渣焊,27.2.1 Shielded Metal Arc-Welding,It is an oldest, simplest, and most versatile welding process. About 50 percent of all large-scale industrial welding operations is performed by this process. The electr

26、ic arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance sufficient to maintain the arc,The weld (焊缝,The base metal (workpiece) (基体金属,The electrode metal (焊条金属,The substances from the coating (来自药皮上的物质,27.2.1 Shielded Metal Arc

27、-Welding,Fig.27-7 Shielded metal are welding,The electrode coating deoxidizes the weld area and provides a shielding gas to protect it from oxygen in the environment,1)研究表明，阴极和阳极产生的热量相近，但由于阴极发射电子消耗的能量较多，故其温度比阳极低一些。 阴极温度为22003500K， 阳极温度为24004200K,2)离子和电子对电极的撞击力电弧焊时，阴极受到正离子的撞击，阳极受到电子的撞击，因正离子的质量远远大于电子的

28、质量，而阴极压降又大于阳极压降，因此撞击力在阴极上较大，而在阳极上较小,阳极区,阴极区,In AC method, the arc pulsates(脉冲) rapidly; This is suitable for welding thick sections and for using large-diameter electrode at maximum currents,The current used may be DC (direct current, 直流) or AC (alternating current,交流,In DC process, the polarity of

29、electrode may be positive or negative. For straight polarity(正极性), the workpiece is positive and electrode negative. For reverse polarity(反极性), the workpiece is connected to negative polarity of power,27.2.1 Advantages of SMAW,Advantages: 1. Relatively simple and versatile. 2. Requiring a relatively

30、 small variety of the electrode 3. The total cost of equipment is typically below 1500. 4. The range of using is wide including construction, shipbuilding, and pipelines, as well as maintenance work. 5. The workpiece thickness welded is commonly 3-19mm, although this range can be easily extended usi

31、ng multiple pass techniques. (see textbook p.781) 6. It is best suited to remote areas,27.2.1 Disadvantages of SMAW,Disadvantages: 1.The productivity is relatively low; 2. The labor costs and material costs are relatively high. 3. The heat sources, the sparks, spatter, and infrared(红外线的) and ultravi

32、olet(紫外线的) radiation is hazarding and even fatality for workers,27.3 Electrodes,Fig.27-12 the structure of electrode Typical coated electrode dimensions are 150 mm-460 mm in length and 1.5 mm 8 mm in diameter,Electrodes are classified,The current (AC or DC,The strength of the deposited weld metal,Th

33、e type of coating,As the thickness of the sections to be welded decreases, the required current and electrode diameter also decrease,27.3 Electrodes,Designations for electrodes The prefix “E” designates are welding electrode. The first two digits of four-digit numbers and the first three digits of f

34、ive numbers indicate minimum tensile strength: E60XX 60,000 psi minimum tensile strength E70XX 70,000 psi minimum tensile strength E110XX 110,000 psi minimum tensile strength The next-to-last digit indicate position: Exx1x all position Exx2x Flat position and horizontal fillets,The typical electrode

35、 consists of wire（焊丝） and coating flux（焊剂）. The coating materials include silicate (硅酸盐） binders and powdered materials such as oxides, carbonates （碳酸盐）, fluorides （氟化物）, and so on. The wire has two following functions: 1. To act as conductor for electricity 2. To supply filler metal for welding poo

36、l,The specifications state : The wire diameter must not vary more than 0.05mm from nominal size; (b) The coatings must be concentric (同轴的） with the wire,焊丝和焊剂相关知识: 1、气焊中焊丝一般不用药皮, 其成分与基体基本相同,原则上要求焊缝与工件达到相等强度; 2、 焊接合金钢、铸铁和有色金属时，熔池中容易产生高熔点的稳定氧化物，如Cr2O3、SiO2 和Al2O3等，使焊缝中夹渣，故在焊接时使用适当焊剂，可与这类氧化物结成低熔点熔渣，以利于

37、浮出溶池； 3、因为金属氧化物多呈碱性，所以一般都用酸性焊剂，如硼砂、硼酸等； 4、焊铸铁时，往往有较多的SiO2出现，因此通常又会采用碱性焊剂，如碳酸钠和碳酸钾等，使用时通常用焊丝蘸在端部送入熔池。 5、焊低碳钢时，只要接头干净，不必使用焊剂,27.2.2 Submerged Arc Welding(SAW，埋弧焊,27.2.2 Submerged Arc Welding(SAW,Fig.27-8 The schematic illustration of submerged arc welding,The molten metal is completely covered by a th

38、ick layer of flux(焊剂）（lime, silica, manganese oxide, calcium fluoride, and other compounds). Flux: 1) prevents（防止） spatter(飞溅） and sparks（火花） and suppresses the intense ultraviolet radiation and fumes. 2) acts as a thermal insulator（绝热层） promoting a deep penetration of workpiece. Remaining fluxes ar

39、e recycled and reused,For welder, the gloves are necessary, but tinted safety glasses and face shields not,埋弧机,焊接参数对焊缝成形的影响,焊接电流、电弧电压、焊接速度、电源种类及其特性 1、焊接电流 焊接电流对熔透深度影响最大，与其几乎成比例关系。电流过大：烧穿和焊缝裂纹；电流过小：电弧不稳定、未焊透、未熔合。 2、电弧电压 电弧电压与电弧长度成正比关系。随电弧电压的提高，焊缝的宽度明显增大，而熔深和余高则略有减小。电压过高：浅而窄的焊道、未焊透和咬边，焊波表面粗糙，脱渣困难；电压过低

40、：高而窄的焊道，边缘熔合不好。 3、 焊接速度 速度过大：咬边和气孔，焊道外形恶化。 4、焊接电源及其特性 直流比交流能更好地控制焊道形状、熔深，且引弧容易。 直流反接（工件接负极）：可获得最大熔深和最佳的焊缝表面。 直流正接（正件接正极）：电弧最大热量集中于焊丝端部，焊丝熔化速度高，焊缝余高增加。适用于浅熔深焊接或表面堆焊。 5、其它：焊丝伸出长度、焊剂粒度和堆散高度、焊丝倾角和偏移量等,埋弧焊缺陷形成原因,1、热裂纹（冶金因素） （1）结晶裂纹：焊接熔池初次结晶过程中形成的裂纹，是一种沿焊缝金属初次结晶晶界的开裂。形成条件： 存在一定数量的低熔点共晶体； 焊缝金属的结晶方式使低熔点共晶体被

41、封闭在柱状晶之间； 焊缝金属在结晶过程中受到足够大的应变作用。 （2）液化裂纹：是紧靠熔合线母材晶界被焊接热局部重熔，并在焊接收缩拉应力作用下产生的,2、冷裂纹（部分冶金因素） 冷裂纹总是在焊接接头冷却到200以下或在室温下延迟一段时间后形成，亦称延迟裂纹。 主要在屈服点大于300MPa的低合金钢和中合金钢中产生。强度愈高，或提高淬硬倾向的合金成分含量愈高，则焊接冷裂纹形成的机遇愈大。 主要由以下因素： （1）焊接区淬硬程度，即马氏体组分在金相组织所占的比率；（2）焊缝金属中的扩散氢含量；（3）焊接接头中残余应力的分布和高低，还包括相变应力。 3、层状撕裂的形成原因（部分冶金因素） 主要以钢板

42、轧制方向分布的非金属夹杂物为裂源，并在钢板厚度方向的焊接拘束拉应力作用下形成。 倾向性取决于: (1）钢中非金属夹杂物MnS、Al2O3和硅酸盐的数量、形状及其分布形式。（2）接头的拘束度。（3）焊接热影响区的热应变时效脆变和氢脆,4、再热裂纹的形成原因（冶金因素） 再热裂纹主要出现于以沉淀硬化机制强化的低合金耐热钢和高铬镍热强钢厚壁接头中。 解释如下：当含有Cr、Mo、V等合金元素的低合金钢埋弧焊时，紧靠熔合线的区域被加热到1000-1350的高温，促使该区的合金碳化物完全固溶，同时奥氏体晶粒急剧长大。当这种焊接接头在500-680的温度范围内热处理时，合金碳化物逐渐从晶体内部沉淀，并使之强

43、化。这种强化不仅阻碍了晶粒本身的整体变形，而且也抑制了晶粒的局部应变。这种，在焊件在热处理过程中产生的蠕变应变就集中于晶界。如蠕变应变超过了晶界的变形能力，即导致晶界开裂，并最终形成宏观裂纹。 5、 气孔形成的原因（冶金因素） 有两种形式： （1）物理溶解：A、母材、焊丝材料中含过量的气体，在重熔时析出而形成气孔；B、焊剂突然中断，电弧暴露，产生氮气孔。 （2）化学反应：A、未烘干的焊剂，其中的水分分解而形成氢气孔；B、焊丝和母材中的碳，与焊件接缝区表面的铁锈、氧化皮反应形成CO气孔,6、未焊锈和未熔合 这两种均是由于焊接参数选择不当和操作不当而引起的缺陷，不象裂纹和气孔之类缺陷与很多冶金因素

44、有关。 原因： （1）焊丝偏离中心；（2）焊接电缆连接焊件位置不当引起电弧偏吹；（3）焊接电流或电压不当；（4）焊件位置下倾角度太大或环缝焊时焊丝位置超前；（5）V形坡焊时，焊丝倾角调整不当。 7、夹渣形成原因 是残留在焊道之间或焊缝与坡口侧壁之间的溶渣，是一处宏观缺陷。与焊缝金属中的非金属夹杂物不同，主要是在多道焊缝中形成。 8、咬边、焊瘤、溢流,27.2.2 Advantages and disadvantages(SAW,Advantages: 1.this process can be automated, so it is economical and the productivit

45、y is high. It can deposits 4-10 times the amount weld metal per hour as the SMAW process. 2. It is used to weld a variety of carbon and alloy steel and stainless steel sheet or plate. 3. The quality of welding is very high due to thick layer of flux protecting the welding zone well. 4.the current us

46、ed may be large(300A-2000A) and heat generated is sufficient, so the penetration of weld is deep and , for thick workpiece even, the sloping（开坡口） is no necessary,Disadvantages: 1. This process is largely limited to welds in a flat or horizontal position because the flux is fed by gravity, although c

47、ircular welding can be made on a pipe, provided that it are rotated during welding (Fig.27-9). 2. Because the arc is invisible , it is difficult to find out the problems during welding 3.The request for the precision of processing and assembling is high. Preparing time before welding is relatively l

48、ong. 另外，只能焊厚板，不能焊薄板；只能焊长焊缝，不适于焊短缝；不能焊活泼金属,27.2.2 Advantages and disadvantages(SAW,Fig.27-9 the circular welding process on a cylinder pressure vessel,27.2.3 Gas Metal-Arc Welding（气体保护焊,In gas metal-arc welding (GMAW), developed in the 1950s and formerly called Metal Inert-Gas (MIG) welding, the weld

49、 area is shielded by an effectively inert atmosphere of argon, helium, carbon dioxide, or various other gas mixtures,Figure 27.9 Basic equipment used in gas metal-arc welding operations. Source: American Welding Society,气体保护焊与焊条电弧焊有很大不同: 焊条电弧焊是通过焊条药皮中稳弧剂来提高电弧的稳定性并通过药皮过渡的合金元素，使焊缝金属获得所要求的物理及力学性能。 气体保护

50、焊是通过保护气体的合理选择及电源特性的相应控制，来实现稳定的电弧过程；同时，通过焊丝或填充丝成分合理选择，使焊缝金属获得所要求的物理及力学性能,气体保护焊是通过电极（焊丝或钨极）与母材间产生的电弧，熔化焊丝及母材，形成焊缝金属。电极、电弧及焊接熔池是靠从焊枪喷嘴喷出的保护气体来保护,Metal Inert Gas Arc Welding, MIG（熔化极惰性气体保护焊）: The inert gases, such as carbon dioxide, argon, are utilized to protect the welded metal from oxidization. Metal

51、 Active Gas Arc Welding, MAG（熔化极混合气体保护焊） The bare electrode, which is consumable, and inert gas is transferred from the welding gun,CO2 CO+1/2 O-Q Metal can be transferred by three methods in the GMAW process: 1. The spray transfer（射流过渡） 2. The globular transfer（射滴过渡） 3. The short-circuiting transfe

52、r (短路过渡,当CO2气体保护焊时,Fe+CO2 FeO+CO (enter into atmosphere) Fe+O FeO (enter into the slag) C+O CO (enter into atmosphere) Mn+O MnO （Mn为脱氧剂） Si+O SiO2 （Si为脱氧剂） FeO+C Fe+Co (a section of which inters into atmosphere, however, other section melts into welding bead and forms pore of CO ) 2FeO+Si 2Fe+SiO2 (

53、entering into the slag) FeO+Mn Fe+MnO (entering into the slag,The zone of high temperature,The zone of low temperature,Notice: (1)FeO应除去，用脱氧剂Si、Mn (2) 脱氧剂常采用Si、Mn联合脱氧,气孔,N,H2,CO,原因：脱氧元素不足，产生过多FeO， 被C还原生成CO,措施：保证足够的脱氧元素Si、Mn,原因：高温熔池溶入大量H2（来自焊丝、工件表 面油污、铁锈及CO2气体中含的水分,措施：CO2具有较强的氧化性，生成不溶入熔池 的羟基，因此H2气孔生成

54、的概率较低,原因：CO2气体保护效果不好或不纯，空气中 的N侵入焊区；它出现的概率最大,措施：加强CO2气体保护和控制CO2纯度。造成 保护效果不好的原因是过小的气体流量、喷嘴被 堵塞、喷嘴与工件距离过大、电弧电压过高、电 弧不稳定或作业区有风等,飞溅 问题,由冶金反应引起，主要是CO 生成并受热膨胀,原因,措施,选用碳含量低的焊丝和合适的 保护气体（如CO2+Ar时，使 Ar含量大于60,作用在焊丝末端电极斑点压力 引起飞溅：正极时较明显，反 极性较小,合理选择焊接电源特性和焊 接工艺参数,采用直流反极性，若用正极性， 应采用活化焊丝,熔滴不正常或焊接工艺参数 不当引起的飞溅,Fig.27-

55、10 The schematic illustration of carbon dioxide Metal-Arc Welding Video 27.4 shows the carbon dioxide Metal-Arc Welding process,27.2.3 The advantages of GMAW,1.This process is suitable for welding a variety of ferrous and nonferrous metals and is used extensively in the metal-fabrication industry. 2

56、.The training of operators is easy because of the relatively simple nature of the process. 3.The process is versatile, rapid, and economical, welding productivity is double that of the SMAW process. 4.The process is suitable for robotized production,优点： （1）高效、节能、节材 （2）连续送进焊丝，易实现自动化 （3）没有焊剂覆盖，便于观察监控

57、（4）与埋弧焊和焊条电弧焊相比，更适合于薄板的焊接 （5）在空间位置焊接作业中，具有独到优势,熔化极气体保护焊应用较广,与非熔化极气体保护焊相比: （1）更适合于较厚工件的焊接，生产效率较高； （2）更适合于自动化焊接。 焊黑色金属：CO2焊或MAG焊 焊有色金属：MIG焊,The disadvantages of GMAW,缺点： （1）由于要另外通保护气体，因此这种工艺不是太方便,在窄小空间及可达性差的地方，不如焊条电弧焊灵便； （2）保护气流的抗风能力有限，如因风力干扰较大，需采取相应措施； （3）焊丝材料范围不广，难以满足某些特殊需要,采用实心焊丝时，向焊缝中过渡合金元素时只能通过焊丝

58、或填丝，不象焊条电弧焊或埋弧焊可通过调整药皮或焊剂的成分而方便实现,27.2.4 Flux-Cored Arc Welding（FCAW）(药芯电弧焊,This process is similar to gas metal-arc welding, with exception that the electrode is tubular in shape and is filled with flux. Cored electrodes produces a more stable arc(电弧更稳定), improve weld contour(形状更好), and produce bet

59、ter mechanical properties (力学性能更好)of the weld. These tubular electrodes are flexible, so they can be provided in long coiled lengths. In this process, the external inert gases may be not needed, however, in GMAW process which is necessary, because those tubular electrodes contain emissive fluxes tha

60、t possess self-shielding feature,Figure 27.10 Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding, showing in Fig. 27.8,药芯焊条,27.2.4 Flux-Cored Arc Welding,Small-diameter electrodes have made the welding of thinner(更薄) materials not only po