制造方法第1-2章-a2王时龙

1、1JOINING PROCESSES TYPE OF JOINTSCHAPTER 1 METAL FORMING2.1 TYPE OF JOINTSCHAPTER 2 JOINING PROCESSES1. Non-permanent joints, e.g., fasteners紧固件, dowel pins合销, cotter pins开口销, grub screws平头螺丝, bolts and nuts螺栓螺母, etc.fastenersdowel pinscotter pinsgrub screwsboltsnutsCHAPTER 2 JOINING PROCESSES2. Per

2、manent joints:a. Mechanical means机械设备, e.g., rivets铆钉, staples订书钉, shrink fits收缩配合.rivetsstaplesvShrink fit 冷压焊 cold pressure weldCHAPTER 2 JOINING PROCESSESc. Liquid state joining, e.g., fusion welding熔焊 (parent metal 母材is melted).d. By additional material, e.g., adhesives粘合剂, soldering焊料, brazing铜

3、焊, welding with filler material填充材料.fusion welding CHAPTER 2 JOINING PROCESSES3. Plastic joining of plastics will be covered in a later chapter.CHAPTER 1 METAL FORMING2.2 WELDINGCHAPTER 2 JOINING PROCESSESWELDING Joining by melting the metal or by applying pressure to the two pieces to be joined. A

4、perfect weld is one where the joint is indistinguishable. Usually, surface oxide layers表面氧化层 and adsorbed gases吸附气体 prevent perfect welding. Hypothetical假设的假设的Case Way of Achieving Welding Welding Defects缺点缺点 Advantages of welding Disadvantages of Welding Weldability可焊性可焊性 Steels Non-ferrous metals有

5、色金属有色金属非铁金属非铁金属CHAPTER 2 JOINING PROCESSESHypothetical CaseIf two piece of metal with perfectly clean surfaces are squeezed挤together in a vacuum真空, a perfect weld should be formed.If tensile forces拉力are applied to separate the parts, failure should occur outside the weld area, since the weld area ha

6、s been strengthened加强 by strain hardening加工硬化. In actual practice, impurities杂志 at the weld interface cause the latter to be weaker than the parent metal.Figure 2-1 Perfect weldingIn vacuumApply pressureCHAPTER 2 JOINING PROCESSESWay of Achieving Welding 1. Fusion welding melt the metal so that the

7、two parts to be joined flow into each other. 2. Pressure welding compress parts together either hot or cold to cause plastic deformation塑性变形. Welding can be facilitated促进 by heating the metal to make the surface oxides flow more easily and the metal more plastic. 3. Fusion welding using added filler

8、 material. This method is used in most engineering applications.Figure 2-2 Pressure weldingCHAPTER 2 JOINING PROCESSES The choice of filler rod 焊条is very important. Ideally, the parent metal and the filler rod should be of the same composition构成 and all cooled at the same slow rate for achieving a h

9、omogenous weld (as strong as the parent metal). For joining different metals, there are filler rods available that are specifically made to join particular pairs of metals.Figure 2-3 Fusion welding with filler materialWelding process; Joint design; Base and filler materialsTypes of welded joints and

10、 welds used in electroslag welding电渣焊: (a) butt joint对接, (b) T-joint丁字接头, (c); corner joint角接接头 (1) butt weld对接焊缝, (2) fillet weld, (3) weld for corner jointTypes of welded joints and welds used in resistance welding电阻焊: (a) butt joint formed by resistance welding, (b) butt joint formed by fusion we

11、lding, (c) lap joint搭接接头 formed by a single row单排 of spot welds, (d) lap joint formed by multiple rows of spot welds,(e) lap joint formed by a single seam weldCHAPTER 2 JOINING PROCESSES By Newtons law of cooling, the weld (fusion zone) cools faster because it is hotter. For steel, there are three d

12、istinct areas in the heat affected zone (HAZ): (a) Overheated zone, above 1000, large grains大晶粒(b) Annealing zone, 900-1000, refined grain structure ( stress relieved )(c) Transition zone过渡区, 700-900, where there is a range of microstructuresFigure 2-4 Heat affected zone (HAZ)If steel is welded and

13、the carbon content is more than 0.3%, the fusion zone might be brittle 脆because of the formation构成 of martensite马氏体. Heat treatment热处理 is needed to temper回火 the weld joint.CHAPTER 2 JOINING PROCESSES Welding Defects:1.Dimensional defects空间缺点a. Warpage弯曲 distortion变形 due to thermal stresses.b. Incorr

14、ect weld size and profile due to poor welding technique. 2.Structural defectsa. Porosity有孔性 due to gases released释放in the metal.b. Inclusions包含物 oxides, slag熔渣.c. Incomplete fusion 不完全焊透of parent metal, insufficient temperature, poor manipulation 操作of heat source 热源inadequate不充分 removal of oxide fil

15、ms.氧化膜d. Lack of penetration.未焊透Figure 2-5 Lack of penetrationCHAPTER 2 JOINING PROCESSES e. Cracking lack of pre-heating or post-heating.Figure 2-6 Cracks in weldf. Surface defects (holes, irregularities) due to bad workmanship.CHAPTER 2 JOINING PROCESSESAdvantages of welding :1. Quick and convenie

16、nt way to join metals of various shapes permanently and strongly.2. Light because bolts and nuts are eliminated.被淘汰 3. Easy to make a cheap prototype.原型、标准、模范4. Cheap because machining is eliminated.5. Does not interrupt stress-flow design (because only the areas near the weld suffer a change in mic

17、rostructure).Disadvantages of Welding:1. Dependent on human factors, e.g. quality of workmanship手艺 and consistency一致性.2. Surfaces must be clean for a good weld.3. Fixtures固定装置 needed to hold the parts together during welding.4. Weld defects are common.焊接缺陷CHAPTER 2 JOINING PROCESSES5. Heat treatment

18、 热处理may be needed.6. Quality control methods needed to check for defects:a. visual inspection (unreliable).目测b. radiographic inspection (X-ray).c. supersonic inspection.超声波检查d. holographic inspection.(ICT, Computerized Tomography )全息检查WeldabilityAlmost all metals are weldable. SteelsLow carbon steel

19、s are excellent for welding. Higher carbon steels and cast iron need special techniques, e.g., pre-heating预热 or post-heating焊后加热 to prevent cracking破裂 and formation of martensite.马氏体组织CHAPTER 2 JOINING PROCESSESNon-ferrous metals Such metals require special welding techniques, depending on their pro

20、perties, e.g., affinity for atmospheric gases, ease of oxidation, thermal conductivity, changes in material properties caused by heat.Note: Steels are considered unweldable if the equivalent C content, Ceq exceeds 0.5%. This is a rule of thumb employed in this region.Mn manganese , Cr chromiumNi nic

21、kel, V tungsten Cu copper Mo molybdenumCHAPTER 1 METAL FORMING2.3 GAS WELDINGCHAPTER 2 JOINING PROCESSESAdvantages:1. Equipment is cheap, portable, versatile.2. No electrical supply needed.3. Can weld thin sheets because temperatureApplications :1. When electricity is not available, e.g.new construc

22、tion sites.2. For welding thin sheets below 2mm.Figure 2-8 Welding methodCHAPTER 2 JOINING PROCESSESWelding flux Flux in the form of powder, grains or paste can be used to protect the weld joint from the atmosphere during welding.Roles of flux 1. Prevents contact of the molten metal with the atmosph

23、ere .2. Removes impurities form the molten weld deposit.3. Provides a slag blanket to decrease the cooling rate and carry off impurities.4. Stabilities the arc .5. Reduces oxides .6. Forms plasma for the arc current .7. Reduces splatter of the weld metal .8. Electrically insulates the electrode.9. A

24、dds alloying elements to the weld.CHAPTER 1 METAL FORMING2.4 ARC WEDDING CHAPTER 2 JOINING PROCESSES ARC WEDDING An electric arc is generated between electrode and workplace. The electrode is also the filler mm (filler rod). Temperatures can reach 6,000Figure 2-9 Arc weldingCHAPTER 2 JOINING PROCESS

25、ES Shielding from atmosphere may be necessary for good welds if the workplace material forms brittle oxides- use flux in the form of coated rod, powder, paste, or use neutral or reducing gas enveloping the arc. The flux will form a protective gas shield.Figure 2-10 Arc welding with gas shieldCHAPTER

26、 2 JOINING PROCESSES Generally we use metal electrodes that also cat as the filler metal. These melt and the electrode must be advanced to maintain the arc gap. There are a variety of filler rods divailable for welding various metallic combinations.Arc Stream The arc is an electrical discharge throu

27、gh a path of ionized particles called “plasma” fourth state of matter ,the other three being solid, liquid and gas.Applications of Arc Welding:1. For joining sections of 2mm thickness or more.2. For faster welding with more localized heating, greater depth of penetration than gas welding.Since it de

28、livers more that to the weld joint than gas welding does.3. For welding materials with high heat conductivity, e.g. aluminum and copper alloys.CHAPTER 1 METAL FORMING2.5 RESISTANCE WELDINGCHAPTER 2 JOINING PROCESSES RESISTANCE WELDING Electrodes press the parts together and carry electricity (usuall

29、y act to them). If electrode and work have nearly the same resistance, they tend to weld together. Low resistance metals (Cu, Ag, Al) are hard, but not impossible, to weld. For heavy usage, the electrodes must be conserved by water.Figure 2-11 Resistance weldingpress-type resistance welding systemCH

30、APTER 2 JOINING PROCESSES Electrodes press the parts together and carry electricity (usually a.c) to them. If electrode and work have nearly the same resistance, they tend to weld together. Low resistance metals (Cu, Ag, Al) are hard, but not impossible, to weld. For heavy usage, the electrodes must

31、 be conserved by water. Applications of Resistance welding: Convenient way of welding sheet metal of roughly the same thickness. Ideally, the electrodes should be of low resistance (e.g. copper which is a very good conductor) and the workpieces are of higher resistance (e.g. various steels) to conce

32、ntrate heat on the latter (P=I2R).CHAPTER 2 JOINING PROCESSES Types of Resistance Welding Spot welding Resistance welding of one spot at a time.Seam welding to form a water-tight or air-tight seal, spot welds can be overlapped to form a continuous seam.Figure 2-12 Spot weldingCHAPTER 2 JOINING PROCE

33、SSESAdvantages :1. Heat is localized.2. Fast.3. No filler metal needed.4. Easily automated for large-scale production.Disadvantages :1. Initial equipment cost can be quite high.2. May be difficult to join sheets of different thicknesses.CHAPTER 1 METAL FORMING2.6 SUBMERGED ARC WELDING (SAW)CHAPTER 2

34、 JOINING PROCESSESSUBMERGED ARC WELDING (SAW) Figure 2-13 Submerged are welding (SAW)SUBMERGED ARC WELDING (SAW) CHAPTER 2 JOINING PROCESSES Submerged Are Welding (SAW) is so called because the weld joint is submerged under a heap of flux grains and is hence shielded from the atmosphere. This has to

35、 be an automated process since the joint cannot be seen. The arc length is maintained automatically. The electrode (consumable) acts as the filler rod (base metal). There are no sparks, splutter or smoke since the weld is submerged. The high currents yield high welding speeds. Applications of SAWFor

36、 welding thick sections and sheets. Commonly used bridges and rails. Generally, not for sheets below 8 mm.CHAPTER 2 JOINING PROCESSES Advantages :1. Automatic feed of electrode and flux.2. Molten flux forms protective coating over weld, hence eye shield is not needed.3. High currents yield higher we

37、lding speeds than other welding methods.Can join thick sections with a single pass (for low C steels, nickel, non-ferrous metals and their alloys, e.g. bronze and brass). Disadvantages1. Needs lots of space and investment.2. Automation is necessary since the joint cannot be seen.CHAPTER 1 METAL FORM

38、ING2.7 METAL INERT GAS WELDING (MIG)2.8 TUNGSTEN INERT GAS WELDING（TIG）CHAPTER 2 JOINING PROCESSES METAL INERT GAS WELDING (MIG) MIG is the same as are welding but with an inert gas shield to protect the weld joint form the atmosphere. In the manual version, a welding gun is used to feed the electro

39、de (filler) and deliver the inert gas. TUNGSTEN INERT GAS WELDING（TIG）Figure 2-14 Inert gas welding (MIG)Figure 2-15Tungsten gas welding (TIG)Metal inert gas welding (MIG)CHAPTER 2 JOINING PROCESSES Also called Gas-Tungsten-Arc Welding (GTA). In TIG, no metal is deposited from the tungsten electrode

40、. A separate filler rod is need. Can weld thin to moderate sections (1-5 mm) Inert gases used. Helium (lighter than air), Argon (heavier than air), or He mixed with Ar (if a gas of the same density as is required). To cut cost, some companies use nitrogen or carbon dioxide, but they are not perfectl

41、y inert.Applications of MIG and TIG: TIG needs more spaced to maneuver than MIG because the former uses a filler rod. MIG is used if only a confined space is available for welding. Both are used for stainless steels, Mg. Al and Ti alloys. Without the inert gas shield, Mg and Al oxidize rapidly in ai

42、r to form brittle oxides. Ti will form titanium carbide, titanium oxide, titanium nitride with the elements in the air.CHAPTER 2 JOINING PROCESSES When stainless steels are welded, there are two potential: 1. Sensitization: Stainless steels are stainless because the chromium in it forms a protective

43、 surface layer of chromium oxide at room temperature (invisible to the naked eye). Between 400 to 850. the chromium forms chromium carbides with the carbon in the steel, leading to chromium depletion and leaving a steel that is no longer stainless. This problem can be alleviated by having very littl

44、e carbon in the steel, as in 316L stainless steel (the letter L means Low C). but the ultra-low carbon steel will not have much strength. 2. Formation of porous chromium oxide layer. When you are weld without inert gas pressed against a stationary metal blank. The friction generated heats up the sha

45、ft/blank interface. When the rotation stops, the metallic joint cools and gets welded together.CHAPTER 1 METAL FORMING2.9 FRICTION WELDINGCHAPTER 2 JOINING PROCESSES FRICTION WELDING Also know as Inertia Welding. A rotating shaft that is chucked in a lathe is pressed against a stationary metal blank. The friction generated heats up the shaft/blank interface. When the rotation stop, the metallic joint cools and get welded together.Figure 2-16 Friction welding CHAPTER 2 JOINING PROCESSES Advantage