Presentation4 BME unit-4 part-2.pptxmechanical

mechanical

1. Unit 4 Part-2 Secondary Manufacturing Process: Welding

2. Fabrication Processes These are secondary manufacturing processes where the starting raw materials are processed by any of the previous manufacturing processes described. It essentially involves joining pieces either permanently or temporarily to perform the necessary function. Welding Welding is an operation whereby two or more parts are united (joined) by means of heat or pressure or both. It is usually used on metals and thermoplastics but can also be used on wood.

3. Classification of welding processes 1. Gas welding 2. Arc welding 3. Resistance welding 4. Solid-state welding 5. Thermo-chemical welding 6. Radiant energy welding Gas welding: - Oxy-acetylene welding - Air acetylene welding - Oxy-hydrogen welding - Pressure gas welding Arc welding: Carbon arc Flux cored arc TIG or GTAW Plasma arc Electro slag & electro gas Shielded metal arc Submerged arc MIG or GMAW Stud arc Resistance welding: Spot Seam Projection Resistance butt Flash butt Percussion HF resistance

4. Solid state welding: Cold welding Explosive welding Friction welding Roll welding Diffusion welding Forge welding Hot-pressure welding Ultrasonic welding Thermo-chemical: Thermit welding Atomic hydrogen welding Radiant energy welding: Electron beam welding Laser beam welding Allied process: Brazing Soldering

5. Common Welding techniques used MIG - Gas Metal Arc Welding (GMAW) Electron Beam Welding (EBW) TIG - Gas Tungsten Arc Welding (GTAW) Stick - Shielded Metal Arc Welding (SMAW) Submerged Arc Welding (SAW) Laser Beam Welding (EBW) Plasma Arc Welding (PAW) Atomic Hydrogen Welding (AHW) Friction welding Resistance welding

6. Types of welded joints The choice of the type of joint depends on the weldment being made and the sheet thickness.

7. Welded joint terminology

8. Electric Arc welding Electric Arc welding is a type of welding process using an electric arc to create heat to melt and join metals. A power supply (using either direct current (DC) or alternating current (AC)) generates an electric arc between a consumable or non-consumable electrode and the base material. An electric arc creates an intense heat of around 6500°F which melts the metal at the interface between two workpieces . The arc can be either manually or mechanically guided along the line of the interface of two materials, while the electrode either simply carry the current or conduct the current and melt into the weld pool at the same time to supply filler metal to the joint.

9. Different Types of Arc Welding Consumable Electrode Methods Gas Metal Arc Welding (GMAW) Shielded Metal Arc Welding (SMAW) Flux Cored Arc Welding (FCAW) Submerged Arc Welding (SAW) Electro-Slag Welding (ESW) Non-Consumable Electrode Methods Gas Tungsten Arc Welding (GTAW) Plasma Arc Welding (PAW)

10.  Gas metal arc welding (GMAW) Gas metal arc welding (GMAW) uses a continuous solid wire electrode which is heated and fed into the weld pool from a welding gun. The two base materials are melted together which causes them to join. The welding gun also feeds an inert shielding gas alongside the wire electrode, which helps protect the process from airborne contaminants.

11.  Process feature of Gas metal arc welding (GMAW) GMAW is a versatile technique suitable for both thin sheet and thick section components. An arc is struck between the end of a wire electrode and the workpiece , melting both of them to form a weld pool. The wire serves as both heat source (via the arc at the wire tip) and filler metal for the joint. The wire is fed through a copper contact tube (contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire. The wire is fed from a reel by a motor drive, and the welder moves the welding torch along the joint line. Wires may be solid (simple drawn wires), or cored (composites formed from a metal sheath with a powdered flux or metal filling). The process offers high productivity, as the wire is continuously fed.

12. Gas Tungsten Arc Welding (GTAW) is an arc welding process that produces the weld with a non-consumable tungsten electrode. Gas Tungsten Arc Welding (GTAW) The arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. When filler metal is required, it must be added separately to the weldpool .

13. Gas welding The gas welding is particularly suitable for joining metal sheets and plates having thickness of 2 to 50 mm. An additional metal called filler material is used for thickness more than 15 mm. This filler metal is used in the form of welding rod. The composition of filler rod is usually same as that of base metal. The filler metal is used to fill up the cavity made during edge preparation. A flux material is also used during welding to remove impurities and oxides present on the metal surfaces to be joined. Different combinations of gases are used to produce hot gas flame, e.g., Oxygen and acetylene, oxygen and hydrogen, oxygen and propane, air and acetylene etc. Gas Welding uses fuel gas (or liquid fuels such as gasoline) and oxygen to weld materials. 

14. Oxyacetylene gas welding set up

15. Schematic of Oxyacetylene gas welding torch and welding process

16. Process features of Oxyacetylene gas welding Oxyacetylene welding relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a relatively hot flame is produced with a temperature of about 3,200⁰C. The chemical action of the oxyacetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene. Three distinct flame settings are used: neutral, oxidising and carburising . Welding is generally carried out using the neutral flame setting which has equal quantities of oxygen and acetylene. The oxidising flame is obtained by increasing just the oxygen flow rate while the carburising flame is achieved by increasing acetylene flow in relation to oxygen flow.

17. Parts of the Flame Parts of the flame are based on the temperature zones. - Inner cone - Inner reducing cone (in case of carburizing flame) Outer zone or envelope The greatest amount of heat of produced just ahead of the inner cone.

18. Types of the Flame

19. Neutral flame: -Neutral flame has two definite zones. -Sharp brilliant cone extending a short distance from the torch tip. -Outer envelope–bluish in colour . -Inner cone develops heat and the outer envelope protects the molten metal from oxidation. -Neutral flame is used for welding steel, SS, Cast iron, Cu, Al, etc. Types of the Flame

20. Carburizing flame/reducing flame: -Percentage of acetylene is more. -Sharply defined inner cone. -Intermediate cone of whitish colour (feather). -Bluish outer cone. -The length of the intermediate cone is an indication of the proportion of excess acetylene in flame. - While welding steel, the presence of more acetylene tends to give the weld a higher carbon content than the parent metal, resulting in a hard and brittle weld. Types of the Flame

21. Oxidizing flame: -Percentage of oxygen is more. -Have two zones. -Small inner cone which has a purple colour . -Outer cone/envelope. -In oxidizing flame, the inner cone is not sharply defined. -This flame is used for welding brass metal. Types of the Flame

22. Resistance Welding The welding processes covered so far are fusion welding processes where only heat is applied in the joint. In contrast, the resistance welding process is a fusion welding process where both heat and pressure are applied to the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name resistance welding.

23. Soldering Soldering is a method of joining similar or dissimilar metals by means of a filler metal whose liquidus temperature is below 450°C. Though soldering obtains a good joint between the two plates, the strength of the joint is limited by the strength of the filler metal used. Soldering is normally used for obtaining a neat leak-proof joint or a low-resistance electrical joint . The soldered joints are not suitable for high-temperature service because of the low melting temperatures of the filler metals used.

24. Soldering

25. Brazing is the coalescence of a joint with the help of a filler metal whose liquidus temperature is above 450°C and is below the solidus temperature of the base metal. The filler metal is drawn into the joint by means of capillary action (entering fluid into tightly fitted surfaces). Dissimilar metals such as stainless steel to cast iron can be joined by brazing. Except for aluminium and magnesium, brazing can join almost all metals. Because of the lower temperatures used, there is less distortion in brazed joints. Brazing

26. Brazing

27. Undercut: This appears like a small notch in the weld interface. This is generally attributed to the improper welding technique or excessive welding current . This is mainly caused by the incorrect manipulation of the electrode while depositing the bead , particularly, in horizontal and vertical welding. Incomplete Fusion: This will be seen as a discontinuity in the weld zone. The main causes for this defect are improper penetration of the joint, wrong design of the joint, or incorrect welding technique including the wrong choice of the welding parameters . The main parameter that controls is the welding current, if it is lower than required, it would not sufficiently heat all the faces of the joint to promote proper fusion. Also, the improper cleaning of the joint hinders the fusion of the metal in the joint. Welding defects

28. Porosity: Porosity in welding is caused by the presence of gases that get entrapped during the solidification process. The main gases that cause porosity are Hydrogen, Oxygen, and Nitrogen. Slag Inclusion: Slag is formed by the reaction with the fluxes and is generally lighter. In view of its low density, it will float on top of the weld pool and would be chipped off after solidification. Hot Cracking: Generally occurs at high temperatures and the size can be very small to visible. The crack is, in most parts, intergranular and its magnitude depends upon the strains involved in solidification. It occurs when the available supply of liquid weld metal is insufficient to fill the spaces between solidifying weld metal, which is opened by shrinkage strains . Liquid cannot reach the regions where it is needed due to inadequate supply or blockage/narrow channels between solidifying grains. Welding defects

29. Cold Cracking: Cold cracking generally occurs at room temperature after the weld is completely cooled . This can be generally seen in the heat-affected zone. The causes are: ● Excessive restraint of the joint which induces very high residual stresses . ● Martensitic transformations make the metal very hard as a result of rapid cooling. Welding defects Lamellar Tearing: It is generally seen at the edge of the heat-affected zone. It appears as a long and continuous visual separation line between the base metal and the heat-affected zone . This is caused by the presence of the elongated inclusions such as Mn, Fe, and S in the base metal. It can also be caused by the weld configuration which gives rise to high residual tensile stresses in the transverse direction.

30. References Manufacturing Technology Volume I (Foundry, Forming and Welding) by P N Rao https://www.youtube.com/watch?v=43KqYUtwQQc https://www.youtube.com/watch?v=Rq_Vuye4HL0 Texts and Figures available on the Web, Books, and Study materials