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![9/4/2012
Example 1-Solution
Rate of heat input to the weld bead = 3500 × f1 × f2
= 3500 × 0 7 × 0 5 = 1225 J/s
0.7 0.5
Heat input = Energy used for heating to Tm + Energy used
for melting
1225 = [Cp(Tm-T0) + Lm ] × A × v
1225 = [480(1760 300) + 247 ×103] × 7860 × 20 ×10-6 × v
[480(1760-300) 10 10 6
Travel speed v = 0.0082 m/s = 8.2 mm/s
45
Summary: Lectures 1-3
• Overview of welding, applications,
ad a tages
advantages
• Welded Joint types
• Fusion & Solid state welding
• Elements of weld setup, Heat Balance,
Power density y
• N.B: Characteristics, micro-structural
zones and concept of lattice continuity in
fusion & solid state welding
46
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Welding lectures 1 4
- Heating rate and peak temperature - Cooling rate - Composition of filler metal - Pre/post heating of base metal - Stress relieving heat treatment Controls the microstructure and properties of the weld. 41 Heat transfer in welding process - Heat input to the weld is from the energy source - Heat is conducted away from the weld zone into the base metal - Heat flow depends on thermal properties of base metal and weld metal - Heat affected zone experiences thermal cycle due to heat input - Rapid heating and cooling rates in welding leads to non-equilibrium microstructures - Control of heat input and cooling rate is important to control weld metallurgy
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Welding lectures 1 4
- 1. 9/4/2012 Casting, Casting Forming & Welding (ME31007) Jinu Paul Dept. of Mechanical Engineering Course details: Welding Topic Hours 1. Introduction to welding science & technology 2-3 2 Welding Processes 4 3 Welding Energy sources & characteristics 1-2 5 Welding fluxes and coatings 1 4 Physics of Welding Arc 1 5 Heat flow in welding 1-2 6 Design of weld joints 2 7. Testing and inspection of weld joints 2-3 8 Metallurgical characteristics of welded joints, Weldability and welding of various metals and alloys 2 Total 19 1
- 2. 9/4/2012 Schedule of Lectures(Welding) Lecture 1 23rd Aug 2012, Thursday, 8.30-9.30 am Lecture 2 24th Aug 2012, Friday, 11.30 am-12.30 pm Lecture 3 30 Aug 2012, Thursday, 8.30-9.30 am Lecture 4 31 Aug 2012, Friday, 11.30 am-12.30 pm Lecture 5 06 Sept 2012, Thursday, 8.30-9.30 am Lecture 6 07 Sept 2012, Friday, 11.30 am-12.30 pm Lecture 7 13 Sept 2012, Thursday, 8.30-9.30 am Lecture 8 14 Sept 2012, Friday, 11.30 am-12.30 pm Lecture 9 L 20 S Sept 2012 Th d 2012, Thursday, 8 30 9 30 am 8.30-9.30 Lecture 10 21 Sept 2012, Friday, 11.30 am-12.30 pm Mid Semester Exam (30 %) 3 Schedule of Lectures (Welding) Lecture 11 04 Oct 2012, Thursday, 8.30-9.30 am Lecture 12 05 Oct 2012, Friday, 11.30 am-12.30 pm , y, p Lecture 13 11 Oct 2012, Thursday, 8.30-9.30 am Lecture 14 12 Oct 2012, Friday, 11.30 am-12.30 pm Lecture 15 18 Oct 2012, Thursday, 8.30-9.30 am Lecture 16 19 Oct 2012, Friday, 11.30 am-12.30 pm Lecture 17 01 Nov 2012, Thursday, 8.30-9.30 am Lecture 18 02 Nov 2012, Friday, 11.30 am-12.30 pm Lecture 19 08 Nov 2012, Thursday, 8.30-9.30 am End Semester Exam (50 %) 4 2
- 3. 9/4/2012 Lecture 1 23rd Aug 2012, Thursday, 8.30-9.30 am Introduction to welding 5 Overview of Joining processes Joining processes Mechanical Brazing Assembly Welding Soldering (e.g., Threaded Adhesive bonding fastners, rivets) 6 3
- 4. 9/4/2012 Joining processes-overview Riveted Joint Threaded fastner Welded Joint Brazed Joint 7 Some application areas of welding Ship building Aircraft industry Automotive industry 8 4
- 5. 9/4/2012 Welding: Application areas • Applications in Air, Underwater & Space • Automobile industry, aircraft industry industry industry, ships and submarines • Buildings, bridges, pressure vessels, girders, pipelines, machine tools, offshore structures, nuclear power plants, etc. • House hold products, farm, mining, oil industry, jigs & fixtures, boilers, furnaces, railways etc. 9 Welding process-Features • Permanent joining of two materials through localized coalescence resulting f l li d l lti from a suitable combination of Temperature & Pressure • Formation of Common metallic crystals at the joints/interface • With or Without filler material 10 5
- 6. 9/4/2012 Welding process-Features • Continuity: absence of any physical disruption on an atomic scale p • Not necessarily homogeneous but same in atomic structure, thereby allowing the formation of chemical bonds Material Metals Ceramic Polymer (similar/dissimilar) Type of bond Metallic Ionic/coval Hydrogen, van der ent Waals, or other dipolar bonds 11 Welding Process: Advantages • Exceptional structural integrity, continuity, fluid tightness, portable equipments • Strength of joints can approach or exceed the strength of the base material(s) • Wide range of processes & approaches • Can be performed manually, semi automatically or completely automatically • Can be performed remotely in hazardous environments (e.g., underwater, areas of radiation, outer space) using robots 12 6
- 7. 9/4/2012 Welding Process:Disadvantages • Precludes disassembly • Requirement for heat in producing many welds can disrupt the base material microstructure and degrade properties; may induce residual stresses • Requires considerable operator skill • Capital equipment can be expensive (e.g., laser beam, vacuum chambers etc.) 13 Types of joints in welding Butt joint Corner joint Lap joint Tee joint Edge joint 14 7
- 8. 9/4/2012 Types of welds 1) Fillet weld Fillet weld Fillet weld Fillet weld on corner joint on lap joint on T-joint 15 Types of welds 2) Groove weld (c) single (a) square groove weld weld, ( ) (b) single bevel g V-groove V groove weld groove weld (d) single (e) single (f) Double V- groove U-groove weld J-groove weld weld for thicker sections 16 8
- 9. 9/4/2012 Types of welds 3) Plug & slot weld • Drill hole/slot on the top plate only • Hole/slot is filled with filler metal 17 Types of welds 4) Spot weld 5) Seam weld • Fused section between the surfaces of two sheets • Mostly associated with resistance welding 18 9
- 10. 9/4/2012 Types of welds 6) Flange weld & Surfacing weld • Surfacing weld is not for joining parts • The purpose is to increase the thickness of the plate or to provide a protective coating on the surface. 19 Lecture 2 24th Aug 2012, Friday, 11.30 am-12.30 pm Weld Microstructure & Concept of continuity y 20 10
- 11. 9/4/2012 Some material sciencebasics… Atoms Lattice Grains • Grain size, Grain boundaries, • Recrystalization ~0.4-0.6 Tm Atoms remain in lattice, but new grains will be formed • Melting Atoms displaced from lattice, free to move 21 Some material science basics… • Metals are crystalline in nature and consists of irregularly shaped grains of i t fi l l h d i f various sizes • Each grain is made up of an orderly arrangement of atoms known as lattice • The orientation of atoms in a grain is uniform but differ in adjacent grains 22 11
- 12. 9/4/2012 Basic Classification ofwelding (a) Fusion welding (b) solid-state welding a) Fusion Welding • Uses heat to melt the base metals • A filler metal is mostly added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. joint • e.g., Arc welding, resistance welding, Gas welding, Laser beam welding, Electron beam welding 23 Micro-structural zones in Fusion welding 1) Fusion zone 2) Weld interface/partially melted zone 3) Heat affected zone 4) Unaffected base metal 24 12
- 13. 9/4/2012 Grain growth in Fusion welding • Direction solidification in fusion zone Epitaxial grain growth Columnar grains • HAZ Possible recrystallization/ grain refinement or phase change • Shrinkage of fusion zone Residual stress on the base metal surrounding HAZ 25 Basic Classification of welding b) Solid state Welding • C l Coalescence results f lt from application of li ti f pressure alone or a combination of heat and pressure • If heat is used, the temperature in the process is below the melting point of the metals being welded • No filler metal is used • e.g., Diffusion welding, friction welding, ultrasonic welding 26 13
- 14. 9/4/2012 Micro-structural zones in Solid state welding • No Fusion zone • Little or no HAZ • Mechanically upset region • Plastic deformation at the interface 27 Role of Temperature in Fusion/ solid state welding • Drives off volatile adsorbed layers of gases, moisture, or organic contaminants g • Breaks down the brittle oxide through differential thermal expansion • Lowers yield/flow strength of base materials helps plastic deformation • Promotes dynamic recrystallization during plastic deformation (if T > Tr) • Accelerates the rates of diffusion of atoms • Melts the substrate materials, so that atoms can rearrange by fluid flow (if T > Tm) 28 14
- 15. 9/4/2012 Role of Pressurein solid state welding • Disrupts the adsorbed layers of gases/organic compound or moisture by macro- or microscopic deformation • Fractures brittle oxide or tarnish layers to expose clean base material atoms • Plastically deform asperities (lattice) to increase the number of atoms that come into intimate contact (at equilibrium spacing) 29 Mechanisms for obtaining material continuity (1) Solid phase plastic deformation Solid-phase deformation, without or with recrystallization (2) Diffusion, and (3) Melting and solidification 30 15
- 16. 9/4/2012 Obtaining continuity 1) Solid-phase plastic deformation • Atoms are brought together by plastic deformation • Sufficiently close to ensure that bonds are established at their equilibrium spacing • Significant lattice deformation • L tti Lattices are left in the strained l ft i th t i d state (distorted) in cold (a) Cold deformation and deformation lattice strain Prevailing mechanism in solid state welding with out heat 31 Obtaining continuity 1) Solid-phase plastic deformation (with heat) • In hot state (0.4-0.5 Tm), the strained lattice recover from the distorted state • Atomic rearrangement & Recrystallization • Grain growth across original interface (b) hot deformation and • Eliminates the original physical dynamic recrystallization interface Prevailing mechanism in solid state welding with heat 32 16
- 17. 9/4/2012 Obtaining continuity 2) Diffusion • Transport of mass through atom movement • Can occur entirely in solid phase or with liquid phase • For dissimilar materials thin layer of alloy at the interface • R t of diffusion Diff Rate f diff i Difference in i a) S lid h ) Solid-phase diff i diffusion composition (Fick’s law) across the original interface (dotted line) Prevailing mechanism in brazing/soldering 33 Obtaining continuity 3) Melting and solidification Liquid provided by melting Establishing a bond upon the parent materials without epitaxial solidification of or with additional filler this liquid • Solidifying crystals take up the grain structure & orientation of substrate/unmelted grains • Prevailing mechanism in most fusion welding process 34 17
- 18. 9/4/2012 Lecture 3 30th Aug 2012, Thursday, 8.30 am-9.30 am Elements of welding set up, power d density & h i heat transfer f 35 Basic elements of a welding setup 1. 1 Energy source to create union by pressure/heat 2. Method to remove surface contaminants 3. Protect metal from atmospheric contamination 4. Control of weld metallurgy 36 18
- 19. 9/4/2012 1. Energy source Classification of Fusion welding based on energy source Energy Types of welding source Oxy fuel gas welding, Exothermic welding/ Thermite Chemical welding, Reaction brazing/Liquid phase bonding Radiant Laser beam welding, Electron beam, Infrared welding/ energy brazing, Imaging arc welding, Microwave welding, Electric-Perm. Gas tungsten arc welding, plasma arc welding, Carbon electrode arc arc welding, atomic hydrogen welding, Stud arc welding Electric- El t i Gas G metal arc welding, Shi ld d metal arc welding, t l ldi Shielded t l ldi Consumable Submerged arc welding, Electrogas welding, Electroslag electrode welding, Flux cored arc welding Electric- Resistance spot, resistance seam, projection welding, Resistance flash/ upset welding, Percussion, Induction welding 37 1. Energy source Classification of solid state welding based on energy source Energy Types of welding source Cold welding, Hot pressure welding, Forge welding, Roll welding, Friction welding, Ultrasonic welding, Friction stir Mechanical welding, Explosion welding, Deformation diffusion welding, Creep isostatic pressure welding, Super plastic forming Chemical + Pressure gas welding, Exothermic pressure welding, Mechanical Pressure thermit forge welding Stud arc welding, Magnetically impelled arc butt welding, Electrical + resistance spot welding, resistance seam welding, Mechanical projection welding, flash welding, upset welding, percussion welding, resistance diffusion welding 38 19
- 20. 9/4/2012 2. Removal of Surface contaminants • Surface contaminants may be organic films, absorbed gases or chemical compounds of the b b d h i l d f th base metals (usually oxides) • Heat when used as source of energy removes organic films and absorbed gases • Fluxes are used to clean oxide films and other contaminants to f i form slag l • Slag floats and solidifies above weld bead protecting the weld from further oxidation 39 3. Protection from atmospheric contamination • Shielding gases are used to protect molten weld pool from atmospheric contaminants like O2 & N2 present in air • Shielding gases could be Ar, He,CO2 • Alternatively, welding could be carried out in y g an inert atmosphere. 40 20
- 21. 9/4/2012 4.Control of weld metallurgy • Microstructures formed in the weld and HAZ determines the properties of the weld • Depends on heating, cooling rates (power, weld travel speed) • Can be controlled by preheating/ post heat treatment • De-oxidants, alloying elements etc. added to control weld metal properties 41 Power density • Defined as the power transferred to work per unit surface area (W/mm2) ( • Time to melt the metal is inversely proportional to power density Welding Process Approx. Power density (W/mm2) Oxy-fuel welding 10 Arc welding 50 Resistance welding 1000 Laser beam welding 9000 Electron beam welding 10,000 42 21
- 22. 9/4/2012 Heat transfer mechanisms in Fusion Welding Heat transf. factor f1= Heat transf. to work / Heat gen. by source Melting Factor f2 = Heat used for melting / Heat tranf. to work Useful heat or energy = f1.f2 43 Example 1 The power source in a particular welding setup generates 3500 W that can be transferred to the work surface with a heat transfer factor f1 = 0 7 The metal to be welded is 0.7. low carbon steel, whose melting temperature is 1760K. The melting factor in the operation is 0.5. A continuous fillet weld is to be made with a cross-sectional area of 20 mm2. Determine the travel speed at which the welding operation can be accomplished? Heat H t capacity of low carbon steel (Cp) 480 J/Kg.K it f l b t l )=480 J/K K Latent heat of melting Lm =247 kJ/Kg Density = 7860 kg/m3 Initial sample temperature T0 = 300 K 44 22
- 23. 9/4/2012 Example 1-Solution Rate of heat input to the weld bead = 3500 × f1 × f2 = 3500 × 0 7 × 0 5 = 1225 J/s 0.7 0.5 Heat input = Energy used for heating to Tm + Energy used for melting 1225 = [Cp(Tm-T0) + Lm ] × A × v 1225 = [480(1760 300) + 247 ×103] × 7860 × 20 ×10-6 × v [480(1760-300) 10 10 6 Travel speed v = 0.0082 m/s = 8.2 mm/s 45 Summary: Lectures 1-3 • Overview of welding, applications, ad a tages advantages • Welded Joint types • Fusion & Solid state welding • Elements of weld setup, Heat Balance, Power density y • N.B: Characteristics, micro-structural zones and concept of lattice continuity in fusion & solid state welding 46 23
- 24. 9/4/2012 Lecture 4 31th Aug 2012, Friday, 11.30 am-12.30 pm Welding Processes 1) Oxy-Fuel gas welding ) y g g 47 Welding Processes- 1) Oxy-Fuel gas welding • Uses oxygen as oxidizer • Acetylene, H2 or Natural gas, methane, propane, butane or any hydrocarbon as fuel • Fuel + Oxidizer Energygy • Acetylene is preferred (high flame temperature-3500 C) 48 24
- 25. 9/4/2012 Gases used inOxy-gas welding Fuel Peak Heat of reaction combustion Temp (C) (MJ/m3) (MJ/ 3) Acetylene 3500 54.8 Methylacetylene- 2927 91.7 propadiene (C3H4) Hydrogen 2660 12.1 Propylene 2900 12.1 Propane 2526 93.1 Natural gas 2538 37.3 49 Oxy-acetylene welding (OAW) operation 50 25
- 26. 9/4/2012 Reactions in Oxy-acetylenewelding • Flame in OAW is produced by the chemical reaction of C2H2 and O2 in two stages Stage 1 C2H2 + O2 2CO + H2 + heat Stage 2 2CO + H2 + 1.5O2 2CO2 + H2O + heat 51 Flames in OAW 52 26
- 27. 9/4/2012 Flames in OAW Neutral flame is used for most applications 53 Flames in OAW- Reducing flame • Reducing flame for removing oxides from metals s ch as al mini m or metals, such aluminium magnesium • Preventing oxidation reactions during welding • To prevent decarburization (i.e., C to CO,) in steels. • Low carbon, alloy steels, monel metal (Ni+Cu+…), hard surfacing 54 27
- 28. 9/4/2012 Flames in OAW-Oxy. flame •The oxidizing flame causes the metal being welded to form an o ide elded oxide. • Useful for preventing the loss of high vapor-pressure components, such as zinc out of brass, through the formation of an impermeable “oxide skin” (here, copper oxide) • Brass, bronze, Cu, Zn & Sn alloys 55 OAW set up • Pressurized cylinders of O2 and C2H2 d • Gas regulators for controlling pressure and flow rate • A torch for mixing the gases • Hoses for delivering the gases from the cylinders to the torch 56 28
- 29. 9/4/2012 OAW Torch 57 Example 1 - OAW An oxyacetylene torch supplies 0.3 m3 of acetylene per hour and an equal volume rate of oxygen for an OAW operation on 4.5-mm-thick steel. Heat generated by combustion is transferred to the work surface with a heat transfer factor f1 = 0.20. If 75% of the heat from the flame is concentrated in a circular area on the work surface that is 9.0 mm in diameter, find (a) t f heat liberated during ( ) rate of h t lib t d d i combustion, b ti (b) rate of heat transferred to the work surface, and (c) average power density in the circular area. (Heat of combustion of Acetylene in O2 = 55×106 J/m3) 58 29
- 30. 9/4/2012 Example 1 - OAW (a) The rate of heat generated by the torch is the product of the volume rate of acetylene times the heat of combustion: RH = (0 3 m3/hr) (55×106) J/m3 = 16 5×106 (0.3 ) 16.5×10 J/hr or 4583 J/s (b) With a heat transfer factor f1 = 0.20, the rate of heat received at the work surface is f1 × RH = 0.20×4583 = 917 J/s (c) The area of the circle in which 75% of the heat of the flame is concentrated is A = Pi. (9)2/4 = 63.6 mm2 The power density in the circle is found by dividing the available heat by the area of the circle: Power density = 0.75 × 917/63.6 = 10.8 W/mm2 59 OAW-Advantages • The OAW process is simple and highly portable bl • Inexpensive equipment • Control over temperature • Can be used for Pre-heating, cutting & welding ldi 60 30
- 31. 9/4/2012 OAW-Disadvantages • Limited energy welding is slow • Low protective shielding welding of reactive metals (e.g., titanium) is generally impossible • Low power density, Energy wastage, total heat input per linear length of weld is high • Unpleasant welding environment • Weld lines are much rougher in appearance than other kinds of welds Require more finishing • Large heat affected zones 61 OAW-Applications • Preheating/post heat treatment • C b used f cutting, grooving, or piercing Can be d for tti i i i (producing holes), as well as for welding • Oxyfuel gas processes can also be used for flame straightening or shaping • Oxidizing flame for welding Brass, bronze, Cu- Zn and Tin alloys • Reducing flame for low carbon & alloy steels 62 31
- 32. 9/4/2012 Pressure Gas welding (Special case of OAW) • Oxyfuel gas used for preheating the weld interface 63 References • Principles of Welding, Robert W Messler • M t ll Metallurgy of W ldi f Welding, J F L J.F. Lancaster t • Welding Science and Technology, Md. Ibrahim Khan • Welding Technology-O.P. Khanna • Manufacturing Engineering and Technology, S. Kalpakjian 64 32