Manufacturing and Joining PDF

# Week 3 ## Manufacturing and Joining - Manufacturing is used for sizing, shaping, and imparting properties - Products in manufacturing vary from simple to complex in geometries - Properties of material which are manufactured: physical, chemical, mechanical, and dimensional properties (straightness,...

# Week 3 ## Manufacturing and Joining - Manufacturing is used for sizing, shaping, and imparting properties - Products in manufacturing vary from simple to complex in geometries - Properties of material which are manufactured: physical, chemical, mechanical, and dimensional properties (straightness, flatness, surface roughness, mechanical and dimensional properties) - Manufacturing processes commonly used: - **Casting**: Shifting from one shape to another. Zero process - **Forming**: Shifting material from one shape to another by deformation. Zero process - **Machining**: Unwanted material is removed. Negative process - **Joining**: Two or more parts are joined to achieve the desired shape and size. Negative or positive process ### Note - **Zero process**: No loss of material during manufacturing. - **Negative process**: Material gets lost as waste during manufacturing. - **Positive process**: Material is added externally during manufacturing. ## Joining - Process of bringing two things together. It is a positive process. - **Mechanical joining**: Uses nuts and bolts, clamps, rivets. - **Chemical or adhesive joining**: Uses epoxy resins, Fevicol, M-seal, quick fix. - **Welding**: Welding, brazing, soldering. - Each joining process differs in load carrying capacity, reliability, compatibility, and service suitability. - **Types of joint**: - **Temporary**: Mechanical joints - **Permanent**: Welding ### Service condition: - Ambient, corrosion, low/high temperature. - Chemical, reliability (mechanical joint - riveted) ## Metallurgical Compatibility ### Nature of loading - Static and dynamic. ### Economy ## Welding vs. Manufacturing Process - Localized heating - Differential heating and cooling conditions - Residual stresses - Partial melting - Unique weld thermal cycle - Chemical, mechanical, and metallurgical heterogeneity. - Reliability ## Dimensional Accuracy & Finish - Weld point - Epitaxial solidification - Welding thermal cycle - Post-welding treatment: DBTT, Creep, HAZ softening, Hardening - DBTT = Ductile to Brittle Transition Temperature ## Factors That Influence Selection of Joining Process (Generally Welding): - **Welding**: By fusion, deformation, and diffusion with the application of heat (pressure, with or without filler material). - **Metal**: Thickness, melting point, thermal expansion. - **Availability** of consumables. - **Criticality** of application. - **Service condition**. - **Precision required**. - **Economy**. ## Advantages of welding: - Permanent joint. - Joint strength as good as base metal. - Economical method of joining. - Done at anywhere. ## Disadvantages of welding - Needs expertise. - High labor cost. - Problematic if disassembly is required. - Hazardous fumes and vapors. - Poor reliability. ## Welding Processes vs Sectors - **Resistance welding**: Automobile - **Thermite welding**: Rail joints in railways. - **Tungsten Inert Gas (TIG) welding**: Aerospace and nuclear reactors. - **Submerged Arc welding**: Heavy engineering, ship work. - **Gas metal arc welding**: Pressure vessel. - **Shielded metal arc welding**: General purpose and repair. ## Fundamental Mechanism of Joining Metals: - **Solid state joining**, with or without recrystallization: - **Cold deformation** and **lattice strain**. - **Hot deformation** and **dynamic recrystallization**. - **Diffusion**: With or without interlayers. - **Use of process accelerators**: Ultrasonic vibration, pressure pulsing. - **Melting** and **solidification**. ## Epitaxial - **When the same filler material is used**: Nucleation and growth mechanism. - **When different filler material is used**: Nucleation and growth mechanism. ## Autogenous welding - Welding in which two workpieces of the same material are welded without the addition of separate filler material. ## Welding Process ### Solid state - Friction stir welding ### Liquid state - Soldering - Brazing ## Classification - **Based on technological**: Approach of joining. - **Based on approach**: Of joining. - **Better classification** would help in: - Basis grouping based on similarity and dissimilarity. - Nomenclature. - Easy communication. - Organization. ## Heat for joining - Used for the purpose of: - **Cleaning**: Vaporizing (moisture), decomposition (tillu oxides), breaking oxides. - **Lowering**: Yield strength, and facilitate deformation (intentional, bulk). - **Dynamic**: Recrystallization. - **Accelerating**: Diffusion. - **Meeting**: Of faying surface (joining surface). ## Pressure for joining - Used for the purpose of: - Disrupting adsorbed gas layer by deformation. - Fracturing brittle oxide layer. - Plastic deformation for increased metallic intimacy. - Large scale plastic deformation for obtaining metallic continuity. ## Purpose of filler metal in joining - To fill gap. - To adjust the chemical composition. - To impart specific properties: Corrosion resistance, thermal expansion, strength, residual stress. - **Surface interaction only**: Soldering, brazing. ## Classification: Technological factors - Joining with or without filler. - Source of energy. - Arc or no-arc process. ## Fusion and pressure welding ## Classification: Approaches of joining - **Welding**: - **Cast welding**: Thermite, ESW. - **Fusion welding**: Where fusion involves. - **Resistance welding**. - **Solid state welding**: Jart principle used for heating. - **Solid state welding**: Uses high heat flow, interfacial bulk deformation. - **Allied processes**: - **Metal deposition**. - **Soldering**. - **Brazing**. - **Adhesive bonding**. - **Weld surfacing**. - **Metal spraying**. ## Note - **Joining/Welding**: Refers only to melting of laying surfaces. - **With filler material (autogenous welding process)**: Gas welding, electron beam welding, laser beam welding, resistance welding, friction welding, ultrasonic welding. - **With filler material**. ## Same as base metal - Solidification occurs by growth mechanism or epitaxial. ## Different from base metal - Solidification occurs by nucleation and growth mechanism. ## No filler used: - Resistance, friction, ultrasonic, electron beam welding. ## Filler may be used: - Laser beam, electron beam, TIG, PAW (plasma arc welding). ## Filler is used: - Shielded metal arc welding, submerged arc welding, gas metal arc welding, electro-slag welding, electro-gas welding. ## Source of energy (Heat) - Chemical energy: Chemical reaction for producing heat. - Mechanical energy. - Electrical energy. - Radiation energy. - Frictional energy: Interfacial friction or impact. - **Arc processes**: GMAW, SMAW, SAW, GTAW, Plasma (PAW), Carbon (CAW), Electro gas (EGW). - **No arc processes**: Friction welding, resistance welding, gas welding, thermite welding. ## Note - Flash butt welding or electro-slag welding are controversial welds. ## Flash Butt Welding: - Used for cleaning and softening. - Flash: An short period. ## Heat for joining. Purpose of heat: - Cleaning. - Softening. - Diffusion. - Melting. ## Chemical Reaction: Gas welding: - **Inner cone**: - $C_2H_2 + O_2 \longrightarrow 2CO + H_2 + 448 kJ/mol$ (12.75 MJ/$m^3$ of acetylene) - **Inner white cone (3100 °C)**: - $4CO + 2H_2 + 3O_2 \longrightarrow 4CO_2 + 2H_2O + Q12 kJ/mol$ (35.77 MJ/$m^3$) - **Touch tip**: Outer blue flame (1275 °C). ## Chemical reaction: Thermite welding - Mixture of metallic oxides and reducing agents. - Oxides of Fe, Mn, Cu, or with reactive metals: Al, Mg. - Exothermic reducing agents reaction: - $Fe_2O_3 + 2Al \longrightarrow 2Fe + Al_2O_3$ - $3CuO + 2Al \longrightarrow 3Cu + Al_2O_3$ ## Electrical Resistance Heating - Heat generated by electrical resistance heating: I²Rt - I = Welding current. - R = Contact resistance or electrical resistance of metal. - t = Time. ## Heat generation by friction - The frictional heat generation is obtained by IMFV. - $n$ = Fraction of energy (energy lost in friction) converted into heat (varies from 0 to 1). - F = Friction force (UN). - v = Relative velocity. ## Heat from severe plastic deformation - Friction stir welding: Interfacial deformation. - Friction stir welding: Bulk deformation. ## Heat input vs. weld performance - **Solidification rate** of the weld pool determines: - Grain size - Inclusion and gas entrapment tendency - Mechanical properties - Alloy segregation tendency - HAZ size - **Cooling rate** which in turn affects the solidification rate. - **Power or energy density of fusion welding process**: Increasing energy density - Gas fusion welding, manual arc welding, MIG welding, plasma arc welding, electron beam welding, laser beam welding. ## Energy density of gas welding - Energy density of LBW (~100) - Gas welding will take maximum amount of heat and LBW will take less amount of heat for melting a certain area. ## Low energy density - High heat input, HAZ ↑, weld size ↑ - Solidification time. ## High energy density - Low heat input, HAZ ↓ , weld size ↓. - High energy density, Hnet ↑, cooling rate ↑. ## Effect of heat input - Increasing damage to an unworked piece, gas welding, arc welding, high energy beam welding. ## Low density of heat source - Leg longer, solidification time, low energy density, high heat input. - Coarse grain structure. - Allows removal of impurities. - Poor mechanical properties. - Segregation of constituents. - Leads to cracking of the weld area. ## Need for protection of the weld pool: - Entrapment of atmospheric gases in a weld pool (which can result in porosity). - Reaction of gases with weld metal. - Contamination of the weld pool from oxides and nitrides. - Porosity occurs due to a difference in solubility of molten metals. ## Protection of weld pool: - By creating vacuum (10⁻³ to 10⁻⁵ torr) - By inert gas atmosphere (He, Ar). ## Effect of gases: - Gases get dissolved inside the weld pool. - Gases form compound inclusions. ## Weld pool protection approach - Forming a developing covering envelope of inert or inactive gases. - TIGW: inert - MIGW: inert (inactive) - PAW : Ar/He - SMAW: CO₂/O₂ ## Forming cover of molten flux (slag) - Used in: - SAW (submerged arc). - EGW (electro slag). ## By creating vacuum - Used in: - Electron beam welding. - Diffusion welding (T ≈ 0.5 to 0.6T_m). ## Note: - O₂% or N₂% ↑ → porosity ↑ and vice versa. - For TIG: O₂% and N₂% is very less. - For MIG: O₂% and N₂% are very high. ## N₂% in weld - Toughness and YS (yield strength) ↑. - Ductility and toughness ↓. - % of O₂ in weld α ductility, toughness, and strength. - % of O₂ in unweld α element transfer efficiency (η_el, mm). - # Week 2 - Principles of fusion welding: - Fusion of faying surfaces. - Solidification. ## Heat sources - Arc. - Gas flame. - Laser beam. - Electron beam. ## Solidification modes - Epitaxial: Occurs only when the composition of the weld metal is similar to the base metal. - Nucleation and growth: Occurs when the composition is different. ## Observed grain structure - Planar. - Cellular. - Dendritic. - Equiaxed. ## Gas welding - Fuel gas used for joining: Oxygen. - Fuel gas: acetylene (C₂H₂) → mostly used (3300 °C). - Propylene. - Propane. - Hydrogen. - Natural gas. ## Outer blue flame (1275 °C) - Fuel gas cylinder. - Oxygen gas cylinder. - Inner cone (highest temp ≈ 3100-3300 °C), heat generation is less. - Intermediate flame feather. ## Temp of flame - Inner cone: Acetylene feather. - Outer envelope. ## Type of flame: - **Neutral**: O₂/C₂H₂ → 1. Preferably used. - **Oxidising**: O₂/C₂H₂ > 1. High temperature. - **Carburizing**: O₂/C₂H₂ < 0.9. ### Note: - Use of carburizing flame in low carbon steel and cast iron leads to an increase in hardness and strength. - In high carbon steel, it causes embrittlement and cracking of the weld. ## Flame velocity depends on: - Fuel/oxygen ratio. - Pressure of gas mixture. - Nozzle design. - Complete combustion. ## To take care of impurities: - Normally paste powder, solid coating liquid is applied on the faying surface before heating. These are called flux. - Commonly used fluxes: Borax. ## Gas welding: Forward welding - Cooling rate↑. - Hardenable steel. - Decrease tendency of cracking. ## Backward welding - Post weld heat treatment. - Reduces tendency of cracking. - Residual stresses ↓. ## Gas welding: Low power density - Flame spread over a large area. - Temperature of heat source is 100-3300 °C. - High heat input. - Cooling rate ↑ of weld metal. - Large HAZ. - Protection of weld pool is not good. ## Soundness of weld ↑ - Welding positions: - Flat. - Horizontal. - Vertical. - Overhead → most difficult position. ## Type of weld: - Groove weld. - Fillet weld. - Bead weld. - Plug weld. - Slot weld. ## Types of joints | Type | Image | |---|---| | Butt joint | [Image of butt joint] | | Lap joint | [Image of lap joint] | | Tee joint | [Image of tee joint] | | Corner joint | [Image of corner joint] | | Edge joint | [Image of edge joint] | ## Welding arc - Electrode. - Cathode drop zone (Vc). - Plasma (Vp). - Anode drop zone (Va). - Power of arc = VI. - Net heat = VI/S (J/mm). - S = Speed of moving arc. - Heat generated by a welding arc = V I X t - For SMAW: 50-70 V. - For GMAW: 20-40 V. - For GTAW/IPA: 10-20 V. ## Arc initiation method: - **Touch start**: GTAW (SMAW). - **Field start**: Workpiece and gas gap. ## When arc is initiated once - Thermal emission of e⁻ occurs. - Ionization of metal vapors. # SMAW (Shielded Metal Arc Welding): - Constant current type power sources are used. - Both AC and DC can be used. - Core wire (d): Diameter in mm. - 250-480 mm. - Diameter of electrode with coating (250-480 mm). - Flux coating. - Arc. - Protective gas shield. - Molten weld pool. - Base metal. ## Coating factor - Ratio of the electrode diameter with flux coating to the diameter of core wire without flux coating. - Coating factor = d/D (varies from 1.2-2.2). - **Light coating**: 1.2-1.35. - **Medium coating**: 1.4-1.7. - **Heavy coating**: 1.8 - 2.2 ## Purpose of coating: - Arc stabilization. - Flux for removing impurities. - Deoxidise (Fe-MN, Fe-Si are used). - Controlled alloying. - Increasing deposition efficiency. - To adjust viscosity (surface tension of slag/molten metal), generally in vertical overhead welding as molten metal falls downward. ## Constituents of coating flux and their role: | Item | Formula | Role | |---|---|---| | Quartz | SiO₂ | Enhance current carrying capacity. | | Magnetite | Fe3O4 | Refining transfer of molten metal drops. | | Calcium carbonate | CaCO3 | Lower arc voltage and release inactive gases. | | Flourspare | CaF2 | Increasing viscosity of molten metal. | | Ferro-manganese & ferro-silicon | Fe-Mn & FeSi | De-Oxidants. | | Cellulose | — | Release shielding gases. | | Potassium water glass | K₂SiO₃ | Bonding agent. | | Rutile | TiO₂ | Increasing slag viscosity and easy re-striking of arc. | - SMAW is used for: - General purpose. - Non-critical applications. ## Zones in welding arc: - **Cathode drop zone** (Vc). - **Plasma drop zone** (Vp). - **Anode drop zone** (Va). - V = Vc + Vp + Va - Melting of the cathode is generally governed by the heat generated in the cathode drop zone. - Heat at the cathode drop zone = VexIxt. ## Melting Rate (MR): - MR = aI + bLI² - aI = Heat generated at anode/cathode drop zone. - bLI² = Heat generated due to electrical resistance heating (I²Rt heating) - a = Coefficient, which accounts for factors affecting cathode/anode drop zone heat. - I = Current flowing through the electrode. - b = Coefficient for material, electrical resistance. - L = Electrode extension. - For small electrode dia. (d), L↑→ I↑ → (bLI²) heat will dominate. - For large electrode dia. (d), L↑→ I↓ → (aI) heat will dominate. ## Polarity - **DC electrode negative**: - DCEN / DCSP - Polarity reverse. - Deep weld, no surface cleaning. - **DC electrode positive**: - DCEP / DESP - DRCP. - Polarity straight. - Shallow weld, surface cleaning. - **AC**: Intermediate. ## Effect of Polarity: - Heat generation/distribution. - Stability of arc. - Cleaning action offered by the welding arc. ## Welding process parameters: - Welding current. - Welding speed. - Arc voltage. ## Welding current: - Directly affects heat generation, which affects: - Melting rate. - Penetration depth. - Cross-sectional area of weld being deposited. ## Arc voltage (V) - Weld thickness α arc stability α depth of weld. ## Welding speed - Speed ↑ → heat delivery to the weld area ↑ → reduced depth of penetration. ## GTAW (Gas Tungsten Arc Welding) - Also known as Tungsten Inert Gas (TIG) welding. - Uses non-consumable electrodes. - Inert gases are used for protection of the weld pool (Ar, He). - Arc length used in this process is very short. - These parameters lead to a clean weld (i.e., less % of O₂ and N₂). ## Electrode: - Tungsten (W) is used (pure tungsten electrode coated with Zr, La, Ce → LIP elements). - Pure coated tungsten electrode increases the current carrying capacity of the electrode. - 1.5 mm dia. of electrode, pure W → 150 Ampere current. ## Power Source - Constant current (CC) type is used. - Current: 5-300 A. - DC (DCEN). - AC (for non-ferrous metal). - Torches: Gas cooled, water cooled. ## Shielding gases: - Ar/He. - Good arc stability. - Shallow penetration. - H₂, N₂ can also be used, economical, costly. ## Arc length in GTAW: - Generally 1-3 mm. - Constant/short arc length (02/N₂ ↑). - In TIG: Heat input (heat) needed is low. - Power density of TIG is greater than SMAW/SAW/GMAW. - Weld size ↓ → HAZ ↓ → CR↑ → quality of the weld joint ↑. ## TIG is used for: - Quality weld joints for critical applications such as nuclear, aerospace, aircraft etc. ## Limitations of TIG: - Welding of thicker plates are tough (limited penetration). - Productivity is not good. ## Gas Metal Arc Welding (GMAW) / MIG - It is a modification of GTAW. - Consumable electrodes are used. - Uses higher current level (only used with ferrous metal). - Protection is good (Ar/He / CO₂ → shielding gas). - Constant voltage power source. - Arc length is fluctuating in GMAW. ## Electrode is consumed - Impurities in weld. - Stability of arc is not good. - O₂/N₂ in GMAW > in GTAW - Heat of arc melts both electrode and base. - Productivity is good, clean weld is made. ## Pulse GMAW: - Stable arc. - Suitable for thin sheets, or very sensitive metal towards heat. - HAZ ↓ , RS ↑ , Hnet ↓ , Distortion ↓, mechanical properties ↑. ## MIG vs TIG welding - MIG weld is not considered as clean as TIG welding. - GMAW offers higher deposition rate for good quality weld joints in industrial fabrication. ## Power source for GMAW: - MIG welding may use either constant voltage or constant current type of power source, depending upon: - Electrode diameter. - Electrode material. - Electrode extension. ## Polarity: - DCEP is preferred over DCEN due to: - Stable arc. - Smooth metal transfer. - Less spatter. - Good weld bead geometry over a range of current. ## DCEN result: - Unstable & erratic arc. - Short circuiting & buried arc. - Lower penetration. - Higher melting rate of electrode. - Unstable arc. ## AC: - Not preferred and no acceptability due to: - Arc extinguishing tendency. - Erratic arc operation. ## Shielding gases: - Arc characteristics. - Metal transfer characteristics: penetration, width of fusion. - Speed of welding. - Quality of weld. - Element transfer efficiency. ## Shielding gases - Arc changes mode of metal transfer from globular to spray and rotary transfer with minimum spatter. - CO₂ produces globular mode of metal transfer, results in weld joint with spattering. - Shielding gas also affects bead geometry due to a difference in heat generation during welding. ## Welding current - Weld penetration. - Deposition rate. - Weld bead geometry and quality of weld. ## Welding voltage - Arc voltage directly affects the width of weld bead. - An increase in arc voltage increases the width of the weld. ## Metal transfer in GMAW - Transfer of molten metal drops from the filler metal to the weld pool generally occurs by: - Touching the weld pool. - Discrete drop moving from tip to weld pool. ## Drop transfer - Can be globular or spray type. ## Type of metal transfer - Depends upon the welding current, electrode diameter, and shielding gas. It can take place through different modes: - Short circuit. - Globular. - Spray. - Dip. ## Role of current pulsation: - Peak current primarily for melting, while low current is for: - Maintenance of the welding arc with low heat. - Allows time for solidification of the weld pool. ## SAW (Submerged Arc Welding) - Consumable electrodes are used. - Power source is either: - Constant current -> 2.4 mm dia. electrode. - Constant voltage -> 2.4 mm dia. electrode. - DCEP / DCSP is used for high heat generation in electrodes. - High MR → DR↑ → productivity (kg/mm) ↑ → self regulating arc. - Granular fluxes are used to protect the weld pool. ## Granular fluxes: - Fused fluxes. - Bonded fluxes. - Agglomerated fluxes. - Mechanical mixed fluxes. ## Electrodes used with constant speed of CV or DCSP: - High electrical resistivity (ρ). - High electrical extension (EE). - Small diameter of the electrode. ## Electrodes used with variable speed, by feed drive system, at CC: - EE short. - Large diameter of electrode. ## Current ↑, DR ↑ - Productive, used in industries. - MRT (melting rate), depth of penetration ↑, used for thick plates (≥ 10 mm). ## Dia of electrode (mm) - 1-6: 150-300 A. - 2: 200-400 A. - 3: 400-600 A. - 6: 700-1200 A. ## Heat input ↑ - Size of weld ↑, HAZ ↑, thermal damage ↑. ## For a given value of current - If dia of electrode increases, power density decreases. ## At constant dia of electrode - If current increases, penetration depth increases. ## Advantages of SAW: - The protecting and refining action produces clean welds. - Since the arc is submerged, spatter and heat losses to the surrounding are minimised. - Both alloying elements and metal powders can be added to the granular flux to control the weld metal composition and increase the deposition rate, respectively. - Using two or more electrodes in tandem further increases the deposition rate. - Because of its high deposition rate workpieces much thicker than those in GTAW and GMAW can be welded by SAW. ## Disadvantages of SAW: - However, the relatively large volumes of molten slag and metal pool often limit SAW to flat position welding and circumferential welding (of pipes). - The relatively high heat input can reduce the weld quality and increase distortions. ## Electro-slag welding: - Primarily used for very thick plates (20-500 mm). - Power source: DC, CV, DCRP. - Molten flux. - Thickness. - Electrode. - Copper shoe. - Heated (55 °C). - Plates being welded. - Molten weld metal. - Solid weld metal. - It is a single pass uphill welding. - Hnet ↑, CR ↑ , HAZ ↑, coarse grain structure. - Post-weld heat treatment is required (normalizing heat treatment to increase toughness, generally). - No angular distortion due to a square groove type joint. - RS ↑ - If the plate thickness is high (more than one electrode can be used based on thickness): - 1 electrode: up to 120 mm. - 2 electrodes: up to 230 mm. - 3 electrodes: up to 500 mm. - It is completed in one go (100% duty cycle). - 40-60 V, 400 - 700 A. - Deposition rate: 10-20 kg/hr. ## ESW (Electro-Slag Welding): - Non-consumable guided tube. - Consumable type of guided tube. ## Guided tube and electrode - Should be of the same composition/material. - This process involves directional solidification. ## Advantages of ESW: - Very thick plate can be welded in one pass. ## Disadvantages (Limitations of ESW): - No edge preparation (square groove is used by process). - No angular distortion. - Single pass process. - Residual stress is less. - There should be no interruption in the process once it starts (the arc is not allowed to break). - Hnet should be high → CR ↑ - HAZ is very wide/coarse. - Low cooling rate causes coarse grain structure in the weld. - Reduces toughness. - Post-heat treatment process required (generally normalizing). ## Electro-gas welding (EGW): - There is very little difference between ESW and EGW. - In EGW, separate gas shielding is used (Ar/CO₂/He). - In EGW, heat of the arc is used to melt the faying surface of the base metal and the electrode. - Cooling rate ↑, Hnet ↑, power density ↑, HAZ ↓. - Mechanical properties ↑, EGW is much better than mechanical properties than ESW. ## Laser beam welding: - LASER = Light Amplification by Stimulated Emission of Radiation. - In a laser, one form of energy (electrical, thermal, chemical) is converted into radiation energy (UV, IR, visible light). - EM radiation achieved is single wavelength (monochromatic radiation) - Laser is used for: - **Heating**: Laser hardening for high carbon steel/cast iron. - **Melting**: Welding, brazing, alloying of the surface layer. - **Control Removal of material**: Machining evaporation. ## Power density (W/mm²) - Evaporation, alloying, welding. - Heating. - Brazing. - Exposure time of laser (s). - CNC table: Relative speed of w/p w/rt laser beam determines exposure time. - Depending upon the combination of power density and the scanning speed, suitable joints are made. For example: - CO₂ Laser: High wavelength laser (10⁻⁶ μm), ≈ 25 kW. - Nd: YAG Laser: Short wavelength laser (1.06 μm), 1-2 kW. (Nd: YAG = Neodymium Yttrium Aluminum Garnet) - **Before using laser, the workpiece must be properly machined and aligned**. - Plates of thicknesses 19 to 32 mm can be welded by lasers. ## Hmet ↑, CR ↑ - Fine grain structure, weld value ↑ , RS ↑. - Sometimes due to high CR, amorphous structure is obtained (e.g. entrapment of gas occurs/porosity and in hardening steel cracking occurs). - There is no use of filler material (autogenous welding). - There is no contamination from outside: - No electrode. - No filler. - Shielding gases may help improve the quality of weld joints. ## Advantages of laser beam welding: - No vacuum requirement. - LBW can produce deep and narrow welds at high welding speeds, with a narrow HAZ and little distortion of workpiece. - It can be used for welding dissimilar metals or parts varying greatly in mass and size. ## Disadvantages of LBW: - Workpiece should be placed accurately. - Good control over the process parameters is needed. - Very high reflectivity of the laser beam by the metal (like Al, Cu) surface is a major drawback. - Equipment cost is very high. ## Laser is directed at the workpiece surface - Conduction mode: Heat by conduction, PDV, depth of pen. ↑ & - Power density ↑, depth of penetration ↑. - Keyhole mode: Depth of penetration ↑. ## Power density of laser - α depth of penetration. # Week 4 ## Solid-Liquid based joining processes - **Soldering**. - **Brazing**. ## Attractive features - No fusion. - Low heat input. ## Material that can be joined has - Metallurgical compatibility - Poor weldability. - Odd position welds. - Different combinations (e.g., Cu-Al). ## Joint configuration: - Lap joint (most common). - Butt joint (especially beveling). ## Molten filler thin/ less viscosity - Laying surface clean - free from impurities, dust, dirt, oil, grease. ## Steps: - Surface preparation. - Placement with proper gap. - Application of heat. - Filler / melting/ distribution by capillary action. - Solidification at the joint. ## Fluxes are used - To avoid gas in the weld and impurities. ## Flux (paste/powder) helps in: - Lowering surface tension. - Avoiding oxidation of filler/base metal. - Improves capillary action. - Formation of slag (flux impurities). - Fluidity of molten metal. ## Soldering: - For Pb-Sn based alloy (solder). - **Temperature range**: 187 °C - 275 °C (low heat). - **Need low heat input**: Mainly used for electronic applications. - **Soldering iron**: Used for heating. ## Brazing - Alloys of Cu, Al, Ni (braze). - **Temperature range**: 450 °C - 800 °C (high heat). - **High heat input**: Compared to soldering. - **Tubes, carbide tools etc**. - **Gas flame (induction is used for heating)**. ## Limitations of soldering/brazing: - Joint strength always less than base metal. - Good for low/moderate temperatures only. - Color mismatch. - Corrosion. - Need flux cleaning (borax, boric acid). ## Principle of soldering: - Lap joints between two sheets using low-melting point: 183-275 °C. - Clearance is controlled: 0.075-0.125 mm for capillary action. ## To ensure good intermetallic bonding surfaces: - Be free from impurities. - Spread ability of molten solder. - Fluidity. - Vapor pressure. - Gravity. - Metallurgical interactions between filler and base metal. - Level of alloying between filler and base metal. ## Strength of soldered joint: - Metallurgical bonds. - Intermetallic compounds formed at the interface: Base metal-solder interaction determines inter-metallic compounds - If no compound is formed at the interface, then the strength is largely determined by the adhesion. ## Soldering materials: - **Lead-tin alloy**: Tin ranging from 5 to 70% and lead 95 to 30%. - High tin lowers the melting point of the alloy and increases the fluidity of molten solder. - Since lead is poisonous, so “lead-free” solders are used. - Tin-antimony solder (Sn-95% & Sb-5%). - Tin-silver solder (Sn-96% & Ag-4%). - Tin-zinc solder (Sn-81-30% & Zn-9 to 70%). - Cadmium-silver solder (Cd-95% & Ag-5%). - Lead-silver solder (Pb-97% & 1.5% tin, 1.5% silver) ## Form of solder and flux for soldering. - Form of solder: Bars and flux cored wires, sheet, foil, ribbon and paste or cream. - Fluxes used in soldering are ammonium chloride, zinc chloride, rosin, and rosin dissolved in alcohol. - These are classified as: - Inorganic fluxes (very active). - Organic fluxes (active). - Rosin fluxes (less active). ## Soldering methods - Soldering irons. - Dip soldering. - Torch soldering. - Oven soldering. - Resistance soldering. - Induction soldering. - Infra-red soldering. - Ultrasonic soldering. ## Soldering methods - **Soldering iron**: Solder is touched to the tip of the soldering iron so that molten solder spreads into the joint surface. - **Soldering flux residue treatment**: Rosin soldering flux can be left on the surface of the joint. Rosin residue flux remove: Alcohol,