NPtel Notes on Manufacturing Processes Till Week 4 PDF

## Week 31 - 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 has to be manufactured: physical, chemical, mechanical and dimensional properties, stra...

## Week 31 - 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 has to be manufactured: physical, chemical, mechanical and dimensional properties, straightness, flatness, surface roughness - Manufacturing processes which are commonly used: - Casting: Shifting from one shape to another - Forming: Shifting of material from one shape to another by deformation - Machining: Unwanted material gets removed - Joining: To achieve desired shape and size, two or more parts are joined to achieve the required product **Note: ** - **Zero process:** Process in which there is no loss of material during manufacturing. - **Negative process:** Process in which material gets lost as a waste during manufacturing. - **Positive process:** Process in which material is added externally during manufacturing. ## Joining - Process of bringing two things together - It is a positive process. ### Types of Joining - **Mechanical joining:** Uses nuts and bolts, claps, 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, compatability, service suitability, choice of joint - **Type of joint:** - **Temporary:** Mechanical joints - **Permanent:** Welding - **Service condition:** Ambient, corrosion, low/high temperature, chemical, reliability (mechanism 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 and Finish - **Weld point** - **Epitaxial solidification** - **Welding thermal cycle** - **Post welding treatment:** DBTT (Ductile to Brittle Transition Temperature), Creep, HAZ softening and Hardening. ### Factors that influence the selection of the 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 conditions** - **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 it's dissembling - Hazardous fumes and vapors - Poor reliability ### Welding Process 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 accelerator:** Ultrasonic vibration, pressure pulsing - **Melting and solidification** ### Epitaxial - When the same filler material is used. ### Nucleation and Growth - When a different filler material is used. **Note:** **Autogenous welding:** Welding in which two workpieces of the same material are welded without the addition of a separate filler material. ## Welding Process - **Solid State:** Friction stir welding - **Liquid State:** Soldering, brazing ### Classification of Joining - **Based on the technological approach of joining** - **Based on the 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:** To 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 gaps** - **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** - **Science of Energy** - **Arc/No-arc process** ### Fusion and Pressure Welding ### Classification: Approaches of Joining - **Welding:** - **Cast welding**: Thermite, ESW - **Fusion welding**: Where fusion involves - **Resistance welding:** (Jart principle used for heating) - **Solid state welding**: Uses high heat/flow or interfacial balk deformation - **Allied processes:** - **Metal deposition** - **Soldering** - **Brazing** - **Adhesive bonding** - **Weld Surfacing** - **Metal spraying** **Note:** - **With filler material:** Autogenous welding process - **Examples:** Gas welding, electron beam welding, laser beam welding, resistance welding, friction welding, ultrasonic welding. - **Without filler material:** **Same as base metal**: Solidification occurs by growth mechanism or epitaxial **Different from the base metal**: Solidification occurs by nucleation and growth mechanism - **Filler may be used** - **Filler is usde** - **Resistance, friction, ultrasonic, electron explosion beam welding** - **Laser beam, election beam, TIG (Tungsten Inert Gas), PAW (Plasma arc welding) welding** - **Shielded metal arc, welding, submerged arc welding, gas metal arc welding, electro-slag welding, electro gas wedding** ## Source of Energy (Heat) - **Chemical energy:** Chemical reaction for producing heat (ex. thermite welding) - **Mechanical energy:** Produces heat for the joining process - **Electrical energy:** - **Radiation energy** - **Frictional energy** - **Interfacial friction or impact** - **Arc process:** GMAW, SMAW, SAW, GTAW, Plasma (PAW), carbon (CAW), electro gas (EGW) **Note:** - **Flash butt welding** and **electro-slag welding** are controversial welds. ## Flash Butt Welding - It's a fast welding method - Used for cleaning and softening ## Heat for Joining - The purpose of heat is: - **Cleaning** - **Softening** - **Diffusion** - **Melting** ## Chemical Reaction: Gas Welding - **Inner cone** (C2H2 + O2 → 2CO + H2 + 448 kJ/mol (12.75 MJ/m³ of acetylene) - **Inner white cone** (3100°C) (4CO + 2H2 + 3O2 → 4CO2 + 2H2O + Q12 kJ/mol (35.77 MJ/m³) ) - **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** - Fe2O3 + 2Al → 2Fe + Al2O3 - 3CuO + 2Al → 3Cu + Al2O3 ## Electrical Resistance Heating - Heat generated by electrical resistance heating: I²Rt - **Where:** - **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 - **Where:** - **n:** Fraction of energy (energy lost in friction) is 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 - The 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:** - Gas fusion welding - Manual arc welding - MIG welding - Plasma arc welding - Election beam welding - Laser beam welding - **Energy density:** - **Of gas welding:** Higher and lower than **LBW** - **Of LBW:** Will take less amount of heat for melting a certain area - **Low energy density:** High heat input, large HAZ, and large weld size - **High energy density:** Low heat input, small HAZ, and small weld size - **High energy density:** Hnet < cooling rate - **Effect of heat input:** - Increasing damage to workpiece - Increasing damage to workpiece - **Heat input to workpiece versus cooling rate:** - **Gas welding** - **Arc welding** - **High energy beam welding:** Increasing penetration, welding speed, weld quality and equipment cost - **Low density of heat source:** Longer solidification time, low energy density, high heat input - Course grain structure - Allows removal of impurities - Poor mechanical properties - Segregation of constituents - Leads to cracking of weld area - **Need for protection of weld pool:** - Entrapment of atmospheric gases in the 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 in the weld pool - Gases form compound inclusion ## Weld Pool Protection Approach - Forming a developing cover/envelope of inert or inactive gases: - **TIGW:** Inert - **MIGW:** Inert (inactive) - **PAW:** Ar/He - **SMAW:** CO2/CO - **Forming cover of molten flux (slag):** - Used in **SAW** (Submerged arc welding), **EGW** (Electro slag welding) - **By creating vacuum**: - **Used in:** Election beam welding, diffusion welding (Temp≈0.5 to 0.6 Tm) **Note:** - 02% or N2% ↑ ⇒ Porosity ↑ and vice versa - For **TIG:** 02%, N2% is very less - For **MIG:** 02%, N2% is very high - N2% in a weld α UTS and YS (Yield strength) - N2% in a weld α Toughness and ductility - % of O2 in a weld α Ductility, toughness and strength - % of O2 in a weld α % Element transten efficiency (ear, si, mm) ## Week 32 - 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 weld metal is similar to base metal - **Nucleation and growth:** It occurs when the composition is different ### Observed Grain Structure - **Planer** - **Cellular** - **Dendritic** - **Equiaxed** - **Weld:** - **Softened Al**: Weak zone of the weld - **Hardened steel:** Cracking/brittle fracture ## Gas Welding - **Fuel gas** used for joining - **Oxygen** - **Acetylene (C2H2):** Mostly used (3300°C) - **Propylene** - **Propane** - **Hydrogen** - **Natural gas** - **Outer blue flame:** (1275°C) - **Heat generation:** It is high here - **Low temperature** - **Inner cone:** (highest temperature ≈ 3100-3300°C) - **Heat generation:** It is less - Intermediate flame feather - **Flame temperature:** - **Inner cone** - **Outer envelope** - **Acetylene feather** ## Type of Flame: - **Neutral:** O2/C2H2 → 1 → Preferably used - **Oxidizing:** O2/C2H2> 1 → High temperature - **Carburizing:** O2/C2H2 < 0.9 **Note**: - **The 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 impairments:** Normally paste, powder, solid coating liquid is applied on faying surfaces before heating - **These are called fluxes.** Commonly used fluxes - borax ## Gas Welding: - **Forward welding:** - **Cooling rate:** ↑ - **Hardenable steel:** Preheated - **decreases tendency of cracking:** - **Backward welding:** - **Post weld heat treatment:** Reduces tendency of cracking - **Residual stresses:** ↓ - **Gas welding (low power density):** - **Flames are spread over a larger area** - **Temperature of the heat source is 100 - 3300°C** - **Gas welding (high heat input):** - **Cooling rate:** ↑ - **HAZ is large** - **Protection of weld pool:** Not good - **Soundness of weld:** ↓ ## Welding Positions: - **Flat:** - **Horizontal:** Difficult position - **Vertical:** Difficult position - **Overhead** : Most difficult position ## Type of Weld: - **Groove weld** - **Fillet weld** - **Bead weld** - **Plug weld** - **Slot weld** ## Type of Joints - **Butt joint** - **Lap joint** - **Tee joint** - **Corner joint** - **Edge joint** ## Welding arc - Electrode - Cathode drop zone (Ve) - Plasma (Vp) - Anode drop zone (Va) - Anode - **Power of the arc:** VI - **Net heat:** VI/S J/mm - **S:** Speed of the moving arc - **Heat generated by a welding arc** - **V x I x τ** - **For:** - **SMAW:** 50 - 70V - **GMAW:** 20 - 40V - **GTAW/PAW:** 10 - 20V ## Arc Initiaton Method - **Touch start:** (GTAW, SMAW) - **Field start** - **When the arc is initiated once:** - **Thermal emission of 'e' occurs** - **Ionization of metal vapor** ## SMAW (Shielded Metal Arc welding) - Constant current type power source is used. - Both AC and DC can be used. - **Core wire (d) (mm)** - **D (Diameter of electrode with coating)** - **Flux coating** - **Protective gas shield** - **Arc** - **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:** 1.6 - 2.2 - **Coating factor:** - **Purpose of coating:** - **Arc stabilization:** (N, K, Ca are used for coating as they have low ionization potential [LIP], and the arc is stable) - **Flux for removing impurities:** - **Deoxidize:** (Fe-Mn, Fe-Si, etc.) - **Controlled alloying:** - **Increased deposition efficiency:** - **Adjusting viscosity (Surface tension):** Of slag (molten metal), generally in vertical or overhead welding as molten metal falls downward ### Constituents of Coating Flux and Their Role | Item | Formula | Role | |---|---|---| | Quartz | SiO2 | Enhance the current carrying capacity | | Magnetite | Fe3O4 | Refining, transfer of molten metal drops | | Calcium carbonate| CaCO3 | Lower arc voltage and release inactive gases | | Flourspar | CaF2 | Increasing viscosity of molten metal | | Ferro-Manganese & ferro-silicon | Fe-Mn & FeSi | De-oxidants | | Cellulose | | Releases 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 the welding arc:** - **Cathode drop zone (Ve)** - **Plasma drop zone (Vp)** - **Anode drop zone (Va)** - **V = Vc + Vp + Va** - **Melting of Cathode:** Is generally governed by the heat generated in the cathode drop zone. - **Heat at cathode drop zone:** VexI x τ ## Melting Rate (MR) - MR = AI + bLI² - **AI:** Heat generated at the anode/cathode drop zone - **bLI²:** Heat generated due to electrical resistance heating (I²Rt heating) - **a:** Coefficient which accounts for factor 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, Reverse polarity - **DC Electrode (positive)** - DCEP/DESP/DCRP, Straight polarity - **AC** - **Deep weld:** No surface cleaning - **Shallow weld:** Surface cleaning - **Intermediate:** ### Effect of Polarity - **Heat generation/distribution:** May be different - **Stability of the arc** - **Cleaning action:** Offered by the welding arc ### Welding Process Parameters - **Welding current** - **Arc voltage** - **Welding speed** ## Welding Current - Directly affects heat generation which affects: - **Melting rate** - **Penetration depth** - **Cross sectional area of the weld being deposited** - **Arc voltage (V) α Weld thickness** α **Arc stability** α **Depth of weld** α **reduction of penetration** - **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. - It uses non-consumable electrodes. - Inert gas is used for the protection of the weld pool (ex. Ar, He). - Arc length used in this process is very short. - These parameters lead to clean weld (ie, less % of O2 and Ne). ### Electrode - **Tungsten (W) is used:** Pure tungsten electrode coated with 2m, La, Ce (LIP elements) - **Pure coated tungsten electrode:** Increases current carrying capacity of the electrode. - 1.5mm dia. of the 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 - **He/N2:** Economical, costly ## Arc Length in GTAW - Generally 1 -3 mm - **Constant/short arc length** ## In TIG - **Heat input (inert)** needed is low. - **Power density:** Of TIG is greater than SMAW/SAW/GMAW - **Weld size:** ↓ ⇒ **HAZ:** ↓ ⇒ **CR:** ↑ ⇒ **Quality of weld joint:** ↑ - **TIG is used for:** - Quality weld joints for critical applications, such as nuclear, aerospace, aircraft, etc. ### Limitation of TIG: - **Welding of thicker plates:** It's tough (limited penetration) - **Productivity:** It's 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:** It fluctuates in GMAW - **Electrode is consumed** - **Impurities in the weld** - **Stability of arc:** Is not good - **O2/N2 in GMAW**: > In GTAW - **Heat of arc melts both electrode and base:** With a constant voltage, torch, power source, and workpiece - **Productivity is good** - **Clean weld is made** ## Week 33 - **Newer variants of Gas Tungsten Arc Welding (GTAW)** - **Pulse GTAW/TIG Welding** - **Constant (DC) current type power source (100 - 1500 A)** - **DCEN** - **Power:** VI - **Heat input:** VI/S/thick - **Facilitates welding of thin/sheets at low heat input** - **Less distortion** - **Ip (Peak current):** Low frequency pulsing, high frequency pulsing - **Ib, Ip:** - **Ib, Itp:** - **Imean:** Ipx tp + Ibxtb/tp+tb - **Power:** Imean x Y - **Background time (Current):** Hnet ↑ ⇒ CRT (Grain size/structure) ↑ ⇒ HAZ ↑ - **Hot wire GTAW:** - **Contact tube** - **TIG torch** - **Ceramic guidance** - **Tungsten** - **GTA power source (DC)** - **Core wire** - **Extension** - **Core wire heating power source (AC)** - **Hot wire GTAW:** - **Conventional GTAW:** - **Kg/hr deposition fate** (Deposition rate versus power) - **Hot wire GTAW:** - **Particularly used when thick plates are to be welded and needed quality welds.** ## Activated Flux GTAW - **Activated flux:** Like SiO2, TiO2, Cr2O3 are used with acetone/ethanol (# g/ml → to make paste) - **Deeper penetration (15mm)** ⇒ **Productivity** ↑ - **Arc construction:** Flux - **Reversal of marangoni convection:** - **High temperature:** High surface tension - **Low temperature:** Low surface tension & vice versa - **Reduced width:** and **increased depth** - **Reduced angular distortion** - **Cross section of weld:** Reduced - **Reduces stresses (residual)** - **Reduces HAZ** ## Cold Metal Transfer Welding - Meaning of ''cold'' is that in spatter-free, HAZ, RS, heat input↓, tempt↓, surface tension ↑, fluidity of molten metal, elemental transfer efficiency ↑, thim sheets (0.3mm) - **Constant speed** - **DCRP:** Constant voltage - **Modes of transfer:** - **Short circuiting transfer:** Workpiece is less - **Globular transfer:** (Gap between electrode and workpiece is less) - **Arc Iap ↑** - **Spray transfer (IT)**: - **Rotational transfer (ITS):** Avoided because - ## Pulse GMAW - **Ip:** (Peak current) - **Ib:** (Base current) - **t:** (Time) - **Stable arc** - **Suitable for thin sheets or very sensitive metal toward heat:** - **HAZ, RS, Hnet, distortion, and 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 the 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 in - **Unstable and erratic arc** - **Short circuiting and buried arc** - **Lower penetration** - **Higher melting rate of electrode** ## 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 the mode of metal transfer from globular to spray and rotary transfer with minimum spatter - They produce globular modes of metal transfer - **CO2** results in a weld joint with spattering, owing to a difference in heat generation during welding, - **Shielding gas also affects:** - Weld penetration - Weld current - Weld bead geometry and deposition rate - Quality of weld ## Welding Voltage - **Arc voltage** directly ***affects the width of the 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 weld pool** - **Discrete drop moving from tip to weld pool** - **Drop transfer** can be globular or spray type. - **Type of metal transfer:** Depends upon: - **Welding current** - **Electrode diameter** - **Shielding gas** - **Can take place through different modes:** - **Short circuit** - **Globular** - **Spray** - **Dip** ## Role of Current Pulsation: - **Peak current:** Primarily for melting - **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:** Either: - **Constant Current:** ~2.4 mmdia - **Constant voltage:** < 2.4mm dia - **DCEP/DCRP:** is used for high heat generation in electrode - **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 (DCRP:** - High electrical resistivity (8) - High electrical extension (EE) - Small diameter of electrode - **Electrodes used with variable speed by feed driver system at CC speed:** - **EE Short** - **EE Large diameter of electrode** - **Current ↑: DR ↑:** 90 -95% - **Productive used in industries:** - **MRT, Depth of penetration:** ↑ (Used for thick plates) ## Advantages of SAW - The protecting and refining action produces clean welds. - Since the arc is submerged, spatter and heat losses to the surrounding are less. - Both alloying elements and metal powders can be added to 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 that 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 plate (20 - 500 mm) - **Power source:** DC, CV, DCRP ## Electro-Slag Welding Setup - **Molten flux:** - **Copper shoe:** - **Contact/guide electrode:** - **Heated (55°C):** - **Plates being welded** - **Molten weld metal:** - **Solid weld metal:** - **It is a single pass up-hill welding:** - **Hnet ↑, CR ↑, HAZ ↑:** Course grain structure - **Post weld heat treatment:** Is required (normalizing heat treatment to increase toughness, generally in steel). - **No angular distortion:** Due to square grove type joints - **RS ↓** ### Electrode Thickness - If the plate thickness is high, more than one electrode can be used, based on thickness. - **1 electrode:** Up to 120mm - **2 electrodes:** Up to 230mm - **3 electrodes:** Up to 500mm - **It is completed:** In one go (100% duty cycle) - **Deposition rate:** 40 - 60V and 400 - 700 A - **DSW:** 10 - 20 kg/hr - **ESW:** Non-consumable guided tube - **Consumable type of guided tube** - **Guided tube show 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 of ESW - No edge preparation (square groove is used by process) - No angular distortion - Single pass process - Residual stress is less ## Limitations of ESW - There should be no interruption in the process once it gets started. - 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 is 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 arc is used to melt the faying surface of base metal and 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 LASER**, one form of energy (electrical, thermal, or light) 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 C steel or cast iron. - **Melting**: Welding, brazing, alloying of the surface layer. - **Control removal of material:** Machining evaporation ### Laser Power Density (W/mm²) - **Evaporation:** - **Alloying:** - **Welding** - **Brazing:** - **Heating:** - **Boring:** - **Exposure time of laser (s):** - **CNC table:** - **Relative speed of workpiece w.r.t. 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-μm) → ≈ 25kW - **Nd:YAG laser:** Short wavelength laser (1.06 μm) → 1-2 kW (Nd: Yttrium Aluminum Garnet) **Before using a laser, the workpiece must be properly machined and aligned.** **Plates of a thickness of 19 to 32 mm can be welded by laser.** ## LBW Advantages - **Hnet ↑, CR ↑:** Fine grain structure, weld value ↑, RS ↑. - **Sometimes due to high CR:** Amorphous structure is obtained, EM entrapment of gas occurs (porosity) and 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 in quality weld joints - **LBW can produce deep and narrow welds:** At high welding speeds, with a narrow HAZ and little distortion of the 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 process parameters is needed.** - **Very high reflectivity of laser beam:** By the metal (like Al, Cu) surface, is a major drawback. - **Equipment cost is very high.** - **Laser is directed on the workpiece surface by two modes:** - **Conduction mode:** Heat by conduction (power density ↑, depth of penetration ↑) - **Keyhole mode:** Depth of penetration ↑ - **Power density of a laser α Depth of penetration** ## Week 34 - **Solid - Liquid based joining processes:** - **Soldering** - **Brazing** - **Attractive features:** - **No fusion** - **Low heat input