Production Engineering II Chapter Three Welding & Bonding Technology AAiT
Introduction to welding Joiningis generally used for welding, brazing, soldering, and adhesive bonding which form a permanent joint between the parts. Weldingis a materials joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/ or pressure. Many welding processes are accomplished by heat alone, others by a combination of heat and pressure, and still others by pressure only.
Welding process is divided into two major groups: 1. Fusion welding and 2. Solid-state welding 1. Fusion welding: use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. The fusion category includes: Arc welding (AW), Resistance welding (RW), Oxyfuel gasswelding (OFW), Electron beam welding, and Laser beam welding. 2. Solid-state welding: joining process in which coalescence results from application of 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 metal being welded, no filler metal is utilized. The solid state category includes: Diffusion welding (DW), Friction welding (FRW), Ultrasonic welding (USW).
Arc welding (AW) A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work Electric energy from the arc produces temperatures 10,000 o F (5500 o C), hot enough to melt any metal Most AW processes add filler metal to increase volume and strength of weld joint
Arc Welding (AW) A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake Figure.1.1 Basic configuration of an arc welding process.
Types of Arc welding electrodes i. Consumable: consumed during welding process. Source of filler metal in arc welding ii. No consumable: not consumed during welding process. Filler metal must be added separately. Arc shielding At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air Mechanical properties of joint can be seriously degraded by these reactions To protect operation, arc must be shielded from surrounding air in AW processes Arc shielding is accomplished by: Shielding gases, e.g., argon, helium, CO 2 Flux
Flux is substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal Protecting the molten weld metal from atmosphere Stabilizes arc Reduces spattering Power source in arc welding Direct current (DC) vs. Alternating current (AC) AC machines less expensive to purchase and operate, but generally restricted to ferrous metals DC equipment can be used on all metals and is generally noted for better arc control
Consumable electrodes -AW processes welding processes which uses consumable electrode: Shielded metal AW (SMAW) Gas metal AW (GMAW) Flux-cored AW (FCAW) Electro gas welding (EGW) Submerged AW (SAW) Non consumable electrodes- AW processes Welding process which uses non-consumable electrodes: Gas Tungsten AW (GTAW) Plasma AW (PAW) Carbon AW (CAW)
Shielded metal arc welding (SMAW) Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding Application: construction, pipelines, machinery structures, fabrication job shops and repair work. The equipment is portable and low cost. Welding stick is clamped in electrode holder connected to power source. Used for steels, stainless steels, cast irons, and certain nonferrous alloys. Not used or rarely used for aluminum and its alloys, copper alloys, and titanium.
Gas metal arc welding (GMAW) Uses a consumable bare metal wire as electrode with shielding by flooding arc with a gas Wire is fed continuously and automatically from a spool through the welding gun. Wire diameter used for GMAW is 0.8 to 6.5mm. Gases used for shielding includes inert gas such as argon and helium, and active gases such as carbon dioxide. Shielding gases include argon and helium for aluminum welding, and CO 2 for steel welding Bare electrode wire plus shielding gases eliminate slag on weld bead. No need for manual grinding and cleaning of slag Used in fabrication operations for welding of ferrous and nonferrous metals. It uses continuous weld wire rather than welding stick
Figure.1.3 Gas metal arc welding (GMAW).
Flux-cored arc welding Adaptation of shielded metal arc welding, to overcome limitations imposed by the use of stick electrodes. Electrode is a continuous consumable tubing that contains flux and other ingredients in its core. there are two versions of FCAW: Self-shielded FCAW-core includes compounds that produce shielding gases Gas-shielded FCAW-uses externally applied shielding gases
Figure.1.4 FCAW, Presence or absence of externally supplied shielding gas distinguishes the two types: (1) self-shielded, in which core provides ingredients for shielding, and (2) gas-shielded, which uses external shielding gases.
Electro gas welding (EGW) Uses a continuous consumable electrode, flux-coredwire or bare wire with externally supplied shielding gases, and molding shoes to contain molten metal When flux-coredelectrode wire is used and no external gases are supplied, then special case of self-shielded FCAW When a bare electrode wire used with shielding gases from external source, then special case of GMAW Figure.1.5 Electro gas welding using flux-coredelectrode wire: (a) front view with molding shoe removed for clarity, and (b) side view showing molding shoes on both sides.
Submerged arc welding (SAW) Uses a continuous, consumable bare wire electrode, with arc shielding by a cover of granular flux Electrode wire is fed automatically from a coil Flux introduced into joint slightly ahead of arc by gravity from a hopper. Completely submerges operation, preventing sparks, spatter, and radiation Used in steel fabrication for structural shape (e.g. welded I-beams); seams for large diameter pipes, tanks, and pressure vessels; and welded components for heavy machinery.
Gas Tungsten arc welding (GTAW) Uses a non-consumable tungsten electrode and an inert gas for arc shielding Melting point of tungsten = 3410 C (6170 F) Tungsten inert gas welding (TIG) or (WIG) Used with or without filer metal When filler metal used, it is added to weld pool from separate rod or wire Application: Aluminium and stainless steel mostly
GTAW Advantages: High quality welds for suitable applications No spatter because no filler metal through arc Little or no post-weld cleaning because no flux Disadvantages: Generally slower and more costly than consumable electrode AW processes Plasma Arc welding (PAW) Special form of GTAW in which a constricted plasma arc is directed at weld area Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream Temperatures in PAW reach 28,000 C (50,000 F), due to constriction of arc, producing a plasma jet of small diameter and very high energy density
Figure.1.8 Plasma arc welding (PAW) Advantages of PAW: Good arc stability and excellent weld quality Better penetration control than other AW processes High travel speeds Can be used to weld almost any metals Disadvantages: High equipment cost Larger torch size than other AW processes Tends to restrict access in some joints
Resistance Welding (RW) A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence Heat generated by electrical resistance to current flow at junction to be welded Principal RW process is resistance spot welding (RSW) Figure.1.9 Resistance welding, showing the components in spot welding, the main process in the RW group.
Components in resistance spot welding Parts to be welded (usually sheet metal) Two opposing electrodes Means of applying pressure to squeeze parts between electrodes Power supply from which a controlled current can be applied for a specified time duration Advantages of RW: No filler metal required High production rates possible Lends itself to mechanization and automation Lower operator skill level than for arc welding Good repeatability and reliability Disadvantages of RW: High initial equipment cost Limited to lap joints for most RW processes
Oxyfuel gas welding (OFW) Group of fusion welding operations that burn various fuels mixed with oxygen OFW employs several types of gases, which is the primary distinction among the members of this group Oxyfuel gas is also used in flame cutting torches to cut and separate metal plates and other parts Most important OFW process is oxyacetylene welding Oxyacetylene Welding (OAW) Fusion welding performed by a high temperature flame from combustion of acetylene and oxygen Flame is directed by a welding torch Filler metal is sometimes added Composition must be similar to base metal Filler rod often coated with fluxto clean surfaces and prevent oxidation
Figure.1.10 A typical oxyacetylene welding operation (OAW) Most popular fuel among OFW group because it is capable of higher temperatures than any other Up to 3480 C (6300 F)
Two stage reaction of acetylene and oxygen: First stage reaction (inner cone of flame) C 2 H 2 + O 2 2CO + H 2 + heat Second stage reaction (outer envelope) 2CO + H 2 + 1.5O 2 2CO 2 + H 2 O + heat The two stages of combustion are visible in the oxyacetylene flame emitted from the torch. When the mixture of acetylene and oxygen is in the ratio 1:1, it results neutral flame. Maximum temperature reached at tip of inner cone, while outer envelope spreads out and shields work surface from atmosphere. Figure.1.11 The neutral flame from an oxyacetylene torch indicating temperatures achieved.
Safety issue in OAW Together, acetylene and oxygen are highly flammable C 2 H 2 is colorless and odorless It is therefore processed to have characteristic garlic odor C 2 H 2 is physically unstable at pressures much above 15 lb/in 2 (about 1 atm) Storage cylinders are packed with porous filler material saturated with acetone (CH 3 COCH 3 ) Acetone dissolves about 25 times its own volume of acetylene Different screw threads are standard on C 2 H 2 and O 2 cylinders and hoses to avoid accidental connection of wrong gases Alternative Gases for OFW: Methylacetylene-Propadiene (MAPP), Hydrogen, Propylene, Propane, and natural gas.
Soldering and Brazing Soldering and brazing process lie some where in between fusion welding and solid state welding. These processes have some advantages over welding process. They can join metals having poor weldability, dissimilar, with very less amount of heating needed. The major disadvantage is joint made by soldering and brazing has low strength as compared to welded joint. The major difference between them is huge amount of heat is requiredin caseof welding, low amountin case ofbrazingandvery low amount in case of soldering. Joints made by brazing are relatively weak and soldering joints are very weak to bear load. Generally, soldering is used to make electrical contacts.
Ways to choose joining methods: There are no hard-and-fast rules. In general, the controlling factors are: thetypesofmetalyouarejoining, cost involved, nature of the products you are fabricating, and the techniques you use to fabricate them. Because of its flexibility and mobility, gas welding is widely used for maintenance and repair work in the field. On the other hand, gas shielded metal arc welding to repair a critical piece of equipment made from aluminum or stainless steel.
1. FILLER METALS Welding Terminology The material that you add to fill space during the welding process isknownasthefillermetal,ormaterial. Two types of filler metals commonly used in welding are welding rods and welding electrodes. welding rods 1. Refers to a form of filler metal that does not conduct an electric current during the welding process. 2. The only purpose of a welding rod is to supply filler metal to the joint. 3. This type of filler metal is often used for gas welding. welding electrodes 1. conducts the current from the electrode holder to the metal being welded. 2. Consumable electrodes :not only provide a path for the current but they also supply fuller metal to the joint (shielded metal- arc welding). 3. Non consumable electrodes : are only used as a conductor for the electrical current (gas tungsten arc welding). The filler metal for GTAWisahandfedconsumableweldingrod.
Welding Terminology 2. FLUXES The term flux refers to a material used to dissolve oxides and release trapped gases and slag (impurities) from the base metal; thus the flux can be thought of as a cleaning agent. In performing this function, the fluxallowsthefillermetalandthebasemetaltobefused. No single flux is satisfactory for universal use; however, there are a lot of good general-purpose fluxes for use with common metals. In general, a good flux has the following characteristics: A. It is fluid and active at the melting point of the fuller metal. B. It remains stable and does not change to a vapor rapidly within the temperature range of the welding procedure. C. It dissolves all oxides and removes them from the joint surfaces. D. It adheres to the metal surfaces while they are being heated and does not ball up or blow away. E. It does not cause a glare that makes it difficult to see progress. F. It is easy to remove after the joint is welded. G. It is available in an easily applied form.
Welding Joints The weld joint is where two or more metal parts are joined by welding. Thefivebasictypesofweldjointsarethebutt,corner,tee,lap,andedge.
A butt joint is used to join two members aligned in the same plane. This joint is frequently used in plate, sheet metal, and pipe work. Corner and tee joints are used to join two members located at right angles to each other. Various joint designs of both types have uses in many types of metal structures. A lap joint, as the name implies, is made by lapping one piece of metal over another. This is one of the strongest types of joints available; however, for maximum joint efficiency, you should overlap the metals a minimum of three times the thickness of the thinnest member you are joining. Lap joints are commonly used with torch brazing and spot welding applications. An edge joint is used to join the edges of two or more members lying in the same plane. It is more frequently used in sheet metal work
Standard Terms (Parts of Joints) 1. Therootofajointisthatportionofthejointwherethemetalsare closest to each other. The root may be a point, a line, or an area, when viewed in cross section.
2. Groove is an opening or space provided between the edges of the metal parts to be welded.
The specified requirements for a particular joint are expressed in terms as bevel angle, groove angle, groove radius, and root opening.
Root penetration refers to the depth that a weld extends into the rootofthejoint.rootpenetrationismeasuredonthecenterlineof the root cross section. Joint penetration refers to the minimum depth that a groove weld extends from its face into a joint, exclusive of weld reinforcement. Weld reinforcement is a term used to describe weld metal in excess ofthemetalnecessarytofillajoint.
Fig: Root penetration and joint penetration of welds
Weld reinforcement Simple weld bead Square groove welds Single Bevel groove welds Double Bevel groove welds
Single V groove welds Fig: Standard groove welds Double V groove welds
Weld Quality Concerned with obtaining an acceptable weld joint that is strong and absent of defects Also concerned with the methods of inspecting and testing the joint to assure its quality Welding defects: 1. Cracks 2. Cavities 3. Solid inclusions 4. Incomplete fusion
Welding cracks Fracture-typeinterruptions either in weld or in base metal adjacent to weld Serious defect because it is a discontinuity in the metal that significantly reduces strength Caused by embrittlement or low ductility of weld and/or base metal combined with high restraint during contraction In general, this defect must be repaired Figure.1.13 Various forms of welding cracks.
Cavities Two defect types, similar to defects found in castings: 1. Porosity -small voids in weld metal formed by gases entrapped during solidification - Caused by inclusion of atmospheric gases, sulfur in weld metal, or surface contaminants 2. Shrinkage voids -cavities formed by shrinkage during solidification
Solid inclusions Solid inclusions -nonmetallic material entrapped in weld metal Most common form is slag inclusions generated during AW processes that use flux - Instead of floating to top of weld pool, globules of slag become encased during solidification Metallic oxides that form during welding of certain metals such as aluminum, which normally has a surface coating of Al 2 O 3
Incomplete fusion Also known as lack of fusion, it is simply a weld bead in which fusion has not occurred throughout entire cross section of joint Figure.1.14 several forms of incomplete fusion
Inspection and testing methods 1. Visual inspection Most widely used welding inspection method Human inspector visually examines for: -dimension, warpage, Cracks, cavities, incomplete fusion, and other surface defects Limitations: Only surface defects are detectable Welding inspector must also decide if additional tests are warranted. 2. Nondestructive evaluation tests (NDT) Ultrasonic testing -high frequency sound waves through specimen to detect cracks and inclusions Radiographic testing -x-rays or gamma radiation provide photograph of internal flaws Magnetic particle testing iron filings sprinkled on surface reveal subsurface defects by distorting magnetic field in part
Welding Symbols Special symbols are used on a drawing to specify where welds are to be located, the type of joint to be used, as well as the size and amount of weldmetaltobedepositedinthejoint. These symbols have been standardized by the American Welding Society (AWS). The reference line becomes the foundation of the welding symbol. It is used to apply weld symbols, dimensions, and other data to the weld. The arrow simply connects the reference line to the joint or area to be welded. The tail of the welding symbol is used only when necessary to include a specification, process, or other reference information.
Welding Symbols Example: Weld symbols applied to reference line
SPECIFING WELDING LOCATION DIMENSIONS APPLIED TO SYMBOLS Filet Weld: Arrow Side Filet Weld: Other Side Filet Weld: Both Side
And of course Do staffs Safe all the time specially Welding!! Use PPE (Personal Protection Equipments)
Personal protection equipments (PPE) Protect your eyes with safety glasses. Protect your body from welding spatter and arc flash with protective clothing: Woolen clothing (possibly cotton) never synthetic! Welding jackets Flame-proof apron Gloves Properly fitted clothing that is not frayed or worn Long-sleeve shirts Straight-leg trousers that cover shoes Fire resistant cape or shoulder covers for overhead work Leathers to protect specific body parts or areas Keep clothes free of grease and oil.
The End