Welding_2 PDF - Manufacturing Processes

ME 3270 Manufacturing Processes Instructor: Mohsen Eshraghi, Ph.D. WELDING PROCESSES 2 WELDING PROCESSES 1. Arc Welding 2. Resistance Welding 3. Oxyfuel Gas Welding 4. Other Fusion Welding Processes 5. Solid State Welding 6. Weld Quality 7. Weldability 8. Design Con...

ME 3270 Manufacturing Processes Instructor: Mohsen Eshraghi, Ph.D. WELDING PROCESSES 2 WELDING PROCESSES 1. Arc Welding 2. Resistance Welding 3. Oxyfuel Gas Welding 4. Other Fusion Welding Processes 5. Solid State Welding 6. Weld Quality 7. Weldability 8. Design Considerations in Welding Two Categories of Welding Processes  Fusion welding - coalescence is accomplished by melting the two parts to be joined, in some cases adding filler metal to the joint  Examples: arc welding, resistance spot welding, oxyfuel gas welding  Solid state welding - heat and/or pressure are used to achieve coalescence, but no melting of base metals occurs and no filler metal is added  Examples: forge welding, diffusion welding, friction welding 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 F (5500 C), hot enough to melt any metal  Most AW processes add filler metal to increase volume and strength of weld joint Arc Welding  A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake Manual Arc Welding and Arc Time  Problems with manual welding:  Weld joint quality  Productivity  Arc Time = (time arc is on) divided by (hours worked)  Also called “arc-on time”  Manual welding arc time = 20%  Machine welding arc time ~ 50% Two Basic Types of Arc Welding Electrodes  Consumable – consumed during welding process  Source of filler metal in arc welding  Nonconsumable – not consumed during welding process  Filler metal must be added separately if it is added Consumable Electrodes  Forms of consumable electrodes  Welding rods (a.k.a. sticks) are 9 to 18 inches and 3/8 inch or less in diameter and must be changed frequently  Weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions  In both rod and wire forms, electrode is consumed by the arc and added to weld joint as filler metal Nonconsumable Electrodes  Made of tungsten which resists melting  Gradually depleted during welding (vaporization is principal mechanism)  Any filler metal must be supplied by a separate wire fed into weld pool Arc Shielding  At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air  Mechanical properties of joint can be 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, CO2  Flux Flux  A substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal  Provides protective atmosphere for welding  Stabilizes arc  Reduces spattering Various Flux Application Methods  Pouring granular flux onto welding operation  Stick electrode coated with flux material that melts during welding to cover operation  Tubular electrodes in which flux is contained in the core and released as electrode is consumed 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 Electrode AW Processes  Shielded Metal Arc Welding  Gas Metal Arc Welding  Flux-Cored Arc Welding  Submerged Arc Welding Shielded Metal Arc Welding (SMAW)  Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding  Sometimes called "stick welding"  Power supply, connecting cables, and electrode holder available for a few thousand dollars Shielded Metal Arc Welding (SMAW) Welding Stick in SMAW  Composition of filler metal usually close to base metal  Coating: powdered cellulose mixed with oxides and carbonates, and held together by a silicate binder  Welding stick is clamped in electrode holder connected to power source  Disadvantages of stick welding:  Sticks must be periodically changed  High current levels may melt coating prematurely Shielded Metal Arc Welding  Shielded metal arc welding (stick welding) performed by a human welder (photo courtesy of Hobart Brothers Co.) 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  Shielding gases include argon and helium for aluminum welding, and CO2 for steel welding  Sometimes it is called Metal Inert Gas (MIG) welding  Bare electrode wire plus shielding gases eliminate slag on weld bead  No need for grinding and cleaning of slag Gas Metal Arc Welding GMAW Advantages over SMAW  Better arc time because of continuous wire electrode  Sticks must be periodically changed in SMAW  Better use of electrode filler metal than SMAW  End of stick cannot be used in SMAW  Higher deposition rates  Eliminates problem of slag removal  Can be readily automated Flux-Cored Arc Welding (FCAW)  Adaptation of shielded metal arc welding, to overcome limitations of stick electrodes - two versions  Self-shielded FCAW - core includes compounds that produce shielding gases  Gas-shielded FCAW - uses externally applied shielding gases  Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g., alloying elements) in its core Flux-Cored Arc Welding  Presence or absence of externally supplied shielding gas distinguishes: (1) self-shielded - core provides ingredients for shielding, (2) gas-shielded - uses external shielding gases 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 Submerged Arc Welding SAW Applications and Products  Steel fabrication of structural shapes (e.g., I-beams)  Seams for large diameter pipes, tanks, and pressure vessels  Welded components for heavy machinery  Most steels (except hi C steel) https://www.youtube.com/watch?v=IRensmZN0wU Gas Tungsten Arc Welding (GTAW)  Uses a nonconsumable tungsten electrode and an inert gas for arc shielding  Melting point of tungsten = 3410C (6170F)  A.k.a. Tungsten Inert Gas (TIG) welding  Used with or without a filler metal  When filler metal used, it is added to weld pool from separate rod or wire  Applications: aluminum and stainless steel mostly Gas Tungsten Arc Welding Advantages and Disadvantages of 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 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) Resistance Welding  Resistance welding, showing 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 and Drawbacks of Resistance Welding  Advantages:  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:  High initial equipment cost  Limited to lap joints for most RW processes Resistance Spot Welding (RSW)  Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes  Used to join sheet metal parts  Widely used in mass production of automobiles, metal furniture, appliances, and other products  Typical car body has ~ 10,000 spot welds  Annual production of automobiles in the world is measured in tens of millions of units Spot Welding Cycle  (a) Spot welding cycle, (b) plot of force and current  Cycle: (1) parts inserted between electrodes, (2) electrodes close, (3) current on, (4) current off, (5) electrodes opened https://www.youtube.com/watch?v=kl8TAVdvE48 Resistance Seam Welding (RSEW)  Uses rotating wheel electrodes to produce a series of overlapping spot welds along lap joint  Can produce air-tight joints  Applications:  Gasoline tanks  Automobile mufflers  Various sheet metal containers Resistance Seam Welding 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  The 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 flux to clean surfaces and prevent oxidation Oxyacetylene Welding Acetylene (C2H2)  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) C2H2 + O2 → 2CO + H2 + heat  Second stage reaction (outer envelope) 2CO + H2 + 1.5O2 → 2CO2 + H2O + heat Other Fusion Welding Processes  FW processes that cannot be classified as arc, resistance, or oxyfuel welding  Use unique technologies to develop heat for melting  Applications are typically unique  Processes include:  Electron beam welding  Laser beam welding  Electroslag welding  Thermit welding Electron Beam Welding (EBW)  Fusion welding process in which heat for welding is provided by a highly-focused, high-intensity stream of electrons striking work surface  Electron beam gun operates at:  High voltage (e.g., 10 to 150 kV typical) to accelerate electrons  Beam currents are low (milliamps)  Power in EBW not exceptional, but power density is very high EBW Vacuum Chamber  When first developed, EBW had to be carried out in a vacuum chamber to minimize disruption of electron beam by air molecules  Serious inconvenience in production  Pumpdown time can take as long as an hour Advantages and Disadvantages of EBW  Advantages:  High-quality welds, deep and narrow profiles  Limited heat affected zone, low thermal distortion  No flux or shielding gases needed  Disadvantages:  High equipment cost  Precise joint preparation & alignment required  Vacuum chamber required  Safety concern: EBW generates x-rays Laser Beam Welding (LBW)  Fusion welding process in which coalescence is achieved by energy of a highly concentrated, coherent light beam focused on joint  LBW normally performed with shielding gases to prevent oxidation  Filler metal not usually added  High power density in small area  So LBW often used for small parts Comparison: LBW vs. EBW  No vacuum chamber required for LBW  No x-rays emitted in LBW  Laser beams can be focused and directed by optical lenses and mirrors  LBW not capable of the deep welds and high depth-to-width ratios of EBW  Maximum LBW depth = ~ 19 mm (3/4 in), whereas EBW depths = 50 mm (2 in) Solid State Welding (SSW)  Coalescence of part surfaces is achieved by:  Pressure alone, or  Heat and pressure  If both heat and pressure are used, heat is not enough to melt work surfaces  For some SSW processes, time is also a factor  No filler metal is added  Each SSW process has its own way of creating a bond at the faying surfaces SSW Advantages over FW Processes  If no melting, then no heat affected zone, so metal around joint retains original properties  Many SSW processes produce welded joints that bond the entire contact interface between two parts rather than at distinct spots or seams  Some SSW processes can be used to bond dissimilar metals, without concerns about relative melting points, thermal expansions, and other problems that arise in FW Friction Welding (FRW)  SSW process in which coalescence is achieved by frictional heat combined with pressure  When properly carried out, no melting occurs at faying surfaces  No filler metal, flux, or shielding gases normally used  Can be used to join dissimilar metals  Widely used commercial process, amenable to automation and mass production  Friction Welding  (1) Rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created Applications and Limitations of Friction Welding  Applications:  Shafts and tubular parts  Industries: automotive, aircraft, farm equipment, petroleum and natural gas  Limitations:  At least one of the parts must be rotational  Flash must usually be removed (extra operation)  Upsetting reduces the part lengths (which must be taken into consideration in product design) Friction Welding Cross section of butt joint of two steel tubes joined by friction welding (courtesy George E. Kane Manufacturing Technology Laboratory, Lehigh University) Friction Stir Welding (FSW)  SSW process in which a rotating tool is fed along a joint line between two workpieces, generating friction heat and mechanically stirring the metal to form the weld seam  Distinguished from FRW because heat is generated by a separate wear-resistant tool rather than the parts  Applications: butt joints in large aluminum parts in aerospace, automotive, and shipbuilding Friction Stir Welding  (1) Rotating tool just before entering work, and (2) partially completed weld seam Advantages and Disadvantages of Friction Stir Welding  Advantages  Good mechanical properties of weld joint  Avoids toxic fumes, warping, and shielding issues  Little distortion or shrinkage  Good weld appearance  Disadvantages  An exit hole is produce when tool is withdrawn  Heavy duty clamping of parts is required 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  Topics:  Residual stresses and distortion  Welding defects  Inspection and testing methods Residual Stresses and Distortion  Rapid heating and cooling in localized regions during FW result in thermal expansion and contraction that cause residual stresses  These stresses, in turn, cause distortion and warpage  Situation in welding is complicated because:  Heating is very localized  Melting of base metals in these regions  Location of heating and melting is in motion (at least in AW) Residual Stresses and Distortion  (a) Butt welding two plates  (b) Shrinkage  (c) Residual stress patterns  (d) Likely warping of weldment Techniques to Minimize Warpage  Welding fixtures to physically restrain parts  Heat sinks to rapidly remove heat  Tack welding at multiple points along joint to create a rigid structure prior to seam welding  Selection of welding conditions (speed, amount of filler metal used, etc.) to reduce warpage  Preheating base parts  Stress relief heat treatment of welded assembly  Proper design of weldment Welding Defects  Cracks  Cavities  Solid inclusions  Imperfect shape or unacceptable contour  Incomplete fusion  Miscellaneous defects