Lecture 6 Joining Processes.pdf

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Mechanical Engineering
Alokesh Pramanik

Lecture 6: Joining Processes Tuesday, 29 October 2013 10:48 AM • Introduction • Solid state welding: Forge welding, friction welding, ultrasonic welding • Fusion welding: electric arc welding, resistance welding, oxyfuel gas welding • Geometryof welds and joints • Defects in welds • Limitations/Drawbacksof welding • DVD on welding Welding • Welding is a process in which two or morematerials, usually metal or alloys are permanently joined together by coalescence • The faying surfaces are the part surfaces I contact or close proximity that are being joined • Welding involves localized coalescenceof the two metallic parts at their faying surfaces • Welding, is usually performed on parts made of the same material, howeversome welding operations can be used to join dissimilar metals • Permanentlyjoins faying surfaces • Usually the most economicalway to join parts in terms of material usage and fabrication costs • Mechanical fastening usually requires additional hardware components(eg. Screws and nuts) and geometricalterations of the parts being assembled (eg holes) • Not restricted to a factory environment,it can be accomplished'in the field • Two forms of welding: Solid state and fusion welding • Solid Sate: welding where pressure and or heat are used to cause diffusion at the joint, causing the parts to fuse together • Fusion welding: Heat is applied to create molten material at the joint, which fuses the parts upon solidification Solid State Welding • Refers to joining processesin which coalescenceresults from the application of pressure alone or a combinationof heat and pressure • If heat is used, the temperatureis below melting point of metals being welded • No filler material metal is added in solid state welding • Forge welding (FOW): coalescence is by solid state fusion between two surfaces held together under pressure at • elevated temperature • Friction welding (FRW): coalescence by heat of friction between two surfaces Forge Welding • One of the simplest methodsof joining metals • Versatile, able to join similar and dissimilar metals • Between similar metals,results from solid-state diffusion • Between dissimilar metals, caused by formationof a lower melting temperatureeutectic between the materials; this weld is often stronger than the individual metals • Temperaturerequired often 50-90%of melting temperature • Care must be taken to avoid overheating to the point that is gives off sparks from rapid oxidation (burning) • Ultrasonic welding (USW): coalescenceby ultrasonic oscillating motionin a direction parallel to contacting surfaces of two parts held together under pressure Friction Welding • Produces a weld by simultaneouslyapplying compressiveforce and relative motion(usually rotation) between the faying surfaces • Heat is produces as the faying surfaces rub together • Heat is produces as the faying surfaces rub together • The compressiveforces causes plastic flow (displacement),expelling material from the joint. The expelled material is called flash • Filler material,flux and shielding gas are not required • Most friction welding is performedby holding one of the two workpiecesstationary while the other is rotated about an axis of symmetrywith the faying surfaces perpendicular to that axis • Environmentallyclean, surface cleanliness not as critical • Narrow heat-affectedzone • Most materials and many dissimilar combinations • Less operatorskill required • Good for mass production • Rapid process • One workpiece must have rotatable axis of symmetry • Critical preparation and alignment • High equipment and tooling costs • Some materials not possible • Special machines required for large parts • Processturns grain direction 90⁰ creating potential for preferential corrosion Ultrasonic Welding • High frequency (10-200kHz)low amplitude (<100 micron) shear vibrations are locally applied to workpiecesbeing held together under normal pressure • Welds almost everymaterial but typically limited to small lap joint welds of thin materials or attaching thin sheet to heavier structural members • Although there is heating, not enough to melt base materials. Rapid reversals of stress along contact interface deform, share and flatten the surface asperities. This break up and disperse oxide films and surface contaminationand allows clean surfaces to coalesceinto a high strength bond. • Good automated alternative to glue, screws or snap-fit designs • Typically used with small parts (eg. Phones, consumerelectronics, toys, etc.) but can be used on parts as large as a small automotiveinstrument cluster. • Solid-state process, no melting of materials • Excellent for aluminium, copper and other high-thermal conductivitymaterials often difficult to join by fusion process • Can join many dissimilar material combinations • Able to weld thin to thick materials • Can weld through oxides and contaminants • No filler metals of gasses required • Low energy required • Fast and easily automated • Restricted to lap joint configuration • Limited in join thickness • More difficult with high strength and high hardness materials • Material may deform under the tooling Fusion Welding • Use heat to melt base metals • A filler metal may be added to moltenpool to facilitate process and provide bulk and added strength to the welded joint • A fusion welding operation where no filler metal is added is called an autogenous weld • Heat is supplied by: Oxyacetylenegas, electric arc, plasma arc, laser • Types of fusion welding: Arc welding (AW), Oxyfuel gas welding (OFW) and resistance welding (RW) • Arc welding: melting of the metals is accomplished by an electric arc, local temperatures5500 to 6600⁰C • Oxyfuel gas welding: melting accomplishedby an oxyfuel gas such as acetylene, top temperaturearound 3500⁰C • Resistance Welding: melting is accomplished by heat from resistance to an electrical current • Resistance Welding: melting is accomplished by heat from resistance to an electrical current between faying surfaces held together under pressure Arc Welding • Fusion welding process in which coalescenceof the metals is achieved by heat from an electric arc between an electrode and the work • Same basic process as arc cutting • Electric energy from arc produces temperatures around 5500⁰C,hot enough to melt any metal • Most AW processes add filler metal to increase volumeand strength of weld joint • An electric arc is a discharge of electric current across a gap in a circuit. It is sustained by an ionized column of gas (plasma) through which the current flows. To initiate the arc in AW, the electrodeis brought in contact with the work then quickly separated by a short distance. Powersources are DC and AC ○ AC machines less expensive to purchase and operate by generally restricted to ferrous metals ○ DC equipment can be used on all metals and is generally noted for better arc control • Two types of AW electrodes: ○ Consumable: consumed during welding process, source of filler metal in arc welding ○ Non-consumable: not consumed during welding process. Made of tungsten which resists melting, gradually depleted during welding (principle mechanism is vaporisation),and filler metal must be supplied by a separate wire fed into weld pool • Forms of consumable electrodes: ○ Welding rods (sticks): 9-18 inches and 3/8 inch or less in diameterand must be changed frequently ○ Weld wire: can be continuously fed from spools with long lengths of wire. Avoiding frequent interruptions • Arc Shielding: At high temperatures,metals are chemically reactive to oxygen, nitrogen and hydrogen in air and degrade mechanical properties of joints. Arc must be shielded from surrounding air which is accomplished by shielding gases (Argon, helium or CO ) 2r flux (A substance that prevents formation of oxides and other contaminantsin welding, or dissolves the and facilitates removal;provides protectiveatmosphere for welding, stabilises arc and reduces spattering) • Submerged arc welding (SAW): uses a continuous, consumable bare wire electrode with arc shielding provided by a cover of granular flux. ○ Electrode wire is automaticallyfed from a coil ○ Flux introduced into joint slightly ahead of arc by gravity from a hopper
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