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Joining Processes WELDING
WELDING Definition: Material joining process. Two parts connected at their contacting surfaces by suitable heat and pressure. Many welding processes are accomplished by heat alone, some others by heat and pressure, and some with pressure only. In some welding operations a filler material is used. Welding operation usually applied to metals but also used for plastics. 2
Joining Processes Parts produced by any of the manufacturing processes can be made into larger, more complex bodies via Joining processes • • Creating a metallurgical bond by adhesion and diffusion Joining by fusion with the use of various heat sources Brazing or soldering with a lower-melting metal Mechanical fastening
Fusion Welding Processes
Sources of Energy for Fusion Welding • Chemical reactions – Burning gases – GAS WELDING • Heat from electricity – Arc – ARC WELDING SYSTEMS – Resistance welding • Light LASER
Three Specific Types of Welding Modules • In this Welding, Cutting, and Brazing module, three specific types of welding are covered. These are listed below: – Oxygen-fuel gas welding and cutting – Arc welding and cutting – Resistance welding
Resistance Welding • Definition: – This is a group of fusion welding processes that use heat and pressure to make the coalescence. • The heat comes from electrical resistance to current flow at the site of the weld. – The processes include: • Spot Welding • Projection Welding • Seam Welding Note: FP&M only does spot welding.
Resistance Welding • Spot Welding – A process typically used in highvolume, rapid welding applications. • The pieces to be joined are clamped between two electrodes under force, and an electrical current is sent through them.
Welding/Cutting Hazards • Potential Hazards – Fires may start by hot materials igniting nearby combustibles. – Burns to the operator may occur if unprotected skin comes into contact with the extremely hot work. – Magnetic fields could easily destroy/disrupt electronic components, stored data if not careful.
Welding/Cutting Hazards • Potential Hazards Cont. : – Metal fumes from vaporizing of the work with the extremely hot arcs may be inhaled into the worker’s lungs. – Certain metals and metal oxide fumes, including zinc, cadmium and beryllium, produce serious illnesses when inhaled. – Fluxes used with welding to create inert atmospheres at the point of the weld also present inhalation hazards. – All welding and cutting must have adequate ventilation to protect the person doing the welding and those working around the welding area.
Weld Joint Structure Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding. Microhardness (HV) profile across a weld bead. A fusion joint is far from homogenous. Degree of inhomogeity increases from pure metals to multiphase alloys.
Typical weld zone in arc and gas welds • The base material adjacent to the melt boundary is exposed to high temperatures, and the properties and structure are changed within the heat-affected zone. • Cold worked base material will show recrystallization in HAZ, with coarse grain sizes. • In either case, a coarse-grained structure of lower strength exits at the melt boundary. Melt
Weldability and Weld Quality - Welding Defects-
Welding Defects 1. Fusion welding defects due wrong heat input, insufficient rate of weld metal deposition, and cooling. 2. Lack of bonding or gas porosity due to surface contaminants, including oxides, oils, etc. 3. Undesirable reactions with surface contaminants 4. Solidification cracks in the weld. 5. Solidification shrinkage coupled with solid shrinkage imposes internal tensile stresses on the structure, may lead to distortion. 6. Gases released or formed during welding (eg CO) can lead to porosity which weakens the joint and acts as a stress raiser.
Weld joint There are 5 basic joint types in welding • Butt joint: Two materials are in the same plane, joined from the edges. • Corner joint: The corners of two materials form a right angle and joined. • Lap joint: Two parts overlaps. • Tee joint: One part is perpendicular to the other, making a T shape. • Edge joint: Edges of the two materials joined. 16
Weld Joints 17
Types of weld 1. Fillet weld: Used in T joints, corner joints, lap joints. 2. Groove weld: Used in butt joints. 3. Plug weld: Used in lap joints. 4. Slot weld: Used in lap joints. 5. Spot weld: Used in lap joints. 6. Seam weld: Used in lap joints. 7. Flange weld: Used in edge joints. 8. Surfacing weld: Not a joining process, it is used to increase thickness of the plate, or provide a protective coating on the surface. 18
Fillet Weld Slot and Plug Weld Groove weld 19
Metal Treatment 1. Preheating the weld zone – reduces energy input, cooling rates in the weld and HAZ, reduces differential shrinkage, residual stresses, and distortion. 2. Postwelding heat treatment of the entire welded structure a. Stress-relief anneal reduces residual stresses to acceptable level. b. Normalizing a steel wipes out most undesirable effects of welding. c. Full heat treatment (quenching and tempering of steels) 3. Peening (hammering or rolling) of weld bead improves the strength of welds.
Oxyacetylene Gas Welding Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.
Oxyacetylene Torch The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are righthanded. Oxygen regulators are usually painted green, and acetylene regulators red.
Oxyfuel gas welding 23
Oxygen-fuel gas welding & Cutting • The elements of Oxygen-fuel gas welding and cutting: – General Requirements – Cylinders – Service Pipe Systems – Pipe System Protection
Oxygen-fuel gas welding and cutting • General Requirements – Focuses on using Acetylene Safely • Flammable • Unstable • Cannot be adjusted above 15 psi – Safe Work Practices • The pressure adjusting screw: – Turning clockwise allows the gas allows to flow. Turning counterclockwise reduces or stop the gas flow. • Blow out cylinder valve – • Turn on cylinder valve first and then adjust the regulator pressure screw. • Never stand in front or behind a regulator when opening the cylinder valve • Open cylinder valve slowly
Oxygen-fuel gas welding and cutting • General Requirements Cont. : – Safe Work Practices • • • Purge oxygen and acetylene passages Light the acetylene Never use oil or grease Do not use oxygen as a substitute for air Keep your work area clean
Oxygen-fuel gas welding and cutting L 10 : Joining processes • Cylinders – Cylinder approval and marking • marked for the purpose of identifying the gas content, with either the chemical or trade name of the gas – Storage of cylinders • • Storage area must be well ventilated Cylinders must be at least 20 feet from combustibles Valves must be closed Valve protection must be in place Inside storage must be limited to 2, 000 cubic feet. Cylinders must be stored in upright position Oxygen must be at least 20 feet from fuel gas or 5 feet with a 1/2 hour fire barrier • Separate oxygen from fuel gas
Oxygen-fuel gas welding and cutting • Cylinders Cont. : – Operating Procedures • Operation must emphasize the absence of oily or greasy substances. Follow these rules of operation: – Cylinders, cylinder valves, couplings, regulators, hose, and apparatus shall be kept free from oily or greasy substances. – Oxygen cylinders or apparatus shall not be handled with oily hands or gloves. – A jet of oxygen must never be permitted to strike an oily surface, greasy clothes, or enter a fuel oil or other storage tank.
Oxygen-fuel gas welding and cutting • Service Pipe Systems – There are special requirements for service pipe systems when using oxygen or acetylene. • Oxygen • Acetylene or Acetylene Compounds
Oxygen-fuel gas welding and cutting • Pipe System Protection – The entire service pipe system must be protected against build-up of excessive pressure and leaks. This protection is accomplished with: • Protective equipment • Regulators • Proper hose and hose connections.
Oxygen-fuel gas welding and cutting • Pipe System Protection Cont. : – Protective equipment is divided into the two categories listed here: • Pressure Relief Devices – The pressure relief device should discharge upwards to a safe location. – Pressure relief valves are required in fuel-gas piping systems to prevent excessive pressure build up within the system.
Oxygen-fuel gas welding and cutting • Pipe System Protection Cont. : – Approved protective equipment shall be installed in fuel-gas piping to prevent: • Backflow of oxygen into the fuel-gas supply system • Passage of a flash back into the fuel-gas supply system • Excessive back pressure of oxygen in the fuel-gas supply system.
Arc Welding 34
Arc Welding and Cutting • Definition: – A fusion process wherein the coalescence of the metals is achieved from the heat of an electric arc formed between an electrode and the work. • Application • Installation • Operation & Maintenance t
Arc Welding & Cutting • Application – Applies to a large and varied group of processes that use an electric arc as the source of heat to melt and join metals. • Installation – Arc welding requires proper installation of equipment. – A critical part of installation is ensuring that proper grounding is completed.
Arc Welding & Cutting • Operation & Maintenance – All connections to the machine shall be checked to make certain that they are properly made. – The work lead shall be firmly attached to the work. – Magnetic work clamps shall be free from adherent metal particles of spatter on contact surfaces. – Coiled welding cable shall be spread out before use to avoid serious overheating and damage to insulation.
Arc Welding & Cutting • Operation and Maintenance Cont. : – During welding operations, cables with splices within 10 feet (3 m) of the holder shall not be used. – Welders should not coil or loop welding electrode cable around parts of their body. – Cables with damaged insulation or exposed bare conductors shall be replaced. – Joining lengths of work and electrode cables shall be done by the use of connecting means specifically intended for that purpose. – The connecting means shall have insulation adequate for the service conditions.
Methods of Arc Welding • Three Types of Welding Methods: – Tungsten Inert Gas Welding (TIG) – Gas Metal Arc Welding (MIG) – Shielded Metal Arc Welding (SMAW)/ Stick Welding
Electric Arc Welding • Heat is produced from electric arc between workpiece and electrode material for melting the workpiece material. • AC and DC are used • An Inert gas shields both electrodes • Most heat energy is due to electron flow to metal Heat input Direct Current Electrode Negative (DCEN): Deeper weld. Direct Current Electrode Positive (DCEP): Shallower and Wider H- heat input, E, Voltage, I, Current, and v the velocity of the arc travels along the weld line
Electric Arc Welding - Classification • The Electrode – Consumable: melts and serves as a filling material – Non-consumable: does not melt, parent metal is used, or a separate filler rod – Coated or Uncoated • Coating – Provides a gaseous shield to prevent oxidation – Lowers the voltage needed to establish the arc – May provide slag-blanket to protect the joint – Add alloying elements to enhance the properties of the joint.
Non-Consumable Electrode Arc Welding • Gas Tungsten Arc Welding ( TIG) • Plasma arc welding • Atomic hydrogen welding
Non-Consumable-Electrode Welding: Gas Tungsten-Arc Welding (GTAW) • Nonconsumable gas tungsten Inert gas welding (TIG) • Weld zone is protected by inert gas • DC with straight polarity is used with steel, cast iron, and stainless • AC with Al, Mg alloys where ac helps in stripping the oxide • Both hand automatic operations are possible • The process demands considerable skill but produces very high-quality welds on almost any material • No weld spatter or slag formation
Consumable Electrode Arc Welding Processes • Shielded metal arc welding • Submerged arc welding • Gas metal arc welding
Consumable Electrode Arc Welding Shielded-Metal Arc Welding Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process.
Consumable-Electrode Welding: Gas Metal-Arc Welding (GMAW) - MIG • Consumable gas metal-arc welding (MIG) • Consumable electrode is metal which melts to become part of the weld seam. • Weld zone is protected by a gas or a flux • No slag is formed • Several layers could be build with little or no intermediate cleaning • It is suitable for most metals • Wire electrode can be supplied in long, coiled lengths which allow uninterrupted welds in any welding position.
Consumable Electrode Arc Welding Gas - Gas Metal-arc Welding Process- (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. (b) Basic equipment used in gas metal-arc welding operations.
Other Welding Processes High Energy Beam Welding • Electron Beam welding (EBW) – Heat is produced by high velocity electron gun in a narrow beam – No filler material – High rate of heating results in greater depth and heat-affected zone is very small – Suitable for welding refractory materials like: molybdenum and zirconium – Requires a vacuum (limitation) – x-ray will be generated around the welding gun which may be cancerous
High Energy Beam Welding • LASER Beam welding (LBW) – Uses a focused high power monochromatic light beam as a source of heat to the metal – Beam can be directed to the welding spot with a lens – Depth of welding similar to electron beam welding – Vacuum is not necessary (advantage) – Workpiece usually needs protection by a gas – Process is suitable for automation – Welding speeds can be upto 7 m/min
Laser beam welding 50
Thermit Welding 51
Explosion Welding 52
Resistance welding 53
Forge welding 54
PCB – printed circuit boards Solder Pads Soldering Top View Side View Circuit Board Resistor
Move soldering iron until tip is touching wire & solder pad
Move solder to touch edge of tip. Solder
Hold until solder melts on tip by wire Solder
Move solder back to touch wire only Solder
Move solder in to form a small pocket Solder
Move soldering iron tip up. This will drag solder up with it. Solder
Look for shinny fillets
Brazing • Definition: – A process which a filler metal is placed at or between the faying surfaces, the temperature is raised high enough to melt the filler metal but not the base metal. • The molten metal fills the spaces by capillary attraction. • Torch Brazing – Oxy-fuel torch with a carburizing flame – First heat the joint then add the filler metal
Safe Work Practices • Electric & Gas Welding – Safety Check: • Ensure electrical cord, electrode holder and cables are free from defects – No cable splices within 10 feet of electrode holder. • Ensure welding unit is properly grounded. This helps to avoid over heating. • All defective equipment shall be repaired or replaced before using.
Safe Work Practices • Electric & Gas Welding Cont. : – Safety Check: • Remove all jewelry – rings, watches, bracelets, etc… • Ensure PPE e. g. . welding hood, gloves, rubber boots or safety shoes, apron are available and in good condition. • Ensure fire extinguisher is charged and available. • Ensure adequate ventilation and lighting is in place. • Set Voltage Regulator to Manufacture’s specifications. • Avoid electrical shock DON’T wrap cables around any body part. • Ensure fittings are tight.
Safe Work Practices • Electric & Gas Welding Cont. : – Safety Check: • Inspect hoses for cuts and frayed areas. • Set gauges to desired PSI. • Ensure that sufficient PPE is made available. • Locate welding screens to protect employee’s – DON’T block your exit. • Ensure that adequate ventilation and lighting are in place.
Fire Protection & Prevention Cont. : • Welding areas should meet the following requirements: – Floors swept & cleared of combustibles 35 ft. radius of work area. – Flammable and combustible liquids kept 35 ft. radius of work area. – At least one fire extinguisher – on site – Protective dividers to contain sparks and slag • Welding curtains • Non-combustible walls • Fire resistant tarps & blankets UW-Eau Claire Facilities Planning & Management
Proper Ventilation for Welding • Ventilation – Proper ventilation can be obtained either naturally or mechanically. • Natural Ventilation is considered sufficient for welding and brazing operations if the present work area meets these requirements: – Space of more than 10, 000 square feet is provided per welder – A ceiling height of more than 16 feet. • Mechanical ventilation options generally fall into two basic categories. – Low vacuum system which takes large volumes of air at low velocities. – High vacuum system that are captured and extracted fumes as near to the work as possible.
Fire Protection & Prevention • Fire hazards must be removed, or – Guards installed, or – Welding/cutting must NOT take place • Hot work permit should be used outside designated areas to ensure that all fire hazards are controlled • Use of fire watch – 1/2 hour after operation ceases
Proper Ventilation for Welding L 10 : Joining processes • Ensure protection from fumes and gases by one or a combination of the following: – Good general ventilation. – Use of a booth. – Local exhaust ventilation on the hand piece. – Air supply to the helmet.
Welding Operators Protection • Welding involves specialized personal protection that must be worn every time you perform welding operations. The following is a list of basic PPE: – – – Fire-resistant gloves Aprons Safety shoes Helmet Ultraviolet radiation filter plate (arc welding) Goggles with filter lenses U
Welding, Cutting and Brazing • Summary – Major hazards include: • • Fire Burns Shock Toxic Exposure – Follow proper procedures to prevent fires – Use appropriate engineering controls – Wear appropriate PPE