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IMPORTANCE OF WELDING
• Welding is the most efficient way to join metals.• Only way to join two or more pieces of metals to make
them act as one piece.• Welding is the most economical method to
permanently join metal parts.• All metals can be joined by one welding process or
another.• There is saying “if it is metal, weld it” and it is true.• Look around, almost everything made of metal is
welded, the world’s tallest building, moon rocket engines, nuclear reactors, home appliances and automobile, barely start the list.
• The use of welding is still increasing.
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Historical Development of Welding
• Welding, one of the newer metalworking trades, can trace its historic development back to ancient times. The earliest example comes from the Bronze Age. Small gold circular boxes were made by pressure welding lap joints together. It is estimated that these boxes were made more than 2000 years ago and are presently on exhibit at the National Museum in Dublin.
• Items of iron and bronze that exhibit intricate forging and forge welding operations have been found in the pyramids of Egypt.
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Historical Development of Welding..contd
• During the Middle Ages, the art of blacksmithing was developed and many items of iron were produced which were welded by hammering. One of the largest welds from this period was the Iron Pillar of Delhi in India, which was erected about the year A.D. 310. It was made from iron billets welded together. It is approx. 25 ft (7.6m) tall with a diameter of 12 in.(300 mm) at the top and 16 in (400 mm) at the bottom. Its total weight is 12,000 lb (5.4 metric tons). It was not until the nineteenth century that welding as we know it today was invented.
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Early metal joining methods - I
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Early metal joining methods - II
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Historical Development of Welding..contd.
• The production of an arc between two carbon electrodes using a battery is credited to Sir Humphry Davy in 1800. In the mid nineteenth century, the electric generator was invented and arc lighting became popular.
• In 1890, C.L. Coffin of Detroit was awarded the first U.S. patent for an arc welding process using a metal electrode.
• In about 1900, Strohmenger introduced a coated metal electrode in Great Britain.
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Historical Development of Welding..contd.
• Resistance welding processes were developed, including spot welding, seam welding, projection welding, and flash butt welding. Elihu Thompson originated resistance welding. His patents began in 1885. He originated a company, the Thompson Electric Welding Company and developed the different resistance welding processes that time in 1900.
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Historical Development of Welding..contd.
• Stud welding was developed in 1930 at the New York Navy Yard, specifically for attaching wood decking over a metal surface.
• Gas tungsten arc welding had its beginnings from an idea by C.L. Coffin to weld in a non-oxidizing gas atmosphere, which he patented in 1890.
• The gas shielded metal arc welding process was successfully developed at Battelle Memorial Institute in 1948 under the sponsorship of the Air Reduction Company.
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Historical Development of Welding..contd.
• In 1953, Lyubavskii and Novoshilov announced the use of welding with consumable electrodes in an atmosphere of CO2 gas.
• Soon after the introduction of CO2 welding a variation utilizing a special electrode wire was developed.
• Flux core wire process invented by Bernard was announced in 1954 but was patented in 1957, when it was reintroduced by the National Cylinder Gas Company.
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Historical Development of Welding..contd.
• Plasma arc welding was invented by Gage in 1957. Plasma arc welding uses a constricted are or an arc through an orifice, which creates an arc plasma that has a higher temperature than the tungsten arc.
• The electron beam welding process, which uses a focused beam of electrons as a heat source in a vacuum chamber, was developed in France. J.A. Stohr of the French Atomic Energy Commission made the first public disclosure.
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Historical Development of Welding..contd.
• The newest welding process is laser welding . The laser originally developed at the Bell Telephone Laboratories was used as a communication device. Because of the tremendous concentration of energy in a small space it proved to be powerful heat source. It has been used for cutting metals and non-metals.
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The Welding Industry and its future
• Welding is now the universally accepted method of permanently joining metals. It is considered a mature industry but it is still a growing industry on a world wide basis.
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Welding Defined
• “Welding as defined by American Welding Society”• Welding is defined as “a joining process that produces
coalescence (Fusion) of materials by heating them to the welding temperature, with or without the application of pressure or by the application of the pressure alone, and with or without the use of filler metal”.
• In less technical language; A weld is made when separate pieces of material to be joined combine and form one piece when heated to a temperature high enough to cause softening or melting and flow together. Pressure may or may not be used to force the pieces together. Filler material is added when needed to form a completed weld in their joint.
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Welding Basics – Weld joints - I
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Welding Basics – Weld joints - II
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Welding Basics – Welding positions
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Welding and allied processes
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Resistance Welding
Resistance welding (RW): a group of welding processes that produce coalescence of the faying surfaces with the heat obtained from resistance of the workpieces to the flow of the welding current in a circuit of which the workpieces are a part and by the application of pressure.
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Processes of Resistance weldingThere are at least eight different resistance welding processes with many variations:
Resistance spot welding (RSW) Projection welding (PW) Resistance seam Welding (RSEW) Lap welding
High frequency (RSEW-HF)Induction (RSEW-I)
Flash Welding (FW) Butt welding Percussion Welding (PEW) Upset Welding (UW)
High frequency (UW-HF)Induction (UW-I)
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Resistance welding processes (Lap Joints)Spot Welding Projection Welding
Seam Welding
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Resistance Welding process (Butt Joints)Flash Welding Upset Welding
Percussion Welding
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Resistance Welding - Principles of Operation.The concept of resistance welding is most easily understoodby relating it to spot welding.
Heat is generated by the passage of electric current through a resistance circuit
. The max, amount of heat is generated at the point of max. resistance
which is at the surface between the parts being weld.
The high current, up to 1,00000 A. at low voltage generatessufficient heat at this resistance point so that the metalreaches a plastic stage.The force applied before, during and after the current flowforges the heated parts together so that fusion will occur.
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Principles of Operation ..contd.Four factors are involved in making a resistance
weld. 1. The amount of current that passes through
the work2. The time the current flow through the work.3. The pressure/force that the electrode
transfer to the work4. The area of electrode tip in contact with the
work.Heat produced in the weld is expressed by the
formulaH=I2xRxTWhere H=Heat energy I=current in amp. R=resistance of the work in ohms.
T=Time of current flow in seconds.
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Weldability of metals – Another factorWeldability of metal is controlled by three factors:1. Resistivity 2) Thermal conductivity 3) Melting temp.The metal with a high resistance to current flow and with a lowthermal conductivity and a relatively low melting temperatureare easily weldable. These three properties can be combined into a formula that willan indication of ease of welding a metal. The formula isW=R/FKtx100Where W=weldability
R=ResistivityF=Melting Temp. of the metal in 0C.Kt=Relative thermal conductivity with copper equal to1.0
If weldability is below 0.25 it is a poor rating. If W is 0.25to0.75It is fair. Between 0.75 and 2.0 weldability is good and above 2.0weldability is excellent.
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METALS WELDABLE BY THE REISTANCE WELDING PROCESS
Metal Weldability Weldability Rating
Aluminiums Weldable 0.75-2+ Magnesium Weldable 1.80 Inconel Weldable 2+ Nickel Weldable 2.15 Brass and bronze Weldable 0.5-10+ Monel Weldable 2+ Precious metals Variable weldable 0.16-3.0 Low carbon steel Weldable 10+ Low alloy steel Weldable 10+ High and medium carbon steel Weldable 10+ Titanium Weldable 50+ Stainless steel Weldable 35+
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Weldability of carbon steels – AWS Paper
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Resistance Spot WeldingSpot welding is the most common of various resistance welding processes. In this process the weld is produced by the heat obtained at the interface between the work pieces. The heat is due to the resistance to the flow of electric current through the work-pieces, which are held together by pressure from the electrodes. Electrodes serve to concentrate the welding current and the pressure at weld area. The size and shape of formed welds are controlled somewhat by the size and contour of the electrodes.
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Spot Welding Machines
• The spot welding system needs at least the following components
Welding Transformer for supplying power A means of applying pressure A controller/contactor Electrode tips for conducting welding current to the work
piece.• Spot welding machines are available in two categories• Single spot welding machine (stationary,portable)• Multiple spot welding machine• More popular machines are stationary single spot welding
machines of either horn or rocker arm type or the press type
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The rocker arm machines have a pivoted on rocking upper electrode arm which is actuated by either the operator’s physical power or by air or hydraulic power.
They are used for a wide variety of work but are restricted to 50 KVA and are used for thinner sheets.
Rocker Arm Spot Welding M/c
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Press Type Spot Welding M/c
For heavier requirements, press type machines are used. This type of machines normally rated at 50 KVA and above.
In this press type of machine, the upper electrode moves in a slide. The pressure and motion on the upper electrode are provided by pneumatic or hydraulic pressure, or are motor operated.
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Portable Spot Welding MachinesWhen the work is too bulky to take to the welding machine, a portable spot welding machine can be used.
The portable machine is moved from one welding location or fixture to another, and a trigger on the gun actuates the welding cycle.
Portable units are normally operated by air or hydraulic pressure. There are two types of portable welding guns.
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Multiple Spot Welding Machine
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Resistance Spot Welding ElectrodesThe working part of the resistance welding machine is the electrode. The electrode is the means for conducting welding current to the work and for providing the force necessary to make welds.Electrode shapes have been standardized by RWMA. Figure given below shows the six standard. nose shapes identified by letters A through F
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Resistance welding electrodes – material specifications
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Electrode coolant parameter
• It is recommended that the water flow to the electrodes should be a minimum of 4 litres per minute for welding two uncoated steel sheet of thickness upto 3.00 mm.
• Higher flow rates are recommended when welding coated steels.
• The internal water cooling feed tube should be arranged to ensure that the water impinges onto the back working face of the electrode.
• The inlet water temperature should not exceed 20 0C and the outlet temperature should not exceed 30 0C. To maintain these temperature levels, the electrode cooling water supply should be independent of transformer and thyristor water cooling circuits.
• Electrode tip dia should not be allowed to increase 30% of initial tip dia as recommended by RWMA standards. When this diameter has been reached, the electrode should be replaced or redressed to its original size and contour.
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Basic periods of Spot Welding
Squeeze time is the time between the first application of the electrode force and the first application of welding current. Weld time is the actual time the current flows. Hold time is the period during which the electrode force is applied and the welding current is shut off. Off time is the period during which the electrodes are not contacting the work pieces.
The inbuilt timer on the machine controls these four different steps.
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Spot welding programme
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Parameters as recommended by M/c Manufacturers - I
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Parameters as recommended by M/c Manufacturers - II
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Welding Parameters as suggested by AWS/RWMA
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Thumb Rule for selecting spot welding parameters for low carbon steel sheets
If t =thickness of thinnest outside sheet in mm. Then required nugget diameter is = 4 x t To achieve this following weld parameters can be selected.
Weld current = 9 x t KA
Weld time =10 x t cycles
Weld force = 200 x t kgf.
Electrode tip dia = 5 x t (Tip dressing needed if dia
increases by 30%)
Note: This rule is for reference only. End results can be achieved by trials only.
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Welding force charts provided by M/c Manufacturers
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Shunting effect in Spot Welding
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Arrangements in Spot Welding
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Quality of spot weld – visual inspection
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QUALITY OF SPOT WELD – DestructiveTest
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MINIMUM WELD NUGGET DIAMETER FOR AUTOMOTIVE RESISTANCE SPOT WELDS
Material Thickness (in mm) Weld Diameter (in mm)
0.38-0.62 3.0 0.63-0.87 3.8 0.88-1.13 4.3 1.14-1.38 5.1 1.39-1.64 5.3 1.65-1.89 5.6 1.90-2.14 6.1 2.15-2.52 6.6 2.53-2.91 7.1 2.92-3.29 7.4 3.30-3.67 7.6 3.68-4.05 8.1 4.06-4.22 8.4
Avg. diameter = D+d/2 as obtained from a peel test Weld nugget diameter taken from AWS D8.7-88 SAE J-1188 Recommended Practices for Automotive Weld Quality – Resistance Spot Welding.
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Spot welding thickness limitations
Parts of widely different thicknesses may be spot welded. For example 0.4mm material can be welded to 150 mm thick piece. The thickness of the thinner component is referred to as the governing metal thickness because it dictates the heat input. However, the thermal mass of the larger component may make RSW applications impractical because the thick section may not reach the fusion temperature before the thin piece is melted.
The limit of welding piece of equal thickness with an uninterrupted flow of current seems to be approximate 3.2 mm. Hence spot welded lap joints are widely used in joining sheet steels up o about 3.2 mm thick and are used occasionally in joining steel 6.4 mm or more in thickness Preferred sheet thickness ratio of two un even sheet thickness sheets should not exceed 1:3 for spot welding.
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Joint designs for optimum access to std spot welding electrodes
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Resistance Projection WeldingProjection welding is somewhat similar to spot welding. To make a projection weld, projections are formed on at least one of the work pieces at the points where welds are desired. The projections, small raised areas, can be any shape such as round, oval, circular, oblong or diamond. They can be formed by embossing, casting, stamping or machining.
The work pieces that have the projections are placed between plain,large area electrodes in the welding machine. The current is turned on, and pressure is applied. Since nearly all of the resistance is in the projections, most of the heating occurs at the points where welds are desired resulting in fusion of the work pieces.
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Projection welding (embossed)
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Weld nugget formation during Projection Welding
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Projection welding machine
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Types of Projection Welds
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Projection Welding parameters suggested by AWS/RWMA
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Common Projection nut welding parameters used in automotive industry
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Quality of Nut welding - visual inspection
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Quality of Projection nut welding - Test methods
Method for peel test by pressing. Method for peel test by torque
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Resistance Seam WeldingSeam welding is similar in some way to spot welding except that the spots are spaced so close together that they actually overlap one another to make a continuous seam weld. In this process roller wheels act as electrodes. These roller type electrodes are usually copper alloy disc 10mm to 16mm thick. Both the upper and lower wheels are powered. Pressure is applied in the same manner as a press type spot welder. Water cooling is not provided internally and the weld area is flooded with cooling water to keep the electrode wheels cool.
This process is quite common for making flange welds, for making water tight joints for tanks, and so on.
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Resistance Seam Welding Machine
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Thank you
