Welding is a process of permanent joining two materials (usually metals) through localized coalescence resulting from a suitable combination of temperature, pressure and metallurgical conditions Welding is commonly classified in following types Pressure Welding: The piece of metal to be joined are heated to plastic state and forced together by external pressure Fusion Welding or Non-Pressure Welding: The material at the joint is heated to a molten state and allowed to solidify
Arc welding Solid State Welding• Shielded Metal Arc Welding • Forge Welding• Submerged Arc Welding • Cold Welding• Metal Inert Gas Welding • Friction Welding• Tungsten Inert Gas Arc Welding • Explosive Welding• Electroslag Welding • Diffusion Welding• Plasma Arc Welding • Ultrasonic Welding Resistance Welding Thermit Welding• Spot Welding Electron Beam Welding• Flash Welding• Resistance Butt Welding Laser Welding• Seam Welding Gas Welding• Oxyacetylene Welding• Oxyhydrogen Welding• Pressure Gas Welding
In hml we use two processes mainly for joining sheet metal, these are• MIG(METAL INERT GAS WELDING): for joining parts of swing arm and main stand, sheet metal thickness varies from 2-7 mm• SPOT WELDING: for joining parts of chain case, metal thickness varies from 0.50-0.75mm
Gas metal arc welding (GMAW), sometimes referred as metal inert gas (MIG) welding, is a welding process in which an electric arc is formed between a consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt, and join. Along with the wire electrode, a shielding gas is fed through the welding gun, which shields the process from contaminants in the air. A constant voltage, direct current power source is most commonly used with GMAW. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.
The typical MIG welding gun has a number of key parts— a control switch, a contact tip, a power cable, a gas nozzle, an electrode conduit and liner, and a gas pipe. The control switch, or trigger, when pressed by the operator, initiates the wire feed, electric power, and the shielding gas flow, causing an electric arc to be struck. The gas nozzle is used to evenly direct the shielding gas into the welding zone—if the flow is inconsistent, it may not provide adequate protection of the weld area.
• MIG torch. (1) Torch handle, (2) Molded phenolic dielectric (shown in white) and threaded metal nut insert (yellow), (3) Shielding gas diffuser, (4) Contact tip, (5) Nozzle output face
There are three basic forms of wire feeders: the ‘push’ system, the ‘pull’ system and the ‘push–pull’ system. As the name suggests, in the push system, the wire is pushed by the wire feed drive rolls along the conduit to the welding torch. The pull system utilizes a set of wire rolls in the torch handle which pull the wire from the wire reel. This arrangement increases the weight of the torch and does not increase the distance over which the wire can be fed, this still being limited to around 3.5m, although the consistency of the wire feed is improved and wire diameters down to 0.8 mm can be used.
The push–pull system is a combination of the above two systems with a set of drive rolls at both the wire reel feeder and in the torch. Here welding is done by two types of machines• Semiautomatic special purpose machine• Robotic machines(automatic)• We use wire feed range between 10-12M/min, and we use push type system for this purpose A push system
Metal transfer in MIG is done in four modes.1. Dip/short circuit mode2. Pulsed mode3. Spray mode4. Globular mode
It is used for low current operation with lower electrode diameter. The molten metal forming on the tip of the electrode wire is transferred by the wire dipping into the weld pool thus causing a momentary short circuit Metal is transferred only during a period when the electrode tip is in contact with the weld pool
Welding current switches automatically from a low level to a higher level in a periodic manner Lower level current also known as back ground current is set below the transition point and the higher level is set well above the transition point in Spray transfer range. Spray type metal transfer is achieved by applying pulses of higher level current , each pulse having sufficient force to detach a droplet. The power supply are specially designed to produce continuous wave forms and frequencies that PULSE the welding current
Either pure Argon or Argon rich with 0.5 to 5% oxygen shield gas is used. With such gas mixture a true spatter free axial spray transfer becomes possible with current above transition point Spray transfer mode can used in welding any position. The metals droplets being very small, short circuit does not occur and spatter is virtually eliminated A superimposed pulsing current higher than the transition current is necessary for spray transfer
It is characterized by a drop size with a diameter greater then a electrode The droplet detach when there weight exceeds the surface tension of the molten metal that holds the drop to the electrode tip It takes place with a positive electrode (DCRP/DCEP) when the current is relatively low regardless of the type of shielding gas The molten drop grows in size with increasing current from its lowest value where the arc can be barely sustained
Welding current Arc voltage Polarity Electrode Gas flow rate Length of stick out Shielding gas composition
Welding current depends upon welded metal thickness and metal transfer mode required according to the parent metal properties For metal thickness T <6mm=100-200amp T6-8mm=200-450amp T>8mm=450-700amp
In MIG process we generally use constant voltage is used This produce self regulation of arc length For current range 150-200 amp it is kept in between 25- 30V for mild steel workpiece Voltage, V Current, A
In MIG we use DCEP (Direct Current Electrode Positive) or reverse polarity Positive terminal to electrode wire, negative terminal to weld fixture DC ensure elimination of arc blow Heating effect is produced on electrode wire mainly for welding sheet metal
Dia of electrode is dependent on welding current With higher current dia should be larger and vice versa It ranges from 0.7mm to 2.4mm depending upon current For current ranging from 100-200amps 0.8-1.2 mm dia is used Electrode is made of same metal as parent metal coated with deoxidizing agents such as copper, it also prevents impurities
For different applications different flow rate is chosen The four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min is generally suitable, while for globular transfer, around 15 L/min is preferred. Here we use dip mode and require good shielding so rate is kept between 20-25L/min
Length of stick out is generally kept between 10-12mm For stable arc it should not be larger It is controlled by self regulation characteristic of MIG
Shielding gases are necessary for gas metal arc welding to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The most commonly used gas is argon, it is generally mixed with other gases such as CO2 Pure argon doesnt provide much penetration with ferrous metals Whereas pure CO2 causes oxide formation As a result, argon and carbon dioxide are frequently mixed in a 75%/25% to 90%/10% mixture.
Improper choice of a welding gas can lead to welding defects hence mixture should be chosen accordingly Here metal transfer occurs in DIP mode and MILD STEEL is welded so 80% AR and 20% C02 is used Ratio of gasses is controlled by a mixer through which gas mixture is supplied to welding apparatus
Common mixes Argon-carbon dioxide• C-50 (50% argon/50% CO2) is used for short arc welding of pipes,• C-40 (60% argon/40% CO2) is used for some flux-cored arc welding cases. Better weld penetration than C-25.• C-25 (75% argon/25% CO2) is commonly used by hobbyists and in small-scale production. Limited to short circuit and globular transfer welding. Common for short-circuit gas metal arc welding of low carbon steel.• C-20 (80% argon/20% CO2) is used for short-circuiting and spray transfer of carbon steel.• C-15 (85% argon/15% CO2) is common in production environment for carbon and low alloy steels. Has lower spatter and good weld penetration, suitable for thicker plates and steel significantly covered with mill scale. Suitable for short circuit, globular, pulse and spray transfer welding.• C-10 (90% argon/10% CO2) is common in production environment. Has low spatter and good weld penetration, though lower than C-15 one; suitable for many steels.• C-5 (95% argon/5% CO2) is used for pulse spray transfer and short-circuiting of low alloy steel.
Some common defects in MIG are Cracks: due to low speed, over deposition Lack of penetration: low current, oil film on parent metal spatter: caused due to high current Blow holes: due to rusting on wire, impurities in shielding gases or non continuous gas flow Porosity: impurities in wire, inclusion of nitrogen of oxygen causes it Under deposition: less torch speed Over deposition: high torch speed
• Over penetration: when the value of current is very high then the melting of parent metal occurs causing over penetration• Bead out: in this weld bead is displaced from its position, caused due to human or machine error
PENITERATION is the most critical to quality parameter in welding It should be 20% of the thickness of parent metal For mild steel work piece with 20% CO2 in shielding gas it gives 20% penetration It can be checked by destructive testing namely PENETERETION TEST In this cross-section of weld is cut, cleaned with abrasive paper After this nitric acid is poured on it, this acid darken the region of weld and parent metal turns out white Now the depth of penetration can be measured by scale