Types of Welding
There are over 70 different welding processes. The most common of which are:
Shielded Metal Arc Welding (SMAW), also known as Manual Metal Arc Welding, MMAW.
This process uses a consumable electrode to support the arc. Shielding is achieved by the melting of the outer flux coating on the electrode. Filler metal is obtained from the electrode core.
- Good for windy, outdoor conditions
- OK to use on dirty or rusty metal
Recommended metal: Steel, stainless steel, cast iron
Skill Level: Moderate
Stick welding is operated in either direct current or alternating current. In direct current, electricity always flows from negative to positive. Just like when you turn on your garden hose and the water begins to flow, when you weld with DC, the current flows out of the negative and back into the positive. This makes for a smooth welding current. In alternating current, the electricity flows back and forth from negative to positive and positive to negative on a sine wave. This makes an erratic flow for SMAW, producing more spatter and a more unstable current.
Direct current electrode positive (DCEP) is what we used to call reverse polarity. Direct current electrode negative (DCEN) is what we used to call straight polarity. In DCEP the electricity flows into the tip of the welding rod and concentrates about two-thirds of the heat, which gives good penetration. DCEP is usually used on thicker steels. In DCEN the electricity flows out of the rod, concentrating about one-third of the heat on the rod. Less penetration makes this a very good choice for thinner steels. On some machines, a switch is available for changing from AC to DCEP or DCEN; on others, the leads have to be changed.
The three major weld defects in SMAW are porosity, slag inclusion, and undercut, the cardinal sin of welding. Porosity is wormholes in the weld. It may be caused by moisture in the flux. These can be turned into tiny steam explosions, or even minute traces of gas left in the steel when it was formed. Slag inclusion occurs when the slag is not chipped and/or cleaned properly and then welded over. A good welder will burn through any exposed slag, but sometimes the slag may be skipped over, leaving some trapped under the weld. Undercut is the cardinal sin because it occurs when the base metal is penetrated, or cut into, without leaving any filler metal. This usually happens when welders use the wrong rod angle, go too fast, or use amps that are too hot. Spatter is the little molten droplets that stick to the steel around your weld. With 7018 you won't get nearly as much spatter as with 6010 or 6011. If the spatter chips off easily, all is well. If it doesn't chip off, you're welding too hot.
Welding puts out radiation through ultraviolet light rays. It's like having a sun at the end of your rod. These rays can sunburn your skin, eyes and even blister your corneas. Always welding with your skin exposed can lead to skin cancer. All too often welders don't want to wear heavy hot leathers or long sleeves on hot days. Do it anyway! Melanoma can be deadly.
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Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding.
This process uses welding equipment with a high-frequency generator. The arc is created between a non-consumable tungsten electrode and the workpiece. Filler metal may or may not be used, and argon inert gas or inert gas mixtures are used for shielding.
- Provides high-quality, precise welds
- Highly aesthetic weld beads
Recommended metal (AC TIG): Aluminum, magnesium alloys
(DC TIG): Steel, stainless steel, copper, brass, and titanium
Skill Level: High
TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. If a flat characteristic power source is used, any contact with the workpiece would damage the electrode tip or fuse the electrode to the workpiece. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminum.
Scratching the surface can start the welding arc. When the short-circuit is broken the main welding current will flow. There is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimized using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts, which last for a few microseconds.
Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Other additives are lanthanum oxide and cerium oxide. These are claimed to give superior performance as in arc starting and lower electrode consumption. It is important to select the correct electrode diameter and tip angle for the level of welding current. As a general rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. Because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip where in the electrode assumes a spherical or 'ball' profile.
Shielding gas is selected according to the material being welded. The following guidelines may help choose a proper gas:
- Argon - the most commonly used shielding gas which can be used for welding a wide range of materials including steel, stainless steel, aluminum and titanium.
- Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. A more constricted and hotter arc permits higher welding speeds. Drawbacks include the risk of hydrogen cracking in carbon steels and weld metal porosity in aluminum alloys.
- Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This allows higher welding speeds and deeper weld penetration. Drawbacks of using helium or a helium/argon mixture are the high cost of gas and difficulty in starting the arc.
TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration. TIG is also widely applied in mechanized systems either autogenously or with filler wire.
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Flux Cored Arc Welding (FCAW).
This is a process that uses a wire-fed welding machine. Metals are melted and joined by heating them with an arc between a continuous, consumable electrode wire and the workpiece. The weld is tubular with flux material contained inside the shielding. Added shielding may or may not be supplied from external gas or gas mixture, depending on the type of flux cored wire being used.
- Works well on dirty or rusty material
- Deep penetration for welding thick sections
- Can be used with or without shielding gas
- Self-shielded wire are best for windy conditions
Recommended to weld: Steel, stainless steel
Skill Level: Low
FCAW not only is easy to teach and to use, it's one of the most flexible welding processes around. FCAW is an excellent process, whether you are a hobbyist or a skilled structural welder in the shop or field. It is a very efficient process to use in all conditions.
FCAW is very similar to gas metal arc welding (GMAW). In GMAW, a solid steel wire is fed from a spool through drive rolls that either push or pull through the gun. The same is true with FCAW. GMAW uses direct current, electrode positive (DCEP), while FCAW uses direct current, electrode negative (DCEN), or direct current electrode positive (DCEP). GMAW uses a shielding gas to protect the puddle from contaminants in the atmosphere such as nitrogen and hydrogen. FCAW uses a tubular wire that has flux in the middle. When the flux burns it emits a shielding gas and thus protects the puddle. The flux turns into slag, just as in the shielded metal arc welding (SMAW) process. Shielding gas is not necessary, but it can be used to make a bead even smoother.
Using FCAW, a welder does not have to stop and change rods as in SMAW. That means longer beads with fewer restarts, high weld deposit, and more production. This means less chance of defects in the start/stop areas. The process uses DCEP and produces deep fusion with a good weld appearance. Also, Smaller diameter wire can be used in all positions and in smaller areas. FCAW can be used with or without shielding gas. Carbon dioxide is very cheap shielding gas to use. The flux contains oxidizers, so the base metal needs minimum cleaning before welding. Post weld cleanup is a breeze because the slag chips off very easily.
Fumes are bad with the FCAW process. It puts out more smoke than a burning steak on a barbeque. Using FCAW in a shop, you should make sure you have good ventilation. These welding fumes are not healthy for you. FCAW is used only on mild steel. It has limited uses for cast iron and stainless, but mild steel is all that is usually used on it.
There are about a million welding projects you can use FCAW for in your shop/garage. FCAW is a great process for the home welder, and there are some really nice, affordable machines out there nowadays. For less than $500 you can get a fully equipped GMAW-FCAW machine that can plug right into your garage's 110 outlet. You can pick up a machine for less, but the ones that are right around $500 come set up completely for GMAW or FCAW. I'd say spend the extra bucks and make your life a little bit simpler.
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Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) Welding or hard wire welding.
A process used with a wire feed welding machine. Metals are joined when heated with an arc. This is between the continuously fed filler metal electrode (solid wire) and the workpiece. Externally supplied gas or gas mixture provide shielding.
- Easy process to learn
- Better control on thinner metals
- Cleaner welds with less slag to clean
- Equipment can be used for Flux Cored welding
Recommended metal: Steel, stainless steel, aluminum, cast iron
Skill Level: Low
MIG is one of the easiest processes to learn. Most people can learn to run good beads with MIG in just a few hours. What is MIG? MIG stands for metal inert gas. In MIG, a spool of solid-steel wire is fed from the machine, through a liner, then out of a contact tip in the MIG gun. The contact tip is hot, or electrically charged, when the trigger is pulled and melts the wire for the weld puddle. In MIG welding, small droplets of molten wire, heated when short-circuited, flow together to make a puddle as they touch the base metal. Inert gas flows out of the gun and keeps the weld puddle shielded from the atmosphere. Thus, metal inert gas. Inert means the gas will not combine with another element. So inert gases, like helium and argon, are used. It was then discovered that carbon dioxide, which is not an inert gas, also worked well. Then someone figured, now we can no longer call it MIG, so let’s call it gas metal arc welding (GMAW).
MIG is usually used in shops and factories, because out in the field, the wind displaces the shielding gas, which, ironically, is there to displace the wind. You have to be careful MIG welding in close quarters, because some of the shielding gases, such as argon, can displace the oxygen in your brain or collapse your lungs, causing you to wake up dead!
MIG is the perfect machine to weld up a storm in your garage. MIG welding is very popular, because it is easily learned and because you can do and make many things with it. A good machine costs at least $400 to $500. You can buy a cheaper machine, but you’ll get what you pay for. Now the nice thing about a good, small MIG machine is that you can plug it right into the 110V outlets in your garage.
MIG Advantages
- High productivity, because you don’t have to stop to change rods or chip and brush the weld frequently.
- Easy to learn and makes great-looking welds.
- Almost no cleanup.
- Can weld on stainless, mild steel, and aluminum.
- Can weld in all positions.
MIG Disadvantages
- Can’t turn your attention else wear.
- Requires a cumbersome bottle of shielding gas.
- Costs money for consumables, such as tips and nozzles.
- Doesn’t work to great on paint, rust, or dirty surfaces.
- Not good for thick steel, because it doesn’t get the proper penetration.
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Plasma Arc Welding (PAW), Plasma Arc Cutting (PAC) and Gouging
Plasma is a gas that is heated to an extremely high temperature and ionized so that it becomes electrically conductive. Similar to GTAW (TIG), the plasma arc welding process uses this plasma to transfer an electric arc to a work piece. The metal to be welded is melted by the intense heat of the arc and fuses together.
In the plasma welding torch a Tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is initiated between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded.
By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of heat to a small area. With high performance welding equipment, the plasma process produces exceptionally high quality welds.
Plasma gases are normally argon. The torch also uses a secondary gas, argon, argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing oxidation of the weld.
A full list of reasons for using the plasma welding process is lengthy but can be summarized into three main features where customers desire the advantages of at least one feature.
- Precision: The plasma process is generally more precise than conventional TIG (remember that enhanced power supplies can create an arc that is different to a conventional TIG arc) Plasma offers the following advantages over conventional TIG:
- Stable, concentrated arc
- Forgiveness in arc length variations (TIG +/- 5%, Plasma +/- 15%)
- Small Part Welding:
- Low amperage capability (many plasma power supplies go down to .1 amps)
- Stable at low amps
- Gentle arc transfer (arc start) with no high frequency noise.
- Short weld times possible (for spot welds - guide wires, tubes etc.)
- High Production Welding:
- Long electrode life offers many more hours of welding than TIG before electrode contamination occurs.
Plasma cutters are ideal for cutting and fabricating metal — from thin sheets, to thick beams. Plasma cutting can be performed on any type of conductive metal - mild steel, aluminum and stainless are some examples. With mild steel, operators will experience faster, thicker cuts than with alloys. Oxyfuel cuts by burning, or oxidizing, the metal it is severing. It is therefore limited to steel and other ferrous metals which support the oxidizing process.
Metals like aluminum and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminum, stainless and any other conductive material. Plasma cutting employs a torch that uses a powerful electric arc to create plasma, made by boosting a gas (nitrogen, argon or oxygen) to a very high temperature. While different gasses can be used for plasma cutting, most people today use compressed air for the plasma gas. In most shops, compressed air is readily available, and thus plasma does not require fuel gas and compressed oxygen for operation. This creates a stream, or cone, of directed plasma that can reach a temperature of 30,000°F.
Handheld torches can usually cut up to 1/2 in (13 mm) thick steel plate, and stronger computer-controlled torches can pierce and cut steel up to 12 inches (300 mm) thick. Unlike laser cutting, for example, the process of plasma cutting is only effective on materials that conduct electricity. Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting. However, for heavy sections of steel (1 inch and greater), oxyfuel is still preferred since oxyfuel is typically faster and, for heavier plate applications, very high capacity power supplies are required for plasma cutting applications.
Plasma cutting is ideal for cutting steel, and non-ferrous material less than 1 inch thick. Oxyfuel cutting requires that the operator carefully control the cutting speed so as to maintain the oxidizing process. Plasma is more forgiving in this regard. Plasma cutting really shines in some niche applications, such as cutting expanded metal, something that is nearly impossible with oxyfuel. And, compared to mechanical mean of cutting, plasma cutting is typically much faster, and can easily make non-linear cuts. The plasma cutting machines are typically more expensive than oxyacetylene. Oxyacetylene also does not require access to electrical power or compressed air, which may make it a more convenient method for some users. Oxyfuel can cut sections thicker than 1 inch of steel more quickly than plasma.
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Submerged Arc Welding (SAW)
Similar to MIG welding, SAW involves formation of an arc between a continuously fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Shielding is obtained from a blanket of granular flux, which is laid directly over the weld area. The flux close to the arc melts and intermixes with the molten weld metal and helps purify and fortify it. The flux forms a glasslike slag that is lighter in weight than the deposited weld metal and floats on the surface as a protective cover. The weld is submerged under this layer of flux and slag- hence the name submerged arc welding.
Material applications
- Carbon steels (structural and vessel construction)
- Low alloy steels
- Stainless Steels
- Nickel-based alloys
- Surfacing applications (wear facing, build-up, and corrosion resistant overlay of steels).
Advantages of SAW
- High deposition rates (over 100 lb/h (45 kg/h) have been reported)
- High operating factors in mechanized applications
- Deep weld penetration
- Sound welds are readily made (with good process design and control)
- High speed welding of thin sheet steels at over 100 in/min (2.5 m/min) is possible
- Minimal welding fume or arc light is emitted.
Limitations of SAW
- Limited to ferrous (steel or stainless steels) and some nickel based alloys
- Normally limited to the 1F, 1G, and 2F positions
- Normally limited to long straight seams or rotated pipes or vessels
- Requires relatively troublesome flux handling systems
- Flux and slag residue can present a health & safety issue
- Requires inter-pass and post weld slag removal.
Prior to welding, a thin layer of flux powder is placed on the workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction. SAW is usually operated as a fully mechanized or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.
SAW is normally operated with a single wire on either AC or DC current. Common variants are: twin wire, triple wire, single wire with hot wire addition, and metal powder addition. All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.
Fluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminum, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as 'active' if they add manganese and silicon to the weld, the amount of manganese and silicon added is influenced by the arc voltage and the welding current level. The main types of flux for SAW are:
- Bonded fluxes - produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes contain metallic deoxidisers, which help to prevent weld porosity. These fluxes are effective over rust and mill scale.
- Fused fluxes - produced by mixing the ingredients, then melting them in an electric furnace to form a chemically homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.
SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with careful choice of electrode filler wire and flux combinations.
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