Types of Welding Metals - Essay Example

INTRODUCTION Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the work pieces and adding a filler material to form a pool of molten material that cools to become a strong joint, but sometimes pressure is used in conjunction with heat, or by itself, to produce the weld. (www.wikipedia.org). A weld is defined as a blend or coalescence of two or more metals by heating then until they reach a critical temperature and flow together. The piece of metal to be welded is called the base metal, work piece, or work. The edges of the base metal are often specially prepared for welding by, for example, machining, shearing, or gouging. There are five basic weld joints: butt, lap, corner, "T," and edge. The American Welding Society has developed a system of symbols that are added to mechanical drawings, to convey precisely how a welding site should be prepared, what type of weld should be made, and any other considerations. This paper focuses on three main types of welding namely arc welding, gas welding, and resistance welding. Arc welding Arc welding is a very popular method of joining metal together for many different applications. It is the processes of joining metal together through fusion which is done by the electric Arc, by electrical current (Arc 1994).An electric arc between the electrode and the work piece generates heat. Sufficient heat is generated to melt the pieces together. The arc reaches temperatures of around 6,000 degrees Celsius which is concentrated at the tip of the electrode (Weldwell 2003). The range of welding current used varies from 5 to 500 amps. The voltage ranges from 20 to 30 volts. The current can be either AC or DC. The type of current is dependent on material thickness. The electrodes have a metal core surround by flux, which melts with the work piece creating a fusion weld. The flux coating forms a gas and slag that shields the molting metal pool. This flux materialis cleaned off once the weld has cooled with a wire brush. The weld should be as strong or stronger then the metal that it is fusing together. There are four common types of welding namely Stick or SMAW(Shielded Metal Arc Welding) Mig or GMAW(Gas Metal Arc Welding) Tig or GTAW (Gas Tungsten Arc Welding). Flux-Core or FCAW (Flux Core Arc Welding). Shielded metal arc welding or stick welding is one of the most commonly used types of welding. (Repp 1994).this is because it is one of the easiest methods and the equipment used is inexpensive. It is used in mobile welding due to its ease of portability. In this process the flux covering the electrode melts during welding. Due to this gas and slag are formed that form a protective layer around the molten weld pool. The slag is chipped off the weld bead after welding. The flux provides a method of adding scavengers, deoxidizers, and alloying elements to the weld metal. Advantages The equipment is simple, inexpensive and portable. It has a moderate welding speed. This process is flexible i.e. in order to tackle different tasks, metals and thickness change of rods or welding parameters is sufficient. Flux provides excellent cleaning action in cases where weld metal preparation is less than ideal (rust, scale, dirt, etc.). It is less sensitive to wind and drifts. Discontinuities Considering the weld discontinuities stick welding suffers from undercut where a groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. Secondly it suffers from incomplete fusion i.e. a weld discontinuity in which fusion does not occur between weld metal and fusion faces or adjoining weld beads. Other discontinuities include porosity of the weld formed, cracks and slag inclusions in the weld. Problems The main problem concerning stick welding is Arc Blow. It is a problem that exists with most electric welding processes. It is caused by the preferential magnetic fields developed near the arc. These are most often caused by the arc current ground path or in the fixture holding the part to be welded. The fields interact with the self induced field around the electrode. When employing DC power the external field will be preferentially in one direction. This builds a stronger field on one side of the arc than the other causing it to move in the direction of the weakened field. The worse situation is when this movement causes the arc to blow backwards and become unstable. This can generate gross porosity in the deposit. The second main problem is Excessive spatter. Spatter occurs when the weld puddle expels molten metal and scatters it along the bead where it then cools and forms a solid mass on the work piece. Excessive spatter creates a poor weld appearance, lowers the efficiency of the process and leads to incomplete fusion in multiple pass welds. Another defect affecting the strength of the weld is poor fusion, though it is often easily visible. It is caused by low current, contaminated joint surfaces, or the use of an improper electrode. Disadvantages It requires high skill level to achieve acceptable results. This process is difficult to weld thin materials. Flux removal (chipping/wire brush) after welding and in-between passes is required. Generally available as 230V units but newer inverter based 115V models are available. Applications Stick welding is dominant in the maintenance repair industry .it is used heavily in the construction of steel structures and in the industrial fabrication. In recent years its use has declined as flux-cored arc welding has expanded in the construction industry and gas metal arc welding has become more popular in industrial environments. Gas Metal Arc Welding is frequently referred to as MIG welding. It is a commonly used high deposition rate welding process. In this wire is continuously fed from a spool and therefore referred to as a semiautomatic welding process. During the welding process the shielding gas, that forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to the molten weld pool. There are three primary metal transfer modes namely Spray transfer, Globular transfer and Short circuiting transfer. The primary shielding gases used are Argon, Argon and 1 to 5 % of oxygen, argon and 3 to 25 % carbon dioxide, or argon/helium. Advantages It has the ability to join a wide range of material types and thicknesses. It requires simple equipment that are readily available and affordable. GMAW has higher electrode efficiencies (around 93-98%) when compared to other welding process. It has higher welder efficiencies and operator factor when compared to other arc welding processes. It has all position welding capability and excellent weld bead appearance. It has lower hydrogen weld deposit (generally less than 5mL/100 g of weld metal. It requires lower heat input when compared to other welding processes and less welding fumes when compared to SMAW and FCAW. Discontinuities In Mig welding discontinuities arise due to undercutting, excessive melt-through, incomplete fusion, incomplete join penetration (A joint root condition in a groove weld in which weld metal does not extend through the joint thickness), porosity, weld metal cracks and heat affected zone cracks. This process is not as versatile as the stick process with respect to maintenance. Also the welding power source, the wire cost, and the shielding gas are expensive when compared to SMAW. Limitations The lower heat input characteristics of the short circuiting mode of metal transfer restricts the use to thin materials. The higher heat input axial spray transfer generally restricts the use to thicker base materials. The higher heat input mode of axial spray is restricted to flat or horizontal welding positions. The use of argon based shielding gas for axial spray and pulsed spray transfer modes is more expensive than 100% carbon-di-oxide. GMAW is not generally suitable for outdoor use, as changes in the atmosphere can cause the shielding gas to dissipate and the quality of the weld to be inferior. It is also unsuitable for underwater welding for the same reasons. Applications GMAW is widely used by the sheet metal industry. It is also used in robot welding, in which robots operate the welding gun and the sheet metal in order to save on time and cost. For the repair of crashed cars, frequently new plates must be welded into the car body. The plates are made of unalloyed and low alloyed steels and have thickness within a range from 0.8 to 2.5 mm .for financial reasons GMA welding method is applied (Prof. Dr.-Ing.U.Dilthey, Welding Institute, Aachen) Gas Tungsten Arc Welding or TIG welding is a commonly used high quality welding process. It is preferred when high quality, precision welding is required. In this welding an arc is formed between a non consumable tungsten electrode and the metal being welded. Gas is fed through the torch to shield the electrode and molten weld pool. If filler wire is used, it is added to the weld pool separately. This process requires a highest skill level. The shielding gases that are used are argon or a combination of argon and hydrogen or argon and helium. Helium is generally added to increase heat input. Addition of hydrogen results in cleaner looking welds and also increase heat input, but it may also promote porosity or hydrogen cracking. A TIG process can achieve, in a single pass, a full penetrated weld in C-Mn steel up to 12mm thickness, without using a bevel penetration or filler wire(Yuschenko K A et al:' A-TIG welding of carbon manganese and stainless steel') Advantages Tig welds are considered as a superior quality welds. Welding can be done with or without filler material. The welding variables like heat can be controlled with precision. Unlike SMAW it is free from spatter and low distortion. It consists of simple equipment, and no moving parts. This process is flexible i.e. wieldable metals range from aluminum to zinc and everything in between. Power source can be used for both SMAW and GTAW processes. Disadvantages Compared to SMAW and GMAW it requires greater welder dexterity and also its deposition rate is low. It is more costly for welding thick sections. It consumes electrode excessively when compared to the other two processes. It is difficult to start the arc. Erratic arc, arc wandering and oxidized weld deposit are the other problems encountered in this process. Applications The process is widely used for welding of pressure vessels, heat exchangers and pipes where tightness is important since the process produces welds with very low pore fractions. TIG-welding is also beneficial for welds with frequently starts and stops and for short welds due to excellent quality with low porosity. The Flux Core Arc Welding process (FCAW) uses an arc between a continuous filter metal electrode and the weld pool. The process is used with shielding from a flux contained within the tubular electrode, with or without additional shielding from an external supplied gas and without application of pressure. The tubular electrode is a filler metal composite consisting of a metal sheath and a core of various powdered materials manufactured in the form of wire. During welding, an extensive protective slag cover is produced on the face of the weld bead. Flux core arc welding is an efficient process readily adaptable to semiautomatic or automatic welding operations and capable of producing high quality weld metal at a high deposition rate. Users of the process include manufacturers or builders of vessels, earth moving equipment, and building and other maintenance applications. There are two major variations of the FCAW process, the self shielded and the gas shielded versions. In the gas shielded method carbon dioxide or a mixture of argon and carbon dioxide acts as the shielding gas and protects the molten metal from the oxygen and nitrogen present in the air by forming an envelope of gas around the arc and over the weld pool. Because air along with nitrogen it contains is excluded the need for denitrification of weld metal is little. The compositions of electrodes are formulated to provide deoxidizers that combine with small amounts of oxygen in the gas shield. Self-shielded flux cored arc welding is often used for field welding because it can tolerate stronger air currents than gas-shielded FCAW. The reason for this is that the high temperature decomposition of some of the electrode core ingredients provides some shielding. The vaporization of these ingredients displaces air from the area immediately surrounding the arc. In addition the wire contains a large proportion f scavengers i.e., deoxidizers and denitrifiers, that combine with the undesirable elements that might contaminate the weld pool. The slag cover protects the metal from the air surrounding the weld. Advantages The benefits offered by FCAW are many. This process produces high quality weld metal deposit and has an excellent weld appearance. It welds many varieties of steels in wide thickness range and has high operating factor and can be easily mechanized. Compared to SMAW it has deposition rate (up to four times) and has high current density. Its electrode deposit efficiency is high and allows economical engineering of joint designs. When compared to GMAW it requires less pre-cleaning and often results in less distortion compared to SMAW. When used with shielded gas it produces exceptionally good fusion compared to short circuit gas metal arc welding. it is highly tolerant to contaminants that cause weld cracking and it is resistant to under bead cracking. Self shielding characteristics of electrodes eliminate the need for flux handling and gas apparatus. Also self shielding tolerates windy conditions in outdoor applications. Limitations Compared to SMAW, the limitation of FCAW are the higher cost of equipment, relative complexity of equipment setup and control, and the restriction on operating distance from the electrode wire feeder. This process generates large volumes of welding fumes and requires suitable exhaust equipment, except for field work. The need for removing slag between passes is an added labor cost. Submerged Arc Welding is defined as "a process which produces coalescence of metals by heating them with a buried arc between a bare electrode and the work. The arc and molten metal are "submerged" in a blanket of granular fusible flux on the work." This process is often referred to as Sub-Arc. This process most commonly uses a continuous, consumed, bare solid wire electrode that is shielded by the flux. The flux acts to stabilize the arc during welding, shielding the molten pool from the atmosphere. Additionally the flux covers and protects the weld during cooling and can affect weld composition and properties. The weld metal properties will vary based on the flux and consumable wire selection. The weld metal chemistry is impacted by both the flux and consumable selection. Both should be closely matched to the base material being welded. Two basic flux types are commercially available. Bonded and fused flux. Where low temperature impact properties are required, the bonded type is selected. When considering resistance to moisture pick up and its suitability to be reused with less processing, the fused flux is selected. Both should be matched with a wire chemistry recommended by the manufacturer. Advantages Partly because it is often automated, it is much faster than regular arc welding. Deep penetration with high quality weld is possible with this method Less distortion occurs from high speed and uniform heat input, especially when automated. The operator can work more easily without safety equipment. Disadvantages Weld may contain slag inclusion. Limited applications of the process - mostly for welding horizontally located plates. Gas welding Gas welding is also called as oxyfuel gas welding (OFW). it refers to process that use gas as the source of heat energy. Oxy acetylene and oxyhydrogen are two types of fuel gases used. Acetylene is commonly used for welding because, when combined with oxygen, the flames temperature reaches up to 5600 degrees Fahrenheit (393 degrees centigrade). This is the highest temperature produced by any fuel gas-oxygen combination. A filler rod may or may not be used worth this type of welding process. The fuel gas and oxygen are contained in separate pressurized tanks or cylinders. Specially designed hoses run from the gas cylinders and connect to the welding torch. The welding torch has valves that control the amount of incoming gases, and a mixing chamber where the gases are mixed. The blended gases flow to the tip of the torch where the flame is ignited by a torch lighter, usually a flint and steel spark lighter. After the flame is lit, it must be adjusted until the correct balance of gases is achieved (a neutral flame). During the years newer methods have replaced oxyfuel gas process. Currently, the oxyacetylene process is used to braze welding, brazing, and soldering. Advantages Gas welding processes are versatile Adapted to different jobs The equipment used is of relatively low cost. Independent from availability of external energy resources. Limitations The welder should have enough skill in manipulating the torch and the filler rod. Reactive and refractory metals are not suited to gas welding. Resistance welding Resistance welding or spot welding (RW) is a process where two or more layers or pieces of metal are stacked together and welded together by a combination of pressure and heat. In this process an electric current along with certain amount of pressure is applied to the area or spot of the weld. Due to metals resistance to flow of electricity it gets heated up as soon as electricity flows through it. This electricity passes from one metal to other when they are in contact. The heat is maximum at the point of contact of the two metals. When the temperature reaches a critical point (which is equal to the melting point of the metal) the metal melt and weld is created at the point of contact. Pressure is applied through air pressure, hydraulic pressure, or mechanical leverage. Electrodes used for spot welding are not consumable, and can be manufactured into specific shapes. In seam welding electrodes are shaped like wheels Advantages This process has high welding rates Low fumes This process is cost effective Easy automation No filler materials are required Low distortions Disadvantages High equipment cost Low strength of discontinuous welds Thickness of welding sheet is limited (upto 6 mm ) Applications Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks, and domestic radiators, pipes of gas oil and water pipelines, wire ends, turbine blades, railway tracks.(Dr . Dmitri Kopeliovich) References Elements of Workshop Technology VOL: I Manufacturing Processes: SK Hajra Choudhury, A.K. Hajra Choudhury, Nirjhar Roy The Procedure Handbook of Arc Welding Lincoln Electric. GMAW welding guide Lincoln electric Welding and the world of metals Miller. Guidelines for Shielded Metal Arc Welding Miller Yuschenko K A et al:'A-TIG welding of carbon manganese and stainless steel'Proc- . Conf. Welding Technology Paton Institute, Abington, October 1993 www.weldingengineer.com www.substech.com www.bernardwelds.com www.welding-advisers.com Read More