SMAW & TIG Welding Shielded metal arc welding (SMAW), Shielded metal arc welding (SMAW), also known as manual metal arc (MMA) welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's most popular welding processes. It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in the construction of steel structures and in industrial fabrication. The process is used primarily to weld iron and steels (including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method To strike the electric arc, the electrode is brought into contact with the workpiece in a short sweeping motion and then pulled away slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the other . As the electrode melts, the flux covering disintegrates, giving off vapors that protect the weld area from oxygen and other atmospheric gases. To strike the electric arc, the electrode is brought into contact with the workpiece in a short sweeping motion and then pulled away slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld. The heat produced by the arc melts the base metal, the electrode core rod, and the coating. As the molten metal droplets are transferred across the arc and into the molten weld puddle, they are shielded from the atmosphere by the gases produced from the decomposition of the flux coating. The molten slag floats to the top of the weld puddle where it protects the weld metal from the atmosphere during solidification FIGURE 1 COATING CORE ROD SHIELDING GASES SOLIDIFIED SLAG WELD METAL WORK PIECE MOLTEN POOL SHIELDED METAL ARC WELDING AC OR DC POWER SOURCE ELECTRODE CABLE ELECTRODE HOLDER ELECTRODE GROUND CABLE WORK SHIELDED METAL ARC WELDING CIRCUIT Equipment & Operation - One reason for the wide acceptance of the SMAW process is the simplicity of the necessary equipment. The equipment consists of the following items. (See Figure 2) 1. Welding power source 2. Electrode holder 3. Ground clamp 4. Welding cables and connectors 5. Accessory equipment (chipping hammer, wire brush) 6. Protective equipment (helmet, gloves, etc.) Welding Power Sources Shielded metal arc welding may utilize either alternating current (AC) or direct current (DC), but in either case, the power source selected must be of the constant current type. DC type of power source will deliver a relatively constant amperage or welding current regardless of arc length variations by the operator The amperage determines the amount of heat at the arc and since it will remain relatively constant, the weld beads produced will be uniform in size and shape. Whether to use an AC, DC, or AC/DC power source depends on the type of welding to be done and the electrodes used. The following factors should be considered: 1. Electrode Selection - Using a DC power source allows the use of a greater range of electrode types. While most of the electrodes are designed to be used on AC or DC, some will work properly only on DC. 2. Metal Thickness - DC power sources may be used for welding both heavy sections and light gauge work. Sheet metal is more easily welded with DC because it is easier to strike and maintain the DC arc at low currents. 3. Distance from Work - If the distance from the work to the power source is great, AC is the best choice since the voltage drop through the cables is lower than with DC. Even though welding cables are made of copper or aluminum (both good conductors), the resistance in the cables becomes greater as the cable length increases. 4. Welding Position Because DC may be operated at lower welding currents, it is more suitable for overhead and vertical welding than AC. AC can successfully be used for out-of-position work if proper electrodes are selected. 5. Arc Blow (Arc Blow 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. ) - When welding with DC, magnetic fields are set up throughout the weldment. In weldments that have varying thickness and protrusions, this magnetic field can affect the arc by making it stray or fluctuate in direction. This condition is especially troublesome when welding in corners. AC seldom causes this problem because of the rapidly reversing magnetic field produced When using a DC power source, the question of whether to use electrode negative or positive polarity arises. Some electrodes operate on both DC straight (W/P –ve and electrode +)and reverse polarity,(W/P +ve and electrode -). Direct current flows in one direction in an electrical circuit and the direction of current flow and the composition of the electrode coating will have a definite effect on the welding arc and weld bead . Figure 3 shows the connections and effects of straight and reverse polarity. Electrode negative (-) produces welds with shallow penetration; however, the electrode melt-off rate is high. The weld bead is rather wide and shallow as shown at "A" in Figure 3. Electrode positive (+) produces welds with deep penetration and a narrower weld bead as shown at "B" in Figure 3. FIGURE 3 DC POWER SOURCE ELECTRODE DC POWER SOURCE ELECTRODE A HIGHER BURN-OFF RATE, LESS PENETRATION DEEP PENETRATION, LOW BURN-OFF RATE WORK PIECE B STRAIGHT POLARITY REVERSE POLARITY WORK PIECE Parts of Electrode welding equipments Electrode Holder - The electrode holder connects to the welding cable and conducts the welding current to the electrode. The insulated handle is used to guide the electrode over the weld joint and feed the electrode over the weld joint and feed the electrode into the weld puddle as it is consumed. Electrode holders are available in different sizes and are rated on their current carrying capacity. Ground Clamp - The ground clamp is used to connect the ground cable to the work piece. It may be connected directly to the work or to the table or fixture upon which the work is positioned. Being a part of the welding circuit, the ground clamp must be capable of carrying the welding current without overheating due to electrical resistance. Welding Cables - The electrode cable and the ground cable are important parts of the welding circuit. They must be very flexible and have a tough heat-resistant insulation. Connections at the electrode holder, the ground clamp, and at the power source lugs must be soldered or well crimped to assure low electrical resistance. The cross-sectional area of the cable must be sufficient size to carry the welding current with a minimum of voltage drop. Increasing the cable length necessitates increasing the cable diameter to lessen resistance and voltage drop. Coated Electrodes - Various types of coated electrodes are used in shielded metal arc welding. Electrodes used for welding mild or carbon steels are quite different than those used for welding the low alloys and stainless steels Types of electrodes There are several types of electrodes that can be used, with different characteristics, and the choice must be based on the particular kind of application. 1. In particular, acid electrodes (silica and iron silicate) are used when materials with good weldability are joined (the electrode doesn’t have any depurating effect and then the weld is subjected to hot cracks) and usually aren’t used for horizontal welding, due to very high temperatures reached. It produce slags that are predominantly silicon dioxide (sand) and have an acidic behavior. Acid slag systems do no refining of the weld metal. 2.Diffuse electrodes are the cellulosic ones, that have an high penetration capacity and allow a strong projection of material from the electrode to the joint, and a welding from every position 3. Basic electrodes (magnesium and calcium carbonates) depurates the weld joint, due to the reaction of magnesium and calcium with phosphorus and sulphur contained in the metal. This kind of electrodes works at relatively low temperatures and are the only that allow an overhead welding. Basic slags do some refining of the weld metal, resulting in lower nonmetallic inclusion content Functions of the coating on covered electrodes a) Shielding of the Weld Metal – The most important function of a coating is to shield the weld metal from the oxygen and nitrogen of the air as it is being transferred across the arc, and while it is in the molten state. This shielding is necessary to ensure the weld metal will be sound, free of gas pockets, and have the right strength and ductility. At the high temperatures of the arc, nitrogen and oxygen combine readily with iron to form iron nitrides and iron oxides that, if present in the weld metal above certain minimum amounts, will cause brittleness and porosity. Nitrogen is the primary concern since it is difficult to control its effect once it has entered the deposit. Oxygen can be counteracted by the use of suitable deoxidizers. In order to avoid contamination from the air, the stream of molten metal must be protected or shielded by gases that exclude the surrounding atmosphere from the arc and the molten weld metal. This is accomplished by using gas-forming materials in the coating that break down during the welding operation and produce the gaseous shield. b)Stabilization of the Arc - A stabilized arc is one that starts easily, burns smoothly even at low amperages, and can be maintained using either a long or a short arc length. c) Alloying Additions to Weld Metal - A variety of elements such as chromium, nickel, molybdenum, vanadium and copper can be added to the weld metal by including them in the coating composition. It is often necessary to add alloys to the coating to balance the expected loss of alloys of the core wire during the welding operation, due to volatization and chemical reaction. Mild steel electrodes require small amounts of carbon, manganese and silicon in the deposit to give sound welds of the desired strength level. A portion of the carbon and manganese is derived from the core wire, but it is necessary to supplement it with ferromanganese and in some cases ferrosilicon additions in the coating d) Concentration of the Arc Stream - Concentration or direction of the arc stream is attained by having a coating crater (depression) form at the tip of the electrodes. Use of the proper binders assures a good hard coating that will maintain a crater and give added penetration and better direction to the arc stream. Furnish Slag for Fluxing - The function of the slag is (1) to provide additional protection against atmospheric contamination, (2) to act as a cleaner and absorb impurities that are floated off and trapped by the slag, (3) to slow the cooling rate of the molten metal to allow the escape of gases. The slag also controls the contour, uniformity and general appearance of the weld f) Characteristics for Welding Position - It is the addition of certain ingredients, primarily titanium compounds, in the coating that makes it possible to weld out-of-position , vertically, and overhead. Slag characteristics, primarily surface tension and freezing point, determine to a large degree the ability of an electrode to be used for out-of-position work. Control of Weld Metal Soundness – Porosity or gas pockets in weld metal can be controlled to a large extent by the coating composition. It is the balance of certain ingredients in the coating that have a marked effect on the presence of gas pockets in the weld metal. The proper balance of these is critical to the soundness that can be produced. Ferromanganese is probably the most common ingredient used to attain the correctly balanced formula. h) Specific Mechanical Properties to the Weld Metal - Specific mechanical properties can be incorporated into the weld metal by means of the coating. High impact values at low temperature, high ductility, and increases in yield and tensile properties can be attained by alloy additions to the coating. i) Insulation of the Core Wire - The coating acts as an insulator so that the core wire will not short-circuit when welding in deep grooves or narrow openings; coatings also serve as a protection to the operator when changing electrodes Disadvantage The weld oxidation and to impurity inclusion, that worsened structural performance of the welded joint. to resolve these problems, nowadays electrodes are covered with appropriate deoxidizing agents, that increase welded joint characteristics. When the operation is finished, the weld needs to be chiseled, in order to remove scrap that forms during welding. The manual character of this process, and the need for consumable electrode substitution, decrease the production rate of this process Tungsten inert gas (TIG) welding Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. GTAW is most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques Gas tungsten arc welding produces exceptionally clean welds no slag is produced, the chance inclusions in the weld metal is and the finished weld requires virtually no cleaning. Argon and Helium, the primary shielding gases employed, are inert gases. Inert gases do not chemically combine with other elements and therefore, are used to exclude the reactive gases, such as oxygen and nitrogen, from forming compounds that could be detrimental to the weld metal. Gas tungsten arc welding may be used for welding almost all metals — mild steel, low alloys, stainless steel, copper and copper alloys, aluminum and aluminum alloys, nickel and nickel alloys, magnesium and magnesium alloys, titanium, and others. This process is most extensively used for welding aluminum and stainless steel alloys where weld integrity is of the utmost importance. Another use is for the root pass (initial pass) in pipe welding, which requires a weld of the highest quality. Full penetration without an excessively high inside bead is important in the root pass, and due to the ease of current control of this process, it lends itself to control of back-bead size. For high quality welds, it is usually necessary to provide an inert shielding gas inside the pipe to prevent oxidation of the inside weld bead. * Gas Tungsten Arc Welding (GTAW) is the current terminology approved by the American Welding Society, formerly known as "TIG" (Tungsten Inert Gas) welding. EQUIPMENT AND OPERATION - Gas tungsten arc welding may be accomplished with relatively simple equipment, or it may require some highly sophisticated components. Choice of equipment depends upon the type of metal being joined, the position of the weld being made, and the quality of the weld metal necessary for the application. The basic equip- ment consists of the following: 1. The power source 2. Electrode holder (torch) 3. Shielding gas 4. Tungsten electrode 5. Water supply when necessary 6. Ground cable 7. Protective equipment GAS TUNGSTEN ARC WELDING CONNECTION SCHEMATIC . Power Sources - Both AC and DC power sources are used in gas tungsten arc welding. They are the constant current type with a drooping volt-ampere curve. Direct current electrode negative (DCEN) is produced when the electrode is connected to the negative terminal of the power source. Since the electrons flow from the electrode to the plate, approximately 70% of the heat of the arc is concentrated at the work, and approximately 30% at the electrode end. This allows the use of smaller tungsten elec- trodes that produce a relatively narrow concentrated arc. The weld shape has deep penetration and is quite narrow. Direct current electrode negative is suitable for welding most metals. Magnesium and aluminum have a refractory oxide coating on the surface that must be physically removed immediately prior to welding if DCSP is to be used Direct current electrode positive (DCEP) is produced when the electrode is connected to the positive terminal of the welding power source. In this condition, the electrons flow from the work to the electrode tip, concentrating approximately 70% of the heat of the arc at the electrode and 30% at the work. This higher heat at the electrode necessitates using larger diameter tungsten to prevent it from melting and contaminating the weld metal. Since the electrode diameter is larger and the heat is less concentrated at the work, the resultant weld bead is relatively wide and shallow. Aluminum and magnesium are two metals that have a heavy oxide coating that acts as an insulator and must be removed before successful welding can take place. Welding with electrode positive provides a good oxide cleaning action in the arc. If we were to study the physics of the welding arc, we find that the electric current causes the shielding gas atoms to lose some of their electrons. Since electrons are negatively charged, these gas atoms now are unbalanced and have an excessive positive charge.Upon striking the work surface, they dislodge the oxide coating permitting good electrical conductivity for the maintenance of the arc, and eliminate the impurities in the weld metal that could be caused by these oxides. Direct current electrode positive is rarely used in gas-tungsten arc welding. Despite the excellent oxide cleaning action, the lower heat input in the weld area makes it a slow process, and in metals having higher thermal conductivity, the heat is rapidly conducted away from the weld zone. When used, DCEP is restricted to welding thin sections (under 1/8") of magnesium and aluminum Alternating current is actually a combination of DCEN and DCEP and is widely used for welding aluminum. In a sense, the advantages of both DC processes are combined, and the weld bead produced is a compromise of the two. Remember that when welding with 60 Hz current, the electron flow from the electrode tip to the work reverses direction 120 times every second. Thereby, the intense heat alternates from electrode to work piece, allowing the use of an intermediate size electrode. The weld bead is a compromise having medium penetration and bead width. The gas ions blast the oxides from the surface of aluminum and magnesium during the positive half cycle. DC constant current power sources – Constant current power sources, used for shielded metal arc welding, may also be used for gas-tungsten arc welding. In applications where weld integrity is not of utmost importance, these power sources will suffice. With machines of this type, the arc must be initiated by touching the tungsten electrode to the work and quickly withdrawing it to maintain the proper arc length. This starting method contaminates the electrode and blunts the point which has been grounded on the electrode end. These conditions can cause weld metal inclusions and poor arc direction. Using a power source designed for gas tungsten arc welding with a high frequency stabilizer will eliminate this problem. The electrode need not be touched to the work for arc initiation. Instead, the high frequency voltage, at very low current, is superimposed onto the welding current. When the electrode is brought to within approximately 1/8 inch of the base metal, the high frequency ionizes the gas path, making it conductive and a welding arc is established. The high frequency is automatically turned off immediately after arc initiation when using direct current Torches - The torch is actually an electrode holder that supplies welding current to the tungsten electrode, and an inert gas shield to the arc zone. The electrode is held in a collet-like clamping device that allows adjustment so that the proper length of electrode pro- trudes beyond the shielding gas cup. Torches may be either air or water-cooled. The air-cooled types actually are cooled to a degree by the shielding gas that is fed to the torch head through a composite cable. The gas actually surrounds the copper welding cable, affording some degree of cooling. Water-cooled torches are usually used for applications where the welding current exceeds 200 amperes. The water inlet hose is connected to the torch head. Circulating around the torch head, the water leaves the torch via the current-in hose and cable assembly. Cooling the welding cable in this manner allows the use of a smaller diameter cable that is more flexible and lighter in weight In Inert gas shielded arc welding processes,a high presssureinert gas flowing around the electrode while welding would physicaaly displace all the atmospheric gases around the weld metal to fully protect it. Shielding Gases - Argon and helium are the major shielding gases used in gas tungsten arc welding. In some applications, mixtures of the two gases prove advantageous. To a lesser extent, hydrogen is mixed with argon or helium for special applications. 2.3.4.1 Argon and helium are colorless, odorless, tasteless and nontoxic gases. Both are inert gases, which means that they do not readily combine with other elements. They will not burn nor support combustion. Commercial grades used for welding are 99.99% pure. Argon is 38% heavier than air and about 10 times heavier than helium. Both gases ionize when present in an electric arc. This means that the gas atoms lose some of their electrons that have a negative charge. These unbalanced gas atoms, properly called positive ions, now have a positive charge and are attracted to the negative pole in the arc. When the arc is positive and the work is negative, these positive ions impinge upon the work and remove surface oxides or scale in the weld area. Argon is most commonly used of the shielding gases. Excellent arc starting and ease of use make it most desirable for manual welding. Argon produces a better cleaning action when welding aluminum and magnesium with alternating current. The arc produced is relatively narrow. Argon is more suitable for welding thinner material and for out of position welding Heaviest of all shielding gases hence require lower flow rate for good shielding action. At equal amperage, helium produces a higher arc voltage than argon Argon-helium gas mixtures are used in applications where higher heat input and the desirable characteristics of argon are required. Argon, being a relatively heavy gas, blankets the weld area at lower flow rates. Argon is preferred for many applications because it costs less than helium. HeliumHelium has higher thermal conductivity so it gives higher arc voltage for given current nad higher heat input. being approximately 10 times lighter than argon, requires flow rates of 2 to 3 times that of argon to satisfactorily shield the arc. Helium rises in turbulent manner and tends to disperse into air.So higher flow rate will be required. It can withstand higher arc voltages and hence used for welding where higher heat input is requiredsuch as for thick sheets or higher conductivity materials as Cu and Al. Argon +5%Hydrogen is used for TIG welding of stainless steel,Nickel and its alloy. The hydrogen helps to increase the arc heating efficiency and reduce amount of oxide formed with stainless steel. Carbon Dioxide is most economical Under the arc, carbon dioxide decomposes to CO and Oxygen which subsequently combines to form carbon dioxide when they cool. It requires slightly higher current which causes more agitation in weld puddle resulting in trapped gases to rise and thus reduce porosity. Carbon dioxide is normally used only DC with electrode positive. Electrodes - Electrodes for gas tungsten arc welding are available in diameters from .010" to 1/4" in diameter and standard lengths range from 3" to 24". The most commonly used sizes, however, are the .040", 1/16", 3/32", and 1/8" diameters. The shape of the tip of the electrode is an important factor in gas tungsten arc welding. When welding with DCEN, the tip must be ground to a point. The included angle at which the tip is ground varies with the application, the electrode diameter, and the welding current. Narrow joints require a relatively small included angle. When welding very thin material at low currents, a needlelike point ground onto the smallest available electrode may be necessary to stabilize the arc. Properly ground electrodes will assure easy arc starting, good arc stability, and proper bead width. Types of Electrodes 1) Pure Tungsten (AWS EWP) Color Code: Green Used for less critical applications. The cost is low and they give good results at relatively low currents on a variety of metals. Most stable arc when used on AC, either balanced wave or continuous high frequency. Pure Ti electrodes are usually preferred for AC welding of Al and Mg. The current carrying capacity is lower than alloyed electrode. 1% Thoriated Tungsten (AWS EWTh-1) Color Code: Yellow Good current carrying capacity, easy arc starting ,longer life and provide a stable arc. Less susceptible to contamination. Designed for DC applications of nonferrous materials. 3) 2% Thoriated Tungsten (AWS EWTh-2) Color Code: Red Longer life than 1% Thoriated electrodes. Maintain the pointed end longer, used for light gauge critical welds in aircraft work. Like 1%, designed for DC applications for nonferrous materials. 4) 5% Thoriated Tungsten (AWS EWTh-3) Color Code: Blue Sometimes called "striped" electrode because it has 1.0-2.0% Thoria inserted in a wedge-shaped groove throughout its length. Combines the good properties of pure and thoriated electrodes. Can be used on either AC or DC applications. 5) Zirconia Tungsten (AWS EWZr) Color Code: Brown Longer life than pure tungsten. Better performance when welding with AC. Melts more easily than thoriam-tungsten when forming rounded or tapered tungsten end. Ideal for applications where tungsten contamination must be minimized. Good arc starting characteristics. The current carrying capacity of electrode depends on: Shielding gas used Length of electrode Cooling of the holders Position of weld type Gas Tungsten Arc Welding is one of the major welding processes today. The quality of the welds produced and the ability to weld very thin metals are the major features. The weld metal quality is high since no flux is used, eliminating the problem of slag inclusions in the weld metal. It is used extensively in the aircraft and aerospace industry, where high quality welds are necessary and also for welding the more expensive metals where the weld defects become very costly. Metals as thin as .005" can be welded due to the ease of controlling the current. 2.3.6.1 The major disadvantages of the process are that it is slower than welding with consumable electrodes and is little used on thicknesses over 1/4" for this reason. Shielding gas and tungsten electrode costs make the process relatively expensive
