Fusion Welding Processes Chapter 30 Fusion Welding Processes Copyright Prentice-Hall

Fusion Welding Processes

Oxyacetylene Flame Types Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.

Oxyacetylene Torch (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, and acetylene regulators red.

Pressure-Gas Welding Process Schematic illustration of the pressure-gas welding process; (a) before, and (b) after. Note the formation of a flash at the joint, which can later be trimmed off.

Arc Welding Processes Most prevalent welding processes that employ an electric arc Shielded Metal Arc Welding (SMAW) Gas Metal Arc Welding (GMAW) Flux Cored Arc Welding (FCAW) Submerged Arc Welding (SAW) Gas Tungsten Arc Welding (GTAW) These processes are associated with molten metal Arc welding processes use an electric arc as a heat source to melt metal. The arc is struck between an electrode and the workpieces to be joined. The electrode can consist of consumable wire or rod, or may be a non-consumable tungsten electrode. The process can be manual, mechanized, or automated. The electrode can move along the work or remain stationary while the workpiece itself is moved. A flux or shielding gas is employed to protect the molten metal from the atmosphere. Different processes lend themselves to welding in various positions: flat, horizontal, vertical, overhead. Different power sources can be used with various processes and materials: AC, DC electrode positive (reverse polarity), DC electrode negative (straight polarity). If no filler metal is added, the melted weld metal is referred to as autogenous. If the filler metal matches the base metal, it is referred to as homogenous. If the filler metal is different from the base metal, it is referred to as heterogeneous. Arc processes are also used to cut materials; plasma arc cutting is an example.

Protection of the Molten Weld Pool Molten metal reacts with the atmosphere Oxides and nitrides are formed Discontinuities such as porosity Poor weld metal properties All arc welding processes employ some means of shielding the molten weld pool from the air Contamination of the weld pool by the atmosphere can cause weld defects. These defects can have an adverse effect on joint efficiency, which may lead to joint failure. Therefore, the weld pool should be protected from the atmosphere until it has completely solidified. A variety of fluxes and shielding gases are employed by the arc welding processes to provide atmospheric shielding.

Welding Flux Three forms Granular Electrode wire coating Electrode core Fluxes melt to form a protective slag over the weld pool Other purposes Contain scavenger elements to purify weld metal Contain metal powder added to increase deposition rate Add alloy elements to weld metal Decompose to form a shielding gas Flux can be coated on the exterior of the electrode as in Shielded Metal Arc Welding (SMAW), placed in the interior core of the electrode as in Flux Cored Arc Welding (FCAW), or poured over the weld pool during the welding process as in Submerged Arc Welding (SAW). During the welding process, the flux melts or vaporizes. The vaporized flux forms a a protective atmosphere around the molten weld pool. The melted flux flows over the surface of the weld pool, further protecting it from atmospheric contamination. This molten flux is referred to as a slag, and solidifies upon cooling as a hard covering over the weld pool. Slag must be removed after each pass of a multipass weld. Any trapped slag is one of many welding defects, and can degrade joint properties. Flux compositions can be designed so that the slag peels away from the weld during cooling. This occurs because of the thermal contraction mismatch between the weld metal and slag. The weld metal shrinks more upon cooling. Weld bead size and shape also affect slag removal. Small weld beads cool more quickly and result in easier slag removal. Weld bead contours, particularly flat ones, that blend well into the surrounding base metal provide better slag removal than undercut beads.

Shielding Gas Shielding gas forms a protective atmosphere over the molten weld pool to prevent contamination Inert shielding gases, argon or helium, keep out oxygen, nitrogen, and other gases Active gases, such as oxygen and carbon dioxide, are sometimes added to improve variables such as arc stability and spatter reduction Shielding gas can be a single pure gas or a mixture of two or more gases. Inert gases, as the name implies, do not react with the weld metal. Argon is often used in the flat and horizontal position, since it is heavier than air. Helium can be used in the overhead position, since it is lighter than air. Helium has the characteristic of producing a “hotter” arc than argon. Active gases, such as oxygen or carbon dioxide, are often added to inert gases in order to improve arc properties. These properties include arc stability and spatter reduction. Carbon dioxide, CO2, is used in the welding of steel. Shielding gases should be free of moisture, which decomposes to hydrogen and oxygen in the welding arc. Moisture in the shielding gas can result in porosity. In steels, hydrogen can lead to cracking. Shielding gas is regulated and measured as a flow rate as it passes over the weld pool. Flow rate is expressed as cubic feet per hour (cfh) or liters per minute.

Gas-Tungsten Arc Welding The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current. (a) The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding. (b) Equipment for gas tungsten-arc welding operations.

Advantages Produces superior quality welds, generally free from spatter, porosity, or other defects Can be used to weld almost all metals Can weld dissimilar metal joints Can be used with or without filler wire Easily automated Can be used in all positions GTAW is a very versatile welding process that can be used on almost all metals. It is free of the spatter which occurs with other arc welding processes. It can be used with filler metal or without filler metal (autogenous). Without filler metal it can produce inexpensive welds at high speeds. This process allows the heat source and filler metal additions to be controlled independently. GTAW produces very clean welds and has found much use in the aerospace and food processing industries, among others. It is particularly useful on smaller sectioned parts and on reactive metals such as titanium.

Limitations Less economical than consumable electrode processes for sections thicker than 3/8 inch Lowest deposition rate of all arc processes Manual GTAW requires welder skill Sensitive to drafts Deposition rates are lower with GTAW than those with consumable electrode arc welding processes. This makes the process unattractive for welding sections greater than 3/8-inch thick. Tungsten inclusions or contamination of the weld pool may occur if the electrode touches the weld pool or proper gas shielding is not maintained. Manual GTAW welding requires slightly more dexterity and welder coordination than with manual GMAW or SMAW. As with GMAW, drafts can blow away the shielding gas, which limits the outdoor use of the process.

Plasma-Arc Welding Process Two types of plasma-arc welding processes: (a) transferred, (b) nontransferred. Deep and narrow welds can be made by this process at high welding speeds.

Shielded-Metal Arc Welding Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process. A deep weld showing the buildup sequence of eight individual weld beads.

Advantages Equipment relatively easy to use, inexpensive, portable Filler metal and means for protecting the weld puddle are provided by the covered electrode Less sensitive to drafts, dirty parts, poor fit-up Can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, aluminum SMAW is by far the most widely used arc welding process. Among it’s many advantages is the fact that the equipment is relatively easy to use, inexpensive, and portable. This makes the process popular for in-the-field repairs and welding in remote locations as well as in the shop. The filler metal and means for protecting the weld puddle are self contained in the covered electrode. It is a versatile process in that it can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, and aluminum.

Limitations Low deposition rate compared to other processes Slag must be removed between each pass Electrodes must be changed often Heat of welding too high for lead, tin, zinc, and their alloys Inadequate weld pool shielding for reactive metals such as titanium, zirconium, tantalum, columbium The covered electrodes produce a slag that must be removed between each welding pass to avoid slag inclusion defects, which can degrade joint properties. SMAW has a low weld-metal deposition rate compared to other arc processes. This is because each welding rod contains a finite amount of metal. As each rod is used, welding must be stopped and a new rod inserted into the holder. A 12-inch rod may be able to deposit a bead of 6-8 inches in length. As such, electrodes have to be changed often. The high heat input of the SMAW process make it unsuitable for welding of low-melting-point materials such as lead and zinc. Also, metals such as titanium, which require very clean welding conditions, are not amenable to the SMAW process.

Submerged-Arc Welding Schematic illustration of the submerged arc welding process and equipment. The unfused flux is recovered and reused.

Advantages Highest deposition rate and deepest single pass weld penetration of all arc welding processes Continuous wire feed High welding current High weld quality Easily mechanized Can be used to weld carbon steels, low alloy steels, stainless steels, chromium-molybdenum steels, nickel base alloys Submerged Arc Welding (SAW) has the deepest single-pass penetration and highest deposition rate of all the arc processes. These characteristics make it especially suited for thick-section welding. Since the arc is completely submerged in the flux, there is no arc radiation. Screens or light filtering lenses are not needed. SAW is generally employed as a mechanized process. It is noted for high weld quality.

Limitations Flux obstructs view of joint during welding Cannot weld in vertical or overhead positions Higher equipment cost than SMAW Must remove slag between passes The flux, which shields the arc and weld pool, also obstructs the view of the joint and molten weld pool. This makes observation of the pool and joint impossible during welding; correction of problems during welding is thus difficult. Due to the presence of a granulated flux, SAW is limited to the flat and horizontal positions. Limited positioning restricts the use of this process in some construction and final assembly applications. As with SMAW and FCAW, SAW produces a slag, which must be completely removed after each pass.

Gas Metal-Arc Welding (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. (b) Basic equipment used in gas metal-arc welding operations.

Advantages Deposition rates higher than SMAW Easily automated No slag removal Continuous wire feed Easily automated Can be used to weld all commercial metals and alloys The major advantage of GMAW is a high deposition rates due to the continuously-fed wire electrode. No time is lost in order to change electrodes as in SMAW. Because a shielding gas is used instead of a flux, there is no slag to remove between passes. This also increases the deposition rate of the process Unlike SMAW, which is a manual process, GMAW is easily mechanized or automated and is very often used in conjunction with robots.

Limitations Equipment is more complex, costly, and less portable that SMAW Restricted access - GMAW gun is larger than a SMAW electrode holder Air drafts can disrupt the shielding gas atmosphere, limiting outdoor use The use of GMAW entails a higher equipment cost than SMAW. The process is less portable in that a shielding gas supply and wire feeder is required. The GMAW torch (or gun) is bulky compared to the SMAW electrode. This restricts the use of the process to applications where there is adequate access to the weld joint by the torch. The weld pool must be constantly protected by the shielding gas. Drafts and outdoor use can cause the shielding gas to be blown away, which can lead to weld defects. The SMAW process is less susceptible to drafts.

Fluxed-Cored Arc-Welding Schematic illustration of the flux-cored arc welding process. This operation is similar to gas metal-arc welding.

Advantages Combines best features of SMAW and GMAW Weld metal composition can be modified by flux Less sensitive to drafts High deposition rate - continuous wire feed Less sensitive to surface condition, e.g. rust, scale Can be used to weld carbon steel, low alloy steels, stainless steels and cast iron The FCAW process combines the best characteristics of SMAW and GMAW. It employs a flux to shield the weld pool, although a supplemental shielding gas can be used. A continuous wire electrode provides high deposition rates. FCAW welds can be larger and better contoured than those made with solid electrodes. The use of a flux means that the process is less sensitive to surface condition such as rust or scale. It can be mechanized. High weld quality is possible with this process.

Limitations Must remove slag between each pass Higher equipment cost than SMAW Generates large volumes of smoke More complex process, requires higher operator skill required than SMAW As with SMAW, the slag must be removed between passes on multipass welds. This can slow down the time required to complete the weldment. The FCAW equipment is more complex, more expensive, and less portable that SMAW equipment. A heavy smoke is produced by this process which must be well ventilated for proper safety and health. Flux core arc welding (FCAW) is presently limited to welding ferrous metals and nickel base alloys.

Electroslag-Welding Equipment used for electroslag welding operations.

Electrode Designations

Weld Bead Comparison (a) (b) Comparison of the size of weld beads: (a) laser-beam or electron-beam welding, and (b) tungsten-arc welding. Source: American Welding Society, Welding Handbook (8th ed.), 1991.

Weld Joint Structure Grain structure in (a) deep weld and (b) shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal (see also Fig. 10.3). (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead. Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding.

Discontinuities and Defects in Fusion Welds Examples of various discontinuities in fusion welds. Examples of various defects in fusion welds.

Cracks in Welded Joints Types of cracks developed in welded joints. The cracks are caused by thermal stresses, similar to the development of hot tears in castings.

Distortion of Parts After Welding Distortion of parts after welding. (a) Butt joints and (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly.

Weld Testing (a) Specimen for longitudinal tension-shear testing; (b) specimen for transfer tension-shear testing; (c) wraparound bend test method; (d) three-point bending of welded specimens.

Welded Joints Examples of welded joints and their terminology.

Weld Symbols Standard identification and symbols for welds.

Weld Design Some design guidelines for welds. Source: After J.G. Bralla.