Manufacturing technology

THE GAS-TUNGSTEN ARC WELDING (GTAW) process is performed using a welding arc between a nonconsumable tungsten-base electrode and the workpieces to be joined. C.E. Jackson defined a welding arc as "a sustained electrical discharge through a high-temperature conducting plasma producing sufficient thermal energy so as to be useful for the joining of metals by fusion." This definition is a good foundation for the discussion that follows.
 
 The physics of GTAW are fundamental to all arc processes and are more straightforward, because the complications of materials (for example, filler and flux) transferred through and interacting with the arc can be avoided. Geometrically, the arc discharge in GTAW is between a rod-shaped tungsten electrode and a planar-shaped electrode, that is, the workpiece. Pure tungsten electrodes are less expensive and, possibly, more environmentally compatible than those with rare earth or other oxide additions. They are used for lower-specification welds, where tungsten contamination that is caused by the molten electrode surface can be tolerated. They are also used for alternating current (ac) welding of aluminum, copper, magnesium, and thin sections of low-alloy and stainless steels.
  Continue reading "Arc Physics of Gas-Tungsten Arc Welding"

DURING FUSION WELDING, the thermal cycles produced by the moving heat source cause physical state changes, metallurgical phase transformation, and transient thermal stress and metal movement. After welding is completed, the finished product may contain physical discontinuities that are due to excessively rapid solidification, or adverse microstructures that are due to inappropriate cooling, or residual stress and distortion that are due to the existence of incompatible plastic strains.

In order to analyze these problems, this article presents an analysis of welding heat flow, focusing on the heat flow in the fusion welding process. The primary objective of welding heat flow modeling is to provide a mathematical tool for thermal data analysis, design iterations, or the systematic investigation of the thermal characteristics of any welding parameters. Exact comparisons with experimental measurements may not be feasible, unless some calibration through the experimental verification procedure is conducted.

Welding Thermal Process. A physical model of the welding system is shown in Fig. 1. The welding heat source moves at a constant speed along a straight path. The end result, after either initiating or terminating the heat source, is the formation of a transient thermal state in the weldment. At some point after heat-source initiation but before termination, the temperature distribution is stationary, or in thermal equilibrium, with respect to the moving coordinates. The origin of the moving coordinates coincides with the center of the heat source. The intense welding heat melts the metal and forms a molten pool.

Some of the heat is conducted into the base metal and some is lost from either the arc column or the metal surface to the environment surrounding the plate. Three metallurgical zones are formed in the plate upon completion of the thermal cycle: the weld-metal (WM) zone, the heated-affected zone (HAZ), and the base-metal (BM) zone. The peak temperature and the subsequent cooling rates determine the HAZ structures, whereas the thermal gradients, the solidification rates, and the cooling rates at the liquid-solid pool boundary determine the solidification structure of the WM zone. The size and flow direction of the pool determines the amount of dilution and weld penetration. The material response in the temperature range near melting temperatures is primarily responsible for the metallurgical changes. Continue reading "Heat Flow in Fusion Welding"

WELDING AND JOINING processes are essential for the development of virtually every manufactured product. However, these processes often appear to consume greater fractions of the product cost and to create more of the production difficulties than might be expected. There are a number of reasons that explain this situation.

First, welding and joining are multifaceted, both in terms of process variations (such as fastening, adhesive bonding, soldering, brazing, arc welding, diffusion bonding, and resistance welding) and in the disciplines needed for problem solving (such as mechanics, materials science, physics, chemistry, and electronics). An engineer with unusually broad and deep training is required to bring these disciplines together and to apply them effectively to a variety of processes.

Second, welding or joining difficulties usually occur far into the manufacturing process, where the relative value of scrapped parts is high.

Third, a very large percentage of product failures occur at joints because they are usually located at the highest stress points of an assembly and are therefore the weakest parts of that assembly. Careful attention to the joining processes can produce great rewards in manufacturing economy and product reliability. The Section "Fusion Welding Processes" in this Volume provides details about equipment and systems for the major fusion welding processes.

The purpose of this Section of the Volume is to discuss the fundamentals of fusion welding processes, with an emphasis on the underlying scientific principles. Because there are many fusion welding processes, one of the greatest difficulties for the manufacturing engineer is to determine which process will produce acceptable properties at the lowest cost. There are no simple answers. Any change in the part geometry, material, value of the end product, or size of the production run, as well as the availability of joining equipment, can influence the choice of joining method. For small lots of complex parts, fastening may be preferable to welding, whereas for long production runs, welds can be stronger and less expensive. Continue reading "Energy Sources Used for Fusion Welding"

For most rotating machines used in the process industries, the trend is toward higher speeds, higher horsepowers per machine, and less sparing. The first of these factors increases the need for precise balancing and alignment. This is necessary to minimize vibration and premature wear of bearings, couplings, and shaft seals. The latter two factors increase the economic importance of high machine reliability, which is directly dependent on minimizing premature wear and breakdown of key components.
 
 Balancing, deservedly, has long received attention from machinery manufacturers and users as a way to minimize vibration and wear. Many shop and field balancing machines, instruments, and methods have become available over the years. Alignment, which is equally important, has received proportionately less notice than its importance justifies. Any kind of alignment, even straightedge alignment, is better than no alignment at all. Precise, two-indicator alignment is better than rough alignment, particularly for machines 3600 RPM and higher. It can give greatly improved bearing and seal life, lower vibration, and better overall reliability. It does take longer, however, especially the first time it is done to a particular machine, or when done by inexperienced personnel. The process operators and mechanical supervisors must be made aware of this time requirement. If they insist on having the job done in a hurry. they should do so with full knowledge of the likelihood of poor alignment and reduced machine reliability. Figure 5-1 shows a serious machinery failure which started with piping-induced misalignment, progressed to coupling distress, bearing failure, and finally, total wreck.
  Continue reading "Alignment and Balancing"

In hydrocarbon processing plants, mechanical seals for pumps and compressors, tube fittings and pipe flanges often use O-rings to prevent fluid flow or leakage. According to application, O-rings can be categorized as static (seal between flange facings) and dynamic (subjected to movement or wobble). Table 10-13 lists the commonly available O-ring materials in decreasing order of preference based on an overall desirability for O-ring sealing service, with cost and availability considered secondary. When following the design steps results in several candidate elastomers for a specific application, this table may be used for final selection. (Letter suffixes identify elastomers compound designations.
 
 Next, the user has to consider temperature limitations of the elastomers. Here Table 10-14 will be helpful.
 
 Chemical compatibility of O-rings with a process fluid and temperature limits will define the method of O-ring production, using full-circle molding, ambient adhesive bonding and hot bonding or vulcanizing. Having no joint and hence no weak point, full-circle molded O-rings are the most common for reliability in operation. Available in a wide range of stock sizes and materials, O-rings of this type also can be custommolded. Ambient adhesive-bonded O-rings of any diameter can be quickly and easily made, using cord stock of most materials except silicone rubber. A simple jig used for cutting square ends and aligning them for bonding gives a smooth joint, which can sometimes be made in place without machine disassembly. Vulcanizing is considered to be an intermediate method in terms of nonstock O-ring delivery, chemical, and temperature resistance. Thermal and chemical resistance of the hot, bonded O-rings is superior to the adhesive-bonded, but inferior to the molded ones. Continue reading "Selection and Application of 0-Rings"

In trying to achieve improved wear resistance it would be well not to neglect proven traditional steel-hardening methods. In surface hardening of alloy steels the core of a machinery part may bc trcated to produce a desired structure for machinability or a strength level of service, whereas the surface may be subsequently hardened for high strength and wear resistance.
 
 Flame hardening involves very rapid surface heating with a direct high temperature flame, followed by cooling at a suitable rate for hardening. The process utilizes a fuel gas plus air or oxygen for heating. Steels commonly flame hardened are of the medium, 0.30 to 0.60 percent carbon range with alloy suitable for the application. The quenching medium may be caustic, brine, water, oil, or air, as required. Normally quenchants are sprayed, but immersion quenching is used in some instances. To maintain uniformity of hardening, it is necessary to use mechanical equipment to locate and time the application of heat, and to control the quench.
 
 As with conventional hardening, residual stresses may cause cracking if they are not immediately relieved by tempering. In some instances residual heat after quenching may be sufficient to satisfactorily relieve hardcning stresses. As size dictates, either conventional furnace tempering or flame tempering may be used. With flame tempering, the heat is applied in a manner similar to that used for hardening but utilizing smaller flame heads with less heat output.
 
 Carburizing is one of the oldest heat treating processes. Evidence exists that in ancient times sword blades and primitive tools were made by carburization of low carbon wrought irons. Today, the process is a science whereby carbon is added to steel within desired limitations to a controlled amount and depth. Carburizing is usually, but not necessarily, performed on steels initially low in carbon.

Continue reading "Hardening of Machinery Components"

Another process that will restore worn or corroded machinery surfaces is industrial plating, usually electroplating. This process is not normally applied on-site but parts in need of restoration have to be shipped to a company specializing in this type of work.
 
 Surface preparation for plating is usually achieved by smooth machining or grinding. In some cases, shot or grit blasting may be suitable. A very rough surface before plating is neither necessary nor desirable. Unless a greater thickness of deposit is required for wear, corrosion allowance, or for bearing material compatibility, there is no need to remove more metal than required to clean up the surface. Sharp corners and edges should be given as large a radius or diameter as possible. Areas not requiring resurfacing will be protected by the plating shop.
 
 Materials that can be repaired belong to the majority of metals used in normal design practice. It is, however, very important that the plating company be informed of the composition or specification.
  Continue reading "Industrial Plating Chrome-Plating of Cylinder Liners"

Many times metals are surfaced with austenitic stainless steels or soft nickel-chromium alloys for the sole purpose of corrosion resistance. For some applications, costly metals such as tantalum, silver or gold are used as surfacings. If a particular application requires a very special material, a surfacing technique probably can be used to put this special metal on only the functional surfaces, with a reduction in cost. In an effort to come up with a viable hard surfacing selection system, a series of wear tests was conducted on fusion surfacing materials from each of the classifications detailed in the preceding pages. Several vendors’ products in each classification were tested, and the welding characteristics of each material determined. Ceramics, tool steels, and special purpose materials were not tested.
 
 The specific procedure for evaluating the fusion surfacings was to make multilayer test coupons, determine the welding characteristics, and run metal-to-metal and abrasive wear tests on the materials that performed satisfactorily in the welding tests. The compositions of the hard surfacing alloys tested are shown in Table 10-1.
 

Continue reading "Hard Surfacing Techniques II - Special Purpose Materials"

     Almost every welding technique can be used to apply a hard-surfacing material. Referring to the definition of hard surfacing-applying by welding or spraying techniques a material with properties superior to those of the basis metal-it can readily be seen that this can be accomplished in many ways. Figure 10-2 illustrates most of the methods used. Each has advantages and disadvantages. Shielded metal-arc welding is the most common and versatile welding technique, but many of the hardsurfacing alloys have not been available in a coated electrode form.
 
 Oxyacetylene welding is the preferred method for applying bare filler metals, in that it minimizes “dilution” or mixing of the filler metal with the basis metal, so that the hardest deposit is achieved with this mode of deposition. Gas welding is slow, and it is difficult to control the deposit profile. Gas tungsten-arc welding is also slow, but provides the most accurate deposit profile of any of the fusion processes; it has the major disadvantage of significant dilution, with a corresponding loss in deposit hardness. Gas metal-arc welding is one of the fastest processes for applying hard surfacing; however (once again) not all surfacing alloys are available as wire that can be roll-driven through the welding gun. (When used for hard surfacing, the gas metal-arc process is often used without a shielding gas, and then is referred to as the “open arc” process.)
 

Continue reading "Hard Surfacing Techniques"

Many repairs of worn machinery surfaces can be achieved by hard surfacing. By definition, hard surfacing is the process of applying, by specialized welding techniques, a material with properties superior to the basis metal.
 
 Perhaps the chief factor limiting wide acceptance of this process today is the aura of mystery surrounding the properties of the various hardsurfacing alloys. There are literally hundreds of hard-surfacing alloys commercially available, each with a strange sounding name and a vendor’s claim that it is the ultimate material for this or that application. Rather than sift through the chaff to determine which should be used on an urgent problem, many machinery maintenance people drop the idea of hard surfacing and rely on more familiar techniques. This section will discuss the various forms of wear, and show how a few hard-surfacing materials can solve most wear problems. The following information is intended to simplify the field of hard surfacing so that maintenance and design engineers can effectively use the process to reduce maintenance and fabrication costs.

Continue reading "Protecting Machinery Parts Against Loss Of Surface"

The fundamental purpose of a bearing is to reduce friction and wear between rotating parts that are in contact with one another in any mechanism. The length of time a machine will retain its original operating efficiency and accuracy will depend upon the proper selection of bearings, the care used while installing them, proper lubrication, and propcr maintenance provided during actual operation. The manufacturer of the machine is responsible for selecting the correct type and size of bearings and properly applying the bearings in the equipment. However, maintenance of the machine is the responsibility of the user. A well-planned and systematic maintenance procedure will assure extended operation of the machine. Failure to take the necessary precautions will generally lead to machine downtime. It must also be remembered that factors outside of the machine shaft may cause problems. Engineering and Interchangeability Data Rings and Bulls-The standard material used in ball bearing rings and balls is a vacuum processed high chromium steel identified as SAE 52100 or AISI-52100. Material quality for balls and bearing rings is maintained by multiple inspections at the steel mill and upon receipt at thc bcaring manufacturing plants. The 52 100 bearing steel with standard heat treatment can be operated satisfactorily at temperatures as high as 250°F (121 "C). For higher operating temperatures. a special heat treatment is required in order to give dimensional stability to the bearing parts.
 
 Seals-Standard materials used in bearing seals are generally nitrile rubber. The material is bonded to a pressed steel core or shield. Nitrile rubber is unaffected by any type of lubricant commonly used in anti-friction bearings. These closures have a useful temperature range of -70" to +225"F (-56" to 107°C). For higher operating temperatures, special seals of high temperature materials can be supplied. Ball Cages-Ball cages are pressed from low carbon steel of SAE 1010 steel. This same material is used for bearing shields. Molded nylon cages are now available for many bearing sizes. The machined cages ordinarily supplied in super-precision ball bearings are made from laminated cotton fabric impregnated with a phenolic resin. This type of cage material has an upper temperature limit of 225°F (107°C) with grease and 250°F (121 "C) with oil for extended service. For periods of short exposure, higher temperatures can be tolerated. Continue reading "Ball Bearing Maintenance and Replacement"

All the large-chip processes use cutting tools of defined geometry which are applied in a controlled manner to remove metal at a predetermined rate. The processes could be classified in many ways, but it is convenient to consider them in terms of the kinematics of the inachine tools. With this in mind, they have been separatcd into four main machine groups:
 
 1. Turning (rotating work)
 
 2.Shaping (reciprocating tool or work)
 
 3. Milling (rotating tool)
 
 4.Drilling and boring (rotating tool)
 
 Turning machines embrace the wide variety of lathes and vertical boring machines which can be controlled manually or automaticially. Automatic control can be achieved using cams, sequential controllers, hydraulic copying devices or numerical programming. All machines in this group are capable of performing six basic operations as shown in Figure 16.1. In addition, copying lathes and numerically controlled lathes can generate mon-parallel forms by traversing the tool simultaneously in two planes.
  Continue reading "Large-chip metal removal processes"

Hard materials are necessary for such industrial applications as abrasives, cutting tools, molds, automobile parts, electronic components, and optical parts. Industry uses hard materials to coat and modify surfaces of industrial materials [I]. Based on their chemical bonding character, conventional hard materials, with a hardness > 15 GPa, are divided into three groups: metallic hard materials (e.g., TiN, Tic, CrN, WC), covalent hard materials (e.g., diamond, Sic, B,C, Si,N,) and ionic hard materials (e.g., A1,0,, ZrO,, TiO,). Carbon also has a role as a component in hard materials. Materials with a hardness >40 GPa, the so-called “super-hard (or ultra-hard) materials” include diamond (C) (hardness: 80-100 GPa, Knoop hardness: Hk 8000- IOOOO), zincblende-type cubic boron nitride (c-BN) (40-60 GPa, Hk: 4000-6000) and carbon boride (B4C) (30-45 GPa, Hk: 3000-4500) [2]. Recently, diamond-like carbon (DLC), carbon nitride, silicon carbon nitride, boron carbonitride, and ceramic nanocomposites are described as super-hard materials. Most of these super-hard materials are composed of the three elements carbon, boron and nitrogen, as shown in Fig. 1. This chapter describes preparation methods and properties of thin films of such super-hard carbon alloys as diamond-like carbon (DLC), carbon nitride (e.g. p-C,N,) and boron carbonitride (B,C&). These materials are carbon alloys and have the mixed bonding states of sp’, sp2 and sp3, and are bonded to the alloying elements hydrogen, nitrogen and boron.
 
 Diamond-like Carbon
 
 Diamond-like carbon (DLC) is defined as an amorphous carbon (a-C) having structural, mechanical, electrical, optical, chemical, and acoustic properties similar to those of diamond. The name “diamond-like carbon” is currently used widely when at least one property is similar to diamond. Table 1 shows the comparison of typical properties between diamond-like carbon and diamond. Diamond-like carbon has properties similar to diamond (Table l), and potential applications in various industrial fields, as indicated in Fig. 2. Diamond-like carbon will be used more extensively in industry in the 21st century.
  Continue reading "Super-hard Materials"

If you thought the modern MIG welder brought great advancements in ease of the learning and operating curves, wait until you try plasma-arc cutting. It's as much of an improvement over gas-torch cutting, in both learning curve and reduced maintenance. We watched as a race-car mechanic in a large shop was being trained by another worker to utilize their plasma cutter. The newcomer to the technology made his first 4-inch by 6-inch part from 3/8-inch steel plate after only fifteen minutes of instruction. After experimenting with different speeds, power levels and materials, he was ready to do real jobs after an hour. You can't learn everything there is to know about the process in your first day, but this story is typical of first-time users.
 
 Unlike welders, most plasma cutters of the shop-type variety are ready to use when you uncrate them. Their power cord is ready to plug in (you don't have to put on your own plug), the torch is usually already hooked up to the front of the machine, and all you have to do is connect your shop air supply before making your first cuts.
  Continue reading "Using a plasma cutter"

In plasma-arc cutting, the technology is much the same as welding, but the arc is even more constricted, with the plasma temperature so high that the arc can cut any metal. The very restricted arc cuts a pencil-thin line through the metal, and a sharp force of gas through the nozzle blows the melted metal from the kerf (see illustration). The velocity and steadiness of the cutting action is such that the edges of the cuts are kept very straight and clean. The benefits of plasma-arc technology in cutting metal are many. The cuts are very clean, with little oxidation of the metal as found in oxy-acetylene cutting, the velocity of the action creates smoother edges that need a great deal less cleanup, and in fact, with the industrial units that are mechanized much like a gas flame-cutter or pattern-cutter, the cuts need no cleanup or preparation for welding (see illustration). There is little or no slag found on the bottom edge of the kerf.
 
 Plasma-arc cutting is considered much safer in many work environments than gas cutting. There are no flammable gasses involved, there is no smoke from the cutting action to bother the weldor, he can watch the process much more closely and there is much less spray of sparks coming from under the cut. If you remember the little sparking toys you had as a kid, where you pushed a plunger and a wheel went around, scraping against a tiny flint making sparks, that is a small-scale version of what you see when plasma cutting. This is in high contrast to gas cutting, where there is a lot of heat generated, and blobs of molten metal are constantly falling on the floor under the cut, creating a fire hazard.
  Continue reading "Plasma-Arc cutting"

Just as the emergence of lower-cost, higher-efficiency MIG machines has made something of a revolution in small shop and home/shop welding in the last ten years, so has plasma technology changed the face of metal cutting. While the technology has been around for a while, it has been seen by most weldors as a high-technology setup that seemed complicated or hard to use. Plasma technology's biggest usage had been in plasma arc welding, or PAW, and it is still used in this form in many industrial applications. The simpler plasma-arc cutting (or PAC) technology got very little play at the time, and it has taken some years for the equipment to filter down to the general welding/cutting marketplace. As with the MIG welders, what was originally considered to be a professionals-only setup has entered the province of the small shop and the amateur fabricator, due mainly to the introduction of less-expensive imported equipment that really opened up new markets with hobbyists. The domestic welding manufacturers suddenly became aware of these newer markets for smaller versions of equipment they had been selling to industry all along and brought increasing competition to the fray. Today there's a wide selection of plasma machines to choose from, from small 110V AC portables to large shop equipment for materials up to one inch thick.

Continue reading "Plasma cutting/welding"

Perhaps a little more difficult to learn than welding steel, precision joints in aluminum or magnesium are among the primary reasons for having a TIG welder, so this should be your next area of practice. You need a well lighted area to do TIG welding because the arc is not as bright as with other electric arc techniques, and on aluminum you really need to see how the puddle is developing. Set your machine to HFAC, with the high-frequency on continuous, or use the square-wave feature if you have it, and adjust the post-flow for 1/8-inch electrodes. Use a clean 1/8-inch electrode (pure tungsten) with the tip shaped for aluminum (not a needle-sharp point), and adjust the flow meter for the argon to 17-20 cubic feet per hour. Set an 1/8-inch-thick aluminum plate that has been cleaned thoroughly down on the welding table. Aluminum tends to age-harden when it is exposed to air, and aluminum oxides will constantly build up on the surface; such oxides are not always visible. Parts to be welded need to be cleaned just before welding to provide the freshest surface. Do not use chemical cleaners for preparing aluminum, unless it is something you have purchased at a legitimate welding supply store. Household or automotive cleaners may contain chemicals that will vaporize during welding and create toxic fumes. Use a stainless-steel wire brush (not steel wool) only, and never use this stainless-steel brush for cleaning plain steel. Since most sandpapers are made from aluminum oxide, and this is exactly the material we are trying to eliminate when cleaning aluminum for heli-arc welding, don't use ordinary sandpapers to pre-clean.
 
 Use an aluminum welding rod, and clean the welding rods before starting, using alcohol and a lint-free cloth to wipe them down. The aluminum part you are welding should be well grounded to your table. If it is lightweight plate, put a heavy weight on part of it to keep it in contact with your grounded table, or attach the work clamp directly to the part. Otherwise, you may find that the bottom of an aluminum part you have welded on your table will have arc burns from intermittent contact with the table. Draw a bead on the aluminum for a short distance, then immediately cut off the arc. Examine this weld and you will probably see that where you stopped there's a small depression at the end of the bead. To avoid this at the end of every weld, don't back off suddenly, but rather ease out of it while slowly moving the tip back over the end puddle, and it should keep from sinking. Continue reading "TIG-welding aluminum"

 When those who are interested in welding talk about fusion techniques, the subject of TIG welding is held in a certain reverence. Its reputation in this regard partly stems from its place in history as the method used for construction of a great many famous aircraft. Although developed initially in the 1920s, TIG (Tungsten Inert Gas welding) wasn't used much because the helium shielding gas was too expensive. The intensified research atmosphere of W.W.II spurred further development as aircraft were being made lighter and lighter and TIG became the preeminent method of joining such non-ferrous materials as aluminum and magnesium. The Linde Corporation (now L-TEC) was the first company to capitalize on the technique, and after the war their trademark name "Heli-Arc" became the de facto generic name for TIG welding. Many weldors today still use the term heliarc more often than TIG as a description of this type of welding, even though a number of other companies have been making TIG equipment for many years.
 
 Besides the romantic beginnings, the TIG process has been considered quite special for other reasons. It does take considerable practice to be good at it, and, because the equipment has always been rather expensive, a weldor who had one and was good with it developed a reputation, particularly in the field of esoteric materials and exotic construction in aircraft and race cars. Basically an outgrowth of arc welding, the TIG process is done with a lightweight torch that uses a tungsten electrode to draw an intense, concentrated arc, shielded by an inert gas. The gas comes out of the torch, all around the electrode and displaces the air from the weld zone to exclude oxygen and nitrogen from contaminating the weld. Although helium was the original gas used, today argon is the most common shielding gas for TIG welding. The tungsten electrode is not considered a "consumable" in the usual sense, since filler metal is supplied by separate, hand-held rods, much like in oxy-acetylene welding. In many ways, heli-arc welding is like a high-tech version of the old-fashioned gas torch. Continue reading "TIG welding"

Welding books written as recently as ten years ago may have recommended oxy-acetylene and arc-welding equipment as the basic tools for home/shop welding. At that time, MIG, or wire-feed, welding equipment was considered too expensive for amateur use, despite its advantages. The MIG equipment was recommended, and indeed was originally designed, for high-production shop work only.

Much has changed in the intervening years, and the introduction of lowercost MIG units and the competitiveness of the marketplace has brought wirefeed welding into an affordable range for the home/shop weldor. An outgrowth of arc welding, the MIG process was conceived as a way to speed up production in industrial applications. The basic difference with MIG, which stands for Metal Inert Gas welding (sometimes also referred to by its technical description as Gas Metal Arc Welding, or GMAW), is that the welding filler rod is automatically fed through the gun whenever welding takes place. The weldor doesn't need to stop to change arc electrodes. The Airco company developed the MIG process right after W.W.II, and it has been refined continuously ever since. The many advantages of the MIG process, besides the basic one of not having to stop to change electrodes, include: being able to weld in all positions; there is no slag removal and the welds are much cleaner and with very little spatter compared to arc welding; the welding can proceed a lot faster than with other methods because the wirefeed rate is automatic and adjustable, which also makes for less distortion of the workpiece because less heat is concentrated on the seam; the process works well on joints that are irregular or have gaps; and the amperage, wire size and wire-feed speed can be tuned down to do ideal, almost distortionless welds on thin sheet metal.

Continue reading "MIG welding"

Like gas welding, electric arc welding has been around for almost 100 years, and the fact that it is still around today illustrates its continued usefulness. The official acronym for arc welding is SMAW, which stands for Shielded Metal Arc Welding. The basic components of the setup include the machine (the power source), a ground lead you clamp to the work anywhere except where the weld is to be made, an electrode lead which runs from the machine to an electrode holder, which is a handle with a clamp that holds consumable electrodes. The electrodes are metal rods covered with a coating.
 
 In use, the weldor strikes an arc against the parent metal with the electrode, which completes the circuit between the two leads and causes a bright light and concentrated heat. Arc welding uses considerable amperage of electricity to generate the intense arc, which melts the parent metal. The central metal core of the electrode melts as the work progresses, becoming the filler metal, while the fluxed coating produces a shielding gas around the welding area that protects the parent and filler metal from impurities in the air. Arc-welding produces slag as you proceed, a thick coating of impurities and deposits left from the rod's coating. This slag must be chipped off with a chipping hammer, which is usually included with the machine. There are a wide variety of welding rods (electrodes) available to suit almost any purpose. The 12-14-inch-long rods are also called "sticks", and you may often hear arc-wetding referred to as stick welding. The rods vary in thickness, according to the thickness of the metal you are welding, and also in alloy and flux-coating content. There are many special-purpose rods, and, because of the variety, rods are usually marked with a number at the beginning of the flux coating, and different colors may also be added to the fluxes for quick identification.
  Continue reading "Arc welding"

This is perhaps the oldest and most versatile of welding setups. For a long time, it was the only setup recommended for the home/shop use, and has been among the least expensive to get started with. The basic combination in a typical gas-welding package are two high-pressure cylindrical tanks, one for oxygen, one for acetylene, a set of gauges and regulators to control the gas flow out of the tanks, a pair of hoses, and a torch. The torch usually comes with a variety of tips, tip cleaners, a spark lighter, and good sets may include a helmet, gloves and often a cutting-torch.

The latter is what really makes the oxy-acetylene system so versatile. It is one of the few welding systems that can do cutting as well as welding. This can be invaluable in both repair and fabrication work. Cutting away damaged or unwanted material is easily done with a properly-used cutting-torch attachment, and you may have many projects where you need to cut an irregular shape out of steel plate. The bulk of tubing and angle-iron cutting is usually done with some kind of saw or an abrasive cutoff wheel, but these tools can only make a straight cut; they can't go around corners. If you need to cut out a circle from a steel plate, you can draw the circle on the plate with a compass and a special hard crayon called a soapstone (which leaves a line you can see even when welding), then use your cutting torch to follow the line and you have your part. Any shape can be cut out. If you make up a cardboard template of the piece you need, trace around the pattern with your soapstone onto the plate, and make it. Cutting with a torch takes skill to closely follow a line, and even then the edges of the metal will require some grinding, filing or sanding to get a smooth edge. Most experienced weldors know just how much to cut outside their pattern size to have an exact-size piece after cleaning up the cut edges with a grinder.

Continue reading "Oxy-acetylene gas welding"

Types of welding If you are reading this book, chances are you have had some exposure to welding through watching a repair done such as having a new exhaust system put on your car, through some hobby interest in the metal arts/crafts area or through some industrial exposure to welding as used in manufacturing and building processes. Obviously you have become interested enough in learning about welding to purchase this book which we feel is an excellent introduction to a field where there are lots of involved textbooks for the person pursuing welding as a profession, but few basic books for someone getting started at the hobby, farm or home/shop level. Perhaps your initial exposure to welding has sparked an interest in doing it yourself. If you are involved in automotive work, you already know how valuable the process can be in fabrication and repairs. Once you have seen it performed, you realize how handy this capability is. You can join pieces of metal to either repair something that was damaged and otherwise scheduled for replacement or build something entirely new, from a barbecue grille to a race car. Once you have the basic skills and the right equipment, you'll find many more uses for welding than you had anticipated. Like a good truck or a specialized tool, once you have a welder, you'll wonder how you ever got along without it! You'll probably find yourself building a materials rack, stocking it with various sizes of tubing and plates, and actually looking for new projects to tackle, from building a workbench to last a lifetime, to storage racks, moveable shop carts, engine stands, shelving, and much more. Continue reading "Types of welding"

Tungsten arc inert gas shielded welding, EN process number 144 abbreviated to TIG, TAGS or GTAW (USA), is an arc welding process that uses a non-consumable tungsten electrode and an inert gas shield to protect the electrode, arc column and weld pool, as illustrated in Fig. 6.1. The welding arc acts as a heat source only and the welding engineer has the choice of whether or not to add a filler wire. The weld pool is easily controlled such that unbacked root passes can be made, the arc is stable at very low welding currents enabling thin components to be welded and the process produces very good quality weld metal, although highly skilled welders are required for the best results. It has a lower travel speed and lower filler metal deposition rate than MIG welding, making it less cost effective in some situations. TIG tends to be limited to the thinner gauges of aluminium, up to perhaps 6 mm in thickness. It has a shallower penetration into the parent metal than MIG and difficulty is sometimes encountered penetrating into corners and into the root of fillet welds. Recommended weld preparations taken from BS 3019 ‘TIG Welding of Aluminium’ are given in Table 6.1.

Process principles

The basic equipment for TIG welding comprises a power source, a welding torch, a supply of an inert shield gas, a supply of filler wire and perhaps a water cooling system. A typical assembly of equipment is illustrated in Fig. 6.2.

For welding most materials the TIG process conventionally uses direct current with the electrode connected to the negative pole of the power source, DCEN. As discussed in Chapter 3 welding on this polarity does not give efficient oxide removal. A further feature of the gas shielded arc welding processes is that the bulk of the heat is generated at the positive pole. TIG welding with the electrode connected to the positive pole, DCEP,results in overheating and melting of the electrode. Manual TIG welding of aluminium is therefore normally performed using alternating current,AC, where oxide film removal takes place on the electrode positive half cycle and electrode cooling and weld bead penetration on the electrode negative half cycle of the AC sine wave. The arc is extinguished and reignited every half cycle as the arc current passes through zero, on a 50Hz power supply requiring this to occur 100 times per second, twice on each power cycle. To achieve instant arc reignition a high-frequency (HF), high-voltage (9– 15 000 V) current is applied to the arc, bridging the arc gap with a continuous discharge. This ionises the gas in the arc gap, enabling the welding arc to reignite with a minimum delay (Fig. 6.3). This is particularly important on the DCEP half cycle.
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MIG spot welding may be used to lap weld sheets together by melting through the top sheet and fusing into the bottom sheet without moving the torch. The equipment used for spot welding is essentially the same as that used for conventional MIG, using the same power source, wire feeder and welding torch. The torch, however, is equipped with a modified gas shroud that enables the shroud to be positioned directly on the surface to be welded (Fig. 7.20). The shroud is designed to hold the torch at the correct arc length and is castellated such that the shield gas may escape.The power source is provided with a timer so that when the torch trigger is pulled a pre-weld purge gas flow is established, the arc burns for a pre-set time and there is a timed and controlled weld termination. The pressure applied by positioning the torch assists in bringing the two plate surfaces together. Because of this degree of control the process may be used by semi-skilled personnel with an appropriate amount of training.

The process may be operated in two modes: (a) by spot welding with the weld pool penetrating through the top plate and fusing into the lower one or (b) by plug welding where a hole is drilled in the upper plate to enable the arc to operate directly on the lower plate so that full fusion can be achieved. Plug welding is generally required when the top sheet thickness exceeds 3mm. The size of the drilled hole is important in that this determines the size of the weld nugget and the diameter should be typically between 1.5 and 2 times the top sheet thickness. Typical welding parameters are given in Table 7.7. Continue reading "MIG spot welding"

As MIG welding is a continuously fed wire process it is very easily mechanised. The torch, having been taken out of the welder’s hand, can be used at welding currents limited only by the torch or power source and at higher travel speeds than can be achieved with manual welding. A typical robot MIG welding cell where the robot is interfaced with a manipulator for increased flexibility and a pulsed MIG power source is illustrated in Fig. 7.19. Greater consistency in operation means that more consistent weld quality can be achieved with fewer defects. The advantages may be summarised as follows:

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The metal arc inert gas shielded process,EN process number 131,also known as MIG,MAGS or GMAW, was first used in the USA in the mid 1940s. Since those early days the process has found extensive use in a wide range of industries from automotive manufacture to cross-country pipelines. It is an arc welding process that uses a continuously fed wire both as electrode and as filler metal, the arc and the weld pool being protected by an inert gas shield. It offers the advantages of high welding speeds, smaller heat affected zones than TIG welding, excellent oxide film removal during welding and an allpositional welding capability. For these reasons MIG welding is the most widely used manual arc welding process for the joining of aluminium.

Process principles

The MIG welding process, illustrated in Figs. 7.1 and 7.2, as a rule uses direct current with the electrode connected to the positive pole of the power source, DC positive, or reverse polarity in the USA. As explained in Chapter 3 this results in very good oxide film removal. Recent power source developments have been successful in enabling the MIG process to be also used with AC. Most of the heat developed in the arc is generated at the positive pole, in the case of MIG welding the electrode, resulting in high wire burn-off rates and an efficient transfer of this heat into the weld pool by means of the filler wire.When welding at low welding currents the tip of the continuously fed wire may not melt sufficiently fast to maintain the arc but may dip into the weld pool and short circuit.This short circuit causes the wire to melt somewhat like an electrical fuse and the molten metal is drawn into the weld pool by surface tension effects. The arc re-establishes itself and the cycle is repeated. This is known as the dip transfer mode of metal transfer. Excessive spatter will be produced if the welding parameters are not correctly adjusted and the low heat input may give rise to lack of-fusion defects.At higher currents the filler metal is melted from the wire tip and transferred across the arc as a spray of molten droplets, spray transfer. This condition gives far lower spatter levels and deeper penetration into the parent metal than dip transfer.When MIG welding aluminium the low melting point of the aluminium results in spray transfer down to relatively low welding currents, giving a spatter-free joint.

The low-current, low-heat input dip transfer process is useful for the welding of thin plate or when welding in positions other than the flat (PA) position (see Fig. 10.3 for a definition of welding positions). It has, however, been supplanted in many applications by a pulsed current process, where a high current pulse is superimposed on a low background current at regular intervals. The background current is insufficient to melt the filler wire but the pulse of high current melts the filler metal and projects this as a spray of droplets of a controlled size across the arc, giving excellent metal transfer at low average welding currents. Continue reading "Mig Welding"

The most significant process for the welding of aluminium to be developed within the last decade of the twentieth century was the friction stir process, an adaptation of the friction welding process. This process was invented at TWI in the UK in 1991 and, unlike the conventional rotary or linear motion processes, is capable of welding longitudinal seams in flat plate. Despite being such a new process friction stir welds have already been launched into space in 1999 in the form of seams in the fuel tanks of a Boeing Delta II rocket (Fig. 8.12). It will soon be used for non-structural components in conventional commercial aircraft and is being actively considered for structural use. Friction stir welding has also been introduced into shipyards with great success and is being actively investigated for applications in the railway rolling stock and automotive industries.

The process utilises a bar-like tool in a wear-resistant material, for aluminium generally tool steel, a tool lasting in the region of 1–2 km of welding before requiring replacement. The end of the bar is machined to form a central probe and a shoulder, the probe length being slightly less than the depth of the weld required. The bar is rotated and the probe plunged into the weld line until the shoulder contacts the surface. The rotating probe within the workpiece heats and plasticises the surrounding metal. Moving the tool along the joint line results in the metal flowing from the front to the back of the probe, being prevented from extruding from the joint by the shoulders (Fig. 8.13).This also applies a substantial forging force which consolidates the plasticised metal to form a high-quality weld. Continue reading "Friction stir welding"

Electron beam welding is, like laser welding, a power beam process ideally suited to the welding of close square joints in a single pass.Unlike the laser beam, however, the electron beam process utilises a vacuum chamber in which is generated a high-energy density beam of electrons of the order of 0.25–2.5 mm in diameter (Fig. 8.. The beam is generated by heating a tungsten filament to a high temperature, causing a stream of electrons that are accelerated and focused magnetically to give a beam that gives up its energy when it impacts the target – the weld line. This enables very deep penetration to be achieved with a keyhole penetration mode at fast travel speeds (Fig. 8.9), providing low overall heat input. The process may be used for the welding of material as thin as foil and up to 400 mm thick in a single pass. The keyhole penetration mode gives almost uniform shrinkage about the neutral axis of the component, leading to low levels of distortion. This enables finish machined components to be welded and maintained within tolerance. The transverse shrinkage also results in the solidifying weld metal being extruded from the joint to give some excess metal outside the joint (Fig. 8.10). The major welding parameters are (a) the accelerating voltage, a 150kV unit being capable of penetrating 400mm of aluminium; (b) the current applied to the electron gun filament, generally measured in milliamperes; and (c) the travel speed.The item to be welded is generally mounted on an NC manipulator, the gun being held stationary. The unwelded joint components are required to be closely fitting and are usually machined. Continue reading "Electron beam welding"

There have been a number of relatively new developments where the laser has been combined with the arc from a conventional welding power source.

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As mentioned above improved focusing has enabled very concentrated beams with energy densities above 40kJ/mm2 to be produced.This has been achieved by using parabolic reflectors or transmissive systems with a focal length of around 150mm. The alloy content affects the energy required to achieve a keyhole with increasing levels of zinc or magnesium requiring less energy.This is attributed to the low vaporisation temperature of these alloying elements assisting in the formation of the keyhole. One corollary of this is that higher welding speeds are possible in those alloys with the higher magnesium contents.

Helium gas shielding of both the root and face of the weld is recommended for the higher magnesium-containing alloys such as 5083 (Al4.5Mg). Over some 3mm in thickness a jet of helium, supplementing the shielding gas, directed at the weld pool also gives improved weld appearance. Helium–argon mixture and pure argon gas have also been used with acceptable results although with a reduced parameter tolerance box.

Wire additions may be used to increase the resistance to hot cracking in those alloys that cannot be autogenously welded such as the 6XXX and 7XXX series of alloys.Wire additions are also beneficial in coping with gaps, a 1.2 mm wire can be used to fill gaps of up to 1.2mm.Wire diameters may be between 0.8 and 1.2mm. Feeding the wire into the leading edge of the weld pool at an angle of around 45° will improve the bead shape on both root and cap. Continue reading "CO2 laser welding"