By Mark Shortt
Editorial Director, Design-2-Part Magazine

In applications ranging from basic MIG to automated laser welding of micro-components, quality and precision remain the ultimate goal of contract welders

Today, the welding capabilities of job shops across North America range from commonly used arc welding--such as MIG (Metal Inert-Gas) and TIG (Tungsten Inert-Gas)--to the high-energy electron- and laser-beam processes. Many of these welding processes are adaptable to automated or robotic welding, which has become a significant growth area for contract welders. But even though high-tech welding is now a reality, most small-to-medium-size shops cannot afford the expensive equipment that it requires. As a result, even those companies that don't use the most advanced state of the art technologies are finding ways to make precision welding an achievable goal.

Electron beam (EB) welding, although a very advanced technology, has been slow to catch on in job shops. "Overall, it gives the best quality weld possible," said Andy Cullison, editor of Welding Journal, a publication of the American Welding Society. "But there are very limited areas where you can apply it." Besides having to be performed in a vacuum chamber, it is a slow process and is used only for critical-type welds, such as aerospace components, or on thick or thin materials, stated Mr. Cullison.

"Lasers are probably the hottest thing going right now as far as new technology adapting itself to the welding industry," said Mr. Cullison. Still, the introduction of laser welding to production facilities remains somewhat limited because of its high cost. In job shops today, lasers are used more for cutting than for welding.

"It's a very expensive piece of equipment for a small or medium-size shop," said Mr. Cullison. And although lasers have proven to be very effective when used on thinner materials, they are not the first choice for materials more than 1/2-inch thick.

By and large, welders have made greater use of technological advances in automation and robotics than in either lasers or electron beam systems. In recent years, many job shops have made the transition to robotic work cells for welding, especially for small-batch production. "It's definitely a growth industry in welding," said Mr. Cullison.

Robotic Welding

Robotic welding, especially robotic MIG, is used extensively by automotive manufacturers for high-volume welding of automobile bodies. Its main advantages are increased speed and consistent or uniform quality of repetitive welds. Often, a robotic system can weld an assembly in a third of the time-or less-required by an individual, according to Tony Mancuso, president, Elrae Industries, Inc., Alden, New York.

Another advantage of robotic welders is their ability to weld continuously in a circular motion. "That's a real nice feature because in one of our applications, we weld a round bushing," said Mr. Mancuso. "We get complete welds around the whole circumference of the unit, whereas a man would basically have to stop and go."

Limited reach and the inability to fit the tip of the robot into tight quarters are among the limitations of robotic welders, according to Mr. Mancuso. Elrae uses a Miller MRK 5 robotic system with a maximum reach of "about five feet," he said. Most of the firm's work calls for general surface welding that involves angular and vertical welds, but not tight quarters, he said.

In automated arc welding, machine units are supplied with controls that feed the wire and move the welding head (or workpiece) along at a pre-programmed rate. Some control systems vary the feed rate of the electrode as the voltage across the arc varies, according to Manufacturing Processes (8th ed., New York: John Wiley & Sons).

By taking a hands-on welder out of the operation, robotic systems have created a need to pay closer attention to details prior to welding. Whereas a manual welder can make adjustments while welding, an automated system runs according to pre-programmed instructions. As a result, welders need to ensure a proper fit-up of the two pieces to be welded together. This means that parts must be machined more precisely prior to fit-up, and they must be clean in the weld joint area.

"You can't weld bad parts-parts that aren't stamped correctly," said Mr. Mancuso. "The reason being that the robot is programmed to go in one direction, and the part needs to be correct in terms of dimension and direction. If the part is off on a slant, the robot still is going to weld straight. So it's really critical that there be a high quality to the components being welded."

Electron-Beam Welding

Established as probably the highest-quality fusion welding process, electron beam welding is frequently used by the aerospace industry for high-performance joining rocket, satellite, and aircraft turbine engine components. The process is performed in a vacuum environment and is also frequently used to weld small components for instrumentation and sensors. It may also be specified for heavy-section, thick material that needs to be welded with a minimal amount of distortion, according to Michael Francoeur, president of Joining Technologies, East Granby, Connecticut.

The process generates heat by use of high-velocity, narrow-beam electrons, the kinetic energy of which is converted to heat as the electrons strike the workpiece. It is a high energy-density process that requires special equipment to focus the beam on a workpiece in a vacuum chamber. The main components of a typical system are the vacuum equipment, the beam delivery system, and electronic controls.

Another major market is the nuclear energy industry, which commonly specifies the process for joining rare alloys and elements; refractory materials, such as hafnium, niobium, tantalum, and tungsten; and for reactive materials that cannot be welded in the presence of oxygen.

Joining Technologies provides electron beam welding of energy panels for fuel cells used by the nuclear power industry. The company is also heavily involved in EB welding of aerospace components and instrumentation and sensors, many of which require an internal vacuum. It has welded components such as vibration sensors, pumphead sensors, diaphragm membranes, needles, and switch assemblies.

Laser-Beam Welding

The high costs of purchasing, upgrading, and maintaining electron beam welding equipment have allowed lasers to make inroads into what were formerly electron beam welding applications. Whereas new electron beam equipment commands sales prices ranging from $800,000 to $1.5 million or higher, laser welding machines are available in the range of $250,000-$300,000. As a result, laser welding is a lot cheaper.

The laser welding process is also faster because the lack of a vacuum chamber eliminates the time required for pumping down the chamber. However, the process also has less power than EB welding, and is therefore usually limited to light-penetration use on materials with a maximum thickness of 1/8 inch.

Mr. Francoeur said that although some lasers can penetrate metal 1/2-inch thick, they are not typically used for welding because problems with shielding gases at the higher energy levels lead to welds that are not as metallurgically sound as EB welds. "The weld pool becomes very violent and the weld just doesn't look that great," he said. "The rule of thumb for us is that typically, a laser beam works very well on materials less than .050 inch thick. Any part above .050 inch deep becomes a candidate for EB welding."

High-volume production is often another requirement for use of laser welding, Mr. Francoeur stated. When a job requires many parts to be welded, EB welding is not as cost-effective because parts have to be moved in and out of the vacuum chamber, he said.

Laser Welding Applications

Job shops currently use lasers to weld small components for the aerospace, medical, automotive, instrumentation and sensors, and electrical/electronics industries, among others. One of the more common uses is for encapsulation of electrical components, such as a switch device that needs to be shielded and hermetically sealed.

Hypodermic needles and tube assemblies for orthoscopic medical devices constitute "classic" medical applications for laser welding, said Mr. Francoeur. The process is also used for welding razor components and for welding tungsten filaments to posts in halogen bulbs.

Precision is, to some extent, built in to the higher-tech welding processes, such as EB and laser welding. But users of the more conventional manual welding processes, such as MIG and TIG, can still do a great deal to achieve consistently high-quality welds. In many cases, their own individual skills and resourcefulness go a long way toward achieving the desired quality.

Gas Metal-Arc Welding

The welding process that is used most extensively by metal fabricating shops is gas metal-arc welding (GMAW), commonly known as metal inert-gas (MIG) welding. Developed in the 1950s, the MIG process is suited for welding a wide variety of ferrous and nonferrous metals. It uses an externally supplied gas, such as argon, helium, or carbon dioxide, to shield the weld area; various other gas mixtures may also be used. To add filler metal, it feeds a solid (consumable) wire electrode through a nozzle into the weld arc. Deoxidizers are also used to prevent oxidation of the molten weld puddle.

Advances in wire technology. According to Dennis Reardon, production manager at Atlas Welding and Sheet Metal, Douglas, Massachusetts, the technology of the welding wire has come a long way in the last several years. Equipment manufacturers have experimented with different types of wire by incorporating various components into the wire to improve flow and procedure a smoother weld. In some cases, they've been successful, he said.

"They've developed a wire that, in a certain position, will flow smoothly, so that you don't have to grind (the weld). You can just hit it with a wire brush, and it looks great."

Gas mixtures. Experienced welders will use various mixtures of gases to suit their purposes. For example, a mixture of three different gases-argon, oxygen, and carbon dioxide-can be used to achieve a very clean weld with no spatter. A tri-mix is also advantageous when working with stainless steel because straight argon is hotter than necessary and would tend to produce undercutting along the seams of the weld.

Mr. Reardon said that Atlas welders are currently using what is known as a "steel mix," which comprises a certain percentage of argon and carbon dioxide to produce a better, smoother bead. "It flows easier, you can run the wire a little faster, and that, in essence, is basically what welding is about-trying to get a s much done as quickly as possible. And that has really helped the industry."

Productivity of the gas metal-arc process is said to be twice that of stick welding, which is commonly used in maintenance applications and heavy field work (general construction, pipelines, and shipbuilding). Advantages of MIG also include rapid speed, clean welds, versatility, and cost effectiveness. Cost of equipment is usually in the range of $1,200 to $3,500. The process can be easily automated and is therefore commonly used in robotic systems.

But because the welding gun should be kept close to the workpiece to ensure adequate shielding, joints that are difficult to reach can pose a problem for MIG welders. The process is also not typically specified for more refined welding or applications in which fracture resistance is critical. Such applications are more likely to require TIG welding.

Gas Tungsten Arc (Tungsten Inert Gas) Welding

Despite the technologically advanced nature of the high-energy electron beam and laser welding processes, they are not suited to every welding application. Some applications, such as fillet welds, are best handled with conventional gas tungsten arc welding (GTAW), commonly known as tungsten inert gas (TIG) welding.

Gas tungsten arc welding, normally thought of as a conventional process, also has a very technical, precision side to it. According to Mr. Francoeur, precision gas tungsten arc welding can be used as an alternative to laser and electron beam welding of micro-components. Power supplies for precision TIG welding today are very sophisticated, offering precisely controlled energy levels, polarity, and currents.

"Welding currents are resolved very, very fine," said Mr. Francoeur, "I mean, they're resolving currents like 0.5 amps, 0.3 amps. They're welding small bellows that are, in some cases, 0.002 inch thick, with GTAW."

A number of factors have contributed to the increased sophistication of power supplies and of TIG welding in general, Mr. Francoeur said. By changing the sin wave on the alternating current (AC), variable-polarity power supplies permit cathodic cleaning and welding of plated or zinc-coated materials. Feedback mechanisms-the circuits that permit closed-loop minitoring of voltages, currents, and arc distance-are also more advanced.

"The ability today to keep the cooling water going through the torch is also very closely controlled. There are new families of electrodes, where they're using new rare-earth elements: there's seriated, there's thoriated, and a few others. There's a family of new electrodes that are also contributing to the advancement of arc starting. And when you couple that with the gas-mixing technology that we have today-the ability to mix raw gases very precisely into new mixtures-you get incredible results.

"So it's not just the power supply; it's really all these things combined that make precision GTAW kind of a new field-an emerging, respected, rebirth of what we would consider conventional."

Achieving Tight Tolerances

Job shops employ a number of measures to help ensure close tolerances on welded parts and assemblies. Early planning can avert or minimize many problems, and has no substitute.

Welders can, for instance, make sure that parts are cut or machined to larger sizes that will allow for the inevitable shrinkage and warping. To anticipate shrinkage, they must consider the typed thickness of the material and size of the required weld. They can also use heat sinks, jigs, and fixtures to minimize heat transfer and distortion. Knowledge of the material, communication between mechanic and welder, and teamwork are essential to achieving quality welds, said Mr. Reardon.

"The biggest thing, from my point of view, is the mechanic working hand-in-hand with our welder. If the mechanic is not sure of how the welder wants to put this together, he goes to the welder and asks 'Do you want this corner-to-corner, do you want it flush, do you want it half-stock-what's the best way to do this?' It depends on the part-some parts need to be very strong, and if you go half-stock on the material, your strength is cut by 35-40%. If you go corner-to-corner, you get more strength with it, because you're filling in that whole corner with weld."

The American Welding Society (AWS) is a worldwide membership organization of 48,000 professionals, including engineers, welders, scientists, educators, and welding foremen. The Society sponsors educational programs and offers books, videos, and software on various welding topics.

Elrae Industries, Inc., performs contract welding of office chair bases and backs, motor brackets for machinery, saddlebag brackets for motorcycles, and electrical components for utility poles, in addition to bushings.

Joining Technologies, provides contract welding services ranging from the high energy-density electron- and laser-beam processes to precision TIG, brazing, hydrogen fuel, and oxy fuel processes. According to President Michael Francoeur, the company has developed a niche in precision, small-component welding for medical devices, electronics, and instrumentation and sensors. It uses robotics in its laser, plasma, and gas tungsten transferred arc welding.

Atlas Sheet Metal and Welding, a division of Granutec, specializes in welding machine tool and belt guards, bins, tanks, hoppers, conveyors, and brackets. In addition to fabricating sheet metal guarding for granulators, the company provides contract welding steel, stainless steel, and aluminum components. The company is scheduled to expand its manufacturing space by some 6,000 square feet when it moves into a new 10,000-square-foot facility late this summer.

Alloy Welding & Manufacturing Co., Inc., performs MIG, TIG, stick, and flux-core welding at its 14,000-square-foot facility in Bristol, Connecticut. The shop welds items ranging in size from 1-oz. spot-welded springs to 10-foot-high mezzanines. Its jobs include metal railings and ladders; industrial tanks, trays, and pans; safety guards for machines; cabinets for electrical components and tool boxes; machine bases and weldments; and repairs to various types of equipment.

References
Amstead, B.H., Ostwald, P.F., and Begeman, M.L. 1987. Manufacturing Processes. 8th ed. New York: John Wiley & Sons.

Cubberly, William H., and Bakerjian, Ramon, editors, 1989. Tool and Manufacturing Engineers Handbook. Desk ed. Dearborn, Michigan: Society of Manufacturing Engineers.

Kalpakjian, Serope.1995. Manufacturing Engineering and Technology. 3d ed. Reading, Massachusetts: Addison-Wesley Publishing Co.