The Science Of Holding Metals Together
Welding is the science of holding metals together through thermal of pressure processes. The process has been practiced since the Bronze Age and had a rebirth in the 19th century with the age of industrialism. It established itself as a part of the manufacturing process during World War II and has been used in automobile and computer construction.
Cars, bridges, skyscrapers, natural gas pipelines, and even computers would be much more expensive, or perhaps nonexistent, were it not for the unheralded technology of welding.
Metals are the biggest component of automobiles, and the manner in which they are joined together determines a vehicle’s overall strength and durability. Today’s cars and trucks are far stronger and safer, require less maintenance, and are more economical to operate than their predecessors of 50 years ago. And one of the main reasons is the thousands of strong, reliable welds that hold them together.
Despite its importance in automobile assembly and countless other manufacturing processes,weldingis a largely unsung technology. It’s even something of an enigma. The term welding has become a popular metaphor for strength and durability, and even for unity among individuals. Many of us speak of stuck drawers that seem “welded shut” and stubborn bolts that appear to be “welded on.” Yet consumers rarely inquire about or inspect welds in the products they buy. In fact, too much of the general public, mention of the term welding conjures only images of cluttered shops, black welding helmets, and showers of sparks.
In reality, however, modern welding is a booming, high-tech subscience of metallurgy. Most of today’s welders are no longer the body-shop variety. Instead, they are welding engineers trained in metallurgy as well as in chemistry, computer automation, thermodynamics, stress analysis, and heat transfer. Although modern welding dates back to only the 1930s, it has revolutionized most types of manufacturing, assembly, and construction. Strong, reliable welds have increased the quality and lowered the cost of countless products. They have furthered the advancement of science and even created a few new industries along the way. From computer circuit microwelds to the massive welded joints of bridges and skyscrapers, welding is literally holding together more and more of our world.
Welding refers to the various techniques of joining metals by fusion (thermal) or solid-state (pressure) processes, but modern welding techniques are almost exclusively of the fusion type. Welds are continuous metal joints that, when properly applied, are as strong as–or even stronger than–the metal itself. Held together by nothing more than atomic or molecular bonding, welds eliminate the need for adhesives and mechanical joining devices such as bolts and rivets.
Welding is possible because of the inherent attraction that similar metals have for each other. In a theoretical model, metals that are properly cleaned and smoothed to maximize surface contact will automatically join by atomic bonding. But cost-effective technologies to achieve this ideal type of contact weld do not yet exist. In the current state of the art, heat generated by electrical arcs and resistance, oxyacetylene flames, lasers, and/or electron beams is necessary to bond metals together.
The first welders
When the art of joining metals together originated in the Bronze Age, it was initially limited to the use of nails and staples, crude adhesives, and to hammering folded seams of bronze sheets. In hammer-forge welding, the first true welding process, metalworkers heated folded seams of bronze sheets nearly to their melting point, then hammered them together. Repetitive cycles of heating and hammering fused the bronze together in strong welds.
Pre-Columbian Amerindian gold workers in South America developed the first welding fluxes, substances used to help metals fuse together. They heated the edges of adjoining gold sheets almost to the melting point, then added powdered copper acetate, a flux obtained by dissolving native copper in vinegar. Without the use of pressure, the flux created an actual atomic fusion of the gold sheets. Strong and nearly undetectable, these joints were identical to the fusion welds achieved by modern jewelers.
During the first millennium A.D., Arab armorers heated iron in contact with carbon to produce a steel that was very hard, but it was too brittle to use for sword blades. To toughen the steel, they interlayered thin sheets of soft, tough iron with the brittle carbon steel, then repeatedly hammered the metals in high heat. These hammer-forge welders thus welded the layers of iron and steel together, producing a durable, hard metal known as Damascus steel that for centuries made the world’s finest sword blades.
In the Middle Ages, European armorers fashioned cannons by hammer-forge welding heavy reinforcing bands around iron barrels, while blacksmiths laboriously hammer-forge-welded the heavy links of anchor chains. As advanced iron-making techniques were developed in the sixteenth and seventeenth centuries, metal workers tended to rely on these newer techniques, instead of seeking to improve hammer-forge-welding techniques.
With the introduction of cast-iron-making technology in the eighteenth century, the need to laboriously hammer-forge-weld many iron products, such as the rims of wagon wheels, was eliminated. By then, only the hammer-forge welding of blacksmiths and the fusion craft work of jewelers kept the art of welding alive.
The rebirth of welding
In the nineteenth century, the rise of industrialism and steam power created the need to rapidly produce strong, continuous joints between large steel plates. Bolts and rivets were poorly suited for the manufacture of steam boilers and pressure vessels, as well as for withstanding the increasing levels of shock and vibration generated by powerful industrial machinery such as railroad locomotives and steam engines. But bolts and rivets were all that was available.
Meanwhile, heavier and faster ships placed unprecedented stresses on the riveted joints of early steel-hulled ships. By the late 1800s, the operation of steel machinery and equipment demanded time-consuming, costly, and constant retightening and replacement of worn and loose bolts and rivets, Welded joints were the obvious solution, but hammer-forge welding, the only known method forwelding iron, was far too slow for large-scale manufacturing.
Then in 1885, Russian researcher Stanislav Olczewiski learned that an electrical arc between an electrode and a grounded piece of steel generated enough heat to melt the steel. Olczewiski then applied the arc to the adjoining edges of two steel plates and, adding metal melted from an auxiliary filler rod, created a molten seam of steel. The molten seam cooled and solidified to join the plates together in a relatively strong, continuous weld.
A year later, Elihu Thomson, a professor at Philadelphia’s Franklin Institute, passed high-amperage electrical current through two adjoining pieces of steel. In a method that became known as resistancewelding, the electrical resistance at the point of contact generated sufficient heat to fuse the steel together.
By 1910, the three original, basic types of modern welding–arc, resistance, and oxyacetylene–were in limited industrial use. But all three methods shared a serious drawback. At fusion temperatures, steel combines readily with atmospheric oxygen and nitrogen to form oxides and nitrides. When coated with oxides and nitrides, droplets of molten metal bond poorly with the surrounding metal, forming brittle welds with hidden weak spots. Since weld strength was notoriously unpredictable, and “quality control” was limited to dropping the welded iron on a concrete floor, welding was limited primarily to repair and maintenance.
The great ship Titanic, launched in 1912, was built before welding became a reliable metal-joining technology. Hence the metal plates that formed its hull were not welded together, as in today’s ships, but joined by rivets. Today those rivets are one of the prime suspects as the weak link dooming that disastrously presumed “unsinkable” ship.
From `Rosie the Riveter’ to `Rosie the Welder’
Realizing that welding required some way to shield the molten metal from atmospheric contact, researchers designed consumable metal electrode :rods coated with various materials that melted or vaporized with the melting of the metal core. The coating vapor or melt provided partial atmospheric shielding and formed a light slag that covered the molten metal. The result was a more ductile weld with fewer weak spots. In the early 1920s, crude shielding techniques improved the reliability ofwelding and led to its first important uses in joining steam boiler plates and manufacturing steel chain links.
Shortly after the inert gases helium and argon became available commercially in 1930, American inventors Henry Hobart and Philip Devers patented a gas shielding method. By continuously releasing a cloud of inert, invisible gas that enveloped the point of the weld, Hobart and Devers completely shielded the molten metal from the atmosphere. Gas shielding made possible consistently strong and reliable welds and marked the beginning of the age of modern welding.
Thanks to gas shielding and improved, coated filler rods, welding had a tremendous impact on World War II. Entire fleets of steel ships were needed immediately to supply the Allied nations and far-flung combat theaters with war materiel. But until this time, building steel ships had been a slow process that involved the tedious drilling, bolting, and riveting of steel plates.
Henry Kaiser, president of the Kaiser Steel Corporation, changed all that by putting welding to its first major use: the mass construction of Liberty ships. With no need to drill, bolt, and rivet hull and superstructure plates, Kaiser’s army of welders, working day and night and bathed in the blinding white light of electrical arcs, launched hundreds of 300-foot-long, steel Liberty ships, weldingtogether each ship from the keel up in a mere 69 days.
Welders constructed submarine pressure hulls that remained leakproof even under the great pressures of depth. They joined together the massive armor plates of heavy tanks, and they welded airframe joints, displacing hundreds of pounds of weight in traditional bolts and rivets. Thus,weldingclearly increased the armor strength of tanks and the bomb-carrying capacities of airplanes.
“Rosie the Riveter,” the popular female poster character who saluted America’s civilian manufacturing effort, may have begun wartime manufacturing, but it was “Rosie the Welder” who finished the job.
World War II proved a turning point in metal fabrication standards. Until the development of shieldedwelding techniques, most metal joints capable of withstanding internal or external pressures, or any significant degree of mechanical stress, employed flanged edges, gaskets, sealant compounds, or bolts, or a combination of all four. Nevertheless, in just five years these various metal joint technologies were significantly displaced by welding as the preferred method of joining metals.
Coming of age
Although welding emerged from World War II as an accepted and proven manufacturing technology, it had been applied almost exclusively to just two types of metals: iron and its steel derivatives. But wartime metallurgical advances now made a number of different metals available for structural and nonstructural use.
Lightweight aluminum, magnesium, and titanium, and their many alloys, were ideal where weight reduction was critical, notably in aviation and aerospace. But welding the chemically active, lightweight metals was difficult, because they formed tenacious, inert oxides at elevated temperatures that interfered with bonding. Even gas shielding did not eliminate the troublesome oxides and traces of residual oxygen. Only when metallurgists developed special, chemically active fluxes that readily combined with oxygen to form noninterfering slags could lightweight metals and their alloys be welded together for aviation and aerospace applications.
Metals such as zirconium and molybdenum, which melt at extremely high temperatures, posed a different challenge. Since the intense heat necessary for welding consumed conventional electrodewelding rods before the metals could fuse, metallurgists developed special tungsten-alloy electrodes that fused at the melting temperatures of the refractory metals.
Designing increasingly complex products sometimes demanded welding two different metals, despite the fact that dissimilar metals rarely formed a strong, common bond. Welding engineers surmounted that obstacle in 1955 by turning to nickel, a metal that is weldable not only to itself but also to steel, stainless steel, and a host of copper and other alloys.
By the mid-1950s, when science began exploring a world of extreme internal and external pressures in space and in the depths of the oceans, metallurgists had established welding techniques for hundreds of standard alloys and even for many “exotic” alloys. The fabrication of manned space capsules and pressurized rocket fuel tanks demanded strong, airtight welding of lightweight metal alloys. Perfect welds were also indispensable in the pressure hulls of deep-diving nuclear submarines and deep-sea submersibles, which must withstand pressures of thousands of pounds per square inch.
Welding also impacted everyday life. In the late 1950s, the automotive industry demonstrated the first welded “unibody” construction. Unibody vehicles, in which frames were welded onto chassis in integral units, eliminated the traditional use of belts and rivets and provided far greater strength and passenger safety.
If welding could build better automobiles, then it could also build better refrigerators, eyeglass frames, lawn furniture, washing machines, and thousands of other products. The elimination of many belts, rivets, and screws would cut both assembly time and costs. Products with welded joints would cost consumers less, yet offer greater operational longevity and reduced maintenance and repair costs. However, before welding could truly revolutionize mass-production manufacturing of consumer products, it needed one more advancement: automation.
Shielded electrical arc and resistance welding methods have proved wonderfully adaptable to automation. In the automotive industry, analog computers controlled the first automatedweldingprocesses with limited success. But by 1970, digital computers had made fully automated, assembly-line welding a reality. Welding began uniting chassis and frames, as well as the components of door, trunk lid, hood, cooling, air-conditioning, heating, seat, and dashboard units. In full-sized passenger cars, welding displaced nearly 100 pounds of bolts, screws, and rivets, while cutting assembly time by about 20 percent.
Welding also built more durable railroad locomotives and rolling stock. Perhaps more importantly, it eliminated a tremendous cause of wear. The familiar “clickety-clack” of wheels passing over bolted rail joints every 39 feet was a traditional part of rail travel. Unfortunately, each clickety-clack signaled shock and vibration that took a severe toll in wear on both rails and trains. Since the 1970s, however, individual rail sections have been replaced by quarter-mile-long rails with smooth, welded joints. By eliminating the clickety-clack, welded rail has saved railroads millions of dollars in annual maintenance and replacement costs.
Welding has even created at least one industry on its own. In the 1950s, oil and coal supplied nearly all the nation’s energy, while natural gas, an abundant resource often associated with petroleum deposits, had virtually no use at all. Because natural gas could not be economically transported to markets, it was simply “flared off,” or burned, at the petroleum wellheads. But thanks to welding, the nation was able to utilize, rather than waste, a huge, nonrenewable energy resource. By the mid-1950s, steel pipelines linked together by millions of welded joints, each impervious to pressurized gas, were delivering natural gas nationwide for residential and industrial use. Today, thanks to 300,000 miles of welded pipeline, natural gas supplies 21 percent of the country’s energy needs.
Advanced welding technologies
In the 1970s, when electricity and oxyacetylene had been welding’s exclusive heat sources for 80 years, researchers began experimenting with new heat sources, including high- and low-power lasers. When focused on metal surfaces, high-power laser beams almost instantly vaporize metal. When properly controlled, however, laser beams can create narrow, deep “penetration burns” with depth-to-width ratios exceeding 10:1. Instead of vaporizing the metal, penetration burns heat a precise zone of metal in an instant. This rapid operation and deep, narrow penetration minimize the volume of metal that is actually heated. In comparison with the much larger volume of metal heated by the original methods, the laser’s low-energy input per weld facilitates rapid cooling, and laser welds can actually be stronger than the metal itself. The deep, narrow penetration of laser weldingis ideal for assembling military armored vehicles, heavy construction equipment, and similar equipment built of heavy metal components.
Laser welding may be applied to both many different types of metals and many different metal thicknesses. Low-power lasers capable of welding metals no thicker than a human hair have greatly impacted the mass production of miniaturized, solid-state electronic components. Being nearly perfect welds, laser welds have extremely low electrical resistance, a big advantage for such microcurrent circuitry as that used in computer boards.
Plasma arc welding is a relatively new method in which shielding gas is forced directly through an electrical arc producing a plasma–ionized gas–which modifies the shape of the arc to increase welding travel speed and lowers the energy input to the metal. Hence the weld cools faster and provides a stronger bond.
The assembly of planned orbital, lunar, and planetary manned space stations relies on electron-beam welding. Like lasers, high-voltage electron beams, accelerated and focused by electromagnetic fields, can instantly vaporize metals or create deep, narrow penetration burns. Electron-beam welding is most effective in vacuums that permit unimpeded electron travel. In orbital tests, American astronauts have already demonstrated the suitability of electron beams forwelding in space.
A science in its infancy
Throughout history, we have found only three ways to join metals: with adhesives, with mechanical devices such as bolts and rivets, and by welding. In strength and durability, welding clearly represents the apex of the science of joining metals.
In their book Welding and Other Joining Processes, authors Roy Lindberg and Norman Braton write, “The art of joining materials has been in existence for thousands of years, yet the science remains in its infancy.” Despite its enormous impact on science, industry, and manufacturing, modern welding has been around for only about 70 years. In testimony to its infancy, welding itself is changing almost as fast as it is changing our world.
In the 1930s, welders used oxyacetylene and electrical methods almost equally. Since then, the use of oxyacetylene has declined steadily. Today oxyacetylene welding systems are valued primarily for their portability and are used more for cutting metal than for welding it. Despite its decline, oxyacetylene welding has recently impacted a most unlikely area: the field of art. Welding has joined casting as a favorite technique among metalwork artists. From tabletop displays to the bigger-than-life “patchwork” creations found in corporate headquarter lobbies and urban malls, gaswelding is becoming an increasingly popular tool in the hands of creative artists.
The future of industrial welding rests, however, on advanced arc and resistance methods, and most importantly on the continued development of plasma, laser, and electron-beam methods. At least for the short term, welding quality is expected to continue to improve, not with new weldingmethods but through advances in the associated technologies of computer automation and quality control.
Automated welding robots are already being replaced by “smart” robots equipped with optical and thermal sensors that scan welds for quality and instantly adjust and correct the weld if necessary. Complementing smart robotic welders are a host of nondestructive testing methods. These include computerized radiography methods that employ X rays, gamma rays, and neutron beams; conductivity methods; magnetic imaging; and ultrasonic techniques, all of which, through digital data processing, will soon be able to confirm the integrity and quality of a weld in real time.
Over the long term, the future of welding may ultimately lie in natural atomic attraction. The development of cost-effective technologies to clean and smooth metal surfaces perfectly, then to mate them together in intimate contact to create natural welds, may one day completely eliminate the need for outside sources of energy in the welding process.
Whatever awaits us in the twenty-first century, rest assured that welding, in one form or another, will hold more of our world together than ever before.