Necessity, the Mother of Evolution & Revolution
Living as we are in the age of rapid technological advance, things are changing at breakneck speeds. And the transformation affects all walks of life. Since ages, change has been the only imperishable entity. Of late however, it has gathered unprecedented momentum.
Welding is among the most popular joining processes, highly valued across the rank and file of the metal industry as it connects materials at the molecular level with joint strength equivalent to that of the constituent materials.
As for arc welding, the flexibility, cost-effectiveness, and broad applicability of the process make it an imperative fusion welding process for the shipbuilding, automobile, power plant, and building-construction industries.
Properties of weld joints, and not of the base materials, command greater influence over the structure’s performance. Metal industries are demanding superior efficiency, productivity, and quality. This has spurred metallurgists to formulate novel materials.
Welding processes need to evolve to join new materials such as composites and steels that retain high tensile strength at extreme temperatures. In some cases, fundamental changes may be essential.
More than that, welding has to survive the competition from bolts, screws, rivets, and emerging joining processes such as adhesive bonding, polymer bonding, and abrasion. This requires welds to be cheaper, faster, and better than these other contestants.
Such a paradigm shift will come only with a radical change in approach. Many welders still look upon welding more as an art than a science. Codification of welding knowledge is essential to change this as is the development of data systems that permit efficient use of such knowledge.
Customer requirements, expertise levels of welders, technical sophistication of welding equipment, and the type of to-be-welded materials set the broad framework for welding processes and their evolution.
‘Smart’ Welding Materials
History is replete with instances where advance in material technology has spurred social and economic progress. Isn’t this how we moved from the stone age to the bronze age and thence to the iron age? And breakthroughs in semiconductor technology landed us in the information technology age.
Context is very important when labeling a material as conventional or advanced for the purpose of welding. For example, structural aluminum is conventional material in the aerospace industry but advanced in the automotive sector.
Properties of conventional materials are well known. You can improve the quality of welding these materials while slashing costs through superior process control and automation.
Often, advanced materials come with exorbitant price tags and highly specific properties. They are therefore used only at absolutely-necessary locations. This amplifies the number of joints and necessitates advanced joining processes including those for bonding dissimilar materials.
And because the properties of joints influence the performance of the structure more than the properties of the main materials, the application of novel materials will be restricted by how easily and effectively we can join them.
Structures are becoming more three-dimensional even as to-be-welded parts are getting smaller. Although steel will continue to be the primary structural material, industries will demand new materials with:
- adaptability to welding with higher heat input, greater speed, and multiple electrodes – this hikes efficiency
- superior tensile strength and toughness
- ability to perform efficiently even at extreme temperatures
- low pre-heat temperature requirement
- negligible or zero environment polluting elements such as lead
- minimum weight / density
- resistance to fire, weather, corrosion, wear, acids, and destructive gases
- low energy needed for fabrication
- negligible or zero impurities such as hydrogen and nitrogen
Metallurgical understanding will be more important than before. For example, hydroelectric power plants use pipes of 950 N/mm2 grade steel. Welding high strength steels (HSS) is tough because stress concentration and residual tensile stresses lower the joint’s fatigue strength.
Using weld metal with 150-200 ppm oxygen and Si-Mn-2.5-3.5% Ni-Cr-Mo composition gives weld joints that do not take away toughness from the 950 N/mm2 grade steel even at low temperatures.
Developments in the form or structure of materials include:
- Heavy Plates that provide better transportation efficiency for container ships
- Thin Sheets minimize automobile weight without compromising on performance or safety
- Steel Pipes serve numerous industries – petrochemical, shipbuilding, automobile, food processing, desalination plants and the like
Making heavy plates, pipes, and thin sheets with new materials not only necessitates transformations in the welding processes used to join them but also in the procedures used for their manufacture.
An example is the Thermo Mechanical Control Process (TMCP), used since the 1980s to make heavy steel plates of 490 MPa tensile strength, high weldability, and a carbon equivalent similar to that of mild steel.
Please note, weld joints lose toughness when the carbon equivalent of the welded materials rises. And hence the emphasis on low carbon equivalent as that of mild steel.
Arc Welding Developments
In arc welding, it is the arc phenomenon that controls the rates of material transfer. Greater controls over parameters that govern the arc improve the efficiency and productivity of the process.
Technological progress in arc welding has focused on:
- improving productivity
- stabilizing weld quality
- saving labor
Targeted automation can provide all three. In order to be successful, automation will need to be simple and inexpensive vis-à-vis the typical complex and extravagant one.
Automation has complemented cutting-edge power sources in their quest for stabilizing and increasing metal deposition rate and penetration depth by employing refined waveforms and current.
Presently, automation is the most prominent growth area in all welding processes because it boosts efficiency and productivity by hiking the deposition factor, operator factor, speed, economy, control, quality, consistency and appearance of welds.
While minimizing the training and operator skill requirements, automation improves safety levels by saving human welders from exposure to the inevitable fumes and radiations. It also spares them from the drudgery of repetitive tasks that normally cut down consistency levels.
Arc welding for shipbuilding has particularly witnessed the development of:
- sensors for weld-line tracking
- adaptive control to respond to changes in work contours, groove shape, and welding position
Currently, the capabilities of arc welding to weld high-strength low-alloy (HSLA) steels without preheating are severely limited. This may change in the near future.
Gas Metal Arc Welding (GMAW) also called Metal Inert Gas (MIG) Welding is inherently flexible. It uses modulated electric current that enables adaption with varying welding positions, materials, and components. Plus, it is used in automatic, semi-mechanized, or fully-mechanized modes.
Better control over energy transfer ensures closer direction of metal transfer. This assures minimum spatter essential for welding surface-coated or highest-strength steels that tolerate only low heat input. Short-arc welding processes direct metal transfer by:
- withdrawing wire during or just after metal transfer
- lowering current in the short circuit phase
More applications will use GMAW because as a continuous-wire process, it can replace Shielded Metal Arc Welding (SMAW), Brazing, Gas Welding, and Resistance Welding. Expected developments in GMAW include improvements in:
- deposition control via thermal management and out-of-position welding
- bead contour control
- productivity by minimizing defects and maximizing deposition rates
Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding delivers elite quality welds, is compatible with automation, and can weld the emerging thin specialty metals. These factors will propel its growth.
Researchers will also extend the use of GTAW for on-site welding of reactive metals such as titanium. But GTAW is slow as quality comes at the cost of speed, something that will limit its development.
Flux Cored Arc Welding (FCAW) is regularly used in the construction of ships and bridges for all-position welding and fillet welding of painted steel plates. The process however suffers from high filler material costs and low filler utilization. Research may improve these areas.
Submerged Arc Welding (SAW) is the most efficient fusion welding process for structural and plate work if you can position the workpieces correctly and guide the welding torch appropriately. Construction of ships, pressure vessels, and bridges use SAW.
However, the process is not very useful for out-of-position welds when workpieces are moved frequently or when multiple welds are necessary. This is because the arc zone and weld remain submerged under a layer of flux. Research may find a way around this inherent limitation.
Hybrid Welding is a combination of arc and laser welding. The blend offers the merits of higher welding speed, superior energy density, low thermal load, excellent gap-bridging, deep penetration, and high tensile strength.
It enables welding thicker workpieces in a single pass, addition of filler material, and changing the metal microstructure. Used mostly for steel, possible developments in hybrid welding include:
- use of stronger power sources and gas-assists for deeper welding
- multiple beam welding
- combination of other welding processes with laser welding
Advances in Other Welding Processes
Electron Beam Welding and Laser Beam Welding are compatible with automation and will therefore witness faster growth. High power density of electron beam welding lowers heat input even in 250 mm-plus thick welds.
You can split the electron beam and weld simultaneously at separate locations on the same workpiece. And with large vacuum chambers of 630 m3 available, welding of large jobs is not an issue. But the process requires a vacuum chamber.
Laser beams have the same high energy density advantage. Plus, they can weld outside vacuum chambers. But lasers are limited to welding materials up to 25 mm thickness only. Developments may extend these capabilities. Lasers welding may find increasing application in automobiles.
Friction Stir Welding (FSW) joins materials at below fusion temperatures. This minimizes metallurgical changes in materials. Aerospace, rail vehicle building, and leak proof welding of hydraulic control parts use FSW.
Research has extended the applicability of FSW beyond aluminum and its alloys. This is likely to continue. The process however requires robust clamping jigs to provide great clamping forces.
Moving ahead, welding processes will witness greater integration with the manufacturing cycle, welding equipment and control systems, metallurgy, and product design.
Increasing automation and advances in filler material with better deposition rates will drive productivity. Simulation and non-destructive testing (NDT) methodologies will garner greater acceptability as researchers seek to verify the compatibility of novel materials with various welding process.
There will also be greater use of information technology systems that facilitate more accurate decision making by enabling quick and easy use of data and analysis compiled from welding experiments.
The optimum process will be one that delivers acceptable quality welds at low cost, high deposition rate, and high operator factor.
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