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Fasteners & Sealing


4 As a family of related yet diverse joining technologies, welding continues to evolve - though designers and production engineers are always demanding greater process improvements. Paul Stevens reports on some of the latest developments in welding that could offer major advances.


Welding developments continue to boost productivity and quality


W


elding is one of the most commonly used methods of joining metals, with designers and production engineers having a broad range of


processes from which to choose. Manufacturers of welding equipment and consumables continue to make incremental improvements to these processes, but some recent developments have the potential to deliver a step-change in quality and productivity, and even give design engineers the opportunity to create new fabrications that would not previously have been feasible. Gas metal arc welding (GMAW) was developed


during the Second World War for joining aluminium and other non-ferrous metals. Metal inert gas (Mig) welding is the most widely used of the GMAW processes, being suitable for everything from small fabrications through to large structures (Fig. 1). Its close relative, metal active gas (Mag) welding differs principally in the type of gas used; often the two processes are simply referred to as Mig/Mag welding. One of the most frequently cited drawbacks


with welding is distortion of the workpiece. However, this can be controlled to a great extent through a combination of joint design, the use of clamping and fixtures, optimised welding procedures and the application of state-of-the-art welding equipment. This last point is included


because, for example, some of the latest Mig welding machines have sophisticated control functions that can reduce significantly the heat input into the weld and, therefore, the distortion. Minimising distortion is one of the most


important factors in a successful and economical weld, especially when repairs are being performed and there is less scope for clamping/fixturing. Uncontrolled or excessive distortion increases the job cost due to the expense of rectification - or replacement in the event of the distorted part being beyond economical repair. It also has to be remembered that controlling distortion by means of clamping and fixturing can lead to the finished component having high residual stresses, which can cause problems later unless stress-relieving procedures are employed. Distortion in welded structures takes place


by three dimensional changes that occur during the process: longitudinal shrinkage parallel to the weld line; transverse shrinkage perpendicular to the weld line; and angular change though rotation around the weld line. One way in which stress and distortion can be minimised is by cooling the joint as it is welded.


Hybrid processes


For faster production rates, laser beam welding (LBW) offers advantages due to its high power density that results in a small heat-affected zone and high rates of heating and cooling. The spot size of the laser can vary between 0.2 and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximised when the focal point is slightly below the surface of the workpiece. Hybrid laser arc welding (HLAW) combines laser


welding and a GMAW process, typically Mig or Mag. This allows for greater positioning flexibility due to the gap-filling properties of the GMAW process, together with the high speed that is attributable to the laser. Weld quality tends to be high as well, since the potential for undercutting is reduced. ESAB’s Hybrio HLAW system was claimed


Fig. 1. Metal inert gas (Mig) welding is the most widely used of the arc welding processes, being suitable for everything from small fabrications through to large structures.


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to be the first such process to be available commercially. This system can weld at three to 10 times the speed of conventional processes, with 80 to 90 per cent less heat input, enabling greater


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