Welding 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.[Page Break]
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.
A continuous or pulsed laser beam may be used, depending upon the application: pulses measured in milliseconds would be used to weld thin materials such as razor blades, while continuous laser beams would be required for deep welds.
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 to be the first such process to be available commercially. This system can weld at three to ten times the speed of conventional processes, with 80 to 90 per cent less heat input, enabling greater performance gains to be achieved than have been possible in a generation of welding process developments (Fig. 2). The technology combines the deep weld penetration and low heat input of laser welding with the superior weld properties and gap tolerance of GMAW.
Laser welding produces narrow, deep welds at very high speeds. This puts less heat into the joint and results in reduced weld shrinkage and distortion; in turn, this avoids the problems and costs associated with unpredictable post-welding fit-up and rework/repairs. Laser-only welding, however, is limited in its ability to produce acceptable welds in joints with wide gaps. Hybrio solves this by using GMAW in tandem with the laser, adding a relatively modest amount of filler metal. The wider weld beads can bridge gaps that are four times wider than those that can be handled by conventional laser-only processes. Incorporating the GMAW process also allows filler metals to be added so as to adjust the weld's metallurgical properties and create beads and fillets. In addition, the slower cooling rate associated with GMAW reduces hardness in the welds. These benefits are said to be especially advantageous when joining high-performance carbon steels and stainless steels.
An important element of the Hybrio process is ESAB's patented adaptive control system that permits monitoring of the weld joint in real time so that the process parameters can be modified to accommodate joint gaps and component mismatches. Described as an advanced, next-generation intelligent control, it is said to broaden the welding performance envelope by a factor of five compared with HLAW processes utilising conventional controls.[Page Break]
Another company that has recently introduced a HLAW process is GE, which claims that its system can revolutionise the way products are made. The company suggests three primary ways in which HLAW can improve manufacturing:
- Production speed: Welding is a fundamental aspect of making many products that contain metal parts. It is also one of the most time-consuming elements of the manufacturing process. HLAW welds faster, meaning that many industrial products can be produced at greater speeds. To achieve this, GE's HLAW system uses high-powered fibre lasers that weld steels up to 2 cm thick in just one pass. To put that in perspective, using HLAW instead of conventional methods to weld the aircraft carrier USS Saratoga could have saved nearly 800 tons of weld metal and reduced the welding time by 80 per cent.
- Reduced shipping costs: Traditionally, welding takes place in a manufacturing environment far from where the finished product is required, so time-consuming and costly transportation is required. However, because GE's HLAW system is portable, large assemblies can be fabricated at the final location.
- Higher efficiency: Because HLAW can weld in just one pass, the new process is far more efficient than its predecessors. This efficiency improvement is applicable in most industrial sectors, including the oil and gas industry, power generation, aviation and rail.
In common with ESAB's Hybrio system, GE's HLAW process uses a combination of laser welding and arc welding. With high-power fibre lasers, steels thicker than 12mm can be welded in a single pass at speeds greater than 1 m/minute. The result is a weld of higher quality than can be achieved with traditional multi-pass welding methods (Fig. 3). Over the years, GE says it has pioneered the use of lasers in manufacturing applications ranging from laser hole drilling in aircraft turbine blades to the first use of lasers for surface treatment of fan blades for improved durability. Lasers are also used to weld filaments for lighting products, lamination spacers for generators, and components for X-ray tubes.
Able to specify processes such as hybrid-laser arc welding and low-stress, no-distortion welding, design engineers can take a fresh look at fabrications that might otherwise have to be assembled in different ways at greater cost or in such a way that performance is compromised. Production engineers can also use these innovative processes to remove cost from existing fabrications. While hybrid-laser arc welding and low-stress, no-distortion welding will not be suitable for every welding application, they represent significant advances in welding technology and warrant close investigation for those projects which could benefit from them.