New developments in advanced welding
Advantages of laser beam welding
Metzbower, in 1981, presented a review on laser technology for thick-section welding, as the technology had been developed up to that time. His paper showed data on welding of the same type of steel with four different welding processes. A butt joint between two pieces of 2.7mm thick HY-130 steel was welded with an 11kW laser beam at speeds between 12.7 and 16.9mm/s. A similar weld was produced with a 40kV electron beam at 21.2mm/s, but this weld showed undercutting and required a defocused cosmetic pass on both sides of the plate of steel. The heat input to the part for the laser weld was 0.7kJ/mm, whereas for the electron beam weld it was somewhat less, about 0.44kJ/mm.
The same weld was done by shielded metal arc (stick) welding (SMAW). The joint preparation of the material was a 60 degree groove and a 120 °C preheat was applied. The welding speed was 3mm/s with a 125A arc at 25 to 30 V. This corresponds to a heat input of 1.1 to 1.18kJ/mm per pass, considerably higher than that of the two deep penetration welding processes. In the SMAW process, seven passes were required to fill the joint completely. The net welding speed used in the laser beam process was 30 times faster than that of the shielded metal arc weld.
Gas metal arc welding was also used to make the weld on 12.7 mm thick HY 130 steel, also with a 60 degree groove weld preparation and 120 °C preheat. The welding speed was about 6mm/s with a 300A, 24 V arc. Five passes were required, with each pass having a heat input of 1.1kJ/mm. The welding speed in the laser process was considerably faster.
The above results typify the advantages of laser beam welding. Thick material can be welded at high speed in a single pass. The narrow width of the deep penetration weld means that the heat input to the weldment is considerably smaller than in the arc welding processes. The laser beam weld had similar characteristics to the electron beam weld, but the electron beam welders require a vacuum system since the beam cannot be transmitted through the air. The lower heat input of the high energy beam welding processes significantly reduces the welding induced distortion and may have beneficial metallurgical consequences.
Repetitively pulsed welders have had considerable success in the medical electronics industry and the electronics industry. Nd-YAG lasers that produce a well-defined energy per pulse at a programmed pulse repetition rate are ideal for welding of thin metal parts; the laser energy can be chosen to be just sufficient to join the two pieces of metal together without overheating or damaging internal components. Many of these lasers monitor the laser pulse energy and have internal feedback circuits to keep the laser energy at a predetermined level.
The output of the Nd:YAG laser is often delivered by fiber optics to the workstation. Since the fibers are essentially loss-less, this means that the laser does not have to be physically close to the welding operation. In many cases, electronics components are assembled in clean rooms or dry rooms. These rooms are expensive to build and maintain; consequently, it is advantageous to have the laser physically outside the room, with the output delivered inside the room by fiber optics.
4.3 Suitability of laser beam welding
Laser welding is an accurate and fast process. The reason that it is relatively fast is that the fusion zone is relatively small. A deep penetration weld heats only the seam and a small area around it, rather than the large area that is welded when using a deep V-groove preparation and arc welding. Sheet metal can be welded rapidly by focusing the beam to a spot size of the order of the thickness of the metal. This accuracy has a disadvantage, however. Weld joints have to be precisely prepared as the process will only tolerate a very small gap between the two parts to be welded. For thick materials, a maximum gap of 3 % of material thickness is quoted (O’Brian, 1991). If a wide gap is used, a weld with an underfill or an undercut results, or, in some cases, part of the beam is transmitted through the gap and is not available for the welding process. Consequently, the laser beam process is used when the expense of a machining process or a precision weld joint preparation process can be tolerated.
For example, coil joining in the steel industry and tailor blank welding in the automotive industry were only successful after a careful examination of the shearing process and optimization of the shearing to produce edges suitable for laser beam welding. Here, the companies decided the benefits of laser welding were so great that the effort involved in developing and implementing methods to prepare high quality edges was justified. However, the need for precision edge preparation has kept laser applications out of many fabricator shop environments.
In many cases, the selection of laser welding is justified on both financial and technical grounds. Laser welding equipment is considerably more expensive than is arc welding equipment. Moreover, the laser process is almost always carried out with automated equipment, which adds to the cost. For safety reasons, laser welding is usually carried out inside a safety enclosure, which is an impediment to loading and unloading weldments rapidly. Businesses will generally only invest in laser welding equipment if they foresee multiyear production runs or sequences of production runs lasting at least as long as the equipment is being depreciated.
The welding process will always be carried out using a process that involves less expensive capital and operating costs unless there is some special reason to use the laser process. It is the low distortion nature of the welding process that has led lasers to be the preferred method for laser welding of gears for automotive transmissions. It is for metallurgical reasons that lasers are the preferred method for weld repair of the tips of blades from gas turbine engines. The speed of the process is the reason it has been adopted for pipeline welding on offshore pipe-laying platforms. The platforms are reputed to cost a million dollars a day to operate, so faster welding saves money.
