By John Lucas
East Granby, Conn. – Proponents of laser beam welding (LBW) and electron beam welding (EBW) each pronounce the singular praises of their favored technology, but often the best solution for a customer is to use both technologies together. Both processes are well suited to joining of components with complex geometries and capable of meeting the most stringent industry demands for metallurgical characteristics of the final assembly.
Having both laser and electron beam technologies in a single facility can streamline the manufacturing process when a component's design incorporates multiple weld joints separately tailored for one process or the other. Examples include sensors, medical devices, and products that require an inert gas or vacuum to be sealed within the finished part.
Laser processing is required either when the size of the final assembly is too large for an EB welding chamber, some component in an assembly is incompatible with vacuum processing (such as a liquid or gas), or when the weld is inaccessible to an electron beam source. Electron beam will be the primary choice when the completed assembly must be sealed with internal components under vacuum, when weld penetrations exceed 0.5 inch, when the material is challenging to initiate laser coupling, or when the weld must not be exposed to atmospheric conditions until it has cooled to an acceptable temperature. Examples are aerospace welding of titanium and its alloys, and many refractory metals such as tungsten, niobium, rhenium, and tantalum.
Laser beam welding – Simpler tooling and shorter cycle times
Laser welding energy sources utilize either a continuous wave (CW) or pulsed output of photons. With CW systems, the laser beam is always on during the welding process. Pulsed systems are modulated to output a series of pulses with an off time between those pulses. With both methods, the laser beam is optically focused on the workpiece surface to be welded. These laser beams may be delivered directly to the part via classical hard-optics, or through a highly flexible fiber optic cable capable of delivering the laser energy to distant workstations.
It is the high energy density of the laser that allows the surface of the material to be brought to its liquidus temperature rapidly, allowing for a short beam interaction time compared to traditional welding methods such as GTAW (TIG welding) and similar processes. Energy is thus given less time to dissipate into the interior of the work-piece. This results in a narrow heat-affected zone and less fatigue debit to the component.
Beam energy output can be highly controlled and modulated to produce arbitrary pulse profiles. Weld seams may be produced by overlapping individual pulses, which reduces heat input by introducing a brief cooling cycle between pulses, an advantage for producing welds in heat sensitive materials.
Salay Stannard, materials engineer for Joining Technologies, an East Granby, CT-based innovator in laser cladding, electron beam and laser welding applications, notes that CW lasers can achieve penetrations up to and exceeding 0.5 inches, while pulsed lasers typically achieve only 0.030-0.045 inches. She says, "These results may vary between laser systems and are largely dependent on processing parameter choice and joint design." FIGURE 1 depicts the construction of a solid-state laser welding system.
|FIGURE 1. Solid state laser welding system.|
Stannard adds, "Since the heat source in this type of welding process is the energy of light, the weld material's reflectivity should be considered. For example, gold, silver, copper and aluminum require more intense energy input. Once melted, the reflectivity is reduced and the thermal conductance of the process progresses to achieve penetration."
As noted, the laser's high power density results in small heat-affected zones and ensures that critical components are unharmed. This has particular advantage for surgical instruments, electronic components, sensor assemblies and many other precision devices. Unlike EBW, LBW does not generate any x-rays and is easily manipulated with automation and robotics. Generally, LBW has simpler tooling requirements as well, and there are no physical constraints of a vacuum chamber. Shorter cycle times translate to cost advantages without sacrificing quality. TABLE 1 lists the advantages of continuous wave and pulse LBW.
Table 1. Advantages of laser welding
Table 1. Advantages of laser welding
Electron beam welding – Higher purity and deeper weld penetration
Widely accepted across many industries, EBW permits the welding of refractory and dissimilar metals that are typically unsuited for other methods. As shown in FIGURE 2, the workpiece is bombarded with a focused stream of electrons travelling at extremely high speed. The kinetic energy of the electrons is converted to heat energy, which in turn is the driving force for fusion. Usually no added filler material is required or used, and post-weld distortion is minimal. Ultra-high energy density enables deep penetration and high aspect ratios, while a vacuum environment ensures an atmospheric gas contamination free weld that is critical for metals such as titanium, niobium, refractory metals, and nickel-based super-alloys.
FIGURE 2. Electron beam welding.
However, the main necessity for operating under vacuum is to control the electron beam precisely. Scattering occurs when electrons interact with air molecules; by lowering the ambient pressure, electrons can be more tightly controlled.
Modern vacuum chambers are equipped with state-of-the-art seals, vacuum sensors, and high performance pumping systems enabling rapid evacuation. These features make it possible to focus the electron beam to diameters of 0.3 to 0.8 mm.
By incorporating the latest in microprocessor Computer Numeric Control (CNC) and systems monitoring for superior part manipulation, parts of various size and mass can be joined without excessive melting of smaller components. The precise control of both the diameter of the electron beam and the travel speed allows materials from 0.001 inch to several inches thick to be fused together. These characteristics make EBW an extremely valuable technology.
The process puts a minimal amount of heat into the work-piece, which produces the smallest possible amount of distortion and allows finish machined components to be joined together without additional processing. TABLE 2 lists the main advantages of EB welding.
|Table 2. Advantages of EB welding|
According to John Rugh, marketing and general sales manager for Enfield CT-based PTR-Precision Technologies Inc., EBW is a process that will be in use for a long time. "Since most EB welding is performed inside a vacuum chamber, it is an excellent fit for joining advanced materials used in such industries as aerospace, power generation, medical and nuclear, which need to be produced in a vacuum environment to protect them from oxygen and nitrogen found in an open air environment."
He adds, "The cleanliness of the welding environment is one variable that you just don't have to worry about. In addition to providing the ideal welding environment, new EB welding controls allow for fast electromagnetic deflection of the beam, which allows the heat input of the weld and surrounding area to be customized for optimum material properties."
For example, this rapid deflection allows preheating, welding and post heating simultaneously just by rapidly moving the beam location, focus and power levels. This provides the ability to weld difficult or "impossible to weld" alloys.
An area where EBW is being increasingly utilized is the manufacture of turbochargers for diesel engines, which are growing in popularity due to their potential to greatly improve engine efficiency. Turbochargers are employed to pre-pressurize the air going into the diesel engine to higher than atmospheric levels, thus providing more oxygen for the combustion process.
Geoffrey Young, general manager of Massachusetts-based Cambridge Vacuum Engineering, commented, "We are seeing many modern passenger car and commercial vehicle engines that are being equipped with turbochargers. Manufacturers of these units had conventionally used inertia friction welding techniques to join the investment cast, Inconel® wheel to the carbon steel shaft. Although this joining method produced a joint of adequate strength, the post-weld machining, grinding and heat treatment operations were expensive and time consuming. An alternative welding process using EBW has been adopted by a number of leading turbocharger manufacturers."
"EBW parts require a minimum of post weld machining and heat treatment and, unlike other fusion welding processes, EBW requires no shielding gases." He adds, "The weld quality is exceptional, the process is extremely efficient (typically 95 percent), all the process parameters are carefully controlled and the process fully automated."
Using EBW in conjunction with laser welding – streamlining the process
According to John Rugh, LBW is commonly used for welding steel sheet metal components and machined components under 1/3 to 1/2 inch thick. Laser welding is also useful for joining parts that are not suitable for processing inside a vacuum chamber.
"Some parts and their associated welding fixtures may be too large to fit into the EB welding chambers available," said Rugh. "Aside from size, if the components being welded contain liquids that would interfere with vacuum pumping, laser welding would be a good choice." It takes minutes to evacuate an EB welding chamber and that time may not be worth it for a less sensitive weld.
If components are of high value, made of a material that would benefit from the vacuum environment such as titanium and nickel alloys, the welds are deeper than 1/3 to 1/2 inch, or if the laser beam has difficulty coupling with the material being welded such as aluminum alloys, EB welding is often the process of choice over laser welding.
Rugh gives the example of gas turbine components where EB welds are used for the deeper welds and welds requiring minimal distortion. The same assembly also had laser welds called out for sheet metal cover details.
While each technology has its benefits, in practical terms, many component designs incorporate both EB and laser welds. In these cases, performing both types of welding at the same facility definitely streamlines the manufacturing process.