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.
EBW – Deeper Weld Penetration and Contamination Free
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
Figure 2. Electron Beam Welding titanium, niobium, refractory metals, and nickel-based super-alloys.
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 millimeters.
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” 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 workpiece, 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.
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.
According to Geoffrey Young, general manager of Massachusetts-based Cambridge Vacuum Engineering, “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.”