Lasers – The Second High Power Density Welding Systems
By Robert Holland

The first high power density welding systems were electron beam. The generation of a high power electron beam in a vacuum environment, accelerating this stream of electrons via high voltage applied between the cathode and anode, and then electromagnetically focusing that total power to a very small spot on the piece to be welded. The power density (power per unit area) was so intense that it immediately vaporized the metal being welded, which then solidified behind the progression of the weld as the aligned weld joint of the part was being moved at a constant velocity under the focused beam. The welding parameters were power, speed, and focus which provided the weld penetration and weld properties desired. The characteristics of an electron beam weld are excellent because the welding takes place in a vacuum environment and oxidation of the weld area is not a problem since any significant oxygen is absent.

Other considerations which detract from electron beam welding are that the part or the electron beam gun has to be manipulated in a large vacuum chamber in order for the part to be welded. These large vacuum chambers require large mechanical and oil vapor diffusion pumps to draw down the vacuum to the high vacuum operating level required. Pump down times can be 20 minutes to a few hours for large systems. In the case of welding the large titanium wing box for the Grumman F15 Fighter the vacuum chamber was extremely large and the vacuum pumps were also large and numerous.

A second consideration is the fact that electron beam welders produce intense X-Rays. The high voltage electron beam welders (150,000 volts) with stationary electron beam guns produced the highest penetrating X-Rays and thus the vacuum chambers and guns had to be lined with 1/8” thick lead. The lower voltage (60,000 volts) movable gun systems had to use 1 ¼ inch thick steel in the construction of the vacuum chamber to attenuate the X-Rays. Both types of electron beam welders had to use thick leaded glass for the viewing ports.

The next iteration of electron beam welders where called non-vacuum welders. With this process the electron beam was generated in a small high vacuum chamber around the electron beam gun only and the electron beam was passed through a small pressure differential orifice into normal atmospheric pressure. The electron beam immediately collided with air molecules and dispersed rapidly. The weld joint needed to be extremely close to the exit orifice (1/8” to 3/8”) or the electron beam spot diameter was too large to do any effective work. Even at a close distance from the exit orifice the weld characteristics lost the deep depth to width ratios associated with high vacuum electron beam welding. The exit orifices were expensive and eroded fairly quickly. The non-vacuum electron beam welder was also a large X-Ray producer and any automation or production material handling equipment had to be baffled and placed in a lead room. Several electron beam in air systems were produced but this technology did not last long.

Electron Beam Welding then progressed into the production welding environment with the advent of “Soft-Vacuum” electron beam welding. With this technology the electron beam is generated in a small high vacuum chamber, directed through a small pressure differential orifice and into a partial vacuum generated by only mechanical displacement vacuum pumps. The electron beam still maintains its deep penetrating narrow weld characteristics, while a smaller vacuum chamber closely sized to the parts to be welded, can be evacuated to the operating vacuum level in a matter of seconds rather than minutes. Many automotive and other parts i.e. flywheels, transmission planet carriers, catalytic converters, hydraulic pistons, hacksaw blades, band saw blades, commutator blanks, torque converters, and spark plugs etc. have been produced on a production basis with “Soft Vacuum” electron beam welding equipment. With some of these systems a unique dial feed table with a sliding vacuum seal was incorporated which allowed the part to be pre-evacuated in the station before the weld station. This set up, totally eliminated the vacuum pump down time from the machine cycle time. Production rates up to 3,000 parts per hour have been achieved using this welding technique.

With the advent of higher power lasers many of the applications that were accomplished by electron beam are now being processed by laser systems. However, many of the close tolerance high value aircraft engine and aerospace components that require deep penetrating non contaminated welds are still being processed by electron beam. The laser is also a much more versatile tool. The various wave lengths available with lasers can offer some very selective results depending on the interaction of that wavelength with the material being processed. For instants, a Nd:YAG laser with a wavelength 1,060 nm can weld a clear piece of plastic to an opaque piece of plastic by passing directly through the clear piece without affecting it and then impinging on the opaque piece heating and melting its surface and effectively producing a leak-tight weld at the interface of the two materials.

Pulsed Nd:YAG lasers are used for intricate highly controlled low penetration welding. This would be the welding of heart pacemakers, heart valves, medical instruments, orthopedic implants, small hermetically sealed electronic enclosures, spot welding razor blades, jewelry welding applications etc. These lasers produce high energy pulses of short duration so a seam weld needs to be produced by overlapping these pulses approximately 75%.

High Power CO2 Lasers have been used for numerous welding applications, many of which have replaced electron beam welding for the same applications. However, the future for narrow deep penetrating electron beam type welds will fall to the new High Power Fiber Lasers. These lasers are extremely efficient, have a long expected solid state pumping diode lifetimes of over 100,000 hours, are virtually maintenance free, and can deliver this laser power through a small flexible fiber optic cable. Manufacturers can use robots to manipulate the fiber optic delivery system to weld a wide variety of large production parts or incorporate it into high speed automated production lines.

The laser beam quality of the fiber laser (ability to focus to the smallest spot diameter) is superior to other high power lasers and provides faster welding speeds for a given power level or increased production rates. Also because of the better beam quality the stand-off distances of the focusing optics to the work piece can be extended to two to four feet for some applications. Great strides in the number of new laser welding applications can be expected in the near future. The high power fiber laser can be an expensive commodity but should be able to greatly increase productivity and equipment up time and easily justify their cost. Remember; automation, robots, and reliable lasers level the playing field with low labor costs when competing on the world stage.