New developments in advanced welding
Current laser beam welding applications
The most recent attempt to survey comprehensively laser applications was performed by the Electric Power Research Institute a number of years ago and is severely out of date (Brushwood, 1984). More recent surveys have relied on discussions with sales representatives from different laser manufacturers and are often incomplete because of considerations of company confidentiality.
The widest acceptance of laser welding applications is in the automotive industries. Laser welding is the method of choice for welding of components of gears; perhaps the largest concentration of high power lasers in the world is near Kokomo, Indiana in the United States, where there are three large automotive transmission plants each with multiple laser welding systems. It has been said that the advent of low heat input welding processes resulted in dramatic changes in the design of gears. Whereas previously a large gear may have had to be machined from a large block of steel, the laser welding process allowed the gear to be made, for example, from a stamped or formed flange welded to a machined hub.
Another area in which lasers have found applications in the automotive industry is that of tailor welded blanks. After welding, the blanks are formed into auto-body parts such as door panels, and holes are cut in the formed parts as needed. This advanced technique enables designers to tailor or optimize the materials in their parts, while keeping the overall weight of a part to a minimum. If a particular location in the part requires a particular type of steel for reasons of strength or corrosion resistance, then a piece of that type of steel is welded to the part while it is still a flat sheet, before being formed into the various complex shapes. The result of instituting a design-for - manufacturing program is reduction in the number of parts and assembly time required per vehicle. Experience has shown that over 20 parts can be eliminated by the use of tailor welded blanks for door parts and frames in a single vehicle. The laser welding process produces parts of superior quality and offers significant advantages over the traditional spot welding operation. The continuous butt weld of the tailor welded blank replaces the discontinuous joint that would result if the parts were joined after forming. Greater dimensional control is achieved. The continuous laser weld eliminates the need for sealant, in addition to achieving greater strength while reducing weight. Automobile manufacturers have adopted tailor welded blanks not only as a cost saving, but also to reduce weight and hence increase fuel efficiency to satisfy legislative requirements.
Other manufacturers of products such as electronic cabinetry and household appliances are also investigating the production of goods from tailor welded blanks. The ability to manufacture tailored blanks can best be utilized by an evolution in design philosophy. In a mass production scenario, engineers should learn to design parts based on raw materials that are optimized for a particular application. Successful implementation of tailor welded blanks requires the development of high-speed, high-quality laser welding processes, producing a minimum of overbead that influences the subsequent punch - and-die forming operations. The development of the laser welded tailored blanks was accompanied by developments in high accuracy shearing processes, producing a smooth edge without any further processing prior to welding.
After a slow 15 year incubation period, laser welding of automobile bodies has been successfully implemented. One of the first applications used the hybrid or laser-assisted arc process with a gas metal arc weld used in conjunction with the laser weld. The filler metal added using the arc process relaxed the tolerances required to make a good weld. Within two or three years of this first installation, one manufacturer was said to be using 240 lasers for body welding.
Another high profile laser installation occurred in the 1980s when Kawasaki Steel Co. implemented laser welding for coil joining (Kawai et al., 1984). By joining the coils of steel produced in the steel mill together to produce one long strip, subsequent processing through the cleaning and chemical cleaning process was simplified. The manufacturing process was essentially turned from a batch process into a continuous process, eliminating the need to feed new strips of steel continually through guide coils. Five kilowatt lasers were used, but auxiliary equipment included a high accuracy shearing mechanism and an abrasive wheel grinder. The grinder allowed ease of transfer of the material over subsequent rolls. An auxiliary wire feed lowered the carbon content of the weld metal when the system was used for joining high carbon steel.
Another instance of using laser joining to turn a batch process into a continuous process was in the application of 45 kW continuous lasers at the Ohita plant of the Nippon Steel Corporation (Anon, 1995). The laser welded rough rolled hot slabs to each other, allowing continuous roll finishing, thereby achieving 20 % higher productivity. It was anticipated that the new process would produce thinner gauge hot steel plate and formability would be improved. In addition, high power lasers have been successfully implemented into pipe mills, welding steel pipe with wall thickness up to 16 mm (Ono et al., 1996).
Repetitively pulsed Nd:YAG lasers, with a close spacing between repetitive pulses, provide a hermetic seal. This ability has been used in sealing of batteries, pacemakers and relays since the early days of the laser (Bolin, 1976, 1983; Fuerschbach and Hinkley, 1997). Applications in the electronics industry include components of electron guns and grids for televisions (Notenboom, 1984), thermocouples, ink cartridges for fountain pens, relays, telephone switching gear, microwave components, lamp electrodes, gyroscope bearings and valve components (Bolin, 1983). A particularly challenging mass production job is the sealing of glass-to-metal feedthroughs into electronic components; the possibility of cracking the glass due to excessive heat input is avoided because of the fine control in the laser welding process.
Laser cladding is a welding operation in which material, usually in powder form, is added to the molten pool and solidifies to produce a surface that has beneficial wear or anti-corrosion properties on top of more easily machinable or less expensive substrate. For example, this process has been applied to automotive valve seats (Matsuyama et al., 2000). When applied to large areas, the process is alternatively called laser hardfacing. The process can also be used to build up worn components by adding the same material as the substrate; in this case, the terminology ‘laser weld repair’ is appropriate. One of the main areas to which this process has been applied, since the mid - 1980s, is in the repair of the tips of the blades of gas turbine engines (Hayes, 1997; Krause, 2001). The added material solidifies epitaxial on the underlaying material, allowing the properties of directionally solidified blades to be maintained in the repair process. The process can be used to reverse machine (that is, to add material instead of machining it away) parts that have been subjected to machining damage, inadvertent damage, or high wear. General Electric Aircraft Engines has applied this process to rebuild turbine spools and disks (Mehta et al., 1984) resulting in considerable cost savings.
Because the laser process involves a low heat input, solidification rates are rapid, leaving little time for segregation of alloy constituents. Under certain processing conditions, a single phase microstructure can be produced via the laser weld repair process, with beneficial surface properties (Hyatt and Magee, 1994). Laser weld repair of nickel aluminium bronze was found to be considerably harder (260 to 377 HV, depending on heat input, versus 265 HV) than similar repairs done by pulsed gas metal arc welding, and have an approximate 30 % improvement in resistance to cavitation erosion (Hyatt and Majumdar, 2000). The laser weld repaired material had a factor of 5 improvement in resistance to cavitation erosion compared to the cast base metal and about a factor of 20 decrease in corrosion current (Hyatt et al., 1998). Benefits of the rapid solidification inherent in the laser process have been observed in other alloy systems.
The laser heat input inherent in the laser weld repair process allows one layer to be deposited on another layer, resulting in the building up of threedimensional structures (Milewski et al., 1998). This allows direct computer aided design (CAD)-to-part manufacture of metallic components, similar to the stereolithography processes that build polymeric components. The process seems to have been developed simultaneously at multiple locations, some targeting general industrial product development, some locations targeting fabricating of complex parts for defense applications. The technology spun off into development of tool and dies, which are made of steels that are challenging to machine. The laser process allowed fabrication of molds with imbedded cooling channels more optimally located than could be achieved by conventional machining processes. The imbedded cooling channels allowed speeding up the injection molding and extraction process, resulting in significant savings to manufacturers. The laser deposition process has allowed large titanium alloy components to be built up for airframes; the traditional manufacturing process required extensive machining of the components from large blocks of the alloy. The laser process achieved a greater utilization of the relatively expensive titanium.
