Repair processes such as welding keep expensive metal components in working order. But sometimes high-temperature welding operations can damage sensitive components in aircraft, tanks and other military vehicles. As a result, these costly parts—including turbine blades, vanes and impellers—are banished to the scrap heap and replaced with new, high-priced components.

Fortunately, a new technology called Laser Engineered Net Shaping (LENS) is rescuing valuable components from the scrap pile at several military facilities. LENS computerizes the process of depositing material on a worn out or cracked surface. Adding material one layer at a time, it produces exceptional metal structures—with material properties that can even surpass those of the base material—through the guidance of 3D CAD files, a CNC interface or basic teach-and-learn software.

The procedure produces significantly less heat than traditional welding processes, making it ideal for fixing tiny, thin parts. Early tests have shown that the technique excels at repairing unweldable parts in a repeatable manner, improving repair quality and saving time and money. New Mexico-based Optomec Design Co. has honed and commercialized the method, which was invented at Sandia National Laboratories.

LENS Users

One facility that is already employing the additive process is the Anniston Army Depot in Alabama, which uses the LENS system to fix several gas turbine engine parts for M1 Abrams tanks. These small, slim metal parts can't bear the heat generated by traditional welding operations, and once damaged, they used to be considered non-repairable and replaced with new components. Now, for a fraction of the cost to replace them, the LENS process restores them to working order.

The U.S. Department of Defense (DoD) is another LENS supporter. Aiming to introduce net shaping as a practical repair option, it recently initiated a $3 million project, as part of the Commercial Technologies for Maintenance Activities. Judging from project cost savings, the return on investment for a LENS system is under six months.

Aside from performing welding-type repairs, the process has also proven effective in making unconventional fixes. For instance, it was able to fix a crack in a 1.5 inch-diameter mold tool made of 420 stainless steel, which confounded several welding techniques. Repeated welding attempts had only exacerbated the damage, warping the material around the crack. Finally, workers sliced off the cracked part of the tool and used the additive process to deposit a new section.

How the Process Works

To begin the repair process, a high-powered laser beam hits an area about 0.02 in. wide on a damaged metal part, creating a molten pool. A nozzle applies a precise amount of metal powder on the pool to add to the material volume. Because the nozzle places the material accurately, masking is not a requirement. The system then deposits layers of material on the substrate, moving back and forth, line by line. With this meticulous layer-by-layer approach, the machine creates a metal version of the CAD model.

The automated procedure takes place inside a sealed chamber, where environmental variables are strictly regulated. For example, the deposition process on sensitive alloys like titanium occurs in an argon atmosphere with oxygen levels below 10 parts per million.

While conventional welding techniques produce a large heat-affected zone (HAZ)—the area in which heat impairs the microstructures of the component under repair—LENS minimizes the HAZ by generating little heat. For instance, in one part composed of Inconel 625, the procedure created a HAZ only 50 microns wide. Because such a HAZ won't harm essential areas of the part, the process can fix components previously classified as non-repairable because of the weakening or distortion caused by heat from conventional welding processes such as tungsten inert gas, metal inert gas, plasma arc, laser cladding and electron beam.

The small molten pool created by the LENS system cools extremely rapidly, typically at 1,000 to 5,000°C per second. And because the deposited material cools and solidifies so quickly, the technique produces extremely strong and ductile features. In fact, in some cases, the repair's mechanical properties exceed those of the original material. As a result, repaired parts can actually be better than new.

Additionally, little material is wasted during the process. Because the nozzle applies material precisely and accurately, cleanup and machining are kept to a minimum. For example, machining away the extra material left after fixing a fan blade took only one minute while machining away excess titanium left by conventional welding takes 30 minutes.

Fixing the Non-Repairable

A single LENS system can bring new life to a wide variety of materials, including 316 stainless steel, Inconel 625, Ti-6A1-4V and gradient-material compositions. The process can also repair components made of unweldable materials such as MAR-M 247 and many nickel-based super-alloys.

This layer-by-layer process restores damaged metal parts that can't handle high-temperature welding. And the repairs resulting from net shaping have superb mechanical properties, which can even surpass those of the base material. Moreover, the procedure fixes parts with efficiency and repeatability, significantly reducing repair times and costs.

Source: Cutting Edge: Net-Shaping Repairs, A Welding Alternative
Fabricating & Metalworking, July 14, 2003
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