Process limits heat-affected zone to repair delicate components
February 27, 2003
At military installations across the country, repair personnel struggle to stretch the life spans of vital pieces of equipment. Sometimes welding can extend the life of damaged components in aircraft, tanks, and other military vehicles. But in some cases, high–temperature welding processes do more harm than good, warping and weakening delicate metal components. Previously such components would be classified as irreparable and replaced with pricey new parts.
Recently, though, several military facilities have been testing a new repair technique developed to save expensive parts from the scrap heap. The technique, known as Laser Engineered Net Shaping (LENS™), produces far less heat than conventional repair techniquesso little that the manufacturer says it won't damage small, thin components such as turbine blades, vanes, and impellers. In early tests the technique has fixed a variety of unweldable parts, resulting in millions of dollars of projected annual savings for the U.S. military.
LENS machines also automate the process of adding material to a worn or fractured surface. Guided by original CAD data, a CNC interface, or simplified teach and learn software, LENS machines deposit metal features one layer at a time in a fast, repeatable manner that also boosts repair quality.
Invented at Sandia National Laboratories, LENS has been developed and commercialized by Optomec Inc., a supplier of additive manufacturing systems.
To start the repair process, a highpowered laser beam strikes a small spot (approximately 0.02 inch wide) on a damaged metal component, producing a molten pool (see Figure 1). A nozzle blows metal powder into the pool to increase its volume. The nozzle positions the material so accurately that there's no need for masking. This process is repeated within a plane to create a single metal layer. Then another layer is deposited on top of that. The machine deposits layer upon layer until it has produced a metal version of the CAD model.
LENS is a computer-controlled, layer-by-layer deposition process that uses a high-powered laser beam to strike a small spot on a damaged metal component, producing a molten pool. A nozzle blows metal powder into the pool to increase its volume, allowing the system to give new life to damaged metal parts that can't stand up to high-temperature welding.
Deposition occurs inside a sealed work area, or "glove box," where environmental variables can be tightly controlled. This is crucial when working with sensitive alloys like titanium, which is repaired in an argon atmosphere containing as little as one part per million of oxygen.
Like common welding techniques, LENS repairs parts by adding metal to a heated surface. But the heat generated by conventional welding processes can produce a large heataffected zone (HAZ), an area in which heat weakens the microstructures of the part being repaired. The integrity of a part can be compromised by the formation of a large HAZ.
LENS generates little heat, keeping the HAZ to a minimum. In one INCONEL® alloy 625 part, for example, LENS produced a HAZ just 50 microns wide. Because the HAZ doesn't spread to critical areas, LENS can fix parts once classified as nonrepairable because of the strength loss or distortion that would be caused by heat from a conventional welding process.
In most cases, the tiny molten pool produced by the LENS system cools at a rate of 1,000 to 5,000 degrees C per second. Rapid material cooling and solidification produce fully dense features with greater strength and ductility than those made by other welding techniques. In some cases, the LENS repair even exhibits mechanical properties that are superior to the original material. This means parts may stay in service longer after repair than before.
LENS produces small, uniform grain structures, which enhances repair quality. The technique can even replicate the singlecrystal structures required for some applications. LENS repairs also exhibit no sink or undercut, further increasing the range of repair possibilities.
A single LENS system can fix parts made of many materials, including 316 stainless steel, INCONEL 625, Ti6A14V, and even gradient material compositions. The process also can repair components made of nonweldable materials such as MARM 247 "rhenium" and many nickelbased superalloys, including Waspaloy®.
LENS nozzles deposit materials in almost the exact amounts required, minimizing postprocess cleanup and machining. After the repair of one fan blade, for example, machining the excess material took just one minute, compared to the 30 minutes of machining needed to remove the large amounts of titanium deposited with conventional welding techniques.
To improve the process, LENS machines eliminate many manual tasks, resulting in faster, less expensive repairs. The machines feature a computerized closedloop control system that helps ensure precise, repeatable repair operations. During each operation, the control system monitors key variables and continually optimizes the process.
Two years ago the U.S. Department of Defense (DOD) launched a $3 million program to establish LENS as a viable repair option. During the program, now viewed by the DOD as a major success, users were expected to recoup their LENS investments in as little as six months.
LENS technology saves money by fixing parts for a fraction of the cost of a new part. Consider a compressor made of thin INCONEL alloy 713LC. LENS repairs to the compressor cost $2,000, compared to the $8,300 purchase price of a new compressor. The bottom line: Military facilities save $6,300 by fixing damaged compressors that used to be scrapped.
The National Center for Manufacturing Sciences (NCMS), its member companies, and the DOD conducted a collaborative research project to employ the LENS system for repair and overhaul techniques at the Anniston Army Depot (ANAD) in Anniston, Ala. Unable to take the heat from conventional welding methods, small, thin metal parts in the gas turbine engines of ANAD's M1 Abrams tanks used to be classified as irreparable and were replaced with new parts. Now a LENS machine fixes them for a fraction of the replacement cost, which is projected to generate more than $6.3 million in savings annually for the DOD 13.5 percent more than original projections.
Completing the first project phase of technology assessment and validation, the NCMSled team has proven not only that LENS technology meets critical performance demands, but initial results also indicate that the projected annual savings will be more than $747,000 over the initial target goal of just less than $5.6 million. Based on NCMS's initial findings, the U.S. Army TankAutomotive Armaments Command (TACOM) already has approved military use of LENS for seven of the repair operations tested in phase I. With one of the methods, TACOM now will have the ability to complete fourthstage seal runner restoration at onethird the cost of replacement.
In addition to these weldingtype fixes, LENS machines can make unconventional repairs beyond the capabilities of any welding system. In one case, several welding procedures failed to fix a crack in a 1¼in.diameter mold tool made of 420 stainless steel. The repeated welding attempts also ruined the material in the area of the crack (HAZ). Finally workers cut off the cracked half of the tool and replaced it with a new half deposited by a LENS system.
The newest LENS machines feature a vision system that lets users see the damaged part on a monitor. Crosshairs appear on top of the part image" users map the repair area by moving the crosshairs with a computer mouse. Then they enter process parameters, such as the length and width of the repair. Using the map and entered data, special software generates the repair file that directs the operation of the LENS machine. Then the system stores this information for future use. If a similar repair is needed, the operator can use the stored file rather than create a new one.
Other uses of LENS technology center on lowvolume manufacturing, in which CAD files drive LENS to create parts that were thought to be impossible to fabricate with conventional techniques. This opens the door for LENS to fabricate replacement components for parts that are not repairable at the time they are needed.
For more information about National Center for Manufacturing Sciences research projects employing LENS technology, contact NCMS at 3025 Boardwalk, Ann Arbor, MI 481083266, 7349950300, fax 7349954004, www.ncms.org.
Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 871851380, 5058434164, www.sandia.gov.
ICONEL is a registered trademark of Special Metals group of companies.Waspaloy is a registered trademark of United Technologies.