by Robert P. Mudge and Nicholas R. Wald
Laser deposition technology is finding applications in laser cladding and repair and laser freeform manufacturing
Laser deposition technology (LDT) is a blanket name that encompasses many processes—direct metal deposition (DMD), laser metal deposition (LMP), laser additive manufacturing (LAM), laser engineered net shaping (LENS), and others—that use a focused laser beam as the heat source for depositing metals. LDT should be considered different from the standard welding or joining processes that use lasers as the heat source, as LDT is always adding new metal to an existing work piece.
FIGURE 1. The LRT process was used to build up the worn bearing seating area in this Ti-6AL-4V bearing housing from a gas turbine engine.
LDT may be characterized as a disruptive additive process that may be used for a variety of repairs and freeform fabrications. Disruptive refers to the technology's capabilities that challenge us to think outside the box. LDT has a powerful ability to apply high-quality metallurgically bonded metal deposits, which may be used for 1) laser repair technology (LRT)—the repair of worn components; 2) laser freeform manufacturing (LFMT)—performing near-net-shape freeform builds directly from CAD files and then back; and 3) laser cladding technology (LCT)—the application of cladding materials. This flexibility is a key ingredient guiding this technology.
Deposits are typically made in a controlled argon atmosphere containing less than 10ppm oxygen. Some cladding work may be performed utilizing a shielding gas system similar to standard MIG type gas shielding. All LDT deposits are metallurgically bonded and exhibit HAZ and dilution zones ranging from 0.005in. to 0.025in. thick. Typical deposit parameters range from <500W with a 1mm spot size, deposition rates less than one cubic inch per hour, and powder utilization rates <20% to 3.3kW with a 3–4mm spot size, deposition rates up to 14 cubic inches per hour, and powder utilization rates up to 80%. Low heat input and minimal distortion are consistent deposit characteristics in all ranges.
Stainless steels, tool steels, nickel alloys, cobalt alloys, titanium alloys, and a variety of hardfacing or cladding alloys are some materials that are successfully being deposited. Some success is being achieved with aluminum, but typically aluminum and copper alloys are very difficult to deposit due to their reflective properties.
FIGURE 2. A high-speed 4340 drive shaft was repaired using a conventional spray process (top). The LCT process was introduced (bottom) for a successful repair.
The high quality, versatility, and flexibility of LDT deposits are the strengths of this technology. It is the reason it is currently being evaluated by the medical and aerospace industries, the Department of Defense (DoD), as well as commercial industries that include electric power generation, oil/gas, chemical processing, and mining. The cost, time, and material savings due to the utilization of LDT is impressive and worthy of additional evaluations.
Low wattage LRT repairs of components cover many aerospace and DoD applications as well as several commercial projects. Many projects have been completed using a typical low power of <500W for the repair of Ti-6Al-4V and Inconel 718 components. Minimal distortion is experienced with this type of repair, which may be used on several aircraft structural components such as wing spars or bulk heads. Several gas turbine engine components as well as land-based turbine blades are all potential candidates for this low wattage repair.
One application employing a low-power LRT repair is shown in Figure 1. This Ti-6AL-4V bearing housing from a gas turbine engine had a bearing seating area in an out-of-tolerance condition and was considered scrap. The LRT process was utilized to build up the worn area, followed by final machining to print tolerances. The housing was repaired with no measurable distortion. It has completed an evaluation run in a test engine and the customer qualification testing is nearing the final engineering acceptance stage, which is the last step in completing this repair as a qualified repair process. This repair costs about 50% of new unit pricing and the LRT process saves all of the materials that would be required to manufacture a new housing. Delivery for the repaired housing is a few days compared to several weeks for a new housing. Similar repairs have been performed on very fine Inconel 718 compressor seals.
The top half of Figure 2 shows a high-speed (8800rpm) 4340 drive shaft that has been repaired using a conventional spray process. Note the severe spalling in the repaired tapered area of the shaft. The user tolerated this recurring problem due to lack of other options for repair. The LCT process was introduced to the customer and now several of these shafts have been successfully repaired over the last 3 years using 420 SS. The LCT repair costs for this shaft are less than 50% of new cost.
Bearing, seal, and coupler surfaces, on shafts typically considered non-repairable by conventional welding techniques, are considered great candidates for build up and repair utilizing the LCT process. Materials such as 4340, 4130, and PH grade stainless steels have been successfully clad. High-speed shafts (up to 12,800 rpm), high horsepower (up to 3500 hp), high-precision shafts (tolerances measured in 0.0005 in.), and large shafts (up to 25,000 pounds) have been repaired.
FIGURE 3. An oil field adapter that has been LCT clad with a tungsten carbide alloy. The main photo shows the as-deposited state and the inset shows the final ground product.
LCT may be also used in the manufacture of new composite components where a component is made from a tough structural base material and then the surface is clad with a specific purpose material. Figure 3 shows an oil field adapter that has been LCT clad with a tungsten carbide alloy. Due to the characteristics and flexibility of the LCT process, many applications are now possible.
LFMT may be used to deposit "freeforms" of near-net-shape metal components, which are nearly 100% dense with mechanical properties comparable to wrought materials, directly from processed CAD files. Freeforms may be thin wall (minimum 0.060 in. thick) or solid builds to any thickness. First a CAD file of the desired part must be provided. Next this file is modified and processed by the system software where the tool path for the laser is generated. A target plate is required as a base to start the build. The target plate may be incorporated into the final part or may simply be removed when the freeform build is complete. Thermal treatments and CMM scans of the completed freeform builds may be required depending on the specific application. The largest freeforms deposited to date weigh in at more than 100 pounds.
One application is to manufacture components with high aspect ratio features such as structural components for aircraft bulk heads or spars. Figure 4 shows a variety of LFMT thin wall freeforms. Special pipes, creative hollow shapes, and hollow stem engine valves are just a few examples of the wide open potential applications for this capability.
Future and conclusions
LDT is a maturing disruptive additive technology that provides new capabilities for creative repairs (LRT) and cladding (LCT) on components that may have previously been considered non-repairable. LDT also has the ability to easily change a CAD file for freeform (LFMT) production revisions versus remanufacturing hard tooling. This freeform ability may revolutionize existing manufacturing processes by employing the concept of simplifying castings and forgings and then applying special features utilizing the LFMT process.
Commercial industries, aerospace companies, and DoD customers are extremely interested in the potential of the technology and are currently investing in several promising and exciting developmental projects. However, because many of these projects are proprietary in nature, the results cannot be published at this time.
The next step is to train engineers to think outside the box to fully utilize the capabilities of this unique technology. As this line of thinking is employed, the family of LDT processes will be ready to deliver high-quality cost-effective deposits.
Editor's Note: All of the projects presented in this article were completed using an Optomec 850R coupled with an IPG 3kW fiber laser. The custom delivery heads that were employed were manufactured by RPM and Associates Inc.
Robert P. Mudge and Nicholas R. Wald (firstname.lastname@example.org) are with RPM and Associates Inc. (Rapid City, SD; www.rpmandassociates.com).