This article focuses on the use of laser in conjunction with metal powder. The use of the laser in conjunction with metal powder could be regarded as an extension of rapid prototyping, which has involved plastic parts almost exclusively. However, terms such as ‘direct metal fabrication’ and ‘rapid manufacturing’ suggest that developers have much more ambitious goals in mind for powder metals. Lockheed Martin Tactical Aircraft Systems in Fort Worth, Texas, is operating a research facility based on the laser engineered net shaping (LENS) process on the same factory floor where the F-16 fighter is assembled for the US Air Force. Optomec Design Co. in Albuquerque builds machines for use with the LENS process. Tests of Sandia National Laboratories' laser engineered net shaping process take place in a tank at Lockheed Martin Tactical Aircraft Systems. Much can be done in direct metal fabrication to improve the thermal conductivity of tooling for injection molding and die casting. The result can be increases in cycle times of as much as 80 percent.
It always seemed to many observers of industry that powder metallurgy was a curious branch of materials science waiting for some kind of technological partner to come along and shake things up. Then, slowly but surely, various processes, such as powder forging and metal matrix composites and hot isostatic pressing, began to take hold.
More recently, the laser entered the powder scene, and now industry is finding a new way to manufacture parts. It's still an esoteric area, hardly commercialized at all, but the incentive is strong. Parts made of powder fused under laser light will require little or no finishing and will involve almost no waste of metal.
The use of the laser in conjunction with metal powder could be regarded as an extension of rapid prototyping, which has involved plastic parts almost exclusively. But terms like "direct metal fabrication" and "rapid manufacturing" suggest that developers have much more ambitious goals in mind for powder metals.
Metal powder has caught the attention of many research engineers at national laboratories and in universities. In a technical report, Eric Whitney and James Sears of Penn State's Applied Research Laboratory identified several laser-based free-form fabrication processes now being developed.
They were the directed light fabrication process at Los Alamos National Laboratory; the selective laser sintering process associated with DTM Inc. and the University of Texas; laser direct casting at the University of Liverpool, England; direct metal deposition at the University of Michigan; the laser engineered net shaping, or LENS, process from Sandia National Laboratories; and Penn State's high-power, laser-based, solid free-form fabrication process.
An Aircraft Launch
Lockheed Martin Tactical Aircraft Systems in Fort Worth, Texas, is operating a research facility based on the LENS process on the same factory floor where the F-16 fighter is assembled for the U.S. Air Force.
About a year and a half ago, Lockheed Martin Corp. joined a consortium that had been formed at Sandia National Laboratories in Albuquerque, N.M., under a cooperative research and development agreement to develop the LENS process. Also involved in the consortium were a combination of Fortune 500 companies and small, high-technology companies: AlliedSignal Inc., Eastman Kodak Co., Hasbro Inc., Laser Fare Inc., MTS Systems Corp., 3M Co., Optomec Design Co., Teleflex Inc., and Wyman-Gordon Co. Los Alamos joined the effort in June.
Lockheed's Fort Worth facility uses a robotically controlled laser and metal powder to create custom parts in a special chamber. The system in use at Lockheed can substitute wire for powder for some assemblies.
The chamber, built by MTS Systems Corp. of Eden Prairie, Minn., is purged with argon, which acts as a protective atmosphere for the powder metals during the melting process. Operating inside the chamber is a six-axis robot programmed to go through the motions necessary to build a particular part. The powder metal is fed through a cable to the articulated arm of the robot. Simultaneously, a laser beam travels through a fiber optic cable. The laser is programmed to arrive at the same point on the robot arm as that of the powder metal. The equipment generates 2 kW of power. It is a YAG (for yttrium, aluminum, garnet) solid state laser from U.S. Laser Co. in Wyckoff, N.J.
The argon-filled chamber is similar to chambers that have been used for years, especially in the gas turbine engine industry, for the gas tungsten are welding of reactive metals like titanium. In that operation and in the LENS process, the argon acts as a shield against contamination from unwanted oxygen and nitrogen from the atmosphere.
According to Brian Rosenberger, team leader for the Lockheed Martin facility, this laser direct manufacturing research facility has been in operation since the first of this year. At present, the process is being used experimentally to produce prototype parts from such alloys as type 316 stainless steel. Lockheed Martin plans to evaluate the processability of 6 Al-4 V titanium, certain aluminum alloys like 6061, and perhaps some alloys from the 7000 series.
Rosenberger said that aluminum is not all that friendly when it comes to laser processing. "There is a reflectivity problem," he said, "but it can often be overcome through increased laser power output." Lockheed Martin will also be looking at a number of the nickel-base alloys.
Rosenberger sees the LENS process eventually reducing the cost of manufacturing. There is a lot of interest in this approach for producing small parts, but Rosenberger also sees tremendous potential in its use for large parts like those currently machined from titanium forgings, which are often several inches thick.
"Under the present system, it can take as long as two years to obtain the tooling for such parts," he said. "In addition, the delivery of titanium billet can be as long as 18 months. One company in this country now is capable of production of 5 lbs. per hour. This type of system can be operated unattended, around the clock. At that speed, a similar type of large titanium part could be produced in 16 to 20 hours."
Another opportunity is in simplifying structures. According to Rosenberger, many subassemblies that are now made of parts fastened or welded together could be built in one piece using the LENS process.
Large-scale assembly using powder metals was tested in a recent Defense Advanced Research Projects Agency program that focused on the flexible fabrication of titanium. Laser deposition technology was one of the programs DARPA used. It produced cylinders, cubes, and rectangular parts weighing 15 to 30 pounds.
Optomec Design Co. in Albuquerque builds machines for use with the LENS process. David Keicher, one of the inventors of the LENS process while an employee of Sandia National Laboratories, is now the director of R&D at Optomec.
James Love, manager of product, design, and development at Optomec, said that many customers seem to be interested in various configurations of the equipment. The machines are getting larger and are more robust than early units. The standard chamber is about 3 cubic feet, with a working envelope of approximately 12 cubic inches. Typically, there is more than one tube for the powder metal feed.
The current deposition rate ranges from a quarter to a full cubic inch an hour.
"We are going through a learning curve on many materials," Love said. "The metals that seem to work include stainless steel, several tool steels, some of the nickel-base alloys, and various titanium alloys. We also have equipment that performs multimaterial parts processing. Aluminum is still something of a problem."
The tool and die makers that support the automotive industry are very interested in the technology. According to reports, they have done much work with H13 tool steel and also have worked with D 2 steel. The controls for this equipment are all PC-based. A motion control card inserted into the system allows for multiaxis automation.
Parts that are produced in the LENS system can be completed within 0.005 inch of the desired finished dimensions. A 10-microinch surface finish can be obtained with post-processing.
A Source for the Lasers
James F. Golden, vice president of marketing at U.S. Laser, said most customers are using a 700-W-capacity laser, but are operating it within a 400-to 450-W power range. Customers either use a fiber optic cable or mirrors and lenses. The cable is needed when robotics are used. The mirrors and lenses operate like those in CO 2 lasers.
Whitney, a research associate at Penn State University's Applied Research Laboratory, said that work with a 14-kW CO2 laser has been applied to numerous alloys, including 6-4 titanium, 5- 2.5 titanium, CP titanium, titanium aluminides, nickel-aluminum-bronze, 17 -4PH stainless steel, and the nickel-base 625 alloy. The lab also has produced graded parts between nickel-aluminum-bronze and alloy 625.
Hal J. Galvin, vice president of business development at MTS Systems and chairman of AeroMet Corp., an MTS subsidiary, said that titanium components pro duced by AeroMet's Lasform process, which is similar to Penn State's, are demonstrating mechanical properties at least equal to those found in titanium. forgings, if not superior. Using the Lasform process, he noted, it is possible to tailor alloy chemistry and to engineer desirable grain structure.
As the structure is built up in the Los Alamos directed light fabrication process, the part is moved in the x and y axes, while the laser head moves in the z direction and can tilt. There is also a rotary axis on which the part can move. A fiber optically delivered Nd-YAG lase r is used in this process. Work is carried out within an inert atmosphere.
Gary Lewis, a metallurgical engineer and one of the inventors of directed light fabrication at Los Alamos, said that the lab has "shown feasibility of producing component shapes out of essentially any metal and intermetallic compounds." The use of the objects would be limited by the metallurgy of the materials, he added.
Los Alamos is trying to commercialize the process, Lewis said.
In laser direct casting, a six-axis computer-controlled machine is capable of building three-dimensional objects. According to the report by Whitney and Sears, a 1.5-kW CO2 laser has been used in this process into deposit stainless steel up to 9 cubic millimeters a second. Travel speeds between 500 and 1,000 nm1 per minute have been used to obtain fully dense, porosity-free deposits. Microstructure of LDC-deposited 316L stainless steel was that of a very fine-grain casting with epitaxial growth between layers.
Brent Stucker, assistant professor of industrial and manufacturing engineering at the University of Rhode Island in Kingston, heads the university's Rapid Manufacturing Center, which is supported by an industry consortium that includes General Electric Co., Laser Fare Inc., Fielding Manufacturing, American Industrial Casting, and DTM Corp. The center is operating with two selective laser sintering machines from DTM and a higher-powered unit from Optomec Design for the LENS process.
With the lower-powered selective laser sintering machine, direct fabrication is a two-step process. In the first step, a green part is fabricated from a powder metal! polymer binder starting stock. The green part is then fed into a furnace, where the binder is removed. This is a relatively inexpensive and accurate means of producing parts of final dimension from powder metal using a laser.
LENS, by contrast, is a one- step operation, but it is also more expensive than selective laser sintering.
At the Rapid Manufacturing Center at the University of Rhode Island, experiments are being performed using matrix composites of copper and ceramic as the starting stock. In this connection, the selective laser sintering machine builds a green part out of a polymer binder. The ceramic in this instance is zirconium diboride. The green part is placed in the furnace to have the polymer removed. The finished part, infiltrated with copper, combines outstanding thermal conductivity and abrasion resistance.
In April of this year, DTM Corp. of Austin, Texas, announced that it would be working with Rockwell International to advance the direct fabrication of metal parts, using the aerospace company's direct metal fabrication process. This process combines selective laser sintering with a process called liquid phase sintering, which fuses one metal in a liquid state with another in a solid state. The combined process will create three-dimensional objects from CAD data.
The following month, DTM announced plans to collaborate with Penn State's Powder Metallurgy Laboratory to commercialize additional metal-based powders for use with DTM's selective laser sintering technology. The company markets equipment for the process under the name Sinterstation. In this program, various metal powders, including aluminum, will be processed in a Sinterstation system from DTM.
Researchers at Penn State will be taking parts in the green state from the Sinterstation chamber and exposing them to liquid phase sintering, which was developed at Penn State.
"Essentially, we will be marrying liquid phase sintering to rapid pro to typing," said Randall M. German, Brush Chair Professor in Materials at the Powder Metallurgy Lab. The aluminum parts will be composed of prealloyed powders. Aluminum parts made from elemental powders, said German, do not work out well in direct manufacturing.
Frank Beaumont, vice president and general manager of Ampal in Palmerton, Pa., an expert in aluminum powder metals, said that you have to go inside a chamber with aluminum. Most powder metal mixtures containing aluminum are blends of elemental powders, he said. Prealloyed powders, in which each particle of the powder contains more than one element, exhibit less reflectivity.
DTM offers two metal-based powders. One is RapidSteel 2.0, a stainless steel infiltrated with bronze, which is used to create mold inserts for plastic injection molding. The other is copper polyamide, a copper and plastic con1.posite used to make mold inserts for short runs of plastic parts.
Parts in the Microwave
Researchers at Pennsylvania State University, which has developed one of the processes for fabricating parts from powdered metals under laser light, have found that they can sinter similar materials using microwaves.
Although conventional kitchen wisdom says metals and microwaves don't mix, the researchers report that the process results in parts with improved mechanical properties, in a fraction of the production time, and using less energy than with conventional furnace heating.
"Solid metals cause problems in microwaves because they reflect, rather than absorb, the microwave radiation," said Dinesh K. Agrawal, professor of materials, senior scientist, and director of Penn State's Microwave Processing and Engineering Center in University Park, Pa. "Powdered metals do absorb microwave radiation, and can be heated and sintered using microwaves," he added. Microwave sintering causes the powdered metal to consolidate and the grains to bond, forming a solid, dense material. At the high temperatures required to produce sintering-roughly 1,200'C-sparking does not occur.
To achieve those high temperatures in the microwave oven, the part must be heated in insulated sintering chambers, so that the heat generated in the green part is not dissipated in the microwave cavity. The insulation material must be able to withstand the extremely elevated temperatures but not absorb the microwave energy, said Agrawal.
Parts that have undergone microwave sintering have shown better mechanical properties, including strength, hardness, and toughness, than products made conventionally. The better performance results from the finer grain sizes in microwave sintered parts-as much as half the grain size of parts produced with conventional thermal heating. Grain sizes are finer because the microwave sintering process is much quicker than thermal sintering, allowing less time for the grains to grow during the sintering step, Agrawal said.
In general, a part that might take several hours to sinter in a conventional furnace could take 90 minutes or less in a microwave oven, he added. Although Agrawal has not yet quantified energy savings, he believes that microwaving parts can use 30 to 40 percent less energy than conventional thermal methods. To prevent oxidation, it's necessary to alter the atmosphere of the chamber with inert gases, such as nitrogen or hydrogen.
Penn State is working with several companies to commercialize the process. One of them is Keystone Powdered Metal Co. of St. Marys, Pa., which is setting up a pilot plant to produce powdered metal gears for motors and automotive applications, said Agrawal.
The microwave sintering process has been demonstrated on pure metals and alloys, including iron, steel, copper, aluminum, nickel, molybdenum, cobalt, tungsten, tungsten carbide, and tin.
The Sandia Effort
Clint Atwood, LENS project manager at Sandia, said that much can be done in direct metal fabrication to improve the thermal conductivity of tooling for injection molding and die casting. The result can be increases in cycle times of as much as 80 percent. It might take six to eight weeks to build a casting for a specific application, whereas the same shape part can be built in days by direct metal fabrication. Chances are that the properties will also be better, he said.
Precision Optical Manufacturing Co. Inc. of Plymouth, Mich., has installed equipment based on the direct metal deposition process, which was developed at the Center for Laser-Aided Intelligent Manufacturing at the University of Michigan in Ann Arbor. Precision Optical is using the system to produce dies from tool steel powder for customers in the automotive industry. The university has put a considerable amount of work into the deposition of H13 tool steels in complex die patterns.
Precision Optical opened for business in September 1998. The equipment needed to perform direct manufacturing of tool steel components was installed in the Plymouth facility in February of this year. The facility is now being used to produce prototype components for various customers.
Dwight M. Morgan, the president of Precision Optical, said his new direct metal deposition system incorporates a 2.4- kW CO2 Trumpf laser. The company's main focus will be on P20 tool steels for plastic injection molding applications and H13 steel for automotive and consumer product applications. The company expects to install a second, slightly larger chamber in the near future.
In direct metal deposition, parts are created by focusing a high-powered laser beam onto a flat tool steel workpiece where a molten pool of metal is formed. A small stream of powdered metal is then injected into the pool, thus increasing its size. By moving the laser back and forth and tracing out a small pattern determined by computer-aided de sign, the solid metal is built, line by line, one layer at a time.
The commercial version of the system uses a CNC motion control system to move the tool steel workpiece past the laser beam, and a CAD/CAM system to define the geometry of the part being produced.
Precision Optical's intent is to commercialize this new technology through the integration of the company's own patent-pending closed loop feedback system with the recently patented direct metal deposition process.
Typically, it can take 30 to 40 weeks to produce a die using present CNC machining technology. According to the National Center for Manufacturing Science in Ann Arbor, Mich., the direct metal deposition process is capable of reducing die production time by 40 percent.
The use of the direct metal deposition process has also been explored for the production of surgical instruments. For surgical tools, it is estimated that the process can reduce the number of steps required for manufacture from 62 to seven.
Terry Feeley, president of the advanced technology group of Laser Fare Inc. in Smithfield, R.I., said that his company has been involved with the lasering of powder metal since 1992. The technology, he said, has been around for 25 years, when it was first used as a cladding technology. For cladding, the process was two-dimensional. For rapid manufacturing, however, industry is now concerned with the fabrication of three-dimensional parts. "There appear to be advantages to parts made by this new technology over wrought structures," Feeley said. "The technology provides the opportunity to design with tailored materials, the ability to shift form one material to another in a single part, and the ability to produce this part in a single sequence."
In all the permutations of processes, materials, and tests, there is a constant theme, because no matter which process is used, the aim is the same: to save money and time, and to turn out a more reliable product.