A radiant plasma lamp offers a unique process to perform surface modification on a wide variety of materials in ways not done before.

Plasma-arc lamp in action

New materials and process development is part of the mission of Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tenn. An invention developed at ORNL' s Infrared Processing Center, Metal & Ceramics Div. provides a unique process to perform surface modification on a wide variety of materials in ways not done before. The process involves the combined advantages of a unique new high-intensity plasma arc lamp and an industrial robot. Their integration made the new technology feasible.

Water window technology for plasma-lamp survivability

The lamp was invented by Vortek Industries (Vancouver, B.C., Canada), and is believed to be the most powerful lamp in the world. It is capable of radiating 3.5 kW/cm2-a brightness rivaling that of the sun. The intensity is so great that without some method to remove/dissipate the heat, the 3.5 kW/cm2 energy-density will instantaneously melt any metal, even tungsten, which has a melting point of 3410C (6170F). The invention grew out of the author's earlier work in the field of radiant heat processing. ORNL researchers recognized the potential of the Vortek lamp to extend radiant heat-processing technology, and ORNL became the exclusive beta test site. Radiant plasma source characteristics include:

  • 300,000 W single source
  • Short-wavelength (0.2 to 1.4 mm) radiant output
  • Constant wavelength independent of power level and anode/cathode wear
  • Availability of 2 to 100% continuously variable radiant output
  • Very short power-level changeover (<20 ms)
  • 55% efficient conversion of electrical to radiant energy
  • Power-scan mode up to 35 cm (~13-3/4 in.) wide
  • Possibility of area heating using reflector design
  • Availability of three separate plasma heads (10, 20 and 35-cm arcs)

ORNL Plasma Infrared Processing Facility

Robot makes the process feasible

The process involves passing the lamp over a substrate/surface to be treated at a controlled rate and for a controlled distance. For example, in a thermal processing application such as heat treating or tempering steel, the objective is to impart the desired time-temperature profile to the workpiece. Key to the success of the process is controlling the lamp's movement within very tight tolerances, which initially presented a problem. The solution to the problem was to use a robot to move the lamp. This required a heavy-duty robot because the lamp and its cooling jacket weigh about 150 lb (70 kg). An existing Cincinnati 776 robot at ORNL-similar to those used to perform spot welding on automotive production lines-had the necessary arm and wrist strengths and was adopted for this work. The robot was an older model, but it had had only 250 hours use, compared with production-line robot service life of up to 50,000 hours (5.7 man-years) between overhauls.

While there was a lot of life left in the robot arm, the robot's controller was outdated (robot controllers become obsolete about every seven years), which made it impossible to get the robot arm to move slow enough with any degree of accuracy. Scanning speed and accuracy is extremely critical when processing using 300,000 W of radiant power. Required results can't be achieved if the placement and distance of the lamp is inaccurate and/or if the sample is scanned slightly faster or slower than required. Therefore, it is necessary to be absolutely sure the distances and tolerances are held extremely tight when the arm is programmed. Changing experimental parameters in the robot program, such as the coating, workpiece shape and dimensions, lamp stand-off distance, arm-speed, etc. also was difficult and tedious.

These challenges were overcome by incorporating a Universal Robot Controller (URCT) and RobotScriptR programming language developed by Robotic Workspace Technologies Inc. (RWT) of Ft. Myers, Fla. RWT configured a URC for the Cincinnati and shipped it to ORNL with the part-programs already prepared. The old controller was disconnected and the URC connected, and the robot was up-and-running in a production mode after only about 3 hours. The first use of the URC (treating flat-stock powder-metal parts) showed that the unit could be operated at a slower speed and with greater accuracy than previously possible.

Fig 3 High-density infrared transient-liquid-phase coating (cermet). Cross section of chromium/Ni-P cermet coating directly fused to 319 aluminum substrate.

URC offers flexibility, accuracy and repeatability

The Universal Robot Controller and RobotScript software allow changing things on the fly. There are ten different programs for that lamp right now, and it is necessary to be able to change readily from one type of experiment to another. The system allows changing tooling using software rather than using hardware, which saves operating costs. At an operating cost of $150-160/hour, tooling changeover via RWT's RobotScript software in about two minutes versus the two to three hours it required using the old software (if it was possible at all) is a significant savings.

An additional benefit is the ability of the URC's standard hardware with its open RobotScript software to communicate with other things within the Infrared Processing Center as the facility evolves. Because it's a PC-based system having standard I/Os, (input/output devices), that arm now can communicate with the rest of the environment; it can take data in, and also send signals out (including via modem, the Internet, etc.), all of which are very important. Handling complex shapes

The URC and its software have the capability to handle more complicated arm movements. It can download CAD drawings, and move the arm in three-dimensional (3-D) space, which is extremely important. For instance, to translate the lamp over a more complicated geometry, a CAD drawing of that part is downloaded, and the arm knows what the part looks like, so it moves in 3-D space in a systematic way around the part. Workpieces might include large forging dies and automotive aluminum die casting dies, where die service lives could be extended by applying a wear-resistant ceramic-like coating on pins extending out of a die-cavity, which are used to make blind holes in the casting. A future application might also involve translating the lamp across large wing structures, or similar components to heat treat the material. Remote operation

Because the robot controller actually is PC-based, as it becomes outdated, you essentially can change out a board or a processor and make a new controller. A RobotScript software-upgrade package allows any new RWT software developments to be input into the URC. Downloading new software can be done over the phone, or can be done over the Internet. This capability opens up another possibility in the future. ORNL has so-called user facilities, which means that a U.S. company can use the Infrared Processing Center to do research. The design of the controller also would allow companies to download their programs and essentially run the facility over the Internet.

Surface modification of metal. Steel is hardened and self-quenched at rates in excess of 1 m/min (3.28 ft/min).
User friendly

The controller's user-interface is highly adaptable; that is, personnel can readily learn how to program the controller.

It's not something that takes weeks of course work, but can be learned quickly, usually within a day or two for the basic-level instructions; that is, how to use the controller, not writing detailed programs.

At the beginning of the process for a completely new job, where the arm is going to be moving in a different geometry, a teach pendant is used to get the initial program, which is saved. Then, a touch screen is used to set/adjust translation-speeds over the workpiece. So, for the initial setup of the arm over a new part or over a rotating lathe, etc., the teach pendant is used. After that, it's all done using the touch screen.

The usual horizontal translation-speed "window" of the arc lamp over the workpiece is from 5 to 40 mm/s (0.20 to 1.5 in./s). Accuracy is about 1 to 2% of the speed; for example, accuracy for a translation speed of 10 mm/s (0.40 in./s) is 0.1 mm (0.004 in.). Safety issues

Safety is a major issue where you are controlling robots. Optical sensors, door sensors and proximity sensors can be installed in a workcell and instantaneously communicate to the arm, and have shut down the arm if there's a safety infraction or concern. Beam sensors are installed in the Infrared Processing Center, so if someone walks through a beam, it shuts equipment down. There also are door interlocks and curtain interlocks because lab personnel can use viewing ports (windows) along the sides of the facility to look in and see the arc lamp and robot running. The arc lamp puts out a substantial amount of ultraviolet radiation (about 5%), so eye protection is a major concern. At each of the view ports, there are welding curtains that automatically come down. If they aren't down and don't activate proximity switches, the lamp won't come on, and the arm doesn't move.

Surface modification of ceramics. Mullite brick has been surface melted using 800 W/cm2 at a scan rate of 1 cm/s.

Application possibilities

Infrared (IR) heating offers a practical industrial process. Benefits of radiant heating include high transfer efficiencies for rapid heating (reduced processing time and operating costs), precise controllability, unidirectional capability (through the lamp design) and the capability of operation in any atmosphere and vacuum. IR processing also is environmentally friendly.

One of the projects at ORNL using the plasma-arc lamp is to develop coatings for casting dies used to make aluminum auto parts. The dies are fitted with AISI Type H13 hot-work tool steel pins used to form holes in the cast part so they don't have to be machined in, which reduces manufacturing costs. The dies are placed in an H-13 steel housing. When liquid aluminum is injected into the dies, it reacts with the H-13 steel, degrading it and gradually making the die unusable.

Using the plasma source, tools are coated with a chromium-carbide coating that protects H-13 steel from attack by liquid aluminum. The plasma lamp is used to precisely and rapidly heat the precursor material on the H-13 steel, making coatings that fuse with the substrate without changing the base material properties. Because the intense radiant heating sets up large temperature gradients so fast, the iron in the H-13 steel has almost no time to dissolve. Thus, heat treating of the coated component is not necessary. Advantages of using a plasma radiant source to fuse coatings include:

  • Large area coverage (3.175 x 35 cm in a line focus and up to 10 x 20 cm in a uniform irradiance)
  • No convective mixing of coating material with the base material
  • Rapid cooling of coating material
  • Minimal effects on base material
  • No degradation of carbide reinforcements in coating
  • No temperature limitation (can readily melt tungsten - melting point = 3410C, or 6170F)

Other potential high-density infrared applications include:

  • Advanced slurry coatings (metal, cermet and ceramic) on metals and ceramics
  • Post-fusing of thermal-sprayed coatings
  • Surface modification of metals and ceramics
  • Rapid recrystallization of sheet materials
  • High heat-flux testing

Wear- and corrosion-resistant coatings, such as tungsten carbide and chromium carbide, fused to a part substrate could be useful in rolling mills and in the chemical process and heavy equipment (e.g., mining) industries.