Oxygen free high conductivity copper (OFHC) is a prime candidate material for arc heater segments as it can effectively dissipate high heat flux loads from the inner bore. However, while serving this function it is also required to maintain an electrically non-conducting surface to avoid the loss of material in the arc. The challenge of fabricating a highly heat conducting material with a highly non- conducting surface is envisioned using coating technology. Smooth, ultrahard coatings that have good thermal conductivity and little or no electrical conductivity can protect these surfaces by preventing arc erosion. In this Phase II multi-year effort (Phase I efforts were successfully completed under the sponsorship of USAF/MMI during 1994-95), in collaboration with Materials Modification Inc. (MMI), the work will continue on the unique approaches that were successfully demonstrated during the phase I efforts. The primary objective is to create and metallurgically bond using Pulsed Electrode Surfacing (PES) technology a layer of insulating films such as AlN, CN, TiC, TiB2 or their multilayer combinations that can provide an erosion resistant surface. The PES technique allows the capability of depositing a very hard arc erosion resistant surface that is integrally bonded and will have superior adhesion when compared to any other form of conventional deposition methods such as physical or chemical vapor deposition, thermal or plasma spraying or chemical precipitation methods. Coatings deposited using PES can be bent to 180o in a conical bend test without peeling or fracturing. The deposited coatings are being characterized using optical, SEM/EDAX and hardness techniques. The coatings will also be exposed to elevated temperatures to ascertain their oxidation resistance.
A second objective is to develop a servocontrolled pulsed electrode surfacing system that can be automated for production and have the capability of coating multiple parts/surfaces at the same time. The new system is expected to be capable of handling complex topography that can be covered rapidly, uniformly and cost effectively. With the use of computer controlled motion devices, the deposition rate and surface quality could be monitored over large areas. Another possible design change to the existing system would be to fit the electrode holder with a gas nozzle that can handle reactive and inert gases such that the surface quality and composition can be manipulated to suit the needs of a given application. After building the servocontrolled PES equipment and optimization of the process control parameters, attempts will be made to coat arc heater segments for actual testing in an operating system.
At the present time most successful methods for the production of the high temperature superconductors and their buffer layers involve physical rather than chemical processing. Current techniques depend primarily on pulsed laser deposition, a method that while reasonably successful, has severe drawbacks in that it must be used in vacuum. Epitaxial oxide buffer layers are deposited on a metallic substrate and are then coated with the superconducting film. Since the oxide buffer layers are the medium for transferring texture to the superconductor while providing protection from contamination, they are one of the key elements to the commercial production of coated conductors. It is, therefore, very important to adapt a non-vacuum process for applying these layers.
Presently, within CLA, UTSI oxide coatings on various substrates are synthesized using high intensity lasers and in-situ reaction technique. These coatings are being developed for a variety of applications, but the process is amenable to adaptation for the buffer layer of the coated conductor. The technique, laser induced reaction coating (LIRC) is conducted in non-vacuum environment and involves the in-situ formation of the required compound and complex coatings onto metals and ceramics. This requires proper selection of precursor material (chemical species) along with strict control of the chemical process via exact control of the energy input. This requirement for control is the reason why lasers are desirable. The high beam quality of laser allows this control with minimal energy fluctuations. Their high power also assists the initiation of the chemical reaction as well as increasing the rate of the entire coating process.
Initial successful efforts have been made to deposit an ultra- thin (approx 100 nm thick) CeO2 layer on pure nickel substrate in the localized environment of the mixture of nitrogen and hydrogen. The figures show scanning electron micrographs of nickel substrate laser-coated with CeO2 film at selected traverse speeds of (a) 100 mm/min , and (b) 500 mm/min . TEA-CO2 laser was used to in extremely short duration pulsed mode (2 ms) to provide suitable thermal conditions to evaporate precursor material, to initiate chemical reaction and finally to entirely avoid change in the substrate material texture. On-going efforts will extend to synthesize various oxide and non-oxide buffer layers on textured nickel substrate.
Laser induced surface modification (LISM) is a process developed under the main theme of laser induced reaction processing which include coating, alloying and joining of materials. LISM in particular is a material processing method which utilizes the high power density available from laser sources to heat the precursor to a temperature below or above melting and a portion of the underlying substrate material to the melting temperature to produce in-situ reaction product. Since melting occurs rapidly and only at the surface and subsurface, the bulk of the material remains cool, thus creating rapid self-quenching and solidification. The wide variety of chemical and microstructural states can be retained because of the rapid quench from the liquid phase. These include chemical profiles where the alloyed element or infiltrated reinforcement material is highly concentrated near the atomic surface and decreases in concentration over shallow depths, and uniform profiles where the concentration is the same throughout the entire modified region. These types of microstructures range from solid solutions with distinct crystalline phases to metallic glasses. Currently, initial efforts are being directed toward modifying structural aluminum (7xxx, 6xxx and 2xxx series) surfaces for enhanced fatigue and wear properties using LISM.
Under the three-year sponsorship of Department of Energy, a laser-induced reaction coating of ceramic on ceramic in non-vacuum environment was developed and studied. Al2O3 ceramic coating on SiC/Al2O3 ceramic composite was deposited in air using a CO2 laser operated in continuous wave mode. The ceramic coating synthesized using this technique was uniform and dense which provided long duration (approx 200 hours) protection from oxidation corrosion at elevated temperatures (1000-1300oC) to the composite. The scanning electron micrograph shows a cross sectional view of laser-induced reaction coated SiC/Al2O3 composite. It is expected that this success will lead to the synthesis of a variety of ceramic coatings on various metal and ceramic substrates for applications such as heat exchangers, incinerators, etc. which involve extreme working conditions in terms of temperature and chemical environment.
Under the one-year sponsorship of Babcock & Wilcox, an autogeneous laser surface treatment of nickel based alloy 690 for enhanced diffusion bonding was developed. Alloy 690 was laser surface scanned using pulsed Nd-Yag laser to homogenize and refine the microstructure within the surface and subsurface region. Rapid cooling rate involved in the process provided metastable structures which in turn modified the diffusion kinetics during the post laser treatment diffusion bonding of the alloy 690. The modified diffusion kinetics allowed to conduct the bonding process at temperature 200oC less and with duration 2 hours less compared to diffusion bonding for as received alloy. Such savings in time and temperature provide an economical process for commercial environment and can be extended to variety of high temperature materials.
During the past year, CLA has continued support of materials development at Arnold Engineering Development Center, AEDC. The Impulse and Arc Heater facilities at AEDC provide high energy flow conditions, and both require materials that can withstand high temperature environments. During the 95-96 year, a vacuum plasma discharge test device has been used to illustrate the difference in performance between coated and uncoated OFHC copper. During the 96-97 year, two facility components were modified by UTSI and delivered to AEDC for testing.
The Impulse facility is a free piston shock tunnel capable of providing operating stagnation pressures over 1,000 atm. UTSI has investigated laser processing techniques for improving the life cycle of the throat insert nozzle for the Impulse facility. A W-20Cu insert was processed with the CLA 3 kW CO2 laser, and delivered to AEDC for future testing.
The Arc Heater facility at AEDC provides high enthalpy test conditions for extended periods by heating a flow stream with a 65 MW electrical arc. Controlling the arc attachment is an important consideration for extending the lifetime of the electrodes used in this facility. The following is a picture of a cathode for the H-3 tunnel. A number of tests were conducted at UTSI to determine if preferential arc attachment could be obtained through selective coating of electrode surfaces. A video recording system was used to observe the attachment of laboratory scale arcs under various conditions. The following figure shows the preferential attachment of the arc to a section of OFHC that was coated with silver.
Based on the results of these tesAEDC's input, a set of machining grooves was placed in an experimental cathode for testing in the H-3 Arc Heater. The machined cathode is shown in the figure. This component was placed in the H-3 facility and tested under normal operating conditions. Preliminary results indicate that wear properties were not affected very much by the grooves and that material addition may be necessary to provide significant differences in performance.
The new high-temperature superconducting materials offer significant advantages for the conservation of energy, particularly in the power generation and transportation industries. UTSI began a DOE program to assist in bringing these ``laboratory'' materials into the commercial arena. CLA is participating significantly in this program in several ways first by applying their knowledge and expertise in laser-based diagnostics to the monitoring and control of superconductor cable fabrication, and second by investigating innovative techniques, such as laser sustained plasma synthesis, to minimize the time and complexity of processing.
The application of high temperature superconducting wires will require knowledge of stress states in the wires. Unlike typical conductors and polymer-based tapes, oxide based superconductor materials are brittle and easily damaged. Limitations for processing, handling, storing, and implementing these wires need to be understood for optimal application. Current work at UTSI includes building finite element models to provide basic capability to predict stress states in these materials and making material characterization for specific production processes. Thus, as the global framework for defining limits is formed, a piece of the required knowledge set is gained. The following is afigure of an Atomic Force Microscopy (AFM) scan of a buffer layer on a metallic substrate. The grain boundaries are clearly evident in this picture, dthis is modeled as shown in the computed deformed mesh and in the computed stress countours in the region of the grain boundary under thermal load. By determining physical characteristics and employing this information in a model, the most appropriate processing conditions can be determined.
The Laser Induced Surface Improvement (LISI) process consists of focusing the high energy of a laser onto a material surface, thereby creating a molten zone. Alloying elements are then introduced into the melt to produce a surface with the desirable protective properties. Since the track widths are generally on the order of millimeters, in order to fully protect a surface, sequential, overlapping tracks are made. As the tracks are overlapped, superposition of both the thermal fields and the compositional fields occurs, generating a series of microstructural regimes both within the surface layer and in the base material. For the LISI process to be fully utilized, the basic laser material interaction issues related to the sequential formation of the surface and the resultant superposition of the fields must be understood. Therefore the objectives are to 1) understand the reactions occurring within the overlap and surrounding regions as a result of the thermal and compositional cycling experienced during the formation of a laser surface modified layer, and 2) to evaluate the process kinetics, solidification and growth morphologies based on thermodynamic and heat-conduction considerations.
The approach for accomplishing the objectives is to conduct a series of precisely controlled and instrumented laboratory experiments and to fully characterize the resultant microstructural regimes. Using a computational model to assist in evaluating the temperature histories, a correlation will be developed between the compositional and thermal histories and the material phases and physical properties. Once accomplished, this research will expand the current fundamental understanding of laser material interactions to include the more complex microstructural regimes developed as a result of the time dependent thermal and compositional superpositions.
For many military and civilian applications, surface degradation by corrosion or erosion is a limiting factor in the effective service life. This is true in particular for many metallic alloys and when such materials are placed in a chemically hostile atmospheric environment where extensive corrosion can result with its associated problems. For existing large-scale structures, such as the Arnold Engineering Development Center (AEDC) wind tunnels, replacement of these alloys with non-corrosive substitutes is cost prohibitive, exceeding $100 M for AEDC, and so other means of treating this problem are being sought.
One option to replacement of the entire structure is to transform the affected surface layer into a non-corrosive alloy. Although at first consideration this would seem unfeasible, the technology developed by The Center for Laser Applications at UTSI, called Laser Induced Surface Improvement (LISI), performs just such a transformation in material surfaces by melting a thin layer and then alloying it with metallic elements to produce a new, more desirable alloy. Since the melting occurs rapidly and only at the surface, the bulk of the material remains cool, thus generating rapid self-quenching and solidification, creating a firmly attached layer of the new alloy. This layer has the enhanced properties of the new alloy.
The equipment capabilities and the multidisciplinary nature of The Center for Laser Applications made development of this process feasible for the Arnold Engineering Development Center. Laser engineers, metallurgists, mechanical and aerospace engineers, and physicists worked together to bring all aspects of the project into focus and develop the system to prove feasibility to AEDC. A demonstration system was designed, tested at UTSI, and transported to AEDC for actual LISI system operation in a segment of wind tunnels at AEDC. Three patches were produced, all of them directly over rust (a significant advantage of the LISI process) and on a wire-brushed surface. The sections will be evaluated during the summer and the decision made on building the production system. When the production LISI processing system is put into operation at AEDC, it will provide a cost reduction of a factor of 10 in the cost to refurbish the wind tunnels. The program continued in the 96-97 year with efforts being aimed at further refinement of the metallurgical properties of the LISI surface. In addition, UTSI is supporting two small businesses which have been selected through the AEDC SBIR program to pursue Phase I projects for development of a system for the application of LISI in the AEDC facilities.
As a path to implementing LISI-based duct refurbishment, AEDC is funding SBIR projects for developing an autonomous robotic application device. UTSI is providing technical support to RedZone Robotics within this program. A first generation wall crawler-type device was brought to AEDC for demonstration in the air ducts. This device was connected to the CLA 3 kW Nd:YAG laser and demonstrated successful application of LISI at UTSI by the use of the Redzone Fury Robot.
The University of Tennessee Space Institute (UTSI) and the Johnson Controls, Inc. (JCI) - Corporate Technology Division have established a partnership for the research and development of laser applications for the JCI Automotive Systems Group (ASG). The ASG represents four (4) plants in Tennessee and Kentucky which produce components for the automotive OEMs located throughout Tennessee and the Southern U.S. Since JCI recently terminated a laser development relationship in Milwaukee which resulted in the availability of a 2.0 kW Nd:YAG laser system, this system, in combination with the expertise of CLA and the proximity of the JCI plants to UTSI, provides an excellent foundation for an effective development program. The program will consist of three main activities:
The first project to be performed is associated with the seat belt housing for the Athens, TN ASG plant.
Bendix Atlantic Inflator Company (Baico), Knoxville, Tennessee, is a major manufacturer of Automotice Airbag Systems. Laser welding is used in the manufacturing process to provide a pressure vessel closure. Maintaining weld quality is critical since hermiticity has to be maintained over the lifetime of the vehicle. The Laser Materials Processing Group within The Center for Laser Applications has been working closely with Baico over the past three years to help them implement and optimize laser welding on the shop floor. Specific tasks undertaken during the last 12 months have included:
Future work will concentrate on assisting Baico with the Ford Project which is scheduled to go into production in early-1977.
The Center for Laser Applications has over the past four years been developing a new technique for laser drilling which yields holes that are substantially free of spatter. Unlike existing methods which attempt to prevent the spatter from adhering to the component, the new technology is based on modifying the drilling mechanism such that the molten material from the hole is ejected at high velocity away from the component. The technology is applicable to both blind holes and through-holes. In the case of Titanium, the technology is particularly effective since it does not require the use of a reactive assist gas -- that is drilling is carried out using Argon. Potential benefits of applying this technology include:
Discussions are currently being held with several companies concerning the use of this technology for their applications.
CLA is participating in this program by helping Tennessee industry appraise, implement and optimize the use of Laser Aided Manufacturing (LAM), the goal of this program being to manufacture better, faster and cheaper in order to maintain our global competitiveness. While in the past our work was mainly concerned with helping existing users of LAM, an effort has been made this year to identify companies that should be using LAM and then to provide the information and methodology necessary for them to carry out a preliminary technical and economic appraisal. To this end we have been working with the Center for Industrial Services' (CIS) director of marketing to promote the use of LAM by publishing and distributing technical literature and holding regional seminars. The goal for 1998 will be to establish a permanent TMEP fellowship at UTSI.
CLA's work with Tennessee Industry has revealed a need to disseminate information on Laser Aided Manufacturing to potential Industrial users. An Industrial survey showed that currently the primary source of information is from equipment vendors; this information is often biased, inaccurate, incomplete, and does not provide the engineer with tools required to identify and appraise an application. We therefore have written a technical bulletin entitled " A Practical Guide To Identifying And Appraising Applications For Laser Aided Manufacturing" which will be distributed Statewide. In addition we are planing a series of regionalized one day seminars, the first of which will be given in Jackson in October 1997.
