In modern industry, ceramic materials applications have become increasingly important. Especially, ceramic materials are being considered for high-temperature structural applications, such as heat exchangers, heat turbines, heat engines and magnetohydrodynamic (MHD) generators. Materials used in these systems experience high-temperature corrosive environments. SiC, Si3N4 and SiC/Al2O3 currently appear to be leading candidate materials because of their high-temperature strength, high thermal conductivity, low thermal expansion and high thermal shock resistance. In particular, SiC particulate-reinforced Al2O3-matrix composites have the potential to combine the corrosion resistance of the alumina with the excellent mechanical and thermal properties of SiC. In spite of these unique properties, it has been reported that SiC, Si3N4 and SiC/Al2O3 are unstable under condensed, hot alkali environments. A possible economical remedy to this corrosion problem is a protective coating on the ceramics without having to change the properties of the bulk material.
In view of this situation, a technique termed Laser Induced Reaction Coating (LIRC) was successfully developed and demonstrated to coat SiC/Al2O3 with Al2O3. As the oxygen diffusion coefficient in Al2O3 is extremely low, it is chemically the most stable oxide ceramic at elevated temperature in an oxidizing environment. In the present technique, alumina coating on the ceramic composite was synthesized using a laser-assisted, in situ reaction process. The technique includes the injection of extremely fine (4 mm size) aluminum as a precursor material into the laser beam at the point of laser-ceramic substrate interaction. The precursor powder was carried by an air-oxygen mixture which also provided the oxidizing environment for the conversion of aluminum into alumina on the surface of the ceramic substrate. By precisely controlling the laser and material parameters, the kinetics of the reaction can be controlled to provide initially a metallic (Al) bond coat at the interface followed by ceramic coating (Al2O3). The bond coat provides favorable wetting conditions for the ceramic coating along with a buffer layer to accommodate thermal and mechanical stresses between two different ceramics. Further, mechanical, chemical and thermal tests demonstrated significant increase in the performance of coated ceramics. The LIRC technique can be extended further to various combinations of materials with careful selection of materials and laser parameters.
Tungsten and tungsten composites have many uses in defense and civilian industry. Unfortunately these materials are among the most difficult to consolidate and shape by classical powder-metallurgy (P/M) processes. Melting and casting are not economically feasible for large-scale production of structural parts. Recent developments in nano-material fabrication and consolidation have encouraged us to conclude that these materials will be much easier (therefore less expensive) to consolidate if they can be fabricated with the correct chemical and physical properties. Consolidation processing of these nano-powders must be accomplished rapidly, in order of minutes, to prevent catastrophic grain growth which will degrade mechanical properties and probably prevent full densification.
As mentioned in the earlier report (CLA Annual Report 1994-95), the successful efforts were conducted in cooperation with Materials Modification, Inc. to demonstrate the ability to synthesize nano-tungsten powder in fairly large quantities 2 lbs/hour in sizes of 7-100 nm in a controlled environment using a commercially available plasma spray gun. This is clearly a highly encouraging step in achieving economically attractive enhanced production rates for large-scale commercialization. Further, three different fast consolidation processes were explored to utilize the unique properties associated with the nano nature of the powder. These consolidation processes included, "Quick HIP", Plasma Activated Sintering (PAS) and "Ultra High Pressure Compaction". Preliminary results from this effort suggest that consolidation of nano-tungsten can be accomplished with minimum growth in grain size using the PAS process. Future efforts will be directed toward optimization of synthesis and consolidation parameters and characterization of the loose and consolidated powders.
Earlier work done within CLA (Jan.-Dec. 1990, U.S. Patent 4,978,601) for the International Lead Zinc Research Organization demonstrated the use of the laser surface modification technique for improved mechanical and corrosion resistance properties of lead alloys. Such improvement in various properties was caused by the high cooling rate and increased solid solubility associated with laser processing which produced refined microstructure with unconventional and nonequilibrium phases. The treatments during this effort were conducted using a CW CO2 laser. In view of these observations, attempts in the present endeavor were further extended to various non-contact energy sources for surface and bulk material treatment of lead alloys. Such non-contact energy sources included lasers (CW CO2, and CW and Pulsed Nd-YAG), plasma torch and infra-red lamps. All lasers demonstrated the feasibility of surface treatments whereas plasma torch and infra-red lamp, being relatively low-energy-density sources, appeared to be suitable only for bulk material processing. These methods are being evaluated for their effects in treated material and also for performance and cost associated with the treatment. The evaluation will indicate the possible implementation of the technique in large-scale commercial production related to lead-acid batteries.
Figure 1: Welding Battery Terminal Using a Focussed Infrared Lamp.
Arc heater segments fabricated from oxygen-free high- conductivity copper (OFHC) need an effective means of dissipating high heat flux loads from the inner bore while maintaining electrically non-conducting surfaces. Smooth, ultrahard coatings that have good thermal conductivity and little or no electrical conductivity can protect these surfaces by preventing arc erosion. In this initial effort, in cooperation with Materials Modification, Inc., methods such as pulse electrode surfacing (PES) and plasma-assisted sintering (PAS) were explored to coat low-carbon steel and OFHC copper with aluminum nitride (AlN), titanium diboride (TiB2), and boron nitride (BN).
Commonly, AlN is deposited with the sputtering technique, however, the stoichiometry and adhesion of these coatings are difficult to control. A particularly effective process for applying thin (5-10 mm), metallurgically bonded layers to metal is PES. The process uses short-duration, high-current electrical discharge pulses to deposit a consumable material onto a metallic substrate in presence of a shield or reactant gas. The variables substrate and electrode materials, reactant gag and power settings dictate the thickness of the deposited layer. The PES generates very little total heat, thus virtually eliminating thermal distortions. Bulk substrate material remains near ambient temperature ensuring no metallurgical changes except those in the fusion zone. The relatively long time between the fusion heating cycles allows the small puddle to solidify rapidly because of the conduction of the heat provided by the cool metal substrate. The result is an extremely fine-grained deposition layer that approaches, and in some materials is, an amorphous structure.
The application of boron nitride in either of its forms (hexagonal or cubic) to a substrate as an adherent layer is difficult because of its strong covalent bonding. To overcome conductivity the difficult energetics involved with obtaining a sound bond between the hBN or cBN coating and the OFHC copper substrate, a second, novel sintering technique of PAS was employed. In this method cBN and hBN powders were placed on an OFHC substrate and placed in a die. Pressure was applied (36-45 MPa) and at the same time a plasma was activated to facilitate sintering of the powders and bonding to the OHFC substrate. The plasma cleans the surfaces of and between the particles and leads to an abnormally clean surface that can be sintered easily at low temperatures, low pressures and short times.
In the next phase of this effort, the coatings will be thoroughly characterized for their chemical, physical and mechanical properties.
During diffusion bonding, opposite surfaces must be energetically activated and brought within the range of their mutual short-range interatomic attractive forces in order for the normal interatomic cohesive forces of metals to result in strong adhesion at this interface. In view of this requirement, surface preparation is one of the important independent variables. In the present effort, the laser surface-modification technique was employed to bring about the changes within the surface and subsurface regions favorable for diffusion bonding. The surface treatments were conducted on nickel based alloy 690 using CW CO2 and pulsed Nd-YAG lasers. The treatment conducted using Nd-YAG laser, caused by its short wavelength and pulsed mode of operation, produced a precipitate- free, chemically homogeneous volume in the surface and sub- surface regions. This volume possessed extremely fine grain structure thereby providing large grain boundary area. These chemical and physical changes appeared to put the regions at a thermodynamically higher energy level compared to the rest of the bulk material under it. When this energetically higher level, laser- treated surface is subjected to conventional diffusion bonding via hot isostatic pressing (HIP), the processing parameters such as temperature, pressure and time appeared to drop by 20%. Such reduction in the processing parameters demonstrate strong promise for large-scale, economical commercial production.
The scale of convective effects that occur during dendritic solidification ranges from microscopic at the dendrite tips and arms, to macroscopic within the bulk fluid. Historically, such convective behavior has received much less attention in the dendritic system than it has for the planar case and the micro-scale convective regimes of the mushy zone and the diffusion layer are not yet adequately characterized. The relative roles of the macro and micro effects and how this relationship changes with gravity level has also not been defined.
The CAST experiment results illustrated the role of convection on the secondary arm evolution during unidirectional dendritic solidification. In particular, the research investigated an anomaly which exists between results from prior earth and microgravity alloy solidification experiments. As early as 1975, if not earlier in the Skylab welds, there has been an apparent enigma when microgravity secondary arm spacings were compared with their corresponding ground-based spacings. In some instances, the microgravity spacings were larger and in other instances the spacings were smaller than those achieved on the ground. Although some attempts have been made to explain this disagreement, a satisfactory inclusive theory has not been proposed.
The experiment flew in the Fluids Experiment System (FES) on the International Microgravity Laboratory-1 where it experienced 10-5 ge's. Identical (except for gravity level) experiments were conducted in earth laboratories. A 28.5 wt% NH4Cl-H2O solution was sealed in an optical-quality quartz cuvette. Thermoelectric devices (TED's) placed at the top and bottom (16 mm 5 8 mm) surfaces provided the capability for controlled heating and cooling.
Bridgman directional solidification was accomplished by simultaneously ramping the top and bottom TED's at a linear rate varying from 1.25x10-3 to 2.22 x10-2 K/s with 5, 10, and 15 K/cm applied temperature gradients for a series of eleven experiments. Single and double exposure holograms were taken at selected intervals during the solidification. Both the microgravity (10-5 ge) and one-gravity experiments were performed using the same cuvette/solidification apparatus and experiment conditions. Eight of the flight experiments were successfully completed and produced a sufficient number of satisfactory holograms for analysis.
Figure 2 presents the data for both the flight and ground- based experiments on a ln/ln scale of secondary arm spacing versus local solidification time. Applying a linear curve fit to the data and using the relationship
d_s = bt_s^n
where ds is secondary arm spacing, b is a constant, and ts is local solidification time, the exponent n is determined to be 0.32 for the flight experiment and 0.16 for the ground-based experiment. Therefore, it appears that the flight experiment follows the predicted relationship of n @ 1/3 for secondary arm spacings as a function of local solidification time whereas the ground-based experiment does not.
Since mass transport between coarsening dendrite arms is generally considered to be caused by interdendritic diffusion, convective fluid motion which impedes the diffusion couple between arms, will reduce the extent of coarsening and result in smaller arm spacings such as obtained on earth. This is illustrated in Figure 3, which describes the diffusion field and resultant coarsening for dendrites in stagnant and flowing liquid. When fluid flow is present, it is the gradient between the individual dendrite solutal field and the incoming fluid composition that determines the extent of coarsening.
It was, therefore, concluded that the flight arm spacings are those obtained from a coarsening mechanism unaffected by fluid flow, while the ground arm spacings are smaller if buoyancy-driven fluid flow occurs and acts in such a way as to reduce the coarsening. The sensitivity of each dendrite arm's diffusion layer gradients to the incoming fluid concentration suggests that convective effects can be important even at low levels of convective motion. This is significant for coarsening studies performed on the ground, for which some level of convective motion is almost assured. The assistance of a baseline case, such as accomplished in microgravity, is therefore useful.
The new high-temperature superconducting materials offer significant advantages for the conservation of energy, particularly in the power generation and transportation industries. During the past year, UTSI began a DOE program to assist in bringing these ³laboratory² 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 ustained plasma synthesis, to minimize the time and complexity of processing.
The high temperatures and pressures in AEDC hypersonic test facilities give rise to challenging materials problems. This project continues to provide materials-related support to the Impulse and, since January 1996, the Arc Heater facilities at AEDC.
The Impulse facility is a free piston shock-tunnel capable of providing operating stagnation pressures greater than 1,000 atm. The primary task in support of the Impulse Facility has been the evaluation of laser processing techniques suitable for improving the performance of refractory materials used in the Impulse Facility nozzle. Operating conditions have been identified, and equipment necessary for processing a facility nozzle has been purchased or developed. This task will be completed in August 1996 with the processing of a W-20Cu nozzle.
The AEDC Arc Heater Facility provides high enthalpy flow conditions by heating a flow stream with a 65 MW electrical arc. The H-1 and H-3 segmented arc tunnels operate under severe conditions for materials. Although arc rotation and back-side water cooling are used to minimize thermal wear effects, the high energy density of the arc attachment creates the potential for erosion of electrode material. The attachment location is especially important for the cathode in which the arc tends to attach downstream of the electrode center. This is undesirable because the water cooling is less effective, and edge effects can become important if the attachment is near the downstream end of the cathode. Erosion increases the frequency of electrode replacement, the likelihood of component failure, and contributes to flow contamination. Thus, it would be advantageous to have some mechanism by which arc attachment could be controlled so that material erosion was minimal and optimum facility operating conditions could be obtained.
In general, the attachment dynamics of high power arcs is not very well understood, and current work at AEDC includes investigations of both electrode design and material selection as contributing factors to arc attachment. It is also expected that fluid dynamics also influences the attachment characteristics, as the airstream is high velocity and high temperature with locally high thermal gradients. Ongoing work includes electrode designs with ridged and flat surfaces and investigation of high temperature coating materials which have desirable combinations of electrical and thermal properties. AEDC technology programs are underway to examine the influence of the different electrode designs, and recent trials with the flat surface design have shown some improvement with arc attachment closer to the electrode center. Other AEDC work has examined the feasibility of applying high temperature, electrically insulating coatings to OFHC copper. The work underway at UTSI is an integral part of this ongoing effort as it addresses the problem of high electrical conductivity (low work function), high thermal conductivity coatings and builds a material characterization test capability.
A main objective of the UTSI tasks supporting the Arc Heater Facility is to provide an electrode surface which is more likely to have arc attachment than the base OFHC copper. In the course of meeting this objective, a more general testing capability is being developed which will also be used for testing of electrically non-conductive surfaces. This will enable cost-effective comparison of potential coating materials and/or application techniques for both electrically resistive and conductive coatings before testing in the Arc Heater facility.
A vacuum plasma discharge test device shown in Figure 4 has been used to illustrate the difference in performance between coated and uncoated OFHC copper. Figure 5 is a comparison of the discharge resistance for two silver coatings and OFHC copper at three applied voltages. Currently, an ambient test apparatus is being developed for testing material coatings in an air environment. A selected coating will be tested in the Arc Heater facility later this year.
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 operation in a segment of their wind tunnels. Figure 6 shows the UTSI investigator, Dr. John Hopkins, with the LISI system in the AEDC wind tunnel duct. 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.
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.
In 1995, the Center for Industrial Services (CIS) requested The Center for Laser Applications to provide assistance to Sandvick to investigate the use of Laser-Aided Manufacturing applied to the production of chainsaw bars. A site visit was made with the CIS Field Engineer to the Milan Plant. This visit revealed an urgent need to find an alternative method of depositing Stellite (a hardfacing alloy) onto the nose of the chainbar, the current method using oxy- acetylene torches being slow and inefficient, but worst of all requiring a dangerous manual-grinding finishing operation. Initially the use of laser cladding was investigated but this proved not to be technically viable. A second visit was made to the company and it was proposed to Sandvick that a radically different approach be taken. Namely, to laser-weld two stellite inserts onto the nose of the chainbar; the major potential benefits of this approach being a reduction in production cost and the elimination of the dangerous machining operation. Laser welding trials were subsequently carried out by the laser materials processing group in The Center for Laser Applications the welds were subjected to analysis and shown to be metallurgically sound. Based on these initial trials, a welding fixture was built and a C.N.C. Program written to enable actual components to be welded. Welding trials were carried out (Figure 7a and Figure 7b) with the Sandvick Engineer in attendance which not only confirmed technical viability but showed that the production cycle- time could be cut from an average of three minutes to less than one minute with Stellite alloy. These economic benefits together with the elimination of a dangerous operation make the laser welding method very attractive. The Center for Laser Applications will continue to assist Sandvick as they plan to implement Laser-Aided Manufacturing in 1997.
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.
