This task focused on liquid phase sintering and infiltration studies of refractory metals and metal composites. Particular emphases was placed on those powered metal compacts which produce liquid alloys in sintering. For this class of materials, heating to a two phase region causes the constituent components to react, forming an alloy liquid which must wet the solid phase. Densification is initially driven by the free energy effects which cause rapid rearrangement. Further densification occurs by evaporation and condensation, surface diffusion, bulk flow, and volume diffusion

Lillybeck, N. P.

Smith, James E., Jr.

English

NASA Technical Reports Server

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Center: Consortium f o r  Materials Development in Space 
The University of Alabama in Hunstville (UAH) 
Task Name: "Powdered Metal Sintering and Infiltration" 
Industrial Participant: Teledyne Brown Engineering 
300 Sparkman Drive 
Huntsville, AL 
Tripty Mookherji 
The University of Alabama in Huntsville 
James E. Smith. Jr. 
Annual Report: September 15, 1986 to September 14, 1987 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
- 8  Center: Consortium f o r  Materials Development in Space 
The University of Alabama in Hunstville (UAH) N8 8 - I 1 8 7 0 
Task Name: "Cast Iron Freezing Mechanisms" 
Industrial Participant: Deere and Company 
Technology Center 
3300 River Dr. 
Moline, IL 61265 
N. P. Lillybeck 
The University of  Alabama in Huntsville 
James E. Smith, Jr. 
Annual Report: September 15, 1986 to September 14, 1987 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  
Introduction 
These two tasks are being worked by the same investigator in his metals lab- 
oratory using essentially the same equipment. Consequently, they are being doc- 
umented in one report rather than in separate reports. 
This task has focused on liquid phase sintering and infiltration studies of 
refral ory metals and metal composites. Particular emphasis is being placed on 
those powdered metal compacts which produce liquid alloys on sintering. For 
this class of  materials, heating t o  a two phase region causes the constituent 
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components to react, forming an alloy liquid which must wet the solid phase. 
Densification is initially driven by the free energy effects which cause rapid 
rearrangement. Further densification occurs by evaporation and condensation, 
surface diffusion, bulk flow, and volume diffusion. 
In unit gravity, sedimentation causes stratification within the sample that 
results in a non-uniform coarsening of microstructure. Sintering, in a reduced 
gravity environment, should produce a smaller grain size, a more uniform 
microstructure possibly with less voids o r  defects, and improved mechanical pro- 
perties. 
Many of the most interesting liquid phase sintered materials generally 
include a refractory metal or ceramic which is sintered or infiltrated at 
temperatures in the range of 1000 - 1800°C. this requires a furnace capable of 
melting such materials while maintaining an inert o r  slightly reducing atmos- 
phere during the sintering o r  infiltration operation. 
Potential uses for liquid phase sintered products include bearings, magnetic 
materials, electrical brushes and contact points, cutting tools, irregular 
shaped mechanical parts for high stress environments, and possibly new and 
improved catalysts for chemical production. Space processing in the areas of 
uniform cutting tools, magnetic materials, and supported inter-metallic alloy 
catalysts offer the greatest research and development potential. 
Furnace Completion 
The construction and retrofit of a TEM-PRES corporation high temperature 
electric furnace has been completed. Only minor technical problems, as will be 
described below, remain u n r e s n l v ~ d .  T h i s  furnace alonp: with numerical modeline: 
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of both isothermal and directional solidification modes, has increased the over- 
all understanding of furnace design, application, control, and integration for 
space processing systems. 
Updated Furnace Control Program 
An updated control program was developed for the high temperature sintering 
furnace. This program was developed to provide complete interactive or auto- 
matic control of the furnace as needed. Since this furnace works at the upper 
margin of electrical furnace performance, an adaptive computer control scheme 
was required for rapid heating during initial stages, with more critical control 
as the furnace achieves its final operating temperature. The program writes a 
complete experimental history of the furnace operating parameters and processing 
temperatures to computer disk. 
Initial Furnace Performance Testing 
The furnace O-ring seals, failure of which was discussed in a previous 
report, were replaced and tested, achieving the desired performance. The fur- 
nace was heated to 250°C t o  test the control program and power systems. To 
date, we have limited the testing to operating temperatures below 300°C. To 
operate the furnace at temperatures above 300°C. a hydrogen reducing environment 
must be used or the molybdenum heating coils will oxidize, becoming unusable. 
For safe operation approximately 50 cc/min of 5% H2-95% N2 or 5% H2-95% He gas 
mixture must flow over the coils. Attempts were made to find rotameters with 
small capacity to mix pure H2 and inert carrier gases. The only commercially 
available flowmeters found capable of providing the hydrogen rates were mass 
flowmeters. Due to the high cost of these flowmeters, a decision was made to 
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use premixed gases. This required a slight modification of the gas regulation 
system to permit the use of premixed gases. With this modification now 
complete, tests will be conducted near the furnace limits. 
Several hardware and software modifications were accomplished to improve 
both furnace temperature control and positioning. A linear bearing guide was 
designed, constructed, and installed to improve crucible alignment within the 
furnace, increase the maximum furnace translation rate, and eliminate potential 
damage to the crucible during furnace translation. Several moderate temperature 
experiments were conducted at temperatures less than 800°C to test the validity 
of the temperature control routines. One sintering experiment was attempted on 
a precompacted AP sample. It was determined after this experiment that addi- 
tional data will be required on both positioning and temperature profiles. To 
expand the number of temperature measurements, preliminary testing of an addi- 
tional data acquisition board and signal conditioner was performed. The data 
acquisition board was tested at a communication distance of 26' which is 16' 
beyond the communication distance recommended by the manufacturer. The test was 
successful and will permit the installation of up to 16 additional 
thermocouplers. The temperature measurement, furnace control, and positioning 
programs are being combined into a system program. 
Development of a New Die and Pre-Sintering Environmental Chamber 
A pre-sintering environmental chamber was designed and built to permit pow- 
dered metal compact preparations under vacuum or  a reducing atmosphere at tem- 
peratures higher than the commercially available system. The white iron powder 
studies to be conducted for John Deere, Inc., along with several other materials 
currently being considered, require either vacuum or a reducing environment to 
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reduce oxidation during preparation. The pre-sintering environmental chamber, 
shown in Figure 1, encloses the test cylinder in a controlled environment. A 
high temperature resistance heater, capable of temperatures to approximately 
lOOO"C, surrounds the test cylinder body. 
directions, cartridge heaters were installed in both the base of the chamber and 
plunger extension as shown in Figure 1. The upper operating temperature of this 
prototype configuration will be limited to approximately 600°C since the chamber 
base was constructed from aluminum. To extended the upper temperature range, a 
base made of stainless steel may be needed. Temperature measurement and control 
techniques along with operating procedures are currently being developed for 
this unit. 
To reduce the heat loss in the axial 
A new test cylinder, with a 0.25 inch diameter bore, was constructed to pro- 
vide samples f o r  testing in the Rapid Melt/Quench Furnace, currently under deve- 
lopment, aboard the KC-135. An initial plunger made of 316 stainless steel was 
tested. This plunger would not tolerate the stress created at 5000 lbs applied 
pressure. A modified plunger made of case-hardened steel corrected the problem. 
With the development of the Rapid Melt/Quench Furnace, the test cylinder, and 
pre-sintering chamber, we now have the necessary elements to pre-screen samples 
in a low gravity environment. 
The 0.25 inch bore test cylinder along with the 2 cm bore and 1.25 inch bore 
test cylinders permits the preparation of 0.25 inch, 2 cm, and 1.25 inch diame- 
ter samples up to 7 cm long. These test cylinders can be used in the environ- 
mental chamber to prepare powder metal samples at sintering pressures exceeding 
25 tons, temperatures up to 6 0 O o C ,  and vacuum o r  reducing atmospheres, for use 
in the high temperature furnace. 
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Further, during the next reporting period, we hope to complete the sample 
pre-sintering experimental apparatus, making refactory sintering experiments 
possible. 
Pre-Sintered Sample Preparation 
Powder metal sample preparation procedures have been developed to insure 
sample uniformity. The powder metal is first sieved to separate a specific par- 
ticle size range. The test cylinder body, plunger, and base are then cleaned 
using a caustic solution and rinsed with distilled water. The parts are then 
hot coated with a sodium sterate and water solution to help prevent galling dur- 
ing the pressing operation. The sterate solution was prepared by mixing 18 gm 
of sodium sterate with 500 ml water while heating to 85°C to dissolve the 
sterate. The press parts are immersed in the solution and cured in air prior to 
use. When cured, the pressing parts have a thin film of sodium sterate which 
melts at high temperatures to lubricate the die. 
For each material studied the pre-sintering temperature, applied pressure, 
and pressing time must be determined experimentally. The low gravity reactive 
sintering studies will a require porous refractory metal skeleton of tungsten o r  
tungsten-carbide, which will be coated with and in-situ formed intermetallic 
alloy. The uniformity of the pre-sintered skeletal material is determined by 
measuring the material's void fraction as a function of position. To determine 
the void fraction distribution of the pre-sintered compact a Reichert 
metallurgical microscope has been integrated with New Brunswick Scientific Co., 
Inc. Biotran I11 imaging system, as shown in the photograph in Figure 2 .  
The Biotran I11 is a precision instrument designed to automate particle 
counting and area measi1rement.s i n  an image plane. A high resolution vidicon TV 
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camera system is coupled to a 4-digit numerical display to count particles or 
any distinct separately bounded group of positive or negative image areas in a 
field of view. The Biotran I11 then calculates the .rcentage ratio of either 
positive to negative o r  negative to positive areas in the field of view. To 
fully utilize the Biotran I11 to experimentally measure the void fractional area 
of a powder metal compact, the contrast between the voids and polished metal 
surface must be enhanced. This is accomplished by blacking the voids to enhance 
contrast. 
To test the integrated imaging system, several 2 cm diameter aluminum com- 
pacts were pre-sintered using the environmental chamber at room temperature. 
Three 16 gm A1 samples were pre-sintered at 10,000 lbs applied pressure for 30 
minutes, 1 hour, and 2 hours, respectively. The 30 minute and 1 hour cylindri- 
cal samples were cut in half lengthwise using a metallurgical saw. The samples 
were polished using standard metallurgical techniques, coated with a fine parti- 
cle black paint, and repolished to retain the paint within the pores. The 
Reichert metallurgical microscope and Biotran I11 imaging system was then used 
to determine the porosity of the sample as a function of position. A photograph 
of the aluminum sample as imaged by the above system is shown in Figure 3. 
Darken areas, outlined in white by the image processor  are voids. The Biotran 
1 1 1  ratios the two image fields as discussed above. This ratio was then used to 
determine the void fraction. A representative plot of  porosity as a function of 
position for the 30 minute pre-sintered aluminum sample is shown in Figure 4 .  
Samples were found to be more compact near the plunger end of the sample with 
this difference diminishing as the sintering time was increased. A study to 
determine optimal pre-sintering conditions is currently underway. 
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Future Plans 
With furnace design, interfacing, and operational verification virtually 
completed, initial testing on lower melting sintered materials such as A1-Fe and 
Cu-Si will continue. Once the controls and measurement systems on the environ- 
mental chamber are completed, the John Deere materials along with the reactive 
sintering of Fe-Ti in a refractory matrix of tungsten and tungsten-carbide will 
be studied. 
In coming months, an axial and radial profile of the furnace will be 
obtained at several operating temperatures. These profiles will be measured in 
all three zones, namely the isothermal zone and the cold over hot and hot over 
cold gradient zones. Once mapped, a better understanding of the sample operat- 
ing conditions, gradients, etc. will permit more rapid sample analysis. We will 
continue our test on AL until we can confirm these operating conditions. 
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ORIGINAL PAGE E _ _  
OF POOR QUALITY 
.. ._.,a - . 
~'igure 2. Photograph of the Reichert Metallurgical.Microscope Integrated 
with the Biotran I11 Imaging System. 
Figure 3 Photograph of the Biotran I11 Image of a Pre-Sintered Aluminum 
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Cast iron freezing mechanisms

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