Metallographic preparation of particulate filled aluminum metal matrix composite material

A method of preparing a particulate filled aluminum metal matrix composite aterial so that it can effectively be examined microscopically and including first rough grinding the sample to produce a flat surface, secondly fine grinding the sample, thirdly rough polishing the sample and fourth and final polishing of the sample to produce a sample with a quality flat and polished surface that can be microscopically examined.

BACKGROUND OF THE INVENTION 
In the past, standard metallographic preparation methods have not been 
acceptable in that their polishing procedures rounded the silicon carbide 
particles and left them in relief and the aluminum was not scratch-free 
and clean using these processes. Accordingly, there is a need for a 
metallographic preparation method for preparation of particulate filled 
aluminum metal matrix composite material having silicon therein. 
Therefore, it is an object of this invention to provide a new preparation 
method that enables one to obtain a flat, scratch-free, representative 
sample of a subject aluminum metal matrix composite material for 
microscopic examination and evaluation. 
Another object of this invention is to provide a preparation method in 
which simple grinding and polishing steps are provided but yet accomplish 
precision samples for examination and evaluation. 
A further object of this invention is to provide a method which brings out 
the shape and size of the individual particulate in the aluminum metal 
matrix in the composite material. 
Still further object of this invention is to provide a preparation method 
in which the silicon carbide particles are not left in relief and a 
process in which the aluminum is left scratch-free and clean. 
Other objects and advantages of this invention will be obvious to those 
skilled in this art. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a metallographic preparation method is 
disclosed for the preparation of particulate filled aluminum metal matrix 
composite material having approximately 25 percent silicon therein. The 
method includes first cutting the sample to size, mounting the sample in a 
plastic to provide a holder for the sample, then rough grinding the sample 
on a belt grinder, next fine grinding the sample on a multiplicity of 
different grit size silicon carbide paper, rough polishing the sample 
after fine grinding using a flat aluminum wheel with nylon polishing cloth 
and diamond bort paste thereon and finally final polishing the sample 
using a 3-step final polishing procedure to produce a sample of fine 
quality which is ready for microscopic examination to determine quality 
and completeness of the polishing method.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, a sample specimen 10 in accordance with this 
invention includes an aluminum metal matrix composite material that is 
filled with silicon carbide. A specific specimen 10 includes for example, 
6061 aluminum alloy with 25 volume percent silicon carbide. A method for 
providing a smooth and flat surface of the specimen is needed in order to 
adequately determine and examine the shape and size of individual 
particulate of the aluminum metal matrix in the composite material. In 
order to properly prepare the surface of sample 10 for appropriate 
examination, the special method as set forth herein and as illustrated in 
the flow chart of FIG. 1 has been found to provide an appropriate method 
for preparing sample surface for proper examination. In accordance with 
this invention, sample 10 which has been cut from a parent metal material 
is incapulated in suitable plastic such as a plastic selected from either 
thermo-setting or epoxy resin material 12 and specimen 10 is mounted in 
material 12 prior to polishing. With specimen 10 mounted in plastic 12, 
the metallographic preparation method for surfacing specimen 10 is divided 
into four stages: 
1. Rough Grinding 
2. Fine Grinding 
3. Rough Polishing 
4. Final Polishing 
Stage I--Rough Grinding 
Sample 10 mounted in holder material 12 is rough ground on a belt surfacer 
at approximately 1725 rpm using a 120 grit silicon carbide belt with water 
and water soluable oil as a libricant with sample 10 being ground until 
the surface is flat and there is no evidence of marks or scratches from 
previous cutting or other marks on the upper surface of specimen 10. The 
purpose of this rough grinding stage is to remove most of the disturbed 
metal resulting from cutting the specimen from the sample material and to 
obtain a flat surface on specimen 10 prior to the fine grinding stage set 
forth hereinbelow. Once sample 10 is rough ground the sample is cleaned by 
running water thereover. 
Stage II--Fine Grinding 
Fine grinding is accomplished in four steps with a pregrinder and 
lubricants as described below: 
______________________________________ 
PAPER LUBRICANT 
______________________________________ 
Step 1 - 220 grit silicon carbide 
Water 
Step 2 - 400 grit silicon carbide 
Water 
Step 3 - 600 grit silicon carbide 
Water 
Step 4 - 1000 grit silicon carbide 
Water 
______________________________________ 
To fine grind sample 10 step 1 is accomplished by moving sample 10 back and 
forth on the 220 grit silicon carbide paper with water lubricant flowing 
over the paper with the back and forth grinding taking place on the 
pregrinder until the 220 grit silicon carbon paper removes scratches 
and/or deformities on the surfaces of the sample and especially in the 
direction of the back and forth movement of the sample. Sample 10 is next 
rotated 90 degrees and moved back and forth on the 400 grit silicon 
carbide paper with water flowing over the silicon carbide paper until the 
sample has scratches and/or other deformities removed from the surface of 
the sample in the direction of the back and forth movement on the silicon 
paper. With step 2 complete, sample 10 is again rotated an additional 90 
degrees in the same direction as previous and moved back and forth on the 
600 grit silicon carbide paper with the water lubricant flowing thereover 
until scratches or other informalities in the surface are removed with the 
sample being moved back and forth in this particular direction. Finally, 
sample 10 is rotated an additional 90 degrees to complete the rotation 
cycle of the sample and moved back and forth on the 1000 grit silicon 
carbide paper with water lubricant flowing thereover until the scratches 
and/or informalities of the surface are removed in this back and forth 
direction also. 
The purpose of the rough grinding stages are to obtain the best possible 
surface that is scratch-free, flat, and relative free of disturbed metal. 
Sample 10 is cleaned after each fine grinding step with running water 
thereover before perceeding to the next finer grit as specified in steps 1 
through 4. 
Polishing--The polishing operation is accomplished by lapping the face of 
sample 10 on a flat wheel of a diameter of about 8 inches and in which the 
wheel is cloth-covered and impregnated with the proper polishing media as 
set forth in the specific polishing steps hereinbelow. Also, the wheel is 
lubricated with a compatible fluid as set forth hereinbelow. 
Stage III--Rough Polishing 
Rough Polishing is done on a flat aluminum wheel of about 8 inches in 
diameter and is covered with a woven nylon polishing cloth charged with 
three (3) micron diameter bort paste. Lapping oil is also used as a 
lubricant on the wheel. Sample 10 is then polished for approximately 20 to 
30 seconds at a wheel speed of approximately 300-400 rpm. Sample 10 is 
then cleaned with liquid detergent and running water. 
Stage IV--Final Polishing 
Step 1. Polishing of sample 10 in this step is done on a flat aluminum 
wheel of about 8 inches that is covered with a woven nylon polishing cloth 
charged with one (1) micron diameter bort paste. Also, lapping oil is 
again used as a lubricant. Sample 10 is then polished at this fine 
polishing step for approximately 15-25 seconds at a wheel speed of 
approximately 300-400 rpm. Sample 10 is then cleaned after this polishing 
step with liquid detergent and running water. 
Step 2. Step 2 polishing is done on a flat aluminum wheel of approximately 
8 inches and is covered with a microcloth impregnatd with a water slurry 
of 0.3 micron alumina powder and a liquid detergent. Step 2 polishing of 
sample 10 is carried out for approximately 10-15 seconds at a wheel speed 
of 700-800 rpm. Sample 10 after polishing step 2 is again cleaned with 
liquid detergent and running water. 
Step 3. The third and last step in the fine mechanical polishing of sample 
10 is accomplished by using a flat aluminum wheel of approximately 8 
inches and covered with a microcloth impregnated with a slurry of 
distilled water and commercial grade magnesium oxide. Sample 10 is 
polished for approximately 2 to 5 minutes at a wheel speed of 
approximately 150-250 rpm. After this polishing step, sample 10 is again 
washed in warm running water, rinsed with alcohol and dried by hot air 
from a heat gun. Sample 10 is now ready for microscopic examination to 
determine quality and completeness of the polishing process. 
Photomicrograph of a sample processed as set forth hereinabove shows 
fracture area of a tensile specimen at 500 diameters. This method of 
processing has been found very effective in preparing particulate filled 
aluminum metal matrix composite material that has been filled with 
silicon.