Method of electrochemical machining (ECM) of particulate metal-matrix composites (MMcs)

A method of machining metal-matrix composite materials using electrochemical machining is provided. The method comprises the steps of: providing a metal matrix composite material in an electrochemical machine, and electrochemically machining the metal-matrix composite material in a nitrate or chloride containing electrolyte at a current density of equal to or greater than approximately 1 A/cm.sup.2. Preferably the metal-matrix composite is comprised of an aluminum alloy or pure Al matrix with ceramic particles, such as silicon carbide.

FIELD OF THE INVENTION 
This invention relates generally to a method of electrochemical machining 
(ECM) of particulate metal-matrix composites (MMCs), and more particularly 
to a method of electrochemical machining of particulate aluminum 
metal-matrix composites. 
BACKGROUND OF THE INVENTION 
Metal-matrix composite (MMC) materials are useful in a variety of 
industries. Generally, MMCs are comprised of a variety of ceramic 
materials contained in a metal matrix. The ceramic materials are usually 
in the form of particles or continuous fibers. Particulate MMCs are 
reinforced with ceramic particles. One such MMC of interest is silicon 
carbide/aluminum (SiC/Al) MMC. SiC/Al MMCs are candidate packaging 
materials for electronic components. Low density and a coefficient of 
thermal expansion relatively close to that of gallium arsenide make SiC/Al 
MMCs attractive for electronic component packaging. Moreover, silicon 
carbide/aluminum is a favored particulate metal-matrix composite where 
superior attributes such as hardness and stiffness, strength at elevated 
temperatures, high thermal conductivity, low coefficient of thermal 
expansion and resistance to wear and abrasion are of primary value. 
Unfortunately, the properties that make MMCs attractive also render them 
difficult to fabricate, and very difficult if not impossible to machine by 
conventional cutting methods (i.e., diamond edge tools) since the 
particles dull and abrade cutting tools. For example, when forming an 
electronic package, typically a network of channels and cavities are 
machined into the packing material in which the electronic components are 
to be mounted. SiC/Al MMCs are difficult to machine using conventional 
cutting methods since the hard SiC reinforcement particles abrade and dull 
the cutting tools. MMCs having high silicon carbide content cannot be 
effectively machined using conventional physical machining techniques. 
Currently, particulate MMCs, including SiC/Al MMCs in particular, have been 
machined by conventional cutting methods with limited success. Excessive 
tool wear and subsurface damage to the work piece have prevented the 
cutting method from being widely accepted. In the case of MMCs with high 
SiC particle volume fraction, components cannot be machined and must be 
cast to near net shapes. Casting can be very expensive for small lot sizes 
and can limit the variety of parts to be fabricated. Additionally, powder 
metallurgy and hot-pressing techniques have been employed to produce MMCs; 
however, only relatively simple shapes can be formed by these methods. 
This difficultly in fabrication has severely limited the use of MMCs. 
Thus, the development of alternative machining methods for MMCs is desired 
and would be a significant advance in the art. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a method to 
fabricate MMC components or materials. 
More particularly, it is an object of the present invention to provide a 
method to fabricate MMC components using electrochemical machining 
methods. 
A further object of the present invention is to provide a method of 
fabricating aluminum MMC components containing ceramic particles such as 
SiC. 
A related object of the present invention is to provide a method of 
electrochemical machining of MMC components. 
Another object of the present invention is to provide a method of 
fabricating MMC components that is economical and allows the fabrication 
of complex shapes. 
These and other objectives are achieved by the method herein of machining 
MMC materials comprising the steps of: providing the MMC material in an 
electrochemical machine; and electrochemically machining the MMC material 
in a nitrate or chloride containing electrolyte at a current density of 
equal to or greater than approximately 1 A/cm.sup.2 ; and flushing the 
electrolyte between the electrochemical tool and the metal matrix 
composite material.

DETAILED DESCRIPTION OF THE INVENTION 
In summary, the inventors have discovered that MMC materials can be 
machined using electrochemical machining (ECM). This is a significant 
advance over the conventional methods of fabricating MMCs which involves 
physical machining with cutting tools. When using an electrochemical 
machining method, there is no tool wear and subsurface damage of the MMC 
when operated under correct conditions. 
According to the method of the present invention, a method of machining an 
MMC material is provided. Specifically, the method comprises the steps of 
providing the MMC material in the electrochemical machine; and 
electrochemically machining the MMC material in a nitrate or chloride 
containing electrolyte at a current density equal to or greater than 
approximately 1 A/cm.sup.2. The inventors have found that the metal-matrix 
is electrochemically dissolved and unbinds the ceramic particles in the 
composite which are then flushed away by the flowing electrolyte. 
Turning to FIG. 1, a simplified schematic drawing of an electrochemical 
machine is shown which may be used to carry out the method of the present 
invention. The electrochemical machine is an electrochemical cell. 
Electrochemical machining is an electrolytic technique where a work piece 
is the anode and a machining tool is the cathode. Material is removed from 
the work piece by anodic dissolution. Specifically, the electrochemical 
machine 10 includes an article or material 12 to be machined and an 
electrochemical machining tool 14. According to the present invention the 
material 12 is comprised of a MMC material. The MMC material 12 is the 
anode of the electrochemical cell. The MMC material 12 is machined by the 
electrochemical tool 14 which is a metal piece formed in a shape that 
conforms to the shape to be machined. The tool 14 is the cathode of the 
electrochemical cell. A power supply 16 provides controlled voltage or 
current to the electrochemical cell and is connected to the MMC material 
12 and the tool 14 such that the MMC material 12 is the anode. Preferably, 
the tool 14 includes a layer of insulation coating 15 disposed on the 
outer surface of the tool 14. The insulation prevents unwanted machining 
in regions of the workpiece adjacent to the insulated layer. 
To remove material from the MMC material 12, an electrolyte is employed. In 
general, the electrolyte is an aqueous solution containing ions that carry 
the ionic current between the anode and the cathode. When the current is 
passed though the cell, metal material is dissolved from the anode (i.e. 
the MMC material 12 ) by anodic dissolution in a pattern that generally 
conforms to the shape of the cathode (i.e. the tool 14). The electrolyte 
18 is disposed between the tool 14 and the MMC material 12. The 
electrolyte may be supplied via a bore in the tool 14 (not shown) or by an 
external conduit. Electrolytes finding use in the present invention are 
nitrate electrolytes such as sodium nitrate (NaNO.sub.3). Other 
electrolytes such as potassium nitrate, potassium chloride and sodium 
chloride could be used. Sodium nitrate is the preferred electrolyte and is 
provided in an aqueous solution having a concentration in excess of 0.1M, 
with a 2M NaNO.sub.3 electrolyte giving good results. 
To machine the MMC material 12, the tool 14 is placed close to the surface 
of the MMC 12 and current is applied to the cell via power supply 16. 
Anodic dissolution begins to occur, and cavity 20 begins to form in the 
MMC material 12. Tolerances of approximately 0.05 mm can be achieved by 
maintaining small distances of approximately 0.025 to 0.75 mm between the 
material 12 and the tool 14. The cavity 20 generally conforms to shape of 
the cathode (i.e. tool 14). Electrochemical machining continues until the 
desired shape of the MMC material is formed. According to the present 
invention a current density of equal to or greater than approximately 1 
A/cm.sup.2 is applied. The current density will vary depending on the 
electrochemical machine used. In an exemplary embodiment, a current 
density in the range of 1 to 10 A/cm.sup.2 is used, with a current density 
of 2.5 A/cm.sup.2 being preferred. 
Of particular advantage, the method of the present invention provides for 
the machining of MMC materials. As discussed above, the hardness of MMCs 
makes them difficult to machine by conventional methods. The present 
invention overcomes such limitations. Specifically, the method of present 
invention provides for machining of aluminum MMCs, and in particular, 
silicon carbide/aluminum (SiC/Al) MMCs. MMCs are successfully machined by 
the inventive method, including MMCs having high silicon carbide content 
(i.e. .apprxeq.40% or higher). In particular, MMCs having discrete silicon 
carbide particles of various concentrations may be machined. The MMC may 
be comprised of a variety of metals and ceramics. Preferably, the MMC is 
comprised of aluminum and silicon carbide. The aluminum metal of the MMC 
may comprise an alloy such as 6061 Al or may be comprised of pure aluminum 
and silicon carbide. In addition, the method of the present invention may 
use other particulate aluminum metal-matrix composites that are reinforced 
with different particles such as silicon nitride, titanium diborides, 
diamond, and the like. The ceramic particles are generally inert and do 
not take part in the electrochemical reaction. 
An important parameter in carrying out the method of the present invention 
is the current density at which the electrochemical machine is operated. 
It was believed that the presence of the silicon carbide particles in the 
metal matrix may significantly effect this parameter. To investigate its 
effects, potentiodynamic polarization experiments were conducted to 
determine the breakdown potential of the MMCs, in particular SiC/6061 Al 
composites. MMC materials having 25 and 40 vol. % of SiC, the balance 6061 
Al, were prepared to determine the effects of SiC content and the alloying 
elements in 6061 Al on dissolution characteristics. These MMCs were then 
compared to monolithic 6061 Al, and ultrapure Al (99.999%) materials. The 
mean diameter of the SiC particles was 3.5 microns. Planar electrodes were 
used for the potentiodynamic polarization experiments. The electrodes were 
polished to a 0.05 micron finish and rinsed in 18 M.OMEGA.-cm water prior 
to polarization experiments. A 2 M NaNO.sub.3 solution was used as the 
electrolyte. The electrolyte was prepared by mixing reagent grade 
NaNO.sub.3 and 18 M.OMEGA.-cm water. The electrolyte temperature was 
maintained at 30.degree. C. Potentiodynamic experiments were conducted in 
solutions deaerated with pre-purified (99.99%) nitrogen gas. An EG&G 
Princeton Applied Research () Model 273 Potentiostat was used to 
perform the potentiodynamic experiments. Electrode potentials were 
measured with respect to a saturated calomel electrode (SCE). 
The results of the potentiodynamic polarization experiments are illustrated 
in FIG. 2. FIG. 2 shows a plot of anodic polarization diagrams for the 
various materials. A comparison of the anodic polarization curves shows 
that all of the materials breakdown at approximately 1.75 V.sub.SCE. 
Hence, neither the alloying elements in the 6061 Al material, nor the SiC 
particles had significant effect on the breakdown potential as compared to 
ultrapure Al. Consequently, the SiC/6061 Al metal-matrix composites must 
be machined at potentials exceeding approximately 1.75 V.sub.SCE in a 2M 
NaNO.sub.3 electrolyte. For electrolytes of lower NaNO.sub.3 
concentration, the potential should increase. For chloride electrolytes, 
the potential should be significantly lower (e.g., .apprxeq.-0.7 
V.sub.SCE). 
Of particular advantage, the method of the present invention provides for 
the fabrication of MMCs having desirable surface finishes. Galvanostatic 
experiments in concert with rotating cylindrical electrodes were conducted 
to determine the effect of the electrolyte convention rate and dissolution 
current densities on the surface finish of the machined MMC. The 
experiments use a rotating electrode as an experimental technique to force 
convection at the electrode; however, during the actual machining process, 
electrolyte convection should be induced by other methods (such as 
pressure gradients and the like). Again, MMC materials having 25 and 40 
vol. % of SiC, the balance 6061 Al, were prepared and compared to 
monolithic 6061 Al, and ultrapure Al (99.999%) materials. The materials 
were fabricated as instrument grade MMCs by Advanced Composite Materials 
Corporation (Greer, S.C.). The mean diameter of the SiC particles was 3.5 
microns. A 2 M NaNO.sub.3 solution was used as the electrolyte. The 
electrolyte was prepared by mixing reagent grade NaNO.sub.3 and 18 
M.OMEGA.-cm water. The electrolyte temperature was maintained at 
30.degree. C. An electrolyte of 2M NaNO.sub.3 produces a better surface 
finish than NaCi electrolytes for monolithic Al alloys. The galvanostatic 
rotating-electrode experiments were conducted in solutions exposed to air. 
Cylindrical electrodes were used for the galvanostatic rotating-electrode 
experiments. The cylindrical electrodes had a thickness of 1 mm, and an 
outer diameter of 11.8 mm. The electrodes were polished to a 0.05 micron 
finish and rinsed in 18 M.OMEGA.-cm water prior to polarization 
experiments. An EG&G Princeton Applied Research () Model 273 
Potentiostat was used to perform the galvanostatic experiments for current 
densities up to 1 A/cm.sup.2. Electrode potentials were measured with 
respect to a saturated calomel electrode (SCE). For galvanostatic 
experiments exceeding 1 A/cm.sup.2, a Sorensen Nobartron DCR 80-10 power 
supply was used. The electrodes were rotated using an EG&G Rotating 
Electrode setup. 
The galvanostatic experiments on rotating cylindrical SiC (40 vol. 
%)/6061Al (SiC.sub.40 /6061 Al) MMC electrodes were conducted at a current 
density of 1 A/cm.sup.2 and rotational speeds between 0 and 5000 RPM. At 
low convection rates (corresponding to a rotation speed with approximately 
100 RPM), hydrogen bubbles adhered to the MMC material and impeded 
dissolution, producing a nodular surface finish which was visible to the 
naked eye. Rotational speeds exceeding 1000 RPM were required to sweep 
away hydrogen bubbles. To produce desirable surface finishes, the 
electrolyte convection rates must be sufficient to flush away hydrogen 
bubbles that adhere to the surface of the SiC/6061 Al MMC electrodes. The 
electrolyte convection rates will vary with the electrochemical machine, 
but what is important is to achieve a rate that flushes away the hydrogen 
bubbles. The bubbles impede dissolution and caused a nodular surface 
finish to develop. At electrode rotational speeds exceeding 1000 RPM, the 
nodular surface was eliminated. Thus, rotational speeds greater than 1000 
RPM produced the best results during the experimental tests. 
The effect of current density on surface finish was studied at a rotational 
speed of 5000 RPM to eliminate interference caused by hydrogen bubbles. 
The galvanostatic experiments were conducted on SiC.sub.40 /6061 Al MMCs 
for current densities ranging from 0.4 to 10 A/cm.sup.2. Experiments were 
also conducted on monolithic 6061-T6 Al and ultrapure Al materials. For 
the SiC.sub.40 /6061 Al MMCs, a dark gray finish was obtained at current 
densities below approximately 3 A/cm.sup.2. The centerline average surface 
roughness (E) was approximately 4.3 microns at 0.4 A/cm.sup.2, and 
decreased to approximately 1.8 microns at 2.5 A/cm.sup.2. A transition to 
a light gray surface finish occurred above 3 A/cm.sup.2. At current 
densities slightly above 3 A/cm.sup.2, the roughness was approximately 2.4 
microns and pits began to develop on the surface and increased in number 
at higher current densities. FIGS. 3 and 4 show SEM micrographs of two 
SiC.sub.40 /6061 Al metal-matrix composite electrochemically machined at 
5000 RPM in aerated 2M NaNO.sub.3 electrolyte at 30.degree. C. In FIG. 3, 
the composite was machined at 2.5 A/cm.sup.2, and in FIG. 4 the composite 
was machined at 10 A/cm.sup.2. As illustrated by the surface finish in 
FIGS. 3 and 4, the preferred current density for machining the SiC.sub.40 
/6061 Al MMCs is approximately 2.5 A/cm.sup.2 for convection rates 
corresponding to a rotational speed of 500 RPM. It is likely that higher 
current densities will also result in smooth surface finishes if the 
convection rates are increased. 
Since the SiC particles were inert, there is a lower limit for the surface 
roughness which is a fraction of the SiC particle size. A mirror finish 
was obtained for the monolithic 6061-T6 Al and the ultrapure Al specimens 
for current densities ranging from 1 To 10 A/cm.sup.2. No distinct 
transitions in surface finish were observed for the 6061-T6 Al or the 
ultrapure Al over the range of current densities. It can be concluded, 
therefore, that the SiC particles play a major role in controlling the 
surface roughness to the extent that the roughness may exceed the SiC 
particle size. 
The surface finish of the SiC.sub.40 /6061 Al MMC was significantly more 
sensitive to dissolution current density than that of either monolithic 
6061-T6 Al or ultrapure Al. The best surface finish for SiC.sub.40 /6061 
Al MMC at convection rates corresponding to 5000 RPM were achieved at a 
current density of approximately 2.5 A/cm.sup.2. It is likely that the 
excellent surface finish can also be achieved at current densities 
exceeding 2.5 A/cm.sup.2 if convection rates are increased. Both 6061-T6 
Al and ultrapure Al had mirror finishes for current densities ranging from 
1 to 10 A/cm.sup.2. 
The foregoing description of specific embodiments and examples of the 
invention have been presented for the purpose of illustration and 
description. They are not intended to be exhaustive or to limit the 
invention to the precise forms disclosed, and obviously many 
modifications, embodiments, and variations are possible in light of the 
above teaching. It is intended that the scope of the invention be defined 
by the claims appended hereto and their equivalents.