U.S. patent application number 11/416921 was filed with the patent office on 2007-11-08 for dry machining of soft metal-modified aluminum castings with carbon-coated tools.
Invention is credited to Carolina C. Ang, Yang T. Cheng, Jean M. Dasch.
Application Number | 20070256763 11/416921 |
Document ID | / |
Family ID | 38660153 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070256763 |
Kind Code |
A1 |
Ang; Carolina C. ; et
al. |
November 8, 2007 |
Dry machining of soft metal-modified aluminum castings with
carbon-coated tools
Abstract
Additions of small but effective amounts of one or more of
bismuth, indium, lead and/or tin to a silicon-containing aluminum
casting alloy markedly improves the dry machinability of a casting
made from the modified alloy. But dry machining of such castings is
synergistically improved by the use of cutting tools with cutting
surfaces coated with microcrystalline diamond or diamond-like
carbon coatings. The use of such carbon-coated tools enables
remarkable improvement in dry machinability even at reduced levels
of the soft metal in the alloy.
Inventors: |
Ang; Carolina C.;
(Bloomfield, MI) ; Dasch; Jean M.; (Bloomfield
Township, MI) ; Cheng; Yang T.; (Troy, MI) |
Correspondence
Address: |
KATHRYN A MARRA;General Motors Corporation
Legal Staff, Mail Code 482-C23-B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
38660153 |
Appl. No.: |
11/416921 |
Filed: |
May 3, 2006 |
Current U.S.
Class: |
148/552 |
Current CPC
Class: |
C22C 21/02 20130101 |
Class at
Publication: |
148/552 |
International
Class: |
C22F 1/04 20060101
C22F001/04 |
Claims
1. A method of making an aluminum alloy article comprising: making
a casting of the article from an aluminum alloy comprising, by
weight, 5% to 18% silicon, a small amount up to about 2% by weight
of one or more machining lubricity-imparting elements selected from
the group consisting of bismuth, indium, lead and tin; the casting
containing a dispersed phase containing the lubricity-imparting
element; and machining a surface of the casting with a cutting tool
to remove cast material without the use of a machining fluid, the
cutting tool being a tungsten carbide cutting tool having a surface
coated with diamond-like carbon or microcrystalline diamond.
2. A method of making an aluminum alloy article comprising: making
a casting of the article from an aluminum alloy comprising, by
weight, 5% to 18% silicon, 1.3% max iron, 0.2% max copper or 2% to
5% copper, 1.3% max magnesium, 0.6% max manganese, about 0.03% to
about 2% of one or more machining lubricity-imparting elements
selected from the group consisting of bismuth, indium, lead and
tin, and aluminum; the casting containing a dispersed phase
containing the lubricity-imparting element; and machining a surface
of the casting with a cutting tool to remove cast material without
the use of a machining fluid, the cutting tool being a tungsten
carbide cutting tool having a surface coated with diamond-like
carbon or microcrystalline diamond.
3. The method of making an aluminum alloy article as recited in
claim 2 comprising making a casting of the article from an aluminum
alloy comprising, by weight, 5% to 13% silicon, 1.3% max iron, 0.2%
max copper, 1.3% max magnesium, 0.6% max manganese, about 0.03% to
about 2% of one or more machining lubricity-imparting elements
selected from the group consisting of bismuth, indium, lead and
tin, and aluminum.
4. The method of making an aluminum alloy article as recited in
claim 2 comprising making a casting of the article from an aluminum
alloy comprising, by weight, 5% to 13% silicon, 1.3% max iron, 2%
to 5% copper, 1.3% max magnesium, 0.6% max manganese, about 0.03%
to about 2% of one or more machining lubricity-imparting elements
selected from the group consisting of bismuth, indium, lead and
tin, and aluminum.
5. The method of making an aluminum alloy article as recited in
claim 2 comprising making a casting of the article from an aluminum
alloy comprising, by weight, 16% to 18% silicon, 1.3% max iron, 4%
to 5% copper, 0.4% to 0.65% magnesium, 0.1% max manganese, about
0.03% to about 2% of one or more machining lubricity-imparting
elements selected from the group consisting of bismuth, indium,
lead and tin, and aluminum.
6. The method of making an aluminum alloy article as recited in
claim 2 comprising making a casting of the article from an aluminum
alloy comprising, by weight, 5% to 7.5% silicon, 1% max iron, 2% to
5% copper, 0.5% max magnesium, 0.6% max manganese, about 0.03% to
about 2% of one or more machining lubricity-imparting elements
selected from the group consisting of bismuth, indium, lead and
tin, and aluminum.
Description
TECHNICAL FIELD
[0001] This invention pertains to the machining of
silicon-containing aluminum castings without the use of a
metalworking fluid for lubrication and/or cooling (dry machining).
More specifically this invention pertains to the markedly improved
dry machining of castings of silicon-aluminum alloys containing
microstructurally-dispersed soft metal additives while using
cutting tools with carbon-based coatings.
BACKGROUND OF THE INVENTION
[0002] Aluminum alloy castings are used in making many articles of
manufacture. In the automobile industry, for example, many engine
and transmission parts, chassis parts, body parts and interior
parts are made of silicon-containing aluminum alloy castings. Many
of these parts such as engine blocks, cylinder heads, crank cases,
transmission cases and the like are initially formed as castings
using sand molding, permanent mold, high pressure die casting and
lost foam processes. These casting processes are capable of forming
complex shapes to reasonably close tolerances. But after the
castings have been trimmed, ground and cleaned by sand blasting (or
various other blast-cleaning processes), many surfaces of the parts
still have to be machined to specified dimensions within very close
tolerances.
[0003] Engine and transmission castings, for example, may require
precision machining processes such as milling, honing, and/or
drilling and reaming. In these machining processes the casting is
carefully positioned in a fixture and a cutting tool, carried and
powered by an operator or computer controlled machine tool, cuts a
cast surface to remove chips of cast metal to bring the surface to
a specified finish and dimension. During the metal removal
operation the machined surface is flooded with a machining fluid
for the purposes of cooling and lubricating the region impacted by
the cutting tool. The lubrication promotes cutting by minimizing
adherence of tool and work. Ultimately, the machining fluid is
drained from the machining area for recovery and re-use, or for
disposal.
[0004] It is an object of this invention to provide a method for
making aluminum alloy castings that can be machined without the use
of a machining fluid. In accordance with this invention such a
practice is termed "dry machining." It is a more specific object of
this invention to provide a dry machining method that uses a
combination of a machinable aluminum alloy casting composition and
a cutting tool with a carbon-based coating on the working surfaces
of the tool.
SUMMARY OF THE INVENTION
[0005] The relatively high silicon content of aluminum casting
alloys increases the difficulty with which they are machined and
has required the use of a machining fluid, typically a liquid based
fluid. The purpose and goal of this invention is to accomplish dry
machining of certain compositionally-modified aluminum alloy
castings without damage of the part and with tool life that is
comparable to fluid lubricated and cooled machining. This invention
provides a synergistic improvement in dry machining of such
castings by using a combination of a relatively small amount of
soft-metal additive in the cast alloy and a carbon-coated cutting
tool. Such combinations have provided benefits in dry machining
tool life that were unforeseen from optimized use of either
practice alone.
[0006] The invention uses a cutting tool, such as a drill, that has
a durable carbon-based coating on the cutting or working surfaces.
The combination of the aluminum alloy composition and the coated
cutting tool permits practical dry machining of aluminum castings
with relatively small amounts of dry machining additive and long
tool life. Indeed, preferred combinations of soft metal
additive-containing aluminum alloys and tungsten carbide cutting
tools with carbon-based coatings have demonstrated surprisingly
successful dry machining benefits.
[0007] In accordance with the invention, suitable
silicon-containing, aluminum casting alloys are modified to contain
relatively small amounts of certain finely dispersed elements that
are softer and lower melting than the aluminum casting alloy matrix
material, and which significantly increase the machinability of
surfaces of a casting into which they are incorporated. These
elements include bismuth, indium, lead and tin and one or more of
them may be added to the casting alloy. These lubricity-imparting
additives are not very soluble in the solidified aluminum-rich
matrix phase of the castings although they may combine with
alloying constituents such as magnesium. Thus, they are dispersed
as very small, globular bodies in the cast metallurgical
microstructure. And in this form, the dispersed phase of low
melting elements surprisingly enables drilling and other metal
removal machining of surfaces of the casting without the use of
machining fluids. Sufficiently low amounts of one or more of soft
elements are added to the casting alloy so that the dispersed,
relatively low melting, soft phase (either as a pure additive phase
or mixed with another constituent of the alloy in a low melting
phase) is present in the solid casting more or less uniformly
through the casting, and surfaces of choice can be machined
regardless of the position of the machined surface.
[0008] Aluminum casting alloys typically contain a significant
amount of silicon to increase the fluidity of the molten phase for
castablity and mold filling. Silicon is also added to reduce the
thermal expansion of the casting, as well as to increase its
corrosion and wear resistance. The silicon content of aluminum
alloys for casting may range from about four percent to about
eighteen percent by weight of the cast alloy. Aluminum casting
alloys for automotive and other applications such as aerospace also
contain suitable amounts of one or more of copper, iron, manganese
and/or magnesium for solid solution strengthening and for formation
of strengthening phases. Other alloying constituents or impurities
such as nickel, zinc, titanium, chromium and rare earth elements
may also be present in the casting alloy to enhance the physical
properties of a cast product.
[0009] But in accordance with this invention, small additions of
one or more of bismuth, indium, lead and/or tin are made to these
casting alloys to enable dry machining of the castings. Typically a
total of at least about 0.03% by weight of low melting elements,
alone or in combination, is added to the melt before casting. The
minimum amount of the additive depends on the casting composition,
its microstructure, and on the selected additive(s). Preferably the
total addition of one or more of these soft, lubricity-imparting
elements does not exceed about two percent by weight of the casting
so that the other properties of the casting are not significantly
altered.
[0010] In general it is preferred to use tungsten carbide cutting
tools in dry machining operations of this invention for a suitable
combination of durability and cost. The cutting surfaces of the
cutting tools are coated with a carbon-based material in dry
machining of aluminum castings. For example, it is preferred to
perform the machining using tungsten carbide drills that are coated
with a diamond-like carbon coating (DLC, a combination of particles
of sp.sup.2 and sp.sup.3 molecularly bonded carbon atoms) or with
microcrystalline diamond particles (sp.sup.3 bonded). The use of
such coated carbide cutting tools often permits dry machining of
aluminum castings with a smaller amount of the soft metal
additive(s) in the cast alloy.
[0011] These and other objects and advantages of the invention will
become more apparent from a detailed description of preferred
embodiments which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is an oblique view of a cast cylinder block for a V8
internal combustion engine for an automobile, and
[0013] FIG. 2 is a bar graph summarizing number of holes drilled (a
measure of cutting tool life) for castings of a representative B319
aluminum alloy composition, with and without lead additions, using
tungsten carbide drills with and without a diamond-like carbon
coating on the cutting surfaces of the drills.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] This invention is applicable, for example, in making cast
parts in large volume for automotive applications. Vehicle engine
and transmission parts are examples of such parts. Most automotive
castings require some machining to produce surfaces to a shape
and/or dimensional specification. The machining requires the use of
high quality and expensive cutting tools such as drills, reamers
and milling and honing tools. Heretofore the machining has also
required the use of machining fluids for part and tool protection.
The machining practices have required close management to produce
high quality cast parts with good tool life and related management
of machining costs.
[0015] This invention is applicable to the making of cast aluminum
parts and enables dry machining of surfaces of the casting without
uneconomical reduction of cutting tool life. Cast aluminum parts
are made from many known casting alloys. Among those commonly used
for automobile parts are, for example, Aluminum Alloys 319.0,
B319.0, 333.0, 336.0, A356.0, 356.0, A360.0, A380.0, 381.0, 383.0,
390.0, and 396.0. The principal alloying components of these
commercial alloys in nominal parts by weight are as follows:
319-Si6Cu3, B319-Si6Cu4Mg, A356-Si7Mg, 333-Si9Cu3, 336-Si12Cu,
356-Si7Mg (Fe), A356-Si7Mg, A360-Si10Mg, A380-Si8Cu3Fe,
381-Si10Cu4Fe, 383-Si10Cu2Fe1, 390-Si17Cu4Fe1, and
396-Si11Cu2.25Fe0.45. Among those used for aerospace parts are, for
example, Aluminum Alloys 4215F--Si5Cu 1.2Mg0.5, 4218G-Si7Mg0.5,
4219G-Si7Mg0.5, and 4241-Si7Mg0.58. These alloys also contain other
elements as impurities or as additives, each of which may affect
the physical, chemical or mechanical properties of the cast
product.
[0016] In accordance with this invention, however, small additions
of one or more of bismuth, indium, lead, and/or tin are made to
aluminum alloys, such as these alloys, for dry machinability. For
many dry machining applications the addition of one, or a
combination, of these lubricity-imparting elements is suitably in
the range of about 0.03% to about 2% by weight of the casting. The
additive-containing aluminum alloy casting is machined with a
cutting tool having a carbon-based coating on the cutting surfaces.
In general, it is preferred to use tungsten carbide cutting tools
in the practice of this invention where the cutting tools are
provided with a DLC coated or microcrystalline diamond coated
cutting surfaces. The use of a cutting tool with a suitable
carbon-based coating in the dry machining of a soft metal
additive-containing aluminum alloy casting can produce dramatic
improvements in tool life.
[0017] FIG. 1 is an oblique, outline view of a cast aluminum engine
cylinder block 10 for a V8 engine. Such an engine component is
often cast from an aluminum casting alloy such as a 319 alloy, a
356 alloy, a 390 alloy, or a 396 alloy. Such castings, especially
if they are of a complex part, such as cylinder block 10, require a
substantial amount of machining in their manufacture to finished
parts. For example, each of the eight cylinder bores 12 (four are
visible in FIG. 1) is honed to a close dimensional tolerance and
degree of roundness. At the top of cylinder bores 12, cylinder
block casting 10 has a deck portion 14 that is machined very flat
to seal with a cylinder head casting, not shown. Several bolt holes
16 are bored or drilled from deck surface 14 for secure attachment
of a cylinder head on each V-portion 18 of cylinder block 10. As is
known and illustrated in FIG. 1, an engine block casting has many
bolt holes, coolant passages, oil passages and the like that are
drilled or drilled and reamed or otherwise machined in the
manufacture of such a cast product. And there is a long succession
of such castings in an engine production line so that machining
operations and the cost of machining tools is very important in
such a manufacturing operation. It is now found that in many
applications of the machining of aluminum castings, a combination
of a carbon coated tungsten carbide cutting tool with a soft, low
melting point element-containing aluminum alloy casting permits
practical, high throughput dry machining of the self-lubricated
cast alloy surface.
[0018] Aluminum alloy B319 is a casting alloy used in cylinder
block, cylinder head and inlet manifold applications. The specified
composition of B319 is, by weight, 5.0% to 7.5% silicon, 3.0% to
5.0% copper, 1.0% max iron, 0.1% to 0.6% manganese, 0.1% to 0.5%
magnesium. 0.3% max nickel, 2.0% max zinc, 0.3% max lead, 0.1% max
tin, 0.15% max titanium, a total of 0.15% other elements and the
balance aluminum. A specific B319 alloy containing <0.02% lead
was used as a starting material in some of the following examples
and tests.
[0019] Drilling tests without any machining fluid were conducted on
a cast plate of B319 alloy to obtain baseline dry machining data.
The macro-hardness of the surface of the plate was determined to be
74 to 80 Brinell and its microhardness was 90 Knoop units. In the
machining tests, commercial one-quarter inch diameter, uncoated
cemented tungsten carbide drills were used to drill closed end
holes to a depth of three-quarters of an inch. The drilling of such
closed end holes is considered a particularly challenging operation
for successful dry machining. Only eleven holes could be drilled in
the unlubricated B319 plate before the drill had to be discarded.
The drilling of the eleven holes required an average power of 3.8
Kw and torque values reaching 2.0 Nm.
B319 Alloys Modified with Lead
[0020] Samples of the B319 aluminum alloy (<0.02% by weight
lead) were then modified by the addition of lead. The
lead-containing B319 material was prepared as follows.
[0021] Lead particles were added to attain the desired amount
(0.03%, 0.05%, 0.08%, and 0.15% by weight in these examples) to
melted aluminum B319 alloy at 1360.degree. F. using a perforated
spoon/ladle. The particles were gently stirred and dispersed into
the melt with the spoon moving the melt in a circular pattern with
the particles held at a level of about two inches below the melt
surface. This was continued for about two minutes and then the melt
was held at temperature for 30 minutes. The alloy melt was then
stirred for one minute and degassed with nitrogen gas using a
rotary degasser at 650-700 rpm for about 15 minutes (for a normal
melt of 30 lbs). The alloy melt was then gently skimmed and the
temperature stabilized at 1310.degree. F. for about 5 minutes
before the crucible was pulled out of the furnace. The alloy,
having cooled to 1260.degree. F., was poured into Zircon sand
molds. Following shakeout and cleaning, the cast plates were heat
treated using a conventional T-5 aluminum alloy heat treatment
schedule to minimize lead segregation.
[0022] B319 aluminum casting alloys were prepared respectively
containing, by weight, 0.03%, 0.05%, 0.08%, and 0.15% lead.
Examination of the cast materials confirmed that the lead was
distributed as fine globules throughout the microstructure of the
casting. The casting also contained eutectic acicular silicon
needles. While the silicon needles make a casting more difficult to
machine, the small amount of soft lead globules were used to
increase its machinability. TABLE-US-00001 Hardness Testing of the
Pb-modified B319 Material: Microhardness Macrohardness Substrate
(Knoop) (Brinell) Conventional B319 90 74 to 80 B319 + 0.15% lead
90 74
[0023] It is seen that the addition of 0.15% by weight of lead did
not appreciably reduce the surface hardness of the cast plates.
But, as will be seen, the lead additions did change the
machinability of the plates especially when tungsten carbide
cutting tools with diamond-like carbon coatings and
microcrystalline diamond coatings were used.
Tool Life Tests using Lead-Containing B319 Castings and Tungsten
Carbide Drills Coated with Carbon-Based Materials
[0024] In the following tests lead-containing B319 aluminum alloy
cast plates were used. Casting alloys and cast plates were prepared
respectively containing, by weight, 0.03% lead, 0.05%, lead, 0.08%
lead, and 0.15% lead. Holes were drilled in the cast material with
uncoated tungsten carbide drills, and carbide drills having a
diamond-like carbon coating (DLC) or a microcrystalline diamond
coating on their cutting surfaces. Tungsten carbide drills are, of
course, commercially available as are tungsten carbide cutting
tools that have DLC coatings or microcrystalline diamond coatings
on their cutting surfaces. DLC coatings comprise a bonded mixture
of graphite (sp.sup.2 carbon) particles and diamond (sp.sup.3)
particles. Cutting tools are also available with microcrystalline
diamond coatings but they are more expensive than DLC coated tools.
These carbon-based cutting tool coatings lower the coefficient of
friction between tool and substrate.
[0025] Dry machining tests were conducted on the lead-containing
cast plate substrates with one-quarter inch DLC coated tungsten
carbide drills, one-quarter inch microcrystalline diamond coated
drills, and, for comparison, with uncoated tungsten carbide drills.
The uncoated drills and microcrystalline diamond coated drills were
rotated at a speed of 61 m/min and fed into the cast substrates at
a rate of 0.18 m/rev. The DLC coated tungsten carbide drills were
rotated at a speed of 213 m/min with a feed rate of 0.18 m/rev.
Rows of 1/4 inch closed holes were drilled to a depth of 3/4 inch
using the respective coated and uncoated tungsten carbide
drills.
[0026] Comparisons in drill life during dry machining were first
made with uncoated tungsten carbide drills and tungsten carbide
drills coated with DLC. Results of these tests are presented
graphically in FIG. 2. As a baseline test, an uncoated tungsten
carbide drill was used to drill one-quarter inch holes in a plate
of commercial B319 alloy with <0.02% by weight lead. Only 11
holes were drilled in the unlubricated plate before the drill had
to be discarded. This value of 11 holes drilled is the left-most
entry bar entry in FIG. 2. In a second baseline test, a plate of
B319 alloy containing 0.08% by weight lead was used as the casting
specimen and one-quarter inch holes were drilled with an uncoated
tungsten carbide drill. The increased soft metal content permitted
the drilling of 58 holes before the drill became unusable. This
data is illustrated graphically in the second-from-left bar entry
in FIG. 2. In a third dry machining test, a tungsten carbide drill
with a DLC coating was used to drill one-quarter inch holes in a
plate of commercial B319 alloy with <0.02% by weight lead.
Ninety-eight, one-quarter inch holes were drilled by dry machining
in the unlubricated plate before the drill became unusable. The
gradual increase in drill life is illustrated in FIG. 2.
[0027] A dramatic increase in drill life was obtained when a
tungsten carbide drill with a DLC coating on its cutting surface
was used to drill holes in a plate of B319 alloy containing 0.08%
by weight lead. More than 4000, one-quarter inch, closed end holes
were drilled before the drill was unable to form additional good
holes. This result demonstrates a synergistic effect for dry
machining in the combination of the use of a carbon-coated tungsten
carbide drill and a lead-containing B319 aluminum alloy.
[0028] A series of comparative dry machining tests was also
conducted using uncoated tungsten carbide drills and tungsten
carbide drills coated with bonded microcrystalline diamond
particles. The following table summarizes tool life and power and
torque requirements on the various substrates and using either
coated or uncoated tungsten carbide drills. In each test the
drilled substrate was a base B319 aluminum alloy or the base alloy
modified with an indicated addition of lead. TABLE-US-00002 Lead
Drill Tool Power Torque (wt. %) Coating Life (kW) (Nm) 0.00 None 11
3.8 2.0 0.03 None 13 3.2 2.4 0.05 None 22 2.8 2.0 0.03 Diamond 760
0.6 1.6 0.05 Diamond >3500 0.5 1.3 0.08 Diamond >3500 0.4 1.1
0.15 Diamond >10,000 0.4 1.2
[0029] The benefits to dry machining of Pb-containing B319 aluminum
alloy using carbon-coated tungsten carbide drills are thus
demonstrated. The tool life and power consumption values are
comparable to those obtained when machining additive-free B319
alloy castings using machining fluids.
[0030] The practice of this invention has been illustrated by the
presence of single lubricious elements in a specific aluminum
casting alloy in a series of drilling tests. However, these
lubricity adding elements may be beneficially used either
individually or in combination in other casting alloys and in other
machining operations. The scope of the invention is limited only by
the following claims.
* * * * *