U.S. patent application number 10/759645 was filed with the patent office on 2005-07-21 for high ductility, high hot tensile strength tungsten wire and method of manufacture.
Invention is credited to Gal, Tamas, Jusztin, Peter, Meszaros, Istvan, Nagy, Attila, Nagy, Gyorgy.
Application Number | 20050155680 10/759645 |
Document ID | / |
Family ID | 34620723 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155680 |
Kind Code |
A1 |
Nagy, Gyorgy ; et
al. |
July 21, 2005 |
High ductility, high hot tensile strength tungsten wire and method
of manufacture
Abstract
A method for manufacturing a high ductility and high hot tensile
strength tungsten wire for incandescent lamp filaments is
disclosed. The method comprises the steps of preparing a tungsten
alloy, swaging a tungsten rod from the alloy, and drawing the
swaged rod to wire size in multiple drawing passes. In the method,
the wire is annealed between predetermined draws. It is proposed
that an annealing is performed before the final drawing pass, by
annealing the wire at a temperature between 1100-1300.degree. C.
There is also provided a tungsten wire for incandescent lamp
filament, which has high ductility and high hot tensile strength.
The tungsten wire of the invention has a cold tensile strength--hot
tensile strength ratio not exceeding 3.5.
Inventors: |
Nagy, Gyorgy; (Erdosor,
HU) ; Meszaros, Istvan; (Szentes, HU) ; Gal,
Tamas; (Gyorossy kert, HU) ; Nagy, Attila;
(Krajcar, HU) ; Jusztin, Peter; (Erdosor,
HU) |
Correspondence
Address: |
Timothy E. Nauman
FAY, SHARPE, FAGAN, MINNICH & McKEE, LLP
1100 Superior Avenue
Cleveland
OH
44114
US
|
Family ID: |
34620723 |
Appl. No.: |
10/759645 |
Filed: |
January 16, 2004 |
Current U.S.
Class: |
148/673 |
Current CPC
Class: |
H01K 3/04 20130101; B21C
1/003 20130101; C22F 1/18 20130101; B21C 37/045 20130101; C22C
27/04 20130101; H01K 3/02 20130101; H01K 1/08 20130101 |
Class at
Publication: |
148/673 |
International
Class: |
C22F 001/18 |
Claims
1. A method for manufacturing a high ductility and high hot tensile
strength tungsten wire for incandescent lamp filaments, comprising
the steps of preparing a tungsten alloy, swaging a tungsten rod
from the alloy, drawing the swaged rod to wire size in multiple
drawing passes, annealing the wire between predetermined draws, in
which an annealing is performed before the final drawing pass, by
annealing the wire at a temperature between 1100-1300.degree.
C.
2. The method of claim 1, in which the final drawing pass after
said annealing is done at a different drawing speed than the
previous drawing passes.
3. The method of claim 2, in which the final drawing pass after
said annealing is done at a slower drawing speed than the previous
drawing passes.
4. The method of claim 3, in which the final drawing pass after
said annealing is done at a drawing speed substantially 0.65 times
the drawing speed of the previous drawing pass.
5. The method of claim 1, in which the wire is drawn from the
swaged rod to final size in twenty to forty drawing passes.
6. The method of claim 1, in which the wire is pre-heated during
the drawing passes.
7. The method of claim 6, in which the wire is pre-heated to
500-900.degree. C. during the drawing passes.
8. The method of claim 1, in which the drawing tools are pre-heated
during the drawing passes.
9. The method of claim 8, in which the drawing tools are pre-heated
to 300-400.degree. C. during the drawing passes.
10. The method of claim 1, in which the wire is further annealed
between drawing passes preceding the final drawing pass.
11. A tungsten wire for incandescent lamp filament, having high
ductility and high hot tensile strength, having a cold tensile
strength - hot tensile strength ratio not exceeding 3.5.
12. The wire of claim 11, having a hot tensile strength between
0.16-0.24 N/mg/200 mm, measured at 1620.degree. C.
13. The wire of claim 11, having a cold tensile strength between
0.50-0.75 N/mg/200 mm, measured at room temperature.
14. The wire of claim 11, being formed as a coil, and having a
mandrel ratio not exceeding 2.
15. The wire of claim 11, comprising additives selected from the
group of Al, K, Si.
16. The wire of claim 11, comprising additives selected from the
group of Th, ThO, YO, LaO, CeO, Re.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a high ductility and high hot
tensile strength tungsten wire for incandescent lamp filaments, and
a method for manufacturing such a tungsten wire.
[0002] Lamps with an incandescent filament have been known for a
long time. In most applications, the filaments are made of a
tungsten wire, which is wound into a coil. The dimensions of the
coil determine not only the light output of the lamp, but also the
optical properties of the light beams emerging from an optical
projector system. Such projector systems are found, among others,
in headlights of automobiles or slide projectors. Lamps with small
filaments have better optical parameters, and allow the formation
of a well-defined projected beam, even with small-sized projecting
optics. Beside, projector systems not only require small filaments,
but also very high lumen output.
[0003] Therefore, coils with extremely small external dimensions
are being produced for automotive lamps and projector lamps. The
small external dimensions mean that the inner diameter of the coils
is also small, in the order of the wire diameter. The inner
diameter of the coil largely corresponds to the diameter of the
mandrel, on which the filament is wound during manufacturing of the
coil. The ratio of the diameter of the mandrel to the wire diameter
is termed as the mandrel ratio. In this manner, coils with a small
inner diameter will also have a small mandrel ratio. Since the
diameter of the filament wire also has a practical lower limit,
filaments with small mandrel ratio are necessary for the best
possible light efficiency. Further, high light output also requires
high filament temperatures. At high temperatures, the sagging of
the filament poses serious problems. Therefore, it is sought to
manufacture so-called non-sag filaments. The non-sag ability of a
filament is closely related to the hot tensile strength of the
tungsten wire from which the filament is made. Hot tensile strength
(hereinafter HTS) is measured at 1620.degree. C., and desired
values are above 0.16-24 N/mg/200 mm.
[0004] During wire production, the wire is annealed (heat treated).
This annealing forms the mechanical properties of the wire to
enable the assembly of the filaments on an automated mounting
machine without breakage. As mentioned above, in some instances the
required optical parameters may be obtained only with coils having
a very small mandrel ratio, in the order of 2 to 1.5, or even
lower. This extreme mandrel ratio requires that the wire remains
ductile on room temperature, otherwise the wire may split or break
during the winding process, particularly at those parts of the
coil, which must endure the largest shaping tension or shaping
stress. Ductility of the wire is closely correlated with its cold
tensile strength (hereinafter CTS), in the sense that a wire with
low CTS has a high ductility, while higher CTS values correspond to
low ductility. CTS is measured at room temperature, and desired
values for high-end, low mandrel ratio filament wires are between
0.5-0.7 N/mg/200 mm.
[0005] It is known in the art that the ductility of the wire may be
influenced with the annealing process. Namely, by the proper
selection of times and temperatures of the annealing in combination
with the parameters of the wire drawing, the desired ductility (or
the CTS) may be accomplished. However, it was noted that HTS values
move in tandem with CTS values. With other words, if the annealing
were directed towards increasing the ductility of the wire (and
thereby lowering the CTS), inevitably the HTS values also
decreased. Conversely, when the annealing were directed towards
increased HTS values, the ductility of the wire decreased.
[0006] For example, U.S. Pat. No. 3,278,281 discloses a process for
manufacturing a non-sag tungsten wire. The process involves the
preparation of a thorium-doped tungsten alloy, which is swaged and
subsequently drawn to wire size. The drawing is done in multiple
drawing passes, with multiple annealing steps between the drawing
passes. This known process proposes annealing after each five
passes, and at temperatures of 1700.degree. C. The resultant wire
has outstanding non-sag properties, but operates best in lamps with
a relatively low efficiency, and is less suitable for high-end
lamps requiring both high temperature and high vibration
resistance.
[0007] Another known process for the manufacture of a tungsten wire
is disclosed in U.S. Pat. No. 4,863,527. This process also involves
the swaging of a tungsten alloy rod, and a subsequent drawing to
size. During drawing, it is proposed to perform multiple annealing
steps, at temperatures around 1560-1620.degree. C. This known
process results in a wire having a relatively low CTS, but high
ductility.
[0008] The publication "The Metallurgy of Doped/Non Sag Tungsten"
by E. Pink and L. Bartha, spublished by Elsevier Applied Science,
London and New York, 1989, further discloses that a tungsten wire
need to be annealed during drawing (see pp. 78-79), because the
wire strength will increase as the wire is drawn to smaller
diameters. According to this literature source, the annealing will
reduce the wire ductility. Depending on the final wire size, a
combination of anneals is used to optimize the properties of the
final wire.
[0009] However, none of the known processes teach a method which
would result in a hight HTS of the wire, while reducing its CTS
value. Therefore, there is a need for a method which is able to
lower the CTS value of a tungsten filament, and accomplishing high
ductility of the wire, while maintaining a high HTS value of the
same wire. Also, there is a need for a tungsten wire which has a
low CTS/HTS ratio. There is also need for a method which
accomplishes these results without the use of any additional or
specific tungsten wire manufactuing equipment, i. e. which does not
require any radical change in exisiting manufacturing
facilities.
SUMMARY OF THE INVENTION
[0010] In an embodiment of the present invention, there is provided
a method for manufacturing a high ductility and high hot tensile
strength tungsten wire for incandescent lamp filaments. The method
comprises the steps of preparing a tungsten alloy, swaging a
tungsten rod from the alloy, and drawing the swaged rod to wire
size in multiple drawing passes. In the method, the wire is
annealed between predetermined draws. It is proposed that an
annealing is performed before the final drawing pass, by annealing
the wire at a temperature between 1 100-1300.degree. C.
[0011] In an embodiment of another aspect of the invention, there
is also provided a tungsten wire for incandescent lamp filament,
which has high ductility and high hot tensile strength. The
tungsten wire of the invention has a cold tensile strength--hot
tensile strength ratio not exceeding 3.5.
[0012] The disclosed method may be performed with standard tungsten
wire manufacturing equipment. By performing the annealing before
the last drawing pass, the cold tensile strength--hot tensile
strength ratio of the wire is unexpectedly lowered, by lowering of
the CTS value, and simultaneously maintaining, in some instances
even increasing the HTS value. Accordingly, the filaments made from
the proposed tungsten wire are resistant against vibration,
tolerate low mandrel ratios, and support high operating
temperatures.
BRIEF DESCRIPTION OF DRAWINGS
[0013] The invention will now be described with reference to the
enclosed drawings, where
[0014] FIG. 1 is a side view of an automotive lamp with a tungsten
filament,
[0015] FIG. 2 is an enlarged view of a tungsten filament,
[0016] FIG. 3 is an illustrative figure explaining the concept of
the mandrel ratio,
[0017] FIG. 4 is a schematic illustration of a wire drawing
process, and
[0018] FIG. 5 is another schematic illustration of a step in the
tungsten wire manufacturing process.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Referring now to FIGS. 1 and 2, there is shown an automotive
lamp 1. The lamp 1 has a sealed lamp envelope 2, typically made of
glass. 1. The envelope 2 has a sealed inner volume 6 filled with a
suitable gas, like argon, krypton or xenon. The inner volume 6
contains a filament 8. The filament 8 is made of a tungsten wire.
In certain embodiments, the filament 8 may be single coiled, or
double coiled (or coil-coiled), as shown in FIG. 2. Such
coiled-coiled filaments are commonly used for higher wattage lamps
or high-end lamps. Often, the filament 8 must also be capable of
high color temperature operation, i. e. in the heated state, its
operating temperature may be above 2900.degree. K., and in extreme
cases it may even reach 3200.degree. K.
[0020] The filament 8 may contain an aluminum-potassium-silicon
(AKS) additive, or other dopants. The dopants are added to the
tungsten alloy during the manufacturing of the filament, as will be
explained below.
[0021] The filament coil is formed during manufacturing by winding
the wire 9 of the filament 8 on a mandrel 10, as illustrated in
FIG. 3. Filaments for high-end lamps require low mandrel ratio, in
order to obtain proper optical and luminous parameters. The mandrel
ratio is defined as the ratio of the diameter d.sub.m of the
mandrel to the wire thickness d.sub.w, i. e. the mandrel ratio is
d.sub.m/d.sub.w (see also FIG. 3). This requires a wire 9 having a
sufficiently high ductility, which corresponds to a relatively low
CTS value, preferably as low as 0.7-0.5 N/mg/200 mm. In the wire
manufacturing method, the ductility needed for a coiling with small
mandrel ratio is increased by annealing the wire during the wire
production, as will be explained below.
[0022] The wire manufacturing method starts with the preparation of
a tungsten alloy, optionally comprising various additives, such as
aluminum, potassium, silicon. Further additives may be selected
from the group of Th, ThO, YO, LaO, CeO, Re. The beneficial effects
of such additives are known in the art, and need not be discussed
here.
[0023] Following the alloy powder preparation, the alloy powder is
pressed and presintered. The pressing and presintering is also made
in a known manner, in order to prepare the alloy powder for the
sintering. Thereafter, the alloy powder is sintered with direct
current. This is a known process step in powder metallurgy. The
specific parameters of the sintering, i. e. temperature, atmosphere
composition and sintering current are dependent of the geometrical
and other parameters of the furnace. Typical values of sintering
current are between 3000 and 6000 A, and the sintering is done in a
hydrogen atmosphere. The sintering of a tungsten alloy is also
disclosed in U.S. Pat. Nos. 6,066,019, 5,742,891 and 4,678,718.
[0024] Following sintering, a tungsten alloy wire is formed from
the sintered alloy ingot. The forming of a filament is done with
known metalworking techniques, e. g. rolling, swaging and wire
drawing. The swaging forms a tungsten rod from the alloy, which is
suitable for drawing to wire size. During swaging, the tungsten rod
may be also annealed and/or re-crystallized. This process step is
known in the art.
[0025] The swaged rod is subsequently drawn to wire size in
multiple drawing passes. As illustrated in FIG. 4, the diameter of
the wire 9 decreases as the wire 9 is forced through a series of
drawing dies 11,12,13, of which only three is shown in FIG. 4.
(FIG. 4 is not to scale.). Typically, the wire 9 is drawn from the
swaged rod to final size in twenty to forty drawing passes,
depending on the final wire diameter. With this method, wire
diameters between 0.3-0.04 mm are customarily produced. The drawing
causes intensive stresses in the crystal structure of the tungsten
wire, which is at least partly compensated by annealing the wire
between predetermined draws, typically after each 3-4-5 or more
drawing passes, depending on the desired result. This annealing may
be done by electric heating, or by heating with a gas burner 15, as
shown in FIGS. 4 and 5. Both types of heating are known in the
art.
[0026] The drawings are not made at room temperature, but the wire
9 is pre-heated during the drawing passes, typically to
500-900.degree. C. The drawing tools contacting the wire 9, i.e.
the drawing dies 11,12,13 can also be heated with a suitable known
heating equipment (not shown), typically to 300-400.degree. C.
[0027] In the proposed tungsten wire manufacturing method, an
annealing is performed before the final drawing pass. During this
annealing, the wire is heated to a temperature between
1100-1300.degree. C., the actual temperature used depending on the
wire diameter. Typically, wires with a larger diameter are annealed
at a higher temperature, and thinner wires at a lower temperature.
As a result of this annealing just before the final drawing pass,
the tungsten undergoes a crystal structure change that improves its
ductility, without adversely affecting the final HTS value of the
wire. This means that the wire will maintain its good non-sag
property, but will not break or split when wound even to small
mandrel ratio coils.
[0028] This step of the method is illustrated in FIG. 5, which
shows the annealing being performed with a gas burner 16 before the
wire 9 is forced through the die 14 during the final drawing pass,
as the wire 9 is drawn to final size.
[0029] In a preferred embodiment, as shown in FIG. 5, the final
drawing pass after said annealing is done at a different drawing
speed than the previous drawing passes. Most preferably, the final
draw is done at a slower drawing speed than the preceding draw. For
example, the last drawing pass--as indicated by the arrow 22--may
be performed at a drawing speed approx. 65% of the speed of the
last but one drawing, the latter being indicated by the arrow 21.
Therefore, the wire 9 is changed from one drawing line to another,
as indicated by the arrow 23 in FIG. 5. Of course, it is also
possible to make the final drawing on the same drawing line, though
it will cause interruptions in a continuous production, hence it is
preferable to use another drawing line for the last drawing.
[0030] The proposed method results in a tungsten wire with
outstanding non-sag and ductility properties. Due to the fact that
the HTS of the wire does not decrease together with the decrease of
the CTS value, it is possible to manufacture tungsten wires having
a cold tensile strength--hot tensile strength ratio not exceeding
3.5.
[0031] For example, with a 240 mg/200 mm size tungsten wire hot
tensile strength values of 0.16 N/mg/200 mm were accomplished. For
the same wire, a cold tensile strength value of 0.52 N/mg/200 mm
was accomplished resulting in a CTS/HTS ratio of 3.25.
[0032] For another wire with a 5.2 mg/200 mm size, hot tensile
strength values of 0.210 N/mg/200 mm were accomplished. For the
same wire, a cold tensile strength value of 0.745 N/mg/200 mm was
accomplished, resulting in a CTS/HTS ratio of 3.43. Such thin and
ductile wires are well suited for small mandrel ratio coils.
[0033] Some illustrative CTS and HTS values obtained with the
method are listed in the table below:
1TABLE I Decrease in Wire Size CTS HTS CTS/HTS mg/200 mm Technology
N/mg/200 mm N/mg/200 mm CTS/HTS ratio, % 5.17 Prior art 0.960 0.217
4.42 5.17 Annealed* 0.745 0.210 3.43 23 41.60 Prior art 0.723
0.1600 4.52 41.60 Annealed* 0.607 0.1770 3.43 25 77.60 Prior art
0.610 0.1550 3.94 77.60 Annealed* 0.570 0.1700 3.35 15 240.00 Prior
art 0.551 0.1740 3.75 240.00 Annealed* 0.520 0.16.00 3.25 14
Annealed* = Annealed before the final drawing pass
[0034] The proposed type of tungsten wire is applicable for all
types of lamps, and it is principally recommended for the
production of special high-end and automotive lamps with double
spiral filaments of small mandrel ratio. A classical example is a
24 V, 21 W stop lamp for automobiles, which is subjected to a high
number of switch on--switch off cycles, beside the intensive
vibration. The application of this wire will largely reduce the
breakage or deterioration of the filaments during manufacture of
the coils, and also increases the lifetime of the lamps.
[0035] With the suggested method, the general mechanical properties
of the filaments of special incandescent lamps with small mandrel
ratio are improved, while it is still possible to produce both the
wire and the filaments with standard manufacturing equipment. This
means in practice that the production facilities for traditional K,
Si, Al doped tungsten wire may be used, while decreasing defect
rate of the filaments during production and use. The improved
ductility of the wire will result in superior filament winding
quality. The wire retains its desired fibrous structure, which is
essential for long-life, non-sag filaments.
[0036] The invention is not limited to the shown and disclosed
embodiments, but other elements, improvements and variations are
also within the scope of the invention. For example, it is clear
for those skilled in the art that beside the annealing step before
the last drawing pass, a number of further annealing steps may be
performed during the various drawing passes, in combination with
re-crystallization or similar heat treatments.
* * * * *