Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Surface alloying of metals is usually accomplished
by welding processes or various flame and plasma spraying
techniques. Welding processes utilize an electrical or
gaseous energy source with a highly variable power level which
is difficult to control. As a result, excess heat is normally
directed to the article being coated and the microstructure
thereof is often adversely affected so that additional heat
treatment is subsequently necessary. While the strength of
the bond between the surface coating and article may be suf-
ficient with such welding processes, an excessively thick
coating is normally provided because of irregularities in the
deposition rate and the lack of the full ability to control
its placement. Such excess coating must, in many instances,
be machined off to achieve the desired final surface contour
at additional expense. Further, welding processes are
basically accomplished at a rather slow material deposition
rate and the accompanying control problems often lead to
impurities in the microstructure, cracks or porosity, or
other forms of harmful metallurgical defects which tend to
decrease the strength of the coating and its bond to the
' article.
s~ By way of example, engine valves of austenitic chrome
nickel silicon steel with Stellite No. 6 coating material fus-
ibly bonded thereto around the annular valve seat portion
~s thereof have been widely adopted by the diesel engine in-
dustry. Stellite is a trademark of the Stellite Division of
.
Cabot Corp., Kokomo, Indiana. These valves are produced
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by a relatively complex manufacturing process utilizing gaseous
torches to extensively heat the valve head prior to~ during,
and subsequent to depositing the Stellite coating material
thereon. This process is known as "puddle welding". The dis-
advantages of this expensive and relatively slow process in-
clude directing an excessive amount of heat into the base
material and slow cooling of the coating. This resu~ts in a
relatively thick overall diffusion zone or region of inter-
stitial bond of approximately 0.020" between the base material
and the coating as well as dendritic segregation and enlarged
crystallization of the coating material with an accompanying
decreased hardness thereof. Further, the thick diffusion zone
represents a region having neither the desirable properties
of the base material or of the coating. For example, it is
known that the iron in the steel degrades the Stellite when
present in only relatively minute amounts. Accordingly, the
service life of these valves is not as high as desired.
On the other hand, flame and plasma spraying tech-
niques apply a coating material to an article without sub-
stantial heating of the article, with the result that a rela-
tively poor metallurgical bond occurs therebetween. Such a
technique is usually limited to a coating thickness of a few
thousandths of an inch having generally low strength. U.S.
~ Patent 3,310,423 issued March 21, 1967 to H. S. Ingham, Jr.
;i 25 discloses a flame spraying process for applying a coating
$ utilizing instantaneous energy bursts from a laser beam to
raise the temperature of the previously heated flame sprayed
s~ particles as they are propelled toward the surface of an
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article. The substantlally molten particles are purported
to adhere to the surface of the article upon contact there-
with with a better bond than conventional flame spraying
processes, which are generally known to have a relatively weak
bond between the coating and the article, and coating porosity.
This method is more expensive because it requires dual heating
and, further, the thickness of the applied layers is restricted
to only a few thousandths of an inch. Because of these
deficiencies, flame spraying processes have been considered
entirely unsatisfactory for hard facing an engine valve,
for example.
Other heating sources such as the possible use of
solar energy or electron beam energy appear to raise more
difficult problems. With solar energy, a large and expensive
heat gathering system is needed in order to obtain sufficient
power density energy levels and the broad spectrum energy
source is continually changing. Likewise, with electron beam
energy a relatively expensive evacuation system is normally
necessary, and this greatly restricts the manufacturing
process of the psrts to be coated. For example, the vapor
pressure of either the coating material or the article could
be adversely affected and some materials might volatilize,
imposing further limits on material choice which is already
- limited by the electron beam welding process itself.
Illustrative of the wide range of efforts to solve
the particular problem of applying a hard surface coating to
the head of an engine valve, for example, are U. S. Patent
3,147,747 to LeRoy O. Kittelson; U. S. Patent 3,362,057 to
; Gerhard Kubera et al; and U. S. Patent 3,649,380 to Max J.
Tauschek. Also, U. S. Patent 3,478,441 to Alexander Goloff
$
/; et al and assigned to the assignee of the present invention
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discloses a method of friction welding a facing material workpiece to an
engine valve to provide an impact resistant seat thereto. However, none of
thes;e methods completely solves the various problems, including the frequent
need to machine off the excess coating, or solves them in a sufficiently
economical manner. One other method, U.S. Paten~ 3,663,793 to James Petro
et al, describes a way of applying a coating to a glass light bulb or the
like for decorative purposes, using heat from a laser beam or an electron
beam. However, such method employs an additional heating step and makes no
mention of improving the wear and impact resistant service life of the coat-
ing and its bond with the parent article to which the present invention isparticularly directed.
Summa and Ob ects of the Invention
ry
It is therefore an object of the present invention to provide an
improved method and apparatus for fusibly bonding a metal powder coating
material to the surface of a metal article wherein a relatively high strength
interstitial bond is obtained.
According to one aspect of the present invention there is provided
a method of fusibly bonding a substantially solid metal powder coating
material to a metal article, which comprises:
Step (a) depositing the metal powder in the form of a band of
; powder onto a surface of the metal article, the band of powder being substan-
tially uniformly levelled and of a preselected width;
Step (b) melting a portion of the band of powder by focusing a
- laser beam thereon;
; Step (c) immediately thereafter melting the surface of the metal
article by the heat of the laser beam on the band of powder at a heat
interaction zone; and
; Step (d) relatively moving the focused laser beam and the heat
interaction zone along the band of powder at a rate sufficient for forming
!'~ 30 a traveling melt and maintaining heat transfer into the article and cooling
, of a trailing portion of the melt at preselected values and subsequent
hardening of the trailing portion of the melt at a relatively fixed distance
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from the heat interaction zone, the relative rate of laser beam movement and
heat transfer being sufficient for providing a solidified surface layer having
physical properties resulting essentially from heating and cooling of only
the powder and a relatively thin, strong region of interstitial bond between
the surface layer and the article having physical properties resulting
essentially from heating and cooling of both the powder and the metal article.
According to another aspect of the present invention there is
provided an apparatus for fusibly bonding a metal powder to a metal srticle,
comprising: depositing means for controllably placing the powder onto a
surface of the metal article in the form of a band of powder of relatively
uniform preselected cross section; laser beam means for controllably melting
the band of powder at a heat interaction zone and the juxtaposed surface of
the article; and holding means for relatively moving one of the article and
the laser beam means at a rate sufficient to provide a traveling melt along
the band of powder and cooling and hardening of a trailing portion of the
melt.
Reference is now made to the accompanying drawings.
Brief Description of the Drawings
' ':
Figure 1 is a perspective of an article holding fixture for fusibly
bonding a metal powder coating material to a metal article in accordance with
the novel method of the present invention.
Figure 2 is a photomicrograph of a polished and etched portion of
; an article and associated hardened coating produced by the method of the
1 ~ present invention at a magnification of approximately 225 times its true size.
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FIG. 3 is a photomicrograph similar to FIG. 2, only
enlarged to a magnification of approximately 860 times its
true size to better show the interface between the article
and coating.
FIG. 4 is an electron microprobe line scan across an
article and associated hardened coating produced by the
method and apparatus of the present invention, and particu-
larly an engine valve having an austenitic chrome nickel
silicon steel base material and a Stellite No. 6 coating at
the valve seat thereof.
FIG. 5 is an electron microprobe line scan of a conven-
tional engine valve produced by puddle welding and having an
~' austenitic chrome nickel silicon steel base material and a
Stellite No. 6 coating at the valve seat thereof for purposes
of comparison with FIG. 4, and at twice the scan length
thereof.
; FIG. 6 is a photomicrograph of a polished and etched
- portion of the conventionally puddle-welded engine valve at
, a magnification of approximately 225 times its true size for
~'` 20 purposes of comparison with FIG. 2.
Descrip'tion 'of Pre'ferred''~mb'o'diment
FIG. 1 of the drawings shows an article holding
~ fixture 10 for fusibly bonding a coating material of
,~ .
fusible powder 12 to the surface of a ferrous metal article 14
~, 25 such as an engine valve or the like in accordance with the
method of the present invention. In the instant embodiment,
Stellite No. 6 powder is effectively bonded to the engine
~,, valve 14 of dissimilar material such as an austenitic chrome
nickel silicon steel. The article holding fixture 10 which
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rotatably supports the engine valve includes a framelike
base 16 which supports an obliquely upwardly facing mounting
plate 18 on which is rotatably mounted an article holding
dlsc 20 having a concentrically disposed cylindrical recess 22
formed therein.
The engine valve 14 includes an upwardly extending
stem portion 24 and a lower head portion 26 adapted to be
received in frictionally sliding relation within the cylin-
drical recess 22 in the rotatable holding disc 20. In this
manner the engine valve is obliquely positioned at an
approximately 45 angle to a horizontal plane by virtue of
the relative disposition of the mounting plate 18 and the
article holding disc 20. This beneficially results in an
annular seat 28 on the head portion of the valve being
maintained in substantially a horizontal plane at, and sub-
stantially ad~acent, the uppermost portion or apogee of the
rotating valve as shown generally by the reference numeral 29.
A variable speed drive motor 30 is suitably attached to the
base 16 of the holding fixture 10 in driving relation with
the article holding disc 20 for rotation of the engine valve
at a relatively fast predetermined speed, for example five
(5) seconds per revolution.
` An angled brace 32 is adjustably secured to the
base 16 of the article holding fixture 10 of the present
invention for holding a gravity fed powder delivery tube 34
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in a generally upright manner above and inwardly ad~acent to
the periphery of the head portion 26 of the valve 14. The
delivery tube has a funnel 36 at the upper end thereof
: adapted to conveniently receive the fusible powder 12,
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and a conical tapered lower end 38 disposed in predetermined relatively
closely spaced relation to the annular seat 28 of the valve 14~
A water cooled, substantially flat reflective mirror 40 is
universally adjustably supported substantially directly above the valve
head 26 on a pair of horizontally aligned spaced pivot connections 42
of a depending yoke 44 and on a vertical pivot connection 46 intermediate
the yoke and an upright arm 48 which is secured to the base 16. A fixed
multi-kilowatt continuous wave laser beam 50 is substantially horizontally
projected on the mirror 40 as shown in broken lines from a suitable source
~not shown). The continuous wave laser beam 50 is then reflected downwardly
by the mirror and focused on the annular seat 28 of the valve 14 to provide
a relatively high energy heat interaction zone as indicated by the reference
numeral 52. This heat interaction zone substantially coincides with the
apogee 29 of the rotating valve.
Effective fusion of the powder 12 and the annular seat 28 at the
; interaction zone 52 is achieved using the laser beam 50 with a cross sectional
beam pattern of substantially doughnut shape. Specifically, the preferred
doughnut shaped laser beam pattern is 3/16 inch O.D. and 1/16 inch I.D.,
which results in a power density of 2.85 x 105 watts per square inch.
The powder delivery tube 34 is disposed arcuately offset from the
apogee 29 of the valve 14 and closely above the annular seat 28 to provide
a powder deposition zone generally indicated by the reference numeral 54.
However, this powder deposition zone is relatively close to the
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apogee so that the annular seat 28 is substantially hori-
zontally disposed and the fusible powder 12 is better
retained thereon. Further, the conically tapered lower
end portion 38 of the delivery tube 34 and its proximal
location to the valve head portion 26 serves to meter the
rate of delivery of the powder to the annular seat. For
example, the powder fills up the space therebetween and no
flow occurs as long as the valve remains stationary, yet flow
occurs at the desired rate with a self-leveling action upon
rotating the valve at the desired speed. s
The article holding fixture 10 of the present
invention, including the powder delivery tube 34, the
reflective mirror 40, and laser beam 50 effectively produces
a hard alloy layer or surface 56 on the engine valve 14.
This surface becomes substantially solid at a solidification
zone 58 which is also relatively close to the apogee 29 of
the engine valve so that the melt of the powder 12 and
adjacent portion of the valve which is liquified at the heat
interaction zone 52 remains substantially horizontal and
2a does not appreciably flow prior to such hardening.
It is extremely significant to note that by utilizing
the manufacturing method of the present invention to apply
the hard alloy layer 56 on the engine valve 14 the hardfacing
; portion of the cost can be reduced to approximatel~ half that
of conventional puddle welding techniques.
.~.
` Operation
While the operation of the present invention is
believed clearly apparent from the foregoing description,
further amplification will subsequently be made in the
following brief summary of such operation. The motor 30 and
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continuous wave laser beam 50 are simultaneously actuated to
initiate the deposition of the powder 12 on the annular seat 28
of th~e engine valve 14 at the deposition zone 54, and to also
impinge the extremely hot pattern of the laser beam thereon
at the heat interaction zone 52. As the article holding disc
20 and engine valve are rotated by the motor in a clockwise
direction as shown by the arrow in FIG. 1, the focused laser
beam relatively rapidly thermally liquifies both the powder
and ad~acent portion of the valve head 26 at the interaction ~-
zone without excessive heating of the entire engine valve. The
resulting melt thereafter travels beyond the heat of the laser
.
beam at the interaction zone, and the rapid transfer of heat there-
from to the relatively cool mass of the engine valve, coupled with
ambient air or room cooling thereof, quickly results in obtaining
i15 the hard alloy surface 56 at the solidification zone 58. This
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alloy surface is relatively thick, for example, 0.050" (1.27 mm),
and has an extremely uniform cross section with an excellent
interstitial bond. Such hard surface has been found so uni~orm
in profile that only a minimum of final machining thereof is
necessary to produce the completed engine valve.
The effectiveness of the fusible bonding method and
apparatus of the present invention is shown by the photomicro-
graph of FIG. 2. This clearly indicates the relatively thin,
~;~ but high quality interstitial bond between the hard metallic
;25 alloy surface 56 and the metallic base material of the engine
~; valve 14. The upper portion of the photomicrograph shows the
,, .
relatively fine microstructure of the Stellite No. 6 surface,
while the lower portion thereof shows the uniform distribution
of fine carbides in an austenitic matrix which is associated
with the austenitic chrome nickel silicon steel of the engine
valve. Between these two regions is a relatively thin overall
diffusion zone or region of interstitial bond which clearly
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illustrates the relatively complete fusion thereof, the absence
of cracks or voids, and the absence o~ oxide inclusions or the
like which would deleteriously affect the structure. Such
thin region of interstitial bond is pictorially noted to be
approximately 0.003" (0.075 mm) thick, and with the dark line
resulting from the etchant used representative of a substantially
planar interface. As is more clearly shown in the more greatly
enlarged photomicrograph of FIG. 3, this planar interface is
noted to be only about 0.0005" (0.012 mm) thick and yet the
uniformity thereof is indicative of a high strength bond.
As best shown in FIG. 4, an electron microprobe line
scan of the engine valve 14 of the present invention indicates
an extremely uniform metallurgical composition of the ferrous
base material and metallic coating, which are respectively
,
disposed to the left and right when viewing the graph. The
` relative qualitative, not quantitative, concentration values
of the elements iron (Fe), chromium (Cr), carbon (C), and
cobalt (Co) are shown, while for illustrative convenience
the concentration values of the primary remaining elements
` 20 nickel, silicon and tungsten have been omitted from the graph.
It is apparent that the smoothness of the scan lines confirms
; the fine grain structure of the coating, as well as the thin
- ~ region of interstitial bond comparable to that shown in the
photomicrograph of FIG. 2. On the other hand, upon inspection
of FIG. 5 showing a line scan of a conventional puddle-welded
engine valve, it is clear that the thickness of the region
- of interstitial bond is approximately 0.020" (0.5 mm), which is
;~; approximately ten times the 0.002" (0.05 mm) thickness of the cor-
:,i.
responding region in FIG. 4. Note is also made in FIG. 5 of the
; 30 very significant extent of the dissolution of the iron (Fe~ pre-
~- sent in the base material into the coating of the prior art engine
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valve, which can deleteriously embrittle and reduce the oxi-
dation resistance thereof. Such dissolution is advantageously
negligible in the engine valve of the present invention as
shown in the relatively abrupt descendency of the iron line
in FIG. 4. Also, the coinciding peaks and valleys in the
chrome (Cr~ cobalt (Co) and carbon (C) scan lines of the prior
art coating in FIG. 5 are indicative of the formation of large
crystals, such as chromium carbides, which accompanies the
high heat input and slow cooling operation of puddle welding.
Note that such peaks are substantially absent in the line
scan of the coating of FIG. 4, indicating its fine grain -
structure.
As best shown in the photomicrograph of FIG. 6, the
prior art coating exhibits relatively large dendritic formations
~ith carbides in a cobalt matrix, and this is dramatically
distinct from the much finer coating structure of the present
invention as shown in FIG. 2. This metallurgical difference
is reflected in Rockwell hardness readings of Rc 65-67 in the
Stellite coating of the present invention, as compared to the
2Q usual Rc 62-65 Stellite coating readings of prior art. Such
increased hardness results in an engine valve having increased
resistance to wear.
Thus, the fusible bondlng method and apparatus of
the present invention has proven extremely effective in hard
`~ 25 facing an engine valve using continuous wave laser energy as
I a heat source. An examination of the microstructure of the
.
hard alloy surface and the metal article has indicated
excellent fusion therebet~een and a relatively narrow but high
strength interstitial bond. Further, the relatively high
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power laser beam has been found to be closely controllable s
to minimize the transmission of heat into the remainder of
the article, and accordingly quicker cooling thereof. On
the other hand, it can easily be focused or collimated to
provide uniform heat distribution over a relatively wide path.
For example, laser beam widths o~ greater than 1/8 inch
(3.175 mm) are easily achieved, whereas electron beam widths
are normally restricted to less than 1/16 inch (1.59 mm). ~`
Greater surface areas can therefore be hard surfaced utilizing
the method of the present invention.
It should also be appreciated that the method and
apparatus of the present invention may be easily adapted to
make a composite metal article by applying a relatively solid
coating material with a preformed shape to the base material.
For example, rather than depositing only the relatively clean
and pure powder 12 on the engine valve 14 as in the instant
case, such powder can be mixed with a suitable binder and
formed into an annular ring. The ring can then be placed on
the valve and sub~ected to the heating energy of the laser beam
50 substantially as described above without departing from the
spirit of the present invention. It is apparent that various
relatively thick coatings of material dissimilar to the base
-~ material, in the order of 0.050" (1.27 mm) thickness, can be
quickly applied in a single pass utilizing these principles.
This obviates the need to make multiple passes which can ex-
cessively heat the article.
While the invention has been described and shown with
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~ particular reference to a preferred embodiment, it will be
.
apparent that other variations mi~ht be possible that would fall
~ithin the scope of the present invention, which is not intended
to be limited except as defined in the following claims.
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