A wetting balance includes a load cell for suspending a specimen above a crucible containing a molten braze pool. A lift supports the crucible for vertical lifting thereof to effect an immersion cycle of the specimen in the pool. A controller operates the lift and determines first contact with the pool by measuring force changes thereat and produces a measured force log during the immersion cycle. In operation, the lift is operated for a predetermined time following first contact for immersing the specimen to a predetermined depth in the pool.

BACKGROUND OF THE INVENTION
 The present invention relates generally to brazing, and, more specifically,
 to wetting balances for measuring wetting force during brazing.
 Soldering and brazing are related processes used for bonding together metal
 components. Soldering occurs at relatively low temperature for typically
 bonding together copper or brass parts, and brazing requires substantially
 higher temperatures for bonding together high strength materials including
 stainless steel and various superalloys typically used in manufacturing
 gas turbine engines, including nickel based superalloys.
 Various brazing materials are tailored to the specific material of the
 parts being brazed for effecting high strength joints therewith. However,
 an effective braze joint requires wetting of the molten braze material
 with the surface of the part so that upon cooling and solidification
 thereof a strong integral metallurgical bond is created.
 Wetting of solder with copper parts generally occurs practically
 instantaneously. Wetting of typical braze materials with corresponding
 parts may occur practically instantaneously or may require a considerable
 time exceeding tens of seconds, and in some cases wetting may not occur at
 all irrespective of the amount of time permitted therefor.
 The wetting time of a particular braze material with a corresponding part
 requiring brazing is a primary parameter in the brazing process. Whether
 brazing is effected manually or automatically, sufficient time must be
 provided to ensure proper wetting for obtaining a full strength braze
 joint.
 Wetting performance of solder and braze materials is typically evaluated in
 commercially available wetting balances. A typical wetting balance
 suspends a specimen from a weighing device such as a micro-balance or load
 cell, and a crucible containing molten solder or braze material is lifted
 for immersing the bottom portion of the specimen into a molten solder or
 braze pool to a preferred depth. The molten solder or braze pool applies a
 force to the specimen which varies depending upon the degree of wetting or
 non-wetting thereof.
 In wetting, the molten pool adheres to the specimen and creates a rising
 positive meniscus having a corresponding wetting angle less than
 90.degree.. In non-wetting, the molten pool does not adhere to the
 specimen and effects an opposite depression or negative meniscus with a
 correspondingly different wetting angle greater than 90.degree..
 By accurately measuring the force applied to the specimen during immersion
 in the molten pool, the wetting angle and wetting force may be
 analytically derived. Accordingly, the wetting performance of various
 solder or braze materials may be quantitatively evaluated.
 However, one form of a wetting balance uses a precision micro-balance beam
 for suspending the specimen and measuring the applied force from the
 molten pool. The balance beam measures force precisely, yet is relatively
 slow in operation for balancing the applied force. And, the specimen
 correspondingly changes immersion depth as the balance beam equilibrates.
 Another type of wetting balance includes a conventional load cell from
 which the specimen is suspended. The load cell, although not as precise as
 the balance beam, measures applied force substantially instantaneously.
 And, the specimen is maintained at a fixed elevation which does not affect
 the immersion depth in the molten pool.
 In both wetting balance types, the specimen is partially immersed in the
 molten pool, and the immersion depth must be accurately determined for use
 in accurately determining the wetting angle and wetting force. Immersion
 depth is typically separately measured by optical observation.
 Accordingly, it is desired to provide a wetting balance having improved
 accuracy and ease of use in measuring applied force and determining
 corresponding wetting force.
 BRIEF SUMMARY OF THE INVENTION
 A wetting balance includes a load cell for suspending a specimen above a
 crucible containing a molten braze pool. A lift supports the crucible for
 vertical lifting thereof to effect an immersion cycle of the specimen in
 the pool. A controller operates the lift and determines first contact with
 the pool by measuring force changes thereat and produces a measured force
 log during the immersion cycle. In operation, the lift is operated for a
 predetermined time following first contact for immersing the specimen to a
 predetermined depth in the pool.

DETAILED DESCRIPTION OF THE INVENTION
 Illustrated schematically in FIG. 1 is a wetting apparatus or balance 10 in
 accordance with an exemplary embodiment of the present invention. The
 balance is an assembly of components configured for measuring force on a
 suitable test specimen 12 during brazing.
 The balance includes means in the form of a load cell 14 which includes a
 holder or sling 16 for suspending the specimen 12 below the load cell and
 measuring vertical force thereon. The load cell may have any conventional
 configuration and preferably includes a tare feature for offsetting the
 entire suspended weight of the specimen when not subject to externally
 applied force. The output of the load cell is an electrical signal
 representative of external force applied to the suspended specimen.
 A crucible 18 is disposed below the load cell 14 in vertical alignment with
 the specimen for containing a molten braze liquid or pool 20 for use in
 conducting a wetting balance test with the corresponding specimen. The
 specimen 12 may have any desired material composition, such as stainless
 steel or various nickel based superalloys typically used in gas turbine
 engines.
 The braze pool 20 may have any suitable material composition for use in
 testing braze performance with the corresponding specimen 13. In this way,
 the wetting performance of the specific combination of specimen and braze
 materials may be evaluated for ensuring effective braze joints of parts
 requiring brazing in any desirable manufacturing application.
 The crucible 18 may have any suitable configuration and is typically formed
 of any inert material for withstanding the high temperature required for
 maintaining the braze material in its molten state. For example, the
 crucible may be formed of a suitable ceramic such as Al.sub.2 O.sub.3.
 Means in the form of an elevator or lift 22 are provided for supporting the
 crucible below the specimen for vertical lifting of the crucible in
 conducting an immersion-emersion cycle for a wetting balance test.
 The lift 22 may have any conventional form including an electrical motor
 driving a screw shaft having a platform at a distal end supporting the
 crucible. In this way, the vertical elevation of the crucible may be
 accurately controlled as it is raised or lowered during the test cycle.
 Means in the form of an electrical control 24 are operatively joined to the
 load cell 14 by a suitable electrical line for obtaining an electrical
 signal corresponding with the measured force from the load cell. The
 controller 24 is also operatively joined to the lift 22 through a suitable
 electrical line to the motor thereof for controlling elevation of the lift
 platform and the crucible 18 resting thereatop.
 In a preferred embodiment, the controller 24 is in the form of a digitally
 programmable computer which may be configured in suitable software for
 controlling the entire wetting balance test cycle and data generated
 therein.
 More specifically, the controller 24 is configured in a programmed cycle
 for automatically operating the lift to elevate the crucible 18 from an
 initial retracted or resting position suitably below the specimen. The
 crucible is lifted vertically upwardly for achieving first contact between
 the specimen and molten pool at which a corresponding first contact force
 is measured by the load cell 15. The lift is also operated to immerse the
 specimen into the braze pool to a predetermined depth H in a predetermined
 time t following detection of the first contact force for self-immersing
 the specimen in the pool.
 At the predetermined depth, the lift is stopped for maintaining the
 immersion depth constant for a predetermined dwell time or interval as
 required for observing wetting between the specimen and braze pool, if
 wetting occurs at all.
 The lift 22 is then operated in reverse for retracting or withdrawing its
 platform and the crucible supported thereon for withdrawing the specimen
 from the pool. And, during this immersion-emersion test cycle, the output
 from the load cell 14 is received by the controller 24 for producing or
 recording a history log 26 of the force F on the specimen due to the
 molten braze material as measured by the load cell over the elapsed time
 of the desired cycle.
 The wetting balance 10 illustrated in FIG. 1 is operated by initially
 suspending the specimen 12 of desired material composition from the load
 cell. The desired braze material being tested is then suitably melted to
 form the molten pool 20 contained in the crucible 18. The crucible is
 supported by the lift 22 at any suitable rest position directly below the
 specimen 12 for positioning the braze pool directly below the specimen.
 The controller 24 is suitably programmed for automatically effecting the
 desired immersion-emersion test cycle by initially operating the lift for
 lifting the crucible and pool therein upwardly toward the specimen for
 controlled immersion of the specimen therein.
 In accordance with the present invention, the load cell 14 is used for
 detecting first contact of the specimen upon immersion into the braze
 pool. Prior to immersion, the load cell 14 measures no external applied
 force and produces an output signal representative of either the starting
 weight of the specimen, or a zero weight due to subtracting the
 corresponding tare weight of the specimen supported in the load cell.
 Upon first contact of the specimen in the braze pool, an initial first
 contact force is detected or measured by the load cell and communicated to
 the controller 24. Any suitable value of detected change in force due to
 first contact of the specimen and pool may then be used to effect the
 predetermined immersion depth H desired.
 The controller 24 merely stops operation of the lift 22 at a predetermined
 time t following detection of first contact corresponding with the desired
 immersion depth H. By simply stopping operation of the lift 22 after a
 suitable time period from detection of first contact, the specimen may be
 precisely immersed at the desired depth H without the need for optical
 control of that immersion depth, or control thereof by other means. The
 wetting balance is thusly configured for self-immersing the specimen in
 the pool automatically and with high precision.
 Upon achieving the desired immersion depth, the lift 22 is turned off for
 then maintaining a substantially constant immersion depth for a desired
 dwell time or interval for evaluating whether or not wetting of the braze
 pool is effected with the immersed specimen.
 Following the desired dwell period, the controller operates the lift 22 for
 retracting or lowering the crucible 18 to correspondingly withdraw the
 specimen from the braze pool to complete the testing cycle.
 During the entire cycle, the load cell transmits the measured force signal
 to the controller 24, and the controller suitably records or logs the
 measured force over the time of the testing cycle to produce a
 corresponding data log 26. The data log may have any suitable form such as
 the force-time graph illustrated generally in FIG. 1, and in more detail
 in FIG. 2.
 The test log 26 illustrated in FIG. 2 is an example of measured force from
 the load cell 14 for a specimen immersed in a braze pool in which wetting
 occurs over a suitable elapsed time. The log is highlighted with six
 corresponding conditions of the specimen being immersed in the braze pool
 designated (a)-(f).
 More specifically, the test starts with the crucible disposed below the
 specimen and being raised until first contact of the specimen with the
 braze pool at condition (a). Prior to first contact, the load cell
 produces an indication of zero applied force on the specimen.
 Lifting of the crucible correspondingly causes the specimen to be immersed
 in the braze pool to the maximum or predetermined immersion depth H
 occurring at condition (b). As the specimen is submerged in the braze
 pool, a buoyancy force is developed on the specimen which pushes upwardly
 on the specimen for developing a negative value of applied force. The
 maximum or peak negative force applied prior to wetting occurs immediately
 upon immersion to the maximum depth at condition (b).
 In the event that the materials of the specimen and braze pool are
 compatible for developing braze wetting, wetting is initiated over time
 following maximum immersion, and includes the two conditions (c) and (d).
 As wetting occurs, the measured force increases from its negative peak
 until it equals that negative force due to buoyancy at condition (c). As
 wetting continues, the measured force F increases further and becomes
 positive. The positive measured force corresponds with wetting which pulls
 the stationary specimen in the downward direction into the pool. The
 measured force continues to rise positive as wetting continues until it
 reaches a substantially steady state or constant positive value beginning
 at condition (d).
 Following the steady state measured force, the crucible is lowered which
 introduces a positive peak force at condition (e) as the specimen is
 withdrawn from the braze pool. The measured force drops to substantially
 zero value at condition (f) after removal of the specimen from the pool,
 and may have a slightly positive value due to any remaining braze material
 adhering to the specimen.
 Wetting force is a term of art and is developed by the balance of surface
 tension acting on a liquid as it wets or does not wet a liquid surface.
 Wetting force is a function of wetting angle which is the angle defined by
 the balance of surface tension developed between the specimen and
 surrounding vapor, the surface tension between the molten liquid and the
 specimen, and the surface tension between the molten liquid and the vapor.
 FIG. 3 illustrates schematically on the left side, wetting of the braze
 pool 20 with the specimen 12 which creates a positive meniscus due to
 capillary rise, with a corresponding wetting angle A less than 90.degree..
 On the right side of FIG. 3, the braze pool is non-wetting with the
 specimen and creates a negative meniscus with a corresponding wetting
 angle A greater than 90.degree..
 Accordingly, the wetting force on a specimen may be derived in different
 manners by measuring applied external force on the specimen from the braze
 pool and calculating wetting force. Or, optical observations of the
 meniscus may be used for determining wetting force.
 One derivation of wetting force FW is represented by the following
 equation:
EQU FW=(9.807/(2(W+b)).times.(F+.rho.WbH)
 In this equation, W is the width of the rectangular specimen beam, b is its
 thickness, H is the depth of immersion, p is the density of the braze
 liquid, and F is the measured force from the load cell having an initial
 value of zero prior to immersion. Force data from the measurement log 26
 illustrated in FIG. 2 may be applied to this equation for producing
 corresponding values of the wetting force for further analysis as desired.
 A particular advantage of the wetting balance illustrated in FIG. 1 is its
 ability to automatically self-immerse the specimen 12 to the predetermined
 immersion depth H following self-detection of first contact with the braze
 pool. The specimen is then quickly and accurately immersed to the selected
 depth, and that depth is accurately maintained constant for the desired
 duration of the test prior to retraction of the crucible.
 The use of the load cell 14 is preferred over a conventional micro-balance
 for quickly and accurately detecting first contact from which the desired
 immersion depth is referenced. In a preferred embodiment, first contact is
 detected by measuring change in force from the load cell which exceeds a
 pre-selected threshold force value suitably above electronic noise and
 preferably less than about 1% of the maximum load rating capability of the
 load cell. Preferably, the threshold force is about 0.1% of the maximum
 load rating capability of the load cell, and corresponds with about 0.05
 gram which is readily detectable by the load cell.
 Upon detection of first contact at this substantially small threshold
 value, the lift 22 continues to operate preferably at a substantially
 constant speed S to lift the crucible at that speed so that the
 predetermined immersion depth H may be obtained by simple clock timing of
 the lift for the desired time corresponding with the desired immersion
 depth.
 By using the controller 24 in the form of a digitally programmable
 computer, the computer may include an internal timing circuit or clock 28
 suitably configured with software for stopping immersion lifting operation
 of the lift 22 at the desired predetermined time following detection of
 the first contact for immersing the specimen to the desired predetermined
 depth H. For a desired immersion depth H, and with a given constant lift
 speed S, the corresponding immersion time t is simply the quotient
 thereof.
 Depending upon the specific material compositions of the specimen and
 braze, wetting may occur almost instantaneously upon immersion, or may
 occur after several seconds, or may not occur at all in a non-wetting
 combination. The immersion-emersion test cycle must be sufficiently long
 for correspondingly long wetting or non-wetting applications, or may be
 relatively short for fast wetting material combinations. In either case,
 it is desirable to accurately measure the applied force on the specimen
 due to immersion in the braze pool for accurately deriving the wetting
 force.
 The load cell 14 preferably has any suitable conventional configuration for
 measuring force on the specimen substantially instantaneously, as opposed
 to the slower acting micro-balances typically used in wetting balances.
 The time response of the load cell should be suitably fast for accurately
 measuring force preferably at 0.1 second intervals or faster.
 In this way, the lift 22 may operate at relatively fast elevating speeds
 for promptly immersing the specimen to the required depth and immediately
 recording the applied loads from the braze pool. In a preferred
 embodiment, the lift speed S is greater than about 10 mm/min and may be up
 to about 600 mm/min, or higher if desired.
 In order to protect the load cell 14 during relative movement between the
 suspended specimen and the crucible, the sling 16 is preferably flexible
 yet permits effective immersion of the specimen in the molten braze.
 More specifically, the sling 16 illustrated in FIG. 1 preferably includes a
 straight rod 16a threaded at its top end for being threadingly joined to a
 threaded stem on the load cell. A flexible wire 16b is suitably suspended
 from the lower end of the rod, and may be formed of a suitable material
 such as platinum or stainless steel.
 A specimen holder 16c is suitably suspended from the lower end of the wire
 by welding or mechanical attachment. And, the specimen 12 is then suitably
 attached to the lower end of the holder by screwing, clamping, or welding.
 The flexible wire may buckle under excessive upward force on the specimen
 during immersion to protect the delicate load cell from overloading. For
 example, if the braze is not sufficiently molten, impact of the specimen
 therewith will buckle the wire and protect the load cell.
 However, the specimen holder is provided with sufficient mass to ensure
 that the specimen is forced into the braze when sufficiently molten. The
 holder acts like an anchor under gravity to immerse the attached specimen
 into the molten braze notwithstanding the flexibility of the wire, which
 wire is weighted by the holder to remain taught.
 The rod, wire, holder, and specimen are preferably coaxially aligned with
 each other to maintain a vertically straight loadpath through the wire to
 the load cell for accurately measuring the small changes in force as the
 specimen undergoes the immersion cycle.
 As the immersion speed increases, the operating time t to achieve the
 desired predetermined depth H decreases. As illustrated in FIG. 2, the
 measured force F from the load cell, as the specimen is rapidly immersed
 in the pool, increases with a steep slope to the peak or maximum negative
 measured force at condition (b) at which the maximum immersion depth is
 first reached within the corresponding travel time t.
 An accurate value of the peak negative force F is measured by the load cell
 14 and may be used in a conventional manner for deriving both surface
 tension and wetting force at condition (b).
 In FIG. 2, wetting may occur between conditions (b) and (d) in the course
 of several seconds. Or, such wetting may occur substantially
 instantaneously in less than a second or fraction of a second. Or, wetting
 may not occur at all irrespective of the amount of testing or dwell time
 following condition (b).
 Fast wetting braze-specimen combinations may be accurately tested
 notwithstanding the speed of immersion, which may occur in less than a
 second. The self-immersing specimen quickly and accurately reaches the
 desired predetermined immersion depth H while the load cell 14 quickly and
 accurately measures the applied force which is conveniently stored and
 displayed by the computer 24 in any desired manner.
 Brazing occurs at temperatures substantially greater than those typically
 found in common soldering. And, a commercially available wetting balance
 configured for testing solder wetting lacks the capability for suitably
 testing brazes.
 Accordingly, the wetting balance 10 illustrated in FIG. 1 preferably also
 includes an isolation chamber 30 in the preferred form of a quartz tube
 sealingly surrounding the crucible 18 and the specimen 12 for isolation
 thereof from the surrounding air environment. The chamber 30 is suitably
 sealingly joined to the load cell 14 and lift 22 with O-rings or other
 seals as desired.
 The chamber may be joined to the lift 22 using a suitable steel expansion
 bellows to provide sealing while permitting lifting motion of the
 supporting crucible. A quartz rod preferably extends between the lift and
 crucible to uncouple heat transfer therebetween.
 A suitable vacuum pump 32 may be operatively joined to the isolation
 chamber 30 for evacuating air therefrom. For example, the vacuum pump 32
 may be operated to evacuate the chamber 30 of air which is substantially
 free or devoid of air and oxygen to less than about 50 part-per-billion
 (ppb), for example. Undesirable oxidation during the testing process may
 be reduced by substantially eliminating oxygen in any conventional manner
 including titanium getter foils.
 The isolation chamber 30 may then be filled with a suitable inert gas 34
 provided from a suitable gas supply operatively joined to the chamber. A
 suitable inert gas is argon, and may also include hydrogen.
 In order to maintain a suitable elevated temperature of the molten braze
 pool 20 during the test, a heating furnace 36 suitably surrounds the
 crucible 18, preferably outside the chamber 30 for heating the pool to
 maintain a desired temperature thereof. The furnace 36 may take any
 conventional form such as infrared radiant electrical furnace for
 achieving molten temperatures up to about 1125.degree. C.
 Soldering typically occurs at solder temperatures less than about
 400.degree. C., with brazing requiring temperatures substantially greater.
 High temperature braze testing requires corresponding low vacuum in the
 isolation chamber, with the air environment being replaced by an inert gas
 for reducing undesirable oxidation during testing. For higher temperature
 braze testing in the exemplary range of 1500.degree. C.-2500.degree. C.,
 greater initial vacuums up to about 10.sup.-6 torr are desirable. And,
 inert gas in combination with hydrogen at greater than about 3%, or even
 pure hydrogen, is preferably used in the testing apparatus.
 Higher temperature operation of the wetting balance may be achieved using
 higher temperature resistance furnaces, or a radiant energy furnace using
 quartz heat lamps and parabolic reflectors, or a radio frequency (RF)
 furnace with a susceptor.
 By suitably heating the braze pool 20 in an environment of inert gas
 substantially devoid of air and oxygen during the immersion testing cycle,
 accurate braze wetting performance may be tested.
 By logging the measured force over time during the testing cycle for
 determining the specific configuration of the wetting performance log 26
 illustrated in FIG. 2, accurate evaluation of the wetting performance for
 specific combinations of specimen and braze material may be obtained.
 For example, the wetting force attributable to a specific combination of
 specimen and braze materials may be readily determined using the exemplary
 equation described above in which the geometric parameters of the specimen
 are given, the density of the braze pool is given, the immersion depth H
 is specified and automatically effected in the manner disclosed above,
 with the measured force represented by the log illustrated in FIG. 2 being
 introduced into the equation for determining a corresponding wetting force
 over the duration of the test.
 The measured force data contained in the log 26 illustrated in FIG. 2 may
 also be used in any conventional manner in evaluating various parameters
 associated with braze wetting, including wetting angle and surface
 tension, for example.
 The self-immersion feature of the present invention may be readily
 incorporated in various forms of wetting balances to advantage. Immersion
 depth may be accurately effected and maintained by timed operation of the
 lifting apparatus following the detection of first specimen-to-pool
 contact. The specimen may be immersed to a specific depth which is then
 maintained constant for the duration of the test. Alternatively, the
 immersion depth may be varied as desired during testing for any suitable
 advantage. In either case, accurate determination of the actual immersion
 depth is readily known by the time interval following first contact during
 which the crucible is elevated at constant speed.
 While there have been described herein what are considered to be preferred
 and exemplary embodiments of the present invention, other modifications of
 the invention shall be apparent to those skilled in the art from the
 teachings herein, and it is, therefore, desired to be secured in the
 appended claims all such modifications as fall within the true spirit and
 scope of the invention.