|
|||||||
|
|
|
|||||
|
|
|||||||
Atmosphere: Dry H2 at
816°C This problem is solved by coating the surface with flux, or if no residual
flux is desired, then pre-electrolytic plating with Ni or Cu. C110 copper is a tough pitch grade, which means it contains copper oxide at
the grain boundaries. When heated under H2, the
H2 permeates through the copper and forms steam,
leaving the Cu porous. This porosity then soaks up the filler metal; therefore,
insufficient filler remains at the surface to form a joint. H2 atmospheres are unsuitable for brazing of
tough pitch copper, and vacuum, argon or nitrogen should be used. Atmosphere: Dry H2 at
1010°C Copper types, such as electrolytic tough pitch, tough pitch and fire-refined,
all contain cuprous oxide and are subject to embrittlement in an H2 atmosphere. Oxygen-free and phosphorus de-oxidised copper
do not suffer from this problem. As explained previously, it is the formation of
steam which causes the problem. Where there are copper oxide stringers or
inclusions, blistering will occur. All the copper parts were electroformed; however, the one which was
successfully brazed had been vacuum annealed at 760°C. This treatment, or use of
dry argon or nitrogen, is recommended before brazing to remove traces of copper
oxide. With 304L stainless steel as one of the components, the assembly could be
brazed in the above atmospheres. If adequate brazing does not occur, it could be
a result of low retort flow rate. An electrolytic nickel plating on the
stainless steel will facilitate the production of a good brazed joint. Atmosphere: Air and torch
brazing If chromium is removed, then corrosion between the surface and filler may
occur and the primary cure for this is to use a nickel-containing filler metal.
Two useful low temperature silver-based fillers are AG7 and AG1, the former is
preferred since it is Cd-free. The nickel forms a nickel-rich layer at the interface thus inhibiting
corrosion. If possible a protective atmosphere is used, since it removes the
need for flux which leaches out the chromium. Atmosphere: Purged with liquid N2 then H2 added to 760°C.
All gas was shut off during the cooling process. Secondly, very dry N2 and H2 gases readily absorb N2
in the transmission lines; therefore, these must also be checked. Finally, the atmosphere flow rate should be such that all outgassing products
are drawn out of the furnace. Typically 0.0045m3/min of gas per m3 of
retort chamber size. Problem: What atmosphere and dewpoint are required and which
base metals may present difficulties under such
conditions? Regarding base materials, chromium irons and chromium containing stainless
steels that do not contain Ti or Al will be readily brazed. With Ti and Al, if
the dewpoint is acceptable, brazing will occur. Alternatively, the parts may be
plated with a layer of electrolytic nickel - the thickness of which is dependent
on the amount of Al and Ti in the base metal. For the filler metals, temperatures less than 1150°C are used to save wear
and tear on furnaces. In general CU2, HTN7 and the silver fillers are useful.
Boron-containing fillers should be avoided, due to formation of nitride
compounds, and zinc and cadmium fillers which vaporise and change the properties
of the filler material. (Cadmium is also poisonous and should be avoided where
possible). Atmosphere: Dry H2 at
800°C. The easiest solution is to use an appropriate flux, or to plate the bronze
with copper or nickel. A further problem to be solved is that the high brazing
temperature of AG7 may cause de-zincification of the brass; therefore a lower
liquidus braze filler, such as AG1 would be preferred, which can be brazed at
640°C. To help avoid de-zincification, the brass could also be nickel or copper
plated. Atmosphere: Vacuum A second point is that HTN5 filler contains Cr, a further carbide former. To
prevent this condition, stop-off or a ceramic separator should be used to
isolate the workpiece from the graphite jig. Atmosphere: Vacuum Ways used to overcome this problem were: Atmosphere: Vacuum A cost-effective approach was to attach a screw-threaded stainless steel
fixture to the cool (200°C) end of the burner nozzle using an Ag-Cu-Ti active
braze filler. Because of very different coefficients of thermal expansion (CTE),
a direct joint between the two materials may have stressed the ceramic to the
point of failure. The solution was to introduce an interlayer of tungsten carbide which has a
CTE closely matched to the ceramic. This ring of tungsten carbide served to
transfer the stress away from the ceramic to the more resilient junction with
the stainless steel. Atmosphere: Vacuum One approach might have been to use a low or graded CTE interlayer, but
modelling indicated that the interlayer would be too large to fit within the
existing design. The solution was to use a corrugated flexible interlayer
fabricated from thin steel, brazed to each material with an active Ag-Cu-Ti
braze. Other considerations are the operation and maintenance of brazing equipment
as well as good house keeping. Appropriate Personal Equipment (PPE) e.g.
footwear, overalls, gloves and eye protection or face-masks should also be
provided. Some braze filler metals contain toxic elements and as such the relevant
safety standard should be consulted prior to use. Similarly, for fluxes, skin
contact and fume inhalation must be avoided. Care should also be taken in storing materials before use and subsequent
disposal of residues, exhaust emissions and other associated effluent. The following sections highlight appropriate safety precautions for the
different brazing processes. The above list is not comprehensive and any safety information
supplied with equipment must be rigorously adhered to and the material
data/safety sheets for all materials and consumables must be consulted before
use.
About the IIW /
Mumbai Branch /
Other Branches /
Coming Events /
Technical Lectures 
THE INDIAN INSTITUTE OF
WELDING - MUMBAI
About the IIW /
Mumbai Branch /
Other Branches /
Coming Events /
Technical Lectures
Mumbai Weldnet /
Trends in welding /
Related Websites /
IIW Forum /
Feedback /
Home
REPRODUCED - COURTESY TWI-UK
Brazing - a guide to best practice
Section 7. Case studies, health
and safety implications definitions.
Case Studies:
C110 Copper plate to 51%Ag/49%W sheet using
AG7
Problem: Parts not brazing and filler metal
disappearing
Solution: Firstly, the atmosphere and brazing
temperature were examined. H2 is not reactive
enough to give good wetting on to tungsten and hence wettability on to a W/Ag
alloy would be minimal.Copper to 304L stainless steel using HTN7
Problem: One part brazed perfectly, whilst the other
blistered; enlarged dimensions, and where the filler metal had been, all that
was left was a dark residue.
Solution: Blistering and the
enlargement of copper parts is usually a sign of the presence of Cu-oxide
particles within the copper.300 series stainless steel tube to copper or brass with
silver filler metals
Problem: Which filler materials and fluxes should be
used?
Solution: Stainless steel may be brazed with a variety
of filler metals, selection is usually made based on the final application. For
torch brazing, stainless steel does not conduct heat as readily as copper or
brass and can be easily overheated. Two standard fluxes may be used: white flux
(FB3-A) or boron-modified black flux (FB3-C). A green colour after brazing is
due to the removal of chromium oxide from the surface of the stainless
steel.Installation of retort furnace for copper brazing AISI
304 stainless steel heat exchangers
Problem:
Parts would not braze and assemblies appeared
green.
Solution: The first consideration is the atmosphere
and all vacuum lines, pumps and valves require leak checking.Installation of continuous mesh belt furnace for
brazing stainless steel using copper, nickel and silver filler metals
Solution: For a straight-through mesh belt
furnace the most commonly used atmospheres for brazing stainless steels are
N2/H2
(20-50%N2 and a dissociated ammonia
75%H2/25%N2).
The dewpoint must be as low as possible to compensate for the increase of
O2 and moisture brought into the furnace on the
belt and parts.Naval brass to aluminium bronze with AG7
Problem: Poor filler flow and hence poor
joints.
Solution: The most likely cause of the problem is
the aluminium content in the bronze, since aluminium readily oxidises, even in a
dry H2 atmosphere. Alternatively, the atmosphere
could cause the Al to off-gas from the surface hence aluminising the filler
metal, which in turn would cause wettability problems.Ni-Cr-Fe with HTN5 filler using a graphite jig
Problem: Graphite jig
had melted into the part
Solution: Pure metals, such as
copper, and nickel, and copper alloys, including copper-nickel, do not pick up
carbon; however, stainless steels (300 and 400 series), titanium, zirconium,
etc, readily alloy to form a low-melting point alloy.Copper to graphite using Ni-Cr-P
Problem: Change of
joint geometries caused graphite to crack during
brazing
Solution: Graphite targets, produced to a particular
design for a number of years, had been brazed with a commercial Ni-Cr-P alloy.
This braze was selected since it was known to wet on to graphite. The required
brazing temperature is a minimum of 940°C and the compounds formed between the
braze and the graphite, which enable wetting, are based on chromium carbide.
Although it produces a relatively brittle compound, the process is well
established and generally accepted.
Stainless steel to sintered silicon carbide
Problem: How to
overcome thermal expansion mismatch for 200°C
service
Solution: A tubular silicon carbide burner nozzle
needed to be attached to a steel burner assembly. Occasional replacement of the
nozzle was anticipated, requiring a non-permanent joint. Machining a
screw-thread onto the nozzle was considered inappropriate, particularly as only
the hot (1400°C) end of the nozzle needed to be fabricated from silicon
carbide.Steel to silicon nitride for automobile engine
tappets
Problem: To introduce
a low CTE (coefficient of thermal expansion) ceramic into an existing high CTE
steel component
Solution: A car engine manufacturer wished
to take advantage of the improved wear properties of silicon nitride by facing
existing steel tappets with the ceramic. The component was required to survive
significant thermal cycling, and the two materials had very different thermal
expansion coefficients.Health and safety implications
Introduction
Although brazing is
well-established throughout industry as a reliable and safe method of assembling
metal components, attention to health and safety precautions is necessary. In
particular, these relate to burns, combustion products from torches and fumes
from fluxes and metals. Additional to this is the need for good ventilation and
general common sense.
Torch brazing
Care has to be taken to operate
burner equipment and gas installations strictly to the manufacturer's
instructions, since all oxyfuel mixtures are potentially explosive. Hoses should
be checked regularly for signs of chafing or cracking and replaced as required.
Non-return valves and flash-back arrestors are also required. Direct hazards are
-
Induction brazing
When high frequency current
is passed through the coil, close proximity of other metal objects causes severe
burns. The manufacturer's instructions regarding maintenance and use must be
followed. Direct hazards are -
Resistance brazing
The health risks from this
process are limited to a spatter risk caused by dirty or high resistance parts,
flux films or insufficient holding pressure. Since cycle times are short and the
volume of fume produced is small, local extraction is usually adequate. The
direct hazards are -
Furnace brazing
For protective atmosphere
brazing, the fumes must be extracted and there is an explosion risk if highly
reducing atmospheres such as hydrogen are used. In vacuum, there are few hazards
except from residual back fill of gases in larger furnaces. For open furnaces,
there are the usual risks with the use of fluxes. The direct hazards are -
Immersion/dip brazing
There is a high risk of
explosion if brazed assemblies are not dried before dipping in the bath, or if
inadequate pre-heat is used. Irritant fumes are emitted by the molten flux and
toxic metal oxide. Splashing by molten liquid is an ever present risk. The
direct hazards are -
References
Definition of terms used in
brazing
As-brazed
Refers to the condition of brazements after brazing, before
any post-brazing thermal, mechanical or chemical treatments.
Automatic brazing
Brazing that requires only occasional or no observation
during brazing, and no manual adjustment of the equipment controls.
Balling up
The formation of spheres of molten brazing filler metal or
flux due to lack of wetting of the base material.
Base material
The metal or alloy that is brazed.
Blind joint
A joint of which no portion is visible.
Braze
A joint produced by heating an assembly to the brazing
temperature using a filler metal having a liquidus above 450°C and a
solidus below those of the base materials. The filler metal is distributed
between the faying surfaces of the joint by capillary action.
Braze interface
The interface between base material and filler metal in a
joint.
Braze welding
A process variation in which a filler metal, having a
liquidus above 450°C and a solidus below those of the base materials, is
used. Unlike brazing, in braze welding the filler metal is not distributed
in the joint by capillary action.
Brazeability
The capacity of a material to be brazed under the
conditions imposed, into a suitably designed structure, and to perform
satisfactorily for the intended service.
Brazement
An assembly whose component parts are joined by
brazing.
Brazer
Person who performs manual brazing.
Brazing
Joining processes which produce coalescence of materials by
heating them to the brazing temperature in the presence of a filler metal
having a liquidus above 450°C and a solidus below that of the base
material. The filler metal is distributed between the faying surfaces of
the joint by capillary action.
Brazing alloy
A brazing filler metal.
Brazing filler metal
The metal which fills the capillary joint clearance and has
a liquidus above 450°C and a solidus below that of the base
materials.
Brazing foil
A brazing filler metal in strip form.
Brazing powder
A brazing filler metal powder.
Brazing procedure
The detailed method involved in the production of a
brazement.
Brazing rod
A brazing filler metal rod.
Brazing rope
A brazing filler metal in the form of fine wires woven
together.
Brazing sheet
A composite sheet consisting of base metal coated with
brazing filler metal.
Brazing shim
A brazing filler metal in strip form.
Brazing tape
A brazing filler metal in strip form.
Brazing temperature
The temperature to which the base material is heated to
enable the filler metal to wet the base materials and form a brazed
joint.
Brazing temperature range
The temperature range within which brazing can be
performed.
Butt joint
A joint between two members aligned approximately in the
same plane.
Capillary action
The force by which liquid, in contact with a solid, is
distributed between closely fitted faying surfaces of the joint to be
brazed.
Carburizing flame
A reducing oxyfuel gas flame in which there is an excess of
fuel gas, resulting in a carbon-rich zone.
Chemical-bath dip brazing
A dip brazing process.
Clad brazing sheet
Metal sheet having one or both sides clad with brazing
filler metal.
Coefficient of thermal expansion
The fractional change in length or volume of a material for
a unit change in temperature at constant pressure.
Coil
An electrical device connected to an induction generator
designed to provide induction heating of a workpiece.
Complete joint penetration
Brazing filler metal penetration for the full extent of the
intended joint.
Copper brazing
Brazing with a copper filler metal.
Corner joint
A joint between two members located approximately at right
angles to each other.
Crack
A fracture type discontinuity characterised by a sharp tip
and high ratio of length to width opening displacement.
Defect
A discontinuity or discontinuities which render a part
unable to meet minimum applicable acceptance standards or specifications.
This term designates rejectability.
Differential thermal expansion
The difference between the dimensional changes of two (or
more) materials having different expansion coefficients, when heated
through a given temperature range.
Diffusion brazing
A brazing process which forms liquid braze metal by
diffusion between dissimilar base materials or between base materials and
filler metal, pre-placed at the faying surfaces. The process may be used
with the application of pressure.
Dilution
The change in chemical composition of a brazing filler
metal caused by the mixture of the base materials or previous brazing
filler metal.
Dip brazing
A process using heat from a molten chemical or metal bath.
When a molten chemical bath is used, the bath may act as a flux. When a
molten metal bath is used, the bath provides the filler metal.
Discontinuity
An interruption of the typical structure of a brazement. A
discontinuity is not necessarily a defect.
Dissolution
The dissolving of filler metal in one or more of the base
materials of a joint.
Edge joint
A joint between the edges of two or more parallel or nearly
parallel members.
Electric brazing
A term for resistance brazing.
Electromagnetic field
The field created when alternating current passes through
an inductor.
Electron beam brazing
A process using heat from a slightly defocused electron
beam.
Erosion
A condition caused by dissolution of the base material by
molten filler metal resulting in a reduction of base material
thickness.
Exothermic brazing
A process using an exothermic chemical reaction between a
metal oxide and a metal or inorganic non-metal as the heat source, with
filler metal pre-placed in the joint.
Face feed
The application of filler metal to the joint, usually by
hand, during brazing.
Faying surface
The mating surface of a member which is in contact with or
in close proximity to another member to which it is to be bonded.
Filler metal
The metal or alloy to be added in making a brazed
joint.
Fillet
A region of brazing filler metal where workpieces are
joined.
Fillet joint
A joint that is designed to have a fillet.
Fit
A term for joint clearance.
Flash coat
In brazing, a thin coating usually less than 0.005mm
thick.
Flat position
The brazing position used to braze from the upper side of
the joint; the face of the braze is approximately horizontal.
Flaw
An undesirable discontinuity.
Flowability
The ability of molten brazing filler metal to flow or
spread over a surface.
Flow brazing
A process that bonds materials by heating them with molten
non-ferrous filler metal poured over the joint until brazing temperature
is attained. The filler metal is distributed in the joint by capillary
action.
Flux
A material used to hinder or prevent formation of oxides
and other undesirable substances in molten metal and on solid metal
surfaces, and to dissolve or otherwise aid removal of such
substances.
Flux coated rod
Brazing filler metal in rod form which is coated with
flux.
Freezing point
A term for solidus.
Fuel gas
A gas usually used with oxygen for heating.
Furnace brazing
A brazing process using a heated furnace.
Gap
A term for joint clearance.
Gas brazing
A term for torch brazing.
Gas pocket
A term for porosity.
Getter
A material used to purify low pressure gases (usually
vacuum furnace atmospheres) by chemically combining with impurities.
Hard solder
A term for silver alloy brazing filler metals.
Hazardous material
A substance that can harm humans.
Heat affected zone
The portion of the base material whose mechanical
properties or microstructure have been altered by heat.
Heat pattern
The area heated by the coil in induction brazing.
Hot crack
A crack that develops during solidification.
Hydrogen brazing
A term for any brazing process which takes place in pure
hydrogen or a hydrogen-containing atmosphere.
Impedance
A combination of electrical resistance, inductance, and
capacitance.
Incomplete fusion
A condition where some of the filler metal did not
melt.
Incomplete joint penetration
Joint penetration that is less than the thickness of the
joint.
Indistinct fillet
A condition where the filler metal did not result in a
fully formed fillet.
Induced current
Circulating currents produced in the workpiece when placed
in an electromagnetic field.
Induction brazing
A process using heat from the resistance of the workpieces
to induced electric current.
Inert gas
A gas that normally does not react chemically with
materials.
Infrared brazing
A process using heat from infrared radiation.
Intergranular penetration
The penetration of a filler metal along the grain
boundaries of a base material.
Joint
The junction of members or the edges of members which are
to be bonded or have been bonded.
Joint brazing procedure
The materials and methods employed in brazing a particular
joint.
Joint clearance
The distance between the faying surfaces of a joint.
Joint design
The joint geometry together with the required
dimensions.
Lack of fill
A term for incomplete penetration.
Lap joint
A joint between two overlapping members in parallel
planes.
Laser
A device that produces a concentrated coherent light
beam.
Laser brazing
A process using energy from a laser beam.
Liquation
The separation of a low melting constituent of an alloy
from the remaining constituents, usually apparent in alloys having a wide
melting range.
Liquidus
The lowest temperature at which an alloy is completely
liquid.
Longitudinal crack
A crack with its major axis approximately parallel to the
joint axis.
Manual brazing
A brazing operation performed and controlled completely by
hand.
Mechanical bond
The adherence of a deposit to a roughened surface by the
mechanism of particle interlocking.
Mechanised brazing
Brazing with equipment which performs the brazing operation
under the constant observation and control of a brazing operator. The
equipment may or may not load or unload workpieces.
Metal-bath dip brazing
A dip brazing process variation.
Metallising
A term for applying a metal layer to ceramic or other
surface in preparation for brazing.
Neutral flame
An oxyfuel gas flame which is neither oxidising nor
reducing.
Non-corrosive flux
Brazing flux which in neither its original form nor its
residual form chemically attacks the base material.
Oxidising flame
An oxyfuel gas flame in which there is an excess
oxygen.
Parent metal
A term for base material.
Partial joint penetration
Joint penetration that is less than complete.
Partial pressure
Pressure, usually of a furnace atmosphere or constituent of
a furnace atmosphere, that is below 1 bar; or the pressure of any
constituent in a gas mixture at any pressure.
Paste brazing filler metal
A mixture of finely divided brazing filler metal with an
organic or inorganic flux.
Peel test
A destructive method of inspection which mechanically
separates a lap joint by peeling.
Penetration
A term for the distance braze metal flows into a
joint.
Porosity
Cavity type discontinuities formed by gas entrapment during
solidification.
Post-heating
The application of heat to an assembly after brazing.
Post-braze heat treatment
Any heat treatment after brazing.
Power density
The electrical power per unit area within an induction
brazing coil.
Pre-coating
Coating the base material in the joint before
brazing.
Preform
Filler metal fabricated in a shape or form for a specific
application.
Preheat
The heat applied to the base material or substrate to
attain and maintain preheat temperature.
Preheating
The application of heat to the base material immediately
before brazing.
Preheat temperature
The temperature of the base material or substrate
immediately before brazing. In a multipass operation, it is also the
temperature in the area immediately before the second and subsequent
passes are started.
Procedure
The detailed elements of a process used to produce a
specified result.
Protective atmosphere
A gas or vacuum envelope surrounding the workpieces, used
to prevent or reduce the formation of oxides and other detrimental surface
substances, and to facilitate their removal.
Quench
Accelerated cooling, frequently in liquid.
Reaction flux
A flux composition in which one or more of the ingredients
reacts with a base material upon heating to deposit one or more
metals.
Reducing atmosphere
A chemically active protective atmosphere, which at
elevated temperature will reduce metal oxides to their metallic
state.
Reducing flame
An oxyfuel gas flame with an excess of fuel gas.
Remelt temperature
The temperature necessary to melt a filler metal in a
completed joint.
Repair brazing
The process of rebrazing a joint that exhibited repairable
defects.
Residual stress
Stress present in a joint member or material that is free
of external forces or thermal gradients.
Resistance brazing
A brazing process which uses heat from the resistance to
electric current flow in a circuit of which the workpieces are a
part.
Salt-bath dip brazing
A dip brazing process variation.
Sandwich brazing
A brazed assembly of dissimilar materials using a
pre-placed shim, other than the filler metal, as a transition layer to
minimise thermal stresses.
Semi-automatic brazing
Manual brazing with equipment which automatically controls
one or more of the brazing conditions.
Semi-blind joint
A joint in which one extremity of the joint is not
visible.
Shielding gas
Protective gas used to prevent or reduce atmospheric
contamination.
Silver alloy brazing
A term for brazing with a silver-containing filler
metal.
Simultaneous brazing
A term for producing several brazed joints at the same
time.
Skull
The unmelted residue from a liquated filler metal.
Slag inclusion
Non-metallic solid material entrapped in filler metal or
between filler metal and base material.
Solidus
The highest temperature at which a material is completely
solid.
Spool
A filler metal package consisting of a continuous length of
wire in coil form wound on a cylinder which is flanged at both ends.
Step brazing
The brazing of successive joints on a given part with
filler metals of successively lower brazing temperatures so as to
accomplish joining without disturbing the joints brazed previously.
Stop-off
A material used on the surfaces adjacent to the joint to
limit the spread of filler metal or flux.
Stress relief cracking
Intergranular cracking in the heat-affected zone or filler
metal as a result of the combined action of residual stresses and
post-braze exposure to an elevated temperature.
Stress relief heat treatment
Uniform heating of a structure to a sufficient temperature
to relieve the major portion of the residual stresses, followed by uniform
cooling.
Susceptor
An electrically conductive material heated by induction and
used to assist in heating a workpiece by radiation.
Thermal expansion
The dimensional change exhibited by solids, liquids, and
gases, which is caused by temperature changes at constant pressure.
Thermal stress
Stress resulting from non-uniform temperature distribution
or differential thermal expansion.
Torch brazing
A brazing process using heat from a fuel gas flame.
Torr
A unit of pressure normally used to describe very low
pressures.
Undercut
A groove melted into the base material adjacent to the
braze and left unfilled by filler metal.
Vacuum brazing
A term for various brazing processes which take place in a
chamber or retort below atmospheric pressure.
Wetting
The condition whereby a liquid filler metal or flux spreads
and adheres in a thin continuous layer on a solid base material.
Workpiece
A part which is brazed.
Mumbai Weldnet /
Trends in welding /
Related Websites /
IIW Forum /
Feedback /
Home
