Objectives
• Define the terms soldering, brazing and braze
welding
• Explain the advantages and disadvantages of
liquid-solid phase bonding
• Describe the functions of fluxes in making
proper liquid-solid phase bonded joints
Introduction
• Soldering and brazing are classified by the AWS as
liquid-solid phase bonding processes
• This means…
– The filler metals is melted
– The base material or materials is not melted
– The phase is the temperature at which bonding takes place
between the filler and base
• The bond between the base material and filler material
is metallurgical because no alloying or melting of the
base metal occurs
• If done correctly, this bond results in a joint that has 5
X’s the tensile strength of that of the filler metal
Soldering & Brazing
• Soldering
– Takes place at a temperature below 804˚ F
• Brazing
– Takes place at a temperature above 804˚ F
• This is the only difference between the two
Brazing
Brazing
Braze Welding
• Parts being joined must
be fitted so the joint
spacing is very small
• This small spacing
allows capillary action
to draw the filler metal
into the joint when the
parts reach the proper
phase temperature
• Does not need capillary
action to pull filler
metal into the joint
Advantages of Soldering and Brazing
•
•
•
•
•
•
•
•
Low temperature
Permanent or Temporary Joining
Dissimilar materials can be joined
Speed
Less chance of damage
Slow rate of heating/cooling
Parts of varying thickness can be joined
Easy realignment
Tensile Strength
• The joints ability to withstand being pulled
apart
• Brazed joints can be made to have a tensile
strength of 4-5 X’s higher than the filler metal
itself
• As joint spacing decrease, surface tension
increases the tensile strength of the joint
Shear Strength
• A joints ability to withstand a parallel force
• Depends upon the amount of overlapping
area of the base parts
• The great the area of overlap, the greater the
strength
Ductility
• The joints ability to bend without failing
• Most soldering and brazing alloys are ductile
metals making the joints they are made with
ductile as well
Fatigue Resistance
• The joints ability to be bent repeatedly
without exceeding its elastic limit and without
failure
• Fairly low for most soldered or brazed joints
• Fatigue failures may also occur as a result of
vibration
Corrosion Resistance
• The joints ability to resist chemical attack
• Compatibility of base metals to filler metals
will determine corrosion resistance
Functions of Flux
• Remove any oxides that form as a result of
heating the parts
• Promote wetting
• Aid in capillary action
Flux in General
• When heated to its reacting temperature must
be thin and flow through the gap provided at
the joint
• As it flows through the joint it absorbs and
dissolves oxides, allowing the molten filler
metal to be pulled in behind it.
• Once the joint is complete the flux material
should be completely removable
Types of Fluxes
•
•
•
•
•
•
•
•
Solids
Powders
Paste
Liquids
Sheets
Rings
Washers
They are also available mixed with filler metal,
inside the filler metal or on the outside of filler
metal
Fluxing Action
• Will remove light surface oxides, promote
wetting, and aid in capillary action
• But they do not eliminate the need for good
joint cleaning
• Flux will not remove oil, dirt, paint, glue,
heavy oxides or other surface contaminants
Soldering & Brazing Fluxes
• Soldering Fluxes are chemical compounds such
as
– Muriatic acid (hydrochloric acid)
– Sal ammoniac (ammonium chloride)
– Rosin
• Brazing Fluxes are chemical compounds such as
• Chemical compounds such as
–
–
–
–
Fluorides
Chlorides
Boric acids
Alkalies
What They Do
• React to dissolve, absorb or mechanically
break up thin surface oxides that are formed
as the parts are being heated
• Must be stable and remain active through the
entire temperature range of the solder or
braze filler metal
• Chemicals react as either acids or bases
• Some dip fluxes are salts
General
• Methods are grouped according to which heat
is applied
– Torch
– Furnace
– Induction
– Dip
– Resistance
Torch Soldering and Brazing
• Oxyfuel or air-fuel torches
• Acetylene is the most often used but is not as
preferable when compared to other fuel gases
– This is due to uneven heating
Torch Soldering and Brazing
Advantages
Disadvantages
• Versatility
• Portability
• Speed
• Overheating
• Skill
• Fires
Furnace Soldering and Brazing
• Parts are heated to their soldering or brazing
temperature by passing them through a
furnace
Furnace Soldering and Brazing
Advantages
• Temperature control
• Controlled atmosphere
• Uniform heating
• Mass production
Disadvantages
• Size
• Heat damage
Induction Soldering and Brazing
• Uses high frequency electrical current to
establish a corresponding current on the
surface of the part
• The current on the part causes rapid and very
localized heating of the surface only
• Little if any internal heating of the part except
by conductivity of heat from the surface
Induction Soldering and Brazing
Advantages
Disadvantages
• Speed
– Very little time is
required for the part
to reach the desired
temperature
• Distortion
• Lack of temperature
control
• Incomplete penetration
Dip Soldering and Brazing
• Two types
– Molten flux bath
– Molten metal bath
Molten Flux Method
• Soldering or brazing filler metal in a suitable
form is preplaced in the joint and the
assembly is immersed in a bath of molten flux
• The bath supplies the heat to preheat the joint
and fuse the solder or braze metal and it
provides protection from oxidation
Molten Metal Method
• Prefluxed parts are immersed in a bath of
fused solder or braze metal which is
protected by a cover of molten flux
• Method is confined to wires and other small
parts
• Once removed from the bath, the ends of the
wires or parts must not be allowed to move
until the solder or braze metal has solidified
Dip Soldering and Brazing
Advantages
Disadvantages
• Mass production
• Corrosion protection
• Distortion minimized
•
•
•
•
Steam explosions
Corrosion
Size
Quantity
Resistance Soldering and Brazing
• Electric current is passed through the part
• Resistance of the part to the current flow
results in the heat needed to produce the
bond
• Flux is usually preplaced
• Material must have sufficient electrical
resistance to produce the desired heating
• Machine used in this process resembles a spot
welder
Resistance Soldering Brazing
Advantages
Disadvantages
• Localized heating
• Speed
• Multiple spots
• Distortion
• Conductors
• Joint Design
Special Methods
• Ultrasonic method
– Uses high-frequency sound waves are used to
produce the bond or aid with heat in the bonding
• Diffusion
– Uses pressure and may use heat or ultrasound to
form a bond
• Infrared Light
– Uses infrared light to heat the part for soldering or
brazing
Material being joined
Strength desired
Joint design
Availability and cost
Appearance
Service (corrosion)
Heating Process to be used
Costs
The type of filler metal used should be selected by
considering as many of the above criteria as possible
General
• Soldering and brazing metals are alloys or two or
more metals
– Each alloy is available in a variety of percentage
mixtures
• Almost all have a paste range
– A paste range is the temperature range in which a
metal is partly solid and partly liquid as it is heated or
cooled
– It is important that joints not be moved during this
stage, if they are they may crumble like dry clay and
destroy the bond
Soldering Alloys
• Usually identified by their major alloying
elements
• The major types of solder alloys are
– Tin-lead
– Tin-antimony
– Cadmium-silver
– Cadmium-zinc
Tin-lead
•
•
•
•
•
•
•
•
Most popular
Least expensive
61.9% tin and 38.1% lead
Melts at 362˚F
No paste range
Most commonly used on electrical connections
Must never be used for water piping
Also not allowed by most codes for use on water
or food handling equipment
Tin-antimony
• Higher tensile strength & lower creep
• Most common is 95/5 or 95% tin, 5%
antimony
• Most commonly used in plumbing because it
is lead free
Cadmium-silver
• Excellent wetting, flow and strength
• Expensive
• High temp solders because they retain their
strength at temperatures above other solders
• Used to join aluminum to itself or other
metals
• Most often seen used in piping for air
conditioning equipment
Brazing Alloys
• Denoted by the letter B to indicate the alloy is
used for brazing
• Next series of letters in the classification
indicates the atomic symbol of metals used to
make the alloy
Copper-zinc
• Most popular brazing alloy
• Available as regular and low-fuming
• Zinc in the braze metal has a tendency to burn
out if overheated
• Overheating is indicated by a red glow on the
molten pool which gives off white smoke
– The white smoke is zinc oxide, if breathed in it can
cause zinc poisoning. Use of low fuming alloy helps
eliminate this problem
• Examples of low fuming alloys are RCuZn-B and
RCuZn-C
Copper-zinc & Copper-phosphorus
A5.8
• Copper-zinc filler rods are often grouped
together and called brazing rod
• 5 classifications
– Copper-zinc
– Navel Brass
– Manganese-Bronze
– High silicon-Bronze
– Nickel-Bronze
Copper-phosphorus
• Referred to as phos-copper
• Good fluidity and wettability
• Used in A/C and plumbing to join copper
piping
Copper-phosphorus-silver
• Referred to as sil-phos
• Similar to copper-phosphorus except the silver
gives the alloy better wetting and flow
characteristics
• Not necessary to use flux when joining copper
pipe
• Most common brazing alloy used in A/C
compressor fittings
Silver-copper
• Can be used to join almost any metal, except
aluminum, magnesium, zinc and a few other
low-melting metals
• Often referred to as silver braze
• Most versatile
• Among most expensive alloys except gold
Nickel
• Used for joining materials that need high strength
and corrosion resistance at elevated
temperatures
• Applications include
–
–
–
–
Joining turbine blades in jet engines
Torch parts
Furnace parts
Nuclear reactor tubing
• When used on copper based alloys, the nickel
may diffuse into copper, stopping its capillary
flow
Nickel and Nickel Alloys A5.14
• Increase being used as a substitute for silver-based alloy
• More difficult to use than silver due to lower wetting and flow
characteristics
• Higher strength than silver
• 7 classes
– BNi-1: high strength, heat resistant, used in jet engine parts
– BNi-2
– BNi-3: high flow rate, excellent for close fitted joints
– BNi-4: higher surface tension than other nickel filler rods,
allows larger fillets and poor-fitted joins to be filled
– BNi-5: high oxidation resistance and high strength at elevated
temps, can be used for nuclear applications
– BNi-6: extremely free flowing, good wetting characteristics,
high corrosion resistance
– BNi-7:high resistance to erosion and can be used for thin or
honeycomb structures
Aluminum-silicon
• Used to join most aluminum sheet and cast
alloys
• AWS type 1 flux must be used
• Must guard against overheating
Copper and copper alloys A5.7
• BCu-1
– Used to join ferrous, nickel and copper-nickel
• BCu-2
– Similar applications to 1
– Contains organic compounds to tie up porosity
Silver & Gold
• Used in small quantities when joining metals
that are under corrosive conditions and high
joining ductility is needed or low electrical
resistance is important
• Increasing price and decreasing availability
Joint Design
• Spacing between the parts being joined affects
tensile strength
• Strongest joints are obtained when the parts are
lapped
• Butt joint strength can be increased by increasing
the area being joined
• Joint preparation is also very important
– Surfaces must be clean and free of oil, dirt, paint,
oxides
– Soldering or brazing should begin as soon the parts
are cleaned to avoid further contamination
Building Up Surfaces and Filling Holes
• Braze metal can be used to build up worn parts
• Ideal for parts that receive limited abrasive wear
because buildup is easily machinable
• Has no hard spots to make remachining difficult
• Good for both round and flat stock
• Low temperature used does not tend to harden
the base metal
• Holes in light gauge metal can be filled and
ground flush leaving a strong patch with
minimum distortion
Summary
• Brazing and soldering are process that have many
great advantages that are often overlooked.
• They are an excellent process for portable
applications and the versatility makes them great
choices for many jobs.
• Their ability to join may different materials with a
limited variety of fluxes and filler metals reduces
the need for a large inventory of materials.