Soldering, Brazing and Braze Welding

Ag Metals IWelding Principles & Applications

Chapter 31

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• 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

Braze Welding• 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• Versatility• Portability• Speed

Disadvantages• 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• Speed– Very little time is

required for the part to reach the desired temperature

Disadvantages• 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• Mass production• Corrosion protection• Distortion minimized

Disadvantages• 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• Localized heating• Speed• Multiple spots

Disadvantages• 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.