Soldering, Brazing and Braze Welding Ag Metals I Welding 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 Brazing 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 Disadvantages Temperature control Controlled atmosphere Uniform heating Mass production 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 Diffusion Infrared Light 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

Heating Process to be used 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.