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Brazing

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Introduction

• Three common processes:

Soldering

Brazing

Braze welding

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Non-fusion welding Advantages & Disadvantages

• Advantages Lower temperature Easy assembly Weld dissimilar metals Allows disassembly/realignment Join metals of different

thicknesses Joint different types of metal

• Disadvantages Lower tensile strength Not efficient method for

thick metal Not efficient method for

large parts.

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Four Requirements of Brazing

• Clean metal

• Appropriate filler rod

• Correct flux

• Heat

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1. Clean Metal

• Molecular bonding requires a clean surface.

• Requires a clean surface not a polished surface.

CleanPolished

• Best method is to grind the surface

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2. Appropriate Filler Rod

• Filler rods are available for many non-fusion processes.

• Brazing: (Figure 24-2, pg 314) Brazing rod are available as bare

rods or flux coated.

• Soldering: (Figure 24-1, pg 314) Solder can be solid or flux core. Flux core can be acid or resin. Can be tin, silver or zinc alloy.

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3. Flux

• Flux must be used with all non-fusion welding processes.

• Three purposes of flux.1. Chemically clean the metal

2. Shield from oxidation and atmospheric contamination

3. Promote wetting

• Flux must be appropriate for the metal and filler material.

• Flux is available in three (3) forms.1. Paste

2. Powder

3. Liquid

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4. Heat

• Sufficient heat must be available to raise the base metal temperature above the melting point of the filler rod.

• The decision on heat source to use is based on the mass of metal that must be heated, the welding process and the availability of equipment.

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Non fusion Heat Sources

Oxyacetylene

Air acetylene

Air propane

Oxypropane

MAPP

Electric soldering iron

Electric soldering gun

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Tinning

• Tinning is the process of applying a thin layer of filler rod to the surface of the metal.

• When soldering sheet metal joint/seams, both surfaces are usually tinned before assembling the joint/seam.

• Makes it easier to get the filler material to fill the joint and bond the metals together.

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Controlling Heat

• Metals are excellent conductors of heat Heat applied to joint moves away from the joint. The greater the mass of metal that must be heated--the greater the

heat requirement.

• Excessive heat will cause the flux to burn. Contaminates the joint. Joint must be re cleaned

• Manipulation of the heat source may necessary to heat both pieces evenly.

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Soldering

• Solder is divided into two categories; Soft Hard

Additional information in pages 170-172

• A process that uses a metal alloy that melts below 840 oF. • May or may not use capillary action.

Capillary action (wicking) is the ability of a substance to draw another substance into it.

• Soft soldering Lead or lead replacement solder Lower tensile strength Copper pipe and sheet metal Stained glass

• Hard soldering Silver based solders Jewelry

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Brazing

• A process that uses a metal alloy that melts above 840 oF, but less than the melting point of the base metal.

• Brazing relies on capillary action to draw the filler metal into the joint or to keep it in the joint.

The capillary effect is a function of the ability of the liquid to wet a particular material.

Requires very small gap between metal surfaces, clean surfaces and flux.

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Brazing Process

1. The joint area is mechanically cleaned and fluxed Flux can be applied before the joint is assembled, or during the

brazing process.

2. The joint is assembled. May require clamping.

3. A heat source is used to raise the temperature of the base metal above the melting of the filler metal (>840 oF). Must insure the joint is not overheated. If this occurs the flux will burn and contaminate the joint. When the joint becomes contaminated it must be recleaned and

refluxed before the welding can continue.

4. The filler metal is added to the joint.5. The filler metal flows into the joint and adheres to the surfaces.6. The heat source is removed and the filler metal solidifies,

bonding the surfaces together.

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Brazing Joints

• The strength of a brazed joint is less than a fusion welded joint.• Joints should be modified to the maximum welded area.

Tensile strength of brazing rod is ~40,000 psi.

Tensile strength of steel electrode is 36,000 to 50,000 psi.

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Braze welding

• A process that uses a metal alloy that melts above 840 oF, but less than the melding point of the base metal.

• Braze welding is the same process as brazing except it does not use capillary action and a visible bead is formed.

Process: 1. The joint area is cleaned and fluxed.

2. A heat source is used to raise the temperature of the base metal to the melting temperature of the filler metal. If excessive heat is added to the

joint, the flux will burn. When this occurs the process

must start over.

3. The filler metal is added to the joint.

4. The filler metal adheres to both surfaces forming a bead. The puddle must be heated until

the key hole collapses.

5. The heat source is removed and the filler metal solidifies, bonding the surfaces together.

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Braze WeldingKey Hole

• When the brazing rod is melted in the puddle, it will bridge across at first--form a key hole

• Sufficient heat must be added to cause the key hole to collapse.

• Failure to collapse the key hole will result in a joint that has incomplete penetration.

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Braze WeldingJoints

The five (5) standard joints can be used for braze welding. Butt

LapEdge

T

Corner

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Questions?