Brazing, Soldering and Adhesives

Question 1

What do brazing and soldering have in common?

Answer 1

Brazing and soldering both use filler metals to join and bond two (or more) metal parts to provide a permanent joint. A filler metal is added in brazing and soldering as in most fusion-welding operations; however, no melting of the base metals occurs.

Question 2

When may brazing/soldering be a better alternative to welding?

Answer 2

(1) the metals have poor weldability
(2) dissimilar metals are to be joined
(3) the intense heat of welding may damage the components being joined
(4) the geometry of the joint does not lend itself to any of the welding methods
(5) high strength is not a requirement

Question 3

What are the similarities between adhesive bonding and brazing/soldering?

Answer 3

It utilizes the forces of attachment between a filler material and two closely spaced surfaces to bond the parts.

Question 4

What are the differences between adhesive bonding and brazing/soldering?

Answer 4

The filler material in adhesive bonding is not metallic, and the joining process is carried out at about room temperature

Question 5

What is brazing?

Answer 5

A joining process in which a filler metal is melted and distributed by capillary action between the faying surfaces of the metal parts being joined. In brazing the filler metal (aka the brazing metal), has a melting temperature that is above 450◦C but below the melting point of the base metal

Question 6

Why - when done correctly - is a brazed joint stronger than the filler material?

Answer 6

This is due to the small part clearances used in brazing, the metallurgical bonding that occurs between base and filler metals, and the geometric constriction that are imposed on the joint by the base parts.

Question 7

What re brazing’s advantages over welding?

Answer 7

(1) any metals can be joined, including dissimilar metals
(2) certain brazing methods can be performed quickly and consistently, thus permitting high cycle rates and automated production
(3) some methods allow multiple joints to be brazed simultaneously
(4) brazing can be applied to join thin-walled parts that cannot be welded
(5) in general, less heat and power are required than in fusion welding
(6) problems with the heat-affected zone in the base metal near the joint are reduced
(7) joint areas that are inaccessible by many welding processes can be brazed, since capillary action draws the molten filler metal into the joint.

Question 8

What are disadvantages of brazing?

Answer 8

(1) joint strength is generally less than that of a welded joint
(2) although strength of a good brazed joint is greater than that of the filler metal, it is likely to be less than that of the base metals
(3) high service temperatures may weaken a brazed joint
(4) the colour of the metal in the brazed joint may not match the colour of the base metal parts, a possible aesthetic disadvantage

Question 9

What are the two types of brazed joint?

Answer 9

butt and lap

Question 10

How has the traditional butt joint been adapted to increase performance?

Answer 10

The traditional butt joint has a limited surface area which jeopardizes the strength of the joint, so, butt joints may be slanted (scarf joint), stepped or increased cross section at the joint.

Question 11

Why are lap joints more commonly used than butt joints?

Answer 11

They provide a larger surface area between the two parts.

Question 12

What are the considerations concerning clearing when brazing two components?

Answer 12

The clearance must be large enough so as not to restrict molten filler metal from flowing throughout the entire interface. Yet if the joint clearance is too great, capillary action will be reduced and there will be areas between the parts where no filler metal is present.

Question 13

What effects optimum clearance?

Answer 13

Base and filler metals, joint configuration, and processing conditions

Question 14

What are typical brazing clearances?

Answer 14

0.025 to 0.25mm at brazing temperature.

Question 15

What considerations should be made of the surface before brazing?

Answer 15

Surfaces must be free of oxides, oils, and other contaminants in order to promote wetting and capillary attraction. Chemical treatments such as solvent cleaning and mechanical treatments such as wire brushing and sand blasting are used to clean
the surfaces. After cleaning and during the brazing operation, fluxes are used to maintain surface cleanliness and promote wetting for capillary action.

Question 16

What characteristics are required for a metal to qualify as a brazing metal?

Answer 16

(1) melting temperature must be compatible with the base metal
(2) surface tension in the liquid phase must be low for good wettability
(3) fluidity of the molten metal must be high for penetration into the interface
(4) the metal must be capable of being brazed into a joint of adequate strength for the application
(5) chemical and physical interactions with base metal (e.g., galvanic reaction) must be avoided.

Question 17

How may filler material be applied during brazing?

Answer 17

Wire, rod, sheets and strips, powders, pastes, preformed parts made of braze metal designed to fit a particular joint configuration

Question 18

What are characteristics of a good flux?

Answer 18

(1) low melting temperature
(2) low viscosity so that it can be displaced by the filler metal
(3) facilitates wetting
(4) protects the joint until solidification of the filler metal

Question 19

Name the four methods of brazing?

Answer 19

Torch Brazing - torch heats the base metal then filler is added with a wire
Furnace Brazing - filler and base metals loaded into a furnace, heated then left to cool
Induction Brazing - heat produced from electrical resistance due to being in the field of a high frequency alternating current
Resistance Brazing - heat is produced by electrical current flowing through the part.

Question 20

What is soldering?

Answer 20

A joining process in which a filler metal with melting point not exceeding 450◦C is melted and distributed by capillary action between the faying surfaces of the metal parts being joined

Question 21

What are advantages of soldering?

Answer 21

(1) low energy input relative to brazing and fusion welding
(2) variety of heating methods available
(3) good electrical and thermal conductivity in the joint
(4) capability to make air-tight and liquid-tight seams for containers
(5) easy to repair and rework.

Question 22

What are disadvantages of soldering?

Answer 22

(1) low joint strength unless reinforced by mechanically means, butt joints should not be used for loads
(2) possible weakening or melting of the joint in elevated temperature service

Question 23

How may soldered mechanical joints be made stronger?

Answer 23

The edges of the sheets may be bent over and interlocked before soldering

Question 24

What considerations are made of the soldering process when used for electrical applications?

Answer 24

For electronics applications, the principal function of the soldered joint is to provide an electrically conductive path between two parts being joined. Other design considerations in these types of soldered joints include heat generation (from the electrical resistance of the joint) and vibration. Mechanical strength in a soldered electrical connection is often achieved by deforming one or both of the metal parts to provide maximum surface area.

Question 25

What are common soldering metals?

Answer 25

Tin and lead - low melting points

Copper and tin - for electrical applications

Question 26

What are the three soldering methods?

Answer 26

Hand soldering - A hand held bit is used to melt the solder, conveying to the joint and withdrawing excess. Most irons use electrical resistance to supply heat.

Wave Soldering - a mechanized technique that allows multiple lead wires to be soldered to a printed circuit board (PCB) as it passes over a wave of molten solder.

Reflow Soldering - a solder paste consisting of solder powders in a flux binder is applied to spots on the board where electrical contacts are to be made between surface mount components and the copper circuit. The components are then placed on the paste spots, and the board is heated to melt the solder, forming mechanical and electrical bonds between the component leads and the copper on the circuit board.

Question 27

What is adhesive bonding?

Answer 27

Adhesive bonding is a joining process in which a filler material is used to hold two (or more) closely spaced parts together by surface attachment.

Question 28

What is an adhesive?

Answer 28

It is a non-metallic substance—usually a polymer.

Question 29

What are adherends?

Answer 29

The parts being joined are called adherends.

Question 30

What is curing?

Answer 30

Curing refers to the process by which the adhesive’s physical properties are changed from a liquid to a solid, usually by chemical reaction, to accomplish the surface attachment of the parts.

Question 31

What may effect the strength of an adhesive bond?

Answer 31

(1) chemical bonding, in which the adhesive unites with the adherends and forms a primary chemical bond upon hardening
(2) physical interactions, in which secondary bonding forces result between the atoms of the opposing surfaces
(3) mechanical interlocking, in which the surface roughness of the adherend causes the hardened adhesive to become entangled or trapped in its microscopic surface asperities

Question 32

What conditions should be implemented for maximum adhesive strength?

Answer 32

(1) surfaces of the adherend must be clean-free of dirt, oil, and oxide films that would interfere with achieving intimate contact between adhesive and adherend; special preparation of the surfaces is often required
(2) the adhesive in its initial liquid form must achieve thorough wetting of the adherend surface
(3) it is usually helpful for the surfaces to be other than perfectly smooth-a slightly roughened surface increases the effective contact area and promotes mechanical interlocking

Question 33

How may be a joint be designed for maximum strength in adhesive bonding?

Answer 33

(1) Joint contact area should be maximized
(2) Adhesive joints are strongest in shear and tension
(3) Adhesive bonded joints are weakest in cleavage or peeling

Question 34

What are the three types of adhesive?

Answer 34

Natural - low stress applications
Inorganic - have low cost but low strength
Synthetic - most important types, cured in a variety of ways for a variety of purposes.

Question 35

What are some example application methods of solvents?

Answer 35

Brushing
Flowing
Manual Rollers
Spraying

Question 36

What are advantages of solvent bonding?

Answer 36

(1) the process is applicable to a wide variety of materials
(2) parts of different sizes and cross sections can be joined—fragile parts can be joined by adhesive bonding
(3) bonding occurs over the entire surface area of the joint, rather than in discrete spots or along seams as in fusion welding, thereby distributing stresses over the entire area
(4) some adhesives are flexible after bonding and are thus tolerant of cyclical loading and differences in thermal expansion of adherends
(5) low temperature curing avoids damage to parts being joined
(6) sealing as well as bonding can be achieved
(7) joint design is often simplified

Question 37

What are limitations of adhesive bonding?

Answer 37

(1) joints are generally not as strong as other joining methods
(2) adhesive must be compatible with materials being joined
(3) service temperatures are limited
(4) cleanliness and surface preparation prior to application of adhesive are important
(5) curing times can impose a limit on production rates
(6) inspection of the bonded joint is difficult