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Processes S2 > Adhesion > Flashcards

Adhesion Flashcards

1
Q

Adhesive

A

Filler material - non-metallic, usually a polymer

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2
Q

Requires

A
  • Flow and wetting of a substrate by a filler (e.g. adhesives or solders)
  • Solidification of the filler/curing of the adhesives
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3
Q

Bonding Theory

A
  • Mechanical interlocking - relies on surface micro roughness, e.g. adhesive must penetrate the micro pores and fissures - joints tend to fail in adherent and adhesives, not the interface
  • Specific adhesion - relies on chemical reaction e.g. intermetallic formation with solders and brazes
  • Interdiffusion - adhesive atoms/molecules diffuse into the substrate
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4
Q

Adhesive Bonding

A

Process by which anadhesives physical properties are changed from liquid to solid, usually by chemical reaction, to accomplish attachment of parts
Requires clean surfaces, tight clearance and longer curing times

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5
Q

Adhesive Form

A

Liquid, paste, solution, emulsion, powder, tape and films

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6
Q

Natural Adhesives

A

Including gums, starch, dextrin, soya flour, collagen
Low-stress applications: cardboard, cartons, furniture, plywood

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7
Q

Inorganic Adhesives

A

Based primarily on sodium silicate and magnesium oxychloride
Low cost, low strength

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8
Q

Synthetic Adhesives

A

Various thermoplastic and thermoplastic polymers (e.g. epoxies, acrylics)
Moat important category in manufacturing

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9
Q

Structural Adhesives

A

Of grate interest in engineering, capable of forming strong, permanent joints between strong, rigid adherends
- May experience large stresses up to their yield point
- Need to be able to transmit stresses without loss of integrity within design limits
- Shear strength >7MPa

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10
Q

Application of Adhesives

A
  • Automotive, aircraft
  • Building products, shipbuilding
  • Packaging industries
  • Foot wear
  • Furniture
  • Bookbinding
  • Electrical and electronics
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11
Q

Surface Preparation

A

For adhesive bonding to succeed, part surfaces must be extremely clean.
Bond strength depends on adhesion between adhesive and adherend, which depends on clean surface.
- For metals - solvent wiping often used for cleaning, and sandblasting improves surface adhesion
- For non-metallic - surfaces can be mechanically abraded or chemically etched to increase roughness

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12
Q

Adhesive Bonding Advantages

(10)

A
  • Maintaining structural integrity without localised stresses induced
  • Aesthetics - no effect on external appearance
  • Versatile - applicable to a side variety of materials - similar or dissimilar
  • Lightweight - no significant weight penalty
  • Bonding porous or dissimilar materials is possible
  • Low T process - no significant distortion
  • Bonding occurs over entire surface area of joint
  • Overall cost-effective
  • Easy to automate
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13
Q

Adhesive Bonding Limitations

(6)

A
  • Limited reliability under hostile ennvironmental conditions (e.g. degradation by T, oxidation, stress corrosion and radiation)
  • Curing times can limit production rates
  • Surface preparation
  • Adhesively bonded joints are generally not as strong as other joining techniques
  • Difficulty of non-destructive testing and inspection
  • A limited range of service Ts
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14
Q

Brazing and Soldering

A

Both joining processes use filler metals to permanently join metal parts, but there is no melting of base metals.
Joint formation:
1. Filler metal melting
2. Joint gap filled by capillary action
3. Filler metal soldification

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15
Q

Filler Metal Melting

A
  • Solders Tm <450 degrees
  • Brazers Tm >450 degrees
    (But below the melting point of the material being joined)
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16
Q

Joint Formation

A
  • The filler metal is distributed by capillary action between faying surfaces of matal parts being joined
  • The filler metal wets the surface to form an intermetallic layer
  • Joint formation relies on wetting.
  • Braze/solder must fill joint before solidifaction
17
Q

Capillary Action

A

Physical tendency of a liquid to be drawn into a small diameter tube or other narrow openings despite force of gravity.
Caused by the adhesive attraction between the liquid molecules and the solid surfaces that define the narrow openings.

2ycos@ / pga = h

y=surface tension, 2a=joint width, h=height due to capillary, @=contact

18
Q

Soldification
Intermetallic Compounds (IMC)

A
  • IMC form at the interface between the base metals and the filler metal during the brazing process
  • IMC can increase the strength of the bond by promoting adhesion between the base metals and the filler metal
19
Q

Bazing Joint

A

The strength and reliability of a brazed joint will be influenced by:
- Cleanliness of the materials being joined
- Joint clearance - gap between two pieces of parent material to be joined
- Filler metal selection

20
Q

Wetting

A

Man factures influence a filler wetting a base metal including:
- Surface roughness
- Surface cleanliness
- Presence of oxide layers
- Temperature
- Brazing time
- Nature of two metals
- Presence or absence of flux
- Degee of oxidation or contamination

21
Q

Filler Metal Characteristics

A
  • Melt T of filler metal is compatible with base metal
  • Low surface tension in liquid phase for good wettability
  • High fluidity for penetration into interface
  • Capable of being brazed into a joint of adequate strength for application
  • Avoid chemical and physical interactions with base metal (e.g. galvanic reaction)
22
Q

Brazed Joints

A

Butt and lap joints common
- Geometry of butt joints are usually adapted for brazing
- Lap joints are more widely used, since they provide larger interface area between parts

Filler metal in a brazed lap joint is bonded to base parts throughout entire surface area, rather than only at edges

23
Q

Brazing Alloys

A
  • Several established ‘families’ of filler metal have been developed fro joining the more common engineering metals
  • Seven categories of filler metals are recognised in ISO something
24
Q

Process

A
  1. Joint design - components set correct distance apart
  2. Cleaning
  3. Oxidation prevention - applying flux or under reducing atmosphere/vacuum
  4. Heating - components are heated
  5. Filler application
  6. Cooling - flux removed by cleaning if needed
25
Q

Brazing and Soldering Processes

A
  • Heat sources - electrical, chemical (combustion, exothermic reactions)
  • Protection from oxidation - fluxes, controlled environment
  • Equipment - torches, furnaces, inductors, baths
  • Brazing - Al, Mg, Ag, Cu etc. based alloys
  • Soldernig - Sn, Pb, In etc. containing alloys
26
Q

Brazing Compared to Welding

A
  • Compatability to join widely dissimilar metals and to do so with min. modification of the materials joined
  • Can be performed quickly and consistently, permitting high production rates
  • Multiple joints can be brazed simultaneously
  • Less heat and power required than PW
  • Problems with HAZ in base metal reduced
  • Joint areas that are inaccessible by many welding processes can be brazed - capillary action draws molten filler metal into joint
27
Q

Brazing Limitations

A
  • Joint strength is generally less than welded
  • Joint strength is likely to be less than the strength of the base metals
  • High service Ts may weaken a brazed joint
  • Colour of brazing metal may not match colour of base metal parts - possible aesthetic disadvantage
28
Q

Soldering

A
  • Joining process in which filler metal (solder) with Tm less than or equal to 450 degrees is melted and distributed by capillary action between surfaces
  • No melting base metals, but filler metal wets and combines with base metal to form metallurgical bond
29
Q

Solders

A

Traditionally alloys of tin and lead (both have low Tm)
- Lead is poisonous and its % is minimised in most solders
- Tin is chemically active at soldering Ts and promoted wetting action for successful joining
- Copper and tin form intermetallic compounds that strengthen bond
- Silver and antimony also used in soldering alloys

30
Q

Advantages of Soldering

(6)

A
  • Lower E than brazing or FW
  • Variety of heating methods available
  • Good electrical and thermal conductivity in joint
  • Easy repair and rework
  • Low joint strength unless reinforced mechanically
  • Joint wealens or melts at elevated Ts
31
Q

Brazing/Soldering Over Welding

A

Use over FW when:
- Metals have poor weldability
- Dissimilar metals are to be joined
- Intense heat of welding may damage components
- Geometry of joint not suitable for welding
- High strength is not required

32
Q

Joint Strength

A
  • Brazes stronger than solders
  • Joint strength typically increases with Tm of joining allows
33
Q

Strength Related to Joint Designs

A
  • Maximise joint area
  • Use interlocking joints
  • Brazing has contraction and residual stress

Processes S2 (8 decks)

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