Adhesion Flashcards
Adhesive
Filler material - non-metallic, usually a polymer
Requires
- Flow and wetting of a substrate by a filler (e.g. adhesives or solders)
- Solidification of the filler/curing of the adhesives
Bonding Theory
- 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
Adhesive Bonding
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
Adhesive Form
Liquid, paste, solution, emulsion, powder, tape and films
Natural Adhesives
Including gums, starch, dextrin, soya flour, collagen
Low-stress applications: cardboard, cartons, furniture, plywood
Inorganic Adhesives
Based primarily on sodium silicate and magnesium oxychloride
Low cost, low strength
Synthetic Adhesives
Various thermoplastic and thermoplastic polymers (e.g. epoxies, acrylics)
Moat important category in manufacturing
Structural Adhesives
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
Application of Adhesives
- Automotive, aircraft
- Building products, shipbuilding
- Packaging industries
- Foot wear
- Furniture
- Bookbinding
- Electrical and electronics
Surface Preparation
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
Adhesive Bonding Advantages
(10)
- 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
Adhesive Bonding Limitations
(6)
- 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
Brazing and Soldering
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
Filler Metal Melting
- Solders Tm <450 degrees
- Brazers Tm >450 degrees
(But below the melting point of the material being joined)
Joint Formation
- 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
Capillary Action
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
Soldification
Intermetallic Compounds (IMC)
- 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
Bazing Joint
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
Wetting
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
Filler Metal Characteristics
- 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)
Brazed Joints
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
Brazing Alloys
- 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
Process
- Joint design - components set correct distance apart
- Cleaning
- Oxidation prevention - applying flux or under reducing atmosphere/vacuum
- Heating - components are heated
- Filler application
- Cooling - flux removed by cleaning if needed
Brazing and Soldering Processes
- 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
Brazing Compared to Welding
- 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
Brazing Limitations
- 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
Soldering
- 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
Solders
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
Advantages of Soldering
(6)
- 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
Brazing/Soldering Over Welding
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
Joint Strength
- Brazes stronger than solders
- Joint strength typically increases with Tm of joining allows
Strength Related to Joint Designs
- Maximise joint area
- Use interlocking joints
- Brazing has contraction and residual stress
