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1. Introduction
For the majority of applications involving sheet steel, spot welding is the preferred method of body construction,
typically up to 10,000 spot welds being used to build a car. Electrodes (often water cooled) squeeze the sheets
together with a force of a few kN and a current of a few thousand amps flows for a few tens of milli - seconds.
2. Adhesives
For joining constructional parts, eg holding bearings in place, fixing sleeves to shafts, anaerobic adhesives,
such as cyanoacrylates or polyacrylate resins are increasingly being used. Here curing action is based on the rapid
polymerisation of the monomer under the influence of catalysts. An absorbed layer of moisture is usually sufficient to
initiate polymerisation and bonding.
For plastics, adhesives are used for the relatively few joints that such construction materials usually necessitate.
For aluminium panels a combination of adhesives and localised spot welding have been developed by Alcan (ref A.1). Some suppliers offer aluminium
sheet pre-treated and pre-lubricated to auto manufacturers ready for pressing. Pressed parts can then be joined by
heat curing structural epoxy adhesive (Ciba Polymers XD4600) and localised spot welding. During the cure stage the adhesive must be able
to displace residual lubricant and wet the pre-treated surface to form a good bond. The subsequent spot welding process
needs careful control of electrode design and approach speed to ensure adhesive is removed from the area of the
spot weld. Generally forces are higher than those needed for steel sheet.
Advantages and Disadvantages of using adhesives are listed below:
Advantages
- Most materials can be joined to themselves or other materials
- Bonded Joints are stiffer than spot welded or mechanical joints.
- Materials to be joined by adhesives can be thinner.
- The high temperature effects of welding are eliminated.
- Components are not weakened by holes, keyways, etc.
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Disadvantages:
- Joints are essentially permanent.
- Adhesives are temperature sensitive, becoming britle at low temperatures
and losing strength at high temperatures. Normal max. temp. is about 150 oC.
- Time is required for hardening.
- Joint preparation is critical and quality control can be difficult.
Ten types of adhesives have been defined by Permabond Adhesives:
| Anaerobic |
Cyanoacrylate |
Epoxy |
Hot melt |
| Phenolic |
Plastisol |
Polyurethane |
Solvent rubber |
Tape |
| Toughened types: |
- anaerobic |
- acrylic |
- epoxy |
Some experts suggest that for working loads above 3.5 MN/m2 only thermoset
type adhesives should be used.
Procedures for making a joint with an epoxy - eg Araldite are:
- Clean
- Degrease
- Abrade surfaces
- Apply pressure for appropriate curing time.
Select adhesive viscosity according to joint gap, thin for close fitting and thick for gap filling.
Where vibration or impact loads are expected, use a low modulus grade.
Avoid operation above 100oC, ref. S1.
Link to information about Structural
Adhesives - Huntsman Araldite
3. Self-Piercing Rivets.
The increased use of aluminium and plastics in vehicles has led to research into alternative
methods of joining sheet materials. One technique gaining popularity is the use of self-piercing
rivets. Key advantages of these is that they can easily join together sheets of different materials,
the hole is formed as a part of the insertion operation and that the innermost sheet is not
pierced. Details of the rivets and an animation of the process can be seen on the
Henrob Limited web site.
The new Jaguar XJ, (which have aluminium bodies) will each have some 3300 self-piercing rivets to join aluminium sheet
and castings, mild steel and high strength steel. The joints are 30% stronger than those produced
by spot welding with a fatigue resistance up to 10 times better. The time to make a joint varies
between 1 and 4 seconds.
Each of these vehicles also have 120 m of Betamate 4601 structural adhesive in them to give back stiffness
lost by using aluminium rather than steel.
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