Weldbonding
by Steve Westgate
Description
Weldbonding is a combination of resistance spot welding and adhesive
bonding. It is a hybrid process and combines advantages of the two
individual processes. A paste adhesive is normally applied to one sheet
and the joint closed. A spot weld is then made through the adhesive. The
electrode force displaces the adhesive to obtain electrical contact
between the sheets and a weld is made in the normal way. As the heating
of the weld is very localised, little damage occurs in the adhesive
around the weld. The adhesive is finally cured to complete the assembly.
Heat curing paste type adhesives are normally used as these are stable
and have a consistent viscosity at room temperature. Typically, such
adhesives are cured in an oven at up to 180°C for 30 minutes. Some
adhesives are available in tape form and incorporate a metal particle
filler which allows initial electrical contact to be made for spot
welding.
Current status
In automotive applications, adhesives and sealants have been welded
through to improve joint strength, load distribution, fatigue
performance and joint sealing. In general, suitable adhesives do not
substantially disturb the spot welding process for steel. The greatest
benefits could be achieved with aluminium alloys where spot welding
alone gives limited joint performance. Much development work has been
conducted and some specialist applications exist for aluminium alloys,
although tight material and process control is required to achieve high
quality weldbonded joints.
Important issues
There is a concern about the long-term durability of adhesive bonds in
hostile environments (e.g. in the presence of moisture, especially when
under load). Thus, the choice of surface treatment is crucial,
particularly for aluminium alloys, and there can be a conflict of
requirements with the spot welding process (normally a low surface
resistance is needed).
When combining the processes, the workpiece and tooling may be more
susceptible to contamination, as a result of adhesive being squeezed out
of the joint than occurs when bonding alone. In addition, health and
safety issues linked to the use of adhesives need to be considered.
Benefits
The process combines the benefits of adhesive bonding for joining thin
sheet assemblies, together with a fast, automated point joining
technique, which eliminates the need for jigging during curing of the
adhesive. Adhesive bonds have limited strength in peel or cleavage, so
the spot welds provide peel-stopping points in the event of overloading,
for example in impact tests.
Risks
Good process control is required to ensure correct joint filling for the
adhesive and to avoid weld quality problems. Material handling needs to
be controlled so that health and safety requirements are met. Welding
through adhesives may create hazardous fume and little information is
available as to the organic compounds produced. Suitable ventilation/fume
extraction should be provided.
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