Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
and copper, provided the surfaces to be joined are carefully prepared to be
free from contamination. In hot pressure welding, the material adjacent to
the weld is softened and the two surfaces are forced together, thus squeezing
out any surface contaminants. The heating may be externally applied or, in
friction welding, the heat is generated by rotating one surface against the
other. The latter technique is widely used in the automotive industry for the
manufacture of welded drive shafts.
‘Diffusion bonding’ can be employed to bond two surfaces without recourse
to plastic deformation, but it is a relatively slow and expensive process
which is only used in special applications.
Fusion welding
‘Welding Handbooks’ are readily available to the engineer to provide a guide
to appropriate welding processes for given alloy compositions, joint design
and joint size. Here we will discuss some of the factors that may affect the
microstructure and properties of fusion welds.
The weld metal
The weld metal is essentially a small casting and the essentials of its structure
can be appreciated by referring to our earlier discussion of the mechanism of
crystallization of metals and alloys in Chapter 1. Cored columnar crystals
form on the still solid component surface along the fusion line. They then
grow along the direction of the steepest temperature gradient in the weld
pool, as indicated in Fig. 3.31. The crystals which grow from the melt initially
share the same orientation as the solid, so it is important to consider whether
grain growth occurs in the component material adjacent to the fusion line, as
this will influence the grain size of the solidified weld metal. In fact, the
grain size in the weld is controlled by the grain size at the fusion line, since
Isotherm
R max
Heat source
Weld centre line
90° Melt
Transition line
Base metal
3.31 Showing growth of columnar crystals in the weld pool.