Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 117
bearing alloy systems of low melting point. A number of lead-free solders
have been developed commercially which meet many of the above criteria,
for example, an alloy of Sn/Bi/Ag, which has a melting point (m.p.) of
138–140 °C. Those with a m.p. >200°C are needed in demanding automotive
applications and such solders are also now commercially available.
If higher strengths are required, then brazed joints are employed.
Al-based brazing alloys
Alloys based on the Al–Si system are used for brazing Al and certain Al
alloys, using chloride–fluoride mixtures as a flux in order to remove the
tenacious oxide film present with these materials.
Cu-based brazing alloys
Cu-based brazing alloys are widely used for joining both ferrous and nonferrous
basis metals, and fall into the three classes of virtually pure copper,
copper–phosphorus (which is self-fluxing in many cases) and copper–zinc
(brass), which often employ a boric acid or borate flux.
Ni-based brazing alloys
Ni-based brazing alloys embrace a wide range of compositions, which may
include Cr, P and Si, and they find their widest use in assemblies of stainless
steels and of nickel or cobalt alloys. Many of the brazed joints retain good
strength at temperatures approaching 1000 °C.
Brazed joints of all compositions are significantly stronger than those in
soft solder, with copper-based joint strengths being in the range 250–
400 MPa and nickel-based joints in the range 300–600 MPa, thus often equalling
that of the metals being joined. It is, however, in welded joints that the
highest strengths are normally encountered.
3.3.3 Welding
An ideal weld would be chemically and physically indistinguishable from
the bulk material: this may sometimes be approached in solid state welding,
but seldom in fusion welding.
Solid state welding
The formation of a sound joint requires either chemically clean surfaces or
sufficient deformation to squeeze out any contaminants such as surface oxides.
Cold pressure welding works well with ductile metals such as aluminium