Bulk defects in a
material are caused by
fabrication of a
device from said
material. Common defect-causing fabrication methods are
casting,
working, and
welding. This
metanode will outline the various defects caused by
fabrication. For more detail, see the
hardlinks. I have placed double parentheses ((
hard link)) around the metanoded subnodes.
Defects caused by casting result
primarily from problems with
cooling and
shrinkage, and also from
gas evolution. The
liquid used usually cools from the outside
inward. This may split the surface causing ((
pipe defects)). This is avoided using a
riser.
If a pocket of liquid is formed with some dissolved gas, gas will evolve as the liquid solidifies, forming a ((blow hole)). This causes undesirable porosity, which acts as a stress riser. The effect is reduce via the introduction of degassing agents, which bond the gas particles.
Forging is a type of working. It basically consists of
squishing a block of material between two
dies. This causes some ((
flash)) to squish out between the edges. It is trimmed off, which can leave a
nucleation point for ((
flash cracks)). Cracks can form and spread from that point into the material. If further work is done to the hunk of stuff, and
flash is
incompletely trimmed, this can cause a folding in of the flash called a ((
cold shut)) defect. The folded-in piece welds incompletely with the surrounding material, which can concentrate stress. Also, bowing can occur if the dies are not properly lubricated, causing cracks inside the material.
In rolling, a material is forced between two narrowly-spaced
rollers. Materials which may have ((
porosities)) are
rolled to force these surfaces together, welding the sides of the pores shut. If the
weld is
incomplete, this may create
defects in the rolled piece. Also, the deformation of the material may cause
non-metallic inclusions in the material to be elongated and/or broken up, causing ((
stringers)).
In
extrusion (think
pasta), a material is
squeezed out of an
orifice by application of
pressure. A high temperature may be applied to
soften the material, perhaps
inadvertently forming an
oxidized skin on the material. Material is forced upward and back away from the
opening, loops around, and then comes out of the opening through the center. In this process the oxidized skin can be moved into the
interior of the material, and will show up on the extruded material as an ((
annular ring of oxide)), the presence of which can cause a
stress concentration.
This also includes
soldering and
brazing. In all of these cases, joining of multiple
pieces of matrial is done by application of
heat,
pressure, or both, with or without filler material, causing
fusion or
recrystallization. This can cause
porosity due to rapid heating and
cooling, and evolution of gases. It can also insert
non-metallic inclusions into the material.
Incomplete fusion may occur due to films or impurities on or in the material, or insufficient temperature and pressure. Finally, high temperatures and high rates of heating and cooling (resulting in
thermal stresses) may cause ((
shrinkage cracks)).