| 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)). |
