09 Materials Processing
Compare major materials-processing methods and learn how process conditions shape material structure, properties, defects, and the outcomes of manufacturing sequences.
How processing affects materials
A manufacturing process does more than determine a component's shape. It can also change the material's structure—including its grains, phases, defects, molecular orientation, and porosity—and therefore its properties. Process selection balances desired geometry and properties with cost, production rate, and likely defects.
and solidification
shapes material by pouring or injecting it into a mold and allowing it to solidify. It can produce complex shapes, including parts that would be difficult to make by cutting or .
In metals, solidification creates grains and often tree-like dendrites. Faster cooling generally produces a finer solidification structure, while uneven cooling can contribute to residual stress or distortion. Shrinkage during solidification, trapped gas, and inclusions can create pores or other defects. These defects reduce the effective load-bearing area and may provide sites where cracks begin. Risers can feed liquid metal into a as it shrinks.
A can achieve a complex shape in one operation, but a highly stressed part may require careful control of solidification and inspection for internal porosity.
by plastic deformation
changes a material's shape by applying forces that cause permanent, plastic deformation. Common methods include rolling sheet or plate, forging parts under compressive loads, pushing material through a shaped opening by extrusion, and drawing wire or tubing through a die.
During , deformation increases dislocation density. The metal usually becomes stronger and harder, but less ductile; deformation can also produce directional properties, called anisotropy. Cold rolling, for example, can strengthen a metal sheet, but if more is needed, an annealing step may restore ductility.
Hot working is carried out at a temperature where recovery and recrystallization can occur during deformation. These processes reduce work hardening and allow large shape changes. Hot working can also replace a cast structure with a more uniform, refined one.
and surface condition
removes material with a cutting tool to produce a shape, dimension, or surface finish. Turning, milling, drilling, and grinding are common methods. is often used to make precise features or finish a part after or .
Cutting imposes intense local mechanical and thermal loads. Depending on the material, tool condition, and cutting settings, the near-surface region may work-harden, develop residual stresses, or undergo microstructural damage. Roughness, cracks, or unfavorable residual stresses can affect fatigue life and dimensional stability. therefore controls not just dimensions but also .
and property changes
changes a material's structure through a controlled sequence of heating, holding, and cooling. Its result depends on the alloy and the time–temperature path.
Annealing can soften cold-worked metal, improve ductility, and reduce residual stresses. Recrystallization may form new, relatively strain-free grains.
Quenching rapidly cools suitably prepared steel. It can form hard martensite, but the result may also be brittle and contain residual stresses.
Tempering reheats quenched steel below its transformation range to reduce brittleness and improve the balance of hardness and toughness.
Age (precipitation) hardening uses to form fine particles that impede deformation in suitable alloys.
is alloy-specific: a cycle that strengthens one material may have little effect or cause unwanted changes in another. Uneven heating or cooling can also cause distortion and cracking.
Polymers are shaped using methods suited to their flow and setting behavior. Injection molding forces softened polymer into a mold and is suitable for complex, repeated shapes. Extrusion pushes material through a die to produce continuous products such as pipes, films, and fibers. Thermoplastics soften for shaping and solidify on cooling; thermosetting polymers are shaped and then cured by a chemical reaction.
Flow during molding or extrusion can orient polymer chains. This orientation may strengthen a product in one direction while making its properties anisotropic. Cooling conditions affect crystallization in semicrystalline polymers and can leave residual stresses that contribute to warping or cracking. Shrinkage can cause dimensional changes or sink marks. Processing conditions and mold or die design therefore help determine the finished polymer's structure and properties.
from powders
Many ceramics are made from powders because their high melting temperatures or brittleness make conventional melt shaping impractical. A typical powder route involves preparing and mixing the powder, a by pressing, extrusion, or slip , drying it, and firing it. Slip uses a liquid suspension of fine particles that deposits against a porous mold.
During firing, bonds particles and densifies the body without requiring the entire component to melt. The powder, , and firing steps affect packing, shrinkage, grain size, and porosity. Poorly controlled packing or firing can leave pores or cause uneven shrinkage; residual pores can reduce strength. can improve density and strength, but excessive grain growth may be undesirable. Because ceramics are often brittle, shaping the before firing can be easier than the fully fired part.
Combining processes in a sequence
Manufacturing often combines processes to achieve different goals at different stages. For example, a steel shaft might be forged to create its basic shape, machined to achieve accurate dimensions, and heat-treated to obtain a desired balance of strength and toughness.
Each step affects the state of the material inherited by the next, so the entire processing sequence matters.