1. PROCESSING and PROPERTIES
5/25/2018 5/25/2018
PROCESSING and PROPERTIES
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2. Intrinsic properties such as strength and resistivity depend on the microstructure and microstructure depends on processing
The ability to tune microstructure and properties is central to materials processing and design, so it brings with it the need for good process understanding and control

3. The figure depicts the process history of an aluminum bike frame
Materials processing involves more than one step
Each process step has a characteristic thermal history
Designers should watch out for unintended side-effects in the joining stage
Design focuses on the properties of finished products, but some of these properties are also critical during processing

4. Metals
Glasses and Ceramics

5. Polymers and Elastomers
The role of shaping processes is to produce the right shape with the right final properties
The first is achieved by controlling viscous flow or plasticity
The second requires control of the nature and rate of microstructural evolution

6. Phase: regions of a material with a specified atomic arrangement
Phase diagram: maps showing the phases expected as a function of composition and temperature, if the material is in its lowest free energy state
Phase transformation: occur when the phases present change – requires a driving force and a mechanism

7. The diagram divides up into single- and two- phase regions, separated by grain boundaries
At any point in the two-phase region, the present phases are those found at the phase boundaries at either end of the horizontal tie-line through the point defining the composition and temperature concerned
Both lead and tin will dissolve in one another to some extent, with the maximum solubility in both cases being at the same temperature
The pure elements have a unique melting temperature
Alloys show a freezing range between the boundaries known as the liquidus and solidus, so there is no longer a single melting point
At the eutectic point, the alloy can change from 100% liquid to 100% two-phase solid

8. Pure iron shows three phases in the solid state - As the temperature increases, iron changes from α-iron (BCC) to γ- iron ( FCC) to δ-iron (BCC) and then melts
The single-phase austenite field has a eutectoid point at its base – at this point the single-phase austenite at a composition of 0.8 wt% C can transform on cooling to a mixture of ferrite and cementite
The austenite field closes at the top in a peritectic point at which single-phase austenite wit this composition transforms on heating to a mixture of liquid and δ-iron

9. When a liquid solidifies, solid first has to appear from somewhere, after which the interface between solid and liquid can migrate to enable atom to switch from one phase to another at the boundary
The two stages of solidification are nucleation and growth

10. Most alloys offer both cast and wrought variants
Metals with an HCP structure such as Cu, Ti, and Ni are primarily cast at high temperatures because of their inherent lack of ductility
Casting and wrought alloys in a given system tend to have different compositions
Casting leads to coarser microstructures and poorer strength and toughness than a wrought alloy
Good castability requires higher levels of alloying additions than wrought alloys to lower the melting temperature

11. In casting, a liquid above its melting point is poured into
a mold where it cools by thermal conduction – it is
relatively cheap and well suited for complex 3-d shapes
New solid forms by nucleation –
new crystals form in the melt, on
the walls of the mold, or on
foreign particles
Crystals grow in opposing
directions and impinge on
one another to form grain
boundaries

12. Deformation processes influence microstructure by various means
Deformation processes exploit the plastic response of metals, that is, their ability to remain intact without damage when subjected to large strains and shape changes
Deformation processes influence microstructure by various means
Temperature – determines the phases present and is relevant during forming and cool-down
Grain size – forming process can strengthen the metal by changing both the size and shape of the grains

13. Forming processes, while increasing the strength of metals, often reduce their ductility
It is often necessary to follow forming with an annealing heat treatment to restore some of the materials ductility

14. In a typical heat treatment, a component is heated to high
temperature, cooled at a controlled rate, and usually reheated
to an intermediate temperature
The figure illustrates the thermal profile and resulting microstructure for a heat treatment process
A solid solution is formed at high temperature followed by precipitation hardening at an intermediate temperature after being cooled

15. Normalizing Quench and temper
Slow cooling from high temperature producing a microstructure of lower strength but high toughness
No follow-up heat treatment
Quench and temper
Cooled faster than the critical cooling rate producing a martensitic microstructure with high strength and low toughness
Then reheated at an intermediate temperature to restore toughness

16. Thermal welding of metals involves heating and cooling which may cause phase transformation in the weld metal and in the heated regions of the weld
The figure shows a weld cross- section with corresponding thermal histories in the weld metal and heat-affected-zone
On the right are typical hardness profiles induced across welds in various metals

17. Surface treatments exploit many different mechanisms and
processes to change the surface
microstructure and properties
Laser hardening is a surface treatment process that modifies
microstructure – the traversing laser beam induces a rapid
thermal cycle, causing phase changes on both heating and
cooling – the track below the path of the laser has a
different microstructure of high hardness

18. Structural sections are commonly made from wrought plain carbon steel that has undergone significant deformation processes
The microstructure consists of ferrite and pearlite which produces high fracture toughness and strength

19. The main alloying element in stainless steel is chromium – nickel as also frequently used
The addition of chromium imparts excellent corrosion resistance to the steel while nickel and chromium provide solid solution hardening

20. Strength and toughness tend to dominate the property profile desired for applications that use ferrous metals
The range of mechanical properties able to be achieved through alloying and processing makes ferrous metals the most versatile group of engineering materials

21. Glasses and Ceramics
Because of their susceptibility to cracking, defect control
is the most important aspect of processing glasses and ceramics –
while some thermal processes can increase the properties of glasses,
changes in composition is generally used to alter the properties of ceramics
Ceramics can be shaped by
filling a mold with loose powder
and compacting it – the main
microstructural evolution is the
shrinkage of porosity during
compaction

22. Processing can play a significant role in determining the final molecular arrangement and thus properties if the molecules are first aligned mechanically by the flow induced during the shaping process, and cooling is rapid enough to freeze the alignment
The most dramatic impact of processing on polymer properties is in making fibers
Drawing fibers creates significant molecular alignment and the covalent bonding along the chain is exploited to markedly increase the specific strength and stiffness of the polymer

24. Hybrid materials offer material property profiles not found for single materials
However, many processing challenges are presented when shaping and joining hybrids

25. SUMMARY
Manufacturing processes are key to producing materials with the desired properties
Processes affect the microstructure of the material
Surface processing can add additional properties such as wear or corrosion resistance
Joining processes can introduce changes in the strength of the object because joints have different strengths than the raw material.
Metals are the most important engineering material to provide a range of strengths and toughness
Ceramics, glasses and polymers can also be modified through:
blending
Cross linking