Lezione 19: Processing and properties

Question 1

Do processes influence the properties of materials?

Answer 1

They do, to a big extent. Intrinsic properties such as strength and resistivity depends on microstructure, and microstructure depends on processing. The ability to tune microstructure and properties is central to materials processing and design, so it’s necessary to have good process understanding and control.

Question 2

What can we deduce from a process history?

Answer 2

Materials processing involves more than one step, and each step has a characteristic thermal history. Designers also should watch out for unintended side effects in the joining stage. Design focuses on the properties of the finished product, but some of these properties are also critical during the processes themselves. 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 micro structural evolution.

Question 3

What differences can we find between cast and wrought (battitura) alloys in metal processing?

Answer 3

• Metals with an HCP (hexagonal close packed) structure (Cu, Ti, Ni) are primarily cast at high T because of their inherent lack of ductility
• Casting and wrought alloys in a given system tend to have different composition. Casting leads to a coarser microstructure and poorer strength and toughness.
• Good castability requires higher levels of alloying additions than wrought alloys to lower the melting T

Question 4

What happens in metal casting solidification?

Answer 4

In casting, a liquid above his melting T is poured into a mold where it cooled by thermal conduction: it’s cheap and complex 3D shaped can be obtained. New solid forms by nucleation: new crystals form in the melt, in the walls of the mold, or on foreign particles. Crystals grow in opposing directions and impinge on one another to form grain boundaries.

Question 5

What happens in deformation processes of metals?

Answer 5

Deformation processes exploit the plastic response of metals (their ability to remain intact when subjected to large strains and shape changes). Deformation processes influence microstructure bu various means:
-Temperature: determines which phases present and is relevant during both forming and cool-down
-Grain size: forming processes can strengthen the metal by changing both the size and shape of the grains.
While these processes can increase strength, they also often reduce the ductility, and it’s necessary to follow forming with an annealing heat treatment (ricottura)

Question 6

Which processes allow metal strengthening?

Answer 6

In order to optimize and/or increase some mechanical properties of metals one or more treatments may be applied

• hardening (plastic deformation)
• alloying
• thermal treatments
• precipitation hardening

Question 7

How does hardening affect the properties?

Answer 7

Hardening (plastic deformation): applied to stainless steel, brings to an increase in yield and tensile strength (E doesn’t depend on microstructure), and a decrease in elongation

Question 8

How does alloying affect the properties?

Answer 8

Alloying: the higher the % of alloying element, the more we see an increase in yield and ultimate tensile strength and a decrease in elongation (so also decrease in fracture toughness bc metal is less ductile) and electric/thermal conductibility

Question 9

How do thermal treatments affect the properties?

Answer 9

Thermal treatments: on some metals, specific thermal treatments can increase mechanical properties as Sy, UTS, hardness and decrease toughness, elongation. For example, quenching (high T heating with fast cooling in oil or water) causes a distorted structure, martensitic, that gives hardness. But then to decrease embrittlement we can procede with tempering (mod. T heating with air cooling)

Question 10

How does precipitation hardening affect the properties?

Answer 10

This is tied to heat treatments too: in a typical heat treatment, a component is heated to high T, cooled at a controlled rate, usually reheated to and intermediate T. A solid solution is formed at high T, followed by precipitation hardening at an intermediate T after being cooled. Effects:

• normalizing: slow cooling from high T producing a microstructure of lower strength but high toughness - no follow up heat treatment
• quench and temper: cooled faster than critical cooling rate producing a martensitic microstructure with high strength and ow toughness, then reheated at an intermediate T to restore toughness

Question 11

Why should be keep an eye on joining processes?

Answer 11

Thermal welding of metals involved heating and cooling, which may cause phase transformations in the weld metal and in the heated regions of the weld.

Question 12

What is surface engineering?

Answer 12

Surface treatments exploit may different mechanisms and processes to change the surface microstructure and properties. Laser hardening is a surface treatment process that modifies the 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.

Question 13

Give some examples of products where process modification of properties is fundamental

Answer 13

• Cast iron disk brake: since most ferrous alloys offer a wide variety of properties required for a disk brake, gray cast iron is a perfect candidate also bc of its machinability. Poisoning is the addition of a small amount of cerium or magnesium, sometimes with annealing treatment too, which improves the morphology of graphite in iron and gives cast iron the needed toughness for a wider range of applications
• Plain carbon steel I-beams: commonly made of wrought plain carbon steel that has undergone significant deformation processes, the microstructure produces high fracture toughness and strength
• Alloy steels and heat treatments for cranks, tools, gears: alloy steels are chosen in this case bc of their high hardenability mainly through quenching and tempering (heat treat.)
• Stainless steel cutlery: here the main alloying element are chromium and nickel. The first one imparts excellent corrosion resistance, the combination provides solid solution hardening

Question 14

What is the most versatile material in engineering?

Answer 14

All in all, the range of mechanical properties able to be achieved through alloying and processing makes ferrous metals the most versatile group of engineering materials. This is because strength and toughness are the most desired properties for engineering applications

Question 15

How are ceramics and glasses processed?

Answer 15

Bc of their susceptibility to cracking, defect control is the most important aspect of processing these materials. While some thermal processes can increase the properties of glasses, change 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

Question 16

How are polymer and elastomers processed?

Answer 16

Processing can play a significant role in determining the final molecular arrangement and properties if the molecules are first aligned mechanically by the flow induces during the shaping process, and cooling is rapid enough to freeze in 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 increase specific strength and stiffness of the material.