Metals - Alloys Flashcards
Describe Homogenisation
Vital pre-hot working step
DC ingots heated for varying lengths of time
Alloy dependent
Reduce micro (and macro) segregation
Poor homogenisation treatments lead to problems during hot working
What is Solution Treatment?
After solidification the alloy is heated to a certain temperature and held for a long time (sometimes as much as 24 hours) to remove the eutectic and to homogenise the solute content
It is then reheated to a lower temperature to dissolve all of the key alloying elements
What is Quenching?
Distortions can be reduced by controlled cooling
Quenching can lead to residual stresses which can cause distortion or even failure
Slower quenching through critical region leads to reduction in achievable strength after ageing
What is Ageing?
Usually the final step
Carried out at 100 to 190 degrees C for 8 to 16 hours
What are the effects of alloying on process and performance?
Deformability is impaired Corrosion resistance is best for pure Al Surface finish is best for pure Al Density is increased by alloying Conductivity is best for pure Al
Why alloy?
Increase strength
Improve fabrication properties
What are the precipitation rules?
When the T decreases, if the solubility of B in A decreases then a precipitate will be formed
The nature and composition of this precipitate will depend on the composition of the alloy
These precipitates (particles, dispersoids) will have a positive effect on strength if we follow the correct heat treatments
Why is Homogenisation used?
Get rid of uneven chemical composition caused by casting
Need to do this otherwise will cause defects in product
Why is Solution Treatment used?
Dissolve all the useful alloying additions
Why is Quenching used?
Retain elevated temperature microstructure
Not to quickly and not too slowly
Why is Ageing used?
Form useful precipitates within grains (not grain boundaries)
Correct ageing time and temperature is key
