Metals- Production of Al Alloys Flashcards
Generic production route
Casting, pre-working treatments, hot working, cold working, post processing, finishing
Homogenisation
Vital pre-hot working step. Cast ingots heated for varying lengths of time (alloy dependent) to allow diffusion to take place. This is to reduce micro and macro segregation. Poor homogenisation leads to problems during hot working
Solution treatment
A post working treatment. Purpose is to take back into solid solution as much as possible all the alloy additions that contribute to age hardening. Temperature control is critical and alloy dependent. Heat to as high a temperature as possible without any melting occurring and for as short a time as possible
Quenching
Quickly cooling the metal back down to room temperature to lock in the microstructure from when it was heated. Post processing technique. Too quick can lead to residual stresses which can cause distortion or failure. Too slow through critical region (400 to 290°C) leads to reduction in achievable strength after ageing.
Quench sensitivity
Some metals are more sensitive to cooling rate than others. Some need a very precise cooling rate
Ageing
Usually final step. Carried out 100 to 190C for 8-16h for artificial ageing. Some alloys naturally age over periods of time. Multi-step ageing is where is held at one temperature then held at a different temperature
Effects of alloying aluminium
Deformability impaired. Corrosion resistance, surface finish, conductivity best when pure Al. Density increased by alloying. But increased strength
Increase in strength for alloying Al
99% pure Al has UTS 90MPa and PS 30Mpa. Strongest age hardenable alloys have UTS 700MPa. Some rapid-solidified powder metallurgy alloys have UTS 1GPa.
How can alloying improve castability?
Add Si or Sn to give greater fluidity. Means it is easier to fully fill the mould for the desired dimensions
Other good things about Al alloys
There are no allotropic changes with temperature (always fcc). Useful solid solubility is limited to a few additions so simpler selection process. Compound formation is common. Can have solid solution strengthening and precipitation hardening.
Heat treatment designations
T1 part solution treated and naturally aged T3 solution treated and cold worked T4 ST and naturally aged T5 artificially aged T6 ST and AA T7 ST and stabilised T8 ST, CW and AA T9 ST, AA, CW
Two types of precipitates for precipitation hardening
Coherent precipitates
Incoherent precipitates
How do coherent precipitates work?
A dislocation edge moves along a slip plane which goes through the particle. When it moves through the particle via shear mechanism, one part of the particle moves relative to the other by a Burgers vector. The strength of the alloy is proportional to the square root of the precipitate size.
How do incoherent precipitates work?
Works by bowing mechanism. The long part of the dislocation tries to move through the precipitates. It has to bend around each one forming bows. These bows stretch until the dislocation finally makes it through and isn’t pinned by the same precipitates any more.
Another name for precipitation hardening
Age hardening
