Light Alloys- Aluminium Work Hardenable Alloys and Heat Treatment Mechanisms Flashcards
What are work hardenable alloys?
Those that cannot be hardened by precipitation mechanisms. Mechanisms of increasing strength limited to:
Refining grain size
Solid solution strengthening
Cold working (increase dislocation density)
Which 3 Al alloy series are work hardenable?
1xxx (at least 99.00 wt% Al)
3xxx (Al-Mn)
5xxx (Al-Mg)
How does the 1xxx series name system work?
Numbers indicate impurity level.
E.g 1145 is 99.45 wt% Al
And 1199 is 99.90 wt% Al
1xxx series: properties, strengthening, applications, other info
Typical impurities are Fe and Si (beneficial). Yield strength can vary from 10MPa in annealed 1199 up to 165MPa in heavily worked 1350. Yield strength increases linearly with volume fraction Al-Fe-Si. Recrystallisation leads to drop in strength but increased ductility. Formation of intermetallics by DC process can affect surface quality. Applications: foil, architectural decorations, power transmission (pure grades).
3xxx series: properties, 3004, recycling
Has 1 wt% Mn which forms dispersoids that refine the grain size (disrupt the structure so help recrystallisation). Annealed has 40MPa and 30%, cold worked has 185MPa ans 4%.
3004 (Al-1.2%Mn-1%Mg) has Mg as a solution strengthener. Is recyclable and used for cans (100billion in US, 30bill in EU). The higher energy content for Al than steel makes it more economical to recycle
3xxx series: applications and strengthening
Used for higher strength than AA1100. Hardening by SS hardening from Mn and from dispersion strengthening via MnAl6 particles. Homogenisation replaces microsegregation of Mn with MnAl6 distribution. Replacing some Mn with Mg (AA3105) gives SS strengthening from Mg. suitable for can stock
Why is there a yield plateau before work hardening on stress strain curve for 5xxx series alloys?
During this deformation is localised into bands surrounded bands of uniformed material (luders bands). E.g forming curved panel from flat sheet. Mg (in 5xxx series alloys) segregates to dislocations on GBs to stop them operating. Only locally when a source get activated will you get a burst of dislocation activity, very localised high strain level.
Solution to yield point phenomena in 5xxx series
Lightly roll sheet (compressive deformation) to activate all dislocation sources throughout the material so don’t get luders bands when forming the panel. However must form just after rolling or Mg atoms will migrate back to dislocation sources during storage at room temperature
5xxx series: what is it?, properties (YS and %), +ves, -ves
Up to 6 wt% Mg. Annealed: YS 40-160MPa, 25% elongation.
Cold work: YS 300MPa, 5% elongation.
Advantages (over steel): much more recyclable, lower density, does not rust.
Disadvantages: weldability (use adhesive bonding instead), repairability, yield point phenomena
Used in some car bodies
5xxx series: strength, general properties, solubility
Up to 3% Mg gives precipitation hardening via Al3Mg2. Stronger than equivalent Al-Mn alloys. High strength, good formability, good corrosion resistance, bright surface finish, weldable. Main strengthening mechanism is SS strengthening by Mg dissolved in Al. Mg has high solid solubility in Al. 17.4% at 450C down to 1.7% at RT. More powerful SS strengthener on a weight basis than copper.
5xxx series: effect of Mg
Strength increases in cold working (strain hardening) due to effect Mg has on lattice strain. Dislocation motion is restricted due to this. Mg atom has higher mobility in Al than a vacancy so moves to areas of higher dislocation density. This further reduces mobility and results in an increase in dislocation multiplication
Which are the heat treatable alloys?
2xxx - Al-Cu(-Mg)
6xxx - Al-Mg-Si
7xxx - Al-Zn-Mg(-Cu)
8xxx - Al(-Li)
What are the different phases considered for the heat treatable alloys?
2xxx: Al-Cu system is α+θ (CuAl2)
2xxx: Al-Cu-Mg system is α+S (Al2CuMg)
7xxx: Al-Zn-Mg system is α+η (MgZn2)
8xxx: Al-Li system is α+δ’ (LiAl3)
First two steps of heat treatments after initial solidification of an Al-Cu alloy
Solution treatment: homogenisation. Alloy heated to temperature below the eutectic temperature and held for up to 24h to remove the eutectic and homogenise the solute content.
Quench to room temperature: cooling quickly prevents CuAl2 formation can Cu cannot diffuse at RT. gives super saturated solid solution.
What happens if you slowly cool a Al-Cu alloy instead of quenching after solution treatment?
Al2Cu will form at the GBs. This is an embrittling intermetallic. Grains themselves are soft.