Light Alloys- Magnesium 3 Flashcards
What are the only ways deformation of Mg can occur at room temperature?
Slip on the {1000} basal planes in the <11-20> direction.
Twinning on the {10-1}, {10-11} and {10-13} planes.
Limited due to hcp structure
New modes of deformation for Mg at T>250C
Slip can occur on pyramidal and prismatic planes.
Means more workable at higher temperatures
What happens to grains in hot rolling of Mg?
Start with equiaxed grains. Go through two rolls (one above and below). While in the region between the rolls there is dynamic recovery (grains start getting longer). Just after rolls can either get static recovery resulting in elongated grains (along rolling direction) or static recrystallisation resulting in formation of smaller equiaxed grains than start
The different directions in rolled Mg sheets
Rolling direction: the direction the metal move through the rolls in.
Normal direction: perpendicular to the rolling direction out of plane of sheet.
Transverse direction: perpendicular to both in plane of sheet.
45°: the direction at 45° to the RD and TD in plane of sheet
Sheet anisotropy in Mg sheets
Stress strain curves always highest in rolling direction compared to transverse and 45° directions (with transverse most often being lowest of 3)
What is limiting dome height?
Related to sheet formability. A dome (bulge) can form on the sheet when near the rolls. I think the limiting dome height is maximum dome height before the metal will fracture on the dome. Grades with higher yield strength generally have lower limiting dome height
What is extrusion?
The process by which a block/billet of metal is reduced in cross section by forcing it to flow through a die orifice under high pressure
Direct and indirect extrusion
Direct: ram on one side of billet forces it through a die orifice on the other side.
Indirect: the die is forced into the billet which flows back through the die orifice in the opposite direction (sealing element on other side)
Alignment of basal planes after extrusion
Basal planes are aligned with the extrusion direction
What happens to the grains during extrusion?
Just before the die orifice there is dynamic recrystallisation. Just after (or a bit later after elongation) is static recrystallisation. Looks from diagrams if small grains start to form before orifice then these form larger grains after the sooner static recrystallisation. Smaller grains form from the delayed static recrystallisation after elongation of grains
Tension and compression stress strains graphs for extruded Mg alloys
In tension is the normal shape you would expect. Compression starts the same but yields earlier then curves back up to cross tension curve then gradient decreases
Advantages of forging Mg
Excellent strength, especially with fibres lying parallel to main load direction. Very good profit pressure-sealed components by preventing porous microstructure. Complex component geometries can be produced (using several forging steps). Used in car wheel for example
Methods of joining for Mg alloys
Arc-welding
Laser-beam welding
Electron-beam welding
Friction welding
Diffusion bonding
The serious problem is pore formation
Corrosion of Mg
High susceptibility to impurities. MgO and Mg(OH)2 are not stable in solution with pH<8.5. Corrosion rate lies between Al and mild steels
Salt fog test
Test of corrosion. Expose the part for a certain amount of time and compare appearance to other materials exposed the same amount of time. Best if cleaner surface and all material still there after exposure
