processing problems on wrought Mg alloys

1. By: Asmaa Mostafa Khalil

2. Wrought Magnesium alloys
 contents
- Introduction
- Composition of diff. Mg alloys
- Disadvantage of Mg alloys
- General deformation behaviour of Mg.
- The Effects of Working Temperature and strain rates

3. Introduction
 Wrought magnesium alloys are manufactured in forms
of rolled sheets and plates, extruded bars, rods and
tubes, forged shapes.
 Warm and hot processing are often used(> 350C)
 Extruded semi-finished products without Zr have full
recrystallised structure and with Zr elongated rectangular
grains due to ZrH
 The ability of magnesium to deform increases as the initial
grain size reduces

4. 

5. Compositions

6. Disadvantages of magnesium alloys:
 Low elastic modulus
 Limited cold workability and toughness (HCP structure)
 Limited creep resistance at elevated temperatures (Tm =
650°C)
 High degree of shrinkage on solidification (high thermal
expansion)
 High chemical reactivity

7. General deformation behaviour of
magnesium
1- Forging of magnesium alloys
Advantages of forged magnesium components compared to commonly used
die cast magnesium parts:
1) Excellent strength, especially with the fibres lying parallel to the main
load direction.
2) Very good properties for pressure-sealed components because of a
forging process in preventing a porous microstructure.
• Grain size and multiphase microstructure are the main problems
in magnesium forging. This can be overcome by additional extrusion process
to give a sufficient grain size for forging.
• Complex component geometries are usually produced in several
forging steps.

8. 2-Deformation
Deformation is limited in Room Temp. due to HCP structure,
This structure restricts its ability to deform because it has
fewer slip systems at lower temperatures (two independent slip
systems)
1) By slip on the {1000} basal planes in the <1120> direction.
2) Twining on the {1012} pyramidal planes.
• At T>250oC slip can occurs on pyramidal and prismatic
planes.
 therefore these alloys are worked hot at the temperatures
within the interval 570-890 °F (300-475°C).

9. Slip system: planes and directions

10. The Effects of Working Temperature and
strain rates
The principle in processing Mg alloys is temperature. In order to achieve
favorable mechanical properties, one must really understand the Mg alloys
behaviour at different deformation conditions namely temperature and
strain rate. It also extremely important for researcher able to correlate the
effect of changing these deformation conditions on the resulting
microstructure and its mechanical properties. Zhang et. al. (2006) studied
the formability of AZ31 Mg alloy sheets on mechanical properties during
hot-rolling
process.
The relationship was illustrated in Figure1, showing the mechanical
properties of AZ31 Mg alloy changed as a function of temperature at a
constant strain rate.

12. Summary of workabilty of AZ alloying systems on different
routes

13. 3- Creep behavior of magnesium
The current interest, however, come from the conceivable
economic and environmental benefits of the use of
magnesium alloys particularly in the automobile’s power-train
applications requiring creep resistance at temperatures up to
200 °C. Thus we must begin by revisiting our knowledge of
general creep behaviour of metals considering magnesium
specifically.
Creep is a time-dependent deformation process that clears
itself even when applied constant stress and below the yield
strength of material. While in an ordinary deformation the
movement of dislocations, on an atomic scale, can cover large
distances at each step, in thermally activated deformation
this movement occurs due to diffusion of atoms and therefore
is restricted to a few atomic distances at each step.

14. stages of creep:
• primary stage creep occurs with increasing
creep rate. Here the work hardening is
higher than the stress recovery.
• In the secondary creep stage (steady-state),
the metal experiences a balance between
work-hardening and recovery processes
resulting in steady-state creep (constant
creep rate or minimum creep rate).
• The tertiary creep stage exhibits increasing
creep rate due to necking
resulting finally in fracture.

15. 4- OXIDATION
• Another major obstacle in processing magnesium is their
high affinity of magnesium to oxygen makes them easy to
oxidation. The oxidation spontanously formed a thin layer
on the surface of magnesium and its alloys upon exposed in
air.
• They found that the lower volume of magnesium
oxide compared to the base material created blisters and crack
easily.

16. Thank you