Investigation of formability of
super plastic magnesium alloy AZ31B
PhD. Student
EMAD ALI HUSSEIN
Dept of Engineering Production and metallurgy
Supervised by
Asst. Prof. Dr. AZAL REFAAT ISMAIL
The aim
• To improve the formability of Magnesium alloy AZ31B by grain refinement .
• Draw FLD .
Magnesium and its alloy
Introduction
Magnesium, with a specific gravity of only 1.74, is the lowest-density metal
available for engineering use. It is produced either by electrolytic reduction of
MgCl2 or by chemical reduction of MgO by Si in the form of ferrosilicon.
MgCl2 is obtained from seawater, brine deposits, or salt lakes. MgO is
obtained principally from seawater or dolomite. Because of the widespread,
easy availability of magnesium ores (e.g., from the ocean), the ore supply is,
in human terms, inexhaustible
Uses
Magnesium is used both as a structural, load-bearing material and in
applications that exploit its chemical and metallurgical properties.
Magnesium is used in a diverse range of markets and applications, each one exploring
its unique properties. For decades, the automotive industry was seen as the greatest
opportunities for structural magnesium. At the same time, magnesium alloy
development has traditionally been driven by aerospace industry requirements.
Application of magnesium for ground and air transportation vehicles reduces their
total weight, reduces fuel consumption, toxic emissions and greenhouse gases, and
increases recyclability, safety and handling.
Applications of magnesium alloy
Applications of magnesium alloy
Physical Properties of Pure Magnesium
Density
Melting point
Boiling point
1.718 g/cm3
650C
1107C
Young’s modulus
Modulus of rigidity
Poisson’s ratio
45 Gpa
16.5 Gpa
0.35
nominal chemistries of wrought magnesium alloys. All values in weight %.
Physical Properties of AZ31B Alloy
Density (g/cm3)
1.77
Melting Point
Liquidus
Solidus
632 C
605 C
Electrical Resistivity (ohm-metres
9.2
108)
Typical Mechanical Properties of Wrought Products
Sheet and Plate
AZ31B
O
H24
Extrusions
AZ31B
F
Forgings
AZ31B
F
255
290
150
220
110
180
21
15
260
200
95
15
260
195
85
9
Superplasticity
Superplasticity is defined as a state in which a solid crystalline
material is deformed well beyond its typical breaking point, often
exceeding 1000% during tension. Such a state is achieved in some finegrained metals and ceramics at temperatures of typically half that of the
absolute melting point. Thus requirements for a material to become
superplastic include a fine grain size, typically below 10 μm, and a fine
dispersion of thermally stable particles that act to pin the grain boundaries
and maintain the fine grain structure at the high temperatures. The
materials must also have a high strain rate sensitivity (>0.5) which
prevents localized deformation at a reduced cross-section (necking). At
the macroscopic scale, a superplastically deformed material experiences
uniform deformation, rather than localized necking, preceeding fracture.
Another cause of premature failure, namely a formation of internal
cavities, is also inhibited.
Superplasticity
In order to achieve superplastic flow in a polycrystalline material :
1- The material must have a very small and stable grain size in range of ~ 2-10 μm .
2- Temperatures at and above of ~ 0.5Tm .
3- Strain rate .
There are two principal classes of superplastic processes termed .
• transformation superplasticity.
• Thermo-mechanical superplasticity.
Thermo-mechanical processing is used to produce alloys for industrial super plastic forming
operation . However , it is now well established that processing through the introduction of
severe plastic deformation provides the capability of producing grain sizes in the sub
micrometer or nanometer range , For example .
1. Process of equal-channel angular pressing (ECAP).
2. Process of equal-channel angular rolling (ECAR).
3. Process of equal-channel angular extrusion (ECAE).
plunger
Roll
Die
Sample
Sample
Die
Roll
ECAP
ECAR
Schematic illustration Thermo-mechanical processing
Forming Limit Diagram
Introduction
The forming limit diagram (FLD) provides a rather complete assessment of
sheet metal formability under various strain paths. The concept of FLD was
originally proposed by Keeler (1965), who concluded, from tests of various metals
biaxially stretched over punches, that there was a critical ratio of major to minor
strain that produced fracture. Goodwin (1968) combined Keeler’s data with
fracture strains gathered on materials stretched under conditions of negative
minor strain to produce the first rudimentary FLD. Currently, the standard
approach to experimentally probe the FLD is to test strips of various widths, which
enforce various strain paths, using the limiting dome height (LDH) apparatus
(Hecker, 1975) .
Forming Limit Diagram
• Sheet metal can be deformed only to a certain level – before local thinning
(necking) and failure occur .
• FLD shows the limit of necking (or failure) as function of minor and major strain
• Strains can be evaluated from the deformation of circle grids plotted on the
surface of sheet metal .
Forming limit diagram test
Sheet metal is one of the most important semi finished products used in the steel
industry, and sheet metal forming technology is therefore an important
engineering discipline within the area of mechanical engineering. Sheet metals
are characterized by a high ratio of surface area to thickness. Sheet metal forming
is basically conversion of a flat sheet metal into a product of desired shape
without defect like fracture or excessive localized thinning.
Patterns of Circle Grids
STRAIN MEASUREMENT
After sheet metal is formed the marked circles will deform into ellipses of different sizes.
Strain is calculated from the following formula.
Forming Limit Diagram
Thank you
for attention