Investigation of formability of super plastic magnesium alloy AZ31B

PhD. StudentEMAD ALI HUSSEIN

Dept of Engineering Production and metallurgy

Supervised byAsst. Prof. Dr. AZAL REFAAT ISMAIL

The aim • To improve the formability of Magnesium alloy AZ31B by grain refinement .• Draw FLD .

IntroductionMagnesium, with a specific gravity of only 1.74, is the lowest-density metalavailable for engineering use. It is produced either by electrolytic reduction ofMgCl2 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

Magnesium and its alloy

UsesMagnesium is used both as a structural, load-bearing material and in applications that exploit its chemical and metallurgical properties.

Applications of magnesium alloy

Applications of magnesium alloy

Physical Properties of Pure Magnesium

Density 1.718 g/cm3 Melting point 650C Boiling point 1107C

Young’s modulus 45 GpaModulus of rigidity 16.5 GpaPoisson’s ratio 0.35

nominal chemistries of wrought magnesium alloys. All values in weight %.

Typical Mechanical Properties of Wrought Products

Sheet and PlateAZ31B O 255 150 110 21 H24 290 220 180 15ExtrusionsAZ31B F 260 200 95 15ForgingsAZ31B F 260 195 85 9

Electrical Resistivity (ohm-metres 108) 9.2

Solidus 605 C

Liquidus 632 C

Melting Point

Density (g/cm3) 1.77

Physical Properties of AZ31B Alloy

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 fine-grained 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.

• 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).

Superplasticity

Roll

Roll

DieSample

Schematic illustration Thermo-mechanical processing

ECAP ECAR

Sample

Die

plunger

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 .

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

Forming Limit Diagram

Thank you for attention