Investigation of formability of super plastic magnesium alloy

Investigation of formability of super plastic magnesium alloy AZ 31 B Ph. D. 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 AZ 31 B 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 Mg. Cl 2 or by chemical reduction of Mg. O by Si in the form of ferrosilicon. Mg. Cl 2 is obtained from seawater, brine deposits, or salt lakes. Mg. O 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 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/cm 3 650 C 1107 C 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 AZ 31 B Alloy Density (g/cm 3) 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 AZ 31 B O H 24 Extrusions AZ 31 B F Forgings AZ 31 B F 255 290 150 220 110 180 21 15 260 200 95 15 260 195 85 9

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. 5 Tm. 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 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

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