Mg wrought alloy processing problems
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This document discusses wrought magnesium alloys, including their composition, processing methods, and deformation behavior. It notes that magnesium alloys have low workability at room temperature due to their HCP crystal structure, but can be processed at temperatures above 300°C. The effects of temperature and strain rate on the microstructure and mechanical properties of AZ31 magnesium alloy during hot rolling are examined. Creep behavior is also discussed, noting the different creep stages and that magnesium alloys have limited creep resistance above 200°C due to their tendency to oxidize.
Mg wrought alloy processing problems
- 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
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- 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