|M.Sc Thesis||Department of Materials Science and Engineering|
|Supervisor:||Prof. Bamberger Menachem|
Magnesium has a special place in the world metallurgical production. Among the advantages of magnesium and its alloys are a) the lowest density of all metallic constructional materials; b) high specific strength; c) good castability, suitable for high pressure die-casting; d) good weldability under controlled atmosphere; e) readily available.
Because of the importance of grain refinement to the development of high-performance magnesium alloys, the research work was initiated to better understanding of the factors controlling grain refinement.
The main goals of the project are to investigate microstructure refining in Mg alloys, when the melt solidifies under Ultrasonic Treatment (UST) or with addition of powerful grain refining modifier. In this project the grain refiner is zirconium.
The first part of the research work, focus on studying the influence of ultrasonic treatment on magnesium alloys Mg-Ca-Zn and Mg-Zn-Sn-Si. The composition of alloys resembles these studied previously. Stable intermetallic Mg6Zn3Ca2 and Mg2Ca, Mg2Sn, Mg2Si phases are expected to form during solidification.
The second part of research investigates the influence of adding different amounts of Zirconium (from 0.2wt%Zr to 1wt%Zr) on the microstructure of magnesium alloy (Mg-Ca-Zn). In addition the alloys with grain refiner, Zr, were solution treated in order to have better understanding of the precipitation mechanism.
Applying ultrasonic field on Mg alloys during solidification has positive effect on microstructure refinement of the cast parts. After ultrasonic treatment the dendrite structure of Mg-Ca-Zn alloy was changed to equi-axed grain structure and DAS of the Mg-Zn-Sn-Si alloy was refined on ~ 50%. Ultrasonic treatment also improves the hardness of these alloys. The Mg2Si and Mg2Sn phases were more uniform dispersed as against these of the reference alloy. The presence of Mg2Ca phase in alloy
Mg-Ca-Zn is significantly suppressed after ultrasonic treatment in comparison to regular solidification according to SEM micrographs and XRD observation.
Zr addition to Mg-Ca-Zn alloy changed its structure from dendrite to equi-axed grain structure, with the average grain size 22µm as against 50µm in the non-refined alloy. It was found that the optimal amount of Zr is 1 wt%. The location of the Zr particles in the center of the grains and TEM analysis indicate that the Zr serves as a nucleation center. This alloy was found suitable for precipitation hardening following the solution treatment and aging at elevated temperatures.