Research On Grain Refinement Of AZ31Magnesium Alloy By Refiner And Combination Of Refiner And External Field

To date, adding carbon containing grain refiner is the main grain refinement method for the widely used Mg-Al magnesium alloys. However, there are some problems of this method such as enviorment pollution, low efficiency and uncertain refinement mechanism etc. Therefore, it is urgent to develop a novel carbon containing refiner or a more efficiency refiner with no carbon containing. Electromagnetic field and ultrasonic field (US) have an strongly influence on the solidification structure of steel and aluminium alloys, but the investigation of combined treatment of the two fields and grain refiner on microstructure and mechanical properties is rarely published; Moreover, the investigation of rifning behavior and affecting mechanism of rare earth elements (Gd, Nd) on Mg-Al alloy are not sufficient enough. Therefore, it is theoreticallyimportant and practically valuable to systematically investigate the effects ofalloying elements, refiner and external fields on grain refinement behavior and mechanism of as-cast Mg-Al alloys.In order to solve the problems of the traditional carbon containing refiner, Al-5C master alloy as a grain refiner was prepared by in-situ reaction sintering method, and the problem of introducing carbon containing refiner into Mg melt had been successfully resolved. Meanwhile, the effects of parameters such as adding amount, stirring way on the microstructure and mechanical properties of AZ31alloy had been investigated. Furthermore, the effects of combination of spiral magnetic field and ultrasonic field with Al-5C master alloy on microstructure refinement of AZ31alloy also had been studied, and theses results indicate that Al-5C refiner has remarkable grain refinement efficiency on AZ31alloy. Imposing spiral magnetic can be contribute to disperse AI4C3particles, thus the refining efficiency of Al-5C refiner are improved, a superimposed refining performances also be proved; nevertheless, the refining performances of combination of US and Al-5C refiner are similar to that of imposing US singly.The investigations of ultrasonic treatment (UT) refining microstructure of Mg alloys, especially for the secondary phases in Mg matrix, are rarely reported. This study investigates the process technology and refining mechanism of AZ31alloy, and found a remarkable refining phenomenon of a-Mg and Al-Mn phase in AZ31alloy by UT. Consequently the refining performance and mechanism of Mg2Si and Mg3Sb2phases in AZ31-lwt%Si and AZ31-lwt%Sb were studied respectively. The results indicate that the secondary phases can be effectively refined by UT when the processing temperature is nearby the precipitation temperature of these secondary phases and the mechanical properties can also be improved. The cavitation effect produced by UT can promote nucleation of secondary phase and affect its growth process. Meanwhile, secondary phases can distribute uniformly under acoustic streaming.Al-3Ti-3B and Al-5TiB2master alloy were prepared by LSM methods. The effects of the two master alloys on microstructure and mechanical properties of as-cast AZ31alloy were investigated. The results indicate that there is no refining effect on AZ31alloy when adding Al-3Ti-3B master alloy with none aided process but having a certain refining effects with hight-temperature stirring process. The refining mechanism should attribute to the elimination of Fe element by reacting with AIB2phase. The grain refinement of AZ31alloy is not evident with adding Al-5TiB2master alloy, which could be owing to TiB2particles cannot acting as heterougnerous substrate of a-Mg.Adding little amount of Gd and Nd into AZ31melt will firstly form Al2Gd and Al2Nd phases which distribute in grain boundary. With the increasing of Gd and Nd addition, Al2Gd and Al2Nd gradually changed into Al4Gd and Al11Nd3phases which mainly distributed inside of α-Mg grain. The refining effect of Nd with a larger GRF value is better than that of Gd element which has a relative lower GRF value. The affecting mechanism of Gd and Nd elements on micro structure of AZ31alloy can be analyzed from the following three aspects:1) Reduction of Al element and Al-Mn compounds by forming Al-RE phases and Al-Mn-RE phases with RE element would decrease Al refining performance;2) The Al-RE compounds percipated prior to α-Mg, which can hinder the growth of a-Mg. But there are large difference in lattice disregistry between α-Mg and Al-RE compounds for which Al-RE compounds cannot act as heterogenous substrate of α-Mg.3) Excess RE elements were discharged in the front of L/S interface which generate constitutional undercooling and promote nucleation. The kinds of Al-RE compunds are changed with increasement of Gd and Nd adding amount, which caused the decrease of Gd and Nd amount and lower the GRF value.