The effects of processing parameters, such as the punch speed, reheating duration and reheating temperature, on microstructure and ultimate tensile strength (UTS) of thixoformed AM60B magnesium alloy have been investigated. The results indicate that low punch speed, short reheating duration or low reheating temperature often produces the defect of cold shuts or shrinkage porosities. On the contrary, gas pores can easily form. In addition, the reheating duration or temperature also has obvious effects on the primary particle size, fraction and morphology. The three parameters can significantly affect the UTS due to their effects on the formation of pores (cold shuts, shrinkage porosities and gas pores). But they can not alter the fracture path during tensile testing and the path is always along the secondarily solidified structures between the primary particles. The effect of pore amount on the UTS is lager than that of the primary particle size, fraction or morphology. In view of their effects on the microstructure compactness and the resultant UTS, the optimized parameters are reheating for 130 minutes at 610 °C and punch speed of 3 m/s.
The microstructural evolution was investigated during partial remelting of 6061 aluminum bulk alloy prepared by cold-pressing of atomized alloy powders. Meanwhile, the effect of heating temperature on semisolid microstructure was also studied. It was found that after partial remelted, a semisolid microstructure with small and nearly spherical particles can be obtained. The microstructural evolution can be divided into three stages: the rapid coarsening of grains and powders, the structure separation and spheroidization of powders, and the final coarsening behavior of primary particles. For most of the primary particles (larger than 10 μm) in the semisolid state, one particle originates from one original powder in the cold-pressed bulk alloy. Furthermore, proper elevated the heating temperature is beneficial to obtain ideal semisolid microstructure.
The effects of Nd or Zr addition on microstructure and mechanical properties of casting Mg-Zn-RE alloy have been investigated. The results indicate that the optimum contents of Nd and Zr are 0.5% and 0.3% respectively. The less or higher contents all increase grain size and accelerate the formation of shrinkage porosity, and thus decrease tensile properties and change fracture regime from transgranular mode to intergranular mode during tensile tests. The Mg-Zn-RE ternary phases transform from W (Mg 3 Zn 3 (Nd,Y) 2 ) in turn into W + Z (Mg 12 Zn(Nd,Y)) and Z as the Nd content increases and their distribution gradually changes from small discontinuous lath form to continuous network form. The grain bonding strength of an alloy with a certain amount of W phase that distributes in a discontinuous form is high than that of the alloy with the Z phase, and thus the W phase is an ideal strengthening phase. The alloy with 0.5 Nd and 0.3% Zr, named ZW21 alloy, has the smallest grains and highest grain bonding strength, and so has the highest comprehensive mechanical properties.
The grain refining technique of AZ91D magnesium alloy by MgCO 3 has been investigated. The refining mechanism and tensile properties of the resulting alloy have also been discussed. The results indicate that MgCO 3 can decrease its grain size from 311 to 53μm. Correspondingly, the tensile properties are obviously improved. The higher the cooling rate from addition temperature to pouring temperature or the higher the addition temperature, the finer the grains. The melt treated by MgCO 3 should be poured as soon as possible because the inoculation fading is quite quick. The microstructure sensitivity to the diameter of a cast rod is relatively high and the microstructures of the rods with large diameters are quite inhomogeneous. The refining mechanism of MgCO 3 belongs to heterogeneous nucleation and the nucleant substrates are believed to be the Al 4 C 3 particles formed from the reactions between the MgCO 3 and the molten alloy.
A novel method, known as powder thixoforming is proposed. The effect of ball milling on the microstructural evolution of a 2024 alloy, prepared by cold pressing ball-milled powders is examined during partial remelting. Additionally the effect of reheating temperature on the microstructure has also been investigated. Results indicate that welding and deformation of the milled powders occurred. A semisolid microstructure with fine spheroidal primary particles results following partial remelting. Microstructural evolution comprises three stages, the rapid coarsening and formation of compact particles, separation of the welded particles with reduction of the particle size, and finally coarsening. Compared with the microstructural evolution of the powder compacts prepared by un-milled powders, the first stage is accelerated, but the second stage is retarded. The microstructural evolution is also accompanied by densification. A suitably elevated temperature is favorable to achieve a compact semisolid microstructure for thixoforming.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.