The Solidification Behavior of Mg-9Al-2Ca Alloy under Furnace Cooling

To understand the solidification pathway and microstructure evolution of Mg-9Al-2Ca alloy,the cooling curve of the alloy solidified under furnace cooling was measured and the water-quenched samples were observed.The experimental results show that the matrix phase of α-Mg dendrites is first generated at 596℃ during the solidification process,then the eutectic phases of Al_(2)Ca and Mg_(17)Al_(12) are formed at 518 and 447℃,respectively,and the solidification is terminated at 436℃.In the process of solidification,the seaweed dendrites of α-Mg get coarser and are gradually transformed into the global dendrites;besides,the secondary dendrite arms spacing(SDAS)of α-Mg as well as the solid fraction are both increased,while the increasing rate of SDAS of α-Mg and the solid fraction in the temperature region of 600-550℃ is faster than that in the temperature region of 550-436℃.And a power function relationship can be used to illustrate the change of the SDAS and the solid fraction with the temperature of solidification.

Formation Mechanism of Chunky Graphite and Its Preventive Measures

The formation mechanism of chunky graphite has been reviewed and studied. The study consisted of a unidirectional solidification method, a small droplet method and a furnace cooling method. Four kinds of iron samples were prepared, namely, the pure Fe-C, Fe-C-S, Fe-C-Ce and Fe-C-Si-Ce alloys, and three kinds of nickel samples, namely the Ni-C, Ni-C-S and Ni-C-Mg alloys. The results of the unidirectional solidification of the Ni-C alloys showed that spheroidal graphite is not observed in the continuous solidified region, in which only flake-like graphite is observed, while spheroidal graphite is usually observed in the quenched liquid region. The existence of spheroidal graphite in the solidified phase is recognized only in the discontinuous growth mode of the Ni-C-Mg alloy solidified at 150 mm/h. This means that the spheroidal graphite is directly crystallized from the melt and entrapped by the flake-like chunky graphite that is formed by the continuous growth mode. In the small droplet method, a small piece of the Fe-C or Fe-C-Ce sample was melted on a pure graphite plate then cooled at a different cooling rate in a He-3%H2 atmosphere. The graphite in the Fe-C-Ce alloy is usually spherical. Nevertheless, the graphite morphology of the final solidified area changed from spherical to chunky and chunky to ledeburite with an increase in the cooling rate. This means that the chunky graphite is formed in the residual liquid region by the solidification into Fe-graphite system. The sample was cooled in a furnace, and the graphite morphology changes from spherical to chunky and chunky to ledeburite with the decrease in the Si content. These phenomena can be confirmed by the cooling curves of these samples.

Melt Quality Evaluation of Ductile Iron by Pattern Recognition of Thermal Analysis Cooling Curves

The melt quality of ductile iron can be related to the melt＇s thermal analysis cooling curve. The freezing zone of the thermal analysis cooling curve was found to indicate the melt quality of the ductile iron, A comprehensive difference parameter,Ω, of the thermal analysis cooling curves was found to be related to the properties of ductile iron melts such as composition, temperature, and graphite morphology. As .Ω approached O, the thermal analysis cooling curves were found to come together with all the properties indicating melt quality about the same. A database of thermal analysis cooling curves related to the properties of the ductile iron melts was set up as a basis for a method to accurately evaluate the melt quality of ductile iron by pattern recognition of thermal analysis cooling curves. The quality of a ductile iron melt can then be immediately determined by comparing its thermal analysis cooling curve freezing zone shape to those in the database.

Effect of Ablation Casting on Microstructure and Casting Properties of A356 Aluminium Casting Alloy

Recently the Ablation Casting Technology was invented as a new casting process to improve foundry products quality. In this study, the effects of processing variables on the porosity content, rnicrostructure and feedability of A356 casting alloy were investigated. Secondary dendrite arm spacing （SDAS） and eutectic silicon mor-phologies were studied to evaluate the influence of Ablation Casting on the rnicrostructure. Casting density was measured in order to identify porosity content and feedability of ablated and non-ablated specimens. In addition, solidification behavior of the samples was investigated by using thermal analysis technique. The cooling curves and the first derivative curves were plotted and compared with each other. Results showed the ablation process could increase solidification rate significantly. In addition, the microstructural evidences revealed that Ablation Casting process results in more fine and homogeneous structure compared to the non-ablated casting. The feedability improved, SDAS reduced to 35% and porosity content decreased to 3.84 vol.% by implementing this process. It concluded the Ablation Casting is an effective process to gain higher quality in aluminum foundry.

Hot Deformation Behavior of V-Microalloyed Steel

Through the expansion curve of continuous cooling transformation at different cooling rates measured by THERMECMASTOR-Z thermal simulator for U75V rail steel,the continuous cooling transformation curve was obtained.The influence on steel microstructure and hardness at different cooling rates was studied.The softening behavior of isothermal deforming in austenite area of 850-1000 ℃ in the interval of passes was also studied by double-pass compression test.The results show that the product of austenite transformation is pearlite when the cooling rate is lower than 10 ℃.When the cooling rate was in the range of 10-50 ℃·s-1,only martensite was received.The hardness of the test steel increases with increasing the cooling rate.Under the condition of deformation of 30% and deformation rate of 3 s-1,the relaxation time for complete recrystallization was shorter than 100 s when deformation temperature was higher than 1000 ℃.When deformation temperature was lower than 880 ℃,complete recrystallization of steel was difficult to achieve even if the relaxation time is extended.

Effect of Ag substitution on structural,magnetic and magnetocaloric properties of Pr_(0.6)Sr_(0.4–x)Ag_xMnO_3 manganites

A systematic investigation on the structural, magnetic and magnetocaloric properties of Pr_（0.6）Sr_（0.4-x）Ag_xMnO_3（x=0.05 and 0.1） manganites was reported. Rietveld refinements of the X-ray diffraction patterns confirmed that all samples were single phase and crystallized in the orthorhombic structure with Pnma space group. Magnetic measurements in a magnetic applied field of 0.01T revealed that the ferromagnetic-paramagnetic transition temperature T_C decreased from about 293 to 290 K with increasing silver content from x=0.05 to 0.1. The reported magnetocaloric entropy change and relative cooling power for both samples were considerably remarkable with a △S_（max） value of 1.9 J/（kg·K）and maximum RCP values of 100 J/kg, under a magnetic field change（？μ0H） equal to 1.8T. The analysis of the universal curves gave an evidence of a second order magnetic transition for the studied samples. The magnetic field influence on both the magnetic entropy change and the relative cooling power was also studied and discussed.