No abstract

In order to predict the distribution of shrinkage porosity in steel ingot efficiently and accurately, a criterion R√L and a method to obtain its threshold value were proposed. The criterion R√L was derived based on the solidification characteristics of steel ingot and pressure gradient in the mushy zone, in which the physical properties, the thermal parameters, the structure of the mushy zone and the secondary dendrite arm spacing were all taken into consideration. The threshold value of the criterion R√L was obtained with combination of numerical simulation of ingot solidification and total solidification shrinkage rate. Prediction of the shrinkage porosity in a 5.5 ton ingot of 2Cr13 steel with criterion R√L>0.21 m･℃ 1/2 ･s -3/2 agreed well with the results of experimental sectioning. Based on this criterion, optimization of the ingot was carried out by decreasing the height-to-diameter ratio and increasing the taper, which successfully eliminated the centreline porosity and further proved the applicability of this criterion.

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Mg-Li alloys have achieved vital applications in aerospace, automotive, and 3C fields for their prominent properties. However, the Mg-Li alloy exhibits poor corrosion and wear resistance due to the high activity of Mg and Li elements and low hardness of Mg. Accordingly, TC4 alloy coating was prepared on the surface of Mg-Li alloy using the cold spray technique to enhance the surface properties. Moreover, the microstructure, microhardness, tribological properties, and corrosion resistance of the coating were systematically investigated. As indicated by the results, the interface between the coating and the Mg-Li alloy substrate was mechanical bonding without significant defects. Several pores exist in the cold sprayed TC4 coating for its high elasticity, toughness, and passivation, resulting in a porosity of 4.3%. The microhardness of the cold sprayed TC4 coating reached 296.3 HV, marking a notable increase in comparison with the substrate. The TC4 alloy coating has better wear resistance than the Mg-Li alloy substrate. The wear volume of the cold sprayed TC4 alloy only accounted for 38% of that of Mg-Li alloy. Abrasive wear was the major wear mechanism of the TC4 alloy coating. In electrochemical tests, the corrosion current density of the TC4 alloy coating (1.426 × 10−5 A/cm2) was two orders of magnitude lower than that of the magnesium-lithium alloy substrate (1.008 × 10−3 A/cm2), and the corrosion potential of the TC4 alloy coating was higher, which indicates that the coating has excellent corrosion resistance.