Study On Oxygen And Nitrogen Removal Process And Fine Grain Casting Of Superlloy K418 For Turbocharger Turbine Wheel

In order to reduce the content of oxygen and nitrogen in the superalloy K418 for turbocharger turbine wheel and improve the tensile properties and low cycle fatigue life of the casting at room temperature and 650?,the K418 alloy was purified by filtration purification and electromagnetic stirring process,and the solidification structure of the alloy was refined by rotating electromagnetic field.By means of metallographic microscope,scanning electron microscope,electron microprobe,tensile testing machine and fatigue testing machine,the effects of vacuum filtration and purification and electromagnetic stirring on the content of oxygen and nitrogen in the superalloy K418 and its mechanism of action,as well as the effects of rotating electromagnetic field on the solidification structure and properties of the alloy were analyzed.The results show that:(1)The content of oxygen and nitrogen in the superalloy K418 can be reduced to 5 ppm and 6 ppm respectively by the filtration purification and electromagnetic stirring processes.Filtration and purification is a physical purification method.The oxides in the melt of K418 alloy can be effectively filtered by blocking,precipitation and adsorption.Electromagnetic stirring after the completion of alloy casting can promote the aggregation of fine inclusions in the melt to the center of the melt and finally to the riser.(2)In the solidification process of superalloy K418,the low-frequency rotating electromagnetic field can be used to obtain uniform and fine equiaonal crystal structure.And on the base of this,the optimal process parameters are obtained by orthogonal experiment with frequency of 15 Hz,current of 200 A,total application time of 180 s and commutation time of 5 s.The grain can be refined to 187 ?m and the equiaxed crystal rate is 100%.(3)The grain refinement of K418 alloy is realized by the electromagnetic stirring generated by the rotating electromagnetic field.On the one hand,the electromagnetic stirring acts to cause the dendrite to break when the alloy melt solidifies,and on the other hand,the melt temperature field gradually tends to uniform,the combination of these two mechanisms promotes grain refinement of the alloy.The dendritic fracture of the alloy melt mainly occurs in the early stage of solidification;in the late stage of solidification,due to the inhibition of dendritic fracture and uniform temperature of the entire solidification liquid,the crystal grains transform to equiaxed crystals,and finally form.small equiaxed crystals.(4)The application of rotating electromagnetic field in the solidification process of superalloy K418 can also improve the casting defects such as elements segregation and shrinkage cavity and porosity,and change the morphology and size of MC carbide,?' phase and(?+?')eutectic structure in the alloy structure.(5)After the grain refinement,both the tensile properties and low cycle fatigue life of the superalloy K418 sample were increased in different degrees at room temperature and 650?.At room temperature,the tensile strength,yield strength,elongation and shrinkage of the fine crystal specimens were increased by 9.4%,17.6%,34% and 27.3%,respectively.The low cycle fatigue life of the fine crystal cast sample under the control of 0.3% total strain amplitude is 1.7 times of that of the common cast sample.Under the condition of the temperature of 650?,fine grain casting specimen tensile strength and plastic is still higher than the common casting samples,and keep at room temperature basic performance levels;Its low cycle fatigue life is 1.65 times longer than that of conventional cast samples.