Thrust force model for ultrasonic-assisted micro drilling of DD6 superalloy

As a typical refractory material,the DD6 nickelbased single-crystal superalloy has important applications in the aviation industry.Ultrasonic-assisted drilling is an advanced machining method that significantly improves machining of refractory materials.The drilling thrust force influences the hole surface quality,burr height,and bit wear.Therefore,it is necessary to predict the thrust force during ultrasonic-assisted drilling.However,there are few reports on the modeling of the thrust force in the ultrasonicassisted drilling of micro-holes.A thrust force prediction model for ultrasonic-assisted micro-drilling is proposed in this study.Based on the basic cutting principle,the dynamic cutting speed,dynamic cutting thickness,and acoustic softening effect caused by ultrasonic vibrations are factored into this model.Through model calibration,the specific friction force and specific normal force coefficients were determined.The model was verified through ultrasonic-assisted drilling experiments conducted at different feed rates,spindle speeds,frequencies,and amplitudes.The maximum and minimum errors of the average thrust force were 10.5%and 2.3%,respectively.This model accurately predicts the thrust force based on the parameters used for ultrasonic-assisted micro-hole drilling and can assist in the analysis and modeling of DD6 superalloy processing.

Effect of Hf on Stress Rupture Properties of DD6 Single Crystal Superalloy After Long Term Aging

The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1040℃for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980℃/250 MPa of the alloy after long term aging was investigated. The results show that the γ＇ coarsening and rafting and no topologically close packed phase （TCP） are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1 040℃ for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980℃/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed.