| Hydraulic turbine,the key component of hydropower unit,is an important hydraulic machinery equipment to realize the conversion of water energy to mechanical energy.Due to the terrible environment,turbine blades were often damaged by corrosion,erosion,cavitation and impact,which worsened the working stability,efficiency,and lifetime.Selecting difficultto-cut materials with high specific strength,wear resistance,high temperature resistance,fatigue resistance and other excellent properties can reduce the failure rate of blade equipment and maintenance costs.However,difficult-to-cut materials were tough for the manufacturing process of turbine blades.The problems of low processing efficiency,poor processing quality and high processing cost have restricted the technical upgrading of hydraulic machinery equipment,and put forward higher requirements for processing technology.Ultrasonic vibration assisted machining was a widely applied method of difficult-to-cut materials.With the view to the turbine blade profile of hydraulic machinery manufacturing,this paper proposed a two-dimensional rotary ultrasonic assisted grinding-electrolysis-discharge generating machining process(2UG-E-DM),and carried out the machining mechanism and experimental research.The specific research work and conclusions are as follows:(1)Based on the compensation of vibration angle and rotation,a method of 2UG-E-DM was proposed when the vibration direction of the workpiece was parallel to the tangential generating facet.The main setups of machining system of 2UG-E-DM are axial and tangential rotary ultrasonic vibration system,including the axial rotary ultrasonic vibration tool handle,the electrolysis-discharge composite device,the tangential ultrasonic vibration system,the tangential rotary platform,etc.The on-line measure and control system of processing parameters were established,including the workpiece’s amplitude and tangential displacement,grinding force,electrical parameters,and the control methods and devices of grinding and electrolysis-discharge machining power supply and C-axis rotation.(2)The material removal mechanism of 2UG-E-DM was discussed from the pointviews of single vibration cycle and three machining surfaces.Based on the electrolysis-discharge machining gap and the equivalent chip of a single abrasive,a mathematical model of material removal rate(MRR)was established.The influence of machining methods and parameters on the material removal mechanism was revealed through comparative tests and single factor tests.The results showed that the plastic grinding and electrolytic dissolution were the main methods,and the discharge machining was the auxiliary,the workpiece surface was smoother and uniform.When the gap was small,the current between electrodes changed periodically with the workpiece vibration,while when the gap was large,the current changed clearly.MRR increased with the increase of rotation and feed speed of the tool,voltage,the amplitude of tool and workpiece.However,when the rotation speed was higher than 4000 rpm,MRR did not increased significantly.(3)Accouting to the material removal mechanism,a model about the relationship between multiple machining energy fields were established thorugh average grinding force and equivalent average current.The effects of processing parameters on the energy ratio and specific energy in 2UG-E-DM were analysised.The results showed that the two-dimensional rotary ultrasonic vibration reduced the average grinding force and the grinding energy,increased the equivalent average current and the electrolysis-discharge energy,and then increased the energy ratio and decreased the specific energy significantly.The process route of coordinating the energy of electrolysis-discharge was to increase the spindle speed,voltage,amplitude of workpiece and tool,and the workpiece amplitude had a greater role in energy coordination,but the rotation speed higher than 4000 rpm and the voltage higher than 5 V were not conducive to the reduction of specific energy.The route of coordinating the energy of grinding was to increase the feed speed of the tool,but the effect on reducing the specific energy was limited when the feed speed was higher than 60mm/min.(4)From the relationship between material removal mechanism and surface generation,the surface morphology and edge damage forming characteristics of 2UG-E-DM were tested,the theoretical models of surface and edge roughness were established,and the evaluation parameters of surface quality were proposed.The results showed that when the amplitude of tool and workpiece increased and the feed speed decreased,the coverage rate of rolling was larger,which was beneficial to reduce the surface and edge roughness.When the rotation speed increased to 3000 rpm,the reduction on the surface roughness was little,but the edge roughness worsened sharply.When the voltage was about 4 V,the rolling coverage rate was greater than two,the rolling and ironing effects of ultrasonic vibration,and the leveling effects of electrolysis-discharge can be exerted for the better surface quality.(5)Through simulation analysis and experimental comparison of single abrasive grinding,the influence of different vibration angle errors on machining mechanism was obtained.The orthogonal test analysis revealed that the main factors in 2UG-E-DM were amplitude of workpiece and feed speed of tool.The multi-objective optimization model was established and the optimal processing parameter combination was obtained:rotation speed of 4015 rpm,voltage of 5 V,tool feed speed of 10 mm/min,workpiece and tool amplitude of 5 μm.The machining examples of different airfoil ruled surface blades were finished on the machining system 2UG-E-DM.The normal clearance at the maximum thickness of the blade body was less than 0.02 mm,the surface and edge roughness was less than 3.3 μm and 4.0 μm,and the specific energy did not exceed 120 J/mm3.Basically achieved the goal of high-efficient and high-quality machining. |
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