Damage Formation Mechanism And Suppression Methods In Rotary Ultrasonic Drilling Of Hard And Brittle Materials Ceramic Matrix Composites

Brittle materials,represented by advanced engineering ceramics,optical glass and ceramic matrix composites,play an important role in various fields such as the aerospace,information and life science industries due to their superior properties.Low efficiency and severe damage in mechanical machining of hard and brittle materials greatly restrict their engineering application.Improving the efficiency and reducing the damage in hard and brittle material processing is one of the research hotpots.Known as a novel unconventional machining method,rotary ultrasonic machining(RUM)has been sufficiently proved suitable for hole-manufacturing of hard and brittle materials with improved drilling efficiency.However,the machining induced damage,especially of the hole exit damage is also a severe and unsolved problem which restricts the application of RUM.Moreover,due to neglecting the adverse effect of material removal on the stability of ultrasonic vibration,the processing superiorities of RUM are not always brought into full play.In this thesis,the damage formation mechanism and control method in RUM of hard and brittle materials & layered composites were mainly studied.Firstly,the existence of critical processing capacity of rotary ultrasonic machine tools(RUMT)was found.During the rotary ultrasonic machining process,when cutting force exceeds a critical value,it will increase abruptly.Simultaneously,ultrasonic vibration will decrease abruptly resulting in the severe suppression of processing superiority of RUM.By monitoring the variation of ultrasonic power,the mechanism of how the thermo-mechanical loading influence the stability of ultrasonic vibration was revealed.Considering the material removal mechanism of RUM,the dependency of critical cutting force on the properties of RUMT was modeled.Mechanistic modelling results indicate that the critical cutting force is an inherent property of RUMT depending on its excitation level,but independent of processing conditions.The experimental observations were in good agreement with the model predicted results.It was proposed that critical cutting force ought to be an index for characterizing the maximum processing capacity of RUMT,by developing a bridge between RUM process and RUMT' properties.The model of critical cutting force would provide promising guideline for the design,manufacture and application of RUMT.And it indicates that the actual cutting force should be well controlled not exceeding the critical cutting force in order to guarantee the effectiveness of RUM in the application of RUMT.Secondly,a novel edge chipping formation mechanism at hole exit considering the effect of machining induced subsurface damage was proposed.Based on the above mechanism,a mechanistic model for the dependency of edge chipping size on the material properties,processing parameters and tool dimension was developed.And a novel experiment method was proposed to evaluate the drilling induced damage.Experimental results showed that RUM produced smaller subsurface damage comparing with conventional grinding,that is also a major factor causing the superiority of RUM in edge chipping reduction by 50%.Furthermore,experimental results from rotary ultrasonic face milling of optical glass verified the subsurface damage reduction of RUM.Then,the effects of tool vibration on surface integrity in RUM of C/SiC composites considering fiber orientation was investigated experimentally.Experimental results showed that the tool ultrasonic vibration could assist the improvement of hole surface integrity in RUM of C/SiC composites by fiber fracture mechanism alteration.With ultrasonic vibration assistance,the fiber cutting direction tended towards 90°.Furthermore,RUM can reduce tearing defects at hole exit by 70%.By comparing the formation mechanism of edge chipping,the decisive effect of cutting force on the formation of tearing defects was established.Finally,a method for maximizing the machining efficiency satisfying the requirement of damage tolerance was proposed based the previous studies.Then,a novel tool design method by constructing a wedge type contact structure between tool end face and workpiece material at the hole exit was proposed to suppress the formation of edge chipping and tearing defects.Accordingly,three novel type tools namely stepped tool,tapered tool and step-taper compound tool were developed.By revealing the mechanism of hole exit damage suppression of novel type tool,combining the previously developed model of hole exit damage,the design criteria of novel type tool were proposed guaranteeing their effectiveness.Experimental results showed that by appropriate application of novel type tool,the hole exit damage could be reduced by up to 50%.