Investigation On The Tool Temperature Characteristics And Key Technologies For Drilling Aerospace Heterostructure

With the development of modern aerospace,national defense industry and other key industries,the performance requirements of applied materials are becoming higher and higher.The traditional single structure material cannot satisfy the using requirement.Aerospace heterostructure composed by two or more different materials can obtain superior comprehensive properties by the synergism between physical and mechanical properties of different materials,which cannot be matched by a single structure.These heterostructure are being widely applied in aerospace due to their improving structural strength and damage tolerance effectively.Different components are assembled using bolts or riveting through connected holes.So the hole quality directly affect the assembly accuracy and play a key role in the stability and reliability of aircrafts.According to the literature reviewing,there are few researches on drilling aluminum/titanium alloy stacks.The temperature distribution along tool cutting edge is significant lacking.It is difficult to develop a reasonable drilling process for the aerospace heterostructure.Therefore,this research aims to study the temperature distribution along cutting edge and drilling process during drilling aerospace heterostructure in order to provide key technical support for the efficient and high quality machining of structural components.Based on the existing problems and study purposes,the following specific contents can be drawn:The temperature distribution along cutting edge during drilling process has been analyzed using a tool-foil thermocouple system.It was found that the temperature gradually decreased from drill center to outer corner along cutting edge,which meant the temperature near drill center was higher than that near outer corner.This result has been verified by an embedded thermocouple system.In addition,the distribution coefficient of friction energy between tool and chip has been determined by Conjugate Gradient Method.The temperature distribution along cutting edge was analyzed theoretically and simulated by finite element method.These results can verify the accuracy and reliability of tool-foil thermocouple system,which can provide technical support for subsequent temperature studies.The performance characteristics during drilling Al/Ti heterostructure have been investigated.Drilling tests have been carried on single layer and heterostructure,respectively.The variation characteristics of machining parameters during drilling were also analyzed.Compared the different temperature distribution in drilling single layer,metal laminates and different sequences,it can be found that when drilling from Al to Ti,the temperature near drill center was lower than that near outer corner.It increased from center to outer corner gradually.However,when drilling from Ti to Al,the result was opposite.Moreover,the evolution law of metallographic structure on hole surface has been discussed.It was found that the metallographic structures were occurred severe plastic deformation and elongated parallel to the hole wall and oriented in the feed direction.Due to the interaction of mechanical force and high temperature,two kinds of phase transformations have been observed:one was the allotropic transformation from a to ? phase and the other was parts of ? phase transformed to minor phase a'(hexagonal martensites).A new 3D schematic has been established to provide better understanding of chip formation and morphology characterization during drilling process.The effect of cutting parameters on the macroscopic and microscopic morphology of chip has been studied.The effect of novel tool geometries on drilling Al/Ti heterostructure has been evaluated.The chip break mechanism has been expounded and drill experiments of three novel tools named multipoint drill,step drill and double cone drill have been conducted.The influence of tool geometry on cutting vibration and temperature distribution along cutting edge has been studied.The results shew that the temperature near drill center was higher than that near outer corner for all these three tools.The temperature distribution trend of double cone drill was gentler than the other two tools.Adhesion was the main wear mechanism in drilling Al/Ti stacks for these three novel tools.Through comprehensive comparison,it concluded that the double cone drill was the most suitable tool for drilling Al/Ti stacks.The influence of different cooling techniques on drilling Al/Ti heterostructure has been discussed.It has clarified that the oil mist was reached the contact surface by the combined effect of drawing force generated by capillary force and the repelling force by surface relative motion,then it can generate a lubricating film to reduce the direct contact area between metals.In addition,the oil mist can transfer heat from contacted surface by convection to achieve its lubrication and cooling function.The temperature distribution characteristics under dry,air cooling and MQL conditions has been comparative analyzed when drilling single aluminum,titanium and Al/Ti laminates.It was revealed that the temperature was gradually decreased from drill center to outer corner when drilling single aluminum and titanium alloys for all different cooling conditions.Air cooling can get the lowest temperature.However,when drilling Al/Ti laminates,the temperature was gradually increased from drill center to outer cormer for all different cooling conditions and the MQL can get the lowest temperature.Adhesion and chipping were the main tool wear mechanism when drilling aluminum and titanium alloys under dry,air cooling and MQL conditions.