Study On Low Damage Machining Method Of Small Hole Of Aluminum Matrix Composites

Aluminum matrix composites have high specific strength and stiffness,low linear expansion coefficient and high thermal conductivity.It has been widely used in structure and functional parts of precision instruments,optical systems,electronic packaging and other fields.Due to the great difference in physical and mechanical properties between SiC particles and aluminum alloy matrix,interface debonding and SiC particles fracture are easy to occur during processing,which can induce the formation of machining damage.The high hardness of SiC particles can also lead to serious tool wear.Especially for small-diameter smooth holes or threaded holes,irreparable edge collapse damage is very easy to occur,and the drill and tap are prone to fracture,or even stuck in the hole and can not be taken out,resulting in workpiece scrapping.The hole machining damage of aluminum matrix composites have become the technical bottleneck of its engineering application,which affects the progress of research and development of key products.Scholars at home and abroad have carried out a lot of research on composites processing technology,but there are still not enough in-depth research or research gaps,such as the research on damage mechanism of aluminum matrix composites hole making,and effective processing technology for small-diameter smooth hole and threaded hole with low damage.In this paper,starting from the formation mechanism of the drilling damage,the strategy of using grinding method to restrain edge damage was proposed based on the understanding of the damage mechanism and the law of its influencing factors.The ultrasonic vibration assisted grinding method was used to realize the low damage and high efficiency machining of small hole.On this basis,helical mill-grinding method was proposed to process the internal thread,and the micro screw mill-grinding tool was developed,so as to solve the knotty problem of machining small diameter smooth hole and threaded hole in engineering application.The main research contents and achievements are as follows:(1)In terms of the drilling damage formation mechanism of aluminum matrix composites,through the observation of the typical characteristics of drilling damage,the hole exit damage evaluation method considering the diameter ratio,height ratio and volume ratio of edge collapse was proposed.The material deformation behavior in the formation process of aluminum matrix composites hole exit damage was studied by means of photographic observation and microscopic observation.In addition,the important factors that affect the edge damage and the law of their influence on the edge damage were experimental studied.The results show that the material deformation during the formation of edge collapse damage can be divided into five stages:stable cutting,chisel edge extrusion,bending deformation,crack growth and damage formation.Under the condition of weak boundary constraint,the support rigidity of the remaining materials at the hole exit is insufficient,resulting in bending deformation.The negative shear plane forms between the intersection of the main and auxiliary cutting edges and the bending point at the bottom of the workpiece.Crack growth and fracture on the negative shear plane cause edge collapse at the exit of the hole.The increase of axial force,drilling temperature and tool wear will make the edge damage more serious.(2)Based on the deep understanding of drilling damage mechanism of aluminum matrix composites,the grinding method was proposed to solve the problem of tool breakage and serious damage during the small hole machining.In order to reveal the grinding mechanism of aluminum matrix composites,the single abrasive scratch test was carried out,and the undeformed chip section model and grinding force model of grinding hole were established.The results show that the macroscopical removal process of aluminum matrix composites is closer to that of metal materials.There are plastic deformation of matrix,fracture and debonding of SiC particles and friction between abrasive particles and workpiece material in the process of single abrasive cutting.The grinding force includes plastic deformation resistance,friction force,particle fracture and debonding resistance.The friction force has the greatest contribution to the axial force and torque of grinding hole,and the thermal effect of friction force has adverse effects on tool life and machining damage.(3)Ultrasonic vibration was introduced in order to solve the problems of low efficiency and tool wear in common grinding hole process.The influence of ultrasonic vibration on the process of abrasive cutting in and out was theoretically analyzed.The influence of machining parameters on the axial force of ultrasonic vibration assisted grinding of aluminum matrix composites small hole was experimentally studied.And the ?2mm small hole processing test was carried out.The results show that reducing the undeformed chip thickness and increasing the ultrasonic amplitude are beneficial to reducing the axial force.But when the amplitude is too large,the axial force will not decrease but rise reversely.And there is an optimal amplitude to minimize the axial force.The optimal amplitude in this paper is close to 7.25 ?m.Under the condition of ultrasonic vibration assisted grinding ?2mm small hole,compared with the common grinding method,the axial force is reduced by 56.8%? 83.2%,and the machining efficiency is increased by more than 5 times.After machining 44 holes,the hole damage is still low,thus the feasibility of this method is verified.(4)In order to solve the problem of machining the small hole thread of aluminum matrix composites,helical mill-grinding method was proposed,the principle error mathematical model of the method was established,and the factors affecting the principle error were analyzed.Based on the principle error model,the parameter of thread machining tool was optimized and a special tool was developed.Taking M2 thread as an example,the machining test was carried out.The surface quality,thread shape accuracy and tool wear condition of thread machining were analyzed.The high-precision thread with pitch diameter error of 0.08%,angle error of 0.3%and accuracy grade H4 was successfully machined,which verified the feasibility of this method.Through the research work and achievements above,the formation mechanism of aluminum matrix composites hole damage and the law of its influencing factors have been revealed,the low damage processing technology of particle reinforced metal matrix composites has been developed,and the machining problems of small holes and small threaded holes have been successfully solved.This paper has provided a reliable technology for the precise and efficient machining of aluminum matrix composites involved in the national key engineering products.