Crack Formation And Distribution During Extrusion Forming Of Copper-aluminum Alloy Planar Micro Spring

With the rapid development of information society,micro-electromechanical systems(MEMS)are commonly used in all walks of life.Planar micro-springs are important structures for transferring force and energy in MEMS.Long-life planar micro-springs are of great importance for ensuring the reliability of micro-electromechanical systems.The fatigue life of aluminium bronze planar micro-springs obtained by extrusion is much higher than that of planar micro-springs prepared by micro-nano processing technology,but the existence of cracks limits the further improvement of the fatigue life of aluminium bronze planar micro-springs.In order to reduce or even eliminate cracks in order to obtain better fatigue life of aluminum bronze planar micro-springs,it is necessary in order to study the law of crack formation and distribution during extrusion forming of the springs.In this paper,QAl5 alloy was invoked as raw material to fabricate planar micro-springs by micro-extrusion.The micro structural changes and mechanical properties of QAl5 alloy under different heat treatment and rolling conditions were studied.The effects of process parameters on the crack generation and distribution in cold extrusion of micro-springs were explored through experiments and finite element simulation.The main conclusions are as follows:The composition and phase composition of QAl5 aluminium bronze is in accordance with the standard.The as-cast structure of QAl5 aluminium bronze is coarse dendrite with elongation of 65.3% after fracture and tensile strength of 237.7 MPa.After annealing,annealing twins are contained in the microstructure of rolled QAl5.Within the annealing temperature range,the number of twins increases with the increase of annealing temperature.The fibre structure of QAl5 is constituted by rolling.The larger the rolling reduction is,the more noticeable the fibre structure is.QAl5 alloy micro-spring columns with line width of 0.3mm,0.5mm and 1mm were successfully fabricated.According to the extrusion characteristics,the extrusion process of spring column can consist of three stages(upsetting stage,die angle transition stage and working belt stage)for analysis.In upsetting stage,the deformation of billet is small and the probability of crack initiation is small.The strain increases gradually after the bullet enters the transition stage of die angle,and when it exceeds the limit value,crack initiation occurs,and crack propagation occurs with the further increase of strain.When the bullet enters the working zone,the original crack will further expand or new cracks will appear according to the strain condition.Experiments and finite element simulation show that the tendency of cracks in the working zone and die corner of the aluminium bronze micro-spring column during cold extrusion is large,and the tendency of internal cracks is much lower than that of surface cracks.Based on the experimental and finite element simulation results,the processes with less crack tendency are rolling reduction of 30%,extrusion speed of 2 mm/min,dry friction and spring linewidth of 1mm.