Experimental Research On Ultrasonic Assisted Punching With Polyurethane Die

Polyurethane die is a kind of punching die, which employs polyurethane rubber as die in place of rigid ones to produce parts with fine surface and no burr on metal and non-metallic blanks. Ultrasonic vibration assisted plastic forming is a kind of non-traditional forming method, which applies ultrasonic vibration with certain vibrating direction and frequency on traditional plastic forming process, and is usually used to decrease flow stress and to increase forming quality. In this paper, ultrasonic vibration is introduced into the punching process with polyurethane die, and punching experiment of parts with two dimensions in sub-millimeter scale is performed.Mooney-Rivlin hyper elastic constitutive model for polyurethane rubber with hardness of 90A and Johnson-Cook constitutive model for 304 stainless steel are built. Ductile Damage criteria is chosen to define the fracture behavior for 304 stainless steel. Blanking process is simulated and the stress conditions of characteristic points are analyzed in ABAQUS.Thin sheet metal of T2 copper,304 stainless steel and 3J21 alloy are punched with polyurethane die. Punching force increases as sheet thickness decreases, and increases as tensile strength increases. Punching force decreases as punch height increases. Higher elongation and thicker sheet lead to higher punch. Fracture surface is composed of rollover and fracture zone, and the influence of punch height on the rollover ratio is not evident.Ultrasonic generator and sandwich piezoelectric transducer are selected. Conical ultrasonic amplifier of 67.5 mm in length is designed using traditional analysis method combined with finite element optimization method. The vibrating amplitudes under different power output of punch end are measured using laser vibrometer, and the amplitude increases with the increasing of power output.Ultrasonic assisted punching experiment is carried out. For T2 copper and 304 stainless steel, ultrasonic vibration leads to the softening of material. Punching force decreases with power output increases. Under power output of 240 W, maximum punching forces decrease by 19.8% and 18.4 %respectively than those without vibration. Ultrasonic vibration also makes the fracture more consistent and rollover evenly distributed. Under power output of 240 W, rollover ratio decrease from 64.3% and 47.7% to 47.8% and 42.7% respectively. For 3J21,the influence of ultrasonic vibration on punching force and fracture surface is not significant.Circular micro punching experiment is performed. Difference in shrinkage of materials after punching results in the difference in punched parts size. Square parts with filleted corner is punched out. The reason of warping on 304 stainless steel punched part is that edges do not break simultaneously, and the tension from the blank results in the displacement of punched part. By applying ultrasonic vibration, the material is softened, and the tension is reduced.304 stainless steel micro punched parts with good size accuracy is punched out.