Theoretical And Experimental Research On Ultrasonic Drawing Of Titanium Wire Based On Wire Oscillation

Drawing is a metalworking process used to reduce or reshape the cross-section of a wire,bar or tube by pulling the workpiece through a single,or a series of drawing die(s)with decreasing bore diameter(s).This metal forming method is widely applied in the production of various industrial materials including many kinds of metallic wires,bars and tubes.Titanium and its alloys,regarded as emerging strategic metallic materials,have an extensive application prospect in the field of both defense industry and national economy.However,they are difficult-to-draw materials,with many disadvantages associated with the conventional manufacturing technology.Conventional titanium wire drawing are generally conducted at elevated temperature,involving complicated surface treatment,lubrication,and multiple vacuum annealing,which is neither energy-saving nor eco-friendly.Problems like large drawing force,high wire breakage ratio exist in the conventional drawing process,and the products have low dimentional accuracy and proor surface finish,making it hard to satisfy the industrial demand.Ultrasonic vibration contributes to decreasing the drawing force,improving lubrication condition within the contacting region,and enhancing the surface finish of the drawing wire,becoming an important plastic metal working method.Therefore,researches on the ultrasonic wire drawing of titanium and its alloys are of great significance to increasing the productivity of titanium wire industries and understanding the mechanism of ultrasonic vibration in metal forming processes.Despite various benefits produced with ultrasonic vibrations employed in wire,bar and tube drawing process,the inherent mechanism has not yet been fully identified with complete quantitative calculation equations about the ultrasonic drawing force remains to be established.In this dissertation,the analytical mathematic model of conventional drawing force is derived based on spherical velocity field distribution model and the upper-bound method.The the contact relations between the inner surface of the die and the outer surface of the wire are discussed according to the theory of stress wave.Based on the results,single die ultrasonic wire drawing process is treated as the forced vibration problem of a moving string and the differential equations are established.Subsequently,the equation is solved based on stress superposition hypothesis,method of mathematical physics,reflection and superposition theory of stress waves,elastic-plastic mechanics,etc.The variation rules of the tension on the string are analyzed.Drawing forces and their averaged values at different drawing speed and ultrasonic amplitude are calculated under three circumstances according to the distance between the die and the drum reel.Result shows that,when the location of the die remains unchanged,ultrasonic drawing force decreases with the increment of the ultrasonic amplitude of the die,while increases with the improvement of the drawing speed.When ultrasonic amplitude of the die and the drawing speed maintain constant,the amount of reduction in drawing force is affected by the distance beteen the die and the drum reel,which maximized when the distance is close to half-wavelength of ultround propagating in the wire,while minimized when the distance approaches to quarter-wavelength.During single-die ultrasonic wire drawing process,it is difficult to realize the separation between the die and the wire.To solve the problem,the method of doubledie ultrasonic wire drawing is proposed.For the second drawing pass,a reversed drawing force is provided by the drawing force of the first pass,which decrease the contacting and friction stresses between the wire and the second die to the point that the die detaches from the wire periodically at its reducing area.In order to validate the correctness of the theoretical analysis and the feasibility of the proposed double-die ultrasonic drawing method,axisymmetric finite element models about the single-die and double-die ultrasonic drawing processes,considering the elastic and plastic behaviors of the material,are established in the FEM software according to the actual length of the wire.Simulation results of the single-die ultrasonic drawing are in accordance with theoretical analysis,reavealing the fluctuating tension on the string as the reason for the decreased drawing force.When ultrasonic vibration is imposed,the continuous process of conventional drawing is turned into intermittent process of ultrasonic drawing.A vibration period of the die is devided into two segments: the processing segment and the non-processing segment.The proportion between the two segments is determined by the amplitude of the die and the drawing speed when the die stays at the same position.In addition,a threshold value,which is determined by the frequency and amplitude of the die,exists for the drawing speed,when surpassed,ultrasonic vibrations would no longer cause a reduction in the drawing force.The influence of the die-reel distance on the drawing force is also in good agreement with the theoretical analysis result.Simulation results of the double-die ultrasonic drawing confirm that: coupling effects exists between the two dies.Beside vibrational amplitude and drawing force,the overall drawing force and drawing forces for each pass are influenced by several other factors about the two dies,including the oscillation modes,their location distribution,and the phase difference.When located at appropriate positions and with suitable phase difference,it is possible to achieve the superposition or counteraction between the ultrasonic amplitudes of the two dies.Under certain conditions,double-die ultrasonic could dramatically reduce the pressure and friction stress between the second die and the wire,thus realizing the cyclic separation between the two surfaces.Based on aforementioned theoretical and simulation analysis,the specific longitudinal ultrasonic drawing system is developed.Modal analyses are conducted to investigate the influence of front-end mass loading and middle transition arc on the vibration characteristics of the ultrasonic horn,as well as the influence of fixation near the nodal plane on the performance of piezoelectric transducers and ultrasonic vibrators.Simulation result shows that the attachment of mass loading will remarkably reduce the first order longitudinal resonant frequency of the ultrasonic horn,while the transition arc is beneficial for improving the corresponding resonant frequency.The resonant frequencies of the piezoelectric transducers and ultrasonic vibrators when constrained near the nodal position are slightly higher than the corresponding free resonant frequencies.Harmonic and transient analyses are conducted to predict the frequency-domain and time-domain response of the transducers and vibrators.Measurement are conducted for the assemblied ultrasonic vibrators in terms of vibration mode and impedance characteristics and two vibrators with similar performance were selected with their output amplitudes calibrated at the output end and the mounting position of the die.The calibration and test results show that although the two vibrators could oscillate synchronously when driven by the same industrial ultrasonic generator,reduction and deviation occur in their output amplitudes and resonant frequencies compared with when working independently.Experimental platform for single-die and double-die ultrasonic titanium wire drawing are established to investigate the influences of drawing speed,ultrasonic amplitudes,as well as the vibration modes,locations,phase difference of the two dies on the variation of the drawing forces.Experimental results are compared with theoretical and simulation results and are in good agreement,which verifies the correctness and rationality of the theoretical analysis and finite element model.The surface morphologies of the drawn wires are inspected with a scanning electron microscope.Results show that: improved drawing velocity and ultrasonic vibration of minor amplitude are beneficial for removing the surface defects,but the effect of ultrasonic vibration would be weakened once surpassing a threshold value.The threshold value would be improved with the increment of the drawing speed.