| According to the traditional theory of brazing, the joining of solder and non-wetting base metal cannot be achieved without using flux in atmospheric environment. However, it can be achieved by ultrasonic-assisted welding with automatically filling brazing. It has obvious contradiction with theories of traditional brazing, and the research of non-wetting filling behavior and mechanism under ultrasonic effect is still relatively scarce. Therefore, the main research goal of this paper is to reveal the physical mechanism of non-wetting filling behavior under ultrasonic, primarily studied the vibration response characteristics of base metal,spreading behavior caused by ultrasonic, the physical mechanism of filling caused by ultrasonic,cavitation characteristics within a narrow gap and its effec ts, etc.The study found that the vibration response on base metal surface under ultrasound is a non-uniform distribution, but it has a regular pattern. There are stronger vibration zones which amplitude vibration intensity may reach several times stronge r vibrations and dead zones which amplitude closes to zero simultaneously. All of the ultrasonic frequency, the physical properties and the shapes,sizes of base metal and ultrasound applied position can alter the distribution characteristics of the vibrati on response. Although ultrasonic amplitude vibration and welding temperature can’t alter the distribution characteristics, it can alter the corresponding position’s vibration intensity. It is extremely important that setting the welding parameters and desi gning the welding structure before welding. They will seriously affect subsequent welding results.This paper studied the spreading behavior of the solder on non-wetting base metal under ultrasonic by the way of combination of fluid simulation and dynamic observation, it found that solder can spread around to film quickly and evenly on non-wetting base metal surface by the effect of ultrasonic, the spreading process is still a non-wetting process. The solder’s spreading process is a process of three-phase interfaces starting periodically spreading forward and stop, retraction alternately, spreading stage accompanied with obvious atomization, spreading length is greater than the retracted length, in order to achieve the overall spreading process. The stronger the ultrasonic vibration is, the faster the spreading process is, with more intense atomization. Alternating flow is due to the similar sinusoidal variation ultrasonic additional pressure gradient in liquid solder caused by ultrasonic, flowing process started in the center of pressure location. Ultrasonic additional pressure gradient is similar sinusoidal function with a certain asymmetry, the degree of the asymmetry of the positive and negative half-wave of this function is represented by a function offset Pc, Pc was the result of non-linear characteristics of ultrasonic propagation inside the solder. Thus, producing a time average pressure which is not equal zero, average ultrasonic additional pressure gradient. When the ultrasonic amplitude is 6μm, the average additional pressure gradient generated near the solder interface is-1195 Pa, the negative indicates the direction is from the center of interface to the gas-liquid interface. Solder instantaneous spreading process conforms that at that moment the ultrasonic additional pressure gradient which from the pressure center point to the three-phase boundary is greater than the surface additional pressure caused by surface tension and friction. The pressure generated by the ultrasonic in solder is o f nonlinear characteristics. Therefore, solder will be effected by the average pressure which is not equal to zero—the average ultrasonic additional pressure gradient. Macroscopically the driving force of the non-wetting spreading under ultrasonic is the average ultrasonic additional pressure gradient which from the pressure center point to the gas-liquid interface in solder due to the nonlinear characteristics of ultrasonic propagation. The average ultrasonic additional pressure increases with the ultrason ic amplitude. The vibration’s non-linear characteristics are weak When vibration frequency is lower than ultrasonic frequency, resulting in acoustic radiation pressure is insuflficient to overcome the resistance to achieve non-wetting spreading. Too high frequency ultrasonic may led solder acoustic levitation, which is against the detriment of spreading on the surface of base metal.The non-wetting filling process under ultrasonic was analyzed, the solder can be achieved under ultrasonic to fill the gap quickly without wetting the base metal. Filling process is accompanied by ultrasonic atomization, ultrasonic cavitation and solder lateral overflow. The velocity changes during the filling process which is consistent with the surface vibration response of the base metal.During the process of non-wetting filling under ultrasonic, the solder is affected by the ultrasonic additional pressure with similar to a sinusoidal variation change with time of nonlinear characteristics, as the same as the spreading process. It generated an ultrasonic additional pressure gradient from the pressure center to the gas-liquid interface within the gap and an ultrasonic additional pressure gradient from gas-liquid interface to the pressure center alternately. Under the effect of p eriodic changes ultrasonic additional pressure gradient, the solder overcome the action of surface additional pressure and friction, solder occurs outward expansion and retraction inward periodic flow instantaneously. As the same as the spreading process, non-wetting filling under ultrasonic is still driven by the average ultrasonic additional pressure from the pressure center to the gas-liquid interface inner gap caused by the non-linear characteristics of ultrasound.Due to the structural characteristics o f the narrow gap, the average ultrasonic additional pressure point inside the gap(when the amplitude is 5μm, it is-9127.2Pa) is greater than that point outside the gap(-484.3Pa, five precents of that inside the gap), in order to achieve directional filling. It is calculated that the average ultrasonic additional pressure increases with the ultrasonic frequency, and decreases with the increasing size of the gap. Wetting angle, solder viscosity will affect the resistance of filling,thus affecting the whole f illing process. The results of the dynamic observation are consistent with dynamics of filling.During the process solder filling, solder and base metal still remain non-wetting state,the oxide film which hinders wetting persists after filling. It will be removed by the ultrasonic cavitation. The structure and distribution of cavitation will directly affect cavitation intensity, thus affecting the removal of the oxide film on base metal surface. By the method of anodizing a layer of oxide film on the surfa ce of 1060 Al, we studied the structure and distribution of ultrasonic cavitation within a narrow gap. It is found that in a thin layer of liquid Sn metal, the cavitation structure composed of a large number of single cavitation bubbles and a small size of cavitation clusters. The greater the ultrasonic amplitude is, the smaller the gap and the solder viscosity are. Due to all of these and the greater density, more uniform distribution of cavitation is. The cavitation effects of the surface nearby base metal consistent with the structure and distribution of cavitation. Ensure cavitation time, increase the amplitude,decrease the gap size, improve the solder fluidity,control the uniformity of the vibration on base metal surface, all of those can improve cavitation effect of removal oxide film and guarantee the uniformity of removal oxide film. |
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