WELDING RESEARCH minum crystal and hence the hardness. Analysis of the Ultrasonic Effect Ultrasonic Effect on the Weld The weld pool was assumed to be a multilayer film, and stress analysis of the liquid film on the surface was conducted. Under the action of the external force, the film was forced to vibrate. The surface microelement was extracted, as shown in Fig. 14. The tension in the unit length of film is regarded as a constant force (T), p represents the axial pressure. The arc axial force acting on surface microelement can be obtained that FF = pdxdy (2) This surface microelement consists of several line microelements with the length of dx and the width of 1. S is the displacement in the vertical direction. The corresponding resultant force of film in the vertical direction is then given by According to Newton’s second law, the forced vibration equation of the weld pool can be obtained (Ref. 30): where is the mass per unit area. Equation 4 is integrated into polar form, and when the boundary conditions meet r = a, S(t,a) = 0 is substituted into the equation S(t,r) = SAejwt (5) where SA is the vibration amplitude. By integrating the displacement amplitude, we can obtain the average amplitude of the entire film (SA): where J0(ka) and J2(ka) are the zeroorder and second-order cylindrical bessel function, respectively. pA is the resultant force, including the arc pressure and the droplet impact force. As the determining factor of the weld pool boundary, SAcan be obtained the amplitude maxima when ka = Mn (Mn is a series of root values about J0(ka)), the resonant of weld pool may be considered to occur. SA is proportional to pA, the greater the increase degree of SA is, the larger the weld. When the ultrasonic is applied to the GMAW process, the increase of the arc pressure and droplet impact force will result in an increase in pA. The arc pressure is associated with the degree of compression of the welding arc, the droplet impact force is closely related to the droplet volume and the droplet transfer frequency, while the arc and the droplet are ultimately controlled by the ultrasonic power, as shown in Fig. 4. Ultrasonic Effect on Element Loss Through EDS analysis, we found that the pulsed ultrasonic had a significant inhibition in the gasification burning of the Zn element. Although the electron temperature of the PUGMAW arc is the highest compared with that of the GMAW and PUGMAW arcs, this experimental result could not just be simply attributed to the fact that PU-GMAW produced a more constrained arc and reduced significantly weld spatter whereas the conventional GMAW process did not. From Fig. 6, it can be seen that the largest weld area is obtained in PUGMAW. The total welding heat input is consistent in three welding processes, so this means that the heat input per unit volume is reduced. Additionally, welding parameters are constant in three experiments. They are the same for the melting volume of welding wire in three weld pools and the most metal from BM entering the welding pool of PU-GMAW, while the Zn element is mainly from the BM of 7A52 aluminum alloy. The schematic of the metal composition of the weld pool is shown in Fig. 15. At last, the effect of ultrasonic streaming within the weld pool should also be considered except the mechanical effect of ultrasonic. When the ultrasonic is applied to deal with the weld pool, acoustic streaming induced by the ultrasonic field is the major form of liquid flow (Refs. 31–33). According to Darcy’s law, the differential form of liquid flow velocity (U) is then given by U = – K ����L (��P+��L g ) (7) where K is the liquid penetration coefficient, g is the acceleration, P is the pressure, L is the liquid density, is dynamic viscosity, and L is volume fraction for liquid phase. From Equation 7, it can be seen the size of velocity is determined by the input power of the ultrasonic wave. If the velocity is large enough so that the time of the arc directly acting on the elements could be shortened, it can also effectively reduce the element loss from the gasification burning. Ultrasonic Effect on Grain Refinement The acoustic pressure characteristics of pulsed ultrasonic is different from that of continuous ultrasonic, the pulse power supply can output higher peak power, there is the higher ultrasonic power input weld pool, and the stronger cavitation can be obtained in the PU-GMA weld pool. The cavitation can reduce the freezing point of the melt, increase mobility, and better refine grain structure. Conclusion The mechanical behavior and the mechanism were comparatively studied for arc shape, droplet transfer, and welds obtained with PU-GMAW in this paper. The following conclusions may be obtained: 1. Under three welding conditions, the arc length, droplet size, and electron temperature in PU-GMAW were the shortest, smallest, and highest, respectively. The cycle time of droplet transfer was reduced from 203 ms in GMAW to 64 ms in PU-GMAW, while the metal transfer frequency increased from about 3 to about 20 Hz. 2. By the analysis of weld properties, it could be found that when ultrasonic was applied, the fusion zone area of the 7A52 aluminum alloy joint in- S – A = pA k2T i - J2(ka) J0(ka) (6) T ��2S 2S + ����x2 ��y2 �� ���� �� ���� dxdy–pdxdy = ��dxdy ��2S ��t2 (4) Fz = T ��2S 2S + ����x2 ��y2 �� ���� �� ���� dxdy (3) 246-s WELDING JOURNAL / JULY 2016, VOL. 95 - - -
Welding Journal | July 2016
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