WELDING RESEARCH about three times the diameter of the welding wire — Fig. 4A. As a result, the metal transfer cycle lasted a long time, 203 ms. When the continuous ultrasonic was used, the diameters of the droplets in U-GMAW were smaller than their respective counterparts in GMAW — Fig. 4B. The metal transfer cycle lasted a short time, 144 ms. Under the action of pulsed ultrasonic, the most characteristic feature was the deformation of the droplet. At first, the droplet size was small, and the droplet shape was approximately spherical. When the droplet diameter exceeded the wire diameter, the droplet started to burst spontaneously — Fig. 4C. The explosion process lasted until the end of the transfer cycle. The resultant droplet shape was approximately columnar. Although the droplet size could not be directly measured due to explosion, the droplet size in PUGMAW was apparently smaller than that in U-GMAW. Since the wire feed speeds were constant in three welding processes, the decrease of the droplet size resulted in a decrease of metal transfer time and an increase in metal transfer frequency. The cycle time was reduced from 144 to 64 ms, while the metal transfer frequency increased from about 8 to about 20 Hz. From Fig. 4, it can also be seen that there is the shortest arc length and the brightest arc to PU-GMAW. According to this calculation result from the spectrum experiment, as shown in Fig. 5, the average electron temperature A B for the PU-GMAW arc could reach 17,000 K and was the highest in the three welding processes. For this reason, 244-s WELDING JOURNAL / JULY 2016, VOL. 95 it was primarily the result of the plasma particle agglomeration. It shows that PU-GMAW was an efficient and stable welding process. The stability could also reduce the spatter and ejection from the molten metal. This caused a higher concentration of molten metal in the plasma region, which led to an elevated electron temperature in turn. Weld Properties With the help of analysis and measurement of photographs, compared with the GMA weld, it could be observed that when ultrasonic was applied, the weld areas were increased, as shown in the left side of Fig. 6. Ultrasonic could change the arc pressure distribution (Ref. 13), while the pulsed ultrasonic further enhanced the force acted on the the arc and droplet. That was the reason why the application of pulsed ultrasonic could increase the weld area much more severely, while the dispersion of the arc and the low additional force acting on the droplet could not bring a larger melt zone. Selected portions of EBSD maps taken from the center of different fusion zones are given in the right side of Fig. 6. The typical solidification structure consisting of an equiaxed structure was confirmed for the 7A52 aluminum weld. The significant difference in grain size was observed for the joints obtained under different welding conditions. For the weld grain of the PU-GMAW joint, the refinement effect was most obvious in the three joints. The size of the grains decreased by the ultrasonic treatment, which should produce complicated heat flows and increase the nucleus number by the cavitation effect. Microstructure The weld center zone was mainly composed of an aluminum matrix solid solution, but also the composition of the wire. The remarkable characteristic in the microstructures at the vicinity of the weld area is shown in Fig. 7. The number density of the precipitated phase particles in the PUGMAW joint obviously decreased compared to that of the conventional GMAW joint. To identify the variation of the identities and intensity of the precipitated phases at different processes, XRD characterization of the welded joint was performed, and the results are shown in Fig. 8. The phase particles mainly consisted of MgZn2 () and Al2CuMg (S) phases. The intensity of the peak of the phase decreased with the addition of the ultrasonic, indicating that the -phase particles were dissolved into the matrix. Similarly, the intensity of the peak of the S phase reached the maximum for the GMA weld. It was also suggested that the S phase particles did not dissolve into the matrix of the GMA weld. Fig. 13 — Microhardness distribution of the weld joint obtained with different welding processes. Fig. 14 — A — Film model of weld pool; B —stress analysis of liquid film. Fig. 15 — Schematic of the metal composition of the weld pool.
Welding Journal | July 2016
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