Sanbao Lin

Grain fragmentation in ultrasonic-assisted TIG weld of pure aluminum

Under the action of acoustic waves during an ultrasonic-assisted tungsten inert gas (TIG) welding process, a grain of a TIG weld of aluminum alloy is refined by nucleation and grain fragmentation. Herein, effects of ultrasound on grain fragmentation in the TIG weld of aluminum alloy are investigated via systematic welding experiments of pure aluminum. First, experiments involving continuous and fixed-position welding are performed, which demonstrate that ultrasound can break the grain of the TIG weld of pure aluminum. The microstructural characteristics of an ultrasonic-assisted TIG weld fabricated by fixed-position welding are analyzed. The microstructure is found to transform from plane crystal, columnar crystal, and uniform equiaxed crystal into plane crystal, deformed columnar crystal, and nonuniform equiaxed crystal after application of ultrasound. Second, factors influencing ultrasonic grain fragmentation are investigated. The ultrasonic amplitude and welding current are found to have a considerable effect on grain fragmentation. The degree of fragmentation first increases and then decreases with an increase in ultrasonic amplitude, and it increases with an increase in welding current. Measurement results of the vibration of the weld pool show that the degree of grain fragmentation is related to the intensity of acoustic nonlinearity in the weld pool. The greater the intensity of acoustic nonlinearity, the greater is the degree of grain fragmentation. Finally, the mechanism of ultrasonic grain fragmentation in the TIG weld of pure aluminum is discussed. A finite element simulation is used to simulate the acoustic pressure and flow in the weld pool. The acoustic pressure in the weld pool exceeds the cavitation threshold, and cavitation bubbles are generated. The flow velocity in the weld pool does not change noticeably after application of ultrasound. It is concluded that the high-pressure conditions induced during the occurrence of cavitation, lead to grain fragmentation in a pure aluminum TIG weld during an ultrasonic-assisted TIG welding process.

Influence of helium content on a ternary-gas-shielded GMAW process

Helium was added to the shielding gas for mild-steel gas metal arc welding. The influence of helium content in a ternary shielding gas (Ar–CO2–He) mixture on arc properties, metal transfer process, and weld bead profile was investigated by numerical simulation and experimental study. As the helium content increases, the arc temperature field becomes more constricted but the central high-temperature zone expands. The current density on the arc axis increases with increasing helium content. The heat flux density on the cathode increases as the helium content increases. The changes in the arc properties lead to the changes in the metal transfer. With increasing helium content, the droplet size increases and the transfer frequency decreases, with approximately linear dependence on helium content. When the helium content is increased from 0 to 25 vol%, the droplet diameter increases by about 80% and the transfer frequency decreases by about 77%, compared to the zero-helium case; this is attributed to changes in the forces on the detaching droplets. With the addition of helium, the base metal fusion area increases and has a more rounded profile; this is attributed to the increased droplet diameter and decreased droplet frequency.

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

ABSTRACT In this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.

A new discovery of arc shape in pulsed ultrasonic wave assisted TIG welding

An entirely new change-rule of an arc shape was found in the pulsed ultrasonic assisted tungsten-electrode inert gas welding process (PU-TIG), i.e., the arc shape had an obvious characteristic of periodic changes under the actions of a pulsed ultrasonic wave in PU-TIG. However, periodic changes of the arc shape were different between the arc white bright zone and the arc dark bright zone. The varying process of the arc shape was that the arc shape went through from expansion to contraction and re-expansion. Meanwhile, the constantly changing acoustic radiation force and electromagnetic force were the main cause of the periodic change of the arc shape in the PU-TIG welding process.An entirely new change-rule of an arc shape was found in the pulsed ultrasonic assisted tungsten-electrode inert gas welding process (PU-TIG), i.e., the arc shape had an obvious characteristic of periodic changes under the actions of a pulsed ultrasonic wave in PU-TIG. However, periodic changes of the arc shape were different between the arc white bright zone and the arc dark bright zone. The varying process of the arc shape was that the arc shape went through from expansion to contraction and re-expansion. Meanwhile, the constantly changing acoustic radiation force and electromagnetic force were the main cause of the periodic change of the arc shape in the PU-TIG welding process.