Grain fragmentation in ultrasonic-assisted TIG weld of pure aluminum

Abstract

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.

Changes of Microstructures and Mechanical Properties in Commercially Pure Titanium after Different Cycles of Proposed Multi-Directional Forging

“…In addition, many dislocations piled up along the band structures, and these structures developed into subgrains with the increase in the number of MDF cycles. At the same time, grains fragmentation (marked in Figure 5b by the blue arrows) was also found in some grains as a result of deformation twinning [21,22]. The SAED pattern in Figure 5b was close to a series of concentric rings with less elongated diffraction spots, indicating that the fraction of high angle grain boundaries (HAGBs) increased, whereas the fraction of LAGBs and the residual stresses decreased.…”

The Mechanism of Ultrasonic Vibration on Grain Refining and Degassing in GTA Spot Welding of Copper Joints

“…It can be concluded that the detached particles from the semi-melted fusion line and fragmented new solidified grain moved to form grains inside the NZ under the effect of USV provided through the solid copper to the liquid in the weld pool ( Figure 8 b,c). The semi-solid particles provided nucleation from the tip of high-density solidified grains; thus, the grain produced a tapered end, as shown in the schematic shown in Figure 13 c. Chen et al [ 21 ] investigated the weld pool of a light metal (pure aluminum) subjected to USV. It formed cavitation at the fusion line and refined the grain size in the nugget zone.…”

Impact of mechanical vibrations introduced by up blasting on the structure and hardness of welds on P235GH steel during MAG welding process

“…The vast majority of methods of introducing mechanical vibrations known from the literature require ensuring perfect contact of the elements of the vibrating system with the basic material [1÷10]. One can find studies in which the authors mention that mechanical vibrations were delivered to the welding zone but do not inform about the method of mutual attachment of the basic material and the vibrating system [1,4,8,9]. Considering that mechanical vibrations are able to improve the structure of welded joints or padding welds so much that they are an alternative to heat treatment, it is worth considering the use of vibrations while eliminating the technical problems of their application and the risk of damage to the vibrating system.…”