Chenglei Fan

Analysis of intermetallic layer in dissimilar TIG welding–brazing butt joint of aluminium alloy to stainless steel

Abstract Intermetallic layer of dissimilar tungsten inert gas welding–brazing butt joint of aluminium alloy/ stainless steel has been studied. A visible unequal thickness intermetallic layer has formed in welded seam/steel interface, and the thickness of the whole layer is <10 μm. The interface with Al–12Si filler metal consists of τ 5-Al8Fe2Si layer in welded seam side and θ-(Al,Si)13Fe4 layer in steel side with the hardness values of 1025 and 835 HV respectively, while the interface with Al–6Cu filler metal consists of θ-Al13(Fe,Cu)4 layer with the hardness of 645 HV. The average tensile strength of the joint with Al–12Si filler metal is 100–120 MPa, and the fracture occurs at θ-(Al,Si)13Fe4 layer, while the joint with Al–6%Cu filler metal presents high crack resistance with tensile strength of 155–175 MPa, which reaches more than 50% of aluminium base metal strength.

Application of rotating arc system to horizontal narrow gap welding

Abstract A rotating arc system for narrow gap horizontal welding was developed for solving the problem that the molten pool sagged due to the gravity. The characteristic of the system was that the synchronous rotation of nozzle and wire was generated by eccentric sleeve driven by motor. This process not only may reduce welding effective heat input due to an increase in the welding instantaneous velocity but also may disperse arc force which could counteract the gravity of upper side of molten metal. Both the two aspects were beneficial for horizontal weld formation. Experimental results indicated that shapely horizontal joint could be obtained in appropriate rotating parameters. The asymmetry of joint formation indicated that the heat affected zone width and microstructure in upper side were larger than that in lower side. The effective heat input difference near each side caused by the instantaneous velocity difference was the main reason why the formation characteristics occurred.

Research on short circuiting transfer mode of ultrasonic assisted GMAW method

Abstract Ultrasonic assisted gas metal arc welding (U-GMAW) has been recently developed to improve the metal transfer characteristics. The ultrasonic wave is applied on the metal transfer process by means of an acoustic field. Welding electrical signal measurement and high speed camera are employed to study the differences of short circuiting metal transfer between conventional GMAW and U-GMAW. Compared with the conventional GMAW, the short circuit frequencies of U-GMAW are obviously increased under the same welding parameters. Moreover, the voltage range of the stable short circuiting transfer is enlarged, and the weld appearances become more uniform with the action of the ultrasonic wave. The high speed video images indicate that the U-GMAW arc is compressed and the electrical field intensity is increased. The decrease in the arc length is the main reason for the increase in the short circuit frequency.

Study on elimination of interlayer defects in horizontal joints made by rotating arc narrow gap welding

Abstract In this work, the characteristics of interlayer defects were studied in horizontal welds made by rotating arc narrow gap welding. The interlayer defects existed in the form of incomplete fusion and slag inclusions, occurring near the lower sidewall. In the rotating arc process, the instantaneous velocity of the arc with respect to the sample is higher when the arc is close to the lower sidewall, whereas it is lower when it is near the upper sidewall. This results in a lower effective heat input to the lower sidewall, and a corresponding reduction in the enthalpy that is transferred along with the deposited metal. Fortunately, this asymmetric nature of rotating arc welding can be exploited to minimise or eliminate interlayer defects. It is shown that, by controlling the direction of rotation for the arc, removing slag, selecting an appropriate groove size and resetting the torch position, defect free horizontal welds in heavy plates can be achieved.

Effect of acoustic field parameters on arc acoustic binding during ultrasonic wave-assisted arc welding

As a newly developed arc welding method, power ultrasound has been successfully introduced into arc and weld pool during ultrasonic wave-assisted arc welding process. The advanced process for molten metals can be realized by utilizing additional ultrasonic field. Under the action of the acoustic wave, the plasma arc as weld heat source is regulated and its characteristics make an obvious change. Compared with the conventional arc, the ultrasonic wave-assisted arc plasma is bound significantly and becomes brighter. To reveal the dependence of the acoustic binding force on acoustic field parameters, a two-dimensional acoustic field model for ultrasonic wave-assisted arc welding device is established. The influences of the radiator height, the central pore radius, the radiator radius, and curvature radius or depth of concave radiator surface are discussed using the boundary element method. Then the authors analyze the resonant mode by this relationship curve between acoustic radiation power and radiator height. Furthermore, the best acoustic binding ability is obtained by optimizing the geometric parameters of acoustic radiator. In addition, three concave radiator surfaces including spherical cap surface, paraboloid of revolution, and rotating single curved surface are investigated systematically. Finally, both the calculation and experiment suggest that, to obtain the best acoustic binding ability, the ultrasonic wave-assisted arc welding setup should be operated under the first resonant mode using a radiator with a spherical cap surface, a small central pore, a large section radius and an appropriate curvature radius.

Novel soft variable polarity plasma arc and its influence on keyhole in horizontal welding of aluminium alloys

Abstract A novel soft variable polarity plasma arc has been proposed to solve the problems in horizontal welding of aluminium alloys (in 2G position). It is defined as an arc with reduced arc pressure while retaining the ability to maintain a stable keyhole. In this paper, its characteristics were studied and its influence on the stability of keyhole was investigated. The results indicate that it has been changed in the energy and pressure distribution compared with ordinary variable polarity plasma arc. It is beneficial for the stability of keyhole especially for the stability of keyhole shrinking process. The keyhole can be established easily. It can also improve the wetting and spreading of molten metal in weld pool. The reason for a stable keyhole is that the soft variable polarity plasma can provide a suitable heat input and a suitable arc pressure.

Molten pool behaviour and weld forming mechanism of tandem narrow gap vertical GMAW

A tandem gas metal arc welding system was utilised for narrow gap welding in vertical down position. In the welding process, the two pulses are asynchronous and two arcs worked in the same molten pool. The molten pool forming process was investigated, and the results suggest that an asymmetric pool was generated, which resulted in the asymmetry of the weld bead. This condition could be improved by decreasing the distance between the two wires. The forces acting on the molten pool and the droplet were analysed. It is pointed out that, in vertical welding with low welding speed, more fusion metal flows down to the head of the molten pool due to the gravity, which leads to the reduction of the penetration depth. Based on the results, a multilayer welding was conducted with a high welding speed and a close distance between two wires, and a high quality weld was acquired.

Flux modification for AC-TIG braze welding of aluminium to stainless steel

Abstract A modified flux consisting of KAlF4 and aluminium powder was developed for AC-TIG braze welding of aluminium to stainless steel. By employing the flux, the arc instability and wetting angle was decreased significantly, and therefore a good joint shape was obtained. The wetting and spreading behaviour as well as the action mechanism of the flux were changed. Microstructure analysis suggested that the intermetallic compound (IMC) thickness increased obviously when the addition percentage of aluminium powder in the flux exceeded 40 wt-%. Results of tensile tests illustrated that a significant improvement in mechanical properties of the butt joint was obtained with the modified flux, and tensile strength increased from 125 to 170 MPa in maximum. Relationships between aluminium powder content in the flux and wetting angle, IMC thickness as well as tensile strength were established. Results indicated that the joint strength was affected both by weld shape and IMC thickness.

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.

Strength Prediction of Aluminum–Stainless Steel-Pulsed TIG Welding–Brazing Joints with RSM and ANN

Pulsed TIG welding–brazing process was applied to join aluminum with stainless steel dissimilar metals. Major parameters that affect the joint property significantly were identified as pulsed peak current, base current, pulse on time, and frequency by pre-experiments. A sample was established according to central composite design. Based on the sample, response surface methodology (RSM) and artificial neural networks (ANN) were employed to predict the tensile strength of the joints separately. With RSM, a significant and rational mathematical model was established to predict the joint strength. With ANN, a modified back-propagation algorithm consisting of one input layer with four neurons, one hidden layer with eight neurons, and one output layer with one neuron was trained for predicting the strength. Compared with RSM, average relative prediction error of ANN was <10% and it obtained more stable and precise results.