S. B. Lin

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.

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.

Temperature simulation of the preheating period in friction stir welding based on the finite element method

Abstract In this paper, a thermal history and distribution analysis of the preheating period of friction stir welding (FSW) of AZ31 magnesium alloy using the finite element method is presented. A moving heat source along the thickness is used to simulate the plunge movement of the pin and the heat flux generated by the friction between the pin and the workpiece is applied as volume heat. Temperature history and distributions at different plunge speeds and rotational speeds are simulated and measured to determine the appropriate preheating parameters. When a rotational speed of 600 r/min and a plunge speed of 3mm/min are used, the maximum preheating temperature can reach to about 450°C, showing that the heat generated by the pin cannot be neglected during FSW and the heat proportion is about 15 per cent of the total heat generated by the tool (including generated by the shoulder and the pin). The high temperature is in the range of the pin diameter and the maximum temperature gradient is on the boundary of the pin. The simulation temperature results corresponded well with the measured temperature during experiments.

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.

Soft variable polarity plasma arc horizontal welding technology and weld asymmetry

Abstract Soft variable polarity plasma arc welding technology has been proposed to solve the problems in horizontal position welding for aluminium alloys. Arc thermal field shape, arc pressure distribution and the characteristic of the weld pool, which are important for keyhole stability, have been changed compared with the ordinary variable polarity plasma arc. Its stability in application can be evaluated by the operational window. The thermal cycle curves, microstructures and microhardness of the horizontal position weld indicate the weld asymmetry. The influence of asymmetry as well as welding defects on tensile strength and fracture behaviour has been studied. The fundamental reason for the asymmetry was considered as the asymmetric fluid flow around the keyhole in the weld pool.

Numerical Analysis on Stress Evolution During GTA-Additive Manufacturing of Thin-Walled Aluminum Alloys

GTA-Additive Manufacturing (GTA-AM) is one of the Wire Arc Additive Manufacturing (WAAM) technologies based on Gas Tungsten Arc Welding (GTAW). During the GTA-AM process, non-uniform heating, as well as rapid heating and cooling of deposited materials lead to inevitable temperature stress, while repeated remelting in subsequent deposition introduces complexity to the evolution of it. Finite element modeling was conducted in this work to investigate the stress evolution during the GTA-AM of thin-walled 5183-Al. Several thin-walled 5183-Al components with different bead widths are included to understand the influences brought about by geometry dimensions. Hole-drilling method is used to measure the residual stress of 5183-Al components manufactured by GTA-AM, which proves the reliability of this numerical model. Results of modeling show that the temperature stress is generated and released alternately and repeatedly as the deposition proceeds. The cycles of equivalent stress are in synchronization with the thermal cycles, in which equivalent stress reaches peak when temperature gets to its valley value. However, as the deposition layers pile up, the peak value of equivalent stress decreases gradually, and finally achieves stability after a certain layer. As for the influences of geometry dimensions, larger bead width results in higher level of stress due to much larger heat input as well as grater constraints.