Conventional Gas Tungsten Arc Research Articles

Butt Welding Joint of Aluminum Alloy by Space GHTA Welding Process in Vacuum

Aluminum alloys have been used widely in constructing various space structures including the International Space Station (ISS) and launch vehicles. For space applications, welding experiments on aluminum alloy were performed using the GHTA (Gas Hollow Tungsten Arc) welding process using a filler wire feeder in a vacuum. We investigated the melting phenomenon of the base metal and filler wire, bead formation, and the effects of wire feed speed on melting characteristics. The melting mechanism in the base metal during the bead on a plate with wire feed was similar to that for the melt run without wire feed. We clarified the effects of wire feed speed on bead sizes and configurations. Furthermore, the butt welded joint welded using the optimum wire feed speed, and the joint tensile strengths were evaluated. The tensile strength of the square butt joint welded by the pulsed DC GHTA welding with wire feed in a vacuum is nearly equal to that of the same joint welded by conventional GTA (Gas Tungsten Arc) welding in air.

Heat input properties of hollow cathode arc as a welding heat source

In order to clarify whether a hollow cathode arc (HCA) can be used as a welding heat source in space, investigations into the fundamental characteristics of HCA were experimentally performed under low pressure conditions. The HCA method enables an arc discharge to ignite and maintain under low pressure conditions; in contrast, low pressure conditions make it extremely difficult for the conventional gas tungsten arc method to form an arc discharge. In an earlier paper, it was shown that the melting process by HCA is very sensitive to process parameters such as the gas flow rate and arc length, and a deep penetration forms when the arc length is long and the gas flow rate is low. In this paper, the distribution of the arc current on the anode surface and the plasma properties of the HCA under low pressure conditions have been made clear and the total heat energy to the anode has been discussed in order to understand the heat input properties of the HCA. The result shows that the HCA in the case of a low gas flow rate is a high and concentrated energy source, and the high energy input to the anode contributes to the deep penetration.

Studies on characteristics of temperature field during GTAW with a trailing heat sink for titanium sheet

The characteristics of temperature field have been studied numerically and experimentally for a titanium alloy sheet when welded with an improved gas tungsten arc welding method, in which a trailing spot heat sink is introduced to control the welding stress and distortion. The impinging jet model is employed to describe the intense heat transfer between the cooling media and the top surface of the workpiece. By comparing this improved technique to the conventional gas tungsten arc welding (GTAW) process, the influence of the spot heat sink is investigated. Results show that a low temperature region exists where the heat sink is applied; a saddle backed temperature field, with the lowest temperature at the centre and higher temperatures at both sides, is formed during the improved welding method; and the time that the metal spends at high temperature is shortened in the improved welding process. Experimental measurements of the thermal histories compare well with the calculations, which prove the validity of the finite element model and the impinging jet heat transfer model in simulating the gas tungsten arc welding process with a trailing heat sink.

Nickel based superalloy welding practices for industrial gas turbine applications

The continued drive for increased efficiency, performance and reduced costs for industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers, these materials must be capable of being welded in a satisfactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes; however, high strength, precipitation hardened materials are prone to heat affected zone and strain age cracking defect formation. A number of factors are found to affect the propensity for defects: composition (aluminium and titanium content), grain size, pre- and post-weld heat treatment, as well as the welding process itself (control of heat input and traverse speed). Process parameter identification is still largely empirical and a fuller understanding of the joining processes is dependent upon the development and application of more sophisticated numerical modelling techniques.

Characteristics of Hollow Cathode Arc Plasma as a Heat Source. Investigation into Melting Mechanism Focusing on Electron Density and Temperature.

A feasibility study has been conducted to determine whether Hollow Cathode Arc (HCA) can be used for a welding heat source in space. The HCA method enables an arc discharge to ignite and maintain under low pressure conditions, where it is extremely difficult for the conventional Gas Tungsten Arc (GTA) method to form an arc discharge. In the previous paper, it has been demonstrated that the melting process by HCA under the condition that the gas flow rate is high (1.7ml/s) is significantly dominated by the diffuse anode and is characterized by shallow penetration, and that the electron temperature is about 1200018000K in arc space. In the present paper, the volt-ampere characteristics and the plasma property of HCA under the condition that the gas flow rate is low (0.17ml/s) have been made clear, and the melting phenomenon have been discussed. The results are summarized as follows, (1) The HCA under the condition that the gas flow rate is low is characterized by high voltage and high electron temperature, and the arc column is concentrated under the cathode.(2) The electron temperature is above 60000K near the arc axis.(3) The electrons with high temperature, that is high energy, contribute to the deep/huge penetration under the condition that the gas flowrate is low.

Gas tungsten arc welding studies on similar and dissimilar combinations of Al-Zn-Mg alloy RDE 40 and Al-Li alloy 1441

The present study is concerned with gas tungsten arc welding of two high strength aluminium alloys, namely, an Al-Zn-Mg alloy (RDE 40) and an Al-Li based alloy of Russian grade 1441. One of the critical requirements of these two alloys is that they should be weldable. In the present work, weldability aspects of these alloys were studied in terms of solidification cracking tendency, microstructure, tensile properties, and microhardness across the welds. These studies were extended to dissimilar welds between RDE 40 and 1441 produced via conventional gas tungsten arc (GTA) welding as well as pulsed current GTA welding. It was found that RDE 40 welds were less sensitive to solidification cracking and weld metal porosity compared with 1441 alloy. The superior weldability of RDE 40 was related to the equiaxed nature of the fusion zone and a lower sensitivity to moisture pickup. It was possible to produce RDE 40-1441 welds without defects. Pulsed current welding of RDE 40 to 1441 showed improved mechanical properties compared with conventional GTA welding, and these were related to the refinement of the fusion zone microstructure.

Observations on welding of 2+O+&bgr; titanium aluminide

The weldability characteristics of an 2+O+&bgr; titanium aluminide of nominal composition Ti–24Al–15.5Nb (at.-%) have been investigated. Conventional gas tungsten arc (GTA) and electron beam (EB) welds exhibited columnar fusion zone grains. Pulsed current and arc oscillated GTA welds exhibited predominantly equiaxed fusion zone grains. The microstructure of GTA welds and pulsed current GTA welds exhibited 2+O phases, whereas arc oscillated GTA welds and EB welds contained 2+O+&bgr;0/&bgr;2; however, &bgr;0/&bgr;2 is predominant in EB welds. The EB welds, which contained 2+&bgr;0/&bgr;2 microstructure, exhibited high strength and ductility compared with GTA welds. The observed microstructural variations are explained on the basis of possible weld thermal cycles and convective currents in the weld pool.

Gas hollow tungsten arc characteristics under simulated space environment

A feasibility study has been conducted to determine whether gas hollow tungsten arc (GHTA) welding can be used for welding in space. As described in a previous paper by the present authors, the GHTA method has been tested in a simulated space environment using aircraft. The test result shows that the method is most promising as a welding process in space. In the present paper, some fundamental characteristics of the GHTA in a vacuum chamber, such as discharge characteristics and plasma properties, have been elucidated and the results of melting tests on stainless steel plate using the GHTA method have been compared with those obtained via a conventional gas tungsten arc method under atmospheric pressure.

Comparison of keyhole and conventional gas tungsten arc welds in commercially pure titanium

Keyhole gas tungsten arc welding (GTAW), a novel process recently developed at CSIRO Manufacturing Science and Technology, Australia, has been used to join commercially pure (CP) titanium. The process enables single pass, out of chamber welding of 12.7 mm thick CP titanium without expensive filler metal addition or joint preparation. The microstructure and mechanical properties of the resultant weld joint have been compared with those of a multipass conventional GTAW joint in CP titanium prepared from plates of the same thickness with a double-V edge preparation and the addition of a matching filler metal. It was shown that the differences in the net heat input and weld thermal cycles associated with the two processes led to similar microstructures, albeit of different degrees of refinement. As a result, the tensile and hardness properties of the two welded joints were similar to each other and comparable to those of the base material. The acicular nature of the fusion zone microstructure in both welds led to a significant increase in their notch toughness over that of the base material. It is concluded that the keyhole GTAW, with its significantly higher productivity combined with the simplicity of proven technology and low capital investment requirements, can be successfully applied in the welding of heavy section CP titanium, without sacrificing the metallurgical quality associated with the GTAW process.

Effects of activating flux on arc phenomena in gas tungsten arc welding

Dramatic increases in the depth of weld bead penetration have been demonstrated by welding stainless steel using the gas tungsten arc (GTA) process with activating fluxes consisting of oxides and halides. However, there is no commonly agreed mechanism for the effect of flux on the process. In order to clarify the mechanism, behaviour of the arc and weld pool in the GTA process with activating flux was observed in comparison with a conventional GTA process. A constricted anode root was found in the GTA process with activating flux, while a diffuse anode root was found in the conventional process. Furthermore, it is suggested that these anode roots are strongly related to metal vapour from the weld pool, which is also related to temperature distributions on the weld pool surface.

低圧下でのＧＨＴＡ放電の特性と母材の溶融特性

In this work, a feasibility study has been conducted to determine if gas-hollow-tungsten-arc (GHTA) can be used for welding purposes in space.As described in the previous paper, the GHTA-method has been tested in a simulated space environment using aircraft. The test result shows that the method is very promising as a welding process in space.In this paper, some fundamental characteristics of GHTA in vacuum chamber, such as ignition voltage and arc voltagecurrent characteristic, have been made clear and the results of melting test on stainless steel plate by GHTA-method have been compared with the case by a conventional GTA (gas tungsten arc)-method under atmospheric pressure.The results are summarized as follows, (1) The ignition voltage of GHTA depends on the ambient pressure : P, and the voltage is minimized under condition of P=26.6 [Pa](2) The arc voltage-current characteristic of GHTA is nearly equal to that of GTA under atmospheric pressure.(3) The penetration shape by GHTA-method is quite different from that by GTA-method under atmospheric pressure. The GHTA method is characterized by shallow and wide penetration.(4) The current distribution on anode surface of GHTA is extremely diffused, and the anode area increases with the arc-length.(5) The diffused anode, which means diffused heat source, gives a satisfactory explanation for the shallow penetration in the GHTA-method.

Optimisation of pulse frequency in pulsed current gas tungsten arc welding of aluminium–lithium alloy sheets

Effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied. It was observed that the structure in the as solidified weld was predominantly columnar in the case of the conventional (i.e. continuous current) gas tungsten arc welding process. The grain structure became finer and more equiaxed with the introduction of current pulsation. Moreover, there was an optimum frequency range over which the grain refinement was a maximum. The same optimum frequency range corresponded with maxima in hardness, ultimate tensile strength, and percentage elongation. Tensile strength increased, in general, after solution treatment and aging (STA). The best combination of tensile properties was achieved for welds deposited under a 6 Hz pulse frequency in the STA condition.

Optimisation of pulse frequency in pulsed current gas tungsten arc welding of aluminium–lithium alloy sheets

Effects of current pulsation frequency on weld bead microstructure, hardness, and tensile properties in AA8090 type aluminium-lithium alloy sheets were studied. It was observed that the structure in the as solidified weld was predominantly columnar in the case of the conventional (i.e. continuous current) gas tungsten arc welding process. The grain structure became finer and more equiaxed with the introduction of current pulsation. Moreover, there was an optimum frequency range over which the grain refinement was a maximum. The same optimum frequency range corresponded with maxima in hardness, ultimate tensile strength, and percentage elongation. Tensile strength increased, in general, after solution treatment and aging (STA). The best combination of tensile properties was achieved for welds deposited under a 6 Hz pulse frequency in the STA condition.