Effect of shielding gas composition and welding speed on autogenous welds of unalloyed tungsten plates

Abstract

Tungsten usually exhibits poor weldability and marked brittleness at room temperature. This cause tungsten welds to be affected by the evolution of cracks along the weld bead, which can be eliminated by using a pre-heating step to reduce thermal straining. In this study, based on the tungsten inert gas welding process, a working envelope, focussed on varying welding speed and five different shielding gas mixtures of argon and helium, has been defined with the view of producing crack-free autogenous welds. The bead appearance and the microstructure of the different welds were correlated to the welding parameters, whose main effects have been analysed. Welding defects such as humping occurred when using gas mixtures with relatively low content of helium, and when using relatively high welding speeds. Crack-free autogenous welds have been produced without preheating when using a high content of helium and relatively low welding speeds. Thus, this study has demonstrated that a helium-rich shielding gas is required for welding thick tungsten plates. Moreover, the low thermal shock induced by the process, coupled with the purity of the tungsten plates used, strongly contributed to avoid the occurrence of any crack.

Microstructure and thermal properties of unalloyed tungsten deposited by Wire + Arc Additive Manufacture

“…The average height of a layer was around 1.2 mm. Please note the choice of 100% He as process-gas for TIG, following from what concluded in our previous study [27]. The total height and length of the deposit were 75 mm and 120 mm, respectively.…”

Development of Wire + Arc additive manufacture for the production of large-scale unalloyed tungsten components

“…In the past, it has been reported that tungsten exhibits limited weldability, due to the low DBTT, the high melting point and the high reactivity with environmental gasses over a large range of temperature [13][14][15][16]. However, in a previous study [17], we have been able to weld tungsten autogenously, avoiding cracks and porosity successfully.…”

“…Table 2 reports the deposition parameters used to produce all the samples analysed in this study. Please note the choice of 100% He as process-gas for TIG, following from what concluded in our previous study [17]. Table 2 WAAM process parameters used for the deposition of unalloyed tungsten.…”

Transferability of the working envelope approach for parameter selection and optimization in thin wall WAAM

“…Aiming at improving the variability proportion that can be quantified for each model (R 2 ), the significance level of each term of the model was evaluated through ANOVA and the less significant terms (p-values>0.05) were discarded. Equations ( 5), ( 6) and (7) present the models used to develop the surfaces in Fig. 22 and their respective R 2 and adjusted R 2 .…”

“…Following this approach, Marinelli et al [7] built a working envelope for autogenous welding with GTAW, evaluating different levels of travel speed and shielding gas compositions (varying Argon and Helium contents). The authors found that higher He contents in the shielding gas and lower travel speeds resulted in weld beads free of cracks.…”