# Tantalum Alloy Welding: Does the Thermal Cycle Influence the Microstructure?
###### Abstract  
The aim of the present work is to verify the microstructural behavior of a B 521 tantalum alloy UNS Grade R05200 after welding, in relation to the welding thermal cycle. The joint design was a 1.5 mm thickness circumferential butt welding, on a 32 mm outside diameter pipe, welded in 1 G position (horizontal, flat, and rotating). The chosen welding process was gas tungsten arc welding (GTAW). The microstructural analysis showed the presence of coarse, dendritic-columnar structures, as well as a hexagonal cell, with no cracks noted. Hardness tests showed an increase in hardness, from 120 HV to 425 HV, in the heat-affected zone. Through finite element methods, the behavior of the temperature field was estimated and compared. tantalum welding; microstructure; heat-affected zone +
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reference to tantalum high alloys, with other elements such as aluminum or titanium, the welding of the metal itself has hardly been explored. According to [4], in form of alloy, tantalum present as distinctive characteristics, the brittle-ductile transition. There is controversy about if BCC (body-centered cubic microstructure) pure tantalum exhibits or not a clear brittle-ductile transition. Recrystallization of the metal, during welding processes, can increase its transition temperature. Contamination during this high temperature period and the presence of metallic and non-metallic impurities can also induce the material brittleness. If such contamination occurs because the transition temperature is increased sufficiently, there will be a weld without desirable metallurgical characteristics. Tantalum has stable performance at room temperature, and when heated, it can be combined directly with non-metallic elements. When tantalum sheet heated to 200-300 \({}^{\circ}\)C, micro-oxidation occurs (O\({}_{2}\), N\({}_{2}\), H\({}_{2}\), and C enter interstitial spaces) and, when heated to above 500 \({}^{\circ}\)C, rapid oxidation occurs [5]. Before welding, welding surface must be cleaned with chemical reagents and all traces of these products removed with water. After the parts are cleaned, they must be protected from oxidation using an inert gas or vacuum chamber [5,6]. Authors in [7] suggests that, during the tantalum welding process, filler material may or may not be used. Also recommends the use of direct current reverse polarity--electrode to positive (less heat input) and high frequency arc starter (improving the success rate of arc start). Helium as inert gas to maintain maximum penetration and counter the reverse polarity effect. Tantalum can be welded to other metals [8], although with some of them it forms brittle intermetallic phases that must be avoided. Successful welding can be made with various techniques such as RW (resistance welding), GTAW (gas tungsten arc welding), PW (plasma welding), or EBW (electron-beam welding). In previous studies [9] it was concluded that, when tantalum is under high temperature processes, it ignites in the air to form tantalum pentoxide, Ta\({}_{2}\)O\({}_{5}\), a white and infusible substance that combines with metal oxides or hydroxides to form compounds called tantalates. Tantalnic acid, TaO\({}_{5}\)\(\times\) H\({}_{2}\)O, is a gelatinous precipitate that is produced by humidity on tantalum pentachloride. When tantalum is heated on oxidizing atmospheres [10], the metal oxidizes to form different compounds. From about 250 \({}^{\circ}\)C, a change in the color of the metal can be observed, this being first gray in color, then it acquires a yellow color, then bluish and finally a fine white powder indicative of the formation of the pentoxide. The different tonality is due to the variation in the thickness of the oxide layer formed. When the thickness of this oxidized layer is of the order of the wavelength of visible light, interferences are produced and colorations are observed.