Assessment of the origin of porosity in electron-beam-welded TA6V plates

Abstract

Experimental and theoretical analysis of the origin of porosity in electron-beam (EB) welding is detailed. The experiments are run with several surface treatments and reasonable welding parameters. The plate faces are characterized before welding with a number of methods, such as scanning electron microscope observation, X-ray photoemission spectroscopy (XPS) and, more significantly, secondary ion mass spectroscopy (SIMS) analysis, elastic-recoil detection analysis (ERDA) for hydrogen analysis, and surface roughness measurement. After welding, pores are sought with X-ray detection, phased-array ultrasonic (US) detection, and destructive control. An original comparison between ERDA and refined SIMS measurements allows a quantitative evaluation of surface pollution with hydrogen, oxygen, and carbon. The theoretical analysis is based on the literature concept that the cavities are nucleated from the adjacent plate faces in the solid state, just before melting. A less classical development is proposed in term of the evolution of bubbles in the weld pool. Once in the liquid, the cavities become bubbles. Their radius oscillates, according to Rayleigh-Plesset equations of bubbles, due to temperature and pressure driving forces. Solidification freezes them as they are, thus, forming pores. The extreme values of the oscillation give a good idea of the range of the size of pores in the weld joint, as the comparison between experiments and prediction states. A criterion of surface cleanliness is set, relating the surface pollution and the surface roughness. Above the criterion, the bubbles remain small during their oscillation. Below the criterion they tend to grow large. All the degraded-surface treatments are in dirty situation (large pores), and the reference surface treatment lies around the criterion for cleanliness.

Hydrogen Transport and Rationalization of Porosity Formation during Welding of Titanium Alloys

“…In practice, these alloys are prone to the formation of porosity. [3,2,29] Pores are often of diameter 0.1 mm or greater and can therefore be detected by NDT methods such as X-ray tomography ( Figure 15). The shape of the pores in these cases is spherical; a scanning electron micrograph of a typical pore in electron beam welded Ti-6Al-4V is shown in Figure 16.…”

“…WHEN titanium alloys [1] are joined using welding-as required for structural applications for which their high strength/density ratio is highly advantageous-they are prone to the formation of gas-related porosity. [2][3][4][5] The pores are located in the fusion zone and are of diameter 0.1 mm or greater; at this size, their presence represents a threat to the mechanical integrity of the component being assembled. For this reason, it is usual to employ nondestructive testing (NDT) methods to ensure their detection before mission-critical parts enter service.…”

“…[7] Presumably, decomposition of these might be anticipated during processing. More recently, the possible role of surface contaminants (specifically based upon carbon and hydrogen) has been studied, [3] the implication being that they are considered responsible for the pore formation. Some unpublished work by one of the authors [8] identified a role of hydrogen-rich contaminants and effects of differing surface preparation.…”

Microstructure and Mechanical Properties of Ti5553 Butt Welds Performed by LBW under Conduction Regime

“…The most common welding techniques to joint titanium and its alloys are Gas Metal Arc Welding (GMAW), such as Metal Inert Gas (MIG); Plasma Arc Welding (PAW); Laser Beam Welding (LBW); and Electron Beam Welding (EBW) [1,[9][10][11][12]. The first three methods fall in the arc welding category with high heat input and low power density of heat source, while the last two techniques belong to the high-energy beam group.…”

TIG Dressing Effects on Weld Pores and Pore Cracking of Titanium Weldments

“…The most likely of these is hydrogen, but other possible agents include oxygen, nitrogen, and carbon dioxide, as well as the inert shielding gases. These gases could be present as a result of any of the following: (a) desorption of the gases' elemental constituents from the parent material or welding consumables; (b) absorption into the weld pool due to inadequate shielding (through either entrapment of air in the shielding gases or a high moisture level in the shielding gases) during welding; (c) entrapment of the shielding gases; or (d) surface contamination [4][5][6][7][8][9]. Various methods, including variation of the process parameters and attention to cleanliness, have been employed to attempt to minimize porosity in titanium weldments.…”