WELDING RESEARCH The Effect of Reduced Pressure on Laser Keyhole Weld Porosity and Weld Geometry in Commercially Pure Titanium and Nickel Laser welds made in vacuum penetrate four times deeper than welds in atmosphere, and contain little or no porosity NOVEMBER 2016 / WELDING JOURNAL 419-s Introduction The type of shielding gas used in partial penetration keyhole laser welding has been shown to have dramatic effects on the amount of porosity that forms in the welds (Refs. 1–4). At atmospheric pressure, inert gases such as Ar tend to produce high levels of porosity in commonly welded metals such as steels and austenitic stainless steels. This type of porosity is the result of voids created in unstable keyholes, and is different from solidification induced porosity that forms as solubility limits are exceeded during cooling of liquid weld pools that are supersaturated with gases such as oxygen and hydrogen. However, when these same metals are welded in nitrogen, instead of Ar, the welds contain little or no porosity under identical laser welding conditions (Refs. 1, 3, 4). Certain metals, such as nickel, produce high levels of porosity when laser welded in Ar or nitrogen (Ref. 1), and other metals, such as titanium, cannot be welded in nitrogen due to its high reactivity with nitrogen. Although the reasons for porosity formation in nickel and titanium aren’t perfectly clear, it is believed the lack of solubility and/or reactivity of inert shielding gas in the metal being welded play an important role in the retention of porosity in the final weld joint (Ref. 1). Because of this, other solutions are needed to minimize porosity in partial penetration Ni and Ti keyhole laser welds. One way to minimize the effects of shielding gas on porosity is to perform the laser weld in vacuum or under reduced pressure conditions, and this technique is being investigated as an alternative to conventional atmospheric laser welding. One study performed by Katayama et al. shows beneficial effects on weld penetration and reduced porosity in reduced pressure laser welds made in aluminum and stainless steel (Ref. 5). It is speculated that reduced porosity occurs in these welds due to changes in the liquid convective path that promote gas pore removal from deep laser welds relative to those made at atmospheric pressure in Ar or He inert gas. More recent studies have also confirmed the bene- BY J. W. ELMER, J. VAJA, AND H. D. CARLTON ABSTRACT The beneficial effect of reduced pressure laser welding in vacuum at 10–1 mBar was investigated in commercially pure titanium and nickel and compared to atmospheric pressure welding in Ar shielding gas. Partial penetration keyhole welds were made in these materials using a continuouswave disk laser operating at 2–4 kW and travel speeds of 12 and 17 mm/s, where moderate to severe porosity was observed under normal atmospheric pressure welding conditions. Additional welds were made in nickel using an electron beam welding process and the same parameters for comparison. Optical metallography, xray radiography, and computed xray tomography (CT) were used to characterize the porosity levels in the welds. Quantitative CT results show a monotonically decreasing pore size distribution for all of the welds, and the distributions can be fit with a twoparameter Wiebull relationship with a beta shape factor that varied between 0.37 and 0.61 depending on the welding conditions. Other results show laser vacuum essentially eliminates porosity in titanium and significantly reduces porosity in nickel, but does not completely eliminate porosity in nickel. Laser vacuum welds have increased keyhole penetration and reduced weld widths compared to their atmospheric pressure counterparts with similar beneficial geometric weld shapes as electron beam welds that are also made in vacuum. Finally, laser vacuum welds showed approximately onehalf the volumetric porosity in nickel than electron beam welds with less porosity at the weld root. KEYWORDS • Laser Vacuum Welding • Electron Beam Welding • Weld Porosity Reduction • Porosity Distribution • Computed Tomography • Keyhole Weld Penetration • Weld Geometry • Porosity Morphology J. W. ELMER (elmer1@LLNL.gov) and H. D. CARLTON are with Lawrence Livermore National Laboratory, Livermore, Calif. J. VAJA is with AWE, Aldermaston, Reading, Berkshire, UK.
Welding Journal | November 2016
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