TWI Industrial Member Report Summary 1011/2012
By A Khan and P Hilton
Nickel alloys are predominantly used in applications requiring high temperature properties, heat resistance, and/or corrosion resistance; for instance: in gas turbines, ethylene plants, and hydrogen production plants (Shinozaki et al, 1999). Tungsten Inert Gas (TIG) welding is the most commonly used fusion welding process to join nickel alloys, because of the high weld quality that can be achieved.
Weld imperfections, such as those associated with bead geometry and sub-surface porosity are of concern given the likely high-performance service requirements of fabricated nickel-based components. This is highlighted by the weld quality criteria stipulated in AWS D17.1:2001 a standard for the fusion welding of aerospace components and it is known that more stringent criteria, usually company-specific to various aero-engine manufacturers also exist. Recent work performed by TWI (Blackburn and Khan, 2011) has indicated that even these stringent criteria can be achieved in titanium alloys when welding with modern 1µm wavelength laser sources of high beam quality. To date, very little work has been reported using high-power, high beam quality, continuous-wave 1µm wavelength lasers for welding of nickel alloys.
This project has been performed to determine the effects of different processing parameters on the quality of welds made in 3 and 5mm thickness Alloy 718 using a high beam quality Yb-fibre laser. The principal focus of the project was to compare the results obtained for bead geometry and sub-surface porosity to Class A of AWS D17.1:2001 and a generic, more stringent company-specific standard. The prevention of liquation cracking was not the main focus of this research, since it is known that this can be avoided by parent material selection, however, an indication of the effects of process parameters on liquation cracking has also been made.
- Assess the weld quality, in terms of sub-surface porosity and weld bead profile, of melt runs produced in 3.35 and 5.0mm thickness Alloy 718 with a high beam quality Yb-fibre laser.
- Compare the results achieved with the criteria in AWS D17.1 and a more stringent typical company-specific standard.
- Identify laser welding envelopes for the production of high quality welds in Alloy 718 with a high beam quality Yb-fibre laser