Laser welding is increasingly becoming the process of choice for joining high temperature nickel- and titanium-based alloys used in airframe and turbine engine applications.
While many of these alloys have been successfully laser welded, a number are susceptible to porosity, cracking, or both during laser welding. Laser and processing parameters affect porosity and cracking. Cracking is also affected by the microstructure and processing history (heat treatment, work hardening) of the material being welded.
Prima Power Laserdyne has undertaken a number of projects to develop laser welding processes that meet the stringent requirements of aerospace and power generation. The subject of the latest study is the nickel-based superalloy Inconel 718.
Inconel 718, often referred to as ‘the workhorse of super-alloys’, is a precipitation hardenable nickel-based alloy designed to provide high yield, tensile, and creep-rupture properties at temperature ranges from cryogenic up to 1400⁰F (760⁰C). Common applications of Inconel 718 (Table 1) are turbine engine and high-speed airframe parts such as discs, blades, casings, buckets, and high temperature bolts and fasteners.
More recently, it has become popular for production of components using metal additive manufacturing (AM).
Table 1: Chemical composition of wrought Inconel 718 alloy (in % by weight.)
Full penetration butt and overlap welds were produced in 2.3 mm thick wrought Inconel 718 sheet using a 20kW QCW fiber laser. The effects of a range of laser and processing parameters, including those shown below, on solidification cracking and porosity formation were investigated during the initial laser welding tests.
- Laser output: Continuous wave (CW) and pulsed
- Average power: 1.5-2.0 kW
- Peak power: 3-5 kW
- Pulse width: 5-8 ms
- Pulse frequency: 80-120 Hz
- Focus position: At focus and ±3 mm
- Shield gases: Nitrogen and Argon
Specimens for optical microscopy were polished and electrolytically etched in 10% oxalic acid for 20 seconds to examine the fusion zone (FZ) and the heat affected zone (HAZ) for porosity and cracking.
Figures 1-4 highlight some of the results observed in this study.
Crack and porosity free welds have been produced in wrought Inconel 718. Furthermore, the study, which involved welding with a range of parameters, has shown that this level of quality can be achieved over a range of parameters.
Once again, we have seen that nitrogen shield gas is associated with much lower porosity than when using argon as the shield gas. Reduced porosity under nitrogen shielding gas is due to the reduced surface tension of the molten pool as compared to that with argon shield gas. Lower surface tension allows gas bubbles to more easily escape from the weld pool.
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