Minimizing Base Metal Cracking in Nickel-Based Alloys – Part 2
The August 2016 issue of The LASERDYNE Technical Journal included an article entitled “Minimizing base metal cracking in nickel-based alloys“. This article introduced R&D conducted by Prima Power Laserdyne specifically aimed at developing laser and processing parameters for eliminating or, at least, minimizing cracking during drilling of nickel based superalloys.
Since this article was published, initial tests have been performed to study the influence of laser and processing parameters on the formation of microcracks during laser drilling of cast Inconel 792 (IN792) (Table 1).
Table 1: Chemical composition of nickel-based superalloy Inconel 792 (at. %)
| Cr | Co | Mo | Ti | Al | C | W | Ta | Zr | B | HF | V | Ni |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 12.4 | 9.0 | 1.9 | 3.6 | 3.1 | 0.12 | 3.8 | 3.9 | 0.1 | 0.02 | 0 | 0 | Bal |
Figure 1 shows the microstructure of the as received, cast Inconel 792 used in these tests. This heat resistant nickel-based alloy is designed for precision investment cast guide vanes and stator parts used in gas turbines.
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| Figure 1: The as-cast microstructure of IN792 alloy is composed of carbides, γ′/γ eutectic,γ′ phase and γ matrix. |
The range of laser and processing parameters shown below were investigated during these initial laser drilling tests:
- Pulse width: 0.8-2.0ms
- Pulse energy: 2.4-22J
- Peak power: 3-11kW
- Average power: 36-330W
- Power density: 18.9- 69.5MW/cm2
- Pulse frequency: 15-20Hz
- Assist gas pressure (oxygen): 5-10bar
Laser drilled holes were sectioned, polished, and electrolytically etched using Oxalic acid for 10 seconds to examine hole quality. Some results this analysis are highlighted in Figures 2 and 3.
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| Figure 2: Laser drilled 0.5mm diameter hole in 2mm thick Inconel 792 alloy at 30 degrees from the surface produced using 0.8ms pulse width, 15Hz pulse rate, 2.4J pulse energy, and 3kW peak power. Metallurgical results: no cracks in the recast layer or base metal. |
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| Figure 3: Laser drilled 0.5mm diameter hole in 2mm thick Inconel 792 alloy at 30 degrees from the surface produced using 0.8ms pulse width, 15Hz pulse rate, 8.8J pulse energy, and 11kW peak power. Metallurgical results: recast layer crack 22.7µm; base metal crack 21.8µm; oxide layer crack 9.8µm. |
Summary
Initial drilling tests involving cast Inconel 792 have shown that:
- With certain laser and processing parameters, it is possible to drill holes without base metal cracking (Figure 2).
- Peak power plays a significant role in the material’s susceptibility to cracking
- The length of cracks, when present, increase with peak power at a fixed pulse width.
Next Steps
While the initial tests have produced valuable results, additional tests are planned. Goals of these tests are:
- Further optimize and refine effective laser and processing parameters to extend the crack-free processing window,
- Identify the effects of heat treatments on the base metal microstructure and on cracking tendency.
- Refine the model for cracking in laser drilling of superalloys.
