Laser Drilling Process Yields Fewer Cracks in Hastelloy X – Part 1
HASTELLOY X is a wrought nickel base alloy with excellent high temperature strength and oxidation resistance. This alloy is one of the most widely used nickel base superalloys for gas turbine engine hot section components, such as transition ducts, combustor cans, spray bars, flame holders, and afterburners.
Laser drilling of cooling holes in this alloy is well established. However, it is also generally accepted that Hastelloy X exhibits a relatively high susceptibility to microcracking during laser processing (cutting, drilling). Cracks in the recast layer often extend into the parent material (also referred to as base metal).
Given the importance of this alloy, Prima Power Laserdyne Applications Engineering has conducted a parametric study to investigate the cracking behavior of Hastelloy X (Table 1) with QCW fiber laser drilling.
Figure 1 shows the microstructure of the as received, cold rolled material.
|Table 1: Chemical composition of Hastelloy X (Wt. %)|
|Balance||20-23||8-10||17-20||0.5-2.5||1.0 Max||1.0 Max||0.30||0.50 Max||0.1 Max||0.05-0.5||0.1 Max|
Laser and process parameters investigated
Table 2 shows the range of laser and processing parameters used for the drilling trials.
|Table 2: Laser drilling parameters|
|Laser parameters||Processing parameters|
|Peak power: 3-15 kW||Material thickness: 2.25 mm|
|Pulse width: 0.3-1.1 ms||Assist gas: Oxygen|
|Pulse energy: 0.9-16.5 J||Gas pressure: 10 bar|
|Pulse frequency: 15-223 Hz||Drilling angles: 90 & 30 degrees|
|Average power: 52.5-247.5 W||Drilling techniques: Percussion & trepanning|
|Power density: 13.7-68.5 MW/cm2||Speed: 40 mm/min|
|Focus position: 0 to ±5 mm|
|Figure 1: Optical microscope image of as-received Hastelloy X showing equiaxed grains and metallic carbides (MC).|
The quality of laser drilled holes produced in this study was evaluated for recast and oxide layer thickness and for cracks in the recast layer and base metal using metallographic cross sections. Results from this comprehensive study have shown a well-defined relationship between laser parameters and the amount of cracking in the recast layer and in the base material.
This relationship is useful in developing processes for drilling holes without base metal cracks. The results can also provide guidance for reducing recast layer thickness and cracks in the recast layer.
Pictured in Figures 2 and 3 are examples of holes produced using laser and process parameters optimized for base metal cracking.
|Figure 2: Trepanned 0.8 mm diameter holes at 90 degrees to the surface in 2.25 mm thick Hastelloy X. Holes were trepanned at 40 mm/min. Metallurgical results – recast layer thickness: 24.9 µm; oxide layer thickness: 20.1 µm; no base metal cracks.|
|Figure 3: Trepanned 0.84 mm diameter holes at 30 degrees to the surface in 2.25 mm thick Hastelloy X. Holes were trepanned at 40 mm/min. Metallurgical results – recast layer thickness: 55.0 µm; oxide layer thickness: 10.0 µm; no base metal cracks.|