Minimizing Base Metal Cracking in Nickel-Based Alloys – Part 1
Aerospace and power generation gas turbines make use of nickel based superalloys that require a large number of small diameter (<1mm) holes to provide cooling of the surfaces of turbine blades, nozzle guide vanes, combustors, and other hot section components. Drilling of these holes by laser is now well established and such holes can be successfully produced either by trepanning or percussion drilling.
With the increased number of cooling holes, greater use of TBC (thermal barrier coatings), and overall drive for increased productivity, laser drilling has become increasingly attractive as an alternate to EDM. Furthermore, designers are searching for materials with even greater thermal properties to further increase turbine engine efficiency.
Some of the materials used in gas turbines, however, exhibit a relatively high susceptibility to microcracking in the recast layer. Such cracks can extend into parent material as shown in Figure 1.
|Figure 1: Typical recast and base metal cracks observed during laser drilling crack sensitive nickel based superalloys.|
There is little information in the technical literature, such as from university research, to provide insight to the causes of and corrective measures that can be taken to control cracking.
However, from our understanding of the metallurgy of nickel based superalloys, formation of microcracks during laser drilling is generally caused by microstructural segregation due to the relatively large solidification temperature range of the alloying elements of nickel based superalloys in the heat affected zone. This leads to formation of brittle phases at regions that experience low solidification rate. Stresses induced by laser drilling can cause these phases to crack.
Nickel based superalloy microstructures exhibit many phases such as intermetallic γ′ phase (Ni3Al,Ti), Cr-Mo boride, and MC carbide (M being Ti, Ta and Nb) which can lead to cracking during laser drilling.
A detailed study of laser drilling of crack sensitive nickel based superalloys is in progress at Prima Power Laserdyne. The objective this study is to gain a better understanding of the factors contributing to cracking in these alloys which is vital to developing laser and processing parameters for eliminating or, at least, minimizing cracking during drilling of these alloys.
As well as optimizing laser parameters, drilling tests are also being carried out to examine the influence of preheat at different temperatures prior to drilling. Heat treatment may, for example, homogenize the alloy’s elemental constituents and reduce grain boundary segregation.