Grinding of fir tree slots of powder metallurgy superalloy FGH96 using profiled electroplated CBN wheel

Broaching is commonly used for machining fir tree slots on turbine disk, which has outstanding advantages and disadvantages, such as high-quality machined surface, high manufacturing accuracy, and high productivity as well as fast tool wear, extremely high processing costs, long preparation time, and poor process flexibility. Utilizing the electroplated cubic boron nitride (CBN) profiled grinding wheels and single-sided local profiled grinding process, the experiments of FGH96 turbine disk slots are carried out. The results show that the high precision of slot profile can be achieved by the developed process. Using the given experimental parameters n = 48,000 rpm, ap = 0.002 mm, vf = 100 mm/min, and 600# electroplated CBN-profiled wheel, the profile error of FGH96 slots is within ±0.012 mm, and the grinding surface roughness is less than Ra 0.8 μm. After four whole slots are machined completely, the grinding wheel still has grinding capability, which proves that 600# electroplated CBN-profiled wheel can meet the grinding needs of FGH96 turbine disk slots.


Introduction
Powder metallurgy superalloy is the preferred material for manufacturing high-temperature load-bearing parts of aeroengines, which have excellent high-temperature mechanical properties and fatigue creep resistance. FGH96 alloy is a typical material of the second generation damage tolerance type superalloy. Because of its mature preparation technology, uniform material structure, and stable mechanical properties, it has been widely used to manufacture turbine disks and other hot components of aero-engines [1][2][3]. However, the excellent mechanical properties at high-temperature lead to poor cutting performance, and the machining of fir tree slots on turbine disk is very difficult especially [4].
At present, broaching is known to be the best process to machine fir tree slots of turbine disk on powder metallurgy superalloy, which can offer very high productivity and is the first selection to serial production of the turbine disk [5,6]. Klocke et al. presented a suitable carbide broach structure, which further improved the machining efficiency and tool life of the broaching. At the same time, the ceramic side milling process was utilized for rough machining fir tree slots, which greatly improved the rough machining efficiency [7,8]. However, major shortcomings of the broaching process are also very prominent, which include high capital cost, large size of machine tools together with their inflexibility, costly tooling, lengthy setup, validation and changeover times, and high cutting forces (up to 10,000 N) [9].
For these reasons, wire-EDM becomes more and more a serious alternative to the established and a critically defined broaching process for fir tree slot production [10,11]. Klocke et al. deeply studied the application of wire-EDM for machining fir tree slots through developing a process monitoring tool for processing quality assessment and correlating surface integrity evaluation, which obtained some good experimental results [10][11][12][13]. The profile error of the slots machined by wire-EDM can be within ±0.01 mm. From technological point of view, wire-EDM can be an alternative to the established broaching process for the production of fir tree slots [14]. But, wire-EDM process also has an outstanding drawback that recast layers with different thicknesses will be formed on machined surfaces, which will have a negative influence on the fatigue performance of a dynamic component [14,15]. Thus, wire-EDM process has limited further application in the field of powder metallurgy superalloy slots machining.
With the development of high speed spindle and electroplated superabrasive grinding wheel technology, highspeed grinding with profiled electroplated superabrasive wheel is gradually applied to the field of fir tree slots machining. Aspinwall pointed out the drawback of broaching fir tree slots, and the experiments of using the profiled superabrasive wheels to machine Udimet 720 fir tree slots were also carried out. Machining data were presented, which verified the feasibility of small diameter, profiled, and electroplated CBN grinding wheels when cutting Udimet 720 fir tree slots [16]. Meanwhile, combined with the electrochemical superabrasive machining, superabrasive grit type and electrical parameters were developed to improve the processing efficiency and flexibility [17]. Lanes et al. proposed a method that using a cup grinding wheel to machine fir tree slots in the theory [18], but the verification experiments were not carried out and the advantages of this process had not been presented.
From the above research results, machining the fir tree slots utilizing profiled electroplated grinding wheel is feasible. But little research achievement on the grinding technology and machining accuracy of FGH96 turbine disk slots is presented. So, utilizing the electroplated CBN-profiled grinding wheels and single-sided local profiled grinding process, the grinding experiments of FGH96 turbine disk slots are carried out. The influence of grinding process and grinding parameters on the profile accuracy of fir tree slots is studied, and the feasibility of this process is verified, which provides an effective process method for machining high-precision fir tree slots of FGH96 turbine disk.

Experimental material and workpiece
The experimental material is powder metallurgy superalloy FGH96, and the chemical composition is listed in Table 1. The mechanical properties of FGH96 at room temperature are shown in Table 2, and based material microstructure is shown in Fig 1. In general, the fir tree slot is straight, as shown in Fig. 2, and the symmetrical plane of the slot has a certain angle with the axis of the turbine disk. The turbine fir tree slots could be pre-machined by wire-EDM, as shown in Fig 3. The profile of slots was measured by a coordinate measuring machine, as shown in Fig. 4.
From Fig. 4, the profile error is between −0.02 mm and +0.03 mm. However, the quality of the machined surface is poor and the surface roughness is about Ra 3.2 μm. Meanwhile, the machined surface and microstructure of FGH96 fir tree slot machined by wire-EDM are shown in Fig. 5. There is an obvious recast layer which thickness is about 15 μm. If the recast layer not only could be removed by precision grinding but also the profile accuracy and surface quality of slots could be ensured, the precision grinding of FGH96 slots will be realized. Therefore, the FGH96 fir tree slots are pre-machined by wire-EDM firstly in order to remove most of the material and only about 0.1 mm is reserved for finishing grinding by the profiled grinding wheel.

Grinding experimental setup
The grinding speed is very important to the grinding efficiency and grinding performance of superabrasive grinding wheel. Because FGH96 is a typical difficult-to-cut material, the highspeed spindle is needed to meet the requirement of grinding linear speed. Therefore, a high-speed spindle which can get maximum rotating speed 60,000 rpm is attached to the machining center, as shown in Fig. 6. At the same time, Blasogrind HC 10 high-speed oil is used as coolant for all the grinding experiments. The outlet pressure of the coolant is 45 bar and the flow rate is 80 L/min.

Structure of profiled grinding wheel and profiled grinding process
According to the structural characteristics of the turbine slot, the structure of profiled electroplated CBN wheel is developed, as shown in Fig. 7. The maximum and minimum diameters of the profiled wheel are about 10.83 mm and 5.15 mm, respectively. At the same time, the diameter of the grinding wheel at any grinding point is less than the width of the fir tree slot, which can ensure that only one side of the grinding wheel is in contact with the profile of slot, as shown in Fig. 8, and that the coolant can fully lubricate and cool the grinding wheel and grinding area. The grinding performance of grinding wheel and grinding surface quality can be guaranteed stably.
Utilizing the above profiled electroplated CBN wheel, it is necessary to translate the grinding wheel for several times and allow the generatrix of the grinding wheel to overlap locally, so as to realize the profiled grinding of one-sided profile of turbine disk slot. Meanwhile, the grinding process of the whole slot tooth is shown in Fig. 9.

Selection of grinding wheel parameters and grinding parameters
FGH96 is a typical difficult-cut material; the grit size of grinding wheel has a direct influence on the machined surface quality and grinding efficiency. So, it is very important to choose the grit size of grinding wheel according to the machined surface quality. Grinding experiments are carried out on the FGH96 workpiece utilizing electroplated CBN grinding wheel with different grit sizes and the grinding parameters n = 48,000 rpm, a p = 0.002 mm, and v f = 100 mm/min. The experimental workpiece is shown in Fig. 10. The surface roughness is measured along the direction perpendicular to the grinding linear velocity by a TIME3220 roughness tester. The measurement results are shown in Fig. 11.
Under the same grinding conditions, when the grit size of grinding wheel increases from 240# to 400#, the surface roughness decreases from Ra 3.01 to Ra 0.72 μm obviously. When the grit size of grinding wheel increases to 700# continuously, the surface roughness still decreases to Ra 0.54 μm moderately. It can be known that the grit sizes have an obviously influence on the surface roughness. According to the machining requirements of turbine disk slot, the surface roughness should be better than Ra 0.8 μm. Meanwhile, the grit size of grinding wheel also has a great influence on the grinding efficiency and the wheel wear. From the above analysis comprehensively, 600# electroplated CBN-profiled grinding wheel is utilized for finish machining FGH96 turbine disk slots, which can make sure the surface roughness stably and meet the use requirements of Ra ≤ 0.8 μm.
On the other hand, the typical microstructure of machined surface and grinding wheels is examined by a scanning electron microscope (SEM), as shown in Fig. 12. The grinding grooves appear on the machined surface along the direction of grinding linear velocity, which caused by the scratch of the higher abrasive grits on the peripheral surface of wheel. The size of abrasive particles has a direct influence on the depth of the grinding grooves.

Experimental results and discussion
Using electroplated profiled CBN wheels, the grinding experiments of FGH96 fir tree slots pre-machined by wire-EDM are carried out, as shown in Fig. 13. The profile of slot is rough ground for 10 times using 400# electroplated profiled CBN wheel; the parameters are n = 48,000 rpm, a p = 0.005 mm, and v f = 200 mm/min. Then, it is fine ground for 5 times using 600# electroplated profiled CBN wheel and the parameters are n = 48,000 rpm, a p = 0.002 mm, and v f = 100 mm/min. The slots after finishing grinding are shown in Fig. 14.    The profile of slots is measured by a coordinate measuring machine. The results are shown in Fig. 15, and the profile error of slots is within ±0.012 mm, which is better than that machined by wire-EDM. Meanwhile, the profile accuracy of 50μm 50μm 50μm 50μm a b Fig. 12 Micromorphology of different wheel surfaces and machined surfaces. a 400# wheel surface and its machined surface. b 600# wheel surface and its machined surface.   At the same time, the surface quality of FGH96 slot is examined by SEM, and it can be seen that the microrecast layer on the surface machined by wire-EDM has been removed completely. The microstructure of the machined surface is evenly arranged and without any damage, as shown in Fig. 16 Meanwhile, the wear and the performance of the grinding wheel are the key factors to ensure the grinding accuracy of the slot. Therefore, it is necessary to detect the wear of grinding wheel after grinding, and the wear degree of grinding wheel surface is shown in Fig. 17.
From the above results, after four whole slots are ground completely, CBN grits on the surface of the profiled electroplated wheel have any wear, but no grits pullout appears obviously. The grits are evenly distributed on the wheel surface and have grinding capability, which can meet the requirements for precision grinding slots. Meanwhile, the grit wear at the groove of the profiled grinding wheel is more serious than that at the inclined plane and flange, because the grinding wheel at the groove has a low grinding speed which affects the grinding performance.

Conclusions
(1) For the FGH96 fir tree slots after rough machining by wire-EDM, single-sided local profiled grinding of slots is presented utilizing 600# electroplated CBN-profiled grinding wheel. In addition, under the conditions of experimental parameters, electroplated CBN-profiled grinding wheel has excellent grinding ability and no obvious wholesale grit pullout is observed, which can ensure the use requirements of FGH96 fir tree slots precision grinding.
(2) Compared with wire-EDM, recast layer appears on the surface of FGH96 fir tree slots machined by grinding, which can avoid the negative influencing of the recast layer on the fatigue performance of turbine disk. (3) Under the grinding parameters n = 48,000 rpm, v f = 100 mm/min, and a p = 0.002 mm, utilizing 600# electroplated CBN-profiled grinding wheel, the machined surface can meet the quality requirement of FGH96 turbine disk slots, the grinding surface roughness is less than Ra 0.8 μm, and the profile error is within ± 0.012 mm. So, an effective process method for the precise and low cost machining of FGH96 turbine disk slots is developed, which can promote the efficient development of turbine disk of new aero-engines.
Author contribution Xun Li: overall framework, experimental planning, draft writing of the manuscript. Bin Qin: grinding experiments operation, experimental result analyzation.