Journal of Failure Analysis and Prevention

, Volume 17, Issue 4, pp 788–795 | Cite as

Application of Direct Metal Deposition Process for Failure Prevention of Oil Pump Gear Shaft in an Aero Engine

  • Srinivasakiran Tumuluri
  • P. Murugeshan
  • R. K. Mishra
  • V. V. Subrahmanyam
Technical Article---Peer-Reviewed
  • 135 Downloads

Abstract

Direct metal deposition process has been adopted for repair of oil pump gear shaft of an aero engine. The process is first optimized for the specific application of oil pump gear shaft material on a specimen and then the actual part was applied with deposition. The deposition on the gear shaft is validated through stringent quality checks and functionality check of the oil pump during bench test and engine test. The successful adoption of the deposition process is able to prevent failures which would otherwise cause catastrophic failure of the aero engine. Oil pumps are now withdrawn from service at a predetermined time prior to its scheduled inspection and overhaul for application of metal deposition.

Keywords

Defect analysis Wear resistance Wear 

Introduction

Oil pump is an important line replaceable unit (LRU) of an aero gas turbine engine. It works under pressure and supplies oil to various parts that need lubrication. Irrespective of the design philosophy, it circulates oil through heat exchangers and scavenge pumps and returns the oil to the oil tank. The elements of the oil pump largely depend on the type of the engine it is intended for and the heat load to be transported. The two most common type of oil pumps are the gear and gerotor, with the gear type being the most commonly used in aero engines. Any malfunction of the oil pump or failure of the drive will have a detrimental effect on the rotating parts of the engine. Cases of gear failure have seen to rise the bearing temperatures or seizure of rotor shafts in aero engines [1, 2, 3, 4].

The oil pump in the present study is a gear type and is mounted on the engine deriving the drive from the gear train of the engine accessory gearbox. This pump gear is supported by plain bearings. Cases of high oil temperature warning or bearing/shaft seizure have led to premature withdrawal of engines or accessories [4, 5]. During investigation, it is observed that the bearing diameter get worn out due to service wear. Wear of gear wheel shaft has also been noticed during repair and overhaul (ROH) which cannot be restored by chrome plating due to thickness constraint, as chrome plating is not be effective beyond 76 µm. thickness. To overcome this limitation of hard chrome plating, a new cutting edge process “direct metal deposition (DMD)” has been adopted in the present case to regain the gear shaft dimension. DMD is a process of building metal layer by layer by high intensity laser beam with controlled parameters [6, 7].

This rapid prototyping process has reached the stage of rapid manufacturing via the DMD technique. The DMD process is capable of producing three-dimensional components from many of the commercial alloys of choice. The DMD process is drawing considerable contemporary interest due to its capability to deliver “Art to Part.” DMD has reduced the lead time for a concept to product by eliminating several intermediate steps and preventing failure of critical parts in aero engine. The most attractive feature of the process is that not only it can produce functional parts but also it can be interfaced with the homogenization design method, heterogeneous solid model and computer-aided design software to produce “Designed Material” with desired properties generally not observed in nature [8].

The present paper highlights how the DMD has been optimized for a specific application of oil pump gear shaft and is able to prevent failures which would otherwise cause catastrophic failure of the aero engine. Oil pumps are withdrawn from service at a predetermined time prior to its scheduled inspection and overhaul. The gear shaft is repaired through DMD and cleared for service so that in-service failure of such shafts could be prevented.

Oil Pump Configuration

Oil pump gear shaft of the subject aero engine is shown in Fig. 1, and composition of gear shaft material is shown in Table 1. During usage, the shaft gets worn out on the supporting bearing areas on both sides of the gear as shown in Fig. 1. In order to prevent in-service failure as well as to reclaim the parts, the state-of-the art technology, i.e., direct metal deposition (DMD) has been adopted [6].
Fig. 1

Oil pump gear shaft

Table 1

Composition of oil pump gear shaft

Element

Ni

C

Cr

Mo

Mn

Si

S

P

Fe

% Age

3.2–3.75

0.12–0.16

0.8–1.1

0.1–0.25

0.4–0.55

0.1–0.35

0.45

0.45

Balance

Direct Metal Deposition

DMD is an additive manufacturing process that uses a high-power laser to fabricate fully dense metal parts by melting metal powders fed through a nozzle. The raw materials in powder form for DMD process are H13 steel, H20 steel, Inconel-625, bronze, nickel, etc. In the present case, H13 steel material has been considered for deposit on pump gear shaft after analyzing the material composition [9, 10].

The requirements of DMD are that there should not be any distortion in the component thereby maintaining dimensional and geometrical accuracies, the heat-affected zone should be minimal, and there should be good interfacial bonding between the substrate and the deposition. Further the bonding should be without any porosity and cracks.

As direct metal deposition is a laser-based thermal process, laser energy, laser scan speed and powder flow rate play an important role in the quality of deposition. Achieving good metallurgical bond between deposition and substrate, control of heat-affected zone (HAZ), control of porosity and control of distortion are some of the challenges of DMD that need high skill and experience during its application [9].

Optimization of DMD Process

Optimization of process parameters plays an important role on the quality of deposition. The following methodology has been adopted in the present part for the application of DMD [7, 8, 11].
  1. a.

    Pre-inspection of shaft for geometrical accuracy.

     
  2. b.

    Optimization of DMD process parameters—laser power, laser scan speed and powder flow rate on cylindrical shaft of same diameter and equivalent material of original pump gear shaft.

     
  3. c.

    Wire electrical discharge machining of deposited cylindrical shaft for microstructure and microhardness studies of depositions.

     
  4. d.

    Characterization of depositions—microstructure and microhardness.

     
  5. e.

    Deposition on pump gear shaft by DMD using optimized process parameters.

     

Pre-inspection

The run-out of the diameter of the pump gear shaft on which the deposition has to be made was measured with a dial indicator by holding the shaft between centers as shown in Fig. 2. A high resolution of 1 μm dial indicator was used to measure the run-out and the values are presented in Table 2. Three readings were measured at bearing areas A and B as shown in the figure. These measurements help to find out the distortions in the shaft after deposition. Initial experimentation was carried out on EN36 cylindrical steel shaft as shown in Fig. 3, with same diameter and having equivalent composition of pump gear shaft material thereby simulating the deposition on pump gear shaft and saving the original part from destructive testing. The cylindrical shaft was held between centers in a custom-made servomotor-controlled rotary attachment as shown in Fig. 4.
Fig. 2

Run-out measurement in pump gear shaft using dial indicators

Table 2

Run-out of pump gear shaft

Sl. no.

Run-out value (µm)

A

B

1

9

8

2

8

8

3

9

9

Fig. 3

Details of EN36 cylindrical steel shaft used for optimization of process parameters

Fig. 4

Cylindrical steel shaft held in between centers in servomotor-controlled rotary attachment

The attachment has the provision for programming the rotational speed as per the requirement. The rotational speed can be varied from 1 to 3000 rpm. The programmed rotational speed determines the laser scan speed during the deposition process. Deposition of H13 steel was carried out by optimizing the DMD process parameters—laser power, laser scan speed and powder flow rate. Eleven sets of process parameters as shown in Table 3 were adopted for experimentation, and 11 depositions were made as shown in Fig. 5 with a deposition width of 3 mm and thickness of 1 mm.

Table 3

Process parameters for optimization

Deposition no.

Laser power (W)

Rotational speed (rpm)

Laser scan speed (mm/min)

Powder flow rate (g/min)

Nozzle standoff distance (mm)

Nozzle tilt angle (°)

1

400

6

415

1.8

10

20

2

200

12

830

3

10

20

3

300

12

830

3

10

20

4

400

12

830

3

10

20

5

400

12

830

3.6

10

20

6

400

12

830

4.2

10

20

7

400

12

830

4.8

10

20

8

400

12

830

5.4

10

20

9

400

12

830

6

10

20

10

400

18

1245

6

10

20

11

200

18

1245

6

10

20

Fig. 5

Depositions of H13 steel on cylindrical steel shaft

Machining of deposited cylindrical shaft was done on wire electrical discharge machine for microstructure and microhardness studies. Figures 6 and 7 show the photograph of the wire electrical discharge machine and wire-EDM-cut deposited cylindrical shaft, respectively.
Fig. 6

Wire electrical discharge machine

Fig. 7

Wire-EDM-cut deposited cylindrical shaft for microstructure and microhardness studies

Characterization of Depositions

Metallographic specimens for all the 11 depositions were prepared and polished for microstructure and microhardness studies as shown in Fig. 8. The material used for preparation of mold was acrylic polymer-based resin. The grinding and polishing of specimens were carried out using SiC abrasive grinding papers of grit size 60, 240, 400, 600, 800 and 1200 grit, followed by polishing with alumina of 0.3 µm grain size on grinder. Microstructure studies of depositions were carried out using ×50 optical microscope. The microstructural analysis was carried out using image analyzer software of microscope to find the “heat-affected zone,” “depth of penetration of deposition,” “deposition height,” presence of “cracks” and “porosity.”
Fig. 8

Metallographic specimen

Results and Discussion

Microstructure

Figure 9 shows the optical micrograph of deposition, interface and substrate for depositions 1 to 11 as mentioned in Table 2, depicting heat-affected zone, depth of penetration and deposition height. It is evident from the microstructure studies that deposition no. 9 has sound deposition without porosity and cracks.
Fig. 9

Optical micrograph of depositions

Volume Deposition Rate and Energy Consumption Rate

The volume deposition rate and energy consumption rate of depositions 1 to 11 are shown in Figs. 10 and 11, respectively. It can be seen that deposition no. 9 has the maximum volume deposition rate with minimum energy consumption.
Fig. 10

Volume deposition rate of depositions

Fig. 11

Energy consumption of depositions

Microhardness

Microhardness testing of the specimens at and around the interface was carried out, and the indentation was measured using image analyzer software. Microhardness was measured at seven different sections—at the interface and at distances of 100, 200 and 300 µm from the interface toward deposition and substrate, using a load of 300 g for test duration of 13 s. At each section, three readings were taken and the average of the three readings was taken for plotting the graph. Figure 12 shows the microhardness of depositions 1 to 11. It is evident from the figure that deposition no. 9 is having the highest microhardness among all depositions.
Fig. 12

Microhardness of depositions

The variation of microhardness of deposition case no. 9 with crack-free dense deposition from deposition side to substrate side at interval of 100 µm across interface is shown in Fig. 13. It is evident from the figure that the transition of microhardness from deposition to substrate across the interface is gradual, which is an indication of good interfacial bond between deposition and substrate. The microhardness at interface is 455 HV. The transition of microhardness toward deposition side is from 455 to 578 HV and toward substrate side is from 455 to 228 HV.
Fig. 13

Variation of microhardness from substrate to deposition across interface of deposition no. 9

Conclusively, microstructure studies shows that deposition no. 9 with laser power of 400 W, laser scan speed of 830 mm/min and powder flow rate of 6 g/min gives sound deposition with maximum deposition rate, minimum energy consumption and maximum microhardness without porosity and cracks.

It is evident from the above figure that the transition of microhardness from deposition to substrate across the interface is gradual, which is an indication of good interfacial bond between deposition and substrate. The microhardness at interface is 455HV. The transition of microhardness toward deposition side is from 455 to 578 HV and toward substrate side is from 455 to 228 HV.

Deposition on Pump Gear Shaft

From the parameters such as microstructural, volumetric deposition rate, energy consumption and microhardness studies on the specimen, it is found that the case 9 has sound deposition quality with higher volumetric deposition rate and higher microhardness without porosity and cracks. Using these optimal process parameters, deposition was carried out on affected oil pump gear shafts. The gear shaft after deposition in shown in Fig. 14. The oil pump fitted with the gear shaft repaired through DMD has undergone various qualification phases. The gear shaft was first passes through stringent quality checks and then was fitted to the oil pump. The oil pump was tested in the rig bench for establishing various flow parameters. The oil pump was then subjected to endurance test on the aero engine in the engine ground test bench. Oil circuit parameters and bearing temperatures along with engine behavior were continuously monitored during the engine endurance test. Post-endurance test, teardown examination of the oil pump and oil circuit elements were carried out. Based on the satisfactory engine performance during endurance test and posttest teardown examination, the DMD process for gear shaft was established and clearance was accorded for the oil pump for service use. The current decreasing trend of failure rate and positive feedback of operators on gear shafts show that DMD has successfully helped to prevent premature failures.
Fig. 14

Gear shaft after deposition of steel using DMD process on worn out areas

Conclusions

Pump gear shaft is an important part of oil pump in an aero engine. The direct metal deposition technology adopted for failure prevention as well as reclamation of such shaft will help the engine in achieve operation. The following conclusions can be from the present shortly.
  • DMD process optimization needs to be carried out in terms of laser power, laser scan speed and powder flow rate.

  • Characterization of depositions for microstructure and microhardness showed that deposition with laser power of 400 W, laser scan speed of 830 mm/min and powder flow rate of 6 g/min at nozzle standoff distance of 10 mm yielded the best deposition quality with maximum volumetric deposition rate and minimum energy consumption without any cracks and porosity.

  • DMD process is found useful in primarily an effective wear-resistant surface for failure prevention as well as in reclaiming aero engine parts.

Notes

Acknowledgments

The authors are very grateful to the General Manger, Engine Division, Hindustan Aeronautics Limited, Bangalore for his kind permission for publishing this paper. The authors also appreciate and acknowledge the Director and Scientists of Central Manufacturing Technology Institute, Bangalore, and Dr. C. K. Srinivasa, Head of Department, Ultra Precision Engineering Department, A. R. Vinod, Scientist of Additive Manufacturing Technology and M. A. Manjunath, Scientist of Nano Manufacturing Technology Center in particular for their contribution and support without which the present work would not have been completed. The authors are also very thankful to the Center for Nano and Material Sciences, Jain University and Regional Center for Military Airworthiness for their guidance and support during work.

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Copyright information

© ASM International 2017

Authors and Affiliations

  • Srinivasakiran Tumuluri
    • 1
  • P. Murugeshan
    • 1
  • R. K. Mishra
    • 2
  • V. V. Subrahmanyam
    • 3
  1. 1.Engine DivisionHindustan Aeronautics LimitedBangaloreIndia
  2. 2.Regional Center for Military AirworthinessBangaloreIndia
  3. 3.Center for Nano and Material SciencesJain UniversityBangaloreIndia

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