Hardness and Micro-structural Study of Inertia Friction Welding of Dissimilar High Temperature Alloys

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Hardness and Micro-structural Study of Inertia Friction Welding of Dissimilar High Temperature Alloys

Ravi Kumar M*1, Dr. Reddappa H N*2, M. Sreenivasa Reddy#3

*1Research scholar VTU-Belgaum, Faculty Dept. of ME RLJIT-Doddaballapur

*2Professor, Dept. of ME BIT-Bangalore

#3Professor, HOD, Dept. of ME RLJIT-Doddaballapur

Abstract the present study is on the development of inertia friction welding (IFW) of dissimilar high temperature alloys (Inconel 718 and Stainless Steel 304) to study the hardness and microstructural properties. Inertia Friction Welding is a solid state joining process which is more advantageous due to the low heat input, production efficiency, simple manufacturing process and environment friendly. Inertia friction welding process can be used to join rods of similar/dissimilar cross sections and materials without using any filler material, shielding gas and flux. The process parameters such as friction force, forging force, forging time, spindle speed and burn-off length are the major factors in determining the strength of the weld joints. The weld joints were tested for their hardness and microstructural properties.

Keywords: Inertia friction welding, Inconel 718, Stainless steel 304, Hardness, Microstructure.

  1. INTRODUCTION

    Inertia Friction Welding is a solid state joining process which is more advantageous due to the low heat input, production efficiency, simple manufacturing process and environment friendly. Inertia friction welding process shown in fig. 1 can also be used to weld different types of ferrous metals, non-ferrous metals and dissimilar metals that cannot be welded by conventional welding processes.

    Alloy 718 is a precipitation hardenable nickel-based alloy designed to display the exceptionally high yield, tensile and creep-rupture properties at temperatures up to 700°C. Alloy 718 has excellent weldability when compared to the nickel-based super alloys hardened by an aluminum and titanium. This alloy has been used for the jet engine and high-speed airframe parts such as buckets, wheels, spacers, and high temperature fasteners and bolts [1]. Stainless steel 304 is the standard "18/8" stainless steel; it is the most widely and versatile used stainless steel, available in a wider range of products, forms and finishes than any other. Grade 304 has outstanding forming and welding characteristics. The balanced austenitic structure of grade 304 enables it to be severely deep drawn without the in-between annealing, which has made this grade dominant in the manufacture of drawn stainless parts such as hollow-are, sinks and saucepans [2]. Super alloys are been joined

    to high strength low alloy steels in the gas turbine engine applications by means of mechanical fastening [2].

    When attempts were made to join these alloys by conventional fusion welding methods, the fusion zone of composition can become significantly enriched in Fe and C due to dilution effects, and this composition modification can significantly alter the solidification behavior and associated cracking tendency. The friction welding process produces a metallurgical bond through the frictional heating and the simultaneous deformation along an interface separating the two materials to be fixed as shown in the figure 1. Heat generated along the interface flows either axially away from the interface or radially along the interface. Since the joining is performed in a solid state, extensive migration of elements will not take place. In addition, as the materials are been subjected to hot working, the weld zone exhibits good grain size. The other advantages of solid state welding are that the welds are free from porosity, segregation and liquation cracking that are common in the conventional fusion welding.

    Figure.1: Inertia Friction Welding Process

  2. MATERIAL & EXPERIMENTAL DETAILS

    1. Materials

      Inconel 718 and Stainless Steel 304 materials are selected for the study, since they are high temperature alloys which are used in aerospace application. Table 1 shows the Chemical Composition of Inconel 718 and table 2 shows the Chemical Composition of Stainless Steel 304.

      TABLE 1

      CHEMICAL COMPOSITION OF INCONEL 718

      C. Selection of welding parameters

      Element

      Content (%)

      Ni (+Co)

      50 55

      Cr

      17 21

      Fe

      Bal

      Co

      1

      Mo

      2.8 – 3.3

      Nb (+Ta)

      4.75 – 5.5

      Ti

      0.65 – 1.15

      Al

      0.2 -0.8

      C

      0.08

      Mn

      0.35

      Si

      0.35

      B

      0.006

      Cu

      0.3

      Element

      Content (%)

      Ni (+Co)

      50 55

      Cr

      17 21

      Fe

      Bal

      Co

      1

      Mo

      2.8 – 3.3

      Nb (+Ta)

      4.75 – 5.5

      Ti

      0.65 – 1.15

      Al

      0.2 -0.8

      C

      0.08

      Mn

      0.35

      Si

      0.35

      B

      0.006

      Cu

      0.3

      • The welding procedure was studied [3].

      • Based on the study, spindle speed, forging force, friction force and burn-off length were selected as the varying process parameters [4].

      • Forging time of 3 seconds was kept constant for all the welding trials.

      • Varying process parameters are tabulated in the Table 4.

        TABLE 4

        Process Parameters

        Level 1

        Level 2

        Level 3

        Forging Force (T)

        2.5

        2.75

        3

        Friction Force (T)

        1

        1.25

        1.5

        Spindle speed (rpm)

        1250

        1500

        1750

        Burn-off length (mm)

        4

        5

        6

        Process Parameters

        Level 1

        Level 2

        Level 3

        Forging Force (T)

        2.5

        2.75

        3

        Friction Force (T)

        1

        1.25

        1.5

        Spindle speed (rpm)

        1250

        1500

        1750

        Burn-off length (mm)

        4

        5

        6

        LEVELS OF PROCESS PARAMETERS

        TABLE 2

        CHEMICAL COMPOSITION OF STAINLESS STEEL 304 [D]

        Element

        Content (%)

        Carbon

        0.048

        Silicon

        0.411

        Manganese

        1.323

        Phosphorous

        0.040

        Sulphur

        0.017

        Nickel

        /td>

        8.859

        Chromium

        18.95

    2. Inertia friction welding machine

    The development of inertia friction welding was carried out by using 3T machine as shown in the figure 2 and machine specifications are shown in Table 3.

    Figure.2: Inertia Friction Welding Machine

    TABLE 3

    FRICTION WELDING MACHINE SPECIFICATIONS [E]

    Manufacturer

    ETA Technology

    Max. Forge Force

    30 KN

    Max. diameter at weld

    10 mm

    Max. stem pin length

    150 mm

    Max. length of head pin

    220 mm

    1. Preparation of specimens

      Inconel 718 material was available in the form of rectangular bar (25 x 15 mm). This was machined to circular bar of 10 mm diameter by turning operation as shown in Figure 3. Stainless steel 304 (bright extruded material) was obtained in the required dimension of 10 mm diameter as shown in Figure 4.

      Figure.3: Inconel 718 rectangular bar machined to round bar

      Figure.4: Bright extruded bar of SS 304

      Friction welding was carried out and the weld produced is characterized by the narrow heat affected zone (HAZ), the presence of plastically deformed material around the weld area and absence of the fusion zone. The weld specimen is as shown in figure 5.

  3. RESULTS AND DISCUSSIONS

    1. Vickers hardness test results

      Hardness is the property of a material that enables to resist plastic deformation, usually by penetration. However, the term hardness may also be referring to resistance to bending, scratching, and abrasion or cutting [16].

  1. Features of flash

Figure.5: Weld specimen

The variation of flash feature with the process parameter such as forging time, forging force, spindle speed, friction force and burn-off are presented in Table 5.

Figure.6: Pictorial representation of flash formed in the welded specimen.

Where, A Diameter of inconel 718 specimen, mm.

B Diameter of SS 304 specimen, mm. C Flash height, mm (inconel 718)

D Flash height, mm (SS 304)

E Flash thickness, mm (inconel 718) F Flash thickness, mm (SS 304)

G Flash width, mm

TABLE 5

WELD JOINT FLASH GEOMETRY DETAILS

Specimen No.

A

B

C

D

E

F

G

1

10

10

15.28

16.42

2.11

4.02

6.13

2

10

10

14.87

16.32

3.06

4.39

7.45

3

10

10

14.55

16.38

3.35

4.99

8.34

4

10

10

15.78

17.28

3.88

4.57

8.45

5

10

10

14.76

16.28

3.09

4.00

7.09

6

10

10

14.79

15.85

2.81

4.41

7.22

7

10

10

15.34

17.22

3.88

4.5

8.38

8

10

10

15.13

16.68

3.27

4.81

8.08

9

10

10

14.62

15.32

2.24

4.13

6.37

Figure 6: Specimen subjected to Vickers Hardness test

TABLE 5:

VICKERS HARDNESS TEST RESULTS

Position No.

Region

Load (kgf)

Indenter

Hardness (HV-5)

1

SS 304

5

Diamond

361

2

HAZ of SS 304

5

Diamond

525

3

HAZ of Inconel 718

5

Diamond

487

4

Inconel 718

5

Diamond

329

5

Weld region

5

Diamond

274

Figure 7: Graph of Vickers hardness test results

Figure 6 shows the confirmatory test specimen subjected to Vickers hardness test at five different regions and corresponding hardness values are tabulated in the table 5 and shown in figure 7. It is observed that the hardness at the weld region is less when compared to that of HAZ and base metal.

  1. Microstructure analysis.

    Analysis of confirmatory test specimen was carried out to study the structure type through optical microscopy. The microstructure of two different materials is as been shown in figure 8 and 9. In the figure 10 the interface structure of two different weld materials can be observed.

    Micro Structure

    The microstructure consists of irregular austenite grains and etch pits at dispersed carbide particles.

    Figure 8: Microstructure of Inconel 718

    The microstructure consists of randomly

    Micro

    distributed, precipitated gama prime Ni3

    Structure

    (AlTi) and carbide particle in matrix of gama

    solid solution.

    Figure 9: Microstructure of Stainless Steel 304

    Conclusion

    In the above photo white portion belongs to SS 304 and dark portion belongs to Inconel 718.

    Figure 10: Friction welded joint region

    CONCLUSION

    The main aim was to develop friction welding process for joining of dissimilar high temperature alloys by varying the process parameters such as Spindle speed, forging force (upset force), friction force and burn- off length. The significant outcomes of the study are listed in this section.

    1. Vickers hardness of 274 HV-5 was obtained at the weld region of confirmatory specimen.

    2. Optical microscopy was performed on the confirmatory specimen and it can be observed that bonding between two work pieces is good.

ACKNOWLEDGEMENT

I sincerely thank to Dr. A. N. Nandakumar, Principal, RLJIT for his support. I express deep wholehearted gratitude to Prof. M. Sreenivasa Reddy, HOD Mechanical Dept., RLJIT. I wish to pay my heartfelt thanks to Mrs. Jayamma and Miss. Savitha S. R for their continues support.

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