Tensile Properties and Microstructural characteristics of Fusion Welded Dissimilar Joints of AISI410 MSS and Su-718 Nickel base

AlloyS.Avinasha*, S.Ravib, , V.Vaithiyanathanc V. Balasubramaniand

a P.G Student, b Professor, d Professor, c Research ScholarCenter for Materials Joining and Research (CEMAJOR),Department of manufacturing Engineering,Annamalai University, Annamalai Nagar - 608002.*Email: avinashsara94@gmail.com

A B S T R A C T

The materials like nickel based super alloys and martensitic stainless steel (MSS) alloys are preferred owing to their superior mechanical and corrosion

resistance. Hence, joining of these dissimilar metals is a challenging task due to dissimilarity in physical and chemical properties such as thermal

conductivity, heat capacity, thermal expansion coefficient and melting temperature. The use of high-energy-density welding processes such as LBW and

EBW produce full-penetration, single-pass autogenously welds, rather than multipass conventional arc welding, are desirable to minimize the above-said

difficulties. In this investigation, an attempt has been made to join AISI 410 martensitic stainless steel with Su718 nickel base alloy sheets of 2 mm

thickness by fusion welding process. The joints were fabricated by two process namely Gas-Tungsten Arc Welding (GTAW), Laser-Beam Welding

(LBW). Tensile properties were evaluated Macrostructure (bead geometries such as penetration, bead width) and microstructure analysis were carried out

using optical microscopy. Microhardness survey was carried out across the weld cross section. From this investigation, it is found that the joint fabricated

by laser welding exhibited superior tensile properties (strength and ductility) compared to other process. This is mainly due to the formation of full

penetration, completely melted and fine grained fusion zone. Moreover, the laser beam welded joints, exhibited narrow heat affected zone and lower

volume fraction of intermetallics at the interface.

KEYWORDS: Martensitic stainless steel Nickel base alloyTensile properties Microstructure Tensile propertiesLaser beam weldingTig welding

1.0 INTRODUCTIONThe AISI 410 martensitic stainless steel are seldom

welded because of its high hardenability and susceptibility to hydrogen induced cracking. However martensitic stainless steel with chromium and carbon content are resistant to various environment such as fresh water, crude oil, gasoline and alcohol and have good machinability.. Because of their good creep ,tensile and fatigue strength properties in combination with moderate corrosion resistance and low cost benefits these steels used in automotive sectors to manufacture gears, shafts, pumps, valve parts and fuel injector.The precipitation hardened nickel based super alloy Inconel 718, a high-temperature application material used in production of hot section components of moderate power generation turbines and aerospace engines due excellent corrosion resistance and high temperature application.. Because of its superior mechanical properties and oxidation resistance at elevated temperatures, Su 718 is particularly suitable for manufactured components in the high temperature regions of aero engines and gas turbines. The conventional arc welding process like Manual metal arc welding (MMAW), gas tungsten arc welding (GTAW) and gas metal arc welding are commonly used to join these material but the problems encountered during fusion welding is reduced mechanical properties. Formation of intermetallic,widerheat affected zone grain growth, loss of material by vaporization.To mitigate the above problems, high energy density process like laser beam andelectron beam welding are used

laser beam welding offers many advantages compared to fusion welding process such as high welding speed, higher stability, reduced distortion,narrow HAZ and finer grains in fusion zone.

[1] the suitable welding parameters are welding speed 200-500mm/min, pulse power 1.7-4.1kW were obtained hardness decreased in HAZ from 178HV to 158HV and increase in from 157HV to 183HV in fusion zone laser welded joint has shear tensile strength 80% of base metal.[2] effect of heat input from LWB on microstructural evolution of super alloy and four different heat input and full penetration was achieved in all welds experiment increasing heat input changed the resulting shape from wine glass to stem less glass with wider surface bead width [3] Welding performed maintaining identical mean power of 1.7kW welding rate increases from 1 to10 microstructure of welds remains same and macro porosity decreases from 1.1% to 0.6% when Welding rate increases, region of macro porosity are distributed randomly throughout welds and micro cracks are not apparent with welds[4] microstructural analysis of fusion zone and heat affected zone reveled that cracking accorded in HAZ and some cracks extend into fusion zone alpha precipitates was found to be major causes of grain boundary liquation and intergranular micro fissuring in HAZ.

* Corresponding author. Tel.: +91 9944200446E-mail address: bahavudeen.ahamed0@gmail.com

2 SOJOM 2018

[5] After the laser melting sulphide inclusion become spherical and size reduced from 2µm to 0.5µm diameter and the SEM investigation showed the total inclusion increased dramatically and calculated total volume of sulphur in stainless steel .while thee pitting potential was increased by over 200mv with inverse dependency on average inclusion size [6] A 1Kw laser source transmitted through 50µm delivery fiber employed to deposited over martensitic stainless steel and four condition were tested to reproduce different temperature fields in base material and the microstructural analysis of HAZ beneath cladding layer with microhardness measurement allowed thermal damage extension [7] Microhardness in fusion zone changes from cellular to columnar dendritic and equiaxed dendritic with increasing energy input and the local hardness reaches its peak in fusion zone to Base metal of outer shell and the inner shell peak hardness accord in HAZ and local softening relative to Fusion zone and HAZ visible at Fusion boundary.

A turbofan engine is the most modern variation of the basic gas turbine engine, there is a core engine, the fan and fan turbine are composed of many blades and are connected to an additional shaft. The BLISK - shaft assembly is generally made up of different materials/ alloys to increase the flexibility in design. The materials like nickel based super alloys and martensitic stainless steel (MSS) alloys are preferred owing to their superior mechanical and corrosion resistance.Sinc the nickel based super alloy and martensitic stainless steel has huge industrial application especially for high temperature application joining of these material is important and from previous literature survey welding of these material had not been welded.Hence in the present investigation an attempt has been made to study the mechanical properties, microstructural characterization of dissimilar joints of AISI410 martensitic stainless steel and Nickel based alloy Su-718 fusion (TIG and LBW) welded joints

2.EXPERIMENTAL WORKIn the present investigation the base metal used is AISI410 martensitic stainless steel and Su718 Inconel 2mm thick rolled plates were used. The base metal were cutted according to the required dimensions (150*150)mm by shear cutter and grinding for butt weld configuration was prepared to fabricate laser beam welded joints Vacuum spectrometer (ARL Model: 3460) was used to study the chemistry of the weld metal Basemetal. Sparks were ignited at various locations and their spectrum was analyzed for the estimation of respective alloying elements. The chemical composition of the Base metal and mechanical properties of base metal are tabulated in table 1

Table 1 Chemical Composition of Base Metals (in Wt %)

Table 1B Mechanical Properties of Base Metal

Material

Tensile Strength(MPa)

0.2 % Yield Strength (MPa)

Elongation (25 mm Gauge

Length) (%)

Micro hardness(HV0.5)

NBA(Su-718) 953 652 62 305

MSS(410) 467 271 34 220

The micro structure of the base metal and weld metal has been reviewed by proper polishing and etching has been done with Picral etchant and Marble’s Reagent to review the microstructure for 410 martensitic stainless steel and Su-718 Inconel respectively. The microstructures of the base metal were analysed using optical microscope and it exhibits martensite and ferrite faces and nickel based super alloy consist of austenitic grins with MC carbides along grain boundary.

Fig. 1 Optical micrograph of Base metals (a) MSS and (b) NBA

The plates were mechanically and chemically cleaned by acetone before welding to eliminate surface contamination. The welding conditions and optimized process parameters for the two Different processes are presented in Table 2. The parameters were optimize by means of trial and error method to obtain full penetration. The optimized condition and parameters are shown in the Table 2. The joints were fabricated by the optimize parameters

Table 2 Welding condition and parameters

The welded joints were sliced and then machined to the required dimensions for preparing tensile and impact specimens. American Society for Testing of Materials (ASTM) guidelines were followed for preparing the test specimens. Two different tensile specimens were prepared to evaluate the transverse tensile properties. The smooth (unnotched) tensile specimens were prepared to evaluate the yield strength, tensile strength and elongation. Notched specimens were prepared to evaluate the notch tensile strength and notch strength ratio (NSR) of the joints. Microhardness survey across the joints was carried out using Vickers microhardness tester.

3.0 RESULTS 3.1 Macrostructure

The macrographs shown in table 3 clearly reveal the difference in weld bead geometry of the two different fusion welding process. Joints fabricated by laser welding exhibits minimum Fusion Zone (FZ) area and minimum heat affected zone (HAZ) area compared to the joints fabricate by TIG welding. Joints fabricate by TIG

(a) (b)

Material C Si Mn P S Cr

AISI410

0.145 0.419 0.868 0.027 0.003 12.86

Su-718 0.022 0.052 0.04 0.001 0.01 19.02

Mo Cu Al Ti Fe Ni

3.30 0.012 0.56 0.08 17.4 52.7

Process Laser Power(W)

Voltage (v)

Main Current (amps)

Welding speed

(mm/min)

Heat input

(J/mm)LBW 2350 - - 2000 70.57

TIG - 9 90 70 694

3 SOJOM 2018

welding process shows larger fusion Zone area and wider weld bead.

Table 3 Macrostructure and bead geometry of two different welding processes

PROCESS CROSS SECTION MACROGRAPH

Width of the Bead

(mm)

DOP

(mm)

FAZ

(mm2)

With of HAZ (mm)

MSS NBA

TIG8.61

6 214.6

71.5 3

LBW1.33

72

2.019

0.780.13

6

3.2 Tensile properties

Fig 3 Load Displacement curves

Photograph of Tensile Specimen (TIG welding)

Photograph of Tensile Specimen (Laser welding) Fig 2 Photograph of Tensile Specimen (after testing)

The tensile test was conducted for the joints fabricate by Laser welding and TIG welding to establish the strength of the weldments under uniformly applied load. It was observed that the tensile strength of the dissimilar welded joints was higher than the lower strength parent metal, i.e., MSS. Hence, the fracture occurred in the MSS BM (far away from the FZ) in all the joints. The tensile properties of the parent metal and welded joints are presented in Table 4. The tensile strength of the parent metals was observed to be 467 MPa for MSS and 953 MPa for NBA. There are no appreciable variation tensile properties of the joints, with respect to its different fusion welding process. The elongation of the parent metals was observed to be 34% for MSS and 62% for NBA, where as the elongation for joint fabricated by laser power of 2350 W is 30%. This suggests that there is a 17% decrease in

elongation. Similarly, the elongation of joints fabricated by other process the elongation of TIG joints are 22% which are 35% lower than those of the parent metal. Out of two different fusion welded joints, the joints fabricated by laser power of 2350 W exhibited minimal change in the ductility values compared to the joints fabricate by TIG welding process The stress (load) -displacement curves of parent metal and welded joints are shown in Fig. 3.

Table 4 Transverse tensile properties of welded joints

Process

0.2% Yeild

strength (MPa)

Tensile strength (MPa)

Elongation in 25 mm

gauge length (%)

Notch strength

ratio

Failure Location

LBW 310 502 30 1.30 MSS-BM

TIG 253 467 22 1.02 MSS-BM

3.3 Microhardness

Fig. 4 Microhardness of different fusion welding process

The hardness profile of the welded joints has been shown in Fig.4 the average hardness of NBA and MSS are 305 and 220 HV, respectively. The average hardness of the fusion zone varies from 240 HV to 300 HV for both the TIG welding and laser welding process depending on indentation.. Hardness is found to be very high in HAZ of joints. The failure location of the welded joints is consistent with hardness distribution profile. The failure occurred in all the joints along the lowest hardness region

3.4 Microstructure

The optical micrographs of FZ of TIG and Laser weldments have been displayed in Table 4. The coarser columnar dendritic structure was observed in TIG weldment with a small amount of cellular dendrites (Fig. 5 (a). Fig. 5 (b) reveals the fine cellular structure and very fine cellular dendritic formed in Laser weldments. The narrow beam promotes constitutional super-

4 SOJOM 2018

cooling in reduced time which leads to a decrease in temperature gradient and resulted in fine cellular dendritic structure. No solidification cracking and under cut is visible

(a) TIG Welded Joints

(b) Laser Beam Welded Joints

Fig.5 Optical micrographs of various regions of welded joints

3.5 Fracture surface analysis

The fractured surfaces of the tensile tested specimen’s welded joints were analyzed using a scanning electron microscope. The fractographs of tensile specimens are displayed in Fig. 6. The modes of failure of the tensile tested welded joints are ductile with acceptable plastic deformation and are evident from the fracture location and fractured surface shown in the Fig. 6. No brittle cleavage fracture was found in any of the tensile tested fractographs.

(A) LASER BEAM WELDED

(B) TIG WELDEDFig. 6 Failure pattern and fractographs of tensile specimens

4. DISCUSSION4.1. Effect of Heat input on macrostructure

The variation in heat input change the size and shape of the weld in laser beam welding with minimum heat input shows nail head shape. In LBW process, the resultant bead structure is narrow due to low heat input (70J/mm). In TIG the bead structure is a wider one due to high heat input (694 J/mm) the increase in main current and voltage more amount of base metal is melted and slower cooling rate also leads to wider weld bead The wider arc column is also a reason for this wide FZ. 4.2. Effect of Heat input on tensile properties

5 SOJOM 2018

The weld metal is comparatively stronger, and the joint properties are controlled by weld metal chemical composition and microstructure. The strong carbide/nitride forming elements, like Nb, Ti, V, etc., have very limited solubility in ferrite and austenite, and normally the precipitates act as fine Dispersion of carbides, nitrides and/or carbo nitrides and contribute to strength due to precipitation hardening. Variation of ductility with microstructure and chemical composition is more complex. In general, all factors, except for grain size, which increase the strength would decrease the ductility. Also the ductility is severely affected by the presence of MnS inclusion in steels and varies with size, shape and volume fraction. The inclusions act as stress raisers, and the cracks easily initiate at the inclusion, either due to cracking of inclusions or decohession of inclusion and matrix. In the case of laser welding the heat input is very low, the material is deformation very low due to the low heat input and faster cooling resulting in a fine grain structure. The load displacement graph clearly indicates that the strength of laser beam welded specimen is more than that of base metal. The increased strength is attributed to the very fine grain microstructure consisting of cellular dendrite and very fine cellular denteritic region. Although the formation of dendrite increases the strength,

4.3. Effect of Heat input on hardness

The fusion zone in laser beam welding microstructure consists of very fine columnar dendrite an columnar dendrite, which could be the reason for higher hardness values compare to TIG . Also low heat input that leads fast cooling rate to room temperature and subsequent transformation to a completely fine columanar dendrite microstructure. Nb-rich phases might cause the higher hardness in weld zone. There is increase in hardness of the weld metal by traversing from 410 steel to Inconel 718 side. Hardness is higher at 410- HAZ of Laser beam welded joints because low heat input involved which mean formation of more martensite structure.

4.4. Effect of Heat input on microstructure

Laser welding process is characterized by high cooling rates compared with TIG , which influence several aspects of weld metal solidification The fusion boundary (FB) located between the 718 HAZ and the WZ.In all cases, epitaxial growth is observed, which typically formed in epitaxial solidification, Secondary phases are formed in the TIG welding process due to high heat input and joints fabricated by laser has lesser amount secondary phases due to low heat input 718 HAZ because of the presence of Nb in this alloy. These phases play an important role in corrosion resistance. Specially, secondary phases can cause a reduction in corrosion resistance of the WZ.

5. CONCLUSIONS

The present study confirmed that dissimilar welding of AISI 410 Martensitic stainless steel and SU-718 Nickel based alloy can be welded without any defects using TIG welding and Laser Beam Welding

1. The joints fabricated using by Laser welding showed superior Yield strength (302 MPa) and (502 MPa)

2. The fusion zone of the Laser Beam welded joints consist of finer grains due to the low heat input

3. The hardness is higher in the Laser Beam Welded joints and failure accord in the lowest hardness distributed region

Reference

[1]Messler Robert Jr W. Joining of metals, alloys, and intermetallics. Joi Mater Struct 2004:535–82.

[2]Hui-Chi Chen , Andrew J. Pinkerton , Lin Li Fibre laser welding of dissimilar alloys of Ti-6Al-4V and Inconel 718 for aerospace applications Int J Adv Manuf Technol (2011) 52:977–987

[3] Sumit K Sharma, Frasanth K Agarwal And Dutta Majumdar (2017)Studies On Electron Beam Welded Inconel 718 Similar Joints Journal Of Material Processing And Manufacturing Vol 7 Pg 654-659.

[4]H. Shah Hosseini, M. Shamanian, A. Kermanpur(2016) Microstructural And Weldability Analysis Of Inconel617/AISI 310stainless Steel Dissimilar Welds International Journal Of Pressure Vessels And Piping Vol 144 Pg 18-24

[5] T. Ramkumara,M. Selvakumarb, P. Narayanasamyc, A. Ayisha Begamd, P. Mathavand, A. Arun Rajd(2017);Studies On The Structural Property, Mechanical Relationships And Corrosion Behaviour Of Inconel 718 And SS 316L Dissimilar Joints By TIG Welding Without Using Activated Flux Journal Of Manufacturing Processes Vol 30 Pg 290-298

[6]K. Devendranath Ramkumar , Sidharth Dev, Vimal Saxena, Ayush Choudhary, N. Arivazhagan, S. Narayanan (2015) Effect Of Flux Addition On The Microstructure And Tensile Strength Ofdissimilar Weldments Involving Inconel 718 And AISI 416 Journal Of Materials And Design Vol 87 Pg 663-674

[7] Agalin.M, T.Vengatashwaran, D.Sivakumar (2014) Effect Of Heat Input On Microstructural And Mechanical Properties Of Inconel 718EB Welds Journal Of Material Science Vol 5 Pg 656-662

[8]S.H. Baghjari, S.A.A. Akbari Mousavi (2013)Pulsed Nd:YAG Laser Welding Parameters And Subsequent Post WeldHeat Treatment On Microstructure And Hardness Of AISI 420 Stainless Steel Journal Of Materials And Design Vol 43 Pg 1-9

[9]Jose´ Roberto Berretta, Wagner De Rossi, Maurı´Cio David Martins Das Nevesc,Ivan Alves De Almeidab, Nilson Dias Vieira Juniorb Pulsed Nd:YAG Laser Welding Of AISI 304 To AISI 420 Stainless Steels Journal Of Optics And Lasers In Engineering Vol 45 Pg960–966

[10]F. Caiazzo ⇑, V. Alfieri, F. Cardaropoli, V. Sergi (2017) Investigation On Edge Joints Of Inconel 625 Sheets Processed With Laser Welding Journal Of Optics And Laser Technology Vol 93 Pg 180–186.

[11]A.T. Egbewandeb, R.A. Bucksona, O.A. Ojoa(2010)Analysis Of Laser Beam Weldability Of Inconel 738 SuperAlloy In Journal Of Material Charactersition Vol 6 1 Pg 5 6 9 – 5 7 4

6 SOJOM 2018