microstructural evaluation and tensile characterization of electron

International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 3, March 2018, pp. 519–528, Article ID: IJMET_09_03_054 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=3 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication

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MICROSTRUCTURAL EVALUATION AND TENSILE CHARACTERIZATION OF ELECTRON BEAM WELDED DISSIMILAR METAL JOINTS OF COPPER AND STAINLESS STEEL 304 R. Ajithraj Assistant Professor and Head, Department of Aeronautical Engineering M. Dev Anand Professor and Director Research, Department of Mechanical Engineering, Noorul Islam Centre for Higher Education, Kumaracoil, Kanyakumari District, Tamilnadu, India ABSTRACT Joining of dissimilar metals is the key area of attention for researchers since it is a complex process due to the difference in the physical and metallurgical properties of both metals. A weld coupled with high quality is the need of aircraft industry. This paper describes about the dissimilar metal joints of Copper- Stainless Steel 304 using Electron Beam Welding for semi Cryogenic engine applications. The Electron Beam welded dissimilar metal joint is also compared with the weld made of TIG welding. The main advantages of EBW in the case of dissimilar metal joints are lack of fusion, lack of penetration and no heat loss which in turn improves the weld quality. The weld has been carried out at BATL, Trivandrum. Various trials have been carried out with Copper-Stainless Steel 304 and finally this dissimilar combination has been welded. The microstructure of both EBW and TIG welded samples are compared. Keywords: Electron Beam Welding; TIG; Copper-Stainless Steel 304; Dissimilar Metal Joints Cite this Article: R. Ajithraj and M. Dev Anand, Microstructural Evaluation and Tensile Characterization of Electron Beam Welded Dissimilar Metal Joints of Copper and Stainless Steel 304, International Journal of Mechanical Engineering and Technology, 9(3), 2018, pp. 519–528. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=3

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Microstructural Evaluation and Tensile Characterization of Electron Beam Welded Dissimilar Metal Joints of Copper and Stainless Steel 304

1. INTRODUCTION Dissimilar metal joints are the key problem that aerospace industry is facing today because of the differences in physical, chemical and metallurgical properties of both metals. Chethan Roy et al., (2014) have studied the dissimilar metal joints of commercially pure copper and AISI 304. They stated that by joining Stainless Steel with Copper which has a good thermal conductivity, the heat dissipation can be improved and prevents the formation of sigma phase. S.K. Albert et al., (2014) studied the microstructure and toughness of dissimilar joints between 1.0%wt. W RAFM steel and AISI 316L (N) SS using EBW process. They reported that fast cooling rate does not provide sufficient time for mixing of both the fused base metals in the fusion zone. As a result, com-positional and micro structural heterogeneities exist in the fusion zone, which in turn results in difference in properties. Sun R and Karppi (1996) made an investigation and reported that there is still a significant need of new combinations. They recognized and reported that the use of the EBW process to join dissimilar metals is still in the growing stage. Rajkumar M.P. et al., (2016) studied the tensile strength in the laser welded dissimilar joints of Stainless Steel and Copper. They optimized the process parameters such as welding speed, laser power and pulse duration to improve the weld strength. This same combination has been accomplished with Electron Beam Welding and the test results have been identified. Gupta R.K. et al., (2017) studied the Micro hardness, mechanical properties and fracture toughness of similar and dissimilar metals of Fe-31Ni-5Co alloy and Co-20Cr-15W-10Ni. They found that in the case of dissimilar metal joints the failure takes place in the base metal which is of less strength. V. Paradiso et al., (2017) studied the dissimilar metal joints of Magnesium and Aluminium alloys by Friction Stir Welding. The microstructure and mechanical characterization has been analysed. The formation of an inter-metallic compound was investigated in this study. CH Muralimohan et al., (2014) studied the microstructure and mechanical properties of Aluminium and Copper joints by friction welding. They identified that the tensile fracture occurs in the aluminium side. Even though many studies have been carried out in this area, only few researchers have focused on dissimilar metal joints using electron beam welding. Electron Beam welding and TIG welding of Copper and Stainless Steel has been carried out in this research. The weldment is subjected to micro structural examination and tensile test and the results have been reported.

2. INPUT PARAMETER QUALIFICATION OF EBW PROCESS Copper and Stainless Steel 304 plates of size 100mm X 60mm X 6mm are joined by Electron Beam Welding to make a welded coupon of size 100mm X 120mm X 6mm with different input parameters like Beam current, Focus Current, Speed and Work distance. M. Gokul Ananth et al. (2013) reported that beam current has the maximum influence in determining the depth of penetration. Several trials have been carried out to finalize the input parameters.

2.1. Selected Input Parameters for EBW Beam Current Beam Voltage Speed Work Distance

: : : :

50mA 60 KV 40mm/s 260mm


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R. Ajithraj and M. Dev Anand

2.2. Selected Input Parameters for TIG Welding Current Filler

: :

100 A Stainless Steel

3. PHYSICAL PROPERTIES OF SELECTED METALS The significant physical properties responsible for melting of Copper and Stainless Steel 304 are as shown in Table 1. Table 1 Physical Properties of Copper and Stainless Steel 304 Metals

Density (Kg/m3)

Melting Point (K)

Specific Heat (J/Kg K)

Thermal Conductivity (W/m K)

Copper SS 304

8940 7850

1356 1723

376.812 530

385 16.2

4. SPECIMEN PREPARATION FOR WELDING Pure copper and Stainless Steel 304 plates have been selected as the dissimilar metals for this study. This metal combination is used for the semi cryogenic engines. The plates are machined for the required size using conventional lathe has been shown in Figure 1.

SS 304

Copper

Figure 1 Machined SS 304 and Copper plates for Welding

The specimen is subjected to cleaning before welding to remove soils, oxides and foreign substances. The process sequence followed in pre weld cleaning of this dissimilar materials have been described in a flow chart as shown in Figure 2.

Figure 2 Process Sequence of Pre Weld Cleaning


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4.1. Degreasing Degreasing removes the surface impurities such as oil, sand etc. Clean the Copper and Stainless Steel 304 plates with cotton soaked with Trichloro ethylene.

4.2. Acid Pickling Prepare a solution with 4% Hydro Fluric acid, 40% Nitric acid and 56% DI water. Dip both the metals in this solution for 10 to 15 seconds. The oxide films present in the metals are removed.

4.3. Rinsing Now rinse the acid pickled metals with DI water to remove the acid present in the metals.

4.4. Drying Dry the metals with hot air. These cleaned metals should be handled only with gloves and stored for welding.

5. TIG WELDING OF COPPER & SS304 Trials have been made to weld Copper and SS 304 using TIG welding. The current value is adjusted in each trial to find the melting of both copper and Stainless Steel. After a series of trials, at a current value of 130 Ampere stainless steel found to be melted. At this value of current the heat generated is sufficient to melt both the metals. Tungsten rod is used to melt the metals since its melting point is much high when compared with Copper and Stainless Steel. SS304 rod is used as the filler material for this weld. Figure 3 and Figure 4 shows the coupons with and without filler material made with TIG welding.

Figure 3 TIG Welding without Filler

Figure 4 TIG Welding with Filler

6. EBW OF COPPER & SS304 The cleaned metals are now subjected to Electron Beam Welding with the selected input parameters. Square butt joint has been selected for this welding. Four operations such as clean, tack, seal and cosmetic has been carried out. High quality weld is achieved with Zero gap joint. Therefore great care must be taken to ensure that the fit up gap should not exceed 0.05 mm. Beam alignment is also an important consideration in the case of Electron Beam


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Welding. Great attention has to be taken to align the beam exactly on the joint. The specimen welded with EBW has been shown in Figure 5.

Figure 5 Electron Beam Welded Specimen

7. POST WELD CLEANING The welded specimen is now subjected to electrolytic etching and the welded joint is inspected visually before further testing. A schematic diagram of Electrolytic etching is shown in the Figure 6.

Figure 6 Schematic Diagram of Electrolytic Etching

10% H2SO4 is used as the electrolytic solution. The welded specimen is considered as anode and connected to an electric circuit of 6V for 5 to 8 minutes at 17 oc. Metal film starts erode from the specimen and finally after etching the surface will be of matt finish. This etched specimen should be handled with gloves for all further processing and should be stored in polythene cover for further testing.


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8. HEAT CONDUCTION IN MELTING OR SOLIDIFICATION DURING EBW During EBW, the high velocity of electrons hits the metals to be welded. The metal melts and then solidifies. During this process heat transfer occurs by a phase change in the conducting medium due to the absorption or discharge of heat in the active zone. During melting or solidification of metals a moving interface exists such as a function of time which separates the two regions of different properties. At this interface heat is absorbed or released and this determines the solidification or melting of the metals. Peter Petrov et al., (1998) reported that the main causes for the non-stationary nature of the heat source are connected with the processes of dissipation of the electron beam in evaporated metal. The factors such as latent heat of fusion, thermal conductivity, thermal diffusitivity, specific heats and density are considered. Initially the heat is removed and the molten metal starts solidification. The substance fills the region x>0 initially. At time τ, the surface separating the liquid and solid phase is at X(τ). Heat conduction occurs from the melted metal through solid phase to the free surface. At the interface the system releases latent heat of fusion.

9. RESULTS AND DISCUSSIONS 9.1. Visual Examination Both the TIG welded and Electron Beam Welded samples are visually examined first. It was observed that a crack has been formed towards the length of the weld in the case of TIG welding in both the specimen with and without filler. It is shown in figure 7 and figure 8. This may be due to the formation of intermittent compounds during welding. Furthermore there may be formation of oxides since the welding is performed in atmospheric conditions.

Figure 8 Crack in the TIG Welded specimen without Filler

Figure 7 Crack in the TIG Welded Specimen without Filler

In the case of Electron Beam Welding a bead of high quality was observed in the weldment. Both the specimens are subjected to microscopic examination for further study.

9.2. Microstructure of Welded Joints The electrolytic etched specimen is subjected to a series of steps to make it more polish and free of scratches. The specimen has been cut using water jet cutting machine. This specimen


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is grinded at the cross section of the weldment. Using sand papers from fine grit numbers to extra coarse grit the weldment is polished at its cross section. Grit size of 100, 240, 600, 800 and 1000 has been used one by one. Now use selvyt cloth and wipe the specimen with great care. Now use diamond paste of 6, 3 and 0.25 microns. Diamond paste of 6 micron will remove all the working scratches on both the metals, 3 micron will give final finish and finally 0.25 micron will give the finest finish of metallographic and optical quality. Soke the selvyt cloth with these diamond pastes one by one and rub the specimen till a mirror finish has been achieved. Great attention should be taken that the polished side should not be touched before the examination in microscope. The weldment of Copper and SS 304 using EBW is shown in Figure 9.

Figure 9 Microstructure of EBW Weldment

Figure 10 Microstructures Showing the Porosity and Cracks in Weldment


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In the Electron Beam welded specimen the weld was found to be both from Copper and Stainless Steel 304 but the formation of flash is more in copper side. This is due to the low melting point and low strength of Copper when compared with Stainless Steel. The microstructural evaluation of the weldment shown in Figure 9 and Figure 10 shows a plastic zone and a heat affected zone at the interface. The depth of penetration of the weld is found to be satisfactory. The porosity is also found to be very minimal. Also the weld is found to be very thin. When examined at the weld there exist few cracks at the weldment. When examined further it is identified that the crack propagates only near to the copper side. This is due to the improper melting and mixing of copper.

Figure 11 Microstructure of TIG Weldment

The microstructure of TIG welding proves that the crack identified by visual inspection to be true. There exists a large gap at the centre of the weld. This might be due to the formation of some oxides during the weld.

9.3. Tensile Testing The joint efficiency of the Electron Beam Welded joint of Copper/SS 304 has been evaluated using tensile testing ASME SEC IX-17 method. The cut specimen for tensile testing is shown in Figure 12.

Figure 12 Specimen for Tensile Testing (Before) Figure 13 Specimen for Tensile Testing (After)


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Figure 14 Graph of Stroke Vs Force

Fmax : 8.34KN UTS : 242.75MPa It was found that the tensile fracture occurs at the weld in all the trails. The failed specimen after testing is shown in Figure 13 and Stroke Vs Force shown in Figure 14. This may be due to the presence of little porosity in the weld. An ultimate tensile strength of 242.75MPa has been recorded and the tensile fracture occurs at 8.34KN. It was found that the ultimate tensile strength of the weld is moreover same as that of the copper and therefore the weld can be treated as a high strength weld.

10. CONCLUSION In this study the microstructure of Copper and SS 304 metal joint using EBW and TIG welding is identified and discussed. Tensile characterization has also been done for the EBW joint. The following conclusions were made in this study: 1. Copper and Stainless Steel 304 has been successfully jointed using Electron Beam Welding with the selected input parameters. 2. TIG welding is not suitable for this metal combination due to the formation of cracks immediately after the weld. 3. The pre weld cleaning and post weld cleaning shows considerable remarks during the microstructure examination. 4. An ultimate tensile strength of 242.75 MPa has been recorded for the weld which is moreover same as that of copper base metal and the tensile fracture for the weld occurs at 8.34 KN. 5. The microstructure shows small pores and cracks in the weld at copper side which shows improper melting and mixing of metal where as fine weld is observed at stainless steel side. 6. The input parameters are responsible for the improper mixing and melting of the base metals. By optimizing the input parameters of EBW fine weld can be obtained without any cracks and pores.


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