CHAPTER-4 EXPERIMENTAL DETAILS. 4.1 SELECTION OF MATERIAL FOR CC GTAW & PC GTAW OF 90/10 & 70/30 Cu-Ni ALLOY WELDS

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1 CHAPTER-4 EXPERIMENTAL DETAILS 4.1 SELECTION OF MATERIAL FOR CC GTAW & PC GTAW OF 90/10 & 70/30 Cu-Ni ALLOY WELDS Hot rolled plates of 90/10 and 70/30 Cu-Ni alloys of 5 mm thickness were selected as test material. The reason is all the shipbuilding industries use 90/10 and 70/30 Cu-Ni alloy plates or pipes of size that ranges from 2-10 mm thickness. Mostly 5 mm thickness is preferred. Hence, 5 mm thickness plates of 90/10 & 70/30 Cu-Ni alloys were taken (Fig: 4.1) and various welding techniques such as GTAW and LBW were carried out for improvement in mechanical properties (Hardness & Tensile Strength). (a) (b) Fig: 4.1 Base Metals of size 1000 x 500 x 5 mm (a) 90/10 Cu-Ni alloy plate (b) 70/30 Cu-Ni alloy plate. 90/10 and 70/30 Cu-Ni alloys of 5 mm thickness were machined to the required dimensions (110 x 100 mm). The chemical composition of the base alloy (90/10 and 70/30 Cu-Ni) and ERCu-Ni 70/30 filler wire of 2 mm diameter are given in Table 4.1,4.2 & 4.3 respectively. ERCu-Ni 70/30 filler wire of 2 mm diameter was selected for CCGTAW & PCGTAW, because filler wire is best suitable for joining both 90/10 & 70/30 Cu-Ni alloy plates. 30

2 Table 4.1 Chemical composition for BM of 90/10 Cu-Ni alloy Alloy Ni Fe Mn Pb Zn S P C others 90/10 Cu-Ni Rest Table 4.2 Chemical composition for BM of 70/30 Cu-Ni alloy Alloy 70/30 Cu-Ni Ni Fe Mn Pb Zn P Si S Ti others Rest Table 4.3 Chemical composition of ERCuNi (70/30) filler material (2 mm dia) Cu Cu Alloy Ni Fe Mn Pb Zn P Si S Ti Oth -ers Cu ER CuNi 70/ Rest A 60 0 sample V- butt joint configurations was prepared to fabricate CCGTAW & PCGTAW joints as shown in Fig: 4.2. The alternate current (AC) GTAW machine used is shown in Fig: 4.3. The initial joint configuration was obtained by securing the plates in position using mechanical clamps. Schematic diagram of arrangement of welding torch, filler wire (manually feeding) and argon gas is shown in Fig: 4.4. The weld was made perpendicular to the plate rolling direction is shown in Fig: 4.5. Single pass welding procedure was used to fabricate the joints. Prior to welding, the base material coupons and filler wires were wire brushed and thoroughly cleaned with acetone. Argon gas was selected for CCGTAW & PCGTAW, because argon is the best shielding gas and it protects the molten weld pool from the atmosphere, otherwise, the molten metal reacts with gases in the atmosphere and produces porosity (bubbles) in the weld bead greatly reducing weld strength. 31

3 Fig: 4.2 Single V butt welds Fig: 4.3 Gas tungsten arc welding machine with automatic welding speed equipment Two types of current modes were used: Continuous Current (CC) and Pulsed Current (PC). Optimum welding parameters based on previous works other than Cu-Ni alloys such as aluminum alloys, Titanium alloys and Magnesium alloys were used. Specific details of CC GTAW and PC GTAW are given in Table 4.4 and 4.5. Table 4.4 Welding parameters used for CCGTAW Welding parameter Selection Arc voltage 18 V Welding current (A) 105A Welding speed (mm/sec) 2.5 mm/sec Filler wire ERCuNi (70/30 ) Shielding gas Argon 32

4 Table 4.5 Welding parameters & their levels used for PCGTAW Welding parameters Levels Peak current (Ip) 200A to 230 A Background current (Ib) 95 A to 125A Speed 140 to 170 mm/min Voltage 18 V Pulse frequency 0.5 Hz to 5 Hz Filler wire ERCuNi (70/30 ) Shielding gas Argon Fig: 4.4 Welding torch arrangement with automatic welding speed & filler wire (manually feeding) Fig: 4.5 A schematic diagram of the GTAW joint bead made perpendicular to the plate rolling direction & tensile test specimen cut from weld. 33

5 Extensive preliminary welding trials at various combinations of welding parameters were carried out to identify optimum welding parameters in each of these two welding techniques, CC GTAW & PC GTAW. The samples were polished by emery papers and disc cloth to remove scratches. Polished surfaces were etched with Glacial acetic acid and Nitric acid (1:1). The microstructures images were recorded with Image analyzer attached to the metallurgical microscope. These alloy welds were tested for mechanical properties. The following tests were performed a) Microhardness by Vicker s microhardness b) Bend test by computer-controlled servo-hydraulic universal testing machine c) Tensile strength by computerised Universal Testing Machine (UTM)) d) Pitting corrosion by dynamic polarisation testing e) Microstructure characterisation by optical microscope, Scanning Electron microscope (SEM) and Energy dispersive spectroscopy (EDS). 4.2 EXPERIMENTAL DETAILS OF 3.5 KW CO 2 LASER BEAM WELDING OF 90/10 & 70/30 Cu-Ni ALLOY WELDS. In addition to the above, LASER Beam Welding (LBW) was also been used for conducting the experiments at various welding speeds (1 m/min, 1.5 m/min,2 m/min, & 2.5 m/min) on 90/10 and 70/30 Cu-Ni alloy welds for further improvement in mechanical properties (tensile strength and hardness),microstructures characterization and pitting corrosion of the weld joints. The hot rolled plates of 90/10 and 70/30 Cu-Ni alloys of 5 mm thickness were machined to the required dimensions (110 x 100 mm) and no filler wire was used. A fast axial flow type CO2 laser resonator generating nearly single mode beam with a 34

6 maximum continuous power of 3kW was used. Laser beam focused a spot diameter of 700 μm using parabolic mirror system with 200mm focal length. LBW machine, machine particulars and schematic layout of C0 2 LASER beam welding are shown in Fig: 4.6, 4.7 & 4.8 respectively. Experimental setup is shown in Fig 4.9. Pre-weld surface cleaning of alloy plates was performed using stainless steel wire brush and nitric acid solution to remove or minimize the surface oxide layer. The specimens were prepared as square butt joints with machined surfaces and were firmly held using fixture to prevent distortion. A pair of 100 mm x 55 mm x 5 mm similar plates of 90/10 & 70/30 Cu-Ni alloys plates were individually taken and mounted in butt joint geometry by computer numerical control table (accuracy within a few micrometers) with the parting line of the joint along z axis. Autogeneous, single pass square butt welds were made in the transverse direction relative to the rolling direction of 90/10 & 70/30 Cu-Ni alloy samples (Fig:4.10). Positive defocusing indicated that the focal point was above the top surface of the work piece, and negative defocusing indicated that the focal point was below the top surface. Shielding was done using argon gas supplied by a 4mm diameter coaxial nozzle with a flow rate of 30 lit/min. Fig: 4.6 Laser welding machine 35

7 Fig: 4.7 Laser welding machine particulars Fig: 4.8 Schematic layout of C0 2 LASER beam welding 36

8 Fig: 4.9 Experiment set up for Laser welding Fig: 4.10 Square butt joint Laser welds were carried out at four different laser welding speeds (1.0 m/min, 1.5 m/min, 2.0 m/min and 2.5 m/min). Optimum welding parameters based on previous works other than Cu-Ni alloys such as aluminum alloys, Titanium alloys and Magnesium alloys. Specific details of LBW are given in Table 4.6. Table 4.6 Laser welding Parameters Welding parameters Levels Laser welding speed 1.0 m / min,1.5 m/ min,2.0 m/min & 2.5 m/min (m/min) Laser power,p (KW) 3.5 KW Shielding gas with flow Argon with flow rate 30 L /min rate(l/min) 37

9 4.3 EVALUATION OF MECHANICAL PROPERTIES CCGTAW, PCGTAW & LBW resulted in significant microstructural evolution within and around FZ and HAZ. This lead to substantial change in mechanical properties after welding. The following tests were conducted to analyse the mechanical properties (Hardness, bend test & tensile test) of the CCGTAW, PCGTAW & LBW of 90/10 & 70/30 Cu-Ni alloy welds MICROHARDNESS TEST Hardness test was carried out using LECO s LV700 Vickers hardness testing machine as shown in Fig: A load of 2 kgf was applied across the weld cross section for 20 sec dwell time. Minimum three readings from each sample on weld centre were taken and an average of three readings was taken as the required hardness value. Study of Hardness along the transverse direction of the weld was conducted with hardness measurements at regular intervals of 2 mm from the centerline of the weld to either side. For measuring hardness, the specimens were metallographically polished with 340 grit paper until the oxides were removed from the surface of the specimen. Fig: 4.11 LECO s LV700 Vickers hardness testing machine 38

10 4.3.2 BEND TEST In order to assess the bond integrity and bond strength, three-point bend test was conducted on CC GTAW, PCGTAW, & LBW joints. The specimen of dimension 100 mm length x 30mm width x 5 mm thk used for bend and is schematically shown in Fig The test was conducted on a computer-controlled servo-hydraulic universal testing machine. The bend test set-up is shown in Fig Fig: 4.12 Bend test specimens. C = 2r + 3t ± t/2 Fig Bend test Bend tests namely face bend and root bend were conducted on these specimens and face bend test specimen is placed in the jig with its face down. The former is depressed until the specimen becomes U-shaped or bend at angle of 180º in the die i.e face is on the tension side. Conversely, the root bend test specimen is placed in the jig with its root down. The former is depressed until the specimen becomes U-shaped or bend at angle of 180º in the die i.e face is on the compression side. This is how the welded plates are usually tested. The idea is that when the test is carried out with welds face or root on both the tension side & compression side.the results must show no cracks to be acceptable TENSILE TEST Specimens of standard ASTM-E8 were taken from BM and weld coupons were cut using EDM wire cutting. Tensile tests were conducted on these cut specimens. The 39

11 dimensions of the ASTM-E8 standard test specimen is shown in Fig Tensile tests were carried out in a computerised Universal Testing Machine (UTM)(Fig:4.15) of capacity 100 kn for finding weld strength. Tensile elongations were measured manually after arranging the fractured pieces together. For each experiment a minimum of three specimens were tested and the average values were calculated. The fractures of the specimens were studied using SEM. Prior to SEM examination, fracture surfaces were ultrasonically cleaned with acetone. Fusion zone 6 Fig: 4.14 Tensile specimen used as per ASTM- E8 Computerised Universal Testing Machine (UTM) and Tensile fracture of the specimen are shown in Fig & Fig: 4.15 Universal Testing Machine (UTM) for Tensile test 40

12 Fig: 4.16 Failure of the sample during tensile testing 4.4 MICROSTRUCTURE CHARACTERISATION STEREO MICROSCOPY Detailed weld macro structures and total crack length measurement examination was carried out using Stereo Microscopy (Leica, DMLM) on top surface of the samples of fusion zones and heat-affected zones OPTICAL MICROSCOPY Detailed weld micro structural examination was carried out using optical microscope (Fig: 4.17) on top surface of the samples of base metal, fusion zones and heat-affected zones. The samples were polished by emery papers and disc cloth to remove scratches. The Polished surfaces were then etched with Glacial acetic acid and Nitric acid (1:1). The microstructures images were recorded with Image analyzer attached to the metallurgical microscope. Microstructure studies on the welds were made with CCGTAW, PCGTAW&LBW SCANNING ELECTRON MICROSCOPY (SEM) AND ENERGY DISPERSIVE SPECTROSCOPY (EDS) JOEL JSM 6610 SEM (Fig: 4.18) micrographs were taken with secondary electron image mode. Energy dispersive spectroscopy (EDS) analysis was carried out to quantify the elements of Ni, Mn, Fe, C, Zn, Pb and Cu using ZAF software. SEM- EDS analysis was used to find out the chemical composition of fusion zones and base metal. 41

13 Minimum of three readings in each sample were taken and their average was taken as the chemical composition. Scanning Electron Microscopy (SEM) with back scattered electron mode was applied on the top surface welds to study the eutectic morphology. SEM micrographs are also useful in identifying the various phases at higher magnifications which are not detected through optical micrography. Welding samples of FZ and Tensile fractures were studied by Scanning Electron Microscopy (SEM) to reveal the fractured features. Fig: 4.17 Optical Microscope (OM) 4.5 PITTING CORROSION Fig: 4.18 JOEL JSM 6610 SEM A Potentiostat software based Gill AC electrochemical system (Fig.4.19) was used to make potentiodynamic polarisation tests to study the pitting corrosion behavior of the base metal and welds. Saturated calomel electrode (SCE) and carbon electrode were used as reference electrode and auxiliary electrode respectively. All experiments were conducted in aerated 3.5%NaCl solution of PH adjusted to 10 [Aida Varea et.al (2012)]. The potential scan was carried out at mv/s with an initial potential of V (OC, open circuit) SCE to the final pitting potential. The specimens of 10 mm x 10 mm were prepared as per the metallographic standard as shown in Fig: The exposure area for these experiments was 1cm 2. The potential at which current increased drastically was ( or less negative potentials) were considered to have better pitting corrosion resistance. The corrosion rate was calculated by polarizing the specimen anodically and cathodically and by extrapolating the tafel regions of anodic and cathodic curves to the corrosion potential. 42

14 Fig: 4.19 Basic Electrochemical System for Dynamic polarisation Fig: 4.20 Dimensions of specimen for corrosion test 43