Study of Weld Quality Characteristics of Inconel 625 Sheets at Different Modes of Current in Micro Plasma Arc Welding Process

Inconel 625 is a Nickel alloy which was used in fabrication of high temperature resistance and corrosion resistance components, such as metal bellows used in expansion joints in aircraft, aerospace and petroleum industry. The paper focuses on studying the effect of continuous current and pulsed current in welding Inconel 625 sheets using Micro Plasma Arc Welding (MPAW) process. Welding was carried out on 0.25 mm thick Inconel 625 sheets using continuous current mode and pulsed current mode separately keeping all other welding parameters constant. Weld quality characteristics like bead profile, micro structure, hardness and tensile properties are investigated and it is found that the usage of pulsed current leads to better weld quality characteristics when compared to continuous current mode.


INTRODUCTION
Inconel 625 is extremely versatile austenitic nickel based super alloys with excellent strength and good ductility at high temperature.Typical applications include aero-engine hot section components, miscellaneous hardware, tooling and liquid rocket components involving cryogenic temperature.Inconel 625 can be joined using variety of welding methods, including Gas Tungsten Arc Welding, Plasma Arc Welding, Laser Beam Welding and Electron Beam Welding.Of these methods, low current PAW (Micro PAW) has attracted particular attention and has been used extensively for the fabrication of metal bellows, diaphragms which require high strength and toughness.PAW is conveniently carried out using one of two different current modes, namely a continuous current (CC) mode or a pulsed current (PC) mode.
Pulsed current MPAW involves cycling the welding current at selected regular frequency.The maximum current is selected to give adequate penetration and bead contour, while the minimum is set at a level sufficient to maintain a stable arc [1,2] .This permits arc energy to be used effectively to fuse a spot of controlled dimensions in a short time producing the weld as a series of overlapping nuggets.By contrast, in constant current welding, the heat required to melt the base material is supplied only during the peak current pulses allowing the heat to dissipate into the base material leading to narrower Heat Affected Zone (HAZ).Advantages include improved bead contours, greater tolerance to heat sink variations, lower heat input requirements, reduced residual stresses and distortion, refinement of fusion zone microstructure and reduced width of HAZ.Pulsed MPAW process parameters are depicted in Figure 1.

EXPERIMENTATION
The chemical composition and tensile properties of Inconel 625 sheets are presented in Tables 1 and 2. These sheets are available in the form of rolls from which the required sizes of 50 mm x 150 mm are sheared using shearing machine (Figure 2) and the pieces are cleaned using Ultrasonic cleaner before welding, to avoid any strains of oil and grease (Figure 3).The selected pair of sheets is fixed in the welding fixture and care has been taken to see that there is no gap between the two edges to be joined.Copper sinks are fixed at the bottom of the welding fixture to minimise the weld distortion and extreme care has been taken for proper butting of sheets without any gap.Welding is carried out at a speed of 260 mm/minute.Square butt joint is selected as the thickness is very small and edge preparation is not required for sheets of thickness less than 1mm (As per 2007 ASME boiler & pressure vessel code).Details about weld joint dimensions are shown in Figure 4. Argon gas with 99.99% purity is used as a shielding gas and to prevent absorption of oxygen and nitrogen from the atmosphere.The same gas is also used as purging gas for fast cooling of weld.The welding has been carried out under the welding conditions mentioned in Table 3.There are many influential process parameters which effect the weld quality characteristics of pulsed current MPAW process like peak current, back current, pulse rate, pulse width, flow rate of shielding gas, flow rate of purging gas, flow rate of plasma gas, welding speed etc. From the earlier works [3,4,5,6] carried out on pulsed current MPAW it was understood that the peak current, base current, pulse rate and pulse width are the dominating parameters which effect the weld quality characteristics.The values of process 22 parameters used in this study are the optimal values obtained from our earlier papers [7,8,9,10,11,12,13,14] .The weld process parameters chosen in the present study and their values are presented in Table 4.  Measurement of Weld Quality Characteristics The weld quality characteristics like weld bead, microstructure, grain size, hardness and tensile properties are studied.

Measurement of weld bead
Three metallurgical samples are cut from each joint leaving the edges of defective portion of the welded length.Defective length of weld is identified visually and also by conducting dye pentrant and X-ray tests and mounted using Bakelite.Sample preparation and mounting is done as per ASTM E 3-1 standard.The transverse face of the samples are surface grounded using 120 grit size belt with the help of belt grinder and polished sequentially using grade 1/0 (245 mesh size), grade 2/0( 425 mesh size) and  The procedure which was discussed in section 3.1 is followed by varying the etching time to reveal the microstructure as per ASTM E407.Micrographs are taken using metallurgical microscope (Make: Carl Zeiss, Model: Axiovert 40MAT) at 100X magnification.
The micrographs of HAZ and weld fusion zone are shown in Figures 7 and 8.  From the SEM iamges it is very clear that the grains in pulsed current mode are smaller than continuous current mode.The smaller size in pulsed current may be due to grain refinement taking place in the fusion zone.

Measurement of hardness
The hardness of the weld fusion zone of the welded samples are measured using Vickers's micro hardness testing machine (Make: METSUZAWA CO LTD, JAPAN, Model: MMT-X7) by applying a load of 0.5 Kg as per ASTM E384.Readings were taken at an interval of 0.3 mm across the weld in both fusion zone and HAZ.The variation of hardness across the weld is represented in Figure 11 and location of hardness measuring points is shown in Figure 12.The tensile properties of continuous and pulsed current mode are presented in Table 5.It is understood that pulsed current specimen failed at Heat Affected Zone (HAZ), where as continuous current specimen failed at Fusion zone.

CONCLUSIONS
Inconel 625 sheets of 0.25 mm thick are joint using pulsed and continuous current mode of MPAW process.From the weld macrographs it is observed that circular uniform ripples are seen in case of pulsed current, which indicates uniform melting of base metal.In case of continuous current mode the weld bead is wider and in irregular fashion.Finer grains are observed in weld fusion zone in pulsed current mode, where as in continuous mode coarse grains are present.The finer grains in the pulsed current mode are due to faster cooling rates of weld molten pool.Because of grain refinement, the hardness and tensile properties of pulsed current mode welding are better than continuous current mode.It is clearly understood that pulsed current mode gives better weld quality characteristics compared with continuous current.

Figure 1 :
Figure 1: Pulsed MPAW process parameters Where IP = Pulse current, Ib = Base current, TP = Peak current duration, Tb = Base current duration There are four independent parameters that influence the process are peak current, base current, pulse and pulse width.The main objective of the present work is to study the weld quality characteristics of continuous current and pulsed current MPAW process.

Figure 4 :
Figure 4: Dimensions of welded joint in mm grade 3/0 (515 mesh size) sand paper.The specimens are further polished using aluminum oxide, diamond paste and velvet cloth in a disc polishing machine.The polished specimens are etched by using Aqua Regia solution to reveal the macrostructure as per ASTM E407.Macrographs are taken using metallurgical microscope (Make: Carl Zeiss, Model: Axiovert 40MAT) at 100X magnification.It is clearly observed that in case of pulsed current uniform circular ripples are seen on the weld surface, where as in continuous current mode the surface is clear without any ripples.The macrographs for continuous current and pulsed current is shown in Figures 5 and6.

Figure 7
Figure 7 Microstructure for continuous current Figure 8 Microstructure for pulsed current

Figure 9 :
Figure 9: SEM image of continuous current Figure 10: SEM iamge of pulsed current

Figure 11 :Figure 12 :
Figure 11: Variation of hardness across the welded joint

Table 2 :
Tensile properties of Inconel 625 sheets

Table 4 :
Important weld parameters

Table 5 :
Tensile propertiesFrom Table5it is clear that the ultimate tensile strength of pulsed current is 798 MPa, where the ultimate tensile strength of continuous current is 784 MPa.It is understood that weld joint strength of pulsed current is better than continuous current.