An Experimental Investigation and Optimization of Microwave Hybrid Heating for Joining of Inconel-625 Alloy

Abstract

Inconel-625 is a nickel-chromium alloy strengthened by additions of carbon, chromium, molybdenum and niobium known for its high strength, excellent weldability, and outstanding corrosion resistance. The alloy finds widespread use in strategic industries due to its exceptional mechanical properties, excellent weldability and resistance to high temperature corrosion on extended exposure to harsh environments. These alloys are preferred for, high temperature as well as aqueous corrosion resistance, high strength at elevated temperatures, toughness and ductility at cryogenic temperatures, definite electrical properties and many other property dependent applications. As present-day industries demand for a technology that is environment friendly and equally competent with the existing processes at reduced costs, the development of novel processes to weld Inconel-625 has to be addressed. Processing of metallic materials through microwave energy is a recently developed technology that is drawing the attention of manufacturing industries. Microwave processed components exhibit similar characteristics at reduced processing times and energy savings to those produced using conventional techniques. In the present work, development of Inconel-625 welded joints using microwave hybrid heating technique (MHH) has been achieved successfully in a low cost 2.45GHz, 900W home microwave oven. Nickel based powder EWAC was employed as interface filler material. Microwave induced joints were examined by uniaxial tensile testing, 3-point bend test, microhardness test, XRD analysis and microstructural studies. Furthermore, an investigation on the effect of process parameters on tensile strength and flexural strength of Inconel-625 plates welded by MHH was conducted. Experiments were planned according to Taguchi L16 orthogonal array (OA) by considering three factors; separator, susceptor and filler powder particle size. Ultimate tensile strength (UTS) and flexural strength (FS) of the specimens welded at 600W and 900W were chosen as response characteristics.The Taguchi method has been used to ascertain the individual effect of separator type, susceptor type and filler powder size on ultimate tensile strength and flexural strength of the welded joints was analyzed. In addition the combination of process parameters that yield the best UTS and FS were determined. Analysis of variance (ANOVA) was performed to determine the contribution of process parameters on the output responses using MINITAB statistical software. The fineness of the interface filler powder was found to be the most significant parameter influencing the tensile and flexural strength of the joints. The strengths of microwave induced joints increased when finer powder (APS 50µm) was used. Graphite separator exhibited better heating characteristics when compared to glasswool regardless of power used for processing. Phase analysis through XRD revealed the formation of chromium carbide in the interface region which in turn contributed to increase the hardness of the joint in the interface region. In addition, the carbides of Niobium and molybdenum and other intermetallics were also observed in the joint region. EDS analysis showed that the amount of Nb, Ti and Mo content was considerably lower in secondary phase of joints developed at 600W which would consequently result in lower amount of Laves volume fraction compared to their counterparts produced at 900W. Further, the specimens produced at 600W also exhibited lower porosity values and higher joint hardness which is attributed to a fine grained structure. All the specimens fractured in the joint zone due to the presence of a hard carbide phase at the interface and fractography study revealed mixed mode fracture. Multi response optimization of the microwave welding process showed that Experiment No. 9 yielded optimum combination of process parameters obtained through GRA which was observed to be separator with combination of graphite and flux followed by coal susceptor and 50µm (APS) filler powder. Confirmation experiment along with subsequent characterization of the microwave induced joints has been carried out to validate the experimental results.