Optimization of Laser Cladded IN625/SS316L Functionally Graded Material for Heat Exchanger Applications

Abstract

Engineering components operating in severe working environments require re- silient materials that can withstand degradation caused by corrosion, erosion- wear, and fatigue. This reduces failure problems in power plants that com- promise the integrity, effectiveness, and lifespan of essential heat exchanger components, such as boiler tubes. The Olkaria V power station in Kenya is one of the facilities that use shell and tube heat exchangers. These heat exchangers were investigated in the present study for failure and it was found that hydrogen embrittlement and sulfide stress cracking caused their prema- ture failure. A recommendation was reached that the frequency of corrosion failure in process industries can be reduced by redesigning tubes to conform to operating conditions and using hybrid materials as surface modifiers. Func- tionally graded materials (FGM) were utilized as advanced hybrid materials that can mitigate degradation by varying material composition. Addition- ally, laser cladding surface modification was employed to join stainless steel 316L and Inconel 625 dissimilar materials on a pure copper substrate to pro- duce quality coatings with enhanced quality characteristics. This attributes include a higher microhardness (MH), and aspect ratio (AR), whereas sur- face roughness (SR), and porosity (P) are minimized. Meanwhile, improper material selection and inappropriate control of the laser cladding process and material parameters affect the quality and performance of fabricated FGM coatings, accelerating failure during service. As such, the present study em- ployed a hybrid optimization of process parameters by combining the Taguchi grey relational analysis (GRA) and artificial neural network (ANN) method to improve output characteristics. Characterization was carried out to observe the microstructure using optical image microscopy and scanning electron mi- croscopy equipped with energy dispersive spectroscopy (SEM-EDS). Phaseidentification was investigated using x-ray diffraction analysis and stress dis- tribution with a residual stress analyzer. Mechanical tests were carried out using a microhardness tester for microhardness and surface roughness profiler for surface roughness. Findings from this study revealed that 600 W laser power, 700 mm/min scan speed, and 1.5 g/min of powder flow rate gave the best optimal process parameters exhibiting desirable material properties with a micro hardness of 257.41 HV, aspect ratio of 3.89 mm, surface roughness of 5.31 ➭m, and porosity of 0.01 %. When varied between 400 and 600 mm/min, the scan speed was found to be the most significant parameter that influences the qualities of the clad, while laser power was varied between 700 and 1000 W, and powder flow rate between 2 and 6 g/min. The outcome of the Taguchi- grey relational analysis showed that it is a powerful tool for optimizing the quality characteristics of FGMs when combined with the artificial neural net- work method because the experimental results were improved by 16.4 %. The Optical and SEM micrographs showed a microstructural transformation from equiaxed structure to columnar and dendritic structure due to steep temper- ature gradient. The FGM was also characterized by increased laves phases and γ-dendrite in the interdendritic area due to solid solution strengthening of IN625, resulting in higher micro hardness properties. Heat treatment was also carried out using water quenching process to alleviate the tensile residual stresses of +ve 8 Mpa induced during laser cladding, to compressive residual stresses of -ve 136 MP, because tensile residual stresses can aggravate stress cracking in boiler pipes. This study has successfully developed optimized Fe/Ni FGM on pure copper that has not been adequately explored and can be successfully used for coating boiler tubes.