PSI - Issue 14

Anigani Sudarshan Reddy et al. / Procedia Structural Integrity 14 (2019) 449–466 Author name / Structural Integrity Procedia 00 (2018) 000–000

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CoCrMo on Cast FSX414 and DMLS SS316L on forged X20Cr13). FSX414 is a cobalt based superalloy and X20Cr13 is a martensitic steel. The substrates FSX414 and X20Cr13 were procured from GE Power, Bangalore. The hybrids were chosen to represent a combination of DMLS material on a cast and a forged substrate, respectively. In both cases of hybrid builds, the substrate and the DMLS build materials were dissimilar in terms of chemical composition. All the samples were fabricated using an EOS M280 machine, at INTECH, DMLS, Pvt Ltd. Bangalore, India by DMLS process. The nominal composition of the all the materials used in this study is listed in the Table 2 were taken from EOS material data sheets for monolithic DMLS samples, Kaustubh Krishna Bawane et al. (2018) for FSX414 and Springler Handbook of condensed matter for X20Cr13. The range of process parameters used for building the different specimens are listed in Table 3. The DMLS processing parameters for all the alloys were optimized to ensure a nearly dense (>99.5%) sample, with each alloy having a specific combination of parameters from amongst this range. Specimens of dimensions 25mm (length) X 8mm (width) X 25mm (height) were built for all the five monolithic samples as shown in the Figure 1(a). The hybrid (DMLS SS316L on forged X20Cr13 was built by depositing a height of 25.4 mm on the substrate dimensions 25.4mm (height) X 5mm (width) and 10mm (length) as shown in the Figure 1(b) and the hybrid of (DMLS CoCrMo on Cast FSX414) was built by welding of DMLS CoCrMo on Cast FSX414 using Nozzaloy weld filler wire (  1mm thick) via Tungsten inert gas welding (TIG) technique, as shown in the schematic in Figure 1(c). The combination of hybrids were chosen to represent direct build up using the DMLS process on top of a conventional substrates and by means of welding a DMLS alloy with a conventionally manufactured substrate, respectively. A suitable fixture assembly was fabricated to position the hybrid parts in the DMLS machine such that there was no distortion of the DMLS build when deposited on top of the substrate. For each alloy, the DMLS build plate included 3 cylindrical specimens in order to evaluate regular ASTM tensile specimen geometry, for comparison with small scale test specimens, as shown in the Figure 1(d). An average of three tests specimens was taken to represent the mechanical properties for both small scale as well as regular specimens. The alloys were heat treated as per the conditions listed in Table 4, respectively. The heat treatment of DMLS SS316L was carried out only in hybrid of (DMLS SS316L on X20Cr13). Heat treatments for IN718, CoCrMo, Ti6Al4V and hybrid of (DMLS CoCrMo on Cast FSX414) were carried out in a vacuum furnace (Delta power controls, using an argon gas quenching facility, at Materials engineering department, IISc, Bangalore) while the steels, Maraging steel and hybrid of (DMLS SS316L on X20Cr13) were heat treated in air, using a Nabertherm furnace. The heat treatments cycles represented stress relieving heat treatment for (CoCrMo, SS316L, and Ti6Al4V) and precipitation hardening in (IN718 and Maraging steel), respectively. The starting powder characterization was done using FEI Quanta200 (Thermo Fisher EDS) Scanning electron microscopy (SEM) and particle size distribution (PSD) by Malvern 3000 for all the materials. Optical microscopy was carried out using a Zeiss Axiocam with Axiovision software to measure the porosity and the as printed and heat treated microstructures. The porosity was quantified using an Image J software. The microstructures were examined after etching in both the as printed and heat treated condition. Table 5 is a summary of the etchants used for the DMLS alloys. Microhardness measurements were taken using Future Tech FM 800 Tester after applying a load of 300 gms and 10 sec dwell time. The small scale tensile specimens were machined out of the monolithic blocks as well as the hybrid samples using EDM wire cutting (as shown in Figure 2a and 2b, respectively). All the specimens used in this study were oriented with the tensile axis along the DMLS build direction. Figure 2c, shows the dimensions of the small scale tensile test specimen having a 6 mm gauge length, 2 mm width, 0.5 mm thickness and 20 mm sample length. The regular specimens were machined as per the dimensions of ASTM E8/E8M as shown in the Figure 3d. All the small scale tests were carried out using an Instron 5987, with 0.006 mm/sec strain rate at room temperature, at the Indian Institute of Science, Bangalore. All the regular specimens were tested using Fuel Instruments and Engineers (FIE) UTM at Geological and Metallurgical Services Pvt Ltd, Bangalore. An average of three tests were recorded for each sample and test condition. Scanning electron microscopy (SEM) was carried out using a FEI Quanta200 to analyse the fractographs of the samples both in as printed and heat treated conditions.