PSI - Issue 42

Litton Bhandari et al. / Procedia Structural Integrity 42 (2022) 529–536 Bhandari et al. / Structural Integrity Procedia 00 (2019) 000–000

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selective laser melting process. The heat treatment was carried under high vacuum at a heating rate of 10°C / min and cooled using furnace cooling.

Table 2. Selective laser melting process parameters Laser Type

Laser power Laser speed Hatch spacing Layer thickness Scanning angle

Ytterbium fiber laser

220 W 660 mm / sec

90 µ m

60 µ m

67°

2.2. Characterization techniques

The cube sample was polished using sandpaper of di ff erent grit sizes and cloth polishing was done using colloidal silica of 0.02 µ m in order to obtain mirror finish. The sample was then etched using Kroll’s reagent (95 ml H 2 O + 2.5 ml HNO 3 + 1 ml HF) for 20 seconds in order to reveal the microstructure. Microstructural and texture analysis was done using Leica optical microscope and EBSD equipped with Zeiss field emission Scanning electron microscope (FE-SEM) on a cube of 10mm x 10 mm x 10 mm at an accelerating voltage of 20 kV. An area of 250 µ m X 250 µ m with a step size of 250 nm was considered for the analysis. Furthermore, MATLAB based MTEX module has been used to carry out post-processing of the EBSD data. X-ray di ff raction (XRD) technique was used to investigate the phase composition of as-fabricated as well as heat treated Ti6Al4V alloy. XRD was conducted in Bruker D8 Advance machine with CuK α radiation with a step size of 0.02°and scan speed of 0.1 seconds / step. Vicker’s Hardness test was then done using Autovick hardness testing machine and a load of 500 gm was applied with a dwell period of 15 seconds. A sample set of 15 was considered to evaluate the standard deviation in the observed data. Tensile tests were carried out on standard dog-bone specimen using Shimadzu universal testing machine (300 kN capacity) and the load was applied parallel to build direction. The tensile test was conducted at a strain rate of 0.1 % / sec and was maintained during the entire test as accordance with ASTM E8. Furthermore, the tension-compression fatigue testing with stress ratio of -1 and at a cycle frequency of 20 Hz was carried out on cylindrical specimens at room temperature using MTS servo-hydraulic fatigue testing machine as per ASTM E466. The tests were continued till 10 7 cycles or till the complete failure, whichever came first. The fracture surfaces were investigated using FE-SEM to find out the micro-mechanism of the fatigue and crack initiating defects size parameter, √ area and its location is then evaluated. The initial as-built microstructure consists of mainly martensitic α ’ microstructure as shown in Figure 2. The prior β grain tends to orient along the build direction as shown by the EBSD results in Figure 3. The formation of these β grains takes place by epitaxial grain growth of the original β phase preferentially along the < 1 0 0 > direction due to layer by layer deposition process. The part of prior β grain of one layer acts as the nucleus point of growth for adjacent layer. The formation of martensitic microstructure is due to rapid cooling (10 3 -10 8 K / sec) during the selective laser melting process. The β phase in such high cooling rate transform into α ’ martensitic microstructure by atomic shear. The martensitic α ’ microstructure thus decomposes into α + β microstructure with the application of heat treatment and α phase nucleates at the boundary of a small portion of α ’ martensitic microstructure and transforms into equilibrium α + β phase. The thickness of the α lamellae increased with the heat treatment. The as-built and heat treated SLM Ti6Al4V alloy mainly consists of α / α ’ phase as indicated by the XRD peaks. The presence of β phase which is body centered cubic crystal is also noticed with the heat treatment as shown in Figure 4. A relatively broader peak is observed for as-fabricated specimen due to α ’ martensitic structure. The solute present in the solvent is unable to di ff use properly during sudden cooling and the movement is hindered. This results in a higher concentration of 2.3. Testing techniques 3. Results and Discussion 3.1. Microstructure and phase evolution