PSI - Issue 14

Digendranath Swain et al. / Procedia Structural Integrity 14 (2019) 337–344 Swain et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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plate (substrate) and feedstock powder materials were similar in chemical composition. Argon containing less than 0.1% Oxygen was used as the shielding atmosphere to avoid hydrogen entrapment. Both the specimens were of similar dimensions: 12 mm (x) × 12 mm (y) × 65 mm (z). The built or lay-up direction was along the z-axis (Figs. 1a and 1b). The heat treatment details for the two specimens are shown in Table 2. The specimen-V3 was heat-treated after the RS measurements were completed in as fabricated condition whereas specimen-H3 was heat-treated without making any RS measurement a-priori. The main objective of heat treatment for the samples is to release the RS developed during 3-D printing. The heat-treatment and RS measurement were carried out after the specimens were removed from the baseplate using wire electrical discharge machining (WEDM). Therefore, no RS measurement was carried out with the specimens attached to the base plates.

Table 2. Heat treatment conditions of the specimens. Heat treatment condition

Specimen V3 Oriented in the print plane

Specimen H3 Oriented in the built direction

Stress relieving at 750  C for one hour followed by furnace cooling

Fig. 1. Digital photograph of (a) specimen-V3 (measurement taken before and after heat-treatment), (b) specimen-H3 (measurement taken after heat-treatment alone) superposed with the co-ordinate system, and (c) typical RS measurement setup on MTS 3000 machine.

2.2. Residual stress measurement method

All RS measurements were performed with incremental hole drilling strain gage method using MTS-3000 RS measurement machine supplied by SINT Technology, Italy. The measurement setup with this machine is shown in Fig. 1c. The stress relaxation was measured using HBM 1-RY61-1.5/120S Type A strain gage rosettes. A typical rosette bonded on specimen-V3 is shown in Fig 1a. Drilling was carried out in 0.025 mm steps with the help of an end mill diameter of 1.8 mm rotating at 3,00,000 rpm with a depth feed rate of 0.2 mm/min. The depth of the drilled hole was 2 mm. The strains measured with the rosette were used for calculating back the RS following ASTM E837 13a which deals with non-uniform RS analysis. The RESTAN software supplied by SINT Technlogy, Italy was used for the RS evaluation. For estimating the RS, Poisson’s ratio and Young’s modulus values of 0.3 and 120 GPa, respectively were input to the software including the hole offset and eccentricity. RS in the longitudinal planes (xz and yz planes) and the xy (print plane) top layers were evaluated up to 1 mm depth. The detailed principle and procedure of RS measurement using incremental hole drilling strain gage can be found elsewhere (Rendler and Vigness (1966), ASTM E837-13a, Vishay Measurements TN 503-6).