PSI - Issue 46

Alok Gupta et al. / Procedia Structural Integrity 46 (2023) 35–41 Alok Gupta et al. / Structural Integrity Procedia 00 (2021) 000–000

37 3

The test configuration and mounting set-up of the bracket assembly for the displacement controlled LCF test is shown in Fig. 2. The LCF testing on the bracket was conducted on a Tinius Olsen H25KS electromechanical testing system fitted with a Tinius Olsen DSCCTOL 25kN load cell and a Severn Thermal Solutions SF2107 furnace. The bracket was installed on the test machine via the bolted top and bottom fixtures. In this test set-up, the bottom fixture remained stationary whereas the top fixture travelled by a pre-set distance. A LVDT was used to control and measure the movement of the bracket. The bracket LCF tests were fully reversed displacement controlled LCF tests (i.e. displacement ratio = −1) at 200  C. Firstly, the bracket was tested at a displacement magnitude of ±1 mm (BT1 test) for a target of 3000 LCF cycles without the failure. In the second test run, the same bracket was tested at ±2 mm displacement (BT2) to understand the cycles to failure when it is tested with elasto-plastic strain levels. To optimize the overall testing time, both BT1 and BT2 tests were carried out at a Displacement Rate (DR) of 0.008 mm/s. Furthermore, it was also required to show that the bracket has redundancy in the load path i.e., the bracket does not show a significant drop in its load carrying capability in the case of a failure of a strut or connector.

Fig. 2. LCF testing on SLM Ti-6Al-4V bracket: (a) elevated temperature test set-up, (b) Furnace enclosure for elevated temperature testing, (c) Bracket installation details, and (d) Saw tooth displacement loading profile.

2.2. Material and bracket manufacturing Ti-6Al-4V (Grade 23) Extra Low Interstitials (ELI) plasma atomised powder, supplied by LPW Technology, was used in this study. The analysis of the powder showed that the mean chemical composition as Ti Bal, Al 6.4, V 4.0, FE 0.19, O 0.12, N 0.02, H 0.0021, C 0.02 (wt. %), which is in line with the ASTM-B348 (2019). The powder size was distributed between 15 – 45  m with mean of the distribution centered at 30  m. The SLM bracket was made using an EOSINT M280 SLM machine. The machine used a laser power of 170 W, operated in argon atmosphere and had a substrate of Grade 5 Ti-6Al-4V material which was preheated to 35  C. The layer deposition speed was kept at 150 mm/s, the laser scan speed was 1250 mm/s and the layer thickness was kept at 30  m. The build orientation of the bracket is shown in Fig. 1. The Wire-EDM cutting process was used to remove the brackets from the substrate plate post SLM build. Subsequently, the build supports were removed and the loosely sintered powder particles were removed using aqua blasting, which also improved the surface finish. The bracket was stress relieved at 650  C for 3 hours in an argon atmosphere followed by furnace cooling to room temperature. 2.3. Fracture characterization The characterization of fracture surface was carried out using a Quanta FE600 scanning electron microscope. The SEM micrographs of fractured LCF specimens were used to analyse the crack initiation sites and area of crack propagation.