PSI - Issue 60

M. Suresh Kumar et al. / Procedia Structural Integrity 60 (2024) 433–443 Suresh Kumar et al., / Structural Integrity Procedia 00 (2023) 000 – 000

436

2.2. Mechanical properties evaluation Tensile testing was performed in accordance with ASTM-E8M at room temperature using an Instron servo hydraulic machine with a cross-head velocity of 2 mm/min. The specimen elongation was measured using an extensometer with a travel of 5.0 mm and a gauge length of 75 mm. The load-displacement data were recorded on an online computer and converted into engineering stress vs strain plots. High cycle fatigue testing was conducted on three different sets of samples, namely, AR, 200 HT, and 300 HT samples, according to the ASTM standard E466 2007 using an Instron servo- hydraulic machine under stress control at stress ratio (R = σ min /σ max ) of 0.1 with loading frequency of 10 Hz and at peak stress range of 160-320 MPa. The data was processed via software to obtain the S-N curve. Three specimens were tested in each condition. The rolling direction of the alloy sheet was kept along the tensile/fatigue loading direction. 2.3. Microstructure, phase analysis and fractography The microstructure of the alloy was characterized using a Carl Zeiss scanning electron microscope (SEM) (Model: EVO18) fitted with an EDX analyzer. Differential scanning calorimetry (DSC) was carried out to identify the pre existing phases in the AR, 200 HT and 300 HT samples. The samples were analyzed at a heating rate of 10°C/min under a nitrogen atmosphere using TA make DSC Model: Q2000. The fracture surfaces of fatigue tested samples were examined using a scanning electron microscope (SEM) at an accelerating voltage of 20 KV. 3. Results and Discussion 3.1. Effect of creep forming temperature on spring back Fig. 2a shows the photographs of creep formed samples, and Fig. 2b depicts the degree of spring back of the alloy sheet after being subjected to creep forming temperatures between 200 to 320°C for 2 hours. The measured spring back in the temperature range between 120-320°C is shown in Fig.3a. Exposure to 120 °C resulted in higher spring back (75%), while at 300 °C the spring back was almost negligible. Fig.3b shows the stress relaxation data at different temperatures and initial stress values for the alloy 5024 from the studies of Zimmermann et al. (2018). From this graph, for instance, at higher temperatures, i.e., 325°C, the applied stress relaxes to a greater extent and reaches a threshold value of ~5 MPa at an initial stress of 240 MPa. Similarly, at lower temperatures, i.e., at 150°C, the threshold value was 100 MPa even at a relatively lower initial stress of 180 MPa. This threshold stress that is responsible for spring back has been attributed to the interaction of dislocations with Al 3 Sc precipitates (Jambu et al. (2002)). Analyzing the spring back data in the present study in light of the reported literature, it is understood that at about 300°C, the applied stress relaxes to a large extent and is responsible for the smaller spring back in the alloy.

Fig. 3. (a) Spring back data for the 5024 alloy samples creep formed in the temperature range 120 to 320 o C; (b) Stress relaxation data for 5024 alloy ( Zimmermann et al. (2018)).