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

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2. Experimental details 2.1. Creep forming experiments and spring back calculations In this study, a commercial grade Al-Mg-Sc-Zr alloy, 5024 H116 sheet of size 1.6 mm was used. The chemical composition of the alloy is given in Table 1.

Table 1. Chemical composition (wt.%) of 5024 H116 alloy

Si

Fe

Mn 0.35

Mg 4.36

Sc

Zr 0.1

Ti

Al

0.32 Balance A single curvature die made of low alloy steel was used for conducting creep forming experiments (Fig.1a). The bend radius of the die was 250 mm. Samples of 200 mm X 50 mm dimensions were subjected to creep forming at an applied normal stress of 210 MPa, which is well below the material's yield strength (~305 MPa). During fixing into the die, the specimen was placed in such a way that its rolling direction lies along the bend radius. The specimen was pressed against the lower die and mechanically secured by the upper die with bolts and nuts. Subsequently, the assembly was loaded into the furnace and heated at a 5°C/min rate to the desired holding temperature. After clamping in the die, the sheet specimens were held isothermally at temperatures in the range 120 – 320°C at steps of 40°C for 2 hours, followed by air cooling to room temperature. The sheets were then unloaded from the die, and the profile was measured to determine the spring back. The spring back ‘S’ of the sheets (S) was calculated using the equation S = (X/Y), where X is the difference between the final and loaded center deflections, and Y is the final center deflections respectively (Fig.1b). S is a dimensionless quantity and ranges from 0 to 1, indicating no spring back to full spring back condition, respectively. The results of the creep forming are shown in Fig.2. To study the effect of CF on the mechanical properties, machined tensile and fatigue test specimens were exposed to simulated creep forming temperatures for 2 hours and air cooled. The samples processed in this study are designated as: As-received: AR; creep forming as CF, e.g., CF at the temperature of 200°C as 200CF; holding the test specimens at the simulated creep forming temperature as HT, for example, holding the specimen at 200°C as 200 HT. 0.34 0.26 0.2

Fig. 1. (a) Creep forming die made of a low alloy steel, and (b) schematic representation for spring back calculation

Fig. 2. Photographs showing (a) curvature of the samples after creep forming at temperatures in the range 200 to 320°C and (b) varying degrees of spring back (bracketed values) at different temperatures of CF; sample in as-received condition (AR) before creep forming is shown in Fig.2a for comparison