A Calibration Approach for Accelerated Creep Testing for Electron Beam Melted (EBM) Ti-6Al-4V Using the WCS Model

Abstract

This study outlines a model calibration approach for an accelerated creep test called the dynamic negative stepped test (DNST) to enable the rapid screening of creep resistant materials. In DNST, stress is stepped decreased based on the attainment of a sufficient minimum-creep-strain-rate (MCSR) at each stress level. Steps are repeated, torturing the material, until rupture occurs. The DNST is advantageous as a screening test for new alloys. The calibration of a constitutive model to DNST data furnishes predictions of the conventional creep response being between 65-6685 hr from the relatively short (<130 hr) DNST Data.

In this study, DNSTs are performed on electron beam melted (EBM) Ti-6Al-4V at 650°C with stepping through 150, 75, 60, and 50 MPa. Six build orientations are tested including 0°, 30°, 45°, 60°, 90°, and V (vertical) direction. The Wilshire-Cano-Stewart (WCS) model is employed to calibrate the experimental data. A unique set of MCSR and stress-rupture (SR) related material constants are obtained for each build direction via optimizing a non-homogenous objective function. The WCS model predictions exhibit remarkable agreement with DNST data. Conventional creep curves are generated to determine which build orientations are likely to demonstrate poor, moderate, and superior creep resistance. Predictions of MCSR and SR curves over a wide stress range are estimated to investigate the extrapolation pedigree of the approach. This will allow the material designers to have more confidence in DNST generated test data for rapid screening of candidate alloys.