Hot Isostatic Pressing for Fatigue Critical Additively Manufactured Ti-6Al-4V

Abstract

The efficacy of hot isostatic pressing (HIP) for enhancing fatigue performance is investigated for additively manufactured (AM) Ti-6Al-4V. The limitations of HIP are probed by varying the initial material state via the selection of AM system, powder chemical composition, and process parameters. We demonstrate that the fatigue performance of HIP’d AM Ti-6Al-4V depends on the as-built quality of the material. Differences in common material attributes, such as pre-HIP defect populations or post-HIP microstructure morphology, are shown to be insufficient to explain the observed discrepancies in performance. This implies that additional microstructure attributes or localized deviations from the expected structure control the failure of this material. Finally, HIP parameters outside ASTM recommendations were explored, where a reduced temperature and high-pressure treatment yielded significantly improved fatigue performance.

Effect of HIP post-processing at 850 °C/200 MPa in the fatigue behavior of Ti-6Al-4V alloy fabricated by Selective Laser Melting

“…Studies on the possible beneficial effects of reducing HIP temperature while increasing pressure are scarce, as previously reviewed. However, the work of Moran et al [22] must be mentioned as a comprehensive comparison between a standard HIP, i.e. at 920 • C and 100 MPa as recommended by the ASTM F2924, and two alternative cycles: a super-Beta HIP at 1050 • C and 100 MPa in which fatigue behaviour was considerably worse due to the α-grain coarsening, and a low-temperature high-pressure HIP (LTHP-HIP) at 800 • C and 200 MPa.…”

“…This LTHP-HIP was demonstrated to minimize microstructure coarsening and resulted in the best fatigue performance. Despite some authors [21,22] claim that the size and shape of defects are not expected to control high cycle fatigue, it is clear that a decrease in defect population will reduce fatigue initiation sites. Therefore, the present works tries to shed light into the relationship between microstructure, defects and fatigue life after a low-temperature high-pressure HIP process.…”

“…Cutolo et al [13] studied the fatigue behaviour using miniaturized samples with a circular cross section of 2.5 mm in diameter. The authors [35] As-built + HT 670 [38] As-built + HT 640 [39] As-built / annealed 820 [10] As-built / annealed 820 [22] Standard HIP 920 investigated two different heat treatments for Ti-6Al-4V produced by SLM, namely, stress-relief (SR) and HIP. For SR, the samples were heated at 850 • C for 2 h followed by air cooling, while for HIP the samples were heated at a temperature of 920 • C at a pressure of 1000 bar for 2 h. All the samples were tested in as-built surface conditions.…”

“…Some authors have carried out treatments at higher temperature, such as Wu et al, HIPping at 1000 • C / 150 MPa for 1 h [19] or Seifi et al [20] HIPping at 950 • C / 100 MPa for 3 h, and they observe as certain material properties decrease with an increased heat treatment temperature as a consequence of microstructural coarsening effects. This has motivated the study of HIP treatments at higher pressures but lower temperatures combined with rapid cooling to limit these microstructural coarsening effects [21,22]. Microstructural changes during the SLM and HIP processes have a significant effect on the mechanical properties of titanium alloys [23].…”

Post-processing of additively manufactured metallic alloys – A review

“…The original model often needs to be re-designed to unleash the full potential of additive manufacturing. The designers need to consider different aspects of the value-chain during the design process, e.g., the support structures, fixturing and clamping for post machining process [11,12] and distortion control and microstructure development during thermal post treatments [13] to ensure the functional requirements are met.…”