the fatigue properties of additively manufactured GR5 titanium alloy based on different post-processing methods
Different post-processing methods, such as heat treatment (HT), hot isostatic pressing (HIP), laser surface modification, etc., are often used to improve the performance of additively manufactured parts. There have been some studies trying to optimize and study the effects of different post-processing methods on the fatigue properties of additively manufactured alloys. However, it is not clear what are the main factors affecting the fatigue performance of additively manufactured titanium alloys, so an optimal post-processing method to improve the fatigue strength of additively manufactured titanium alloys has not yet been determined. Furthermore, the effect of applying heat treatment before HIP treatment on improving fatigue strength, especially for Ti-6Al-4V alloy, has hardly been explored.
This article studies and compares the tensile and fatigue properties of Ti-6Al-4V alloy prepared by powder bed laser selective melting method in the initial state, heat treatment state and hot isostatic pressing state. Fill the research gaps in this field.
The initial state Ti-6Al-4V alloy obtained by 3D printing has the highest yield strength, tensile strength and hardness; the Ti-6Al-4V alloy under hot isostatic pressing heat treatment machine processing conditions (HT+HIP+M) is stronger in terms of strength. Low, but has the highest ductility. This is due to the coarsening of α lath and the decomposition of α ′ martensite structure and the reduction of phase boundary dislocation density during heat treatment. The specific static strength and tensile ductility data are shown in Table 1.
Table 1 Static strength and tensile properties results obtained by different post-processing methods of additively manufactured titanium alloys. AB represents the initial state, HT represents the state after heat treatment at 850℃ for 2 hours, and HIP represents the state maintained at 920℃ and 120MPa for 2 hours.
Figure 1 Microstructure of additively manufactured titanium alloy under different post-processing conditions. (a) AB: Initial state (b) HT: 850℃ for 2 hours; (c) HIP: 920℃, 120MPa for 2 hours; (d) HT + HIP state. The direction marked on the right side of the figure is the stacking direction of the material when printing.
Figure 1 shows the microstructure of additively manufactured titanium alloys under different post-processing states. This picture is a backscattered image taken under an electron microscope. In this image, the phase with white contrast is the β phase, and the phase with black and gray contrast is the α phase. The unique microstructural characteristics of printed titanium alloys are marked in the figure, including the original β phase grain boundaries and Widmanstatten structure. The results in Figure 2 show the TEM test results under different post-processing conditions.
Figure 2 TEM observation results of additively manufactured Ti-6Al-4V in different post-processing states and initial states. (a) AB; (b) HT; (c) AB + HIP; (d) HT + HIP.
Heat treatment, that is, HT+HIP conditions, was applied before hot isostatic pressing, and then the sample surface was polished (HT + HIP + M). It was found that the fatigue strength of Ti-6Al-4V was significantly improved under this condition, compared with The initial condition was improved by almost 5 times, while heat treatment alone increased its fatigue strength by almost 3 times compared to the original condition. The specific SN curve is shown in Figure 3.
Figure 3 SN curves under different post-processing conditions. The specific post-processing method is shown in the legend in the upper right corner of the figure. Among them, M means that the sample surface is polished, and no M means that the sample surface is in the state after printing is completed.
Heat treatment (HT) and hot isostatic pressing (HIP) lead to changes in the microstructure within the additively manufactured Ti-6Al-4V alloy. After heat treatment, the metastable acicular α ′ martensite structure decomposes into α + β lamellar structure, and the thickness of α lamellae gradually increases as the heat treatment temperature increases. The HT treatment improves the fatigue strength compared with the AB state, mainly because of the obstruction of α-colonies (crystal groups with similar grain orientation) in the α + β lamellar structure and the tensile residual stress introduced by the 3D printing process. relaxation. The reason why HIP treatment improves the fatigue strength is not only the two points mentioned in the HT state, but also promotes the elimination of defects within the material. The specific X-ray μCT test results are shown in Figure 4 below.
Figure 4 Three-dimensional reconstruction renderings of internal defects under different post-processing states. (a) AB; (b) HT; (c) AB + HIP; (d) HT + HIP.
Paper citation information:
Bhandari L, Gaur V. Different post-processing methods to improve fatigue properties of additively built Ti-6Al-4V alloy[J]. International Journal of Fatigue, 2023: 107850.
