PSI - Issue 68

Xiangnan Pan et al. / Procedia Structural Integrity 68 (2025) 1038–1044 X. Pan et al. / Structural Integrity Procedia 00 (2025) 000–000

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of this paper is to incorporate the detailed microstructures and specific failure types, borrowing the FIP concept, and to use a few tensile quantities to indicate the HCF and VHCF properties of AMed and conventional titanium alloys.

Nomenclature δ f

elongation at fracture engineering strain engineering stress applied stress amplitude fatigue resistance at 10 applied maximum stress

ε σ

σ a

i cycles under stress ratio R = –1

σ f- i

σ max

UTS, ultimate tensile strength

σ u

yield strength corresponding to 0.2 % residual tensile strain

σ 0.2

elastic modulus

E E i

input energy under the i

th cycle.

maximum energy that can be contained and/or dissipated at N f

E N f

number of cycles to failure

N f

stress ratio, the ratio of minimum stress to maximum stress

R

2. Defects and microstructures of titanium alloys Pure titanium has two main solid phases: one with a crystal structure of hexagonal close-packed (HCP) lattice at room temperature, called α; the other with a body-centered cubic (BCC) lattice at high temperature, called β (Leyens and Peters, 2003; Lütjering and Williams, 2003). Titanium can be alloyed by regulating the thermal stabilities of α and β phases through the solid solution of other chemical elements in them. The stability and volume of β phase define the four types of titanium alloys mentioned, of which near α and α+β are the two most commonly used. As the titanium alloy cools from above the phase transition point to room temperature, the prior β grain transforms into multiple LM colonies of different orientations, each colony consisting of α lamellae with a same orientation and an inter-lamellar matrix of residual β. If the cooling rate is increased from furnace to water, the number of LM colonies will increase, the thickness of lamellar α will decrease and more β phase will survive (Pegues et al., 2020). In AM process of PBF LB, the very fast cooling rate causes the α phase to not complete the transformation and be replaced by the martensitic α’ and/or α’’ laths (Thijs et al., 2010; Xu et al., 2017; Zhao et al., 2012). Equiaxed α grain can be recrystallized by annealing after plastic deformation of the near α and α+β titanium alloy (Pan et al., 2021; Zhao et al., 2012). For titanium alloy with EM, the volume fraction of equiaxed α grains must be greater than 30 ~ 50 % and the rest are the transformed β islands or LM domains (Pan et al., 2024b). If both lamellar and equiaxed α grains are present in a titanium alloy and the volume fraction of equiaxed α does not surpass 30 %, then it can be referred to as BM. When equiaxed α grains are completely absent and only the lamellae or martensitic laths remain in the titanium alloy, it is called generalized LM. If the prior β phases are all equiaxed grains and have not been plastically deformed, the transformed microstructure is a special LM, named Widmanstätten structure. The prior β and/or the LM can be plastically deformed by rolling, forging or other thermal-mechanical methods and then shaped to interweave like a basket, so that the resulting microstructure is called BW. For conventionally manufactured titanium alloys, the qualified material is free from both void (e.g. holes) and inclusion (e.g. oxides) type defects of microstructural inhomogeneities. For AMed titanium alloys with PBF-LB, it is very difficult to eliminate the void type defects, such as lack of fusion (LoF), gas pore and keyhole (Gao et al., 2024; Molaei et al., 2020; Pegues et al., 2020). After a thermal-mechanical treatment of hot isostatic pressing (HIP), the most void type defects will be collapsed and healed. Fig. 1 gives an example of AMed Ti-6Al-4V alloy via PBF-LB without and with HIP, showing the distributions of metallurgical defects and microstructures. Attributed to the HIP, the AM defects of void types densely distributed in the specimen (Fig. 1a) are being substituted by the sparsely distributed inclusions (Fig. 1b). The directly printed state is an LM with martensitic laths (Fig. 1c), and the HIP state is still maintained the LM with lengthened and thickened α lamellae (Fig. 1d).