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• An in situ cryogenic tensile X-ray tomography testing system is developed. • Damage evolution is revealed through time-lapse X-ray tomography. • A void nucleation model is proposed considering the orientation of inclusions. • The Gurson-Tvergaard-Needleman model is extended to low temperatures. This work investigates the failure behaviour of tungsten inert gas (TIG) welded 2219 aluminum alloy butt welds deformed over a range of cryogenic temperatures relevant to liquid hydrogen/oxygen propellent storage tanks. A novel in situ tensile testing machine, able to deform samples at temperatures down to 83 K, was designed to be accommodated on synchrotron computed tomography beamlines. In this way we were able to observe the accumulation of damage from void nucleation through growth to coalescence and ultimate failure by time-lapse X-ray microtomography during tensile straining at 293 K and 123 K. The results reveal that micro-void nucleation is triggered by θ (Al 2 Cu) precipitates and α-Al + θ (Al 2 Cu) eutectics in the form of particle fracture and interface decohesion. After nucleation, these freshly nucleated voids, along with pre-existing welding defects, extend with straining along the loading direction. This leads to void linkage, a very short void coalescence stage and subsequent catastrophic failure. The probability distribution function that describes the rate at which voids nucleate by interface decohesion considering the particle orientation was determined through the Eshelby inclusion theory. The evolution of void aspect ratio and void growth were modelled using the Gurson-Tvergaard-Needleman model and Le Roy model, respectively. This suggests that pre-existing defects grow faster, but that the numerous closely spaced nucleated voids play a significant role in failure. [Display omitted] [ABSTRACT FROM AUTHOR]

• Fatigue strength and life of 2219 welded joints at 77 K are improved compared to RT. • The maximum pore defects near the weld surfaces are predicted by EVS. • Fatigue strengths at RT and 77 K are predicted using EVS and El-Haddad model. • A fatigue life prediction model at room and low temperatures is proposed. This study investigated the tensile and high-cycle fatigue (HCF) properties of a tungsten inert gas (TIG)-welded 2219 aluminum alloy at room temperature (RT) and 77 K for the first time. Defect-sensitivity-based fatigue strength and life prediction models were proposed to link welding defects with fatigue properties at cryogenic temperatures. The findings revealed that the yield strength, ultimate tensile strength, and fatigue strength of 2219 welded joints at 77 K were significantly improved by 29.7 %, 34.1 %, and 60.4 %, respectively, compared to those at RT. The largest possible pore defects near the weld surface were predicted using a combination of laboratory X-ray computed microtomography and the extreme value statistics (EVS) method. Furthermore, the fatigue strength was predicted by considering the maximum defect size in terms of the El-Haddad-based Kitagawa–Takahashi (K–T) diagram. The predicted fatigue strength was 95.5 MPa with an error of 5.4 % at RT, whereas at 77 K, the predicted value was 144.5 MPa with an error of 10.8 %. Finally, a fatigue life prediction model at both RT and 77 K was proposed based on a modified K–T diagram and Murakami's S - N prediction model. The combination of the fatigue life prediction model and the maximum possible defect size based on EVS can provide slightly conservative predictions of the fatigue life without conducting a large number of HCF tests. [ABSTRACT FROM AUTHOR]

• Post-heat treated LPBF 316L steel exhibits a cyclic hardening followed by non-saturated cyclic softening stage. • The cyclic deformation is primarily influenced by the evolution of dislocation density and dislocation patterns. • Anisotropy in fatigue life is attributed to the anisotropic microstructure, defects and yield strength. • Ratchetting strain and ratchetting strain rate are highly dependent on the applied stress levels. This study has investigated the cyclic deformation and ratchetting-fatigue interaction of 316L stainless steel fabricated by laser powder bed fusion (LPBF) and subjected to a 900 °C/2 h post-heat treatment, considering both the vertical and horizontal orientations. Strain- and stress-controlled low cycle fatigue (LCF) tests are conducted to explore the cyclic deformation and the effects of the mean stress and stress amplitude on the fatigue life and ratchetting deformation. A detailed analysis of the cyclic deformation mechanism is conducted through X-ray microtomography and transmission electron microscope (TEM) observations. The results show that post-heat treated LPBF 316L steel exhibits cyclic hardening followed by a non-saturated cyclic softening stage. This behavior is attributed to the evolution of dislocation density and dislocation patterns, and the formation of surface cracks. The ratchetting strain and its rate are sensitive to the mean stress and stress amplitude. The fatigue lives of vertically built specimens are slightly higher under strain-controlled loading conditions, but significantly lower under stress-controlled loading conditions than those of horizontally built ones. Furthermore, ratchetting deformation can promote fatigue damage, resulting in a reduction in fatigue life. [ABSTRACT FROM AUTHOR]

• The tensile properties of WAAM 2219 over the temperature range (77 ∼ 573 K) were obtained. • The effects of temperature on void nucleation and evolution were explored by a combination of SEM and EBSD observations, and X-ray micro computed tomography. • The effects of temperature on deformation and fracture mechanisms were investigated. • True stress and strain responses over a wide temperature range (77 ∼ 523 K) and strain rate range (0.0001 ∼ 0.01 s−1) were predicted using an improved Arrhenius constitutive model. This work investigates the deformation and damage mechanisms of wire + arc additive manufacturing (WAAM) 2219 aluminum alloy over a broad temperature range of 77 K to 523 K. It is found that the strength and work hardening rate increase significantly with decreasing the temperature from 523 K to 77 K, and the fracture mode transitions from transgranular fracture mode to a mixture of intergranular and transgranular fracture ones. The large elongations to fracture are very similar for the specimens tested at 77 K and 523 K. This is mainly attributed to higher uniform elongation before necking at 77 K and higher post-necking elongation at 523 K. The cracking of α-Al + θ (Al 2 Cu) eutectics is the primary mechanism of micro-void nucleation, independent of the test temperature. The nucleated micro-voids, along with larger gas pore defects introduced by WAAM, extend along the tensile direction. In addition, as the temperature increases from 77 K to 523 K, the fracture mechanism transitions from high strain localization to extensive necking, leading to higher dislocation density, more grain fragmentation and micro-void nucleation, and more significant grain elongation and void growth. True stress and strain responses over the studied temperature range (77 ∼ 523 K) and strain rate range (0.0001 ∼ 0.01 s−1) are well predicted using an improved Arrhenius-type constitutive model, by considering the activation energy of deformation as a function of temperature. [ABSTRACT FROM AUTHOR]

• Both the tensile strength and ductility of the HRAM-processed alloy are improved. • The HRAM-processed alloy is characterized by a high dislocation density and large α laths. • High dislocation density and large α laths lead to the improved strength and ductility, respectively. • The fatigue crack growth resistance of HRAM-processed alloy is isotropic. Hybrid in situ rolled wire + arc additive manufacturing (HRAM) is a novel printing technology capable of fabricating large-scale components by combining in situ micro-rolling with standard wire + arc additive manufacturing (WAAM), which enhances the ductility and tensile strength of the resulting parts. This study attempts to identify the aspects of the microstructure that are critical to the property improvement. The microstructure of the WAAM- and HRAM-processed Ti6Al4V alloys exhibited a basketweave morphology composed of α laths and β phases. The HRAM-processed alloy was characterized by a higher dislocation density and larger α laths compared to those of the WAAM-processed alloy. The relatively high dislocation density and large α laths are responsible for the improvement in the tensile strength and ductility, respectively. The spatial evolution of cracking for the HRAM-processed alloy was investigated using a specially designed uniaxial tension/compression rig mounted on the synchrotron X-ray computed microtomography platform. The failure originated from the nucleation of micro voids and surface cracks. [ABSTRACT FROM AUTHOR]