No abstract

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No abstract

Reduced hot cracking susceptibility is essential to ensure the flawless manufacturing of nickel superalloys typically employed in welded aircraft engine structures. The hot cracking of precipitation strengthened alloy 718 mainly depends on chemical composition and microstructure resulting from the thermal story. Alloy 718 is usually welded in a solution annealed state. However, even with this thermal treatment, cracks can be induced during standard industrial manufacturing conditions, leading to costly and time-consuming reworking. In this work, the cracking susceptibility of wrought and investment casting alloy 718 is studied by the Varestraint test. The test is performed while applying different welding conditions, i.e., continuous tungsten inert gas (TIG), low frequency pulsed TIG, continuous laser beam welding (LBW) and pulsed LBW. Welding parameters are selected for each welding technology in order to meet the welding quality criteria requested for targeted aeronautical applications, that is, full penetration, minimum cross-sectional welding width and reduced overhang and underfill. Results show that the hot cracking susceptibility of LBW samples determined by the Varestraint test is enhanced due to extended center line hot cracking, resulting in a fish-bone like cracking pattern. On the contrary, the minor effect of material source (wrought or casting), grain size and pulsation is observed. In fact, casting samples with a 30 times coarser grain size have shown better performance than wrought material.

Collagen gels have been extensively used as three-dimensional (3D) cell culture systems. To enhance their mechanical properties, the manufacture of collagen-based gels with agarose has been proposed. However, little is known about the stability of these gels under cold storage conditions. The consequences of cold storage on biological tissues for clinical applications are known to be significant; yet, they have not been considered on hydrogels used for in vitro experiments. This work studies the effect of extended cold storage on the stability of collagen and collagen-agarose hydrogels using rheometry and scanning electron microscopy. In addition, cell-matrix interactions of adipose-derived stem cells (ADSC) have been studied using these gels. Results show that both the storage modulus (G') and loss modulus (G″) of pure collagen gels gradually decrease with extended cold storage along the 30 days of the study, while G' and G″ increase in collagen-agarose gels under the same conditions. Moreover, significant changes in both moduli of collagen-agarose gels were only found after 30 days of cold storage, while in the case of collagen gels significant changes were already detected after 7 days. Finally, a reduction in the ability of ADSC to remodel the gel after prolonged cold storage was observed. To the best of our knowledge, this is the first work proving that cold storage of hydrogels prior to cell culture might have a significant impact on their mechanical properties and cell-matrix interactions.