Enhanced Plastic Stability: Achieving High Performance in a Al6xxx Alloy Fabricated by Additive Manufacturing.

Additive manufacturing (AM) facilitates the creation of materials with unique microstructural features and distinctive phenomena as compared to conventional manufacturing methods. Among the various well-fabricated AM alloys, aluminum alloys garner substantial attention due to their extensive applications in the automotive and aerospace industries. In this work, an Al6xxx alloy is successfully fabricated with outstanding performance. A nucleation agent is introduced to diminish the susceptibility to cracking during the AM process, thereby inducing a heterogeneous microstructure in this alloy. However, the introduction of ultrafine grains induces plastic instability, evidenced by the presence of Lüders band. This work investigates the evolution of the Lüders band and the strategy to reduce their undesirable effect. The heterogeneity destabilizes the band propagation and thus deteriorates the ductility. Through a T6 heat treatment, the local Lüders strain decreases from 10.0% to 6.2%, leading to a substantial enhancement in plastic stability. With the increase in grain growth and the enlargement of coarse grain regions, the mismatch between the local and macroscopic Lüders strain disappears. Importantly, the strength and the thermal conductivity are concurrently increased. The findings demonstrate the significance of ensuring plastic stability to achieve improved strength-ductility trade-off in AM alloys with heterogeneous microstructures.

Additive manufacturing of alloys with programmable microstructure and properties.

In metallurgy, mechanical deformation is essential to engineer the microstructure of metals and to tailor their mechanical properties. However, this practice is inapplicable to near-net-shape metal parts produced by additive manufacturing (AM), since it would irremediably compromise their carefully designed geometries. In this work, we show how to circumvent this limitation by controlling the dislocation density and thermal stability of a steel alloy produced by laser powder bed fusion (LPBF) technology. We show that by manipulating the alloy's solidification structure, we can 'program' recrystallization upon heat treatment without using mechanical deformation. When employed site-specifically, our strategy enables designing and creating complex microstructure architectures that combine recrystallized and non-recrystallized regions with different microstructural features and properties. We show how this heterogeneity may be conducive to materials with superior performance compared to those with monolithic microstructure. Our work inspires the design of high-performance metal parts with artificially engineered microstructures by AM.

Application of quantitative T1, T2 and T2* mapping magnetic resonance imaging in cartilage degeneration of the shoulder joint.

To investigate and compare the values of 3.0 T MRI T1, T2 and T2* mapping quantification techniques in evaluating cartilage degeneration of the shoulder joint. This study included 123 shoulder joints of 119 patients, which were scanned in 3.0 T MRI with axial Fat Suppression Proton Density Weighted Image (FS-PDWI), sagittal fat suppression T2 Weighted Image (FS-T2WI), coronal T1Weighted Image (T1WI), FS-PDWI, cartilage-specific T1, T2 and T2* mapping sequences. Basing on MRI images, the shoulder cartilage was classified into grades 0 1, 2, 3 and 4 according to the International Cartilage Regeneration & Joint Preservation Society (ICRS). The grading of shoulder cartilage was based on MRI images with ICRS as reference, and did not involve arthroscopy or histology.The T1, T2 and T2* relaxation values in the superior, middle and inferior bands of shoulder articular cartilage were measured at all grades, and the differences in various indicators between groups were analyzed and compared using a single-factor ANOVA test. The correlation between T1, T2 and T2* relaxation values and MRI-based grading was analyzed by SPSS software. There were 46 shoulder joints with MRI-based grade 0 in healthy control group (n = 46), while 49 and 28 shoulder joints with grade 1-2 (mild degeneration subgroup) and grade 3-4 (severe degeneration subgroup) in patient group (n = 73), accounting for 63.6% and 36.4%, respectively. The T1, T2 and T2* relaxation values of the superior, middle and inferior bands of shoulder articular cartilage were significantly and positively correlated with the MRI-based grading (P < 0.01). MRI-basedgrading of shoulder cartilage was markedly associated with age (r = 0.766, P < 0.01). With the aggravation of cartilage degeneration, T1, T2 and T2* relaxation values showed an upward trend (all P < 0.01), and T1, T2 and T2* mapping could distinguish cartilage degeneration at all levels (all P < 0.01). The T1, T2 and T2* relaxation values were significantly different between normal group and mild degeneration subgroup, normal group and severe degeneration subgroup, mild degeneration subgroup and severe degeneration subgroup (all P < 0.05). Quantitative T1, T2 and T2* mapping can quantify the degree of shoulder cartilage degeneration. All these MRI mapping quantification techniques can be used as critical supplementary sequences to assess shoulder cartilage degeneration, among which T2 mapping has the highest value.

Contrast opacification difference of mural artery and the transluminal attenuation gradient on coronary computed tomography angiography for detection of systolic compression of myocardial bridge.

PURPOSE: Myocardial bridges (MB) have traditionally been considered a benign condition, but recent studies have demonstrated that the clinical complications can be dangerous. The transluminal attenuation gradient (TAG) obtained from coronary computed tomography angiography (CCTA) data (Retrospective ECG-triggered method) has been used in detecting significant stenosis in coronary artery caused by atherosclerosis. Contrast opacification difference (COD) was the parameters calculated as the change between attenuation of mural artery and the median attenuation of presumptive vessel segment; it was evaluated along with TAGstandardized (TAGs) and MB length for predicting MB with systolic compression (MB-SC) in patients diagnosed as MB in left anterior descending coronary artery (LAD) by CCTA or invasive coronary angiograph (ICA). METHODS: A total of 107 MB patients were divided into three groups based on systolic compression (SC), including: Group 1 (MB without SC); Group 2 (MB with mild SC); and Group 3 (MB with significant SC). ANOVA and Kruskal-Wallis analysis indicated TAGs showed the most significant differences for MB identification. RESULTS: This study revealed that TAGs decreasing and COD increasing were dominated in MB with significant SC. CONCLUSIONS: COD had a higher sensitivity and a higher negative predictive value for detecting MB with significant SC than TAGs.

Biliary tract and pancreatic surgery complicated by acute pancreatitis: a clinical analysis.

OBJECTIVE: To study the clinical treatment features of biliary tract and pancreatic surgery complicated by acute pancreatitis. METHODS: A retrospective analysis of 21 cases of biliary tract and pancreatic surgery complicated by acute pancreatitis in the Department of General Surgery in our hospital during May 2005 to July 2011 was performed; the clinical treatment features were analyzed in terms of surgical option, onset interval of acute pancreatitis after last surgery, length of stay in hospital and Ranson score. RESULTS: There was no statistic difference between the two groups (A: The onset interval of acute pancreatitis after last surgery < 0.5 year. B: The onset interval of acute pancreatitis after last surgery > 0.5 year) in pathogenetic condition and length of stay in hospital. All patients were discharged after treatment, a follow-up of 6-18 months found no recurrence of pancreatitis. CONCLUSION: There is no relevance between the treatment feature and onset interval of biliary and pancreatic surgery complicated by acute pancreatitis. The disease is still treated meanly with symptomatic and supportive treatment, while the etiological treatment is also particularly important.