The Effect of Direct Laser Deposition Process Parameters on Microstructure and Mechanical Properties of Ti-6Al-2Sn-4Zr-6Mo

Blown powder Direct Laser Deposition (DLD) is a type of Additive Manufacturing (AM) that is of interest to the aerospace industry as a method of performing high-integrity repairs of critical components. The properties of the deposited material are largely influenced by process parameters such as beam power, velocity, hatch spacing, beam radius and powder feed rate. It is critical for a high-quality repair, that the effect of these process parameters on the solidification microstructure and hence the mechanical properties are fully understood. The work presented here focuses on quantifying the effect of process parameters on DLD of the α+β titanium alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6246). This alloy demonstrates high strength and good corrosion resistance and is a suitable replacement for Ti-6Al-4V in aerospace applications. This is due to its ability to perform at higher temperatures which is important as gas turbine engines push towards higher efficiencies and hence elevated operating temperatures. A Design of Experiment (DoE) was used to map a potential process window that would be suitable for Ti-6246 DLD repair of compressor bladed disks (Blisks). The aim was to identify combinations of process parameters that resulted in a fully-dense defect-free build that produced repeatable mechanical properties comparable to the parent Ti-6246 blisk material. Ten deposits were built with five different parameter sets using an RPM 557 laser deposition machine. Tensile specimens were machined from the build for uniaxial tensile testing. Small sections of each build were also retained for microstructural analysis, with the aim to correlate process parameters with the size of the resultant α+β lamellar microstructure. The α-lath width was found to generally increase with decreasing line energy density (beam power divided by velocity), although the effects of additional process parameters such as powder feed rate is also important and the influence of this is also explored. The results from this work were used to determine response surfaces relating process inputs such as energy density to process outputs such as 0.2% yield stress. These were then used to provide recommendations for future work with the aim of optimizing the DLD process window for Ti-6246 as a suitable repair method. The experimental work was supported by the development of a thermal model. This helped to inform how process parameters influenced the laser deposition conditions. The thermal model was calibrated against a thin-wall aerofoil-type build and reasonable agreement was found between predicted and measured melt depths for a range of process parameters. The thermal model also can help to provide predictions about the how further optimisation of the process window may affect mechanical properties. Some of the key findings and outcomes of this work are: • Development of an automated process to measure the size of Ti-6246 α+β lamellar microstructure produced by DLD. This automated process was validated using manual measurement techniques and was found to be a robust and trustworthy method that significantly decreases the time to gather microstructural data. • Size of the α-laths were generally found to be <1µm, apart from a dendritic zone at the top of each of the builds which has remained fine due to lack of coarsening from repeated thermal cycles. • Definition of a process window for the DLD of Ti-6246 which can produce dense builds with minimal defects (as revealed by both SEM and XCT analysis). • Testing of Ti-6246 DLD builds showed mechanical properties (tensile strength, 0.2% yield stress and elongation) comparable to parent forged material and within requirements set by Rolls-Royce for repair purposes. • Linear regression and response surface analysis showed that laser beam velocity (v) had the most effect on mechanical properties, particularly the 0.2% yield stress. Hatch spacing had little to no quantifiable effect on the mechanical properties. • Recommendations for process optimisation and productivity gains include increasing the hatch spacing and/or beam velocity to increase productivity. • Development of a Gaussian-based thermal model used to define a new parameter – melt pool saturation level (MPSL), this being the ratio between melting capacity of the laser and the actual amount of material being melted during the DLD process. • The MPSL was used to calculate an upper limit to the PFR and DLD process inputs were used to define a lower limit or “aspirational” PFR. Hence, the model developed in this work is useful in an industrial setting as it can reduce the number of test deposits needed to down-select the best process parameters and therefore define a suitable process window

Student reading lists: evaluating the student experience at the University of Huddersfield

Purpose: To evaluate the bespoke Resource List Management System (MyReading) at the University of Huddersfield. Design/methodology/approach: An online survey was designed to assess student use of MyReading and their views on potential improvements. The survey used closed questions designed to obtain quantitative data. Thematic analysis was used to analyse qualitative data obtained from open questions to obtain. Findings: The paper supports findings of another recent study which found that reading lists are perceived as more important by students than by lecturers. A variety of positive and negative themes emerged which pointed to this conclusion. Positive themes were: the perceived helpfulness of reading lists; students' view of MyReading as a starting point for independent further reading; ease of use of MyReading and the time saving afforded to students; the value students place on their reading lists as being "quality assured" by lecturers. Negative themes were: inconsistency in the length and structure of lists; concerns that some lists are not regularly updated; lack of awareness of functionality revealing training needs for students and lecturers; suggestions for future enhancements to MyReading. Another finding from the quantitative data is a clear link between low use of reading lists by students in certain Schools and low use of other library resources. Practical implication: The research provides guidance to universities regarding future development of Resource List Management Systems and promotional and training needs. Originality/value: The study adds rich information to the existing body of qualitative research on students' perceptions of their reading lists

The tripeptide feG inhibits leukocyte adhesion

© 2008 Mathison et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens

Home Delinquency Rates Are Lower Among ACA Marketplace Households: Evidence From a Natural Experiment

This brief uses administrative income tax data coupled with survey responses from roughly 5,000 households living near the poverty line to estimate how access to the Affordable Care Act’s health insurance Marketplaces have affected households’ experiences of extreme illiquidity, which is measured by delinquencies on home payments. To estimate this relationship, we exploit a natural experiment underway in states that did not expand Medicaid and created by the eligibility rules for Marketplace subsidies. Results suggest that insured households living near the poverty line are better able to make timely rent and mortgage payments compared with similar, uninsured households. Given housing instability’s well-documented links with downstream financial and developmental outcomes, findings presented in this brief suggest that the Affordable Care Act’s health insurance programs have improved the short-term financial well-being of low- and moderate-income households in a way that may have longer term implications

Variation in pigmentation gene expression is associated with distinct aposematic color morphs in the poison frog Dendrobates auratus

Background: Color and pattern phenotypes have clear implications for survival and reproduction in many species. However, the mechanisms that produce this coloration are still poorly characterized, especially at the genomic level. Here we have taken a transcriptomics-based approach to elucidate the underlying genetic mechanisms affecting color and pattern in a highly polytypic poison frog. We sequenced RNA from the skin from four different color morphs during the final stage of metamorphosis and assembled a de novo transcriptome. We then investigated differential gene expression, with an emphasis on examining candidate color genes from other taxa. Results: Overall, we found differential expression of a suite of genes that control melanogenesis, melanocyte differentiation, and melanocyte proliferation (e.g., tyrp1, lef1, leo1, and mitf) as well as several differentially expressed genes involved in purine synthesis and iridophore development (e.g., arfgap1, arfgap2, airc, and gart). Conclusions: Our results provide evidence that several gene networks known to affect color and pattern in vertebrates play a role in color and pattern variation in this species of poison frog

The subgingival plaque microbiome, systemic antibodies against bacteria and citrullinated proteins following periodontal therapy

Periodontitis (PD) shows an association with rheumatoid arthritis (RA) and systemic inflammation. Periodontal pathogens, namely Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans, are proposed to be capable of inducing citrullination of peptides in the gingiva, inducing the formation of anti-citrullinated protein antibodies (ACPAs) within susceptible hosts. Here, we sought to investigate whether periodontal treatment influenced systemic inflammation and antibody titres to P. gingivalis, A. actinomycetemcomitans, Prevotella intermedia and ACPA in 42 systemically health patients with periodontal disease. Subgingival plaque and serum samples were collected from study participants before (baseline) and 90 days after treatment to analyse the abundance of specific bacteria and evaluate anti-bacterial antibodies, C-reactive protein (CRP), tumour necrosis factor α (TNF-α), interleukin 6 (IL-6) and ACPA in serum. Following treatment, all patients showed reduced periodontal inflammation. Despite observing a weak positive correlation between CRP and IL-6 with periodontal inflammation at baseline, we observed no significant reductions in any indicators of systemic inflammation 90 days after treatment. In contrast, anti-P. gingivalis IgG significantly reduced post-treatment (p &lt; 0.001, Wilcoxon signed rank test), although no changes were observed for other antibody titres. Patients who had detectable P. gingivalis in subgingival plaques had significantly higher anti-P. gingivalis IgG and ACPA titres, suggesting a potential association between P. gingivalis colonisation and systemic antibody titres

Whole-genome approach to assessing human cytomegalovirus dynamics in transplant patients undergoing antiviral therapy

Human cytomegalovirus (HCMV) is the most frequent cause of opportunistic viral infection following transplantation. Viral factors of potential clinical importance include the selection of mutants resistant to antiviral drugs and the occurrence of infections involving multiple HCMV strains. These factors are typically addressed by analyzing relevant HCMV genes by PCR and Sanger sequencing, which involves independent assays of limited sensitivity. To assess the dynamics of viral populations with high sensitivity, we applied high-throughput sequencing coupled with HCMV-adapted target enrichment to samples collected longitudinally from 11 transplant recipients (solid organ, n=9, and allogeneic hematopoietic stem cell, n=2). Only the latter presented multiple-strain infections. Four cases presented resistance mutations (n=6), two (A594V and L595S) at high (100%) and four (V715M, 32 V781I, A809V and T838A) at low (&lt;25%) frequency. One allogeneic hematopoietic stem cell transplant recipient presented up to four resistance mutations, each at low frequency. The use of high throughput sequencing to monitor mutations and strain composition in people at risk of HCMV disease is of potential value in helping clinicians implement the most appropriate therapy