Microstructural evolution and properties of pure titanium grade 2 subjected to severe plastic deformation

One of the most probable fields of applications of Severe Plastic Deformation (SPD) processes is that of biological implants, and the most investigated metallic material in this context is titanium and its alloys. The motivation for the present work follows a general tendency which aims to substitute the Ti-6Al-4V alloy for commercially pure Ti; a prerequisite for such replacement being the mechanical strength enhancement of the latter. Lower cost and the absence of the harmful Al and V justify the substitution. For the purpose of strength enhancement a number of SPD processes are available, but very recent investigations showed that the combination of Equal-channel angular pressing (ECAP) with rolling or extrusion gives excellent results. In this work SPD was applied to commercial Ti Grade 2 employing (i) four ECAP passes; (ii) same followed by rolling, at room temperature and (iii) same followed by rolling at 100oC. The focus of the investigation is centred on process (iii), its influence on mechanical properties, work hardening capacity, sensitivity to strain rate, grain size and microstructure. Highest mechanical strength among all other process conditions was produced by only two ECAP passes plus rolling at 100oC. This was achieved by the establishment of a very small grain size and a high density of dislocations. The sub-zero mode of deformation also affected positively the high cycle fatigue behaviour, the Charpy fracture energy and the thermal stability; that was determined by microhardness measurements and calculation of the activation energy for recrystallization employing DSC. Additionally were investigated a series of properties here called technological properties of utmost importance for the final product: Results show that sub-zero SPD is harmful to machinability, benefits corrosion and produces screws for bone implants whose torque limit is above this product specifications but its torque fracture angle is smaller than for the Ti-6Al-4V alloy.Universidade Federal de Sao CarlosUm dos prováveis campos de aplicação da Deformação Plástica Severa (DPS) é o de implantes biológicos, e o material mais investigado neste contexto é o titânio e suas ligas. A motivação para o presente trabalho segue uma tendência geral que objetiva a substituição do Ti-6Al-4V por Ti comercialmente puro, pré-requisito para isso sendo o aumento da resistência mecânica do segundo material. Diminuição de custo e ausência do Al e V justificam a substituição. Para realizar o aumento de resistência diversos processos de DPS estão disponíveis, mas investigações muito recentes mostraram que a sequência extrusão em canal angular (ECAP) seguida de laminação ou extrusão obtém excelentes resultados. Neste trabalho DPS foi aplicado a Ti comercial Grau 2 empregando: (i) quatro passes ECAP; (ii) idem, seguido por laminação em temperatura ambiente; (iii) idem, seguido por laminação a -100oC. O foco do trabalho está centrado no processo (iii), em sua influência nas propriedades mecânicas, endurecimento por deformação, sensibilidade à taxa de deformação e microestrutura. Os maiores resultados resistência mecânica foram obtidos com apenas dois passes ECAP seguidos de laminação a -100oC. Isto foi conseguido por apreciável redução de tamanho de grão e alta densidade de discordâncias. A deformação sub-zero também afetou positivamente o comportamento em fadiga de alto ciclo, a energia de fratura Charpy e a estabilidade térmica; esta última determinada por medidas de microdureza e cálculo da energia de ativação para recristalização aparente via DSC. Também foram estudadas uma série de propriedades aqui chamadas de propriedades tecnológicas, que são críticas para o produto final. Os resultados mostram que o processamento SPD sub-zero prejudica a usinabilidade, aumenta a resistência à corrosão e pode produzir parafusos para implantes ósseos cujo limite de torque está acima das especificações para este produto, embora com ângulo de fratura sob torque menor do que o da liga Ti-6Al-4V

Severe plastic deformation by equal channel angular pressing: product quality and operational details

As a technique, Equal Channel Angular Pressing (ECAP) is simple and inexpensive. However, if die manufacture and operational details are not carefully planned and implemented, difficulties arise, which can interfere with the product characteristics and the pressing operation itself. The present paper offers guidelines on die design and manufacture, emphasizing geometry, material and heat treatment. Further, operational parameters such as lubrication, pressing temperature, deformation routes, die closure procedure and the influence of channel cross section on maximum acceptable load are described. Additionally, the effects of those variables on the product characteristics (deformation level and homogeneity) and integrity, plus process control and safety, are discussed

Corrosion Behavior of CW6MC Nickel Cast Alloy (Inconel 625) Welded by Shielded Metal Arc Welding

The aim of this study concerns the effect of multi-pass shielded metal arc welding (SMAW) on the corrosion behavior of CW6MC cast nickel alloy. Using optical and SEM techniques the welded joint is analyzed. Vickers microhardness mapping and potentiodynamic polarization in NaCl and H2SO4 solutions are also evaluated. Both the Laves phase and NbC-type carbides are identified in the base metal (BM) and weld metal (WM) regions. The main microstructural difference observed between these regions is the morphology aspect and fineness of the dendritic arrays. The welding process promotes the finer columnar grains formation with refined intermetallic particles in the WM than equiaxed grains of the BM, which in turn results in higher microhardness values in the former region. However, no substantial changes were observed in the corrosion behavior between the BM and WM regions, considering both acid and saline media. Nevertheless, during the multi-pass SMAW process, some non-metallic micrometric inclusions (Mo and S-rich regions) can be constituted to occur in the WM region. This is associated with a significant drop in the corrosion performance of this region when the electrochemical tests are evaluated

Effect of lung recruitment and titrated Positive End-Expiratory Pressure (PEEP) vs low PEEP on mortality in patients with acute respiratory distress syndrome - A randomized clinical trial

IMPORTANCE: The effects of recruitment maneuvers and positive end-expiratory pressure (PEEP) titration on clinical outcomes in patients with acute respiratory distress syndrome (ARDS) remain uncertain. OBJECTIVE: To determine if lung recruitment associated with PEEP titration according to the best respiratory-system compliance decreases 28-day mortality of patients with moderate to severe ARDS compared with a conventional low-PEEP strategy. DESIGN, SETTING, AND PARTICIPANTS: Multicenter, randomized trial conducted at 120 intensive care units (ICUs) from 9 countries from November 17, 2011, through April 25, 2017, enrolling adults with moderate to severe ARDS. INTERVENTIONS: An experimental strategy with a lung recruitment maneuver and PEEP titration according to the best respiratory-system compliance (n = 501; experimental group) or a control strategy of low PEEP (n = 509). All patients received volume-assist control mode until weaning. MAIN OUTCOMES AND MEASURES: The primary outcome was all-cause mortality until 28 days. Secondary outcomes were length of ICU and hospital stay; ventilator-free days through day 28; pneumothorax requiring drainage within 7 days; barotrauma within 7 days; and ICU, in-hospital, and 6-month mortality. RESULTS: A total of 1010 patients (37.5% female; mean [SD] age, 50.9 [17.4] years) were enrolled and followed up. At 28 days, 277 of 501 patients (55.3%) in the experimental group and 251 of 509 patients (49.3%) in the control group had died (hazard ratio [HR], 1.20; 95% CI, 1.01 to 1.42; P = .041). Compared with the control group, the experimental group strategy increased 6-month mortality (65.3% vs 59.9%; HR, 1.18; 95% CI, 1.01 to 1.38; P = .04), decreased the number of mean ventilator-free days (5.3 vs 6.4; difference, −1.1; 95% CI, −2.1 to −0.1; P = .03), increased the risk of pneumothorax requiring drainage (3.2% vs 1.2%; difference, 2.0%; 95% CI, 0.0% to 4.0%; P = .03), and the risk of barotrauma (5.6% vs 1.6%; difference, 4.0%; 95% CI, 1.5% to 6.5%; P = .001). There were no significant differences in the length of ICU stay, length of hospital stay, ICU mortality, and in-hospital mortality. CONCLUSIONS AND RELEVANCE: In patients with moderate to severe ARDS, a strategy with lung recruitment and titrated PEEP compared with low PEEP increased 28-day all-cause mortality. These findings do not support the routine use of lung recruitment maneuver and PEEP titration in these patients. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01374022