Cold gas dynamic manufacturing: A non-thermal approach to freeform fabrication

This paper reports on the development of a novel freeform fabrication technique using a cold spray (CS) system. In the CS process, metallic powder particles are accelerated in a supersonic gas jet and impacted with a substrate at speeds in excess of 600 m/s. The non-melting nature of its deposition mechanism ensures that the sprayed material is free from thermally induced tensile stresses, while the underlying substrate remains unchanged. The process is seen as a viable method for additive manufacturing because of its high deposition rates and controllable spray jet. A process was developed to investigate the potential of non-thermal freeform fabrication and was coined Cold Gas Dynamic Manufacturing (CGDM). Here, additive and subtractive techniques were combined to enable the production of complex geometries. Whereas most CS facilities concentrate on the application of wear or corrosion-resistant coatings, CGDM is dedicated to the production of freeform components, whilst still retaining an inherent coating ability. The process can produce functional forms using novel manufacturing strategies that are unique to CS. This paper presents information on the process, and details the various strategies employed during component fabrication. It was possible to construct components from many materials, including titanium, which exhibited freeform surfaces, internal channels and embedded devices. A breakdown of the process economics is also provided, with and without helium recycling.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S-B) twinning mechanism is conservative. The possible wider significance of the S-B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

High-temperature electron backscatter diffraction and scanning electron microscopy imaging techniques: in-situ investigations of dynamic processes

In-situ heating experiments have been conducted at temperatures of approximately 1200 K utilising a new design of scanning electron microscope, the CamScan X500. The X500 has been designed to optimise the potential for electron backscatter diffraction (EBSD) analysis with concomitant in-situ heating experimentation. Features of the new design include an inclined field emission gun (FEG) column, which affords the EBSD geometrical requirement of a high (typically 160 degrees) angle between the incoming electron beam and specimen surface, but avoids complications in heating-stage design and operation by maintaining it in a horizontal orientation. Our studies have found that secondary electron and orientation contrast imaging has been possible for a variety of specimen materials up to a temperature of at least 900 degrees C, without significant degradation of imaging quality. Electron backscatter diffraction patterns have been acquired at temperatures of at least 900 degrees C and are of sufficient quality to allow automated data collection. Automated EBSD maps have been produced at temperatures between 200 degrees C and 700 degrees C in aluminium, brass, nickel, steel, quartz, and calcite, and even at temperatures >890 degrees C in pure titanium. The combination of scanning electron microscope imaging techniques and EBSD analysis with high-temperature in-situ experiments is a powerful tool for the observation of dynamic crystallographic and microstructural processes in metals, semiconductor materials, and ceramics.     

Standoff distance and bow shock phenomena in the Cold Spray process

Cold Spray involves the deposition of metallic powder particles using a supersonic gas jet. When the nozzle standoff distance is small, a bow shock is formed at the impingement zone between the supersonic jet and the substrate. It has long been thought that this bow shock is detrimental to process performance as it can reduce particle impact velocities. By using computational fluid dynamics, Particle Image Velocimetry and Schlieren imaging it was possible to show that the bow shock has a negative influence on deposition efficiency as a result of a reduction in particle velocity. Furthermore, the existence of the bow shock was shown to be dependent on the length of the nozzle's supersonic potential core. Experiments were carried out with aluminium, copper and titanium powders using a custom-made helium nozzle, operating at 2.0 MPa and 20 °C, and a commercial nitrogen nozzle operating at 3.0 MPa and 300 °C. In all cases, it was found that there is a direct relationship between standoff distance and deposition efficiency. At standoff distances less than 60 mm, the bow shock reduced deposition efficiencies by as much as 40%.     

Interfacial deformation mechanisms in hexagonal-close-packed metals

Deformation processes involving interfacial dislocation mechanisms in twin boundaries of hexagonal-close-packed (hcp) metals are described. The topological properties of individual defects, namely their Burgers vectors, b, and step heights, h, are defined rigorously, and the magnitude of the diffusional flux of material required for motion of a defect along an interface is expressed quantitatively in terms of b, h, and the material’s density. This framework enables interactions between defects to be treated and, in particular, enables identification of processes that are conservative. Using these topological arguments, it is shown that sessile interfacial defects in twins need not block further twinning and that the recently discovered Serra-Bacon (S—B) twinning mechanism is conservative. The possible wider significance of the S—B-type mechanism that causes localized lateral growth of twins is also considered briefly in the context of the deformation of hcp and martensitic materials. This article is based on a presentation made in the symposium entitled “Defect Properties and mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Lousiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Epigenetic Modifications and Non-Coding RNA in Diabetes-Mellitus-Induced Coronary Artery Disease: Pathophysiological Link and New Therapeutic Frontiers

Diabetes mellitus (DM) is a glucose metabolism disorder characterized by chronic hyperglycemia resulting from a deficit of insulin production and/or action. DM affects more than 1 in 10 adults, and it is associated with an increased risk of cardiovascular morbidity and mortality. Cardiovascular disease (CVD) accounts for two thirds of the overall deaths in diabetic patients, with coronary artery disease (CAD) and ischemic cardiomyopathy as the main contributors. Hyperglycemic damage on vascular endothelial cells leading to endothelial dysfunction represents the main initiating factor in the pathogenesis of diabetic vascular complications; however, the underlying pathophysiological mechanisms are still not entirely understood. This review addresses the current knowledge on the pathophysiological links between DM and CAD with a focus on the role of epigenetic modifications, including DNA methylation, histone modifications and noncoding RNA control. Increased knowledge of epigenetic mechanisms has contributed to the development of new pharmacological treatments (“epidrugs”) with epigenetic targets, although these approaches present several challenges. Specific epigenetic biomarkers may also be used to predict or detect the development and progression of diabetes complications. Further studies on diabetes and CAD epigenetics are needed in order to identify possible new therapeutic targets and advance personalized medicine with the prediction of individual drug responses and minimization of adverse effects.     

Critical Assessment 10: Tensile elongation of strong automotive steels as function of testpiece geometry

Total elongation as measured in tensile tests on strong steels is adopted in product specifications and development goals, and sometimes assumed to relate to formability. The quantity, however, is measured in a variety of ways in the reported literature and is often used to make unjustified comparisons. Using purposely generated experimental data, the effect of specimen geometry on the measured total elongation is critically assessed.