Laser Cladding

Laser cladding is a modern technology whose uses include, for example, the creation of protective coatings to reduce wear and corrosion on engine parts and tools. The process is now also often used for the purpose of Additive Manufacturing to fabricate whole parts layer-by-layer. The aircraft and automotive industries are examples of industries in which it is much employed. This account considers the theory of a number of aspects of the process in detail. The first to be studied is the interaction of the laser beam directly with the powder that is being deposited; the effects of gravity, beam shadowing, and particle heating are investigated. This is followed by a discussion of themechanisms by which the particles adhere to the surface of the work piece and are absorbed into it. In order to understand the process, a study of the melt pool and the associated temperature distribution is necessary; it is then possible to infer the final bead geometry. An inevitable consequence of a thermal process such as laser cladding is the induced thermal stress and resulting distortion of the work piece. The fundamentals are discussed, a numerical model presented and in addition a simple heuristic model is given. The use of induction-assisted laser cladding as a means of preventing the formation of cracks is discussed

Detailing Radio Frequency Heating Induced by Coronary Stents: A 7.0 Tesla Magnetic Resonance Study

The sensitivity gain of ultrahigh field Magnetic Resonance (UHF-MR) holds the promise to enhance spatial and temporal resolution. Such improvements could be beneficial for cardiovascular MR. However, intracoronary stents used for treatment of coronary artery disease are currently considered to be contra-indications for UHF-MR. The antenna effect induced by a stent together with RF wavelength shortening could increase local radiofrequency (RF) power deposition at 7.0 T and bears the potential to induce local heating, which might cause tissue damage. Realizing these constraints, this work examines RF heating effects of stents using electro-magnetic field (EMF) simulations and phantoms with properties that mimic myocardium. For this purpose, RF power deposition that exceeds the clinical limits was induced by a dedicated birdcage coil. Fiber optic probes and MR thermometry were applied for temperature monitoring using agarose phantoms containing copper tubes or coronary stents. The results demonstrate an agreement between RF heating induced temperature changes derived from EMF simulations versus MR thermometry. The birdcage coil tailored for RF heating was capable of irradiating power exceeding the specific-absorption rate (SAR) limits defined by the IEC guidelines by a factor of three. This setup afforded RF induced temperature changes up to +27 K in a reference phantom. The maximum extra temperature increase, induced by a copper tube or a coronary stent was less than 3 K. The coronary stents examined showed an RF heating behavior similar to a copper tube. Our results suggest that, if IEC guidelines for local/global SAR are followed, the extra RF heating induced in myocardial tissue by stents may not be significant versus the baseline heating induced by the energy deposited by a tailored cardiac transmit RF coil at 7.0 T, and may be smaller if not insignificant than the extra RF heating observed under the circumstances used in this study