Volume 26, Issue 3, August 2014
Index of content:
- Technical Articles
26(2014); http://dx.doi.org/10.2351/1.4869257View Description Hide Description
Laser-assisted chemical vapor deposition (LACVD) is an attractive maskless process for growing locally carbon nanotubes at selected places on substrates that may contain temperature-sensitive components. This review gives a comprehensive overview of the reported research with respect to laser assisted CVD for the growth of carbon nanotubes. The advantages and disadvantages of local growth using laser sources are discussed, with a focus on structural quality and properties, such as length, position and alignment, and process control. The paper is divided into two parts. The first part deals with the influence that the main parameters for nanotube growth—gas, catalyst and thermal energy—have on the growth of carbon nanotubes by laser-assisted synthesis. The second part deals with the attempts and successes to control different aspects of local nanotube growth using a laser-assisted growth method.
Laser cladding with scanning optics: Effect of scanning frequency and laser beam power density on cladding process26(2014); http://dx.doi.org/10.2351/1.4868895View Description Hide Description
Scanning optics is an effective way to manipulate a laser beam for laser cladding. The numerical adjustment of the scanner gives a great deal of flexibility to the cladding process. However, the effect of the scanned beam on the cladding process itself has not been studied very thoroughly so far. This study concentrates on explaining how the scanning frequency and power density of the laser beam affect the stability of the cladding process. The results showed that both of these factors significantly influence the process stability and the outcome of the cladding process. If the local specific energy input was over 2.46 J/mm2, the process was noticed to be unstable. This limit was cross when scanning frequency was under 40 Hz. Power density's limit value for stable process was found to be 191 kW/cm2 and higher power densities than this was found to produce unstable process. If the cladding process was found to be unstable, dilution increased significantly and process started to resemble more laser alloying.
26(2014); http://dx.doi.org/10.2351/1.4869499View Description Hide Description
LMD (Laser Metal Deposition) is a technique used to repair damaged components and directly form three dimensional structures. From the aspect of productivity, improvement of the shape accuracy of the deposit is required to reduce machining process after LMD. Objective of this work is to control the build-up shape by adjusting the weld pool size. Configurations of deposits made with various LMD process conditions were measured in cross section. In addition, the signal intensity of heat radiation, plume emissions and laser reflections from the weld pool was monitored by photodiodes intergrated in laser processing head during LMD process. The results showed a strong correlation between weld pool size and signal intensity of heat radiation. Adaptive shape control system, which consists of in process monitoring system and PID (Proportional-Integral-Derivative) controller using that correlation, was developed to control build-up shape. Due to the liner relationship with weld pool size, laser power was selected as the PID output parameter adjusted by the heat radiation signal. Since the target value of the heat radiation signal can be changed flexibly during the LMD process in our PID control system, deposit shape is successfully controlled as required with sufficient profile accuracy of the build-up layers.
26(2014); http://dx.doi.org/10.2351/1.4870675View Description Hide Description
Optical scattering in electrospun poly(ε-caprolactone) (ES-PCL) nanofibers was studied. Femtosecond laser beams with wavelengths of 775 and 387.5 nm were directed onto PCL nanofiber meshes of different thicknesses, and the reflection and transmission were measured by using an integrating sphere. Meshes were prepared by electrospinning PCL in acetone and dichloromethane (DCM). The absorption and scattering coefficients of the samples were calculated using a three-flux scattering approximation. The PCL/acetone meshes had finer fibers, smaller pore size, and 50% larger scattering coefficients than the PCL/DCM meshes. In addition, somewhat higher scattering coefficients were measured at shorter wavelength in both PCL/Ace and PCL/DCM nanofibers. However, in all cases, scattering coefficients were 15 to 30 times the absorption coefficients; thus, scattering was the dominant factor in optical attenuation in both types of meshes and at both wavelengths.