Advanced Materials Research Vols. 383-390

Abstract: A mathematical model is developed for the simulation of keyhole and molten pool formation during laser beam welding. Heat transfer from the key hole to the weld pool and details of the fluid flow play a major role in determining the weld bead profile. The simulation is performed based on the control volume method (CVM) and a volumetric heat source is used to model the laser beam and the equations of mass, momentum and energy are solved. The effect of Peclet number, Marangoni number, and Nusselt number is studied by varying process parameters like welding speed and beam power. The simulated bead profiles are compared with the experimental profile and are found to be acceptable.

Abstract: In today’s highly competitive market, laser cutting which has a characteristic of “make to order” and high product variety is under pressure to reduce costs, to increase productivity and to respond to the rapidly changing demands from customers. To maintain the competitive advantage, companies need to have a real-time dynamic scheduling system, which can handle large combinations of jobs, allowing sequencing of jobs to achieve multi-objective goals. Motivated by a real-life scheduling problem in a sheet metal processing company in Malaysia, this research addressed single machine scheduling problem with sequence-dependent setup times and group technology assumption to minimize makespan and with the secondary objective of minimizing setup times. The focus of this paper is on developing a simple heuristic algorithm based dynamic scheduling system. This algorithm has been coded in vb.net and is integrated with a database system. The scheduling system developed is verified and validated by comparing to the actual production run. Results show that the algorithm model can find good solutions within short computational time.

Abstract: This work presents the results of a study to determine the wear behavior of components manufactured by direct metal laser sintering (DMLS). Wear is an important issue in using layer manufactured parts for functional application. Two different bushes were selected for the functional testing for wear behavior studies. Specimens (bushes) were manufactured by DMLS technology and was tested for wear behavior and compared with bushes manufactured by conventional manufacturing methods. Components were manufactured by using the process parameters like sintering speed, hatch spacing, post contouring speed, hatch type and infiltration with an optimized value. Testing was carried out for bushes, used for an automobile engine starter motor. A comparative study for the wear behavior was carried out and results are discussed.

Abstract: In the present study, a low average power pulsed Nd:YAG laser is used for bead-on-plate welding of low carbon steel plate. The effect of laser process parameters on the weld quality and weld bead profile, welding penetration (P), welding surface width (W) and welding pool area (A), is studied. Focal point position, welding speed, pulse duration, and energy are selected as the process variables. The formation of a keyhole and weld bead profile is found to be highly dependent on laser pulse parameters. Results show that the maximum penetration accurse when the focal point is above the surface, in pulse duration of 6 ms, welding speed of 2 mm/s and pulse energy of 16.67 J.

Abstract: Direct Metal Selective Laser Sintering (DMSLS) is a layer-by-layer additive process for metal powders, which allows quick production of complex geometry parts. The aim of this study is to analyse the improvement of DMSLS with “EOSINT M270”, the new laser sintering machine developed by EOS. Tests were made on sintered parts of Direct Metal 20 (DM20), a bronze based powder with a mean grain dimension of 20 μm. Different properties and accuracy were evaluated for samples manufactured with three different exposure strategies. Besides mechanical properties, the manufacturing process was also examined in order to evaluate its characteristics. The quality of laser sintered parts is too affected by operator experience and skill. Furthermore, critical phases are not automatic and this causes an extension of time required for the production. Due to these limitations, DMSLS can be used for Rapid Manufacturing, but it is especially suitable to few sample series.

Abstract: In this study bead-on-plate tests on 3 mm-thick Ti6Al4V sheets were performed using a Yb:YAG thin disk laser in continuous mode to evaluate the effects of focus position on the bead characteristics. Among the various solid-state lasers, the disk laser has better beam quality than conventional rod geometries thanks to the absence of thermal lensing effect. An experimental plan was conducted using nine focus positions with constant power and welding speed. The beads were characterised in terms of geometry, defects, hardness and microstructure. This work is a preliminary study to select the best values of the focal position for subsequent butt welding tests. Analysing the results obtained, the best value of defocusing was -4 mm because, among full penetrated beads, it showed lower undercut and excessive penetration, lower melted area and smaller grain size in the melted area.

Abstract: Aluminum alloy 2024 is extensively used in automotive and aerospace industries, but its application is limited due to the susceptibility to generate porosity during the welding process. Nevertheless, benefits from laser welding are clearly demonstrated. In addition, the use of a disk laser allows to obtain significant reduction in focus diameter and increased beam quality compared to traditional rod or slab lasers. The aim of the work is to discuss porosity formation as influenced by the thermal input provided, so bead-on-plate specimens in different conditions have been prepared. Porosity content is examined in relation to the fused zone extent and discussed considering interaction between laser and material. Higher thermal inputs are beneficial in full penetrative welds.

Abstract: Quantum dots applied in solar cells will be of great importance to enhance the quantum tunneling efficiency and improve the photogenerated current transport. In this study, a new easy-to-operate technology was developed to fabricate germanium-silicon quantum dots in a SiOx matrix. The quantum dots were formed by first deposited germanium-rich SiO on quartz substrate using pulsed laser deposition technique and then annealed under a comparatively high temperature. We have demonstrated a stable and low-cost fabrication process which is much cheaper than the epitaxy method to provide for the fabrication of high density germanium-silicon quantum dots. Quantum dots with diameters of 3~4 nm embedded in the amorphous SiOx layer were clearly observed. The morphological features of the thin film were characterized. The optical properties were performed by Raman spectroscopy, photoluminescence spectrum and XRD test respectively to verify the crystallization of quantum dots in the SiOx matrix. Reflectance spectrum displayed a high light absorption rate in a spectra region from 300 nm to 1200 nm, evidencing that germanium-silicon quantum dots have promising features to be used as absorber for photovoltaic application.

Abstract: This paper presents an on-the-fly method to synchronize a laser scanner and a linear stage for fast and wide area fabrication. Demands for high-precision laser processing with wide area have been increasing for a number of applications such as in solar cell batteries, display parts, electronic components, and the automobile industry. An ultra high-speed laser scanner system is limited by its size of scanning area, for example, its scanning area is only 50 × 50 mm2. Recently, a laser scanner system that could scan a wide area was developed; however, it was found that the scanning quality of this scanner decreased with an increase in the scanning area. The developed method is different from the existing step and scanning method in that it our method ensures continuous stage movement, which may improve the processing speed and in turn the laser fabrication quality.

Abstract: Influence of using two oxide layers in the both sides of active layer with different position and aperture size on Vertical Cavity Surface Emitting Laser (VCSEL) performance is analyzed showing effects on the output power, single mode operation and threshold current. In addition, for improving speed we use ion implant area along with thick oxide layer to minimize parasitic elements. As the result, the proposed design exhibits much better stability of the fundamental mode over a wider current range, much higher output power, lower threshold current, than the conventional one with a high frequency response.