III. VINCULACIÓN CON LOS ORDENAMIENTOS JURÍDICOS APLICABLES EN MATERIA
III.5. Programa de Ordenamiento Ecológico Local del Municipio de Isla Mujeres, Quintana Roo,
Stationery laser spot on the powder bed
The aim of this chapter is to elucidate the effect on geometrical characteristics of a focussed laser beam at various laser power outputs on a Ti 6Al 4V powder bed. Experiments were carried out to analyse the influence of DMLS parameters particularly laser power on the melt pool size and morphology of the synthesized laser spot. A stationery Nd:YAG laser beam with power varying from 20W to 200W was struck on loose Ti6Al4V powder for ten seconds. A period of ten seconds was selected as this will produce specimens with measureable size even at the lowest laser power. The as built structure due to this condition was reported and analysed prior to a powder consolidation mechanism. Results showed that at each laser power a blob structure or a spherical droplet was formed consisting of a melted core and surrounded by incompletely sintered powder particles similar to a hemispherical shape. The specimen sizes varied from 1mm to 8mm in diameter with increasing laser power. Metallographic analysis showed that the neck size between powder particles varied with the laser power depending on the location and distance from the centre of the laser beam spot. The spherical droplets or blobs had a bigger radial distribution compared with their depth with increasing laser power.
4.1 Introduction
Direct metal laser sintering (DMLS) is considered to be a feasible way of producing complex parts for low production runs. Many studies have shown that this technique, which employs additive manufacturing principles, is capable of producing functional parts with mechanical properties comparable to more conventionally made parts[1-‐4]. Therefore, much of the research on sintering processing was directed towards extending the use of this technology in critical areas such as aerospace and biomedical applications where customised design is of prime importance. However, the emphasis of many of the studies was more on the applications rather than on fundamental studies of the sintering mechanism, particularly on laser-‐material interaction. This is because, many
researchers had difficulty understanding the sintering process which involves a large number of mutually influential, complicated parameters [1, 5].
Based on typical laser sintering definitions, a sintered structure consists of particles bonded by necks where only partial or surface melting has occurred (binding at the interfacial grain contact area). However, with the advances in laser and powder technologies, the DMLS process is indeed capable of producing fully density parts where the powder particles are fully melted during the process. Of particular interest is the heat transfer mechanism where, during laser-‐ material interaction, a sufficient amount of energy (heat) is absorbed by the powder which causes phase changes from solid powder to liquid and finally back to solid. This interaction happens very quickly, within microseconds, depending on the processing parameters. Subsequently, residual stresses develop in a laser sintered part and this influences the integrity of the part[6, 7]. With a greater understanding of process-‐microstructure relationships, one can modify and manipulate the process parameters so that desired properties can be achieved. 4.2 Experimental & Processing Conditions
The loose titanium alloy powder used is known as EOS Ti6Al4V with an average particle size of 40µm. The powder was sieved using a 63µm filter and placed in the powder container in the building chamber with a layer height of 10mm and without any substrate. This was to provide a sufficiently large enough processing zone without the influence of a substrate. Argon gas was flushed through until the oxygen level dropped to 0.01% and the chamber temperature was set 80˚C throughout the experiment. Powder scanning was accomplished using a Nd:YAG laser beam (1.06µm wavelength). The processing parameters were adjusted so that the laser strike on the powder bed was accomplished in accordance with the set parameters.
There was a long enough gap between each laser strike so that each sintered structure, corresponding to its laser power, could be distinguished. The scan speed was set to zero which means that the laser was kept motionless during the experiment and the time duration for each strike was 10 seconds. As-‐built specimens were collected and labelled corresponding to the laser power used.
A Quantachrome Instrument, Ultrapycnometry 1000 was used to calculate the density of the laser sintered part. The operation of this instrument is based on Archimedes principle and Boyle’s Law. Nitrogen gas was used instead of Helium which can penetrate the finest pores of the sample near to 0.25nm.
Microstructural examination was performed using an optical microscope and Scanning Electron Microscope (SEM) equipped with an EDS analysis system. Phases were identified determined by the XRD method using the Philip X-‐Ray Diffractometer. Microstructural examination was carried out on the polished cross-‐section of specimens, etched with the Kroll’s reagent (100ml of distilled water, 3ml of HF, 6ml of HNO3). Measurement of the cracks, particle sizes and other structural features were made using the Image Analysis software on the optical microscope.
4.3 Results
A crucial aspect of the DMLS process is the heat flow from a laser strike on the powder bed with a highly focused beam occurring predominantly downwards through the previously solidified layer or substrate. In this experiment, the powder bed was set at 10mm thick in order to avoid the influence of the substrate. Within this relatively long exposure time, the material is continuously heated to above or below the β transus temperature with each laser beam pass. Therefore, the microstructure occurring in a layered structure may be complex and the degree of complexity will depend on the thermal history.
Figure 4.1: The features of a blob, showing a hole and cross section