Further studies in selective laser melting of stainless and tool steel powders
Introduction
One current trend in production is the shortening of lead times for product development. New processes, especially those in the field of layer manufacturing, support this trend. In addition, they open new possibilities for manufacturing [1]. One group of such processes, selective laser sintering or SLS, has become popular for rapidly manufacturing freeform parts using a wide range of materials [2]. Laser sintering of metallic materials shows great promise, but significant further research and understanding are still required for obtaining high-quality parts.
This paper is built on previous studies on selective laser melting (SLM) of single tracks [3] and single layers [4] of ferrous alloy powders in the surface of a deep powder bed by a scanning laser beam. SLM is an emerged name for the direct route of SLS when the complete melting of powder occurs rather than the sintering or partial melting [5]. The aim is to create a strong part that is usable without further post processing other than surface finishing. In the single track work [3], where M2 and H13 tool steel and 314S-HC stainless steel powders were examined, different track forms were identified, depending on the laser power and scan speed that was used. Explanations for the transitions between these various forms of track were given in terms of the melt pool dimensions, temperatures and temperature gradients that existed. In the single-layer work [4] through studying the topography of melted layers from H13 tool steel, the onset of porosity was addressed.
Subsequently the melted masses of single tracks and single layers were investigated at various process parameters [3], [4]. Since at constant laser power and (for layers) scan spacing, delivered energy density to the bed reduces with increasing speed, the initial expectation was that for any level of laser power, melted mass would reduce as scan speed increased. However, the reverse was observed in both single-track and single-layer works. In fact, there existed scan speed ranges in which the mass of melted track or layer increased (or fluctuated in the case of layer) with scan speed, at constant laser power and scan spacing. To study this, the amount of heat required to melt the observed mass was compared to the incident energy. An explanation was given in terms of absorptivity of the laser energy into the bed increasing with speed. But the observation for single track was not much emphasized as it was thought that the conditions, in which the increase occurred, being associated with the formation of elliptical section (non-flat) tracks, were not practically useful for processing. However, the simulated results of single-layer melting have shown that the variability of layer mass for the speed range is not predictable with a constant absorptivity α value. Indeed, understanding the causes of α variation becomes of central importance to fundamental studies of single-layer formation. This paper expands the investigation towards M2 tool steel and 316L and 314S-HC stainless steel powders to identify these material behaviours in single-layer melting, in conditions in which dense layers have been formed, that might form the foundation for the subsequent layers. The questions that it addresses are the variation of α in the SLM process and effects of scan spacing and thermal history on the melted mass. In an alternative approach for explanation of mass variations, an idea based on the process efficiency is developed later.
Section snippets
Modelling
An existing finite element thermal model for laser sintering of polymers [6] has been developed to predict the mass of melted metallic material by the scanning laser beam. It is a transient heat conduction three-dimensional model in which the temperature rise caused by the travelling laser beam is calculated [7]. The model takes into account the latent heat and the influence of both porosity and temperature on the thermal properties of the powder bed. In the temperature calculation, thermal
Experimentation
Single layers have been produced, by a scanning CO2 laser beam, in the surface of beds made from gas-atomized powders of composition and size fraction listed in Table 1. All powders were obtained from Osprey Metals Ltd, UK and, were spread and levelled in a flat tray of area 120 mm×150 mm to a depth of 5 mm. Square areas 15 mm ×15 mm were melted in a research SLS machine. The SLS equipment has been described before [12], [13]. A 250 W continuous wave CO2 laser beam, focused to a spot diameter of 0.6
Results and discussion
Fig. 1 shows the measured masses of the 15 mm ×15 mm squares of 316L and M2 as function of scan speed. It can be seen that both materials reveal more or less similar trends in variation of layer mass with scan speed. In all cases depending on the scan speed, four individual regions are distinguished. The first region is a low scan speed range, marked AB in Fig. 1(a), in which mass reduces rapidly with increasing speed. There is then a range BC in which mass increases with speed, the third CD
Conclusion
Experimental studies on melting single layers of M2 high speed steel and 316L and 314S-HC stainless steels in the surface of powder beds, by a raster-scanning laser beam have confirmed that variations of layer mass with scan speed are not consistent with what is expected from the delivered energy to the powder bed. Two explanations are suggested. The first one, which was supported by the simulation as well, suggests that absorptivity may increase with increasing scan speed. In the second one
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