Further studies in selective laser melting of stainless and tool steel powders

https://doi.org/10.1016/j.ijmachtools.2006.09.013Get rights and content

Abstract

Previous reported work on the selective laser melting of a H13 tool steel powder bed surface has shown that there is a scan speed range in which the layer mass increases/fluctuates with increasing speed. This paper expands the investigation towards M2 tool steel and 316L stainless steel powders to identify if they reveal similar behaviours. Wide ranges of scan spacings and scan speeds have been examined, at selected laser powers. Furthermore, the masses of the layers have been compared with those predicted from an existing finite element thermal model. It has been found that at a constant laser power, the variation of mass with scan speed is much less than might be expected from a constant assumed absorptivity into a powder bed.

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

References (19)

  • F. Abe et al.

    The manufacturing of hard tools from metallic powders by selective laser melting

    Journal of Materials Processing Technology

    (2001)
  • M. Shiomi et al.

    Finite element analysis of melting and solidifying processes in laser rapid prototyping of metallic powders

    International Journal of Machine Tools and Manufacture

    (1999)
  • T.C. Tszeng et al.

    Thermal analysis of solidification by the temperature recovery method

    International Journal of Machine Tools and Manufacture

    (1989)
  • G.N. Levy, R. Schindel, J.P. Kruth, Rapid manufacturing and rapid tooling with layer manufacturing (LM) technologies,...
  • D.T. Pham et al.

    A comparison of rapid prototyping technologies

    International Journal of Machine Tools and Manufacture

    (1997)
  • T.H.C. Childs et al.

    Selective laser sintering (melting) of stainless and tool steel powders: experiments and modelling

    Proceedings of the Institution. of Mechanical Engineers, Part B: Journal of Engineering Manufacture

    (2005)
  • T.H.C. Childs et al.

    Raster scan selective laser melting of the surface layer of a tool steel powder bed

    Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture

    (2005)
  • T.H.C. Childs et al.

    Selective laser sintering of a crystalline and a glass-filled crystalline polymer: experiments and simulations

    Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture

    (2001)
  • C.M. Taylor, T.H.C. Childs, Morphology of direct SLS-processed stainless steel layers, in: proceedings of the solid...
There are more references available in the full text version of this article.

Cited by (170)

  • Enhancement of electrical conductivity and corrosion resistance by gold-nickel coating of additively manufactured AlSi10Mg alloy

    2022, Journal of Materials Research and Technology
    Citation Excerpt :

    Laser powder bed fusion (L-PBF) is currently the most widely deployed technique in the field of metal AM. The metals that have been successfully produced by AM are including titanium and its alloys [12,13], nickel-based alloys [14,15], iron-based alloys [16,17], and aluminum and its alloys [18,19]. The ability to use high power and fast scanning speed with applying high layer thickness has made aluminum alloy, particularly AlSi10Mg, one of the most effective materials for L-PBF manufacturing.

  • A comprehensive review on laser powder bed fusion of steels: Processing, microstructure, defects and control methods, mechanical properties, current challenges and future trends

    2022, Journal of Manufacturing Processes
    Citation Excerpt :

    These models are also helpful to predict the complex temperature field of molten melt pool, development of microstructure, residual stresses, distortion, warping and etc. Some of the researchers attempted to correlate experimental and modelling results of LPBF fabricated steels [81–83]. Childs et al. investigated the link between range of laser powers and scan speeds with respect to the formed melt tracks through experiments and modelling of LPBF of M2 tool steel, H13 tool steel, and 314S-HC stainless steels [81,82].

View all citing articles on Scopus
View full text