INSTABILITY OF SCAN TRACKS OF SELECTIVE LASER SINTERING OF HIGH SPEED STEEL POWDER H.J. Niu and I.T.H. Chang School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK (Received March 5, 1999) (Accepted in revised form August 10, 1999) Keywords: Laser sintering; Steel and surface Introduction Selective laser sintering (SLS) is one of the leading commercial rapid prototyping (RP) processes, which produces three dimensional fully functional and prototype parts directly from polymers, metals and ceramic materials without the use of any tooling (1). In this process, a laser is scanned across the surface of a loosely packed power bed, allowing the sintering of the powders into the required shape. The whole part is therefore constructed layer by layer from the bottom to the top. However, as SLS is carried out line by line, laser scan causes melting along a row of powder particles, thereby forming a track of molten region of cylindrical shape. This cylinder is likely to break up into a row of spheres so as to reduce the surface area, leading to the formation of the agglomerates, i.e. “balling” phenomenon as observed in the SLS processing of pure metals and alloys, such as tin, stainless steel and high speed steel (1–3). The instability of the liquid cylinder was originally described by Rayleigh (4) and further developed by Nichols and Mullins (5). Their analyses show that a cylinder of water is unstable in front of sinusoidal fluctuations when the wavelength  is greater than the cylinder circumference 2R. The reduction of the surface energy with decreasing surface area is the driving force for the break-up of the liquid cylinder. However, for high energy density processes, such as SLS, a steep thermal gradient is developed between the centre and edge of the melt pool at the surface. As the surface tension is a function of temperature, the presence of a temperature gradient causes a corresponding variation of the surface tension between the centre and edge of the melt pool. The surface tension gradient will induce a Marangoni flow from a region of low surface tension to a region of high surface tension (6). This fluid flow will produce an extra force exerted on the molten track of SLS samples and influence the balling phenomenon. SLS of the water atomised M3/2 high speed steel (HSS) powder with different particle sizes tends to produce poor surface density with large agglomerates and inter-agglomerate pores (7). However, SLS of the gas atomised M2 HSS powder results in a homogeneous structure with a highly dense surface (7). This is attributed to high packing density, spherical shape and a low oxygen content of the gas atomised M2 powder particles. In this work, different sizes of water/gas atomised M3/2 and M2 HSS powders were laser-sintered by scanning a single track on the powder bed at various laser powers and scan rates. Pergamon Scripta Materialia, Vol. 41, No. 11, pp. 1229 –1234, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00276-6 1229