THE EFFECTS OF PRE-ALLOYED STEELS POWDER COMPOSITIONS ON BUILD QUALITY IN DIRECT METAL LASER RE-MELTING. SP Akhtar, CS Wright and M Youseffi Engineering Materials Research Group, University of Bradford, Bradford, UK Abstract This paper describes recent studies investigating the relationships between alloy composition and processability of a range of pre-alloyed stainless and tool steel powders melted using CO 2 and Nd:YAG lasers. Factors considered in this paper that influence processability are particle size and shape, particle distribution, fluidity, wetting, and scan conditions. However, thermodynamic modelling, validated by experiment, suggests that alloy composition is a significant factor affecting processability. A short solidification range corresponds to a significant increase in build quality. Introduction Selective laser sintering (SLS) is one of the more important solid freeform manufacturing processes developed in the last 10-15 years [1]. One of the main applications of SLS is for the manufacture of prototype or low volume production tooling using steel powders. Commercial systems are available which generate parts from either polymer coated steel powders [DTM Rapid Tool 2.0 TM ] or by processing a mixture of metal powders that contains a low melting point component [EOS Direct Tool TM ]. The need to post process, i.e. infiltrate with bronze, or the reliance on a low melting point liquid, leads to components with poor mechanical and tribolological properties. This has led to a number of studies aimed at investigating Direct Metal Laser Re-melting (DMLR) pre-alloyed metal powders with the aim of eliminating the above disadvantages. These studies generally made use of commercially available powder stainless steel [2, 3], and tool steels [4 - 6]. Stainless Steel powders have been DMLR processed to give multi-layer samples with densities of +99% [3] compared with only 60% for HSS [6]. This paper presents some results of ongoing investigations into the melting / densification behaviour of pre-alloyed metal powders during DMLR which has the ultimate aim of identifying the factors which control the ability to fabricate fully dense, multi- layer components. Experimental Procedure Five types of powder were used in this investigation. Annealed water atomised M2 (WA-M2), gas atomised M2 (GA-M2), gas atomised H13 (GA-H13) with varying contents of carbon and water atomised P58 (WA-P58), an experimental high speed steel (Fe-14Mo-4Cr- 14C) . The water atomised M2 was sourced from Powdrex, UK and the gas atomised powders were obtained from Osprey Metals Ltd, UK. Water atomised powders were sieved in compliance with ASTM standard E11 to give the following size fractions; >75-150µm, >38- 75µm and <38µm for water atomised powders. Identical size fractions were obtained for gas atomised powders from the suppliers. Direct metal laser re-melting studies were performed on specially constructed machines at the Universities of Leeds and Liverpool. The Leeds machine includes a 250W continuous wave (CW) CO 2 laser. Initial studies were conducted at a beam size of 1.1mm diameter. This was subsequently reduced to 0.55mm diameter. Sintering was performed in an 141