Selective Laser Melting of Duplex Stainless Steel
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Duplex stainless steels are a special class of alloys characterised by the equal amounts of austenite and ferrite in the microstructures, achieved through a balanced composition of the alloying elements. The dual phase structure makes them strong, tough and resistant to corrosion in extreme environments, combining the good properties of both the constituent phases. As a result, duplex steels became very useful and important in a wide range of applications including oil and gas, food and beverages, civil construction and architecture, and medical devices and implants. While the dual phase structure is essentially the reason for all the useful attributes of duplex steels, the same may also act as a hindrance to their performance, during and after the application of secondary processes. Thermo-mechanical interactions might lead to a loss of the equal phase duplex structures and machinability attributes are not often encouraging due to work hardening and formation of strong and continuous chips. Considering the significant properties and the applications, and problems associated with the secondary processing, the material system kept the challenges of improving existing methods and finding alternative ways of manufacturing alive. The past decade saw a tremendous growth in the additive manufacturing technologies, as a few of the more promising techniques grew beyond the simple rapid prototyping solutions. In particular with the metal powders, the selective laser melting process became a strong contestant in terms of the near net shape consolidation and higher densities. At the time this research project started, there was no evidence of any a priori data related to the laser based processing of duplex stainless steel powder through a layer-wise consolidation. This was the research gap identified and the research proposal was developed to answer the questions that will arise encompassing the possible immediate responses of duplex steels to laser melting conditions, process-structure-property relationships and abnormalities if any that may arise from the structures altered significantly due to the non-equilibrium conditions. Experimental plans were then drawn, appropriate equipment identified and the proposed research was conducted as planned, achieving the set objectives. Duplex stainless steels were proved to be suitable for processing by selective laser melting. Structural distortions were noted to arise due to the laser interactions, but post-process heat treatment procedures were observed to be effective in rectifying the problems to varying degrees. The maximum bulk density levels reached were at around 95%, while the mechanical and other properties showed no significant losses. The transformation of the duplex stainless steels from the hard to the soft magnetic states as a result of the structural alterations induced by selective laser melting is an additional research outcome. Further, the machinability of the material in the as-built state was found to be better than in the wrought case. As far as the published literature commonly available goes, the initial experimental trials conducted as part of this research were the first to apply selective laser melting to duplex steel powders. Other original contributions include the establishment of the ensuing metallographic and mechanical attributes of the samples printed with varying conditions and establishment of the anomalous magnetic responses and the unexpected improvement in the machinability responses.