Investigation Into Manufacturing Metal Parts Via Indirect Selective Laser Sintering

Selective laser sintering (SLS) is one of the most potential technologies among Rapid Prototyping & Manufacturing technologies (RP&M). It can manufacture complex metal parts of any types, therefore it has irreplaceable advantage over other traditional manufacture technologies. Nowadays,much attention is paid to manufacturing metal parts via SLS due to the enlarged application domain and high value of metal parts. Although it is reported recently that metal parts of some certain alloy systems have been produced via SLS in the world, there are even some technical problems, such as the complexity of preparation of powder materials, the high cost of powder materials and the difficulty of composition adjustment of powder materials. In addition, the investigation deficiency of forming mechanism, forming process and post process of SLS green shapes still exists. Therefore it is impossible to simply apply the recent investigation achievements to manufacture metal parts with high complexity and properties.For the existent problems, new special SLS composite metal powder materials are developed in this work, and the forming mechanism, the forming process of these powder materials and the post process of green shapes composed of these powder materials are investigated systematically and thoroughly. Some metal parts with high complexity and injection mould with conformal cooling channels also are manufactured via indirect SLS using abovementioned powder materials. Additionally, manufacturing dense metal parts via indirect SLS with the supporting of cold isostatic pressing (CIP) and hot isostatic pressing (HIP) is also investigated in this work, which develops a new direction for indirect SLS. Hence the work provides a good contribution to the application of indirect SLS both theoretically and practically.Aspect of forming theory: based on the surface and thermodynamic theories, the forming mechanism of polymer/metal mixed powders is studied. Exposed by laser, polymer material melts and flows to wet and bond the metal particles, and the final result is that the internal energy of powder system decreases to a relative stable state. The interfacial action and evolvement between epoxy and metal particles are also analyzed through interface theory, and under laser irradiation, the two surfaces are bonded through hydrogen bonds. Recurred to the theory of the effects of laser on materials, the absorption law of laser energy among powders is studied, and the concepts of both laser energy testing area and binding effective area are put forward.Aspect of forming materials: iron alloy system composite powders composed of alloy element powders are developed with epoxy as a binder material, and following the binder content calculating equation deduced by the packing relation between the binder particles and metal particles,the weight percent of epoxy among the composite powders is decided as 4%. Through sintering, infiltration and thermal treatment combined with the analysis of SEM microstructure and mechanical properties, the better composition of composite metal powders is determined as Fe8Cu4Ni0.5C. Sintered at 1200℃for 1h, infiltrated with Cu at 1200℃and quenched at 890℃followed by being tempered at 200℃, the copper phase in Cu-infiltrated Fe8Cu4Ni0.5C alloy is up to 50% or so and some mechanical properties of this Cu infiltrated alloy are: yield strength-483MPa, tensile strength-546MPa, Yang's modulus-116GPa, elongation-<3%, Brinell hardness->200. Sintered at 1200℃for 1h, infiltrated by liquid epoxy at 100℃and cured at 160℃for 6h, the tensile strength of this epoxy reinforced alloy is over 90MPa, its hardness (HRC) is 25-35.Aspect of forming process: not only recurred to the orthogonal experiments of both strength and dimension of green shapes but also connected with laser transmission characters, the absorption and loss of laser energy, the roller's rolling behavior and forming efficiency, the SLS forming process is studied and optimized. The final optimal SLS parameters are: laser power-14-15W, scan speed-2000mm/s, scan spacing-0.1mm, layer thickness-0.1mm, powder bed temperature-53℃. The dimension error of green shapes is below 1.5% on the condition of no shape error, and their bending strength reaches 2.42MPa using the optimal parameters.Aspect of manufacturing injection mold: Green shape of injection mold with internal conformal cooling channels is formed using the composite metal powders developed in this work and optimal forming parameters. After decomposition, vacuum sintering,dropping infiltration of liquid bronze (or infiltration of epoxy and curing) and being simply machined, the mold can be used for plastic injection. Some good quality plastic parts are obtained though plastic injection using the mold finally.Aspect of the composite manufacture composed of SLS and CIP(HIP): Green shapes of the parts with high complexity are formed by SLS, and CIP and HIP are also applied to press the parts to a certain density. Metal parts with high complexity are manufactured without shape error.In conclusion, this work further develops SLS technology in the direction of intelligent and high efficiency manufacturing metal parts with high complexity, high accuracy and fine property, and makes a good contribution to the enlarged effects exerted by SLS technology on manufacturing.