Selective laser sintering of alumina

Selective Laser Sintering (SLS), a Solid Freeform Fabrication (rapid prototyping) technique has been applied to form parts of a single phase high temperature ceramic, alumina. Two approaches of forming the single phase alumina (i) using an Al inorganic binder and (ii) using an organic binder were investigated.; In the first approach, the aluminum melts under the laser and bonds the alumina particles. Some of the aluminum reacts with the ambient (air) to form alumina. The residual aluminum is oxidized in a subsequent heat-treatment step. The reflectivity of the alumina and aluminum powders are measured and these are related to the SLS parameters when using an aluminum binder. Linear expansion of the parts with oxidation heat-treatment is also examined. Green density decreases with increasing composition of aluminum but remains constant as a fraction of theoretical density. With increasing incident energy density the density of green parts increased. During the oxidation heat-treatment the extent of oxidation increases with time at heat-treatment temperature and with temperature. Some of the oxidized samples were infiltrated with CrO{dollar}sb3{dollar} solution to increase the density. They were subsequently heat-treated to obtain a solid solution of Al{dollar}rmsb2Osb3{dollar}-{dollar}rm Crsb2Osb3{dollar}. Infiltration with epoxy was done on some oxidized samples. The strength increased by a factor of 20 to 60 MPa after epoxy infiltration.; In the second approach the Selective Laser Sintering (SLS) process is used to prepare samples from Al{dollar}rmsb2Osb3{dollar}/polymer binder powders. Al{dollar}rmsb2Osb3{dollar} particles coated with polymer by spray-drying are found to form bars that were approximately twice as strong as could be formed from mixtures of alumina and polymer binder at the same binder level and processing conditions. In mixed systems, bar strengths were found to increase nearly in proportion to increases in polymer binder content over the range 20-40 vol% binder. Parts made in any particular laser scanning mode show optimum values for strength and density as the laser energy density is systematically increased from 2-8 cal/cm{dollar}sp2{dollar}. These optima result from the counteracting influences of energy density on binder fusion and thermal degradation. The optimum energy density is mode or geometry sensitive and shifts to lower values as the laser scanning vector is reduced. This behavior is probably the result of the lower heat losses. Equivalently better utilization of laser energy is associated with the shorter scan vectors. Some of the SLS fabricated bars were infiltrated with colloidal alumina, fired to remove the binder, and sintered at 1600 {dollar}spcirc{dollar}C to achieve alumina bars with 50% relative densities, interconnected porosity, and strengths between 2 and 14 MPa. The strengths obtained after the 1600 {dollar}spcirc{dollar}C sintering treatment were also found to be sensitive to SLS build orientation.