Nanjing University Of Aeronautics And Astronautics The Graduate School

As an important branch of Rapid Prototyping (RP), Selective Laser Sintering(SLS) of metal powder, which is also named Direct Metal Laser Sintering (DMLS),is a recently developed technique which owns the capability of fabricating metalparts with the arbitrary geometry. Due to the limitation of material properties andprocessing conditions used, lots of defects exist in the parts processed by DMLS,such as "balling" effect, warping deformation, low sintered density, and poormechanical strength. Balling effect is the most serious defect which is unique toDMLS. The occurrence of balling resulted in the formation of discontinuoussintered tracks and high porosity in the sintered structure. This may in turnlowered the density and strength of the sintered parts, increased the surfacecoarseness of the parts, and decreased the dimension precision. In the presented paper, some experiments on DMLS of 316 stainless steelpowder were carried out. Basing on these, the mechanism of balling occurrenceand the effects of process parameters (laser power, scan speed, and layer thickness)on balling effect were investigated. Using optimized powder system and processparameters, two specimens without obvious balling particles were sinteredsuccessfully, in approximate oblong shape. At different process parameters, DMLS of 316 stainless steel powder with theparticle size of 45μm were done. It was found that the balling was serious duringlaser sintering, and sintered specimens were made up of big balling particles. Thesize of balling particles reduced with increasing scan rate and layer thicknesswithin the range 0.25-0.4mm. The size of balling particles increased withincreasing laser power below 500W; while the laser power increased above 500W,the balling trend showed a decrease. DMLS of a powder mixture, which was blended with two kinds of 316stainless steel powder with different particle size of 45μm and 80μm, wassuccessfully carried out. At the laser power of 750W and the scan rate of 0.02m/s,the part was sintered, with sound surface quality and without obvious balling. Themicrostructures of the sintered part were mainly consisted of directional crystalsand isometric crystals.The result showed that a bimodal mixture produced by mixing coarse and finepowders, each of which has the particle size in Gauss distribution, can be used toreduce the trend of balling. It also was found that pre-alloyed CuP powder was useless to improvesintering ability of 316 stainless steel powders. Using the QJ102 as an additive,the part without obvious balling was sintered. It showed that QJ102 was aneffective additive in restraining balling occurrence during laser sintering of 316stainless steel powders.