摘要
作为快速成形技术的一个重要分支,利用选择性激光烧结技术烧结金属粉末成形金属零件或块体是 20 世纪 90 年代初发展起来的能直接制造任意几何形状的金属零部件的一项新技术。由于材料和工艺因素的限制,直接用金属粉末激光烧结制造金属零件还存在一定不足之处,球化是其中的突出问题之一。球化效应的产生使得烧结线不连续,成形过程难于持续进行;又导致成形件的内部孔隙增加,成形件强度和密度减小、表面粗糙度增大及尺寸精度降低。 本文对 316 不锈钢粉末进行了激光烧结实验,从材料和工艺参数两方面入手,初步分析和探讨了球化产生机理和影响球化的因素,并在优化粉末体系和工艺参数的基础上,成功烧结出具有一定形状、无明显球化的块体。
在对单组元粒度为 45μm 的 316 不锈钢粉末进行激光烧结的研究中发现,球化现象在较宽广的工艺条件下表现十分严重,成形困难,烧结试样由较大的球形颗粒构成。球化颗粒尺寸随扫描速度和粉层厚度(0.25-0.4mm 范围内)的增加而减小;球化颗粒尺寸随激光功率的增大而增大,但激光功率增大超过500W 时,球化颗粒尺寸则随功率的增大而有所减小。
本文还进行了双组元 45μm 与 80μm 不同粒度 316 不锈钢粉末的混合粉末体系和在 45μm 的 316 不锈钢粉末中添加特制的添加剂的激光烧结实验研究。研究发现,对该双组元粉末,在 750W 的激光功率、0.02m/s 的扫描速度下,成形出具有较好表面质量、无明显球化的块体。研究结果表明,采用颗粒尺寸成双峰分布的混合粉末体系有助于减小球化倾向。
实验还发现,316 不锈钢粉末的激光烧结对添加剂具有一定的选择性。选用 CuP 作为添加剂对改善 316 不锈钢粉末的球化没有明显作用,烧结结果不是很理想;选用钎剂 102 作为添加剂,在 750W 的激光功率、0.02m/s 的扫描速度条件下,可以烧结成形出具有一定形状的块体试样,研究发现,球化现象得到明显抑制,结果表明,对 316 不锈钢粉末的激光烧结成形,钎剂 102 是一种抑制球化的合理添加剂。
对以上两个烧结块体的微观组织分析表明,烧结试样组织以柱状晶和等轴晶两种组织形态交替叠加而成,形成整个烧结件的组织周期性重复出现。
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.
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