模具钢18Ni300选区激光熔融工艺优化方法的研究

STUDY ON PROCESS OPTIMIZATION METHOD OF SELECTIVE LASER MELTING FOR THE TOOLING STEEL HHN18NI300

果城

11749152

硕士

机械工程

朱强

2019-06-04

2019-06-28

哈尔滨工业大学

深圳

增材制造技术是基于离散堆积原理，通过材料的逐渐累积来实现材料成型的技术，这种制造技术可以突破加工模具的限制，实现复杂结构的自由制造，为模具钢18Ni300随形冷却模具的制造提供了新的解决思路。选区激光融化（SLM）具有产品致密度高、性能优越、加工周期短、原材料利用率较高等优势，受到工业界的关注，已成为实现规模化应用的主流增材制造技术之一。现有的SLM工艺及应用研究均以原装进口粉末为实验材料，为了探究SLM技术的实用性，本论文以国产18Ni300粉末作为实验材料，对粉末特征表征、SLM成型工艺、热处理制度进行了系统的研究。 首先对粉末特性进行了表征。粉末的化学成分满足DIN-17350标准，粉末的粒度分布、粉末的综合性能（松装密度、振实密度、休止角、崩溃角）均满足EOS出筛标准。粉末组成相大部分是马氏体，存在少量奥氏体。粉末的表面光滑，整体圆整度较高，有卫星粉存在。粉末内部有孔洞构成空心粉，但是空心粉含量很少。小颗粒粉末多见等轴晶粒，大颗粒粉末枝晶较明显。通过工业CT扫描打印的腔体，发现铺粉较均匀，在腔体壁和结构转角存在大的孔隙。通过Thermo-calc软件对凝固相变过程模拟，得出相随温度的变化路径。 然后研究了18Ni300的选区熔融工艺。研究结果显示：1) 试件的致密度随着激光功率的升高而升高，最终趋于一个稳定值。当扫描间距适当，随着激光功率的升高，表面粗糙度先降低后升高。2) 过高扫描速度会减少熔池所获得的能量，而降低试样的致密度，过低扫描速度导致熔体汇集球化，降低致密度，同时使表面质量恶化。3) 在能量供给不足时，降低扫描间距可以有限提高打印件的致密度。当能量供给充足的时候，扫描间距对致密度的影响不显著，但提高扫描间距会使得表面粗糙度上升。4) 打印件的硬度随着致密度的上升而逐渐上升至平稳状态，致密度随着表面粗糙度的上升而先减小后增大。5) 金相组织分析表明，试样的组织为很小的胞状结构和条束状结构，且条束状结构可以穿过熔池的边界，并朝向熔池的中心区域。成型件为立方织构，且晶界大多为小角度晶界。 最后研究了打印件的热处理制度。进行固溶时效处理后，内部的亚晶结构和熔池边界消失，取而代之的是板条马氏体和原有的奥氏体晶界，合金元素通过固溶处理重新吸收进入基体。直接进行时效处理，原始的胞状结构得以保存，且硬度和强度略高于固溶时效处理的试样，但延伸率略低于固溶时效的试样。SEM对断口分析表明，原始打印件为塑性断裂，断裂机理为微孔聚集断裂。经过固溶时效处理和时效处理以后变为脆性断裂，断裂形式为准解理断裂。

Additive manufacturing technology is based on the principle of discrete stacking, through the gradual accumulation of materials to achieve material forming technology, this manufacturing technology can break through the limitations of processing molds, to achieve the free manufacturing of complex structures, for the manufacture of mold steel 18Ni300 conformal cooling mold Provide new solutions. Selective zone laser melting (SLM) has the advantages of high product density, superior performance, short processing cycle and high utilization rate of raw materials. It has attracted the attention of the industry and has become one of the mainstream additive manufacturing technologies for large-scale applications. The existing SLM process and application research are based on imported powder as experimental materials. In order to explore the practicality of SLM technology, this paper uses domestic 18Ni300 powder as experimental material to systematically characterize powder, SLM manufacturing process and heat treatment system. the study. First, the characteristics of the powder were characterized. The chemical composition of the powder satisfies the DIN-17350 standard, and the particle size distribution of the powder and the comprehensive properties of the powder (loose density, tap density, angle of repose, and collapse angle) satisfy the EOS standard after sieving. Most of the powder composition phase is martensite, and a small amount of austenite is present. The surface of the powder is smooth and the roundness of the powder is generally at high standard, and satellite powder is present. The inside of the powder has pores to form a hollow powder, but the hollow powder has a small content and has less influence on printing. Due to rapid cooling, the small particle powder is more common in equiaxed grains, and the large particle powder dendrites are more obvious. Through the industrial CT scanning of the printed cavity, it was found that the powdering was relatively uniform, and there were large pores in the cavity wall and the structural corner. The simulation of the properties by the Thermo-calc software yields a path of change with temperature. Then the selective melting process of 18Ni300 was studied. The results of the study show: 1) The density of the sample piece increases as the laser power increases, eventually reaching a stable value. When the scanning pitch is appropriate, as the laser power increases, the surface roughness first decreases and then rises. 2) Excessive scanning speed reduces the energy obtained by the molten pool, and reduces the density of the sample. Too low scanning speed leads to melt collection spheroidization, reduce the density, and deteriorate the surface quality. 3) When the energy supply is insufficient, reducing the scanning pitch can increase the density of the sample, but there are still many voids. When the energy supply is sufficient, the influence of the scanning pitch on the density is not significant, but increasing the scanning pitch causes the surface roughness to rise. 4) The hardness of the print gradually rises to a steady state as the density increases, and the density decreases first and then increases as the surface roughness increases. 5) Metallographic analysis shows that the microstructure of the sample is a small cell structure and a bundle structure, and the bundle structure can pass through the boundary of the molten pool and face the center of the molten pool. The molded part has a cubic texture, and the grain boundaries are mostly small-angle grain boundaries. Finally, the heat treatment system of laser melting 18Ni300 in selected area was studied. After the solution treatment and aging treatment, the internal sub-grain structure and the molten pool boundary disappear, and the lath martensite and the original austenite grain boundary are replaced, and the alloy elements are reabsorbed into the matrix by solution treatment. Directly aging treatment, the original cell structure was preserved, and the hardness and strength were slightly higher than those of the solution treated with aging, but the elongation was slightly lower than that of the solution with aging. The SEM analysis of the fracture shows that the original print is plastic fracture and the fracture mechanism is micropore aggregation fracture. After solution aging treatment and aging treatment, it becomes brittle fracture, and the fracture form is quasi-cleavage fracture.

选区激光熔融 马氏体时效钢 粉末特性 工艺参数 热处理

selective laser melting maraging steel powder characteristics process parameters heat treatment

中文

联合培养

南方科技大学

果城. 模具钢18Ni300选区激光熔融工艺优化方法的研究[D]. 深圳. 哈尔滨工业大学,2019.