选区激光熔化工艺参数对气雾化316L不锈钢粉末成形制品性能的影响
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摘要
本研究系统考察了激光功率和扫描速度对316L不锈钢粉末选区激光熔化工艺成形熔道、制品微观组织及力学性能的影响,并分析了各类缺陷的形成原因。研究结果表明:在低激光功率和高扫描速度条件下,熔道中出现了大量球状颗粒,这些颗粒之间的空隙恶化了下一层粉末的熔化条件,这正是成形制品中熔道分布混乱以及孔洞、裂纹产生的根本原因,进而导致成形制品力学性能降低;在高激光功率和低扫描速度条件下,熔池快速升温/冷却的热应力作用增强,使得成形制品的熔道交界处也存在孔洞和裂纹等缺陷。在本研究实验条件下,激光功率为350 W,扫描速度为1750 mm/s时,SLM成形制品的力学性能最为优异,其中抗拉强度为731 MPa、屈服强度为638 MPa、断后伸长率为40.0%,致密度为96.27%。
Recently, an additive manufacturing method, a supplement to the traditional manufacturing industry,has gained a lot of attentions due to the advantages of rapid preparation of complex parts. With the improvement of additive manufacturing technology, the range of additive manufacturing raw materials has been expanded from the organic materials with low melting points to the metal materials with the high melting points, which extends its application range to the fields of aerospace and automobile manufacturing. The selective laser melting(SLM) is a mainstream technology of additive manufacturing with the metal materials. However, the supply of high quality metal powder has become an obstacle for the SLM development. Currently, the amount of stainless steel powder, such as 316 L, used in molds accounts for about 20% of the global 3 D printing metal materials. In this study, the 316 L stainless steel powder with the low oxygen content, fine particle size and good circularity was prepared by the vacuum gas atomization. The chemical composition and physical properties, including the angle of repose, the bulk density and the tap density, of the 316 L stainless steel powder all meet the requirements of SLM process. In addition, the effects of selective laser melting(SLM) process parameters, including the laser power and scanning speed, on the melt channel, microstructure and mechanical properties of SLM forming products were investigated in detail. The formation reasons of various defects were also analyzed. The results could be summarized as follows. Under the conditions of the low laser power or the high scanning speed, due to the insufficient laser energy, the powder in the molten pool is not completely melted, resulting in the increasing viscosity of the molten pool and the poor wettability between the molten pool and substrate. Therefore, a large number of spherical particles appear in the melt channel, and in the severe cases, the melt channel could be even broken. In addition, there is a large amount of voids between the spherical particles, which further deteriorates the melting condition of the next layer of powder. It is the root cause of disordered distribution of the melt channels and the generation of holes and cracks in the longitudinal and horizontal sections of SLM forming product. During the stretching process, the above defects are the source of cracks causing the fracture of SLM forming product,which further leads to a decrease in the mechanical properties of SLM forming product, such as the tensile strength, yield strength, elongation at break and relatively density. Under the conditions of the high laser power and low scanning speed, the Marangoni flow inside the molten pool enhanced and exhibits instability. In addition,the rapid heating or cooling processes of molten pool enhances the thermal stress in the SLM forming product.The above phenomenon leads to the fact that the defects, including holes and cracks, could also be found in the SLM forming product. And these defects are generally present at the junction of adjacent melt channel in the same or different powder layers. Furthermore, under the experimental conditions of this work, when the laser power is 350 W and the scanning speed is 1750 mm/s, the mechanical properties of SLM forming products are best,which includes a tensile strength of 731 MPa, a yield strength of 638 MPa, an elongation of 40.0% and a relatively density of 96.27%.
Recently, an additive manufacturing method, a supplement to the traditional manufacturing industry,has gained a lot of attentions due to the advantages of rapid preparation of complex parts. With the improvement of additive manufacturing technology, the range of additive manufacturing raw materials has been expanded from the organic materials with low melting points to the metal materials with the high melting points, which extends its application range to the fields of aerospace and automobile manufacturing. The selective laser melting(SLM) is a mainstream technology of additive manufacturing with the metal materials. However, the supply of high quality metal powder has become an obstacle for the SLM development. Currently, the amount of stainless steel powder, such as 316 L, used in molds accounts for about 20% of the global 3 D printing metal materials. In this study, the 316 L stainless steel powder with the low oxygen content, fine particle size and good circularity was prepared by the vacuum gas atomization. The chemical composition and physical properties, including the angle of repose, the bulk density and the tap density, of the 316 L stainless steel powder all meet the requirements of SLM process. In addition, the effects of selective laser melting(SLM) process parameters, including the laser power and scanning speed, on the melt channel, microstructure and mechanical properties of SLM forming products were investigated in detail. The formation reasons of various defects were also analyzed. The results could be summarized as follows. Under the conditions of the low laser power or the high scanning speed, due to the insufficient laser energy, the powder in the molten pool is not completely melted, resulting in the increasing viscosity of the molten pool and the poor wettability between the molten pool and substrate. Therefore, a large number of spherical particles appear in the melt channel, and in the severe cases, the melt channel could be even broken. In addition, there is a large amount of voids between the spherical particles, which further deteriorates the melting condition of the next layer of powder. It is the root cause of disordered distribution of the melt channels and the generation of holes and cracks in the longitudinal and horizontal sections of SLM forming product. During the stretching process, the above defects are the source of cracks causing the fracture of SLM forming product,which further leads to a decrease in the mechanical properties of SLM forming product, such as the tensile strength, yield strength, elongation at break and relatively density. Under the conditions of the high laser power and low scanning speed, the Marangoni flow inside the molten pool enhanced and exhibits instability. In addition,the rapid heating or cooling processes of molten pool enhances the thermal stress in the SLM forming product.The above phenomenon leads to the fact that the defects, including holes and cracks, could also be found in the SLM forming product. And these defects are generally present at the junction of adjacent melt channel in the same or different powder layers. Furthermore, under the experimental conditions of this work, when the laser power is 350 W and the scanning speed is 1750 mm/s, the mechanical properties of SLM forming products are best,which includes a tensile strength of 731 MPa, a yield strength of 638 MPa, an elongation of 40.0% and a relatively density of 96.27%.
引文
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[9]史玉升,鲁中良,章文献,等.选择性激光熔化快速成形技术与装备[J].中国表面工程,2006,19(10):150-153.
[10]Gu D D,Shen Y F.Balling phenomena during direct laser sintering of multi-component Cu-based metal powder[J].Journal of Alloys and Compounds,2007,432(1-2):163-166.
[11]Nikolay K T,Sergei E M,Igor A Y,et al.Balling processes during selective laser treatment of powders[J].Rapid Prototyping Journal,2004,10:78-87.
[12]Gu D D,Shen Y F.Balling phenomena in direct laser sintering of stainless steel powder:Metallurgical mechanisms and control methods[J].Materials&Design.2009,30(8):2903-2910.
[13]Song B,Zhao X,Shuai L I,et al.Differences in microstructure and properties between selective laser melting and traditional manufacturing for fabrication of metal parts:A review[J].Frontiers of Mechanical Engineering,2015,10(2):111-125.
[14]Shiomi M,Osakada K,Nakamura K,et al.Residual stress within metallic model made by selective laser melting process[J].Cirp Annals-Manufacturing Technology,2004,53(1):195-198.
[15]Mercelis P,Kruth J P.Residual stresses in selective laser sintering and selective laser melting[J].2005 Solid Freeform Fabrication Symposium,2005,06-08(8):254-265.
[16]成汉华.焊接工艺学[M].北京：水利电力出版社,1995.
[17]Deev A A,Kuznetcov P A,Petrov S N.Anisotropy of mechanical properties and its correlation with the structure of the stainless steel 316L produced by the SLMmethod[J].Physics Procedia,2016,83:789-796.
[18]付立定.不锈钢粉末选择性激光熔化直接制造金属零件研究[D].武汉:华中科技大学,2008.
[2]LüL,Fuh J Y H,Wong Y S.Laser-induced materials and processes for rapid prototyping[M].Singapore:Springer Science&Business Media,2013.
[3]Karunakaran K P,Shanmuganathan P V,Jadhav S J,et al.Rapid prototyping of metallic parts and moulds[J].Journal of Materials Processing Technology,2000,105(3):371-381.
[4]Kruth J P,Froyen L,Van V J,et al.Selective laser melting of iron-based powder[J].Journal of Materials Processing Technology,2004,149(1-3):616-622.
[5]Abe F,Osakada K,Shiomi M,et al.The manufacturing of hard tools from metallic powders by selective laser melting[J].Journal of Materials Processing Technology,2001,111(1-3):210-213.
[6]杨永强,刘洋,宋长辉.金属零件3D打印技术现状及研究进展[J].机电工程技术,2013,42(4):1-7.
[7]曾光,韩志宇,梁书锦,等.金属零件3D打印技术的应用研究[J].中国材料进展,2014,33(6):376-382.
[8]Badrossamay M,Childs T H C.Further studies in selective laser melting of stainless and tool steel powders.International Journal of Machine Tools and Manufacture[J].2007,47(5):779-784.
[9]史玉升,鲁中良,章文献,等.选择性激光熔化快速成形技术与装备[J].中国表面工程,2006,19(10):150-153.
[10]Gu D D,Shen Y F.Balling phenomena during direct laser sintering of multi-component Cu-based metal powder[J].Journal of Alloys and Compounds,2007,432(1-2):163-166.
[11]Nikolay K T,Sergei E M,Igor A Y,et al.Balling processes during selective laser treatment of powders[J].Rapid Prototyping Journal,2004,10:78-87.
[12]Gu D D,Shen Y F.Balling phenomena in direct laser sintering of stainless steel powder:Metallurgical mechanisms and control methods[J].Materials&Design.2009,30(8):2903-2910.
[13]Song B,Zhao X,Shuai L I,et al.Differences in microstructure and properties between selective laser melting and traditional manufacturing for fabrication of metal parts:A review[J].Frontiers of Mechanical Engineering,2015,10(2):111-125.
[14]Shiomi M,Osakada K,Nakamura K,et al.Residual stress within metallic model made by selective laser melting process[J].Cirp Annals-Manufacturing Technology,2004,53(1):195-198.
[15]Mercelis P,Kruth J P.Residual stresses in selective laser sintering and selective laser melting[J].2005 Solid Freeform Fabrication Symposium,2005,06-08(8):254-265.
[16]成汉华.焊接工艺学[M].北京：水利电力出版社,1995.
[17]Deev A A,Kuznetcov P A,Petrov S N.Anisotropy of mechanical properties and its correlation with the structure of the stainless steel 316L produced by the SLMmethod[J].Physics Procedia,2016,83:789-796.
[18]付立定.不锈钢粉末选择性激光熔化直接制造金属零件研究[D].武汉:华中科技大学,2008.