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金属学报  2023, Vol. 59 Issue (5): 623-635    DOI: 10.11900/0412.1961.2021.00248
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尺寸设计对选区激光熔化304L不锈钢显微组织与性能的影响
侯娟1,2(), 代斌斌2, 闵师领2, 刘慧2, 蒋梦蕾2, 杨帆2
1中广核工程有限公司 核电安全监控技术与装备国家重点实验室 深圳 518172
2上海理工大学 材料与化学学院 上海 200082
Influence of Size Design on Microstructure and Properties of 304L Stainless Steel by Selective Laser Melting
HOU Juan1,2(), DAI Binbin2, MIN Shiling2, LIU Hui2, JIANG Menglei2, YANG Fan2
1State Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment, China Nuclear Power Engineering Co., Ltd., Shenzhen 518172, China
2Academy of Materials and Chemistry, University of Shanghai Science and Technology, Shanghai 200082, China
引用本文:

侯娟, 代斌斌, 闵师领, 刘慧, 蒋梦蕾, 杨帆. 尺寸设计对选区激光熔化304L不锈钢显微组织与性能的影响[J]. 金属学报, 2023, 59(5): 623-635.
Juan HOU, Binbin DAI, Shiling MIN, Hui LIU, Menglei JIANG, Fan YANG. Influence of Size Design on Microstructure and Properties of 304L Stainless Steel by Selective Laser Melting[J]. Acta Metall Sin, 2023, 59(5): 623-635.

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摘要: 

采用选区激光熔化(SLM)技术制备304L不锈钢样品,通过改变扫描道次(T)和打印层数(L)实现不同的温度梯度与凝固速率,研究尺寸效应对微观组织与力学性能的影响。具有不同T × L组合的SLM 304L不锈钢样品沿打印构建方向形成柱状晶组织,并且尺寸效应影响柱状晶结构。随尺寸增大,凝固组织沿散热方向择优生长的程度高,柱状晶的连续性更好,由低长径比的“米粒形”向近等轴的“短窄形”、进而向高长径比的“长条形”过渡,晶粒粗化现象明显。在较大尺寸样品中,随着不断远离基板也观察到了类似现象。尺寸效应对力学性能的影响主要体现在随着打印尺寸增加,材料的屈服强度下降而塑性延伸率上升,但当尺寸增加至一定程度后力学性能变化趋于稳定。尺寸效应对SLM 304L不锈钢的析出相组成和含量无明显影响。结果表明,影响力学性能的主要影响因素为柱状晶的尺寸和分布,在较大尺寸的样品中,“长条形”柱状晶占比高,导致材料强度降低而塑性增加。结合ANSYS ADDITIVE对凝固速率和温度梯度的模拟仿真结果,对尺寸效应通过影响凝固过程进而对材料微观组织和力学性能产生影响的机理进行了阐述。

关键词 选区激光熔化304L不锈钢尺寸效应凝固特征力学性能    
Abstract

As one of the most promising metal additive manufacturing methods, selective laser melting (SLM) is very attractive for fabricating complex-shaped structure components of austenitic stainless steels in the nuclear field. SLMed austenitic 304L stainless steel has been demonstrated to have excellent mechanical properties and superior corrosion resistance due to the unique hierarchical microstructure produced by the ultrafast cooling rate and high-thermal gradient. Scanning tracks (T) and depositing layers (L) are key factors for geometry design by affecting the processing efficiency and the solidification structure of the components. Hence, it is essential to clarify the effecting mechanism of sample size and geometry on material performance. In this work, samples of different sizes are designed to study the influence of geometry on the microstructure and mechanical properties of 304L stainless steel components made via SLM. Various solidification conditions are achieved by varying the temperature gradients and cooling rates by adjusting T and L. Metallographic microscopic observations in samples with various T × L combinations demonstrate that a columnar structure is formed along the build direction, which is significantly impacted by the geometry effect. The columnar grains grow preferentially along the heat dissipation direction with an increase in the sample size. The columnar grains gradually change from having a low length-diameter ratio (LDR) with a rice grain shape to a higher LDR with a short rod-like shape and then a long strip shape. Grain coarsening could also be identified along with the formation of “long strip” columnar grains. Moreover, consistent microstructure evolution behavior is observed in large-sized samples. The influence of geometry on the mechanical properties is examined via tensile testing to demonstrate the decrease in yield strength and increased plastic elongation with the rise in sample size. As the sample size increases, the mechanical properties become consistent. The comprehensive analysis concludes that grain size and columnar grains play critical roles in determining the mechanical properties according to the Hall-Petch relationship. In larger-sized samples, “long strip” columnar grains with a high proportion could lead to a decrease in material strength and an increase in plasticity. The geometry mechanism affecting the solidification process, microstructure formation, and mechanical properties of 304L stainless steel processed by SLM is explored by combining the solidification rate and thermal gradient simulation results using ANSYS ADDITIVE.

Key wordsselective laser melting    304L stainless steel    build geometry    solidification theory    mechanical property
收稿日期: 2021-06-18     
ZTFLH:  TG142.7  
基金资助:国家自然科学基金项目(52073176);深圳市国际合作研究科技计划项目(GJHZ20200731095203011);核电安全监控技术与装备国家重点实验室开放课题(CSO-102-001)
作者简介: 侯 娟,女,1983年生,副教授,博士
图1  选区激光熔化(SLM)用304L不锈钢粉末形貌及不同尺寸效应样品在成形基板的排布示意图
SampleCNPSCrCuMnNiOSiMoFe
Powder0.0060.0130.0270.00118.950.0330.0169.480.0290.0560.87Bal.
SLMed0.0150.0130.0270.00319.700.0320.0549.620.0310.0650.83Bal.
表1  原始粉末及SLM 304L不锈钢化学成分 (mass fraction / %)
LT
1220100200300
5T1-L5T2-L5----
10--T20-L10--T300-L10
100--T20-L100T100-L100T200-L100T300-L100
500--T20-L500T100-L500T200-L500T300-L500
700--T20-L700T100-L700T200-L700T300-L700
1000--T20-L1000T100-L1000T200-L1000T300-L1000
表2  不同扫描道次(T)与打印层数(L)组合的尺寸效应样品及编号方法
图2  沉积层与层之间旋转67°成形及扫描路径采用“skywriting”扫描策略示意图
图3  拉伸测试样品尺寸及SLM制备过程示意图
图4  SLM 304L不锈钢沿打印构建方向显微组织的OM像
图5  不同尺寸效应SLM 304L样品沿打印构建方向显微组织的OM像
图6  SLM 304L不锈钢T100系列样品显微组织的OM像
SampleGrain size / μmRice-shaped columnarNarrow-short columnarStrip columnar
(0-50 μm2) / %(51-150 μm2) / %(≥ 151 μm2) / %
T100-L10013 ± 225.250.724.1
T100-L50016 ± 221.345.633.1
T100-L70018 ± 118.742.338.0
T100-L100020 ± 217.645.047.4
表3  T100系列样品内部晶粒尺寸与柱状晶分布统计结果
图7  SLM 304L不锈钢T200系列样品显微组织的OM像
SampleGrain size / μmRice-shaped columnarNarrow-short columnaStrip columnar
(0-50 μm2) / %(51-150 μm2) / %(≥ 151 μm2) / %
T200-L10015 ± 220.946.232.9
T200-L50019 ± 218.744.337.0
T200-L70020 ± 217.345.137.6
T200-L100022 ± 116.142.441.5
表4  T200样品晶粒尺寸与柱状晶分布统计结果
图8  SLM 304L不锈钢T300系列样品显微组织的OM像
SampleGrain size / μmRice-shaped columnarNarrow-short columnarStrip columnar
(0-50 μm2) / %(51-150 μm2) / %(≥ 151 μm2) / %
T300-L10018 ± 212.037.051.0
T300-L50020 ± 211.732.056.3
T300-L70023 ± 210.530.059.5
T300-L100025 ± 19.723.167.2
表5  T300系列样品晶粒尺寸与柱状晶分布统计结果
图9  T100、T200和T300系列样品XRD谱
图10  T300-L100和T300-L1000样品析出相的EBSD分析
图11  不同尺寸效应样品的室温拉伸应力-应变曲线
SampleYield strengthUltimate tensileElongation
MPastrength / MPa%
T100-L100541.5 ± 17691.6 ± 1452.7 ± 0.6
T100-L500478.7 ± 13667.0 ± 1661.0 ± 0.2
T100-L700475.7 ± 15654.7 ± 2060.1 ± 0.9
T100-L1000477.8 ± 9665.9 ± 2366.6 ± 0.3
T200-L100504.6 ± 15682.8 ± 1754.9 ± 0.7
T200-L500476.6 ± 8656.1 ± 1959.9 ± 0.9
T200-L700470.2 ± 11649.1 ± 2263.3 ± 0.4
T200-L1000464.2 ± 15646.3 ± 1467.0 ± 0.5
T300-L100486.8 ± 13674.6 ± 2456.6 ± 0.8
T300-L500470.9 ± 18669.2 ± 1859.3 ± 0.9
T300-L700472.9 ± 8667.8 ± 1663.0 ± 1.0
T300-L1000459.8 ± 12662.7 ± 1663.1 ± 1.2
表6  SLM 304L不锈钢不同尺寸效应样品室温拉伸强度与塑性
XYZGRG × RG / R
mmmmmmK·m-1m·s-1K·s-1K·s·m-2
51151168970.3075157381016640315
51251430650.3010154815117086594
51547892720.3078147409415559688
表7  SLM成形过程中凝固速率(R)和温度梯度(G)的模拟结果
图12  SLM 304L不锈钢样品中沿打印构建方向柱状晶形貌及分布随尺寸效应演变示意图
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