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试验所用金属粉末为316L不锈钢钢球形粉末,颗粒尺寸为10~45 μm,微观形貌如图1所示。使用前将316L不锈钢金属粉末烘干5 h以上,粉末的化学成分如表1所示。
表 1 316L不锈钢粉末化学成分(质量分数%)
Table 1. Chemical composition of 316L stainless power (wt.%)
C Mn Si Cr Ni P S Mo Fe ≤0.03 <5.00 <1.00 16.0-18.0 10.0~25.0 ≤0.045 ≤0.03 <5.00 Allowance -
成型设备采用德国SLM公司进口的SLM-125HL打印机,激光器为IPG光纤激光器,功率为400 W,可成型不锈钢粉末(304、316)、钛合金、镍合金、铝合金等粉末材料。压缩试验设备采用长春机械院研发的SDS100型电液伺服疲劳试验机,该设备可以进行拉伸、压缩、疲劳等常规的力学性能试验。
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试验选取多孔结构孔隙率、平均孔径、比表面积作为影响因素,以弹性模量和抗压强度为力学性能指标,分别针对体心立方结构和正十二面体结构设计五种不同大小的多孔样件,选取前期优化的最佳工艺参数[17-19]进行成型试验,工艺参数如表2所示。
表 2 工艺参数
Table 2. Technological parameter
Laser power /W Scan speed /mm·s-1 Hatch spacing/mm Powder-bed depth/mm Scanning strategy 250 650 0.06 0.02 Single-direction sweeping 制备过程选区激光熔化系统的工作原理如图2所示。样件制备完成后进行纵向压缩试验,分析应力应变曲线,对处理的数据加以分析,得出最终结论。
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多孔结构是一种内部具有复杂孔洞的腔体结构,通常由棱径和壁板相互联结从而形成网状体。对于多孔结构的力学性能,学者Gibson和Ashby做了大量的研究。研究发现,孔隙率与弹性模量、强度之间的关系符合幂函数特征,并创立了Gibson-Ashby模型[20],即
$$\dfrac{{{E^*}}}{{{E_s}}} = C{(\dfrac{{{\rho ^{\rm{*}}}}}{{{\rho _s}}})^m}$$ 式中:
${E^*}$ 为316L不锈钢多孔结构的弹性模量;${E_s}$ 为其固体的弹性模量,大小为210 GPa;${\rho ^*}$ 为316不锈钢多孔结构的密度;${\rho _s}$ 为其固体的密度,大小为7.98 g/cm3;C和m为几何比例常数,Gibson和Ashby研究得出,开孔多孔结构的C=1、m=2。Karageorgiou V在研究三维生物材料支架的孔隙度与成骨时发现,孔隙率大于50%更有益于骨细胞的增长[21],生物相容性有明显的提高。而人体骨骼的弹性模量为0.9 ~1.7 GPa[22],人工植入体的弹性模量在此范围内,且抗压强度越高,生物相容性和优越性越好。笔者利用Solidworks软件,通过改变单元体杆长度,控制单元体的孔隙率、比表面积、平均孔径,且试验样件满足国家标准(GB/T7314—2005),结合Gibson-Ashby模型,设计出的多孔单元孔隙率均大于50%,并预测出不同单元的弹性模量(表5),预测的弹性模量值为0.250~3.049 GPa。孔隙率的增大,势必对多孔结构的弹性模量和抗压强度产生一定影响,为了将多孔结构的弹性模量更加接近人骨的弹性模量,减轻或消除“应力屏蔽”效应,所设计的正十二面体结构杆长L1 = 1.5~1.9 mm,体心立方结构杆长L2 = 2~4 mm,直径均为D = 0.4 mm。多孔单元设计相关参数如表3所示,多孔单元如图3所示。
表 3 多孔单元参数设计
Table 3. Parameter design of porous element
Sample Parameter Design 1 Design 2 Design 3 Design 4 Design 5 Regular dodecahedron structure Volume to area ratio 4.33 3.82 3.40 3.04 2.74 Average pore diamete/mm 3.29 3.56 3.78 4.00 4.22 Porosity 55.13% 60.21% 65.46% 69.88% 72.44% Body centered cubic structure Volume to area ratio 2.12 1.26 0.95 0.70 0.54 Average pore diamete/mm 1.90 2.48 3.06 3.64 4.22 Porosity 79.64% 86.82% 90.71% 93.15% 94.74% -
文中所设计的多孔结构模型,通过对多孔单元进行布尔阵列,构造出5×5×5单元体叠加累积的多孔样件,如图4所示。
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利用SDS100型电液伺服疲劳试验机,在样件纵向上施加载荷,压缩的位移速度为1 mm/min,采样频率为10 Hz。压缩停止后,计算机生成位移-载荷数据文件,数据采样结束。
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分形几何是由Mandelbrot(1983)发展起来的一门新的数学分支,用来描述自然界中不规则以及杂乱无章的形体特征,而分形插值是其重要的组成部分。
分形插值[23]是根据分形几何的自相似性原理和迭代函数系的理论将已知数据插值成具有自相似结构的曲线或曲面,任何一个局部都与整体自相似或统计自相似。分形插值,根据整体与局部相似的原理将插值数据点的变化特征映射到了相邻点之间的局部区域,在相邻的两个信息点之间得到局部波状起伏的形状,从而可以得到两信息点之间的局部变化特征。对于大多数的实际情况而言,在相邻两信息点之间并
不是线性变化或光滑过渡的,而是存在局部变化的特征。因此,对于具有分形特征的形体,两信息点之间有更多更精细一级的波状起伏,用分形插值法分析其结果更加符合实际。利用MATLAB强大的数据分析、图像处理功能,可以实现离散数据点的分形插值拟合。选区激光熔化成型试验过程中,存在零件打印时间长、成本高的问题,故而试验样本数量不能太大。所以,利用分形插值方法能够求得相邻两信息点之间的未知特征,较好地解决在有限样本数据中分析试验规律的问题。
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图5为选区激光熔化成型出的多孔结构样件,其表面无明显缺陷。图6为选区激光熔化成型出多孔结构样件的显微特征,通过显微镜对成型质量进行观察,发现样件表面有少量的粉末黏附,多孔结构的梁柱上有轻微的金属粉末烧结残渣。综上可知,样件成型效果较好。
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数据采集完成后,取每种设计样块应力应变数据的均值作为最终的结果,通过Origin软件进行应力−应变曲线的绘制。如图7所示,成型的多孔316L不锈钢多孔结构在压缩过程中,由于材料本身塑性较强,无纵向断裂现象,导致应力应变曲线未有断崖式下降,总体成上升趋势。
图 7 多孔316L不锈钢压缩应力应变曲线。(a) 正十二面体多孔结构;(b)体心立方多孔结构
Figure 7. Compression stress and strain curves of porous 316L stainless steel. (a) Regular dodecahedron porous structure; (b) Body centered cubic porous structure
由上述316L不锈钢的压缩应力-应变曲线,通过对样件的变形行为分析,可以得出其压缩过程可以分为三个阶段:
(1)弹塑性变形阶段
在液压柱对样件施加载荷的过程中,316L不锈钢多孔结构内部是承受压缩应力的最主要部分,此阶段,因为压缩应力较小,多孔结构内部发生了弹性变形,进而样件的应力−应变曲线呈线性关系,且施加压缩载荷卸除,样件由于弹性变形可逆性,能够恢复初始状态。
(2)局部塑性变形阶段
当施加载荷逐渐增大,多孔结构变形进入局部塑性变形阶段,此阶段应力−应变曲线呈非线性关系,且随着应变的增加,应力的增长趋势越来越缓慢。
(3)完全塑性变形阶段
此阶段中,多孔结构样件的变形已十分明显,应变值的增大,应力基本上保持不变或者变化较为缓慢,应力−应变曲线也较为平缓。
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通过表4抗压强度数据发现,选区激光熔化技术成型的316L不锈钢多孔结构抗压强度在43.19 ~160.31 MPa之间,满足人骨的抗压强度(102.9 ~140.7 GPa)[24]要求。正十二面体结构的抗压强度幅值变化比体心立方结构小,因为正十二面体多孔结构梁柱数量比点阵多孔结构多,所以自身稳定性较强,能够承受更多的压缩应力。
通过表5弹性模量数据发现,选区激光熔化技术成型的316L不锈钢多孔结构弹性模量在0.375 ~1.716 GPa之间,满足了人骨组织弹性模量在0.9 ~1.7 GPa的要求,其发展趋势与预测值趋势大致相同。实心316L不锈钢的弹性模量为210 GPa,正十二面体和体心立方多孔结构大幅度的降低了材料的弹性模量,且可通过调节多孔结构的相关参数,达到与人骨组织相同的弹性模量,实现减轻“应力屏蔽”效应的目的,提高了生物相容性。正十二面体和体心立方多孔结构弹性模量较Gibson-Ashby模型误差分别为46.61%、27.94%,体心立方结构弹性模量更加符合预测模型。主要原因是正十二面体单元存在着大量的悬垂结构,导致成型后挂渣和球化现象比体心立方结构更加明显,成型质量相对体心立方结构较差。
表 4 压缩试验计算的抗压强度
Table 4. Compressive strength calculated by compression test
Sample Design number Average compressive
strength/MPaRegular dodecahedron structure Design 1 160.31 Design 2 150.65 Design 3 134.95 Design 4 129.25 Design 5 111.75 Body centered cubic structure Design 1 158.03 Design 2 92.07 Design 3 66.97 Design 4 53.05 Design 5 43.19 表 5 压缩试验计算的弹性模量
Table 5. Elastic modulus calculated by compression test
Sample Design number Average elastic modulus/GPa Predicted value by Gibson-Ashby formula/GPa Deviation Regular dodecahedron sample Design 1 1.042 2.729 46.61% Design 2 1.223 2.201 Design 3 1.030 1.877 Design 4 0.925 1.769 Design 5 0.919 1.347 Body centered cubic structure Design 1 1.716 3.049 27.94% Design 2 1.070 1.350 Design 3 0.672 0.720 Design 4 0.553 0.464 Design 5 0.375 0.250 -
通过图8(a)分形插值曲线可以发现,正十二面体多孔结构在孔隙率为55%~62%时,随着孔隙率的增大,弹性模量增大,并在孔隙率为62%左右达到峰值后,弹性模量随着孔隙率的增加而呈现出下降趋势,正十二面体结构受孔隙率影响较小。体心立方多孔结构随着孔隙率的增加一直保持线性下降的趋势,且体心立方结构受孔隙率影响较大。
图 8 (a)孔隙率与弹性模量关系;(b)孔隙率与抗压强度关系(RD:正十二面体多孔结构;BCC:体心立方多孔结构)
Figure 8. (a) Relationship between porosity and elastic modulus; (b) Relationship between porosity and compressive strength (RD: porous structure of regular dodecahedron; BCC: body centered cubic porous structure)
从图8(b)分形插值曲线可以发现,孔隙率对316L不锈钢多孔结构抗压强度影响趋势与弹性模量大致相同,都随孔隙率增大抗压强度减小。正十二面体多孔结构随孔隙率的增大,抗压强度下降较为缓慢,而孔隙率对体心立方多孔结构影响更为显著。
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通过图9(a)分形插值曲线可以看出,平均孔径对正十二面体多孔结构弹性模量的影响呈先增大后减小的趋势,且幅值变化较小,平均孔径达到4 mm后,弹性模量变化趋于稳定。体心立方多孔结构随着平均孔径的增大,始终保持下降趋势,且平均孔径在2~3 mm区间内下降趋势较为迅速,3~4.5 mm区间内较为缓慢,幅值变化较大。
图 9 (a)平均孔径与弹性模量关系;(b)平均孔径与抗压强度关系(RD:正十二面体多孔结构;BCC:体心立方多孔结构)
Figure 9. (a) Relation between average pore diameter and elastic modulus; (b) Relation between average pore diameter and compressive strength (RD: porous structure of regular dodecahedron; BCC: body centered cubic porous structure)
通过图9(b)分形插值曲线可以看出,平均孔径对316L不锈钢多孔结构的抗压强度影响与对弹性模量的影响基本相同,都随着平均孔径的增大抗压强度减小,且两者的变化趋势都十分明显,正十二面体多孔结构更趋向于线性下降。
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通过图10(a)分形插值曲线可以看出,比表面积对316L不锈钢多孔结构弹性模量的影响有所不同,正十二面体多孔结构弹性模量随着比表面积的增大先上升后减小,在比表面积为3.8附近取其峰值,而体心立方多孔结构与比表面积一直保持正相关线性增长,最值差明显高于正十二面体多孔结构。
图 10 (a)比表面积与弹性模量关系;(b) 比表面积与抗压强度关系
Figure 10. (a) Relationship between volume to area ratio and elastic modulus ; (b) Relationship between volume to area ratio and compressive strength
通过图10(b)分形插值曲线可以看出,316L不锈钢多孔结构的抗压强度随着比表面积的增大而逐渐增大,且均呈现出线性增长,体心立方多孔结构线性相关性更强。比表面积的改变,体心立方多孔结构的抗压强度变化范围要比正十二面体多孔结构大,二者受比表面积影响十分显著。
Mechanical properties of 316L stainless steel porous structure formed by selective laser melting
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摘要: 为了减轻或消除人工植入体的“应力屏蔽”效应,提高生物相容性,需要对选区激光熔化(SLM)技术成型多孔结构进行力学性能研究。通过制备316L不锈钢体心立方(BCC)、正十二面体(RD)两种多孔结构,分别进行成型件纵向压缩试验,建立了Gibson-Ashby模型,预测了多孔结构弹性模量值。采用分形插值法,分析了孔隙率、平均孔径、比表面积对多孔结构弹性模量和抗压强度的影响程度。分析试验表明,316L不锈钢多孔结构样件在孔隙率为55.13%~94.74%,平均孔径为1.90~4.22 mm,比表面积0.54~4.33时,其弹性模量为0.375 ~1.716 GPa,抗压强度为43.19~160.31 MPa。对比人骨弹性模量0.9 ~1.7 GPa,满足植入体要求。孔隙率、平均孔径、比表面积对正十二面体多孔结构的弹性模量和抗压强度的幅值变化影响较小,对体心立方多孔结构影响较大。正十二面体多孔结构抗压强度为111.75~160.31 MPa,体心立方多孔结构的抗压强度为43.19~158.03 MPa,正十二面体多孔结构的力学性能比体心立方结构性能更好,为选区激光熔化技术制备316L不锈钢多孔结构的人工植入体研究提供依据。Abstract: In order to reduce or eliminate the "stress shielding" effect of artificial implants and improve the biocompatibility, mechanical properties of porous structures formed by selective laser melting (SLM) technology need to be studied. Through the preparation of 316L stainless steel body core cubic (BCC) and regular dodecahedron (RD) porous structures, the longitudinal compression test of the molded parts was carried out respectively, and the Gibson-Ashby model was established to predict the elastic modulus value of the porous structure. The effects of porosity, average pore diameter and specific surface area on elastic modulus and compressive strength of porous structures were analyzed by fractal interpolation. The analysis results show that when the porosity of 316L stainless steel porous structure sample is 55.13%-94.74%, the average pore diameter is 1.90-4.22 mm, and the specific surface area is 0.54-4.33, the elastic modulus is 0.375-1.716 GPa, and the compressive strength is 43.19-160.31 MPa. The elastic modulus of human bone was 0.9-1.7 GPa, which meets the requirements of implants. Porosity, average pore diameter and specific surface area have little influence on the elastic modulus and the amplitude of compressive strength of the dodecahedral porous structure, but have greater influence on the body-centered cubic porous structure. The compressive strength of the dodecahedron porous structure is 111.75-160.31 MPa, and the compressive strength of the body centered cubic porous structure is 43.19-158.03 MPa. The performance of the dodecahedron porous structure is better than that of the body centered cubic structure. This paper provides the basis for the research on the preparation of 316L stainless steel porous structure by selective laser melting.
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表 1 316L不锈钢粉末化学成分(质量分数%)
Table 1. Chemical composition of 316L stainless power (wt.%)
C Mn Si Cr Ni P S Mo Fe ≤0.03 <5.00 <1.00 16.0-18.0 10.0~25.0 ≤0.045 ≤0.03 <5.00 Allowance 表 2 工艺参数
Table 2. Technological parameter
Laser power /W Scan speed /mm·s-1 Hatch spacing/mm Powder-bed depth/mm Scanning strategy 250 650 0.06 0.02 Single-direction sweeping 表 3 多孔单元参数设计
Table 3. Parameter design of porous element
Sample Parameter Design 1 Design 2 Design 3 Design 4 Design 5 Regular dodecahedron structure Volume to area ratio 4.33 3.82 3.40 3.04 2.74 Average pore diamete/mm 3.29 3.56 3.78 4.00 4.22 Porosity 55.13% 60.21% 65.46% 69.88% 72.44% Body centered cubic structure Volume to area ratio 2.12 1.26 0.95 0.70 0.54 Average pore diamete/mm 1.90 2.48 3.06 3.64 4.22 Porosity 79.64% 86.82% 90.71% 93.15% 94.74% 表 4 压缩试验计算的抗压强度
Table 4. Compressive strength calculated by compression test
Sample Design number Average compressive
strength/MPaRegular dodecahedron structure Design 1 160.31 Design 2 150.65 Design 3 134.95 Design 4 129.25 Design 5 111.75 Body centered cubic structure Design 1 158.03 Design 2 92.07 Design 3 66.97 Design 4 53.05 Design 5 43.19 表 5 压缩试验计算的弹性模量
Table 5. Elastic modulus calculated by compression test
Sample Design number Average elastic modulus/GPa Predicted value by Gibson-Ashby formula/GPa Deviation Regular dodecahedron sample Design 1 1.042 2.729 46.61% Design 2 1.223 2.201 Design 3 1.030 1.877 Design 4 0.925 1.769 Design 5 0.919 1.347 Body centered cubic structure Design 1 1.716 3.049 27.94% Design 2 1.070 1.350 Design 3 0.672 0.720 Design 4 0.553 0.464 Design 5 0.375 0.250 -
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