激光选区融化成形多孔硬骨支架的建模、仿真及实验研究

金属3D打印技术成为当前最具有发展潜力和发展前景的工业制造技术之一,通过SLM激光选区烧结技术,选取合理的烧结参数,将金属粉末烧结成型。建立了不同孔径的多孔支架复杂三维模型,并通过有限元分析进行应力、应变的模拟分析,获得了优化后的多孔支架三维模型,为后续的实验研究分析建立理论基础,然后通过SLM烧结技术制备316L不锈钢多孔支架,通过后期热处理实验、压缩试验、金相实验,对多孔试样进行力学性能分析、硬度测试以及表面微观组织分析。通过模拟分析获得优化后的多孔支架孔径尺寸,获得了更适于人体骨骼缺损部位承重的多孔支架,可对后续研究进行指导。实验研究发现300μm孔径支架强度和弹性模量都高于天然骨,而成形多孔结构的金属件保证了骨骼修复体的生物力学性能,具有良好的力学性能。     

钛合金变形Gyroid单元梯度多孔结构设计与分析

研究一种具有径向和轴向孔径梯度的变形Gyroid单元多孔结构参数化设计方法,采用激光选区熔化成形(selective laser melting, SLM)技术,制备出孔隙率为60%和75%的钛合金变形Gyroid单元梯度多孔结构样件。使用有限元法(finiteelementmethod,FEM)对4组梯度多孔支架模型及2组均质模型进行静力学仿真分析,对制备的钛合金梯度多孔样件进行力学性能测试,并与已测试过的均质样件进行力学性能对比分析。有限元计算结果与力学性能试验结果共同表明：变形Gyroid单元多孔结构力学性能随孔隙率的升高而降低,孔隙率相同时,径向梯度多孔支架力学性能优于均质多孔支架,更适用于皮质骨的骨缺损修复,轴向梯度多孔支架力学性能相比均质多孔支架有所减弱,更适用于松质骨。     

多孔钛骨组织工程支架设计及孔结构表征

针对目前骨组织工程支架微孔结构难以准确设计制备的问题,提出了一种基于点云的参数化建模+3D打印新方法。通过提取cube(C)、diamond(D)、gyroid(G)3种结构的型面函数点云数据,完成对不同孔结构特征的参数化建模。通过对模型有限元力学分析,对不同孔结构特征的多孔钛骨组织支架进行力学设计与订制。借助激光选区熔融(SLM)3D打印技术,完成对不同孔特征的骨组织支架快速成型。对多孔钛骨组织支架进行了相关材料学表征,包括孔结构表征与力学性能测试。结果表明:参数化模型的快速成型制造,能够有效地设计制备钛合金骨组织工程支架的孔结构特性,且可有效设计订制支架的力学性能,从仿生的角度实现多孔钛合金骨组织工程支架生物学功能的设计优化。     

中国造出世界最大金属零件高精度3D打印装备

正由华中科技大学完成的"大型金属零件高效激光选区熔化增材制造关键技术与装备(俗称激光3D打印技术)"最近通过了湖北省科技厅成果鉴定。该成果深度融合了信息技术和制造技术等特征的激光3D打印技术,体积为(500×500×530)mm3,由4台激光器同时扫描,为目前世界上效率和尺寸最大的高精度金属零件激光3D打印装备。该装备攻克了多重技术难题,解决     

316不锈钢藕状多孔结构的选区激光烧结制备

研究了一种新型的制备金属藕状多孔结构技术——选区激光烧结，着重说明该技术的基本原理和工艺过程，并利用此制备技术成功获得了藕状多孔试样。利用SEM分析了316不锈钢藕状多孔试样的微观孔隙特征，并测定其孔隙率，结果表明其孔径大小分布均匀（2-4μm），平均孔隙率约为60％，孔隙贯通性良好；初步探讨了选区激光烧结制备316不锈钢藕状多孔结构的成形机制。     

激光选区熔化金属3D打印设备现状与发展趋势

通过对激光选区熔化金属3D打印设备结构、主要设备供应商以及研究机构的分析,对国内激光选区熔化金属3D打印设备发展进行了总结,并对其发展提出建议。     

激光选区熔化成型多孔Ti6Al4V合金及其热处理研究

Ti6Al4V合金是骨科中应用最为广泛的钛合金,激光选区熔化成型多孔Ti6Al4V合金能够降低其弹性模量,减少应力遮挡现象,促进骨组织的长入,但成型件的塑性通常较低。为此设计了单元尺寸在1~2 mm、孔径约在500~1200 μm、孔隙率约在60%~90%之间的菱形十二面体多孔结构并采用激光选区熔化技术成型了压缩试件,通过压缩仿真和实验对其力学性能进行研究,并研究了退火热处理对其力学性能、显微组织的影响。仿真结果表明,对模型的支杆直径进行误差补偿可获得更为准确的结果。实验结果表明当单元尺寸为1.5 mm时,试件的抗压强度在78.16~242.94 MPa之间,弹性模量在1.74~4.17 GPa之间,与人体皮质骨的力学性能相近。经820 ℃退火2 h后试件的抗压强度、弹性模量与成型态基本保持相当,而塑性有所提升,因此更适用于骨科植入体。     

基于TPMS结构的多孔钛制备与表征

多孔钛因具有与人体骨组织相近的弹性模量和允许骨长入的孔隙结构而备受关注。其孔隙结构特征不仅影响骨长入效果,而且决定了多孔钛的力学性能。通过三周期极小曲面(triply periodic minimal surfaces,TPMS)隐函数参数的精确调控可以构建出理想的孔隙结构模型。本实验针对TPMS模型中常用的G单元模型,研究了G单元模型隐函数参数对孔隙率、孔径、杆径等孔隙结构特征的影响规律,设计出了孔隙率约为77%,孔径分别为300(G300)、500(G500)微米的均质孔隙结构;模仿自然长骨径向梯度结构模型,构建了相应的G单元仿生梯度孔隙结构。采用选区激光熔化(SLM)增材制造技术制备了相应的多孔钛样件,利用数字显微镜和扫描电镜观测多孔钛的孔隙结构特征,发现SLM多孔钛实测孔隙率低于设计孔隙率,实测孔径小于设计值,实测杆径大于设计值。力学性能检测结果显示,G300和G500多孔钛弹性模量分别为2.04和3.12GPa,其最大抗压强度分别为63.5和103.5MPa,梯度孔隙结构多孔钛弹性模量和最大抗压强度分别为6.3 GPa和186.9 MPa。研究结果表明,G单元梯度孔隙结构多孔钛是一种理想的承重部位骨缺损修复体。     

激光选区熔化GH5188合金组织和性能研究

针对GH5188高温合金支板的激光选区熔化制造工艺,设计了不同的增材制造工艺方案,对比分析了不同的增材制造工艺参数和热处理工艺参数对产品的微观组织和力学性能的影响,总结了GH5188高温合金支板的激光选区熔化工艺设计要点。最终获得了满足要求的激光选区熔化GH5188高温合金支板零件。     

Er改性铝合金的选区激光熔化成形性及力学性能

通过选区激光熔化(SLM)技术制备了Al-Mg-Mn-Er-Zr铝合金,系统研究了不同工艺参数对合金粉末成形性以及时效处理对沉积态样品力学性能的影响。结果表明：高激光能量密度下获得的样品不存在微裂纹,样品的孔隙率较低,最低约为0.4%。样品以柱状晶为主,熔池边界分布有少量等轴晶,平均晶粒宽度约为5μm。样品中主要包含α(Al)、Al₆Mn和L12型晶体相。在350℃下,样品的硬度和压缩屈服强度随时效时间的延长先增加后降低,最大值分别为(171±1) HV和(555±12) MPa。     

Macro-micron-nano-featured surface topography of Ti-6Al-4V alloy for biomedical applications

One of the critical issues in the development of novel metallic biomaterials is the design and fabrication of metallic scaffolds and implants with hierarchical structures mimicking human bones. In this work, selective laser melting(SLM) and electrochemical anodization were applied to fabricate dense Ti-6 A1-4 V components with macro-micron-nanoscale hierarchical surfaces. Scanning electron microscopy(SEM), 3 D laser scanning microscopy(3 D LSM), contact angle video system, fluorescence microscopy and spectrophotometer were used to investigate the properties of the samples. The results reveal that the SLMed post-anodization(SLM-TNT) exhibits enhanced or at least comparable wettability, protein adsorption and biological response of mesenchymal stem cells(MSCs) in comparison with the three reference configurations, i.e., the polished Ti-6 Al-4 V(PO-Ti64), the SLMed Ti-6 A1-4 V(SLM-Ti64) and the polished Ti-6 A1-4 V post-anodization(PO-TNT). The improved cytocompatibility of the samples after SLM and anodization should be mainly attributed to the nanoscale tubular features,while the macro-micron-scale structures only lead to slight preference for cell attachment.     

3D Analysis of Porosity in a Ceramic Coating Using X-ray Microscopy

Suspension plasma spraying (SPS) is a new, innovative plasma spray technique using a feedstock consisting of fine powder particles suspended in a liquid. Using SPS, ceramic coatings with columnar microstructures have been produced which are used as topcoats in thermal barrier coatings. The microstructure contains a wide pore size range consisting of inter-columnar spacings, micro-pores and nano-pores. Hence, determination of total porosity and pore size distribution is a challenge. Here, x-ray microscopy (XRM) has been applied for describing the complex pore space of the coatings because of its capability to image the (local) porosity within the coating in 3D at a resolution down to 50 nm. The possibility to quantitatively segment the analyzed volume allows analysis of both open and closed porosity. For an yttria-stabilized zirconia coating with feathery microstructure, both open and closed porosity were determined and it could be revealed that 11% of the pore volumes (1.4% of the total volume) are closed pores. The analyzed volume was reconstructed to illustrate the distribution of open and closed pores in 3D. Moreover, pore widths and pore volumes were determined. The results on the complex pore space obtained by XRM are discussed in connection with other porosimetry techniques.     

Monte Carlo simulation of uniform corrosion process under potentiostatic conditions

The aim of this paper is to study the corrosion resistance of industrial coatings of metallic alloys including certain amount of process-induced porosity. Surface damages during a simulated corrosion process are investigated using Monte Carlo simulation technique and compared to corrosion response (dissolution currents). In this simulation, a 3D grid of the coating is subject to an electrolyte attack under potentiostatic conditions. The surface damages are related to a controlled dissolution process governed by dissolution probability, roughness, porosity size and fraction. In order to validate the model results, theoretical I = f(t) curves are compared to corrosion behaviour of Fe-40Al samples pointing out critical parameters affecting current density and exposed area. Predictions relating the porosity level to corrosion behaviour are established and discussed. The main conclusions focus on a primary effect of pore size and connectivity and a secondary effect of porosity level in the considered process window (porosity levels between 5.8% and 7.8%). The experimental work shows that the porosity effect could not explain all observed trends and a deeper examination of the microstructure reveals another candidate (unmolten particles) which is expected to vary both pore connectivity and dissolution events.     

A three-phase simulation of the effect of microstructural features on semi-solid tensile deformation

A direct finite-element microstructure model for prediction of the deformation behavior of semi-solid metallic alloys is presented. The 2D model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure, liquid surrounding the grains, and micro-porosity. An elasto-plastic empirical constitutive equation is derived for the solid grains, while the liquid is approximated as a perfectly plastic material with a very low yield stress. The resulting three-phase model was used to investigate the effects of fraction solid, porosity, and grain size on the constitutive behavior of a semi-solid aluminum alloy, AA5182. The model predictions were validated against experimental data at high fraction solid. These simulations reveal a strong correlation between semi-solid grain size and yield stress, and between porosity and strain localization. The application of direct finite-element simulations is shown to be an effective technique for examining the effects of microstructure phenomena on the macro constitutive behavior of semi-solid materials.     

316L Stainless Steel with Gradient Porosity Fabricated by Selective Laser Melting

To fabricate 316L stainless steel part with a pore gradient structure, the method using selective laser melting (SLM) technique is exploited. Scan tracks feature, densification, and tensile property of SLM-produced samples prepared via different scan speeds were investigated. The results show that the porosity is strongly influenced by scan speed. On this basis, a gradient changed scan speed was applied in every SLM layer for the purpose of producing a gradient porosity metal. The results indicate that the structure exhibits a gradually increased porosity and a reduced molten pool size along the gradient direction of scan speed variation. The forming mechanisms for the gradient porosity were also addressed.     

Sintering characteristics of plasma sprayed zirconia coatings containing different stabilisers

Zirconia powders with different types of stabiliser (Y₂O₃, Dy₂O₃ and Yb₂O₃) have been air plasma sprayed onto metallic substrates. The coatings were detached and dimensional changes during heat treatment were measured by dilatometry. Ytterbia-stabilised specimens exhibited the highest rates of shrinkage, in both in-plane and through-thickness directions. However, it was noted that these specimens had higher initial porosity levels, and a finer microstructure, than coatings containing the other stabilisers. In-plane stiffness and through-thickness thermal conductivity were also measured after different heat treatments. These increased at greater rates for specimens with higher porosity levels (ie the Yb-stabilised coatings). Changes in pore architecture during heat treatments were also studied. Fine scale porosity is rapidly reduced during heat treatment. This correlates with enhanced inter-splat bonding and healing of intra-splat microcracks. In general, the sintering behaviour, and consequent changes in microstructure and properties, appear to be more sensitive to the pore architecture than to stabiliser type. This is correlated with theoretical expectations that it is grain boundary and surface diffusion which will dominate the sintering behaviour, rather than lattice diffusion, and these are more likely to be affected by pore structure, and possibly by the presence of certain types of impurity, than by stabiliser content. It is also noted that thermal cycling appears to retard sintering, at least in terms of the rate of shrinkage. This effect, which could be of practical significance, is briefly discussed.     

Fabrication of Bioceramic Bone Scaffolds for Tissue Engineering

In this study, microhydroxyapatite and nanosilica sol were used as the raw materials for fabrication of bioceramic bone scaffold using selective laser sintering technology in a self-developed 3D Printing apparatus. When the fluidity of ceramic slurry is matched with suitable laser processing parameters, a controlled pore size of porous bone scaffold can be fabricated under a lower laser energy. Results shown that the fabricated scaffolds have a bending strength of 14.1 MPa, a compressive strength of 24 MPa, a surface roughness of 725 nm, a pore size of 750 μm, an apparent porosity of 32%, and a optical density of 1.8. Results indicate that the mechanical strength of the scaffold can be improved after heat treatment at 1200 °C for 2 h, while simultaneously increasing surface roughness conducive to osteoprogenitor cell adhesion. MTT method and SEM observations confirmed that bone scaffolds fabricated under the optimal manufacturing process possess suitable biocompatibility and mechanical properties, allowing smooth adhesion and proliferation of osteoblast-like cells. Therefore, they have great potential for development in the field of tissue engineering.     

Pore size scaling for enhanced fracture resistance of nanoporous polymer thin films

Poly(arylene) ether (PAE) polymer films containing controlled nanometer-sized pores are shown to exhibit increasing fracture resistance with porosity. Such surprising behavior is in stark contrast to widely reported behavior for the fracture toughness of porous solids, which decreases markedly with porosity. A ductile nano-void growth and coalescence fracture mechanics-based model is presented to rationalize the increase in fracture resistance of the voided polymer film. The model is shown to explain the behavior in terms of a specific scaling of the size of the pores with pore volume fraction. It is demonstrated that the pore size must increase with close to a linear dependence on the volume fraction in order to increase rather than decrease the fracture energy. Independent characterization of the pore size as a function of volume fraction is shown to confirm predictions made by the model. Implications for the optimum void size and volume fraction are considered for superior fracture resistance of the nanoporous films.     

An investigation on selective laser melting of Al-Cu-Fe-Cr quasicrystal: From single layer to multilayers

In this study, the effects of processing parameters on the microstructure of Al-Cu-Fe-Cr quasicrystalline (QC) coatings fabricated by selective laser melting (SLM) are investigated. A qualitative analysis on the XRD patterns indicates that the phase composition for the SLM processed coating mainly consisted of Al-Cu-Fe-Cr quasicrystals and α-Al (CuFeCr) solid solution, and with increasing laser energy input or coating thickness, the volume fraction of QC i-Al₉₁Fe₄Cr₅ reduced and those of QC d-Al₆₅Cu₂₀Fe₁₀Cr₅ and crystalline θ-Al₂Cu increased. The formation of cracks during the coating building procedure from single layer to multilayers is also discussed. For the coatings with the same layer number, the pores and balling particles diminish as laser power increases, due to the growth of melting degree. At the early stage of fabrication, with increment of layer number (or coating thickness), pores and balling particles decrease considerably because the molten pool solidified more “slowly”. However, after the layer number increases continuously from 10 to 20, the porosity no longer decreases, and some big size pores, microcracks and fractures appear, especially for the sample obtained at lower laser power. A wavy-like pattern composed chiefly of Al and QC phases, is formed at the interfacial region between substrate and coating due to Marangoni effect.     

时效温度对激光选区熔化TC21钛合金微观组织及硬度的影响

SLM成形TC21钛合金经不同温度时效处理后进行显微组织观察和硬度测试,较为系统地探究了时效温度对其组织和硬度的影响。结果表明,时效温度较低时,随着温度的升高,次生α相呈弥散针状析出,且随着温度升高弥散度增大,同时β析出相体积分数也随着温度的升高而增加。时效温度过高时,次生α相粗化,形成尺寸较大的片状α相,强化效果下降。当时效温度为450 ℃时,所得SLM成形TC21钛合金的显微组织最为弥散、均匀,硬度达到最大值575 HV5,较熔凝态硬度提高43.3%。因此,时效温度应控制在450 ℃。