Finite Element Simulation of Stainless Steel Porous Scaffolds for Selective Laser Melting (SLM) and Its Experimental Investigation

Abstract

In recent years, bone defect and bone tissue damage have become common clinical diseases. The development of bionic bone has had an important impact on the repair and reconstruction of bone tissue. Porous scaffolds have the advantages of adjustable pore size and controllable shape, which can solve the problem of mismatch in the process of bone repair, but traditional processing methods cannot overcome the challenge of the preparation of complex porous scaffolds. Therefore, 316L porous stainless steel scaffolds with different pore sizes (200 μm, 300 μm, 400 μm and 500 μm, respectively) were prepared by selective laser melting. Stress and strain were simulated and analyzed by using a finite element simulation method. Combined with a heat treatment experiment, a hardness test, a metallographic observation and a compression test, porous scaffolds were studied. The mechanical properties and microstructures of the scaffolds were studied and analyzed, and the optimized porous scaffolds were obtained. With reasonable melting parameters, the porous scaffolds that could meet the mechanical property requirements of load-bearing bone restorations were prepared by SLM.

1. Introduction

The use of 3D manufacturing technology produces 3D products by adding materials layer by layer [1,2,3,4] and is a kind of manufacturing method that is opposite to the traditional material removal processing method. By adding materials, and converting 3D CAD into STL facet files and other model data, we can directly manufacture a 3D physical entity model that is completely commensurate with the corresponding mathematical model. Because it does not need billets, it can realize the production and manufacture of products only by using the graphical data of a model made by a computer, can simplify the manufacturing procedure of products, shorten the development cycle of products, improve the production efficiency; it has been widely used in aerospace, automobile, motorcycle, household appliances, biomedicine, cultural creativity and other technological fields. It also has a unique position in engineering, scientific research, teaching and other fields of application. The laser-based powder bed fusion of metals (PBF-LB/M) [5] process is used in current Rapid Prototyping (RP)technology. PBF-LB/M is now considered to be a promising manufacturing technology, and we intend to apply it to orthopedic implants under load-bearing conditions [6]. Rehme et al. [7] studied and obtained the influence of laser parameters on the realization of microcrystalline lattice structures of various configurations. Figure 1 is the schematic diagram of the basic principles of SLM technology.

Why do we need to introduce biological concepts into additive manufacturing? How can people “copy” the ingenious and complex biological structures created by nature into industrial manufacturing? Increasingly, intelligent design software and 3D printing technology offer a shortcut to creating bionic structures [8]. Many of nature’s materials are highly adaptable, while man-made materials are not. The density of steel material is higher than other materials. Ceramic is fragile, while plastic is elastic. The parts made from these materials exhibit the properties of the source material throughout the process. On the other hand, many objects in nature exhibit adaptive density, elasticity, and brittleness in different areas for multiple purposes (e.g., combinations of muscles, veins, and bones) [9,10].

In theory, most orthopedic surgeries now use standardized implants. Here, 3D printed personalized implants are mainly used to solve problems that cannot be solved by traditional surgery, as well as to meet high-end medical needs [11]. For example, in bone tumor repair surgery, personalized 3D-printed implants can meet the needs that cannot be met by traditional methods; this is a crucial demand for doctors and patients. As a new technology, 3D-printed personalized implants will coexist with traditional technologies fora long time. They will gradually replace traditional technologies after their long-term accumulation, on the basis of crucial-need cases [12,13]. The elastic modulus of stainless steel, titanium and titanium alloy is much higher than that of human bone, and it is easy to a produce “stress shielding” effect after implantation; this will affect the normal development of bone tissue near the implantation material [14]. Therefore, as a bone implant material, it is necessary to further improve the biological activity of stainless steel material to promote bone tissue regeneration [15,16,17]. Porous scaffolds also have the advantages of good penetration, adjustable pore size and porosity, and a similar elastic modulus to natural bone tissue, which can effectively prevent the generation of the stress shielding effect [16].

Natural bone tissue is a complex tissue with a precise porous structure. The ideal design of bone tissue engineering requires that the implant structure conforms to the anatomical structure of natural bone tissue [18]. In this paper, porous scaffolds for medical biology were prepared by SLM technology [19,20]. Porous scaffold is a promising bone graft, which provides a cell-friendly biological environment and simulates the bone structure [21]. This porous scaffold has good biocompatibility, bone tissue integration ability, good bone binding ability and appropriate mechanical properties [22]. The optimized porous scaffold not only replaces the damaged bone tissue, but also overcomes the shortcomings of the short-term implantation of porous scaffolds, manufactured by traditional technology. A 3D porous structure is conducive to cell adhesion, migration and proliferation, and provides the necessary living space for nutrition, metabolism and metabolic pathways to provide transport channels. Finally, implants and organisms are successfully integrated to achieve regeneration [23,24]. The homogenization calculations are of great significance for the study of porous composites [25,26,27].

Porous scaffolds have the advantages of adjustable pore size and controllable shape; this can solve the problem of mismatch in the process of bone repair, but traditional processing methods cannot overcome the challenge of preparing complex porous scaffolds. Therefore, 316L porous stainless steel scaffolds with different pore sizes (200 μm, 300 μm, 400 μm and 500 μm, respectively) were prepared by selective laser melting. Stress and strain were simulated and analyzed by the finite element simulation method. Combined with the heat treatment experiment, hardness test, metallographic observation and compression test, porous branches were studied. The mechanical properties and microstructures of the scaffolds were studied and analyzed, and the optimized porous scaffolds were obtained. With reasonable melting parameters, the porous scaffolds that could meet the mechanical property requirements of load-bearing bone restorations were prepared by SLM. The optimal porous scaffolds, obtained by the combination of finite element simulation and experimental analysis, can meet the mechanical property requirements of load-bearing bone prostheses.

2. Materials and Methods

2.1. Materials

Indeed, 316L Stainless Steel (SS) is one of the most commonly-used medical metal materials in the world [28]. It has the advantages of strong corrosion resistance, high strength, good processability and low cost. At present, 316L stainless steel is widely used in clinical medicine. It is widely used as human implant because of its excellent mechanical properties and good human compatibility. Because of the complex shape and the various sizes of artificial joints, traditional manufacturing technology has found it difficult to meet the market demand. SLM can produce porous scaffolds with complex shapes, and has been gradually applied in the field of biomedical sciences. Figure 2 shows the microscopic images of the morphology of 316L stainless steel powder, via scanning electron microscopy (ZEISS SUPRA 55, Mainz, Germany). It can be seen that 316L stainless steel powder has spherical distribution and the powder diameter is uniformly distributed around 40 μm.

Table 1. Chemical composition of 316L stainless steel powder (%).

Brand Name	C	Si	Mn	P	S	Ni	Cr	Mo
00Cr17Ni14Mo2	≤0.03	≤1.00	≤2.00	≤0.035	≤0.03	12.0–15.0	16.0–18.0	2.0–3.0

shows the chemical composition of 316L Stainless Steel Powder (%).

Table 2. Mechanical properties of 316L stainless steel (JIS G 4305-2005).

Distinguish	Ys Yield Strength (MPa)	Ts Tensile Strength (MPa)	El Elongation (%)	Hv Vickers Hardness	Remarks
Standard	≥205	≥520	≥40	≤200	2 B/1.5 t
SLM material	310	620	53	155

gives the mechanical properties of 316L stainless steel (JIS G 4305-2005).

2.2. Finite Element Models

The finite element analysis software (version 17.0) is ANSYS 17.0, and the porous scaffold model is modeled by SolidWoks software (version 2016). In order to ensure that the computational accuracy was improved while saving computational resources, the model cell mesh was divided using a tetrahedral mesh with a mesh size of 0.05 mm. The static mechanical model was a cylinder with a diameter of 10 mm and a height of 10 mm. The boundary conditions of the finite element model were consistent with the compression experiments, and the calculated load was an axial downward compression force of 3000 N. The boundary conditions at the bottom of the model were set to fully constrained. A large number of studies have shown that the ideal large hole size for bone tissue engineering is approximately 300–900 μm, and the porosity is between 60% and 95%. Considering that the optimal size range of bone growth and angiogenesis is 100–500 μm, the internal structure of the scaffold selected in this paper is connected to pore structure, and the pore size is set as 200 μm, 300 μm, 400 μm, and 500 μm, respectively. In the simulation, the elastic modulus of the stainless steel material E is selected as 196 GPa, and Poisson’s ratio ν is selected as 0.3.

2.3. Manufacture of Porous Scaffolds

In this study, a laser power of 150 W, a scanning spacing of 0.06 mm, a scanning speed of 500 mm/s and a powder layer thickness of 0.035 mm were selected to construct a porous bone scaffold. Figure 3 shows the solid model of the 3D implant scaffolds with different apertures. The complex 3D model of porous scaffolds with different apertures was constructed by using Soildworks software. Figure 3a has a porosity of 200 μm, Figure 3b has a porosity of 300 μm, Figure 3c has a porosity of 400 μm, and Figure 3d has a porosity of 500 μm. Figure 4 shows 316L stainless steel porous scaffolds with different pore diameters, formed by SLM.The porous solid 316L stainless steel implant scaffold was prepared by melting metal powder with SLM technology. The pore diameter of the scaffold increased from 0 μm, 200 μm, 300 μm, 400 μm and 500 μm, respectively.

3. Results and Discussion

3.1. Finite Element Analysis

A large number of studies have shown that the ideal macro-pore size for bone tissue engineering is approximately 300–900 μm, and the porosity is between 60% and 95% [28,29,30]. Considering that the optimum size range of bone growth and angiogenesis is 100–500 μm, the internal structure of the scaffold selected in this paper is connected to pore structure. The aperture sizes are set to 200 μm, 300 μm, 400 μm and 500 μm, respectively. The porous metal parts ensure the biomechanical properties of the bone restorations. They have good corrosion resistance, wear resistance and mechanical properties [31]. They are especially suitable for the manufacture of artificial restorations. As the porous structure is applied to human bones, the stress of human bones is mainly compressive stress; therefore, this paper mainly conducts compression test experiments on the porous structure test pieces, and takes experimental data and simulation results as analysis objects. However, due to the complexity of the porous structure in this study, the finite element mechanical numerical simulation analysis of the porous structure is only limited to the range of linear elastic deformation, and its force direction is shown in Figure 5.

3.2. Simulation Numerical Results Discussion

The stress–strain analysis of porous scaffolds with different pore sizes was performed by using finite element simulation analysis, and the optimized scaffold porosity structure was obtained. Figure 6 shows the deformation, stress and strain nephograms of porous scaffolds with different pore sizes, and the ends of the porous scaffolds were compressed unilaterally. From Figure 6a–d, it can be seen that the deformation at the upper end of the specimens is higher than the other part of specimen, and that the deformation limit of the scaffold was higher than that of deformation limit of natural bone. For the stress distribution of scaffolds, it can be seen from Figure 6e–h that the stress at the upper and bottom of the scaffolds is higher at the beginning stage. With the deformation proceeding, under the condition that the porosity of the porous scaffolds is similar, the stress distribution can be obtained from the stress-deformation nephogram of the scaffolds. The porous scaffolds with a diameter of 300 μm can effectively relieve the local stress concentration. Figure 6i–l support position, using the principle of equidistant points, and its strain, is plotted as shown in Figure 7. It can be seen that, with the increase in porosity, the effective strain presents a gradient rise.

The numerical simulation of the selective laser melting of porous scaffolds with different apertures was carried out by finite element analysis. Selective laser melting was carried out under the same conditions. The average elastic modulus of the scaffolds with different apertures was obtained, as shown in

Table 3. Compressive strength and elastic modulus of different apertures.

Aperture Size (μm)	Compressive Strength/MPa	Modulus of Elasticity (GPa)
300	514	10.38
400	488	6.38
500	465	5.56

. The elastic modulus of human bone with a higher transplantation rate ranges from 10 to 30 GPa [32]. On the one hand, the design of scaffolds is to obtain a high-strength structure; on the other hand, it is also a process to obtain a perfect match between the mechanical properties of the scaffold and the surrounding bone tissue. If the elastic modulus of the scaffold exceeds the elastic modulus of the natural bone after implantation, it is easy to produce a stress shielding phenomenon; this causes osteoporosis around the implanted bone tissue and easily leads to fracture. Therefore, it can be concluded from the above analysis that among the porous scaffolds, the one with the pore size of 300 μm can withstand the greatest pressure, and the one with the pore size of 300 μm is more suitable for the load-bearing role of the human bone implant.

3.3. Compression Experimental Analysis

The porous scaffolds with an aperture size of 200 μm, 300 μm, 400 μm and 500 μm, respectively, were subjected to compression tests. Their test loads were 28.4 KN, 51.9 KN, 25.5 KN and 23.8 KN in turn. It can be seen that the strength of the scaffold model is the best when the aperture is 300 μm. Figure 8 shows the curves of the test force and the deformation obtained from the compression tests of the scaffolds with different diameters. In the case of the same deformation, the porous scaffolds with the diameter of 300 μm can bear the greatest test force (in addition to solid scaffolds).It can be concluded that the pressure-bearing capacity of porous scaffolds with an aperture size of 300 μm is higher than that of porous scaffolds with an aperture size of 400 μm and 500 μm, respectively.

3.4. Microstructure Analysis

All Scaffolds are uniformly manufactured under the same conditions, with regular holes. Figure 9 gives the optical and SEM morphology of the porous scaffolds. From Figure 9, it can be seen that the prepared porous scaffolds have a compact and uniform structure, without defects. In an observation from Figure 9(A1–A4), SLM forms a porous scaffold surface and there is uniform melting. It can be seen that the pore shape is basically circular, there is no obvious crack, the forming quality is relatively complete, and there is no over-melting phenomenon. The scaffold has a well-connected macroporous structure, many micro-spheres are observed on the wall surface of the scaffold frame gap, and the size range of micro-spheres is 30–50 μm. The scaffold pore hole shown in the picture does contain some non-melting or adhesive globular particles. The spheroidization of SLM can be observed from the SEM image of the scaffold in Figure 9(B1–B4). Metal powders are melted via selective laser melting. It can be seen from the figure that the prepared porous scaffold has a dense and uniform structure, and that the forming part is mainly composed of columnar crystals with the same growth direction. The columnar crystal arrangement has the same directionality and uniform distribution, and the periphery is densely distributed with fine equiaxed crystals. Due to the high temperature and high gradient solidification characteristics of the rapid molding technology, the resulting structure is fine, uniform, and the microstructure is arranged regularly.

3.5. Experimental Analysis of Heat Treatment

Because SLM-forming technology involves a complex metallurgical process, including metal powder through high-speed laser scanning, instantaneous melting, solidification, the cooling process, and the process forming workpieces, extensive residual stress is produced; however, bubbles and poor sintering defects are also produced, which not only affect the microstructure of the porous scaffolds, but also affect the mechanical properties of the materials.

In this paper, the method of heat treatment is used to improve the mechanical properties of materials and eliminate the participating stress. Stress relief annealing is used to heat the temperature below the phase transformation temperature, and then cool it slowly in the air. Therefore, no structural transformation occurs during the heat treatment process. This experiment adopted a heat treatment process with a heating temperature of 800 °C, heat preservation of 100 min and furnace cooling. The hardness values of supports with different pore sizes, before and after heat treatment, were found. The hardness was measured by a Vickers hardness tester, the test load was 1000 N, each sample was measured five times, and the average value was taken.

Table 4. Hardness change before and after heat treatment.

Aperture Size (μm)	Before Heat Treatment (HV)	After Heat Treatment (HV)
300	276	224
400	268	224
500	262	220
Solid	258	220

shows the Vickers hardness changes in four different structure scaffolds, before and after heat treatment. The experimental results show that the heat treatment method can reduce the hardness of the scaffold well and match the mechanical properties of human bone.

4. Conclusions

The porous scaffolds with different pore sizes, prepared by SLM, have great potential to replace bone tissue and precisely control porosity and pore size. Through the combination of finite element simulation analysis and experimental study, the scaffold model with the most suitable mechanical properties—porous scaffolds and different pore sizes—was obtained, and the mechanical properties of human bone tissue were better matched through heat treatment. The main conclusions are as follows:

(1)

The deformation at the upper end of the specimens is higher, and the deformation limit of the scaffold is higher than that of deformation limit of natural bone. The porous scaffolds with a diameter of 300 μm can effectively relieve the local stress concentration. With the increase in porosity, the effective strain presents a gradient rise. In addition, the one with the pore size of 300 μm is more suitable for the load-bearing role of the human bone implant.

(2)

The prepared porous scaffold has a dense and uniform structure, and the forming part is mainly composed of columnar crystals with the same growth direction. The columnar crystal arrangement has the same directionality and uniform distribution, and the periphery is densely distributed with fine equiaxed crystals. Due to the high temperature and high gradient solidification characteristics of the rapid molding technology, the resulting structure is fine, uniform, and the microstructure is arranged regularly.

(3)

Through finite element simulation analysis, the reasonable pore diameter of the porous scaffold is obtained, and the optimized pore size of the porous scaffold is obtained. The elastic modulus of the porous scaffold with a pore size of 300 μm isclosest to the elastic modulus required by human bone. Therefore, the comprehensive mechanical properties of the scaffold with the pore size of 300 μm arethe best match with that of human bone. The identification of the porous scaffold that is more suitable for the load-bearing effects of human bone can guide the follow-up research.