**3. Medical implants with lattice structures made from Titanium by using the Selective Laser Melting Technology (medical case study example)**

#### **3.1. Introduction**

**2.4. Conclusion**

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(Romania), as it is possible to observe in **Figure 17**.

As a conclusion of made research, it is possible to state that the Selective Laser Melting technology is easy to be understood in principle, but it is not so easy to be controlled. There are a lot of aspects that have to be taken into consideration when speaking about the accuracy of the injection moulding tools made by Selective Laser Melting (SLM), starting with the properties of the raw material, the optical system and ending with the scanning strategy or the technological parameters that are used in the manufacturing process. As related to the technological parameters (laser power, scanning speed, powder bed temperature, etc.), as it has been proven by the finite element analyses that were made, it is very difficult to find a set of technological parameters that would be unique and universally valid in all cases of moulds manufactured by Selective Laser Melting from H13 Tool Steel material. The accuracy of the injection-moulded tools made by Selective Laser Melting technology will be different, being dependent on the geometry of the tools (the size and the shape) and the accuracy of the process. Research still needs to be done in the future regarding the determination of scale factors that can be applied in the pre-processing stage onto the 3D model that has to be manufactured using the Selective Laser Melting equipment. The injection moulding tools were successfully manufactured on an SLM 250 HL equipment in the SLM Solutions GmbH Company from Luebeck (Germany) and tested in the injection moulding process of four type of plastic materials (Acryl Butadiene Styrene – ABS, Polypropylene – PP, Polyamide armed with glass fibers - PA+30% GF and Poly Oxy Methylene– POM) at Plastor SA Company from Oradea

**Figure 17.** Injected plastic materials obtained at Plastor SA Oradea using the injection moulding tools made by SLM.

The recent researches developed in the bio-medical field proved the high interest that exists in this field regarding the possibilities of manufacturing customized medical implants using Additive Manufacturing (AM) methods, such as Selective Laser Sintering (SLS), Selective Laser Melting (SLM) or Electron Beam Melting (EBM) [18–22]. These types of AM methods allow the manufacturing of fully dense metallic parts with complex geometrical shape, starting from a 3D model realized with a computer aided design (CAD) program and exported in an "\*.stl" format [23]. Fully dense metallic structures are required to be produced in the case of custom‐ ized medical implants made by SLM, especially in some regions where the implants needs to be fixed into the human bone with titanium screws. There are also regions of the implants where the porous structure is required for a better osseointegration process of the human tissue through the surface of the medical implant made by SLM. Obtaining structures with a wellcontrolled level of porosity in this case is very important and could be achieved in different ways, such as by designing an implant having different types of lattice structures (geometrical configuration of cells) by adjusting the technological parameters (laser power, hatching distance, etc.) and the scanning strategy during the SLM manufacturing process, or by mixing in different ratios the raw powder material (titanium, in this case) with other types of biocom‐ patible materials (e.g. hydroxyapatite, PMMA, etc.) [24]. The research presented in this chapter, made at the Technical University of Cluj-Napoca (TUC-N), was focused on the finite element analysis of the strain and stress of several models that were especially designed to have different types of lattice structures (size and geometrical configuration of cells). The samples designed and made by SLM at TUC-N using the MCP Realizer II SLM 250 equipment were analyzed afterwards by using a Scanning Electron Microscope JSM – 5600 LV (JEOL) type, in order to determine which is the optimum size and configuration of the cells to be recommended from the structural point of view to be used within the design and manufacturing process of a customized medical implant to be made by SLM. Taking into account the results obtained at TUC-N, a customized medical implant was manufactured by SLM from TiAl6V4 material for a German Medical Institute, by using the SLM 250 HL equipment from SLM Solutions GmbH Company (Luebeck, Germany), at the end.

#### **3.2. Design of the lattice structures using the SolidWorks CAD program**

Six types of models similar to the ones presented in **Figure 18** were designed using SolidWorks, with different types of lattice structures (geometrical features), as following:


 **Figure 18.** Models with different geometrical features designed using SolidWorks.

The extrusion and width of the cells was designed as having 0.2 mm in the case of all models that were designed using SolidWorks. The cellular structure has been obtained by copying the shape of a single cell along the X and Y-axes directions. The models designed in this way were further analyzed using the Abaqus finite element program, in order to determine the stress and strain of the samples and their mechanical behaviour, in the case of a particular pressure load that has been applied in the uniaxial direction onto the top surface of the designed models.

## **3.3. Estimating the mechanical behaviour of samples with different types of lattice structures manufactured by SLM**

The mechanical behaviour of all models presented in **Figure 18** was analyzed using the Abaqus 6.9-3 FEA program. Several material characteristics, such as the elastic modulus (E=114 GPa), Poisson ratio (*υ* =0.31) and yield strength (σc= 775 MPa) for the TiAl6V4 metallic powder material were taken into consideration in the analyses, as they are specified in the datasheet of the material provided by the supplier company on its website. [17]

The next step consisted in establishing the movement restrictions along the X, Y, and Z-axes, as illustrated in **Figure 19**.

Applications of the Selective Laser Melting Technology in the Industrial and Medical Fields http://dx.doi.org/10.5772/63038 177

 **Figure 19.** Movement restriction applied on the Z-axis direction.

**•** circular shape (diameter of a single cell: d=1 mm)

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**•** pyramidal shape (base size of a single cell: l=1.5 mm).

 **Figure 18.** Models with different geometrical features designed using SolidWorks.

The extrusion and width of the cells was designed as having 0.2 mm in the case of all models that were designed using SolidWorks. The cellular structure has been obtained by copying the shape of a single cell along the X and Y-axes directions. The models designed in this way were further analyzed using the Abaqus finite element program, in order to determine the stress and strain of the samples and their mechanical behaviour, in the case of a particular pressure load that has been applied in the uniaxial direction onto the top surface of the designed models.

**3.3. Estimating the mechanical behaviour of samples with different types of lattice**

of the material provided by the supplier company on its website. [17]

The mechanical behaviour of all models presented in **Figure 18** was analyzed using the Abaqus 6.9-3 FEA program. Several material characteristics, such as the elastic modulus (E=114 GPa), Poisson ratio (*υ* =0.31) and yield strength (σc= 775 MPa) for the TiAl6V4 metallic powder material were taken into consideration in the analyses, as they are specified in the datasheet

The next step consisted in establishing the movement restrictions along the X, Y, and Z-axes,

**structures manufactured by SLM**

as illustrated in **Figure 19**.

As one may notice, the "Displacement" type of restriction has been selected in this case. The degrees of freedom were locked accordingly along all three axes by selecting the adequate facet of the model and by setting up the corresponding displacement value to zero.


 **Figure 20.** Applied load specified in the analysis.

The pressure applied in a single direction onto the top surface of the model was determined by using formula (1):

$$P = \frac{F}{A\_p} \tag{1}$$

A force value F = 3200 N, which corresponds to four-times higher the force exerted by a person weighing 80 kg has been considered in all analyzed cases. The value of the contact area Ap in mm2 was different from case to case, due to the fact that the geometrical configuration of the cells was different. The Ap value was determined for each case by using the tools that are available within the SolidWorks CAD program. The pressure load determined by using formula (1) was introduced for each particular case, as shown in **Figure 20**.

The mesh has been generated as illustrated in **Figure 21**. The selected shape of the finite elements was tetrahedral, with an approximate global size of 0.25 mm and a maximum curvature control having a deviation factor of about 0.1 mm.


 **Figure 21.** Element-type control and mesh-size control defined for the FEA.

#### **3.4. Results of the finite element analyses made with Abaqus**

The equivalent von Mises stress, displacement and equivalent strain were determined for all the six samples presented in **Figure 18**, using the finite element analysis method. The analysis allowed formulating recommendations with reference to the optimum values ranging in the standard limits that exist in this field.

In order to determine the equivalent von Mises stress, for the calculus made within Abaqus FEA program, it has been considered the fifth theory for multiaxial stresses:

$$\sigma\_{\rm{ach}} = \sqrt{\frac{1}{2} \left[ \left( \sigma\_{\rm{x}} - \sigma\_{\rm{y}} \right)^{2} + \left( \sigma\_{\rm{y}} - \sigma\_{\rm{z}} \right)^{2} + \left( \sigma\_{\rm{z}} - \sigma\_{\rm{x}} \right)^{2} + \left( \tau\_{\rm{xy}} \right)^{2} + \tau\_{\rm{yz}}^{2} + \tau\_{\rm{zz}}^{2} \right]} \right] \geq 0; \sigma\_{\rm{ach}} \leq \sigma\_{\rm{c}} \tag{2}$$

If we consider in formula (2) that the tangential stresses are equal,

$$\begin{cases} \boldsymbol{\tau}\_{\mathcal{X}\mathcal{Z}} = \boldsymbol{\tau}\_{\mathcal{Z}\mathcal{X}} \\ \boldsymbol{\tau}\_{\mathcal{Y}\mathcal{Z}} = \boldsymbol{\tau}\_{\mathcal{Z}\mathcal{Y}} \\ \boldsymbol{\tau}\_{\mathcal{X}\mathcal{Y}} = \boldsymbol{\tau}\_{\mathcal{Y}\mathcal{X}} \end{cases} \tag{3}$$

a particular state of uniaxial stress is obtained. The simplified formula was used within Abaqus FEA program in order to perform the interpretation of the stress results.

The stress energy was also calculated according to formula (4):

available within the SolidWorks CAD program. The pressure load determined by using

The mesh has been generated as illustrated in **Figure 21**. The selected shape of the finite elements was tetrahedral, with an approximate global size of 0.25 mm and a maximum

The equivalent von Mises stress, displacement and equivalent strain were determined for all the six samples presented in **Figure 18**, using the finite element analysis method. The analysis allowed formulating recommendations with reference to the optimum values ranging in the

In order to determine the equivalent von Mises stress, for the calculus made within Abaqus

 t

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formula (1) was introduced for each particular case, as shown in **Figure 20**.

 **Figure 21.** Element-type control and mesh-size control defined for the FEA.

FEA program, it has been considered the fifth theory for multiaxial stresses:

**3.4. Results of the finite element analyses made with Abaqus**

 ss

If we consider in formula (2) that the tangential stresses are equal,

standard limits that exist in this field.

 ss

s

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curvature control having a deviation factor of about 0.1 mm.

$$E\_{def} = \frac{1}{2} \sigma\_{ach} \times \mathcal{e}\_{ach} \tag{4}$$

where *Edef* - represents the stress energy; *σech* - equivalent stress von Mises; *εech* - equivalent strain.

As one may notice in **Figure 22**, the values of the equivalent stress von Mises were lower than the yield strength σc= 775 MPa specified in the material datasheet provided by the supplier of the TiAl6V4 material. The minimum value of the equivalent stress von Mises (102.50 MPa) has been determined in the case of the sample with lattice structure designed as having cells with a rhombic shape, while the maximum value (278.56 MPa) was obtained in the case of the sample designed with cells having a conical shape. If we consider a value of 212 MPa for the mechanical resistance of the trabecular bone, the adequate solution would be to design and produce the medical implant as having a rhombic shape of the cell, in this case [25].

 **Figure 22.** Distribution of the equivalent stress von Mises as computed by the Abaqus FEA program.

The displacement and equivalent strain values were negligible in all cases that were analyzed, being considered too low for affecting the bone or customized medical implant made from titanium powder by SLM [25].

#### **3.5. Manufacturing samples with lattice structures and medical implant by SLM**

The manufacturing of the six samples was realized using the MCP Realizer II SLM 250 from the National Centre of Innovative Manufacturing from the Technical University of Cluj-Napoca (TUC-N) presented in **Figure 2**. After the metallic supports were removed, the manufactured samples made by SLM were obtained as presented in **Figure 23**.

 **Figure 23.** Samples manufactured using the MCP Realizer II SLM 250 equipment [26].

After the manufacturing process, the structure of the samples was analyzed by using a Scanning Electron Microscope (SEM) JSM-5600 LV (JEOL) – type that is available at the Technical University of Cluj-Napoca (TUC-N).

The best geometrical configuration of the lattice structures that were manufactured by SLM was the rhombic one (see **Figure 24**). By analyzing the image presented in **Figure 24.a**, a series of appreciations regarding the metallic grains distribution on the surface of the sample was made. As it is possible to observe in this image, the grains are distributed in a uniform manner.

**Figure 24.** Metallographic analysis of the sample with rhombic geometrical shape manufactured by SLM. (a) Sample realized by SLM (70X) (b) Sample realized by SLM (600 x).

By zooming in the surface analyzed area (600x), it was possible to observe that the presence of secondary pores was at the lowest level in this case, conferring higher resistance connections of the grains in the lattice structure. By analyzing the image presented in **Figure 24b** it is possible to observe that the grains that were partially melted formed bridge connections with the grains stated in a semi-liquid phase of about 50 μm.

**3.5. Manufacturing samples with lattice structures and medical implant by SLM**

manufactured samples made by SLM were obtained as presented in **Figure 23**.

 **Figure 23.** Samples manufactured using the MCP Realizer II SLM 250 equipment [26].

Technical University of Cluj-Napoca (TUC-N).

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realized by SLM (70X) (b) Sample realized by SLM (600 x).

After the manufacturing process, the structure of the samples was analyzed by using a Scanning Electron Microscope (SEM) JSM-5600 LV (JEOL) – type that is available at the

The best geometrical configuration of the lattice structures that were manufactured by SLM was the rhombic one (see **Figure 24**). By analyzing the image presented in **Figure 24.a**, a series of appreciations regarding the metallic grains distribution on the surface of the sample was made. As it is possible to observe in this image, the grains are distributed in a uniform manner.

**Figure 24.** Metallographic analysis of the sample with rhombic geometrical shape manufactured by SLM. (a) Sample

The manufacturing of the six samples was realized using the MCP Realizer II SLM 250 from the National Centre of Innovative Manufacturing from the Technical University of Cluj-Napoca (TUC-N) presented in **Figure 2**. After the metallic supports were removed, the

> Taking into account the obtained results, a customized medical implant was manufactured by SLM from TiAl6V4 material for a German Medical Institute, using the SLM 250 HL equipment from SLM Solutions Gmbh Company from Luebeck, Germany, presented in **Figure 8**.

> This equipment was selected for the manufacturing process of the customized implant presented in **Figure 25**, due to the fact that this type of equipment, as compared to the MCP Realizer SLM 250 equipment from the Technical University of Cluj-Napoca allows the possibility of using different type of scanning strategies in the same deposited layers, in such way that at the end, in some areas of the implant (e.g. the area where fixing screws are required) this areas will result with a density of 100%, while in other areas (e.g. areas where the medical implant is getting in contact with the human tissue), the areas will be a porous one, the osseointegration process being facilitated in a significant way in that region.


**Table 5.** Technological parameters used for manufacturing the customized medical implants by SLM using the SLM 250 HL equipment.

 **Figure 25.** Customized medical implant manufactured from Ti6Al4V material using the SLM 250 HL equipment.

A metallographic analysis of the medical implant was made using the JSM-5600 LV (JEOL) Scanning Electron Microscope (SEM) from the Technical University of Cluj-Napoca. The analyzed area was a mixed area that is close to the contact area of the fixing screws (an area with 100% density of the material) and an area with a controlled porosity given by the geometrical shape of the lattice structure, as it is possible to observe in **Figure 26**.

 **Figure 26.** (a) Lattice structure (2x), (b) welding lines (25x) of the customized medical implant made by SLM.

As it is possible to observe in **Figure 26.a**, the laser beam has followed very accurately the geometrical path of the bore that was designed for fixing the implant with screws. The density in this area is 100%, conferring a higher mechanical strength that is required for fixing the medical implant onto the femoral head.

By zooming in the image of the analyzed area, as it is possible to observe in **Figure 26.b**, the size and the distribution of the pores resulted in the structure of the material was homogenous and uniformly distributed (wall thickness of the cells was approximately 1 mm), an aspect that is important for the future proliferation of the human tissue within the structure of the medical implant within the osseointegration process, at the end.

#### **3.6. Conclusions**

As it was possible to observe by analyzing the results obtained after the finite element analyses, the sample with the cells having a rhombic shape has proved to be optimal from the mechanical resistance point of view as compared to the results obtained in the case of the other analyzed samples. Taking into account the obtained results, a customized medical implant was manu‐ factured by SLM from TiAl6V4 material for a German Medical Institute, using the SLM 250 HL equipment from SLM Solutions GmbH Company from Luebeck. A metallographic analysis of the medical implant was made using the JSM-5600 LV (JEOL) Scanning Electron Microscope (SEM) from the Technical University of Cluj-Napoca, proving the fact that the laser beam has followed very accurately the geometrical path of the bore that was designed for fixing the implant with screws, and that the size and the distribution of the pores resulted in the structure of the material which is homogenous and uniform, with positive consequences for the future proliferation of the human tissue within the structure of the medical implant within the osseointegration process. Further researches are required to be done in the future regarding the possibilities of manufacturing customized medical implants with a well-controlled level of porosity, made from new types of biocompatible materials (e.g. titanium samples coated with hydroxyapatite or PMMA). Finding a proper method for the stress-release of customized medical implants made from titanium based alloys, during or after the Selective Laser Melting (SLM) process still presents an important challenge for the future researches, as well.
