Surface Finishes for Ti-6 Al-4 V Alloy Produced by Direct Metal Laser Sintering

The increase of life expectancy in recent decades has increased the population and, consequently, increased the surgeries number of partial or total replacement of some part of the human body1,2. Currently, additive manufacturing allows production of custom prosthetic implants, fitting them directly to patient needs. It can be used in many medical specialties including neurosurgery, maxillofacial surgery, craniofacial and plastic surgery, oncology, dentistry and orthopedics3-8. The main metallic materials used in orthopedic implants are stainless steel alloys, cobalt-chromium alloys and titanium alloys1,9,10. The Ti-6Al-4V alloy is considered biocompatible to the human body. Biocompatibility is the ability of a material to fulfill its function in a patient for a specific application9, and biomaterials are natural or synthetic substances that are tolerated transiently or permanently in the human body11. One of the main desirable properties of biomaterials for orthopedic implants is the low modulus of elasticity due to bone reabsorption, besides biocompatibility, corrosion resistance (degradation), mechanical strength, fatigue resistance and wear resistance, which is especially the case of metallic materials. The surface of an implant has a direct influence on its anchorage in bone9,12-14. It is responsible for direct contact with patient tissues and is a key factor for osseointegration, protein adsorption, interaction with cells, the creation of the interface between the implant and the body and as well as for the development of tissues. Topography, chemical characteristics, charge and wettability are the most important properties relating the surface with osseointegration13,15. Osseointegration is the anchoring of an implant to the living bone, obtained through contact between them, where bone cells migrates to the implant surface, reaching stability and a durable anchoring, allowing the transmission and distribution of loads to surrounding tissues12,13,16. Osseointegration helps in the healing process and in stability and durability of the implant13. Rough surfaces, porous coatings and surfaces with osteoconductivity and osteoinductivity in body fluids are shown to be good surfaces for osseointegration15. Titanium rough surfaces are effective to further a mechanical attachment with the tissues, increasing the stress distribution at the interface and causing osseointegration15. On the other hand, surfaces with high roughness can enhance the development of bacteria in areas with low blood flow17. Furthermore, the increased roughness causes an increase in surface area and, consequently, an increase in the quantity of ions released by the implant surface18. This study aimed to analyze the surface finishes of blasting, chemical etching and electropolishing in Ti-6Al-4V alloy produced by direct metal laser sintering (DMLS) process.


Introduction
The increase of life expectancy in recent decades has increased the population and, consequently, increased the surgeries number of partial or total replacement of some part of the human body 1,2 .Currently, additive manufacturing allows production of custom prosthetic implants, fitting them directly to patient needs.It can be used in many medical specialties including neurosurgery, maxillofacial surgery, craniofacial and plastic surgery, oncology, dentistry and orthopedics [3][4][5][6][7][8] .
The main metallic materials used in orthopedic implants are stainless steel alloys, cobalt-chromium alloys and titanium alloys 1,9,10 .The Ti-6Al-4V alloy is considered biocompatible to the human body.Biocompatibility is the ability of a material to fulfill its function in a patient for a specific application 9 , and biomaterials are natural or synthetic substances that are tolerated transiently or permanently in the human body 11 .One of the main desirable properties of biomaterials for orthopedic implants is the low modulus of elasticity due to bone reabsorption, besides biocompatibility, corrosion resistance (degradation), mechanical strength, fatigue resistance and wear resistance, which is especially the case of metallic materials.
The surface of an implant has a direct influence on its anchorage in bone 9,[12][13][14] .It is responsible for direct contact with patient tissues and is a key factor for osseointegration, protein adsorption, interaction with cells, the creation of the interface between the implant and the body and as well as for the development of tissues.Topography, chemical characteristics, charge and wettability are the most important properties relating the surface with osseointegration 13,15 .Osseointegration is the anchoring of an implant to the living bone, obtained through contact between them, where bone cells migrates to the implant surface, reaching stability and a durable anchoring, allowing the transmission and distribution of loads to surrounding tissues 12,13,16 .Osseointegration helps in the healing process and in stability and durability of the implant 13 .Rough surfaces, porous coatings and surfaces with osteoconductivity and osteoinductivity in body fluids are shown to be good surfaces for osseointegration 15 .
Titanium rough surfaces are effective to further a mechanical attachment with the tissues, increasing the stress distribution at the interface and causing osseointegration 15 .On the other hand, surfaces with high roughness can enhance the development of bacteria in areas with low blood flow 17 .Furthermore, the increased roughness causes an increase in surface area and, consequently, an increase in the quantity of ions released by the implant surface 18 .
This study aimed to analyze the surface finishes of blasting, chemical etching and electropolishing in Ti-6Al-4V alloy produced by direct metal laser sintering (DMLS) process.

Experimental Procedures
Cylindrical samples with dimensions of 10 mm x 5 mm (diameter x height) were produced from commercial powder of Ti-6Al-4V (Figure 1), donated by the National Institute of The implant's surface is responsible for direct interaction with the human body.For cases where osseointegration must be favored and the risk of bacteria proliferation is lower, rough surfaces are more suitable, while for implants where the risks are higher, a reduced surface roughness is required.This study aimed to analyze and produce different surface finishes on samples of Ti-6Al-4V alloy produced by additive manufacturing technique of Direct Metal Laser Sintering (DMLS).Surfaces of the samples were analyzed in the as-built condition, after blasting, after chemical etching, after electropolishing and two different combinations of these methods.The surfaces were studied using the technique of scanning electron microscopy, and surface roughness and mass measurements.The lower roughness value was obtained after a combination of blasting and chemical etching.Blasting results in a surface with uniform roughness while chemical etching cleans the surface and reduces its roughness.

Guilherme
Biofabrication (INCT -BIOFABRIS), by direct metal laser sintering (DMLS) process using the additive manufacturing equipment EOSINT M270 from EOS GmbH Electro Optical Systems (Figure 2).DMLS is a layer by layer process which uses a laser beam that is directly exposed to the metal powder, fusing and consolidating thin layers.A DMLS process scheme is shown in Figure 3 19 .
Processing parameters are shown on Table 1.The blasting was performed on the upper surface of the sample, using grit with an average particle size of 200 µm.The chemical etching step was carried out using a solution containing 2% of hydrofluoric acid and 20% of nitric acid.The samples were completely immersed in the solution at room temperature for 25 minutes, using a rod for agitation.The electrochemical polishing on the samples was performed using the Struers LectroPol-5 equipment.A solution containing 5% of perchloric acid (60%) in acetic acid was used as electrode, and a 0.5 cm 2 mask for polishing of the top surface of the sample was selected.The samples were then polished at room temperature of 25 °C for 5 minutes using a voltage of 55 V under a current of 0.3 A.
The surfaces of the samples were analyzed in Zeiss -EVOMA15 scanning electron microscope with Smart SEM software using the secondary electron emission.Mass measurements were performed using a Shimadzu -AX200 scale at each step of surface finishing.Also, roughness measurements for each step of surface finishing of the samples were made using a Mitutoyo SJ-201 rugosimeter.Five measurements were made for each sample, turning them 30º at each measurement.The values shown are the mean values and their standard deviations.

Results and Discussion
Figures 4a and 4b show the mean values for surface roughness average (Ra) and mean roughness depth (Rz) of samples in conditions as-built (AB), after blasting (B), after chemical etching (CHE), after electropolishing (EP) and after different combinations of methods.The B -CHE combination gave the lowest surface roughness value.While blasting is responsible for reducing surface roughness and making it more uniform, chemical etching reduces even more the roughness.The electropolishing finish presented high values of roughness, even when made after B -CHE treatment, showing concordance with the work of Pyka et al. 20 .
Table 2 shows the results for mass analysis, where the reduction is the percentage mass reduction relative to the as-built condition, and the relative reduction is the percentage mass reduction relative to the previous step.The mass measurements are plotted in Figure 4c.The CHE treatment showed the highest mass reduction.This happened due to two factors: the CHE process works by removal of material by surface oxidation, causing ionization of atoms that come off the matrix; the sample was completely immersed in the reagent, causing all the surfaces of the sample to be attacked.For EP and B the mass reduction occurs on a smaller scale.However, it is noteworthy that only the top   surface of the sample (25% in area of total surfaces) was treated by reducing the effective area of material removal.The blasting process removes material due to the shape of the grit, which has cutting edges that remove material on colliding with the surface.Electrochemical polishing also removes materials by ionizing surface atoms.
In Figure 5, it is possible to observe the surfaces of the samples in the AB condition (a), after CHE (b), after EP (c), after B (d), after B and CHE (e) and after combining the three treatments (f).In the AB condition the top surface lines that characterize the laser building strategy of zigzag and the hatch spacing of 100 µm can be seen.In the CHE condition, it can be seen that material has been removed and the grains of the microstructure were lightly revealed, due to the reaction of the reagent with the alloy material.However, the traces of the scanning lines of the laser are still visible.Moreover, after EP, the laser scanning lines are no longer visible.Furthermore, it is possible to see that, although the surface looks essentially flawless, there is an undulation on the surface, as shown by shading in some regions.After B finishing the laser scanning lines also disappear completely.The blasting process works by deforming and removing material from the surface, obtaining as a result a very rugged surface.However, some grit grains attached on the surface of   the sample.The B -CHE combination showed the smallest surface roughness value.The CHE finishing cleans the previous step attached grits on the sample surface.Also, there is no trace of scanning lines, and the microstructure grains were revealed.Finally, the B -CHE -EP finishing showed the same appearance as for the EP treatment, indicating that the process is independent of the two preceding treatments.

Conclusions
The lower surface roughness value was obtained after combining blasting and chemical etching.Blasting is responsible for leaving a surface with uniform roughness, while the chemical etching is responsible for cleaning the surface and reduce its roughness.Electropolishing showed a mirrored surface finish, but with a high roughness value, showing ineffectiveness in lowering the surface roughness of the material.The mass analysis showed a reduction in weight of the samples after all the treatments.This occurs because all treatments have as principle material removal.

Figure 5 .
Figure 5. Micrographs obtained by SEM for each condition.As-built (a), after chemical etching (b), after electropolishing (c), after blasting (d), after blasting and chemical etching (e) and after combining the three treatments (f).

Table 2 .
Values for mass measurements.