A Study on Metal 3D Printing Internal Pore Control for the Fabrication of Metal Scaffolds

Byeong-Ju Jin; Ki-Young Park

doi:10.5781/JWJ.2022.40.4.6

Abstract

In the treatment of diseases using medical bone scaffolds, it is necessary to design a customized bone scaffold according to the type of individual damage to restore normal bone function based on the damage information of each patient. In addition, the bone scaffold should serve to promote the regeneration of damaged bone in the patient. Compared to a rigid scaffold, a patterned scaffold inserted into a living body promotes bone formation more effectively. Therefore, using a metal 3D printer, it is important to incorporate artificial voids by controlling process variables so that the metal scaffold that replaces damaged bone can form a good bond with live tissue cells. In this study, Ti-6Al-4V ELI, which is actively used in the fields of medical implants and medical devices because of its excellent biocompatibility, was used to fabricate a metal hybrid scaffold that also provides external support. As for the process method, specimen production was conducted by controlling process parameters of the selective laser melting (SLM) method, which is one of the metal 3D printing techniques. Specimens fabricated for each scenario were analyzed by measuring flexural strength, and by imaging via optical microscopy (OM) and scanning electron microscopy (SEM). For OM analysis, the porosity was measured for each case and compared with the flexural strength. Thus, it was possible to adjust the strength by varying the internal pores via the process variables of the metal 3D printer. Accordingly, basic research was conducted to fabricate a metal osseous scaffold with strength similar to that of bone.

Key words: Metal 3D printer, Metal scaffold, Ti-6Al-4V powder, Process values

1. Introduction

For the treatment of diseases using medical bone scaffolds in orthopedic and plastic surgeries, customized scaffolds should be designed according to the type of individual damage to restore normal bone function based on the damage information of each patient¹^-³⁾. The design of hybrid scaffolds includes external/internal support forms. Customized fabrication according to the degree of damage and requirements of each patient is required to induce restoration of bone function by inserting an alternative scaffold for internal support in vivo. Furthermore, in order to promote the regeneration of fractured bone that is intuitively related to the treatment of a patient’s diseased part, it is crucial to fabricate an artificial pore during the additive manufacturing process using a metal 3D printer so that bonds with living tissue cells can be formed well through control of the internal shape of metal scaffolds that replace broken bones⁴^-⁶⁾. The metal 3D printing process using additive manufacturing has a melting point of about 1,000-2,000°C. Hence, the technical difficulty is high and it is advantageous for fabricating products that are difficult to manufacture using the traditional methods⁷^-⁹⁾.

Therefore, this study examined the fabrication of metal scaffold using Ti-6Al-4V ELI powder to support the outside among internal and external hybrid scaffolds. With respect to the fabrication of external scaffold, the unmelted powder is removed from the molten pool while the molten pool is formed and sintered from metal powder by laser in the selective laser melting (SLM)) manufacturing process, which is one of the representative processes of metal 3D printing. Internal pores may be generated due to the balling effect¹⁰⁾. Hence, we tried to control internal pores by controlling process variables. Based on this, basic research on the fabrication of metal scaffolds with similar strength to bone was conducted.

2. Experimental Method

2.1 Experimental setup and materials

The metal 3D printer used in this study is ORLAS Creator RA of the SLM type (Fig. 1). The detailed specifications of this printer are shown in Table 1 below. The specimen was prepared using Ti-6Al-4V ELI alloy powder from AP&C with a particle size distribution of 15-45 ㎛. The chemical composition is shown in Table 2.

Fig. 1

ORLAS creator RA metal 3D printing system

Table 1

ORLAS creator RA spec

Parameter	Range
Building volume	∅100*110mm
Shielding gas	Argon/Nitrogen
Layer thickness	20-100µm
Mark speed	Jog	20m/s
Mark speed	Operation	6m/s
Laser type	Yb Fiber laser
Laser power	Max. 250W
Spot size	Min. 40µm

Table 2

Ti-6Al-4V properties reference

Ti-6Al-4V ELI(Grade 23, 15-45µm)
Ti	Al	V	Fe	C	N	H	Etc.
Bal.	5.5-6.75	3.5-4.5	0.05-0.25	0.02	0.02	0.01	<0.4
Tensile strength(Annealed)					860 MPa

2.2 Experimental design

In this experiment, flexural specimens with a size of 3mm*10mm*80mm were fabricated, and the stacking direction of the output is shown in Fig. 2. The experimental design was established in three levels through the Taguchi Design of Experiments as shown in Table 3. As for experimental variables, Laser Power (W), Mark Speed (mm/s), Line Thickness (㎛) and Base Angle (degree) were selected, which are major variables that determine the volumetric laser energy density (VED)¹¹⁾, which has an impact on the microstructure and mechanical properties of metal 3D printer. Regarding the base angle, the mechanical properties have little effect, but there is a difference in deformation depending on the angle¹²⁾. Therefore, variables were set based on the base angle of 67 degrees suggested by Dimter et al.¹³⁾. The meaning of each variable is shown in Fig. 3.

Fig. 2

Specimen size and printing direction

Table 3

Taguchi design of experiments

Case No.	Laser power (W)	Mark speed (mm/s)	Line thickness (µm)	Base angle (degree)
1	52	500	60	45
2	52	800	100	67.5
3	52	1100	140	90
4	82	500	140	67.5
5	82	800	60	90
6	82	1100	100	45
7	120	500	100	90
8	120	800	140	45
9	120	1100	60	67.5
Fixed variable		Layer thickness: 25µm, Shielding gas: Ar 99.99% Oxygen level: Min 0.2% Raster: ZigZag Spot size: 40µm

Fig. 3

Metal 3D printing process variable

3. Experiment Results

3.1 OM Analysis

3.1.1 OM measurement and porosity analysis

For OM analysis, the OM was measured by cutting the specimen in the transverse direction orthogonal to the longitudinal direction, which is the same direction as the stacking direction, at the center of the fabricated specimen. Then the porosity was verified through image analysis of the metal and pore regions. For the cross-section pore analysis method, the metal image (metal color) and the pore image (black) in the total image pixels (3,145,728 pixels) taken were classified and the pixel value occupied by the pores in the total image pixels was confirmed as shown in Fig. 4. Table 4 shows the results of measuring the number of metal and pore pixels in the OM images in the stacking and cross-sectional directions, respectively, under the same experimental conditions. The porosity of each specimen calculated by the number of pixels is shown in Table 5 and as a graph in Fig. 5. Regarding the porosity trend by experiment condition, it could be seen that many pores occurred when the VED value was low as expressed by the equation for VED, the energy density per volume (VED(J/mm3)=PVht, where P is the laser power (W), V is the mark speed (mm/s), h is the line thickness, and t is the layer thickness).

Fig. 4

A method of measuring porosity in pixels in an image

Table 4

Metal and porosity area result data

Case No.	Porosity area(Pixel)		Metal area(Pixel)
Case No.	Stacking direction	Cross section	Stacking direction	Cross section
1	334,557	1,515,301	2,811,171	1,630,427
2	Not Stacked
3	Not Stacked
4	197,597	287,700	2,948,131	2,858,028
5	8,732	33,398	3,136,996	3,112,330
6	709,382	759,669	2,436,346	2,386,059
7	1,028	4,324	3,144,700	3,141,404
8	33,179	88,766	3,112,549	3,056,962
9	1,057	25,172	3,144,671	3,120,556

Table 5

VED and porosity values for each process condition

Case No.	Laser power (W)	VED (J/mm³)	Porosity (%)
1	52	69	29.4
2	52	26	Not Stacked
3	52	14	Not Stacked
4	82	47	7.71
5	82	68	0.67
6	82	30	23.35
7	120	96	0.09
8	120	43	1.94
9	120	73	0.42

Fig. 5

Graph of porosity values for each process condition

Furthermore, the specimen could not be fabricated in Cases 2 and 3 where the VED value was 26 (J/mm³) or less in this experiment because the powder was continuously eliminated during the coating process due to insufficient bonding strength between the layers. As a result, the specimens could be fabricated from Case 6 where the VED value was above 30 (J/mm³). The pore was measured as the lowest in Case 7, where the VED value was the highest. In Case 1, where the Mark Speed and Line Thickness values are low, the VED value is high but the laser output is low as 52W. Consequently, the applied metal powder could not be melted and the porosity was high because sufficient energy density per unit area could not be secured. This indicates that the minimum laser power is required. In an experiment excluding Cases 2 and 3 where the VED value was 26 or less, the porosity decreased according to the VED value in the laser output range of 82-120W.

3.1.2 Analysis of mechanical properties

The flexural strength results according to the porosity for each experiment according to the porosity were examined as shown in Fig. 6 below. The influence of experimental variables on porosity was analyzed using the Taguchi method for the result of flexural strength. Among the control variables shown in Table 3, the variables were analyzed to have an effect on porosity in the descending order of laser power, base angle, mark speed, and line thickness. In other words, the higher the laser power and base angle, the lower the porosity became. However, it was analyzed that an appropriate level of control is required for mark speed and line thickness because they produced the lowest porosity at the intermediate level among the variable levels. Moreover, a regression equation for predicting the porosity trend under the metal 3D printing process conditions was derived from the results of this experiment, as shown in Equation (1) below, where LP denotes laser power, MS denotes mark speed, BA denotes base angle, and LT denotes line thickness.

(1)

PoPorosity=63.5−0.257LP−0.0035MS−0.346BA−0.057LT

Fig. 6

Flexural strength and porosity results

3.2 SEM analysis

After measuring the flexural strength, the fracture surface was measured with a magnification of 40-3,000 times using a field emission scanning electron microscope (FE-SEM), and the results are shown in Fig. 7. The Ti-6Al-4V alloy produced by a metal 3D printer of the SLM type underwent rapid cooling at a temperature above the β transformation point (1,050°C) and generated martensite. Therefore, due to the high strength and hardness, a wall interface was observed in the event of fracture in every specimen, which is considered to be a brittle fracture.

In Case 7, where the strength was the highest, it was difficult to observe the unmelted powder. However, unmelted powder was observed on the fracture surfaces of Cases 1, 4, 5, 6, 8, and 9, which had relatively low strengths.

Fig. 7

SEM Measurement result of the fracture surface of the bending specimen

The occurrence of unmelted powder implies the generation of pores inside the laminate. The strength value shows a decreasing trend as the distribution of powder observed in the SEM image increases.

4. Conclusions

In this study, the metal bone scaffold process was controlled to achieve a bone-like strength through a metal 3D printer. The experimental plan for evaluation of process variables was established by Taguchi Design of Experiments, and OM analysis was performed on the fabricated specimen cross section to verify the porosity under each condition. A flexural strength test was conducted to confirm the correlation between porosity and strength. Furthermore, the fracture shape was verified by SEM measurement of the fracture surface, and composition analysis was performed according to the state of the fracture surface. The following conclusions were drawn from this study.

1) In the OM measurement of metal 3D printing specimens, the porosity of the cross-section was measured in pixel unit by cutting the specimen in the stacking and longitudinal directions. The possibility of internal pore control was confirmed by controlling the major process variables of metal 3D printing.
2) The flexural strength was measured and its correlation with porosity was analyzed. As a result, an inverse proportion was found. The regression equation derived from the measurement results confirmed the possibility of fabricating metal bone scaffolds with similar strength to bone.
3) The distribution of the unmelted powder on the fracture surface according to the strength was observed through SEM. The result confirmed a decreasing trend of the flexural strength according to the distribution of unmelted powder. Martensite was generated on the fracture surface due to rapid cooling, and all fractures occurred at the wall interface.

This study performed porosity control experiment and cross-sectional analysis according to process variables to fabricate a scaffold with bone-like strength using a metal 3D printer. The result suggest the possibility of controlling porosity through process variable control. In order to apply the scaffold to medical devices, it is necessary to precisely control internal pores through expanded research on metal 3D printing process variables and to develop pore prediction technology through research on monitoring devices.