Fabrication of Ti-6Al-4V Scaffolds by Direct Metal Deposition
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Direct metal deposition (DMD) is a rapid laser-aided deposition method that can be used to manufacture near-net-shape components from their computer aided design (CAD) files. The method can be used to produce fully dense or porous metallic parts. The Ti-6Al-4V alloy is widely used as an implantable material mainly in the application of orthopedic prostheses because of its high strength, low elastic modulus, excellent corrosion resistance, and good biocompatibility. In the present study, Ti-6Al-4V scaffold has been fabricated by DMD technology for patient specific bone tissue engineering. Good geometry control and surface finish have been achieved. The structure and properties of the scaffolds were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tension test. The microstructures of laser-deposited Ti-6Al-4V scaffolds are fine Widmanstätten in nature. The tensile and yield strengths of the as-deposited Ti-6Al-4V were 1163 ± 22 and 1105 ± 19 MPa, respectively, which are quite higher than the ASTM limits (896 and 827 MPa) for Ti-6Al-4V implants. However, the ductility of the as-deposited sample was very low (∼4 pct), which is well below the ASTM limit (10 pct). After an additional heat treatment (sample annealed at 950 °C followed by furnace cooling), both strength (UTS ∼ 1045 ± 16, and YS ∼ 959 ± 12 MPa) and ductility (∼10.5 ± 1 pct) become higher than ASTM limits for medical implants.
KeywordsBone Tissue Engineering Furnace Cool Powder Feed Rate Direct Metal Deposition Laser Scanning Speed
Tissue engineering constitutes a promisingly alternative approach to the repair of damaged tissue or organs. Different biomaterials have been used as scaffolds, including bioactive metallic, ceramics, and polymers for bone tissue engineering. Ideally, the materials should exhibit adequate mechanical and biological properties. The primary roles of scaffold are (1) to serve as an adhesion substrate for the cell, facilitating the localization and delivery of cells when they are implanted; (2) to provide temporary mechanical support to the newly grown tissue; and (3) to guide the development of new tissues with the appropriate function. Moreover, the scaffold should be biocompatible. A titanium alloy, specifically Ti-6Al-4V, is widely used as an implant material for biomedical applications due to its relatively low modulus, good biocompatibility, and enhanced corrosion resistance when compared to more conventional stainless steels and cobalt-based alloys. The principal focus of this study is to produce ideal scaffolds of Ti-6Al-4V by the direct metal deposition (DMD) technique for patient specific bone tissue engineering with good mechanical and metallurgical properties.
Rapid prototyping, also known as solid freeform fabrication, is a manufacturing process that quickly produces physical prototypes directly from computer aided design (CAD) solid models using a special class of fabrication technology. Direct laser deposition is a rapid prototyping method that can be used to manufacture near-net-shape components from their CAD files in one step. The DMD[4,5] technology developed at the University of Michigan, direct light fabrication (DLF)[6,7] developed at the Los Alamos National Laboratories, and laser engineering net shaping (LENS)[8,9] developed at the Sandia National Laboratories in the early 1990s are all successful examples of direct laser deposition technology. The basic principles of DMD, DLF, and LENS processes are similar in that they use a focused laser beam as a heat source to melt metallic powder and create a three-dimensional (3-D) object. In addition, the DMD and LENS have a feedback control system that provides a closed-loop control of dimension during the deposition process. In the present investigation, the DMD system developed at the University of Michigan was used to fabricate complex shaped metallic scaffolds for medical implants, which are very difficult to process for conventional methods.
First, the CAD model of the component is sliced into a series of parallel layers with a certain build-height. During the DMD process, powder is fed at a controlled rate into the focal point of a high-power laser where the individual particles are melted. The CAD files control both movements of substrate in the X-Y plane and laser beam in the Z direction to add successive layers, thus directly generating a three-dimensional component without the necessity of any machining. There are many variables in this process that can affect the material properties and the microstructure of the resulting part, such as laser power, powder feed rate, and laser scanning speed.
The aim of the present work is to produce ideal Ti-6Al-4V scaffolds and to study the microstructural development and mechanical response for their applicability as hard tissue biomaterials.
2 Experimental procedure
2.2 Scaffold Fabrication
For mechanical tests, tensile specimens were fabricated according to the ASTM E 8 standard (gage length: 25 mm, width: 6.25 mm, and thickness: 0.7 mm) by the same laser deposition parameters that have used for scaffold fabrication. The specimens were then polished using different (240 to 1200) emery papers to improve the surface smoothness for the tension test. The tensile axis of all specimens was perpendicular to the deposit direction. The tensile specimens were tested in a universal testing machine (Instron 55005) at a crosshead speed of 0.5 mm min−1. Three tests were conducted for each condition to ensure the reproducibility of the results. It should be mentioned that tensile fracture occurred near the middle of the gage section.
In order to improve the ductility of the laser-deposited samples, several tensile specimens were annealed at 950 °C and 1050 °C followed by either air cooling (AC) or furnace cooling (FC). The heat treatment was performed in a horizontal cylindrical furnace under an argon atmosphere for 1 hour.
The microstructures of the laser-deposited scaffold were investigated by optical microscopy (OM) (Nikon eclipse ME600), scanning electron microscopy (SEM, PHILIPS1 XL30 FEG scanning electron microscope), and transmission electron microscopy (TEM, JEOL2 2010 transmission electron microscope operating at 200 kV). The as-deposited samples were investigated using X-ray diffraction (Rigaku rotating anode XRD) with Cu K α radiation. Specimens for OM and SEM were cut out from different parts of the scaffold. A Kroll’s (10 mL HNO3, 5 mL HF, and 85 mL H2O) etchant was used for the OM and SEM. For transmission electron microscopy/selected area electron diffraction (TEM/SAD), 3 mm in diameter samples were mechanically punched out. Thereafter, the samples were thinned by ion milling with low accelerating voltage (3.5 kV) to minimize the beam effects on the microstructure.
The surface roughness of the DMD samples parallel to the build direction was measured over a length of 3 mm using a stylus profilometer (Form Talysurf 50, Taylor-Hobson). The roughness value was calculated by averaging the data from five measurements.
3.1 Surface Morphology
Surface Roughness of the As-Deposited Ti-6Al-4V Scaffold after Sand Blasting and Chemical Etching
Ra (average roughness)
Rq (root-mean-square roughness )
Rt (maximum peak-to-valley height)
3.2 XRD Analysis
3.3.1 Optical microscopy
3.3.2 Scanning electron microscopy
3.3.3 Transmission electron microscopy
3.4 Mechanical Properties
Tensile Properties of Direct Metal Deposited Ti-6Al-4V after Different Heat Treatments
Yield Strength (MPa)
Ultimate Strength (MPa)
1105 ± 19
1163 ± 22
4 ± 1
950 °C/air cool
975 ± 15
1053 ± 18
7.5 ± 1
950 °C/furnace cool
959 ± 12
1045 ± 16
10.5 ± 1
1050 °C/air cool
931 ± 16
1002 ± 19
6.5 ± 1
1050 °C/furnace cool
900 ± 14
951 ± 15
7.5 ± 1
An optical micrograph (Figure 6(a)) shows that prior β grains are columnar in nature, whose axis is approximately parallel to the Z-axis, i.e., the build direction of the deposit. During laser deposition, cooling of a molten pool occurs mostly via the substrate and via the surrounding atmosphere. Heat loss through the substrate leads to more rapid cooling via the substrate than through convection and radiation. This leads to directional growth of the grains counter to the cooling direction and, subsequently, to the formation of columnar grains. The maximum height of the individual deposited layer is around 0.3 mm. However, we found that the length of columnar grains varies from 5 to 15 mm. Therefore, the columnar grains, nucleated at the first deposited layer, were continuously grown during deposition of the following layers. Recently, several researchers have also reported similar microstructural observation of laser-deposited Ti-6Al-4V.[15, 16, 17]
It is very difficult to predict the microstructure of the DMD sample because of its very complex thermal history. Recently, Qian et al. have developed a finite element model for temperature history prediction in direct laser deposition samples. According to their model, during laser deposition, the very top layer cools from the liquid at a rate of ∼7 × 104 K s−1 directly to a temperature significantly below the martensite start temperature. Ahmed et al. have shown that Ti-6Al-4V completely transformed to the martensitic structure at the cooling rate above 410 K s−1. However, in the present study, no clear martensitic microstructure was observed even at the topmost layer of the as-deposited sample. Thus, it should be noted that the cooling rate of the present deposition process was less than 410 K s−1. The SEM investigation revealed that the as-deposited microstructure is mainly various morphological forms of diffusion-controlled α. The observed microstructure is mostly a mixture of coarse and fine α laths. The observed microstructural difference at different locations of the sample can be explained in terms of the complex thermal history of the as-deposited sample. During DMD processing, the temperature of the pre-existing layers can exceed the β transus (∼1000 °C) during the deposition of subsequent layers. As a result, a new heat-affected zone formed locally in the deposit every time the laser passed. This thermal excursion could result in coarsening of primary α laths, as found in Figure 9. Hence, the fine α laths are probably a result of the rapid cooling of the laser-deposited layer during the initial deposition process, whereas coarse α laths are a result of diffusion-controlled growth of the α laths during thermal excursion, with an additional layer added to the pre-existing layer.
The Ti-6Al-4V scaffold for the porcine CRU bone tissue engineering has been successfully fabricated by DMD technology. It should be noted that the additional treatment including annealing and sandblasting was necessary in order to meet essential mechanical and surface criteria recommended for bone tissue engineering. The ductility of the as-deposited tensile sample was below the ASTM limit for Ti-6Al-4V implants. After a suitable heat treatment (sample annealed at 950 °C followed by furnace cooling), both strength (UTS ∼ 1045 ± 16, YS ∼ 959 ± 12 MPa) and ductility (∼10.5 ± 1 pct) become higher than ASTM limits for medical implants.
This work was supported by the Army Tank Command R&D group via Allion Corporation. Todd Richman was the program manager.
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