Laser Melted Ti 6Al 4V Research Articles

Chip formation characteristics of selective laser melted Ti–6Al–4V

In this research work, chip formation characteristics of selective laser-melted (SLM) Ti–6Al–4V in both ‘as built’ and ‘heat treated’ conditions are studied and compared with conventional wrought Ti–6Al–4V. Machined chips and partially deformed chips were collected from turning trials and quick stop experiments to study the nature of chip formation characteristics. Chip formation studies reveal that, ‘segmented’ or ‘saw tooth’ chips were produced during machining of SLM Ti–6Al–4V materials. The tendency to form segmented chips was higher in SLM Ti–6Al–4V materials as compared to conventionally produced wrought Ti–6Al–4V. In addition, cracks were found to be a common feature in primary and secondary deformation zones of SLM Ti–6Al–4V chip samples, illustrating that periodic crack initiation is the root cause of ‘saw tooth’ formation during machining. Furthermore, the tendency to form build up edge during machining was less in SLM Ti–6Al–4V materials compared to wrought Ti–6Al–4V, influencing the machined surface finish.

Tribocorrosion Study of Ordinary and Laser-Melted Ti6Al4V Alloy

Titanium alloys are used in biomedical implants, as well as in other applications, due to the excellent combination of corrosion resistance and mechanical properties. However, the tribocorrosion resistance of titanium alloy is normally not satisfactory. Therefore, surface modification is a way to improve this specific performance. In the present paper, laser surface-modified samples were tested in corrosion and pin-on-disk tribocorrosion testing in 0.90% NaCl under an average Hertzian pressure of 410 MPa against an alumina sphere. Laser-modified samples of Ti6Al4V were compared with ordinary Ti6Al4V alloy. Electrochemical impedance showed higher modulus for laser-treated samples than for ordinary Ti6Al4V ones. Moreover, atomic force microscopy revealed that laser-treated surfaces presented less wear than ordinary alloy for the initial exposure. For a further exposure to wear, i.e., when the wear depth is beyond the initial laser-affected layer, both materials showed similar corrosion behavior. Microstructure analysis and finite element method simulations revealed that the different behavior between the initial and the extensive rubbing was related to a fine martensite-rich external layer developed on the irradiated surface of the fusion zone.

Specific Yielding of Selective Laser-Melted Ti6Al4V Open-Porous Scaffolds as a Function of Unit Cell Design and Dimensions

Bone loss in the near-vicinity of implants can be a consequence of stress shielding due to stiffness mismatch. This can be avoided by reducing implant stiffness, i.e., by implementing an open-porous structure. Three open-porous designs were therefore investigated (cubic, pyramidal and a twisted design). Scaffolds were fabricated by a selective laser-melting (SLM) process and material properties were determined by conducting uniaxial compression testing. The calculated elastic modulus values for the scaffolds varied between 3.4 and 26.3 GP and the scaffold porosities between 43% and 80%. A proportional linear correlation was found between the elastic modulus and the geometrical parameters, between the elastic modulus and the compressive strengths, as well as between the strut width-to-diameter ratio (a/d) and elastic modulus. Furthermore, we found a power-law relationship between porosity and the modulus of elasticity that characterizes specific yielding. With respect to scaffold porosity, the description of specific yielding behaviour offers a simple way to characterize the mechanical properties of open-porous structures and helps generate scaffolds with properties specific to their intended application. A direct comparison with human bone parameters is also possible. We generated scaffolds with mechanical properties sufficiently close to that of human cortical bone.

Microstructural evolution and microhardness of a selective-laser-melted Ti–6Al–4V alloy after post heat treatments

Microstructure and hardness of a powder-bed-type selective laser melted Ti–6Al–4V alloy after post heat treatments at from 300 °C to 1020 °C were systematically investigated by using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and Vickers hardness (HV) tester. Long columnar original β grains together with the inside dominated parallel acicular martensite in the side view, and chessboard pattern in the top view, were found in the as-received specimen. The subtransus heat treatment does not enable modification of the morphology of the original columnar β grain, only leading to the acicular α′ martensite decomposition into the α platelet and whether surrounded β phase or transformed α′ phase depending on the heating temperature; while the supertransus heat treatment would thoroughly break up the original long columnar β grain, leaving only big original equiaxial β grain filled with the new forming weave-type acicular α′ martensite like the supertransus heat treated wrought specimen. Vickers hardness evolution strictly follows the trend of the microstructural change as the heating temperature increasing, and the double peak phenomenon of the Hardness–Temperature plot should be attributed to substructural refinement effect at around 500 °C, martensitic refinement effect at around 1000 °C, and softening effect resulting from the completely decomposition of the martensite at around 875 °C.

基于粉末特性的选区激光熔化Ti6Al4V表面粗糙度研究

基于粉末特性的选区激光熔化Ti6Al4V表面粗糙度研究

Mechanical properties of regular porous biomaterials made from truncated cube repeating unit cells: Analytical solutions and computational models

Additive manufacturing (AM) has enabled fabrication of open-cell porous biomaterials based on repeating unit cells. The micro-architecture of the porous biomaterials and, thus, their physical properties could then be precisely controlled. Due to their many favorable properties, porous biomaterials manufactured using AM are considered as promising candidates for bone substitution as well as for several other applications in orthopedic surgery. The mechanical properties of such porous structures including static and fatigue properties are shown to be strongly dependent on the type of the repeating unit cell based on which the porous biomaterial is built. In this paper, we study the mechanical properties of porous biomaterials made from a relatively new unit cell, namely truncated cube. We present analytical solutions that relate the dimensions of the repeating unit cell to the elastic modulus, Poisson's ratio, yield stress, and buckling load of those porous structures. We also performed finite element modeling to predict the mechanical properties of the porous structures. The analytical solution and computational results were found to be in agreement with each other. The mechanical properties estimated using both the analytical and computational techniques were somewhat higher than the experimental data reported in one of our recent studies on selective laser melted Ti–6Al–4V porous biomaterials. In addition to porosity, the elastic modulus and Poisson's ratio of the porous structures were found to be strongly dependent on the ratio of the length of the inclined struts to that of the uninclined (i.e. vertical or horizontal) struts, α, in the truncated cube unit cell. The geometry of the truncated cube unit cell approaches the octahedral and cube unit cells when α respectively approaches zero and infinity. Consistent with those geometrical observations, the analytical solutions presented in this study approached those of the octahedral and cube unit cells when α approached respectively 0 and infinity.

Specific heat treatment of selective laser melted Ti–6Al–4V for biomedical applications

The ductility of as-fabricated Ti–6Al–4V falls far short of the requirements for biomedical titanium alloy implants and the heat treatment remains the only applicable option for improvement of their mechanical properties. In the present study, the decomposition of as-fabricated martensite was investigated to provide a general understanding on the kinetics of its phase transformation. The decomposition of asfabricated martensite was found to be slower than that of water-quenched martensite. It indicates that specific heat treatment strategy is needed to be explored for as-fabricated Ti–6Al–4V. Three strategies of heat treatment were proposed based on different phase transformation mechanisms and classified as subtransus treatment, supersolvus treatment and mixed treatment. These specific heat treatments were conducted on selective laser melted samples to investigate the evolutions of microstructure and mechanical properties. The subtransus treatment leaded to a basket-weave structure without changing the morphology of columnar prior β grains. The supersolvus treatment resulted in a lamellar structure and equiaxed β grains. The mixed treatment yielded a microstructure that combines both features of the subtransus treatment and supersolvus treatment. The subtransus treatment is found to be the best choice among these three strategies for as-fabricated Ti–6Al–4V to be used as biomedical implants.

Anisotropy in the impact toughness of selective laser melted Ti–6Al–4V alloy

Apparent anisotropies have been found in the mechanical properties of selective laser melted Ti–6Al–4V alloy. This study investigated the influences of the vertical and horizontal building directions on the impact toughness of selective laser melted Ti–6Al–4V, and a mechanism for the anisotropy in toughness is proposed. Disc-shaped building defects were clearly observed, even though the relative density was as high as 99.5%. The directionality of these defects reduces the load-bearing cross-section of a vertical specimen relative to that of a horizontal specimen; consequently, the impact energy of a horizontal specimen is 96% higher than that of a vertical specimen.

Fatigue performance evaluation of selective laser melted Ti–6Al–4V

Additive Manufacturing of titanium components holds promise to deliver benefits such as reduced cost, weight and carbon emissions during both manufacture and use. To capitalize on these benefits, it must be shown that the mechanical performance of parts produced by Additive Manufacturing can meet design requirements that are typically based on wrought material performance properties. Of particular concern for safety critical structures are the fatigue properties of parts produced by Additive Manufacturing. This research evaluates the fatigue properties of Ti–6Al–4V specimens produced by the Selective Laser Melting additive manufacturing process. It was found that the fatigue life is significantly lower compared to wrought material. This reduction in fatigue performance was attributed to a variety of issues, such as microstructure, porosity, surface finish and residual stress. There was also found to be a high degree of anisotropy in the fatigue performance associated with the specimen build orientation.

Microstructure and tensile properties of selectively laser-melted and of HIPed laser-melted Ti–6Al–4V

Ti–6Al–4V samples have been prepared by selective laser melting (SLM) with varied processing conditions. Some of the samples were stress-relieved or hot isostatically pressed (HIPed). The microstructures of all samples were characterised using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and the tensile properties measured before and after HIPing. It was found that the porosity level generally decreased with increase of laser power and laser scanning speed. Horizontally built samples were found to have a higher level of porosity than vertically built samples. The as-fabricated microstructure was dominated by columnar grains and martensites. HIPing closed the majority of the pores and also fully transformed the martensite into α and β phases. The as-fabricated microstructure exhibits very high tensile strengths but poor ductility with elongation generally smaller than 10%. The horizontally built samples show even lower elongation than vertically built samples. HIPing considerably improved ductility but led to a reduction in strength. With HIPing, the SLMed samples were found to show tensile properties comparable with those thermomechanically processed and annealed samples.

Cracking Behavior and Formation Mechanism of TC4 Alloy Formed by Selective Laser Melting

Crack is the most common and great destructive defect in selective laser melting(SLM) Ti6Al4V(TC4) alloy process. A TC4 alloy test specimen is fabricated by progressive alternative scan strategy in SLM process. The cracking behavior and formation mechanism is discussed in TC4 alloy SLM processing using scanning electron microscopy and X-ray energy spectrum analysis. The experimental results show that the crack is considered as cold cracking in SLM TC4 alloy part, which has typical characteristic of the transgranular fracture. By using the progressive alternative scan strategy, the basket-like martensitic structure is formed in SLM process. The high residual stress is caused by high temperature in forming process of SLM part. Under the residual stress, the crackle will occur in the martensitic structure because it has poor cracking strength. Finally, the coarse crack decomposes into smaller cracks in the process of expansion and the extension is terminated. Further research shows that by improving the process parameters, the microstructure of TC4 alloy will be changed and the residual stress will be weakened. At the same time, achieving to the purpose of weakening or eliminating cracks.

The effect of pore geometry on the in vitro biological behavior of human periosteum-derived cells seeded on selective laser-melted Ti6Al4V bone scaffolds

The specific aim of this study was to gain insight into the influence of scaffold pore size, pore shape and permeability on the in vitro proliferation and differentiation of three-dimensional (3-D) human periosteum-derived cell (hPDC) cultures. Selective laser melting (SLM) was used to produce six distinct designed geometries of Ti6Al4V scaffolds in three different pore shapes (triangular, hexagonal and rectangular) and two different pore sizes (500μm and 1000μm). All scaffolds were characterized by means of two-dimensional optical microscopy, 3-D microfocus X-ray computed tomography (micro-CT) image analysis, mechanical compression testing and computational fluid dynamical analysis. The results showed that SLM was capable of producing Ti6Al4V scaffolds with a broad range of morphological and mechanical properties. The in vitro study showed that scaffolds with a lower permeability gave rise to a significantly higher number of cells attached to the scaffolds after seeding. Qualitative analysis by means of live/dead staining and scanning electron micrography showed a circular cell growth pattern which was independent of the pore size and shape. This resulted in pore occlusion which was found to be the highest on scaffolds with 500μm hexagonal pores. Interestingly, pore size but not pore shape was found to significantly influence the growth of hPDC on the scaffolds, whereas the differentiation of hPDC was dependent on both pore shape and pore size. The results showed that, for SLM-produced Ti6Al4V scaffolds with specific morphological and mechanical properties, a functional graded scaffold will contribute to enhanced cell seeding and at the same time can maintain nutrient transport throughout the whole scaffold during in vitro culturing by avoiding pore occlusion.

Relation between pore structure and fatigue behaviour in sintered iron-copper-carbon : K.D.Christian, R.M.German. (Pennsylvania State University, Pennsylvania, USA), Int. J. Powder Metallurgy, vol 31, no 1, 1995, 51–61

Apparent anisotropies have been found in the mechanical properties of selective laser melted Ti–6Al–4V alloy. This study investigated the influences of the vertical and horizontal building directions on the impact toughness of selective laser melted Ti–6Al–4V, and a mechanism for the anisotropy in toughness is proposed. Disc-shaped building defects were clearly observed, even though the relative density was as high as 99.5%. The directionality of these defects reduces the load-bearing cross-section of a vertical specimen relative to that of a horizontal specimen; consequently, the impact energy of a horizontal specimen is 96% higher than that of a vertical specimen.