Ti6Al4V Powder Research Articles

Sintering study of Ti6Al4V powders with different particle sizes and their mechanical properties

Ti6Al4V powders with three different particle size distributions (0–20, 20–45, and 45–75 μm) were used to evaluate the effect of the particle size distribution on the solid-state sintering and their mechanical properties. The sintering kinetics was determined by dilatometry at temperatures from 900 to 1260°C. The mechanical properties of the sintered samples were evaluated by microhardness and compression tests. The sintering kinetics indicated that the predominant mechanism depends on the relative density irrespective of the particle size used. The mechanical properties of the sintered samples are adversely affected by increasing pore volume fraction. The elastic Young’s modulus and yield stress follow a power law function of the relative density. The fracture behavior after compression is linked to the neck size developed during sintering, exhibiting two different mechanisms of failure: interparticle neck breaking and intergranular cracking in samples with relative densities below and above of 90%, respectively. The main conclusion is that relative density is responsible for the kinetics, mechanical properties, and failure behavior of Ti6Al4V powders.

Surface carburizing during hot isostatic pressing

ABSTRACTThe product preparation and surface carburizing of Ti–6Al–4V alloy are usually carried out separately, which leads to a low production efficiency and high processing costs. In this study, hot isostatic pressing (HIP) was proposed to complete the product preparation and surface carburizing of Ti–6Al–4V alloy simultaneously. Ti–6Al–4V powder compact with carburized layer is successfully fabricated using this method. X-ray diffraction and Energy dispersive spectrometer analyses illustrate the carburized layer is mainly composed of C, TiC and Ti. Scanning electron microscope shows a uniform TiC layer and an obvious diffusion region in the carburized layer and the total thickness is about 104 μm. The Ti–6Al–4V region is composed of equiaxed grains and typical lathlike structures. The Vickers hardness of the carburized layer especially the TiC layer is higher than the Ti–6Al–4V region. These results indicate that HIP is a promising process for product preparation and surface carburizing of Ti–6Al–4V alloy.

Critical influences of particle size and adhesion on the powder layer uniformity in metal additive manufacturing

The quality of powder layers, specifically their packing density and surface uniformity, is a critical factor influencing the quality of components produced by powder bed metal additive manufacturing (AM) processes, including selective laser melting, electron beam melting and binder jetting. The present work employs a computational model to study the critical influence of powder cohesiveness on the powder recoating process in AM. The model is based on the discrete element method (DEM) with particle-to-particle and particle-to-wall interactions involving frictional contact, rolling resistance and cohesive forces. Quantitative metrics, namely the spatial mean values and standard deviations of the packing fraction and surface profile field, are defined in order to evaluate powder layer quality. Based on these metrics, the size-dependent behavior of exemplary plasma-atomized Ti–6Al–4V powders during the recoating process is studied. It is found that decreased particle size (increased cohesiveness) leads to considerably decreased powder layer quality in terms of low, strongly varying packing fractions and highly non-uniform surface profiles. For relatively fine-grained powders (median particle diameter 17 μm), it is shown that cohesive forces dominate gravity forces by two orders of magnitude leading to low quality powder layers not suitable for subsequent laser melting without additional layer/surface finishing steps. Besides particle-to-particle adhesion, this contribution quantifies the influence of mechanical powder material properties, nominal layer thickness, blade velocity, as well as particle-to-wall adhesion. Finally, the implications of the resulting powder layer characteristics on the subsequent melting process are discussed and practical recommendations are given for the choice of powder recoating process parameters. While the present study focuses on a rigid blade recoating mechanism, the proposed simulation framework can be applied to systems using alternative recoating tools such as soft blades, rakes or rotating rollers.

Effect of HIP post-treatment on the HIPed Ti6Al4V powder compacts

ABSTRACTHot isostatic pressing (HIP) is an ideal fabrication technique for products with complex structure, but also a reliable post-treatment method. The individual effect of the HIP or HIP post-treatment (HIPPT) on the mechanical properties of powder compacts have been investigated, but little attention has been paid to their combined effect. In this study, the effect of HIPPT on the densification, microstructure and mechanical properties of HIPed powder compacts were investigated. The relative density shows a regional increase after HIPPT. The interlayer spacing of lamellar α phase is increased, equiaxial grains are coarsened and high-angle grains are decreased. Moreover, an obvious increase in elongation is detected, which demonstrates that HIPPT is beneficial to improve the properties of HIPed powder compacts.

Numerical and experimental analysis of the effect of volumetric energy absorption in powder layer on thermal-fluidic transport in selective laser melting of Ti6Al4V

A volumetric heat source is used in numerical modeling of selective laser melting (SLM) of Ti6Al4V powder. Single track and multi-track SLM simulations are performed by varying the two key process parameters-laser power and scan speed. The model is validated with the published experimental results for melt pool shape, size and temperature. The predictions are in good agreement with the experiments at low to medium energy density. The validated model is used for investigating the thermo-fluidic transport during SLM of Ti6Al4V and examining the dependence of the melt pool characteristics on the process parameters. As-solidified porosity is calculated numerically for the multi-track simulations and its formation is delineated with the transport phenomena. The predicted porosity compares reasonably well with the experimental values. Solidification parameters, such as temperature gradients and cooling rate are calculated at the instantaneous location of the solidification front and analyzed. This analysis suggests the formation of fully columnar grains of different sizes along the width and depth of the melt pool. Overall, the model provides a good description of thermo-fluidic transport in SLM of Ti6Al4V powder and the resulting temperature field, melt pool characteristics, as-solidified porosity and the expected grain structure. Based on the current analysis, an optimum processing window of 50–70 J mm−3 energy density is suggested for SLM of Ti6Al4V powder.

Self-Sufficient Modeling of Single Track Deposition of Ti–6Al–4V With the Prediction of Capture Efficiency

Understanding the capture efficiency of powder during direct laser deposition (DLD) is critical when determining the overall manufacturing costs of additive manufacturing (AM) for comparison to traditional manufacturing methods. By developing a tool to predict the capture efficiency of a particular deposition process, parameter optimization can be achieved without the need to perform a costly and extensive experimental study. The focus of this work is to model the deposition process and acquire the final track geometry and temperature field of a single track deposition of Ti–6Al–4V powder on a Ti–6Al–4V substrate for a four-nozzle powder delivery system during direct laser deposition with a LENS™ system without the need for capture efficiency assumptions by using physical powder flow and laser irradiation profiles to predict capture efficiency. The model was able to predict the track height and width within 2 μm and 31 μm, respectively, or 3.3% error from experimentation. A maximum of 36 μm profile error was observed in the molten pool, and corresponds to errors of 11% and 4% in molten pool depth and width, respectively. Based on experimentation, the capture efficiency of a single track deposition of Ti–6Al–4V was found to be 12.0%, while that from simulation was calculated to be 11.7%, a 2.5% deviation.

Investigation of the wear and corrosion behaviors of Ti5Al2.5Fe and Ti6Al4V alloys produced by mechanical alloying method in simulated body fluid environment

In this study, wear and electrochemical corrosion behaviors of the Ti5Al2.5Fe and Ti6Al4V alloys produced with the mechanical alloying method were examined in simulated body fluid environment. Ti5Al2.5Fe and Ti6Al4V powders were produced by grinding elemental powders in the mechanical alloying device for 120 min. The sintered alloys were characterized SEM, XRD, hardness and density measurements. Wear tests were performed in simulated body fluid environment using a pin-on-disk type wear testing device, under three different loads at four different sliding distances with 1 ms−1 sliding speed. Corrosion tests were performed using the potentiodynamic polarization technique and a cyclic polarization measurement. The density of the MA'ed Ti5Al2.5Fe and Ti6Al4V alloys respectively was measured as 4.314 g/cm3 and 4.427 g/cm3 and its hardness was found as 706.6 HV and 630 HV. Also, it was found dominant α-Ti phase in the alloys' structure. The wear resistance of Ti5Al2.5Fe alloy is higher than that of Ti6Al4V alloy. According to the corrosion test results, the Icorr value of the Ti5Al2.5Fe alloy was measured to be 18.24 mA/cm2, and the Icorr value of the Ti6Al4V alloy was measured to be 43.58 mA/cm2. The corrosion resistance of Ti5Al2.5Fe alloy is higher than that of Ti6Al4V alloy. After corrosion tests, formation of pits of different sizes on the surface of Ti5Al2.5Fe and Ti6Al4V alloys was found to dominate the pitting corrosion mechanism.

A Microscale Study of Thermal Field and Stresses during Processing of Ti6Al4V Powder Layer by Selective Laser Melting

Due to localized heating and cooling in the Selective Laser Melting based additive manufacturing process, a steep temperature gradient forms in the heat affected zone which induces unwanted thermal stresses in the component. In this work, a transient coupled thermo-mechanical model is developed to study the evolution of stresses at microscale on a spot during laser processing of Ti6Al4V powder layer in the heating and the subsequent cooling stage. The thermal model accounts for melting, solidification, fluid flow in the melt pool, variation of the material properties with different phases and volume reduction in the porous powder layer upon melting. The thermal model is validated by comparing the predicted melt pool size for different laser powers with the benchmark experimental data. The thermal model is fully coupled with the mechanical model, accounting plasticity, used to calculate the resulting stress field. The temporal evolution of temperature, various stress components and von Mises stress is described. During heating, compressive stresses are observed in the region below the melt pool. A zone of tensile stresses is developed in the surrounding region to balance these compressive stresses. During the cooling stage, tensile stresses are found in the solidified melt pool region and balancing compressive stresses are found in the region underneath. Determination of critical locations of excessive residual stress suggests that in the solidified melt pool and surrounding region in the solid substrate the residual stresses exceed the yield strength of the material. This implies yielding in that region which can cause material failure. It is quantified that the magnitude of residual stresses increases with laser power and laser interaction time and decreases with pre-heating of solid substrate.

Structural integrity and dispersion characteristics of carbon nanotubes in titanium-based alloy

Over the years, carbon nanotubes have attracted much attention in engineering materials research due to their outstanding and superlative properties. Owing to the demands for lightweight materials with excellent mechanical and thermal properties for diverse industrial applications encouraged the incorporation of carbon nanotubes into titanium alloys. However, there are various challenges associated with the incorporation of carbon nanotubes into titanium alloys which includes; uniform dispersion and retaining the structural integrity of carbon nanotubes after dispersion. Past works have emphasized the importance of homogeneous dispersion with less structural damage to the carbon nanotubes in the various metal matrix. Therefore, this research focused on dispersion of 0.5 wt. % multiwalled carbon nanotubes in Ti6Al4V using low energy ball milling and evaluating the dispersion characteristics and structural integrity of MWCNTs in Ti6Al4V after dispersion. Various characterization techniques such as high-resolution transmission electron microscopy, scanning electron microscopy, X-Ray diffraction and Raman spectroscopy were adopted to ascertain the microstructural evolution, morphology, interfacial reaction and structural integrity of the carbon nanotubes in the Ti6Al4V powders before and after dispersion. The results indicated homogeneous dispersion of carbon nanotubes with less structural damages which are confirmed from the (ID/IG) ratio of the Raman spectra and the TEM images.

Identification of a suitable volumetric heat source for modelling of selective laser melting of Ti6Al4V powder using numerical and experimental validation approach

Using a suitable heat source is crucial to correctly simulate the selective laser melting (SLM) process as the heat source represents the absorbed heat energy, which governs the thermal and fluid flow phenomena associated with SLM. The discontinuous mass distribution in the powder bed causes absorption of incident laser energy over a substantial depth in the powder bed. Therefore, the conventional way of including laser beam energy as a surface heat flux in the modelling of SLM process is likely to provide inaccurate results. This draws attention towards the use of a volumetric heat source in SLM modelling. A number of models for the volumetric heat sources are present that make a selection very difficult. Therefore, several heat sources are investigated in this study using numerical modelling. A three-dimensional model of SLM of Ti6Al4V powder has been developed. The model comprises of a mathematical description of the SLM process along with models for thermal, physical and optical properties of the powder bed. The temperature field, melt pool dimensions, temperature gradient and cooling rate are discussed in detail. The simulation results are compared with the published experimental results to identify a suitable heat source for SLM modelling. Among different heat sources considered, the Gaussian exponential volumetric heat source is found to provide the best matching results with the experiments. Hence, it is identified as the suitable heat source for use in modelling of SLM.

Dynamic research on Ti6Al4V powder HIP densification process based on intermittent experiments

The standard hot isostatic pressing (HIP) process at 930 °C/120 MPa for three hours was interrupted into four stages to dynamically study the HIP process on Ti6Al4V powders. The surface chemistry composition of Ti6Al4V powders and the thickness of surface contaminants were analyzed by X-ray photoelectron spectrometry (XPS) technology, and the results show that the oxide layer on the surface of the Ti6Al4V powders is less than 9 nm. Powders compaction simulation was adopted to simplify the analysis of the powder HIP process, and the results show that a large amount of plastic deformation accumulated around the particle boundaries. The microstructure and mechanical properties of specimen of the Ti6Al4V alloy at different HIP stages were investigated. It was found that with the movement and deformation of particles, the final microstructure of the sample shows a typically grid-like morphology composed of the equiaxed α phase and the lamellar β phase, and the α phase is transmitted from the α′ phase. Moreover, the tensile strength of samples fabricated in each interrupted experiment shows that with the prolongation of the HIP process, the overall mechanical properties of the HIP parts are gradually improved, and their final performance reaches the level achieved by forging.

Microstructure and wear resistance of compositionally graded Ti[sbnd]Al intermetallic coating on Ti6Al4V alloy fabricated by laser powder deposition

In this study, a compositionally graded TiAl intermetallic coating has been successfully fabricated by laser powder deposition (LPD) with Ti6Al4V and AlSi10Mg powder. The microstructure, composition distribution and phase structure of the graded TiAl intermetallic coating were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), electron probe microanalysis (EPMA) and micro-area X-ray diffraction analysis (XRD). In order to evaluate the wear resistance of the graded TiAl intermetallic coating, the sliding wear test was performed. Besides, the morphologies of the worn surface were analyzed by SEM and confocal microscope. The results show that the coating had a crack-free and well-graded compositional microstructure. Intermetallic Ti3Al, TiAl, TiAl3 and Ti5Si3 hard phase were the dominant constructive phases of the graded TiAl intermetallic coating. These phases enhanced the micro-hardness of the graded coating and consequently improved its wear resistance. The average friction coefficient and the wear rate of the graded TiAl intermetallic coating were 0.3704 and 1.989 × 10−4 mm3/Nm, respectively, which were significantly lower than those of the substrate (0.5667 and 8.384 × 10−4 mm3/Nm). The wear mechanism of the coating was mainly abrasive wear. Based on our finding, the graded TiAl intermetallic coating is a promising wear resistant coating for Ti6Al4V alloy.

Effect of oxygen content in new and reused powder on microstructural and mechanical properties of Ti6Al4V parts produced by directed energy deposition

Some AM processes such as directed energy deposition (DED) have typical powder usage efficiencies ranging between 40 and 80%. Since, for a given alloy, powder cost is proportional to its purity, choosing a less expensive powder or reusing powders is interesting for economical and environmental reasons. The work summarized below studied the effect of oxygen content in Ti6Al4V powders on mechanical properties of AM parts fabricated by DED. Three different powders with increasing oxygen content were used to produce specimens and characterize its effect on microstructure and tensile properties before and after heat treatment. Our results show that no clear oxygen pick up was measured from either powder recycling or during printing of parts. Only coarsening of the particle size distribution and the presence of fragmented particles was observed for the recycled powder. Comparing the chemistry of parts vs that of powder feedstock it was determined that for all the tests, the Al content was slightly lower in the parts and that no significant loss of vanadium was noted when printing with new (fresh) powders. On the other hand, V loss was significant in parts made with recycled powders, although still leaving them within acceptable chemistry to respect their original grade 5 classification.

Powder-scale multi-physics modeling of multi-layer multi-track selective laser melting with sharp interface capturing method

As a promising powder-based additive manufacturing technology, selective laser melting (SLM) has gained great popularity in recent years. However, experimental observation of the melting and solidification process is very challenging. This hinders the study of the physical mechanisms behind a variety of phenomena in SLM such as splashing and balling effects, and further poses challenges to the quality control of the products. Powder-scale computational models can reproduce the multi-physics process of SLM. In this study, we couple the Finite Volume Method (FVM) and Discrete Element Method to model the deposition of powder particles, and use the FVM to model the melting process, both with ambient air. In particular, a cutting-edge sharp surface capturing technique (iso-Advector) is incorporated into the Volume of Fluid Model to reconstruct the interface between different phases during the melting process. Iso-Advector is then used to capture and reconstruct the interface between molten material and ambient air, which is further used as a solid boundary for spreading the next powder layer. As such, 3D geometrical data is exchanged between these two stages repeatedly to reproduce the powder spreading-melting process of SLM incorporating different scan paths on multiple powder layers. To demonstrate the effectiveness of the powder-scale multi-physics modeling framework, typical scenarios with different fabrication parameters (Ti–6Al–4V powder) are simulated and compared with experimental observations available in literature.

Additive manufacturing of porous metals using laser melting of Ti6Al4V powder with a foaming agent

Recent years have seen researchers paying attention to the fabrication of porous structures by using additive manufacturing techniques suitable for the production of small batches. This paper focuses on the fabrication process and the compressive characteristics of a porous metal manufactured using the direct energy deposition (DED) process, which is a 3D printing technology for metals. Materials with structures containing internal pores were manufactured by spraying a mixture consisting of Ti6Al4V powder and a foaming agent (Na2CO3) onto a Ti6Al4V substrate, and subsequently fused with the substrate using a high-power laser beam. The ensuing molten pool undergoes rapid solidification, resulting in the formation of a metal layer. Before the molten metal solidifies, the carbon dioxide gas generated from Na2CO3 created pores inside the deposited layer. In this study, pored structures fabricated under various conditions were analyzed and compared from the viewpoint of compressive behavior. The results showed that the densities of the layered porous structures could be controlled by varying process parameters such as the laser power, scanning speed, powder feed rate, and mix ratio of the foaming agent. The most dominant parameters to obtain a pored structure were the amount of foaming agent and powder feed rate. The compression tests revealed that the porous materials manufactured using DED collapsed due to fracturing of the internal porous structures. After these structures fractured, the materials experienced densification, followed by crumbling. In addition, decreasing the laser power and scanning speed caused the compressive strength to decrease due to the high number of internal pores. Furthermore, before finally fracturing, the compressive strain decreased for specimens with increased porosity. The compression tests proved that the porous structure was able to effectively absorb compression loads.

Experimental validation of a numerical thermal model of the EBM process for Ti6Al4V

Electron Beam Melting (EBM) is an additive manufacturing (AM) process that uses an electron beam to melt metallic powders. Although the use of an electron beam in the AM field is relatively recent, several applications are already available in the aerospace and medical fields. To increase the applicability of the EBM process and to make it more reliable, different modelling techniques will be helpful tools. Modelling may have the potential to reduce the optimization time when compared with experimental trial and error approaches. In fact, by means of numerical modelling, process stability could be reached by exploring virtually what-if scenarios. However, experimental validation is necessary to ensure the accuracy of the modelling. The aim of this paper is to provide a validation of the effectiveness and reliability of a Finite Element (FE) thermal model by comparing numerical results and experimental measurements.Two different scanning strategies were studied: MultiBeam and continuous line melting. In MultiBeam melting, separated short melt lines (MultiBeam lines) are activated at different points along the predefined melt track. The electron beam jumps sequentially between the short lines until a complete melt track is finished. Continuous line melting means that the complete predefined melt track is melted from the start point to the stop point in one single sequence. The following parameters were investigated: beam speed, beam size and length of MultiBeam lines. A novel experimental setup was used in which single line tracks were melted in an Arcam Q10 system. Standard Arcam Ti6Al4V powder was used and the layer thickness was set to 0.05 mm. Microscope images were used to acquire the width of the melt pool at different positions along the melt tracks. The experiments were replicated by numerical simulations and a good agreement between simulations and experimental data was found.

Microstructure and mechanical properties of Y2O3 reinforced Ti6Al4V composites fabricated by spark plasma sintering

The paper presents the fabrication of (0.6–3.0 wt%) Y2O3-Ti6Al4V composites by spark plasma sintering (SPS) with the sintering heating rate of 100 °C/min and the sintering temperature of 900 °C. Ti6Al4V powders and Y2O3 powders were admixed by rocking mill for 8 h at 30 Hz. Scanning electron microscopy (SEM) equipped with complete energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were used to characterize the as-received Ti6Al4V powders, Y2O3 powders, the admixed composite powders and the sintered samples. The microhardness, the compressive yield strength and the ultimate strength of as-sintered samples at room temperature were enhanced up to 464.1HV, 1346 MPa and 1583 MPa respectively with a plastic strain of 19.1%, when 2.0 wt% Y2O3 was introduced. The mechanical behaviors of 2.0 wt% Y2O3-Ti6Al4V composite at 450 °C were also carried out with the yield strength of 832 MPa and the ultimate strength of 1088 MPa. Compared with the sintered Ti6Al4V, the 2.0 wt% Y2O3-Ti6Al4V composite has higher yield strength and ultimate strength at elevated temperature with the increment of 54% and 37%, respectively. The mode of fracture was transformed from ductile fracture to a combination of ductile and brittle fractures with the increase of Y2O3 content.

Effects of Cr and Fe Addition on Microstructure and Tensile Properties of Ti–6Al–4V Prepared by Direct Energy Deposition

The effects of Cr and Fe addition on the mechanical properties of Ti–6Al–4V alloys prepared by direct energy deposition were investigated. As the Cr and Fe concentrations were increased from 0 to 2 mass%, the tensile strength increased because of the fine-grained equiaxed prior β phase and martensite. An excellent combination of strength and ductility was obtained in these alloys. When the Cr and Fe concentrations were increased to 4 mass%, extremely fine-grained martensitic structures with poor ductility were obtained. In addition, Fe-added Ti–6Al–4V resulted in a partially melted Ti–6Al–4V powder because of the large difference between the melting temperatures of the Fe eutectic phase (Ti–33Fe) and the Ti–6Al–4V powder, which induced the formation of a thick liquid layer surrounding Ti–6Al–4V. The ductility of Fe-added Ti–6Al–4V was thus poorer than that of Cr-added Ti–6Al–4V.

Influence of Powder Morphology and Microstructure on the Cold Spray and Mechanical Properties of Ti6Al4V Coatings

The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.

Manufacturability of Overhanging Holes Using Electron Beam Melting

This study aims to investigate the manufacturability of overhang round holes with and without support. The experiments were conducted by electron beam melting (EBM) using a Ti6Al4V powder. A large number of overhanging holes with and without support were fabricated and evaluated. The geometrical accuracy, mechanical properties, and microstructures were utilized as a measure of the process performance. It was demonstrated that overhanging features can be built successfully without support up to a certain dimension (or threshold value). Beyond that value, a minimal support structure can be employed to achieve the most suitable trade-off between production time, cost, and accuracy.