Experimental Investigation on Machinability of α/ß Titanium Alloys with Different Microstructures.

In the current study, Ti-6Al-4V (Ti64) and Ti-6Al-7Nb (Ti67) alloys were prepared by vacuum arc melting. The produced samples were then subjected to different heat treatment regimes. The evolved microstructures and their corresponding hardness were investigated. Computerized drilling tests using TiAlN-coated high-speed steel bits were performed to assess the machinability of the prepared specimen regarding cutting force, tool wear, and thickness of the deformed layer. It was observed that Ti64 specimens that were water quenched from either α/ß or ß range contained martensitic phase. In Ti67, samples showed martensite only when water quenched from the ß-phase range (1070 °C). Formation of martensite resulted in higher hardness and hence led to higher cutting forces and increased tool wear during the drilling process. Machined samples with higher hardness formed a thicker subsurface deformation area (white layer) and increased burr heights. Surface roughness in Ti64 workpieces was generally higher compared to Ti67 specimens. The coat of the drilling bit was partially attacked in the as-cast specimens, which was evident by elemental N on the machining chips. The machining tool deteriorated further by increasing the workpiece hardness through martensitic formation, where elements such as Cr, V, Fe, etc. that came from the tool steel were detected.

Influence of heat treatment processes on microstructure evolution, tensile and tribological properties of Ti6Al4V alloy.

The influence of heat treatment processes on microstructure, tensile and tribological properties of Ti6Al4V alloy was investigated. The specimens were heated for 30 min at 925 °C and then cooled at various rates by water quenching, air cooling, and furnace cooling. After that, the samples were aged for four hours at 600 °C. Three phases make up the microstructure: primary α-phase (αp), secondary α-phase (αs), and retained ß-phase (ßr). Cooling in the air and water followed by aging (AC + Aging and WQ + Aging) resulted, αs-phase precipitating inside ßr-phase. The highest hardness of 35 HRC was recorded for WQ + Aging specimen due to existence of a high amount of ßr-phase and precipitation of αs-phase. On the other hand, the lowest hardness of 26 HRC was obtained for the FC specimen. AC specimen achieved the highest elongation value of 14%. However, WQ + Aging specimen exhibited the highest ultimate tensile strength of 1028 MPa. For WQ + Aging and AC + Aging specimens, the ideal balance of strength and elongation was discovered. The wear resistance of solution-treated specimens was significantly improved by the aging process and 125% improvement could be achieved in WQ compared to WQ + Aging specimens.

Metallurgical investigation of early failure of heater pipes in a gas complex.

An early corrosion failure of the piping system of a gas heater was reported by a gas complex company. Local corrosion rates of 0.90 and 0.66 mm/year were observed in the heater piping system. An investigation including visual examination, macrostructure, microstructure (SEM, EDS and XRD), thickness gauging, chemical analysis, and mechanical testing, was performed. The results showed that corrosion damage occurred on the external surface of the pipes. Corrosion occurred at the cold sides of the pipes and elbows. The corrosion pattern is broad and shallow pitting. Some elbows showed an early stage of carbide spheroidization and pearlite decomposition. The EDS microanalysis revealed that the level of sulfur, chlorine, and nitrogen was substantially high in the rust samples. The XRD of the corrosion products showed that the main oxyhydroxide was Akaganeite. The analysis of results showed that the flue gas dew point corrosion was the mechanism of damage, and the root cause was the operation of the heater at low temperatures and the frequent outings of service, combined with evident material drawbacks including low levels of Si, Cu, Ni, Cr, and Mo. These elements should be at their maximum allowable limits of the SA 105 and SA 106 to improve the corrosion resistance of the steel piping components.

Influence of Microstructure and Alloy Composition on the Machinability of α/ß Titanium Alloys.

A comparative study was conducted for the machining of two α/ß titanium alloys, namely Ti-6Al-4V (Ti64) and Ti-6Al-7Nb (Ti67), using wire electric discharge machining (WEDM). The influence of cutting speed and cutting mode on the machined surfaces in terms of surface roughness (Ra), recast layer (RL), and micro-hardness have been evaluated. Rough cut (RC) mode at a cutting speed of 50 µm/s resulted in thermal damage; Ra was equal to 5.68 ± 0.44 and 4.52 ± 0.35 µm for Ti64 and Ti67, respectively. Trim-cut mode using seven cuts (TRC-VII) at the same speed decreased the Ra to 1.02 ± 0.20 µm for Ti64 and 0.92 ± 0.10 µm for Ti67. At 100 µm/s, Ra reduced from 2.34 ± 0.28 µm to 0.88 ± 0.12 µm (Ti64), and from 1.42 ± 0.15 µm to 0.90 ± 0.08µm (Ti67) upon changing from TRC-III to TRC-VII. Furthermore, a thick recast layer of 30 ± 0.93 µm for Ti64 and 14 ± 0.68 µm for Ti67 was produced using the rough mode, while TRC-III and TRC-VII modes produced layers of 12 ± 1.31 µm and 5 ± 0.72 µm for Ti64 and Ti67, respectively. Moreover, rough cut and trim cut modes of WEDM played a significant role in promoting the surface hardness of Ti64 and Ti67. By employing the Response Surface Methodology, it was found that the machining mode followed by cutting speed and the interaction between them are the most influential parameters on surface roughness. Finally, mathematical models correlating machining parameters to surface roughness were successfully developed. The results strongly promote the trim-cut mode of WEDM as a promising machining route for two-phase titanium alloys.

Effect of Bi and Ca on the Solidification Parameters of Sr-Modified Al-S-Cu (Mg) Alloys.

The effect of tramp elements, mainly Bi and Ca, on the thermal characteristics of Sr-modified Al-Si-Cu and Al-Si-Cu-Mg alloys has been investigated using thermal analysis, X-ray radiography, and field emission scanning electron microscopy (FESEM) techniques. The high affinity of Bi to interact with Sr results in an increase in the Al-Si eutectic temperature, and hence an increase in the size of eutectic silicon particles. In contrast, the Ca-Sr interaction seems to have no significant effect on the alloy thermal behavior. The effect of these interactions on porosity formation has been discussed. Hot zones may be formed in thin cavities, in particular, near the bottom of the mold, leading to formation of unexpected coarse porosity, mostly shrinkage type. The study also highlights the significance of other parameters on porosity formation, such as no melt degassing, SrO, Al2O3 (strings or bifilms), as well as the presence of iron-based intermetallics.

Effects of microstructure and alloy composition on hydroxyapatite precipitation on alkaline treated α/ß titanium alloys.

The current work aims at exploring the effects of the microstructure and alloy composition on enhancing the bone osseointegration in Ti-6Al-4V (Ti64) and Ti-6Al-7Nb (Ti67) alloys. This was revealed by investigating the alloy susceptibility to grow hydroxyapatite (HA) on their surfaces after immersion in simulated body fluid (SBF). The specimens were produced by two methods: forging and casting in order to study the influence of the microstructure on the precipitation process. The surface conditions investigated were the polished, alkaline and the hydrothermally treated. It was found that precipitation on both of Ti64 and Ti67 occurs after about 4 weeks and considerably dissolve back with further immersion. Precipitation process is enhanced at some pH levels lower than the neutral level. Forged Ti67 has less reactivity with Hank solution than Ti64 specimens; the reverse is true for cast specimens. In case of the alkaline treated specimens, precipitations on cast specimens were denser than on the forged ones. For the hydrothermally treated specimens, high amounts of Ca and P were observed on cast Ti67 indicating that hydrothermal treatment is considered the best surface modification treatment for alloy Ti67.

The Influence of Microstructure on the Passive Layer Chemistry and Corrosion Resistance for Some Titanium-Based Alloys.

The effect of microstructure and chemistry on the kinetics of passive layer growth and passivity breakdown of some Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and TC21 alloys, was studied. The rate of pitting corrosion was evaluated using cyclic polarization measurements. Chronoamperometry was applied to assess the passive layer growth kinetics and breakdown. Microstructure influence on the uniform corrosion rate of these alloys was also investigated employing dynamic electrochemical impedance spectroscopy (DEIS). Corrosion studies were performed in 0.9% NaCl solution at 37 °C, and the obtained results were compared with ultrapure Ti (99.99%). The different phases of the microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical composition and chemistry of the corroded surfaces were studied using X-ray photoelectron spectroscopy (XPS) analysis. For all studied alloys, the microstructure consisted of α matrix, which was strengthened by ß phase. The highest and the lowest values of the ß phase's volume fraction were recorded for TC21 and Ti-Al-Nb alloys, respectively. The susceptibility of the investigated alloys toward pitting corrosion was enhanced following the sequence: Ti-6Al-7Nb < Ti-6Al-4V << TC21. Ti-6Al-7Nb alloy recorded the lowest pitting corrosion resistance (Rpit) among studied alloys, approaching that of pure Ti. The obvious changes in the microstructure of these alloys, together with XPS findings, were adopted to interpret the pronounced variation in the corrosion behavior of these materials.

In-vitro and in-vivo, cytotoxicity evaluation of cast functionally graded biomaterials for dental implantology.

An innovative technique of casting the titanium implant alloys and its calcium phosphate coating using centrifugal casting method was proposed in the current study. This one-step, self-coating technique results in formation of functionally graded biomaterials (FGBMs) of calcium phosphate/Titanium alloys. Two groups of samples (Group 1 and Group 2) were prepared utilizing self-coated titanium­niobium (Ti-6Al-7Nb) and titanium- vanadium (Ti-6Al-4V) alloys respectively. The cast samples were then prepared for in-vitro cytocompatibility evaluation, in-vivo systemic toxicity and osseointegration evaluation. Cytocompatibility was examined in-vitro on human bone osteosarcoma cells (Saos-2). In-vivo surgical implantation of the samples in the mandible of the experimental dogs was correspondingly implemented. Tests for systemic toxicity, X-ray images, Dual Energy X-ray Absorptiometry (DEXA) and histopathological analysis were obtained for each dog's mandible sample. In-vitro cytocompatibility studies revealed high cytocompatibility of Group 2 in comparison to Group 1. In-vivo results proved safety of Group 2 as regards to systemic toxicity. On the other hand, results of the X-ray images, DEXA and histopathological analysis revealed comparable osseointegration potential of both groups. These results suggest the use of Group2-FGBMs in dental implant manufacturing and prove that FGBMs prepared by centrifugal casting as self-coated implants have good biocompatibility and osseointegration capability.

Surface modification of investment cast-316L implants: microstructure effects.

Artificial femur stem of 316L stainless steel was fabricated by investment casting using vacuum induction melting. Different surface treatments: mechanical polishing, thermal oxidation and immersion in alkaline solution were applied. Thicker hydroxyapatite (HAP) layer was formed in the furnace-oxidized samples as compared to the mechanically polished ones. The alkaline treatment enhanced the precipitation of HAP on the samples. It was also observed that the HAP precipitation responded differently to the different phases of the microstructure. The austenite phase was observed to have more homogeneous and smoother layer of HAP. In addition, the growth of HAP was sometimes favored on the austenite phase rather than on ferrite phase.

Fabrication of Al-Al3Ti/Ti3Al Functionally Graded Materials under a Centrifugal Force.

Fabrication of Al-Al3Ti functionally graded materials (FGMs) under the centrifugal force has recently attracted some attention. The controlled compositional gradient of the fabricated FGMs, the low cost of the process, and the good mold filling, are the main advantages of the centrifugal method (CM). Using the conventional CM techniques such as the centrifugal solid-particle method and centrifugal in-situ method, FGMs rings with gradually distributed properties could be achieved. As a more practical choice, the centrifugal mixed-powder method (CMPM) was recently proposed to obtain FGMs containing nano-particles selectively dispersed in the outer surface of the fabricated parts. However, if a control of the particles morphology, compound formulas or sizes, is desired, another CM technique is favored. As a development of CMPM, our novel reaction centrifugal mixed-powder method (RCMPM) has been presented. Using RCMPM, Al­Al3Ti/Ti3Al FGMs with good surface properties and temperature controlled compositional gradient could be achieved. In this short review, this novel method will be discussed in detail and the effect of RCMPM processing temperature on the reinforcement particles morphology, size and distribution through the fabricated samples, will be reviewed.