Cast Titanium Alloys Research Articles

Modeling of Alpha-Case Formation and Its Effects on the Mechanical Properties of Titanium Alloy Castings

Titanium (Ti) castings generally contain a hard surface layer of alpha-case (α-case) that is enriched of oxygen. For dental Ti prostheses, the hard and brittle α-case can be a significant fraction of the thickness of the entire work piece and must be controlled properly to ensure minimum properties. The objective of this article is to present a theoretical framework for predicting α-case formation in dental Ti castings as a function of cooling rate and the influence of α-case on the tensile ductility and wear resistance. First, a diffusion analysis is presented to describe the oxygen profile and microhardness distribution within the α-case in Ti castings of CP-Ti and Ti-6Al-4V. The reduction in tensile ductility by the α-case is then predicted using an existing tensile ductility model. Furthermore, the improvement in wear resistance by the α-case is predicted using a recent wear model. These results are then used to assess the feasibility of controlling the α-case thickness to optimize both tensile ductility and wear resistance in Ti castings.

Titanium and Titanium Alloy Castings for Biomedical Application

The aim of this study is to optimize the economic net-shape forming of titanium and titanium alloys for the biomedical application. The alpha-case formation reaction between titanium, and Al2O3, ZrO2, CaO stabilized ZrO2 and ZrSiO4 mold were examined in a plasma arc melting furnace. Regardless of the thermodynamic approach, α-case formation reactions still remain to be eliminated with the complex chemical milling processes. The reason why the α-case generated cannot be explained by the conventional α-case formation mechanism. However, from the experimental results and thermodynamic consideration, it can be confirmed that the α-case is formed not only by interstitial oxygen atoms but also by substitutional metal atoms dissolved from mold materials. Based on the interstitial and substitutional α-case formation mechanism, α-case controlled net-shape forming of titanium and titanium alloys can be possible for the biomedical application.

Friction Stir Processing of Investment-Cast Ti-6Al-4V: Microstructure and Properties

Investment-cast titanium components are becoming increasingly common in the aerospace industry due to the ability to produce large, complex, one-piece components that were previously fabricated by mechanically fastening multiple pieces together. The fabricated components are labor-intensive and the fastener holes are stress concentrators and prime sites for fatigue crack initiation. The castings are typically hot-isostatically-pressed (HIP) to close internal porosity, but have a coarse, fully lamellar structure that has low resistance to fatigue crack initiation. The as-cast + HIP material exhibited 1- to 1.5-mm prior β grains containing a fully lamellar α + β microstructure consistent with slow cooling from above the β transus. Friction stir processing (FSP) was used to locally modify the microstructure on the surface of an investment-cast Ti-6Al-4V plate. Friction stir processing converted the as-cast microstructure to fine (1- to 2-μm) equiaxed α grains. Using micropillars created with a dual-beam focused ion beam device, it was found that the fine-grained equiaxed structure has about a 12 pct higher compressive yield stress. In wrought products, higher strength conditions are more resistant to fatigue crack initiation, while the coarse lamellar microstructure in the base material has better fatigue crack growth resistance. In combination, these two microstructures can increase the fatigue life of titanium alloy castings by increasing the number of cycles prior to crack initiation while retaining the same low-crack growth rates of the colony microstructure in the remainder of the component. In the current study, high-cycle fatigue testing of investment-cast Ti-6Al-4V was performed on four-point bend specimens. Early results show that FSP can increase fatigue strength dramatically.

Self-propagating high-temperature synthesis of tool steel

The process of fabrication of a high-hardness alloy with the help of self-propagating high-temperature synthesis due to combustion of thermit from powdered cinder, aluminum, and titanium carbide is studied. The effect of the mass fraction of the titanium carbide powder and of additives of powdered titanium diboride, molybdenum, and alloy cast iron and the effect of the fineness of the blend and the heat treatment mode on the combustion process, the chemical composition, the structure, and the hardness of the alloy obtained are estimated.

Nd:YAG laser penetration into cast titanium and gold alloy with different surface preparations

This study investigated the effect of surface preparation on the Nd:YAG laser penetration into cast titanium and gold alloy. Cast blocks of each metal were given four different surface preparations: (i) coloured with black marker; (ii) air-abraded with 50 microm Al2O3; (iii) ground with SiC points and (iv) polished with 1 microm Al2O3 (mirror-polished). Two blocks with each of the surface preparations were abutted and laser-welded at their interface using the voltages of 210-260 V in increments of 10 V. After the welded blocks were mechanically separated, the laser penetration was measured using computer graphics. Regardless of the surface preparation, an increase in voltage increased the laser penetration for both metals. The laser penetration into titanium prepared with black marker and air-abrasion was significantly deeper than into the titanium ground with SiC points and mirror-polished. Although there were no statistical differences in penetration among the surface preparations for the gold alloy, the penetration in the mirror-polished specimens was shallower than any of the other preparation methods at higher voltages of 240-260 V. The results obtained in this study suggested that broken metal frameworks with finished surfaces should be painted with black marker or air-abraded before laser welding.

Laser Welding of Cast Titanium and Dental Alloys Using Argon Shielding

This study investigated the effect of argon gas shielding on the strengths of laser-welded cast Ti and Ti-6Al-7Nb and compared the results to those of two dental casting alloys. Cast plates of Ti, Ti-6Al-7Nb, gold, and Co-Cr alloy were prepared. After polishing the surfaces to be welded, two plates were abutted and welded using Nd:YAG laser at a pulse duration of 10 ms, spot diameter of 1 mm, and voltage of 200 V. Five specimens were prepared for each metal by bilaterally welding them with three or five spots either with or without argon shielding. The failure load and percent elongation were measured at a crosshead speed of 1.0 mm/min. The factor of argon shielding significantly affected the failure load and elongation of the laser-welded specimens. The failure loads of argon-shielded laser-welded CP Ti and Ti-6Al-7Nb were greater compared with the failure loads of specimens welded without argon shielding for both three- and five-spot welding. Regardless of argon shielding, the failure loads of the laser-welded gold alloy were approximately half that of the control specimens. In contrast, the failure loads of the nonshielded laser-welded Co-Cr alloy were greater. The percent elongations positively correlated with the failure loads. The use of argon shielding is necessary for effective laser-welding of CP Ti and Ti-6Al-7Nb but not for gold and Co-Cr alloy.

Grain refinement of cast titanium alloys via trace boron addition

Abstract The grain size of as-cast Ti–6Al–4V is reduced by about an order of magnitude from 1700 to 200 μm with an addition of 0.1 wt.% boron. A much weaker dependence of reduction in grain size is obtained for boron additions from >0.1% to 1.0%. Similar trends were observed in boron-modified as-cast Ti–6Al–2Sn–4Zr–2Mo–0.1Si.

Effect of surface reaction layer on grindability of cast titanium alloys

The purpose of this study was to investigate the effect of the cast surface reaction layer on the grindability of titanium alloys, including free-machining titanium alloy (DT2F), and to compare the results with the grindability of two dental casting alloys (gold and Co-Cr). All titanium specimens (pure Ti, Ti-6Al-4V and DT2F) were cast using a centrifugal casting machine in magnesia-based investment molds. Two specimen sizes were used to cast the titanium metals so that the larger castings would be the same size as the smaller gold and Co-Cr alloy specimens after removal of the surface reaction layer (alpha-case). Grindability was measured as volume loss ground from a specimen for 1 min using a handpiece engine with a SiC abrasive wheel at 0.1 kgf and four circumferential wheel speeds. For the titanium and gold alloys, grindability increased as the rotational speed increased. There was no statistical difference (p>0.05) in grindability for all titanium specimens either with or without the alpha-case. Of the titanium metals tested, Ti-6 Al-4V had the greatest grindability at higher speeds, followed by DT2F and CP Ti. The grindability of the gold alloy was similar to that of Ti-6 Al-4V, whereas the Co-Cr alloy had the lowest grindability. The results of this study indicated that the alpha-case did not significantly affect the grindability of the titanium alloys. The free-machining titanium alloy had improved grindability compared to CP Ti.

Microstructure formation in Ti–Si composite subjected to high temperature gradients

Abstract Titanium eutectic composite Ti–Si–Al–Zr alloy castings were subjected to high thermal loads and temperature gradients to simulate extreme conditions of a gas turbine component. The microstructure of the affected zones was studied and characterized. The results can be applied to the development of a new generation of heavy-duty components with properties exceeding known cast titanium alloys and intermetallics.

Effects of post material on stress distribution in dentine

To investigate stress distribution of different material restored post-cores in dentine and provide a theoretical guidance for clinical use. A three-dimensional finite element model of maxillary central incisor restored with post-core and PFM crown was constructed by SCT scan technology. Based on this model, stress distribution in dentine was analyzed before and after post-core restorations with 6 different materials, including cast Ni-Cr alloy, cast titanium alloy, cast gold alloy, glass fiber reinforced composite, polythene fiber reinforced composite, and common composite resin. When the tooth was restored with cast Ni-Cr alloy post and PFM crown, the maximum tensile stress and Von Mises stress in dentin at post apex increased 152% and 162% respectively, compared with a tooth restored only with PFM crown. If polythene fiber reinforced composite was used as post material, the stress distribution did not significantly change. When the other materials were used for the post, the stress distribution changed greatly. The elastic modulus of post-core materials affected the stress distribution pattern in dentine. The materials with elastic modulus similar to that of dentin, such as polythene fiber reinforced composite, may be suitable for post restoration.

Model-based melt rate control during vacuum arc remelting of alloy 718

Vacuum arc remelting (VAR) is used widely throughout the specialty metals industry to produce superalloy and titanium alloy cast ingots. Optimum VAR casting requires that the electrode melting rate be controlled at all times during the process. This is especially difficult when process conditions are such that the temperature distribution in the electrode has not achieved, or has been driven away from, steady state. This condition is encountered during the beginning and closing stages of the VAR process, and also during some process disturbances such as when the melt zone passes through a transverse crack. To address these transient melting situations, a new method of VAR melt rate control has been developed that incorporates an accurate, low-order melting model to continually estimate the temperature distribution in the electrode. This method of model-based control was tested at Carpenter Technology Corporation. In the first test, two 0.43-m-diameter alloy 718 electrodes were melted into 0.51-m-diameter ingots. Aggressive start-up and hot-top procedures were used to test the dynamic capabilities of the control technique. Additionally, a transverse cut was placed in each electrode with an abrasive saw to mimic an electrode crack. Accurate melt rate control was demonstrated throughout each melt. The second test used an electrode size and grade proprietary to the host company. Because it was not stress relieved after the primary casting process, the electrode was known to possess multiple cracks that make accurate melt rate control impossible using standard VAR controller technology. This electrode was also successfully melted with good melt rate control using the model-based controller.

Fatigue Characteristics of Dental Cast Titanium Alloy, Ti-6Al-7Nb, Conducted with Thermochemical Heat Processing

Fatigue Characteristics of Dental Cast Titanium Alloy, Ti-6Al-7Nb, Conducted with Thermochemical Heat Processing

Localization of plastic strain under shock-wave loading of titanium alloy with a crack

The effect of shock-wave loading with an amplitude of 4 GPa and a duration of 10 µs on the localization of plastic flow and the dynamic fracture of VT-1 cast titanium alloy containing a disk-shaped crack is considered. It is found that the crack size decreases in the direction of shock wave propagation, the dislocation density grows and adiabatic shear bands appear near the crack, and secondary cracks originate in the areas of localized flow near the crack edges. The strain, strain rate, dislocation density, rate of dislocation generation, and dislocation velocity near and away from the healing crack are estimated.

Wear resistance of experimental Ti–Cu alloys

After using cast titanium prostheses in clinical dental practice, severe wear of titanium teeth has been observed. This in vitro study evaluated the wear behavior of teeth made with several cast titanium alloys containing copper (CP Ti+3.0 wt% Cu; CP Ti+5.0 wt% Cu; Ti–6Al–4 V +1.0 wt% Cu; Ti–6Al–4 V+4.0 wt% Cu) and compared the results with those for commercially pure (CP) titanium, Ti–6Al–4 V, and gold alloy. Wear testing was performed by repeatedly grinding upper and lower teeth under flowing water in an experimental testing apparatus. Wear resistance was assessed as volume loss (mm 3) at 5 kgf (grinding force) after 50,000 strokes. Greater wear was found for the six types of titanium than for the gold alloy. The wear resistance of the experimental CP Ti+Cu and Ti–6Al–4 V+Cu alloys was better than that of CP titanium and Ti–6Al–4 V, respectively. Although the gold alloy had the best wear property, the 4% Cu in Ti–6Al–4 V alloy exhibited the best results among the titanium metals. Alloying with copper, which introduced the α Ti/Ti 2Cu eutectoid, seemed to improve the wear resistance.

In vitro wear assessment of titanium alloy teeth

Purpose Wear of commercially pure (CP) titanium prosthetic teeth has frequently been observed. The greatest wear has been found when the same grades of CP titanium are used for both maxillary and mandibular teeth. This study examined the wear behavior of teeth made with cast titanium alloy and compared these results with those for CP titanium and gold alloy teeth.Materials and Methods All tooth specimens were cast with grade 3 α titanium, 3 metastable β alloys [Ti‐15Mo‐2.8Nb‐0.2Si (Timetal 21 SRx), Ti‐13Nb‐13Zr, and Ti‐15V‐3Cr‐3Sn‐3Al], and 2 α+β alloys (Ti‐6Al‐7Nb and Ti‐6Al‐4V). As a control, Type IV gold alloy was also cast conventionally. All teeth (both maxillary and mandibular) were secured in an in vitro 2‐body wear testing apparatus that simulated chewing function (60 strokes/min; grinding distance, 2 mm under flowing water). Wear resistance was assessed as volume loss (mm3) at 5 kgf (grinding force) after 50,000 strokes. The results (n= 5) were analyzed using analysis of variance or Fisher's exact test (α= 0.05).Results Of the titanium teeth, the wear of 2 of the metastable β alloys (Timetal 21 SRx and Ti‐15V‐3Cr‐3Sn‐3Al) was found to be significantly (p <0.05) higher than that of CP titanium and the 2 α+β alloys. The Type IV gold alloy exhibited better wear resistance than all of the titanium teeth tested. No correlation was found between wear loss and hardness among all the metals tested.Conclusions Among the titanium teeth examined, the α+β alloys exhibited significantly less wear than the other types of titanium. The dental casting gold alloy tested exhibited the best wear resistance among all of the metals tested.

Electrochemical characterization of cast titanium alloys

A reaction layer forms on cast titanium alloy surfaces due to the reaction of the molten metal with the investment. This surface layer may affect the corrosion of the alloy in the oral environment. The objective of this study was to characterize the in vitro corrosion behavior of cast titanium alloys. ASTM Grade 2 CP titanium, Ti–6Al–4V, Ti–6Al–7Nb and Ti–13Nb–13Zr alloys were cast into a MgO-based investment. Experiments were performed on castings ( N=4) with three surface conditions: (A) as-cast surface after sandblasting, (B) polished surface after removal of the reaction layer, and (C) sandblasted surface after removal of the reaction layer. Open-circuit potential (OCP) measurement, linear polarization, and potentiodynamic cathodic polarization were performed in aerated (air+10% CO 2) modified Tani–Zucchi synthetic saliva at 37°C. Potentiodynamic anodic polarization was subsequently conducted in the same medium deaerated with N 2+10% CO 2 gas 2 h before and during the experiment. Polarization resistance ( R P) and corrosion rate ( I CORR) were calculated. Numerical results were subjected to nonparametric statistical analysis at α=0.05. The OCP stabilized for all the specimens after 6×10 4 s. Apparent differences in anodic polarization were observed among the different surfaces for all the metals. A passivation region followed by breakdown and repassivation were seen on specimens with surfaces A and C. An extensive passive region was observed on all the metals with surface B. The Kruskal–Wallis test showed no significant differences in OCP, R p, I CORR or break down potential for each of the three surfaces among all the metals. The Mann–Whitney test showed significantly lower R P and higher I CORR values for surface C compared to the other surfaces. Results indicate that the surface condition has more effect on corrosion of these alloys than the surface reaction layer. Within the oxidation potential range of the oral cavity, all the metal/surface combinations examined showed excellent corrosion resistance.

Influence of boron addition on microstructure and mechanical properties of dental cast titanium alloys

As-cast titanium alloys prepared using a dental cast machine with a series of boron additions have been studied using an optical microscope, XRD, SEM, and hardness and tensile testing. It has been shown that a small amount of boron addition induces a significant refinement of as-cast structure and improvement of mechanical properties. Titanium borides, TiB particles, are observed in Ti matrix. Tensile ductility of cast Ti–B and Ti–0.5Si–B alloys is improved obviously when boron content is about 0.086–0.14 mass%. This is primarily due to the role of borides precipitated at the prior β boundary and refinement of the prior β grains. Cast Ti–B alloys with a good combination of greater tensile ductility and strength can be obtained with very low boron addition.

Effect of Mold Material and Binder on Metal-Mold Interfacial Reaction for Investment Castings of Titanium Alloys

The aim of the present paper is to investigate the combined effect of mold material and binder for investment casting of titanium and titanium alloys. A plasma arc melting furnace was used for melting titanium alloy, and the interfacial reaction of titanium castings was determined by optical microscope, SEM, EDS analysis and hardness profile. The mold materials examined were ZrO2 ,A l 2O3, CaZrO3 and CaO. Machined graphite mold was examined for comparison. The result shows that the titanium castings produced using ZrO2 and Al2O3 mold had a clear reaction whereas a negligible reaction occurred in the castings in CaO, CaZrO3 and graphite molds. CaZrO3 is regarded as a promising mold material for titanium castings from the viewpoints of thermal stability against molten titanium alloy, sufficient mold handling strength and slurry viscosity control.

Effect of Thermohydrogen Treatment on the Structure and Properties of Titanium Alloy Castings

Effect of Thermohydrogen Treatment on the Structure and Properties of Titanium Alloy Castings

An introduction to the extraction, melting and casting technologies of titanium alloys

The processes of Ti extraction, melting and casting are outlined. In addition, the major process technologies for the precision casting of titanium alloys are studied through a series of basic experiments on such fundamental phenomena as the absorption of oxygen into the sub-surface of the castpart from the molding material, mold filling and defect formation, and the formation of microstructures. Quantitative model for the phenomena being developed based on experimental results may support the production of titaniums alloy castparts free from the surface hardened layer and casting defects, and with uniform microstructure throughout the castpart.