Evaluation of different bonded investments for dental titanium casting

The properties of several different investments were investigated including phosphate bonded, magnesia bonded, and alumina cement investments.Measurements included the setting expansion, thermal expansion, and compressive strength of investments, as well as the tensile strength, elongation, Vickers hardness (VHN) and surface roughness of titanium castings. For phosphate bonded investment, the setting expansion after being mixed with its own mixing solution was 2.10%, which was larger than the other investments; the thermal expansion was −0.25% at 200 ^∘C, the compressive strength 14 and 5 MPa after heating. For titanium cast in phosphate bonded investment, the hardness on its top surface was 655 Hv, the tensile strength was 379 MPa, the elongation was 19.4%, and the surface roughness was 2.29 μ m. Athough the thermal expansion of phosphate bonded investment is small, the setting expansion is large enough to compensate for the shrinkage of titanium castings. As its thermal expansion at T ≥ 600^∘C was constant and its heating-cooling cycle was almost reversible, these two properties can reduce the thermal shock and thus avoid cracking of the investment.     

Effect of casting method on castability of titanium and dental alloys

Titanium, once considered to be difficult to cast because of its relatively high melting point and strong chemical affinity, can now be acceptably cast using newly developed casting apparatus. The objectives of this study were to examine the castability of commercially pure (CP) titanium using an ultra high-speed centrifugal casting machine and a pressure difference-type casting unit and to compare the castability of titanium with that of conventional dental casting alloys. To determine castability, two types of patterns were used: a mesh pattern of 22 × 24 mm cut polyether thread sieve, and a saucer pattern (24 mm diameter) perforated to create four T-shaped ends. The casting equipment significantly affected the mold filling of both patterns . The castability indices obtained from both patterns of CP titanium cast in the centrifugal casting machine were significantly better than the indices of the castings produced in the pressure-difference casting unit. The radiographs of the saucer pattern cast in the centrifugal casting machine showed some pores that were fewer and smaller in size than the pores found in castings made in the pressure-difference unit. When the ultra high-speed centrifugal casting machine was used with the manufacturers recommended mold material, the castability of titanium was similar to that of gold alloy or Ni-Cr alloy cast by conventional means.     

Evaluation of the reactivity of titanium with mould materials during casting

A methodology for evaluating the reactivity of titanium with mould materials during casting has been developed. Microhardness profiles and analysis of oxygen contamination have provided an index for evaluation of the reactivity of titanium. Microhardness profile delineates two distinct regions, one of which is characterised by a low value of hardness which is invariant with distance. The reaction products are uniformly distributed in the metal in this region. The second is characterised by a sharp decrease in microhardness with distance from the metal-mould interface. It represents a diffusion zone for solutes that dissolve into titanium from the mould. The qualitative profiles for contaminants determined by scanning electron probe microanalyser and secondary ion mass spectroscopy in the as-cast titanium were found to be similar to that of microhardness, implying that microhardness can be considered as an index of the contamination resulting from metal-mould reaction.     

Dental magnesia-based investments for cast titanium crown: setting and fitness

Titanium crowns were cast using experimental magnesia-based investment mixed with hydrolysed sol. Ten types of hydrolysed sol were used, adding a catalyst to each sol whether the setting (hardening) of the magnesia investment powder appeared within 3 h after mixing. A penetration test using a Vicat needle to measure needle depth as setting was carried out after mixing of the investment powder, showed that the penetration depth decreased gradually with increasing holding time at room temperature. Setting time was shown to be more or less than 3 h or dependent on the gelation. The total magnitude of expansion (setting + thermal) ranged from 0.5 to 0.9% using the selected mixtures. Titanium cast crowns were made in the range 0.02–1.3 mm (occlusal) and 0.03–1.5 mm (cervical), depending on the expansion value of the particular magnesia-based investments mixed with the hydrolysed sols.     

A newly developed calcia/titanium modified magnesia-based investment mold for titanium casting

Calcia (CaO) investment has the high stability with melted pure titanium (Ti) but the poor manipulation usually leads to low success rate of complete castings. Magnesia (MgO)-based investment can produce the high success rate of pure Ti casting though. However, the MgO-based investment should be accompanied with Ti powder to meet the accuracy of Ti castings. In this study, a pure titanium casting using Ti powder modified MgO-based investment and 0.1–0.5 wt.% CaO additive was investigated. Experimental results showed that pure Ti cast with a modified MgO-based dental investment with Ti/CaO resulted in a higher success rate of the complete casting and a decrease in the interfacial surface reaction at high casting temperatures as compared to unmodified MgO-based investments with Ti powder. This newly developed Ti modified MgO-based investment and 0.1–0.5 wt.% CaO additives had an acceptable marginal accuracy with a marginal gap of less than 50 μm; the 0.4 wt.% CaO additive gave the best accuracy. Based on a micro-chemical analysis, the addition of 0.1–0.5 wt.% CaO to the Ti modified MgO-based investment significantly reduced the reaction layers and casting defects in pure Ti casting, which had proven to enhance Ti–porcelain bonding; the investment modified with 0.5 wt.% CaO had the thinnest reaction layer.     

钛合金精密铸造陶瓷型壳性能的研究

获得高质量精密铸造钛合金铸件的关键是制备出高质量的陶瓷型壳 .本文对Y/Y、ZY/ZY和TJ/ZC等三种面层材料的性能进行了研究 ,主要包括TG/DTA测试、型壳强度测试以及热膨胀性能测试 .实验表明 ,TJ/ZC材料是钛合金精密铸造理想的面层材料 .     

Laser bonded microjoints between titanium and polyimide for applications in medical implants

Bioencapsulation of medical implant devices, and neural implant devices in particular, requires development of reliable hermetic joints between packaging materials that are often dissimilar. Titanium-polyimide is one of the biocompatible material systems, which are of interest to our research groups at Wayne State University and Fraunhofer USA. We have found processing conditions for successful joining of titanium with polyimide using near-infrared diode lasers or fiber lasers along transmission bonding lines with widths ranging from 200 to 300 m. Laser powers of 2.2 and 3.8 W were used to create these joints. Laser-joined samples were tested in a microtester under tensile loading to determine joint strengths. In addition, finite element analysis (FEA) was conducted to understand the stress distribution within the bond area under tensile loading. The FEA model provides a full-field stress distribution in and around the joint that cause eventual failure. Results from the investigation provide an initial approach to characterize laser-fabricated microjoints between dissimilar materials that can be potentially used in optimization of bio-encapsulation design.     

铸造钛合金及其熔炼技术的发展现状

钛合金具有密度低、高温性能好、耐腐蚀性能好等优点,逐渐成为一种优异的航空航天结构材料.综述了钛合金的成形特点和发展现状,以及钛合金熔炼技术的发展现状.     

Setting expansion of gypsum-bonded investment in dental casting

The values of setting expansion of investment under uniaxial stress have been determined at conditions designed to obtain the difference of setting expansion between that parallel to the loading direction and that perpendicular to the loading axis. The setting expansion curves were represented by $$a(t) = a_0 (1 - P/E')[1 - exp( - kt)]$$ along the loading direction and $$a(t) = a_0 (1 + \nu 'P/E')[1 - exp( - kt)]$$ perpendicular to the loading direction, wherea(t) is a setting expansion,a ₀ = 0.009,ν′ = 0.2,E′ = 5 kg cm^?2,k = 0.032 min^?1,P applied stress, andt the time (min). On the basis of these results, a method to estimate the value of setting expansion under restrictive force was developed. According to this method, the setting expansion of the investment could be calculated by substituting?ε/?t forε, ka ₀ exp (?kt)/E′ for 1/E, ν′ forν, andka ₀ exp (?kt) forαT in the theory of elasticity.     

Dynamic freeze casting for the production of porous titanium (Ti) scaffolds

This paper proposes dynamic freeze casting as a new manufacturing technique for producing porous Ti scaffolds with a uniform porous structure and good ductility. In this method, Ti/camphene slurries with various initial Ti contents (15, 20, and 25 vol.%) were frozen at 44 °C for 12 h in rotation, which allowed for the extensive growth of camphene crystals and the uniform construction of walls made of Ti particles. All the fabricated samples showed spherical-like pores surrounded by dense Ti walls that were uniformly formed after sintering at 1300 °C for 2 h in a vacuum. The porosity decreased from 71 to 52 vol.% with an increase in Ti content from 15 to 25 vol.%, whereas the pore size decreased from 362 to 95 μm. On the other hand, the compressive strength and stiffness increased considerably from 57 ± 4 to 183 ± 6 MPa and from 1.3 ± 0.5 to 5.0 ± 0.8 GPa, respectively, due to the decrease in the porosity of the samples.     

Statistical failure analysis of adhesive resin cement bonded dental ceramics

The goal of this work is to quantitatively examine the effect of adhesive resin cement on the probability of crack initiation from the internal surface of ceramic dental restorations. The possible crack bridging mechanism and residual stress effect of the resin cement on the ceramic surface are examined. Based on the fracture-mechanics-based failure probability model, we predict the failure probability of glass-ceramic disks bonded to simulated dentin subjected to indentation loads. The theoretical predictions match experimental data suggesting that both resin bridging and shrinkage plays an important role and need to be considered for accurate prognostics to occur.     

The influence of casting technique on elongation and other mechanical properties of low-ductility dental casting alloys

A study was made of the effect of mould orientation on the mechanical properties of eight dental cobalt-chromium alloys prepared by centrifugal casting. In general, tensile test bars cast with their length horizontal rather than vertical to the induced gravitational field yielded superior elongation and ultimate tensile strength values. Proof stress was less affected by casting conditions.     

Antibacterial hydroxyapatite coatings on titanium dental implants

Titanium and its alloys are often used as substrates for dental implants due to their excellent mechanical properties and good biocompatibility. However, their ability to bind to neighboring bone is limited due to the lack of biological activity. At the same time, they show poor antibacterial ability which can easily cause bacterial infection and chronic inflammation, eventually resulting in implant failure. The preparation of composite hydroxyapatite coatings with antibacterial ability can effectively figure out these concerns. In this review, the research status and development trends of antibacterial hydroxyapatite coatings constructed on titanium and its alloys are analyzed and reviewed. This review may provide valuable reference for the preparation and application of high-performance and multi-functional dental implant coatings in the future.     

Multiscale modeling of titanium investment cast dental prostheses

Titanium alloys are attractive materials for biomedical applications due to their superior biocompatibility. However, the use of titanium alloys for dental and maxillofacial prostheses is particularly challenging because each prosthesis has a unique, complex shape. A multiscale model for the processing of Ti prostheses was developed that combines a commercial macrocode for process simulation with an in-house code to explicitly track the development of the microstructure including surface reactions. The model was applied to simulate the dental titanium investment casting process. The macroscopic heat transfer model is coupled with a microscale simulation incorporating the release and diffusion of impurity elements at the mold–metal interface. The penetration depth of impurity species was simulated to determine the amount of alpha-case formed. The microstructural modeling results indicate that the critical factor is the time for which liquid Ti is in contact with the mold and that the incorporation of silicon from the mold increases this time by depressing the solidus temperature.     

Numerical study of porosity in titanium dental castings

A commercial software package, MAGMASOFT (MAGMA Giessereitechnologie GmbH, Aachen, Germany), was used to study shrinkage and gas porosity in titanium dental castings. A geometrical model for two simplified tooth crowns connected by a connector bar was created. Both mold filling and solidification of this casting model were numerically simulated. Shrinkage porosity was quantitatively predicted by means of a built-in feeding criterion. The risk of gas pore formation was investigated using the numerical filling and solidification results. The results of the numerical simulations were compared with experiments, which were carried out on a centrifugal casting machine with an investment block mold. The block mold was made of SiO₂ based slurry with a 1 mm thick Zr₂ face coat to reduce metal–mold reactions. Both melting and casting were carried out under protective argon (40 kPa). The finished castings were sectioned and the shrinkage porosity determined. The experimentally determined shrinkage porosity coincided with the predicted numerical simulation results. No apparent gas porosity was found in these model castings. Several running and gating systems for the above model casting were numerically simulated. An optimized running and gating system design was then experimentally cast, which resulted in porosity-free castings. © 1999 Kluwer Academic Publishers     

Osseointegration evaluation of laser-deposited titanium dioxide nanoparticles on commercially pure titanium dental implants

The nanotechnology field plays an important role in the improvement of dental implant surfaces. However, the different techniques used to coat these implants with nanostructured materials can differently affect cells, biomolecules and even ions at the nano scale level. The aim of this study is to evaluate and compare the structural, biomechanical and histological characterization of nano titania films produced by either modified laser or dip coating techniques on commercially pure titanium implant fixtures. Grade II commercially pure titanium rectangular samples measuring 35?×?12?×?0.25?mm length, width and thickness, respectively were coated with titania films using a modified laser deposition technique as the experimental group, while the control group was dip-coated with titania film. The crystallinity, surface roughness, histological feature, microstructures and removal torque values were investigated and compared between the groups. Compared with dip coating technique, the modified laser technique provided a higher quality thin coating film, with improved surface roughness values. For in vivo examinations, forty coated screw-designed dental implants were inserted into the tibia of 20 white New Zealand rabbits’ bone. Biomechanical and histological evaluations were performed after 2 and 4 weeks of implantation. The histological findings showed a variation in the bone response around coated implants done with different coating techniques and different healing intervals. Modified laser-coated samples revealed a significant improvement in structure, surface roughness values, bone integration and bond strength at the bone-implant interface than dip-coated samples. Thus, this technique can be an alternative for coating titanium dental implants.     

Evaluation of the microstructure and mechanical properties of diffusion bonded joints of titanium to stainless steel with a pure silver interlayer

Solid-state diffusion bonding of commercially pure titanium to 304 stainless steel using an Ag interlayer was carried out at 825–875 °C under a uniaxial pressure of 8 MPa for 20 min in vacuum. The microstructural observations revealed that the resultant joints were composed of the remnant Ag interlayer, TiAg intermetallic phase and Ti–Ag solid solution. An optimized bonding strength of up to 414 MPa was achieved. Fracture took place through the remnant Ag interlayer during tensile tests and the interfacial TiAg phase exhibited no detrimental effect on the bonding strength. Extensive dimples were observed on the fracture surfaces, indicating that the joints were ductile in nature.     

厚度和宽度对连铸板坯轻压下率的影响

结合压下率理论模型,研究了厚度为210 mm、230 mm、250 mm,宽度为1000 mm、1150 mm、1300 mm的连铸板坯轻压下率的变化规律.结果表明:板坯的厚度越大,轻压下率沿着铸机的流线线性减少越缓;板坯的厚度对轻压下率和平均压下率的影响比较大;压下率的入口值和出口值与板坯的厚度呈线性反比;板坯厚度增加10%,平均压下率减少10%;板坯的宽度对轻压下率和平均压下率的影响较小.