Investment Molds Research Articles

Development of a Bioactive Surface on a Co-Cr-Mo Alloy during Investment Casting or Heat Treatment

Two different techniques were used to promote a bioactive surface on a cobalt base alloy: i) the cobalt alloy melt was cast into wollastonite-coated cavities of an investment mold, or ii) wollasonite-encapsulated as-cast samples were heat treated at 1220°C for 1 h, this is the typical treatment performed to this alloy for improving its mechanical behavior. In vitro bioactivity was assessed by immersing samples in a simulated body fluid for 21 days. Potentially bioactive layers were obtained in both of the cases. A thicker apatite layer was formed on the samples obtained by investment casting. However, since the heat treatment needs to be performed, the heat treatment method is also a promising technique for promoting the bone-bonding ability of this Co alloy.

A Study of hBN-Y<sub>2</sub>O<sub>3</sub> Composite Investment for Casting Ti-6Al-4V Alloy

An hBN-Y2O3 composite investment moulding material was developed for investment casting Ti-6Al-4V alloy. Hexagonal boron nitride (hBN) powders plus a few Y2O3 powders were used as the investment mould material to prepare the shell mould with colloidal yttria as binder. The results show that the hBN-Y2O3 composite investment mould can be used to cast Ti-6Al-4V alloy, and has lower cost compared with the traditional yttria based investment mould, because of chemically inert of hexagonal boron nitride and its lower density. Moreover the interfacial reaction between the Ti and hBN-Y2O3 composite shell mould was investigated with measurement of microhardness and scanning electron microscopy (SEM) with energy dispersive (EDS).

An investigation on microstructural and mechanical properties of solid mould investment casting of AZ91D magnesium alloy

In this work, AZ91D magnesium alloy was cast in solid plaster mould using vacuum assistance. The influence of process parameters and wall thickness on the microstructure and tensile properties of cast specimens was studied. Within the range of experimental parameters, it was found that casting and mould preheating temperatures have minor influence on mechanical properties. However, gating design proved to affect the microstructure and tensile properties of cast specimens. Compared to bottom filling, top filling led to casting defects that seriously impaired the mechanical properties. Generally, tensile properties increased with a decrease of section thickness, this improvement being explained by a reduction of dendrite arms spacing and grain size. Finally, the mechanical properties of solid mould investment cast specimens were compared with those obtained by foamed plaster casting, sand casting, permanent mould casting and die-casting.

Fracture toughness of ceramic moulds for investment casting with ice patterns

Ice patterns can be used to make ceramic investment moulds for metal castings. Owing to the characteristics of ice, the ceramic mould must be made at subzero temperatures and consequently, requires a different formulation than shells built at room temperature. Success of this process depends greatly on the fracture toughness of mould materials. The present paper describes the experimental results of fracture toughness of mould materials processed from different compositions. The Taguchi method was used to reduce the trial runs. The parameters considered included the ratio of fibre containing fused silica and aluminosilicate powders, the volume of binder and the volume of catalyst. The microstructure and green fracture surface of test bars were also examined to understand the underlying mechanism. While conducting the four point bend test on ceramic mould samples, some samples had exceedingly low strengths appearing as outliers in the Weibull analysis. Examination of these low strength ceramic samples improved understanding of failure of mould materials. Sound moulds have been made for the investment casting process with ice patterns based on the analysis of experimental results. The casting of an M8 bolt is used to demonstrate that metal castings of complex geometry can be fabricated using ice patterns. The measured tolerances are within the required tolerance range.

Pure titanium casting into titanium-modified calcia-based and magnesia-based investment molds

Calcia is the most stable oxide for molten titanium at high temperature. In this study, pure titanium casting into calcia- and magnesia-based investments was investigated. Experimental results show that adding the proper amount of titanium powder into calcia- and magnesia-based investments can increase the thermal expansion value, at 800 °C, of both investments. Pure titanium castings in the calcia-based group have better expression at the interface reactivity and the VH. However, castings in magnesia-based group have better results for the investing and the dimensional accuracy.

Characterization of Al–Mn particles in AZ91D investment castings

Manganese is currently added to Mg–Al alloys in order to improve the corrosion behavior of cast components. A part of this manganese is dissolved in the magnesium matrix and the balance is found as fine Al(Mn,Fe) particles dispersed within castings. For AZ91D specimens prepared using the plaster mould investment casting process, these particles were observed in very large quantity at the surface of castings. These particles were characterized by scanning electron microscopy and electron probe microanalysis. It was found that they consist of Al 8Mn 5 phase and that their morphology and size depend on local solidification conditions. Their presence at the surface of the castings is related to low solidification rates and reduced thermal gradients at the mould/metal interface.

Curing kinetics of ceramic slurries used in investment casting with ice patterns

Ice patterns can be used to make ceramic investment moulds for metal castings. Using ice means that the ceramic slurries must be poured around the pattern and cured at sub freezing temperatures. Success of this process depends significantly on the curing kinetics of the slurries. This paper describes the experimental results of an investigation into the curing kinetics of slurries with differing compositions. The variables considered include volume percent solids, temperature and the volume of the catalyst. Spindle speed was also included as an experimental variable to see if the viscosity exhibited shear rate dependency. The Taguchi method of experimental design was used to reduce the number of trial runs. The curing kinetics of the slurries was studied by measuring the viscosity variation of the slurries with time. The data suggests that there is an initiation and propagation stage and a reactant depletion stage during the slurry gelation process. Three models are evaluated for fitting the time dependence of the viscosity data: a simple exponential in time, a Dougerty–Krieger exponential where there is a defined end point time, and a two stage model developed by superposing a simple exponential for the second stage with an Avrami kinetics model for the first stage. The results indicate that the two stage model can best describe the curing. Significance and effects of the process variables are discussed. For stage one, the characteristic time is strongly dependent upon the amount of catalyst, the spindle speed and the solids loading. For stage two, the characteristic time is strongly dependent upon the amount of catalyst, as well as temperature. A cylindrical ice part is used as an example to investigate the dimensional accuracy. The measured outer diameters of the castings are very close to the nominal outer diameter.

Bioactivation of a cobalt alloy by coating with wollastonite during investment casting

Cobalt alloy samples were bioactivated during investment casting. The cavities of the investment mold were previously coated with wollastonite. Additionally, before coating with wollastonite, some mold cavities were filled out with graphite rods to avoid a chemical reaction between the wollastonite powder and the investment material. Half of the cast samples were heat treated at 1220 degrees C for 1 h. To perform the in vitro bioactivity assessment, the cast and heat-treated samples were immersed in a simulated body fluid solution (SBF) for a period of 21 days. The surface of the samples before and after immersion in SBF was characterized by SEM, EDX, and XRD analyses. During the casting, particles of pseudowollastonite were embedded on the metallic surface. After immersion of the samples in SBF, a ceramic layer was formed on both the alloy obtained by using the investment mold and the alloy obtained by using the graphite-filled cavity. The ceramic layer was thicker on the alloy cast in the investment mold. The layer was identified as hydroxyapatite by XRD analysis, in all the cases. The heat-treated samples after immersion in SBF showed the formation of a thin homogeneous layer consisting of fine grains of apatite.

Economic net-shape forming of TiAl alloys for automotive parts

Alpha-case formations between investment molds and TiAl alloys were investigated for the economic TiAl alloys net-shape forming. In the case of TiAl alloys, there were no α-case formation reactions. There were neither interstitial nor substitutional α-case formations since TiAl alloys have both negligible solubility of oxygen and low activity in molten states. The fluidity of TiAl alloys is increased with mold preheating temperature since they have a peritectic reaction that appears in the form of envelopment, surrounding each particles of the primary constituent. The results of the investment casting of TiAl alloys confirm that the casting route in our study can be an effective approach for the economic net-shape forming of TiAl alloys.

Net-Shape Forming of TiAl Automotive Parts for Fuel Efficiency

Alpha-case formations between investment molds and TiAl alloys were investigated for economic TiAl alloys net-shape forming. The α-case reactions between mold and TiAl alloys were characterized by using optical microscopy and a hardness tester. In the case of TiAl alloys, there were no α-case formation reactions. The α-case was formed not only by interstitial oxygen elements but also by substitutional metallic elements dissolved from mold materials. There were neither interstitial nor substitutional α-case formations since TiAl alloys have both negligible solubility of oxygen and low activity in molten states. The results of the investment casting of TiAl alloys confirmed that the casting route in our study can be an effective approach for the economic net-shape forming of TiAl alloys automotive parts.

Hydroxyapatite coating on a cobalt base alloy by investment casting

A cobalt alloy was cast into preheated molds previously coated with hydroxyapatite powder. Two molds were used, one made of investment material and the other of pure graphite. Selected samples were heat treated. Both heat and non heat treated samples were immersed in simulated body fluid for 21 days at 37 °C. A ceramic layer, identified as hydroxyapatite, was formed on all the samples. A thicker layer was formed on the sample cast into the investment mold without heat treatment. A chemical interaction between the investment mold and hydroxyapatite takes place leading to a higher in vitro bioactivity.

Evaluation of Ti-Cr-Cu Alloys for Dental Applications

This study examined the characteristics of as-cast Ti-Cr(7–19%)-Cu(3–7%) (all percentages in this article are mass%) alloys to evaluate their suitability for dental applications; studies on the alloy structures and mechanical properties, grindability, and corrosion behavior were included in the investigation. The alloys were centrifugally cast and bench-cooled in investment molds. The x-ray diffractometry of the as-cast alloys bench-cooled in the molds indicated the following phases: α+β+ω in the 7% Cr and 7% Cr+3% Cu; β+ω in the 13%Cr; and β in the 13%Cr+3% Cu through the 19%Cr+3% Cu alloys. The strengths of the binary β Ti-Cr and ternary β Ti-Cr-Cu alloys with 13 and 19% Cr were approximately two times higher than those of CP Ti. The alloy ductility was dependent on the chemical composition and thus, the microstructure. The 7% Cr alloys were extremely brittle and hard due to the ω phase, but the ductility was restored in the 13 and 19% Cr alloys. The hardness (HV) of the cast 13 and 19% Cr alloys was approximately 300–350 compared with a value of 200 for CP Ti. The grindability of the cast alloys was examined using a rotating SiC wheel at speeds (circumferential) of 500 and 1250 m/min. At the higher speed, the grindability of the 13 and 19% Cr alloys increased with the Cu content. The grindability of the 13% Cr alloy with 7% Cu was similar to that of CP Ti. Evaluation of the corrosion behavior in an artificial saliva revealed that the alloys are like many other titanium alloys within the normal intraoral oxidation potential. The wear resistance testing of these alloys also showed favorable results.

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.

Mechanical properties of cast Ti-Hf alloys

This study examined the mechanical properties of a series of Ti-Hf alloys. Titanium alloys with 10 to 40 mass % Hf were made with titanium and hafnium sponge in an argon-arc melting furnace. Specimens cast into magnesia-based investment molds were tested for yield strength, tensile strength, percentage elongation, and modulus of elasticity. Vickers microhardness was determined at 25 to 600 microm from the cast surface. X-ray diffractometry was also performed. Commercially pure Ti (CP Ti) and pure Ti prepared from titanium sponge were used as controls. The data (n = 5) were analyzed with a one-way ANOVA and the Student-Newman-Keuls test (alpha = 0.05). The diffraction peaks of all the metals matched those for alpha Ti; no beta phase peaks were found. Alloys with Hf > or = 25% had significantly (p < 0.05) higher yield and tensile strength compared to the CP Ti and pure Ti. There were no significant differences (p > 0.05) in elongation among all the Ti-Hf alloys and CP Ti, whereas the elongation of alloys with Hf > or = 30% was significantly (p < 0.05) lower than that of the pure Ti. The cast Ti-Hf alloys tested can be considered viable alternatives to CP Ti because they were stronger than CP Ti and had similar elongation.

Investment Casting of Near-Net Shape Gamma Titanium Aluminide Automotive Turbocharger Rotor

The objective of this study was to optimize the casting design of gamma titanium aluminde automotive turbocharger rotor by means of the practical experiment and numerical simulation. Gamma titanium aluminide rotors were produced by centrifugal casting methods on a laboratory scale. Based on the metal-mold reaction of gamma titanium aluminide, the investment molds were manufactured by an electro-fused Al2O3 mold. The experimental results showed that the castings failed to reach the end of the cavities due to insufficient centrifugal force and a lower fluidity compared to the other metals. Although the satisfactory results were not obtained in the numerical simulation, it was concluded that numerical simulation aided to achieve understanding of the casting process and defect formation in gamma titanium aluminide turbocharger rotor castings.

Effect of Heat History on the Corrosion of Ferritic Stainless Steels Used for Dental Magnetic Attachments

This study investigated the effect of heat history on the corrosion of keepers used for dental magnetic attachments. Ferritic stainless steels of SUS 444 and 447J1 were prepared with heat treatments in the temperature range of 550-850 degrees C for 1-5 hours. The stainless steels were electrochemically and metallurgically examined by anodic polarization curves in a 0.9% NaCl solution and by microstructural observation using an electron probe microanalyzer with WDS. Heating both kinds of stainless steel at 650-750 degrees C for two hours or more led to the deterioration of their corrosion resistance. For example, there was evidence of a reduction in the breakdown potentials and an increase in the current densities of the anodic polarization curves. These phenomena were attributed mainly to the precipitation of the sigma (FeCr) or chi (Fe18Cr6Mo5) phase, which sometimes resulted in intergranular corrosion. When dental alloys are cast in investment molds embedding the keepers, the heat time required for thermal expansion of the molds should be kept under one hour.

加熱炉内での石こう系埋没材鋳型の熱的挙動 （II）X線回折

加熱炉内での石こう系埋没材鋳型の熱的挙動 （II）X線回折

Pure titanium casting into zirconia-modified magnesia-based investment molds

Objective. Molten titanium is highly reactive with common mold materials at elevated temperatures. The aim of this investigation was to improve the accuracy of pure titanium casting by adding unreactive zirconia into magnesia-based investment material. Methods. An automatic thermal expansion laser-recording machine (TEM-1000/Pantos, Nippon Co.) was used to measure thermal expansion of investment materials. An automatic argon-casting machine (Castmatic-S, Iwatani Co.) was used to cast pure titanium samples. A stereomicroscope was used (Nikon SM-2T, Japan) to measure marginal discrepancy on a metal die. A Vickers microhardness indenter (MXT-50, Matsuzawa Seiki Co.) determined the Vickers hardness (VH) of the titanium samples. Interfacial reactivity of the titanium was evaluated with an X-ray diffractometer (Rigaku D/max VIII, Tokyo, Japan). A dental X-ray machine was used to examine internal porosity of the castings. Data was analysed with paired t-test ( p<0.05), ANOVA/Tukey's HSD test ( p<0.001) and Spearman's correlation test ( p<0.05). Results. Addition of 5.0–6.0 mass% of zirconia to a magnesia-based investment material significantly increased its thermal expansion value ( p<0.05). ANOVA indicated significant marginal discrepancy differences within the zirconia-added group ( p<0.001). There was a significant correlation between thermal expansion at 750 °C and marginal discrepancy ( p<0.05). The addition of zirconia decreased interfacial reactivity and the VH of titanium. Significance. Under appropriately adjusted conditions, the addition of zirconia to magnesia-based mold materials may be used to produce high quality pure titanium castings.

加熱炉内での石こう系埋没材の熱的挙動 （I）重量変化およびDTA

加熱炉内での石こう系埋没材の熱的挙動 （I）重量変化およびDTA

Effect of burnout temperature on strength of gypsum-bonded investments

Objective. To investigate the variation of the strength of gypsum-bonded dental investments with burnout temperature. Methods. The disc-rupture test was employed at burnout temperatures ranging from 450 to 800 °C for four products (Beauty Cast, Cristobalite, Novocast (all WhipMix) and Deguvest California (Degussa)). In this test, an investment disc is created inside an investment mold such that it forms a diaphragm across the mold space, being an integral part of the mold. Copper was cast into molds of this type, and whether the disc had ruptured or not was determined by inspection when cold. The amount of copper cast in successive tests was varied in staircase fashion up and down depending on whether the disc survived or failed. The strength of the investment was represented by the 50% point of the transition from survival to rupture. Results. The strength of gypsum-bonded investment is temperature sensitive, there being marked differences between the products tested. Overall, the strength ranking (temperature range average/MPa) was: Cristobalite (10.1)>Beauty Cast (7.8)>Novocast (5.1)>Deguvest California (3.4). The strength ranking at high temperatures differs from that known for the room temperature values. Significance. The use by manufacturers of room-temperature strength data conveys no information about high temperature behavior. Investment properties should be optimized by reference to behavior under casting conditions.