The use of casting investments for soldering gold implant bars: a pilot study

The use of gypsum- and phosphate-bonded casting investments for soldering gold implant bars was investigated. Eighteen gold implant bars were sectioned and divided into two groups of nine specimens each. One group of bars was invested with gypsum-bonded investment and the other group invested with phosphate-bonded investment. The accuracy of fit of each group was measured at the gold cylinder-abutment interface in microns using a stereomicroscope. The use of gypsum-bonded investment resulted in a more accurate fit as compared with phosphate-bonded investment.

Joining titanium materials with tungsten inert gas welding, laser welding, and infrared brazing

Titanium has a number of desirable properties for dental applications that include low density, excellent biocompatibility, and corrosion resistance. However, joining titanium is one of the practical problems with the use of titanium prostheses. Dissolved oxygen and hydrogen may cause severe embrittlement in titanium materials. Therefore the conventional dental soldering methods that use oxygen flame or air torch are not indicated for joining titanium materials. This study compared laser, tungsten inert gas, and infrared radiation heating methods for joining both pure titanium and Ti-6Al-4V alloy. Original rods that were not subjected to joining procedures were used as a control method. Mechanical tests and microstructure analysis were used to evaluate joined samples. Mechanical tests included Vickers microhardness and uniaxial tensile testing of the strength of the joints and percentage elongation. Two-way analysis of variance and Duncan's multiple range test were used to compare mean values of tensile strength and elongation for significant differences (p < or = 0.05). Tensile rupture occurred in the joint region of all specimens by cohesive failure. Ti-6Al-4V samples exhibited significantly greater tensile strength than pure titanium samples. Samples prepared by the three joining methods had markedly lower tensile elongation than the control titanium and Ti-6Al-4V rods. The changes in microstructure and microhardness were studied in the heat-affected and unaffected zones. Microhardness values increased in the heat-affected zone for all the specimens tested.

A comparison of infrared- and torch-soldering of Au-Pd and Co-Cr metal-ceramic alloys using a high-fusing solder

PURPOSE

The purpose of this study was to compare the bond strengths and fracture modes of soldered joints formed by infrared and gas-oxygen torch heating of a high-fusing, gold-based solder in two metal-ceramic alloys.

MATERIALS AND METHODS

Pairs of half-dumbbell-shaped specimens of either an Au-Pd or a Co-Cr alloy were positioned with a 0.25-mm gap and were joined using a high-fusing, gold-based solder by either gas-oxygen torch heating or infrared heating. The resulting specimens were subjected to a heat treatment that simulated ceramic firing. Each specimen was fractured in tension at a loading rate of 0.5 cm/min, and its bond strength was measured. The halves of the specimen were rejoined using the other heating method and were heat treated, and the specimen's bond strength was again measured. Fractured cross-sections were examined at a magnification of 40x to determine the following: the fracture mode (adhesive, cohesive, or mixed); the percentage of adhesive fracture; the presence or absence of voids, porosities, or flux inclusions; and the percentage of the cross-section that was discolored.

RESULTS

Three-factor analyses of variance showed that neither the heating method, the particular specimen tested, nor the order of testing significantly affected the bond strength (P < .05). For each alloy, significantly fewer infrared-heated joints than torch-heated joints contained voids, porosities, or flux inclusions visible at 40x (chi 2 test, P < .05). All fractures were either entirely cohesive in the solder or mixed cohesive and adhesive. When infrared heating was used, entirely cohesive fractures occurred more frequently in the Au-Pd alloy specimens than in the Co-Cr alloy specimens (chi 2 test, P < .05). The coefficients of variation of the bond strengths for the infrared-heated joints were smaller than those for the torch-heated joints.

CONCLUSIONS

Although the two heating methods produced solder joints that had strengths that were not significantly different, infrared-heated joints showed less scatter in bond strengths. It was suggested that, in the hands of most technicians, fewer infrared-heated joints would contain defects visible at a magnification of 40x. The presence of such defects may increase the probability of in vivo failure caused by cyclic stresses.

A new technology for direct restorative alloys

OBJECTIVES

The purpose of this study was to develop a new technology for preparing mercury-free metallic dental restorative materials.

METHODS

The novel approach relies on the cold welding of surface treated silver particles. At ambient temperature, intermetallic compound formation takes place spontaneously at the silver-tin interface. The ability of a loose powder to consolidate at ambient temperature under moderate pressure and within a short time duration was investigated for various mixtures of elemental silver, tin, and pre-alloyed silver-coated powders. Surface treatment aimed at removing silver surface oxide layers is done with a dilute acid. The compressive and the transverse rupture strengths of several of the consolidated powder mixtures were determined.

RESULTS

Cold-welding of surface-treated powder particles takes place across the silver-silver interface and is promoted by exposure to a mild acid. Powder mixtures containing approximately 50% silver powder, 40% silver-coated silver-tin intermetallic compound particles, and a small silver-tin fraction deposited from an aqueous solution display very good condensability, depending on the amount of silver-silver interface area available. The attained rupture strength values (200 Mpa) were higher than those of amalgams; the compressive strengths (120 MPa) and hardness values (100 KHN), however, were lower than those found for amalgams.

SIGNIFICANCE

Mercury-free silver-tin powder mixtures can be processed for use as metallic composite dental restorative materials.

Porcelain bond to laser-welded titanium surfaces

OBJECTIVES

The purpose of this investigation was to determine if laser-welding titanium had an effect on the bond strength of porcelain to metal. This is an important consideration since the metal copings for many multi-unit porcelain restorations are fabricated as single units and then either soldered or welded.

METHODS

The bond strength of porcelain to laser-welded titanium was measured with a four-point bending test and the interface was examined using SEM/EDX analysis.

RESULTS

The result showed no statistically significant differences (p<0.05) in bond strengths between laser-welded surfaces and mechanically machined surfaces and neither were there any alterations in composition identified after the welding. The use of a special bonding agent did not improve the bond strength significantly.

SIGNIFICANCE

Fusing porcelain to laser-welded areas in titanium fixed restorations implies no deterioration of bond strength.

Spark erosion: a sophisticated technique to solve a difficult problem

Even with the most careful planning, situations arise that require sophisticated treatment alternatives. The Milled Bar, of which the spark erosion bar is one type, is described in detail. A two-degree tapered meso (primary) bar with anterior friction pins attached to osseointegrated implants provides frictional retention when supporting a secondary bar with swivel lock attachments. The two components of the prosthesis are accurately associated with each other using spark erosion and plasma arc welding technology.

Addition of an implant pillar to an existing implant prosthesis

When an implant is lost, it is sometimes desirable to replace it to ensure adequate support of the prosthesis. Such an addition may necessitate a costly and time-consuming remake procedure. A technique is offered that, in some circumstances, may allow incorporation of the newly placed implant into the existing restoration.

Investigation of stiffness and microstructure of joints soldered with gas-oxygen torch and infrared methods

The stiffness of soldered connectors fabricated with the gas-oxygen torch technique was compared with those fabricated with the infrared technique. Untreated solder materials were used as controls. Untreated solder materials of gold-palladium, palladium-silver, and chromium-cobalt alloys exhibited significantly higher stiffness than the soldered specimens fabricated with either method. There was no significant difference between the gas-oxygen torch and the infrared groups of the gold-palladium and palladium-silver alloys. Photomicrographs revealed that the soldered specimens displayed porosities. For the chromium-cobalt alloy, the infrared group demonstrated significantly higher stiffness than the gas-oxygen torch group. Photomicrographs also disclosed that the chromium-cobalt alloy soldered with the gas-oxygen torch technique had a significant heat-affected zone.

A comparison of hydrogen/oxygen and natural gas/oxygen torch soldering techniques

The tensile strength of connectors joining bars of palladium-silver alloy and formed using hydrogen/oxygen flame soldering was compared to those formed using conventional gas/oxygen soldering. As-cast specimens served as the control. No significant differences were found between the as-cast, hydrogen/oxygen flame soldered, or the natural gas/oxygen flame soldered specimens.

Evaluation of soldered connectors of two base metal ceramic alloys

Soldered connectors for two base metal ceramic alloys (nickel-chromium and cobalt-chromium) were compared by use of four different techniques: (1) infrared preceramic soldering, (2) gas and oxygen preceramic soldering, (3) porcelain furnace postsoldering under vacuum, and (4) porcelain furnace postsoldering without vacuum. A control group was established with solid cast specimens of each alloy. No statistically significant difference was noted between infrared and torch preceramic soldering techniques for either of the two alloys. However, the joints postsoldered under vacuum were significantly superior to postsoldered connectors without vacuum (p < 0.0001). No significant differences were observed among techniques 1, 2, and 3, although the three groups were substantially superior to technique 4 for both alloys (p = 0.05). The control group for both alloys was appreciably stronger than the soldered groups (p < 0.0001), and the nickel-chromium samples within the control group were significantly stronger than the Co-Cr samples.

Mechanical properties of laser welds of titanium in dentistry by pulsed Nd:YAG laser apparatus

The use of titanium in dentistry is increasing, and an adequate method of joining titanium units of restorations is needed. This study examined the welding of titanium with a normal pulse Nd:YAG laser. Laser welding of titanium was effective when performed in an argon atmosphere. ANOVA for the three-point bending test showed a correlation between the bending strength of a weld, the atmosphere under which the irradiation is performed, and the intensity of the irradiation.

Fatigue life of preceramic soldered and postceramic soldered joints

This research investigated the fatigue life (ie, the number of cycles of a given stress that can be sustained by a sample prior to catastrophic failure) of metal ceramic soldering. One metal ceramic alloy was joined using four postceramic solders and one preceramic solder. All specimens received appropriate heat treatment simulating ceramic application although no porcelain was applied. Five specimens were fabricated for each solder type, and a 35,000 psi (241.4 MPa) fatigue stress was applied to each solder joint during specimen testing. The test variable was the number of stress cycles required to fail each specimen. The postceramic solders exhibited significantly higher fatigue lives than did the preceramic solder. Scanning electron microscope analysis showed that the preceramic solder not only exhibited a larger grain size, but also showed micropores between these grains.

Mechanical properties of metal connectors soldered by gas torch versus an infrared technique

PURPOSE

The ultimate tensile strength and percentage elongation of solder joints of three alloy systems were tested.

MATERIALS AND METHODS

Solder joints were formed using either an infrared soldering machine or gas-oxygen torch. Intact solder materials were used as controls.

RESULTS

There were no significant differences in percentage elongation among different treatment methods of the solders or among various solder materials. All intact solder materials possessed significantly higher ultimate tensile strength than the joints made with either method. For high-noble and noble alloy solders, there were no significant differences in ultimate tensile strength of joints made with either method. For base metal alloy solder, the ultimate tensile strength of joints made with the infrared technique was significantly higher than that made with the gas-oxygen torch technique.

CONCLUSIONS

The infrared technique can be used as an alternative to the gas-torch technique for soldering high-noble and noble alloys. It is superior to the gas-torch technique for soldering a cobalt-chromium alloy.

Studies on laser- and plasma-welded titanium

In recent years, titanium has become a material of major interest in prosthetic dentistry. Due to its chemical properties, titanium has to be processed differently from conventional alloys. In this paper, two different methods of welding were investigated. Specimens machined from pure titanium rods were fused either by laser welding or plasma welding. Hardness profiles and light microscopy images were taken in the region of the weld. The mechanical properties were tested by alternating bending fatigue tests up to 3 million cycles. Light microscopy images and hardness profiles showed a larger heat-affected zone after plasma welding compared to laser welding. No significant differences comparing fatigue strength could be found between the two methods of welding. However, extreme loads led to earlier fatigue in the plasma-welded specimens. SEM images of the laser-welded joints showed fractures in the welding zone, while the plasma-welded specimens fractured mostly beyond the heat-affected zone. From these results, it can be assumed that both methods are suitable for welding titanium. At the moment, laser welding is the more suitable technique in dentistry because of its lower thermal alteration of the workpieces.

Comparative analysis of two techniques for soldered connectors

The aim of this study was to compare and analyze gas torch and an infrared technique as a heat source. Ney Oro B2, Ney Eclipse and Ney 76 alloys were selected for this purpose. Sixty standardized rods were invested, cast, and sectioned from their respective sprue buttons. The rods were then invested for soldering and sectioned at their midpoints. Ten specimens of each alloy were soldered with the gas torch flame. The Instron universal testing device was used to test the flexural strength of the resultant joints. Fractured specimens were analyzed under electron microscopy.

Comparison of tensile strength of solder joints by infrared and conventional torch technique

Fabrication of a fixed partial denture may require a soldering step. This study compared soldering by a conventional torch procedure with an infrared soldering technique. Comparisons were made for tensile strength, porosity, and time efficiency between the two methods. No significant difference was found in ultimate tensile strength between the two types of solder joints and the nonsoldered control samples. Random samples photographed with a scanning electron microscope revealed no difference in joint porosity between the two techniques. Torch soldering took consistently less time that infrared soldering.

The fit of fixed partial dentures joined by infrared soldering

This study determined the accuracy of fit of three-unit fixed partial dentures joined by an infrared soldering technique compared with one-piece fixed partial denture castings and individually cast crowns. Wax patterns of prepared Ivorine teeth, maxillary left central incisor and maxillary left canine, were injection molded; a plastic rod was used as a pontic. One group of patterns was cast as one-piece dentures; the other group was sectioned in the connector area, cast individually and then joined by infrared soldering. Castings were seated on their respective dies, embedded in epoxy resin, and sectioned. Gap distances between the casting and the die were measured at specified marginal sites with a profile projector. Results showed that the fit of infrared-soldered fixed partial dentures was significantly better than that of one-piece castings and was comparable with the fit of single crowns. The gap openings measured in all castings were within the range of clinical acceptability.

Development of a new investment for high-frequency induction soldering

This study developed a new investment for induction soldering using high frequency induction heating. Ninety-five mass% magnesia clinker and 5 mass% dental stone were selected for the main constituents. The magnesia investment itself was scarcely affected by induction heating, so the addition of metal powders such as Fe, Ni, Co were investigated. Among these three powders, the addition of 10 mass% cobalt powder was most effective. This investment needed only 40 seconds of high frequency induction heating to go from room temperature to 900 degrees C without preheating. Thermal expansion of this investment in a vacuum atmosphere was 1.25% at 1000 degrees C. Dimensional changes during induction soldering were measured using German-silver and silver solder. When the new magnesia investment containing 10 mass% Co powder was used, the dimensional change was -0.2%. This contraction was less than when a magnesia investment without metal additives was used.