Effect of temperature and flux concentration on soldering of base metal

The present study used the acoustic emission (AE) technique to evaluate interactions among soldering temperature, flux treatment, and the resultant ultimate tensile strength (UTS). Scanning electron microscopy (SEM) was used to examine fracture surfaces of the solder joints. Specimens were cast from removable partial denture alloy and then placed in a jig with a gap distance of 1.0 mm. A high-frequency soldering machine with an optical pyrometer was used for soldering at 1150 degrees C and 1200 degrees C, respectively. The flux concentrations were 67% and 75%. The soldered specimens were subjected to tensile test at a crosshead speed of 0.05 mm/min. During testing, acoustic emissions in the frequency range of 100--1200 kHz were collected, filtered, recorded, and processed by a sensing device. The results were analysed by ANOVA and Tukey LSD test. UTS at different temperatures showed no significant difference according to either mechanical or acoustic results. But in the 1200 degrees C group, the UTSs and AE counts showed significant differences (P<0.05) at both flux concentrations. SEM showed that the 1200C group had better dendritic crystal structure than did the 1150 degrees C group. In the 1200 degrees C group specimens with 67% flux had fewer flux inclusion bodies and dendritic crystals than did specimens with 75% flux. The 75% flux subgroup produced high-amplitude (60--70 dB) acoustic signals within the elastic deformation zone, while the 67% flux subgroup produced similar signals within the plastic deformation zone, either beyond the 0.2% yield point or before fracture.

[Comparison of mechanical properties of laser-welded wrought and cast titanium]

OBJECTIVE

This study was aimed to compare the mechanical properties of laser-welded wrought and cast titanium.

METHODS

According to relevant reports and clinical demands, the specimens of TA2 were machine milled or cast into certain sizes using LZ centrifuge-vacuum-pressure titanium casting machine. This experiment involved four groups including the laser-welded wrought titanium group, the laser-welded cast titanium group, the wrought titanium group and the cast titanium group. There were 6 complete specimens in each group and the specimens were ultrasonically cleaned with ethyl and distilled water before they were welded with home-made Nd:YAG apparatus, and comparisons of mechanical properties were made among these groups. All tensile specimens were mounted on the Instron universal testing machine to test tensile loads, yield loads and absolute elongations were measured. The cross section of fractured surface was observed under S-2700 Hitachi SEM and Vickers' microhardness values in different areas were detected with AKASHI microhardness tester.

RESULTS

It was showed that the tensile load of laser-welded wrought titanium made no difference from that of the original wrought titanium, but the yield load of laser-welded wrought titanium was bigger than that of the original wrought titanium. The tensile load and yield load of cast titanium were slightly bigger than those of wrought titanium. The tensile load and yield load of laser-welded wrought titanium made no statistic difference from those of laser-welded cast titanium. The absolute elongations of cast or welded titanium were lower than those of wrought or non-welded titanium. The fractured surface of cast titanium under SEM appeared lath-like structure and that of welded cast titanium honeycomb-like structure. The honeycomb-like structure of wrought titanium was more obvious than that of laser-welded wrought titanium. The microhardness value of laser-welded cast titanium was slightly bigger than that of laser-welded wrought titanium.

CONCLUSION

The mechanical property of laser-welded wrought titanium might be not apparently different from that of laser-welded cast titanium.

[The researches on titanium and titanium alloy in dental use]

Although titanium and titanium alloy have been used in dental field for several years because of their excellent biocompatibility, corrosion resistance and mechanical properties, many practical problems remain to be solved. This review focuses on the practical problems and the prospects of the researches on titanium, including the development of new titanium alloy, the surface treating, the corrosion in oral, and the processability.

Marginal fit of metal ceramic restorations subjected to a standardized postsoldering technique

The fit of a fixed partial denture (FPD) is fundamental for the clinical success of a restoration. When a postsoldering procedure is needed, a high-precision laboratory technique is necessary to not affect the fit of the FPD. This article evaluates whether a standardized postsoldering technique affected the marginal fit of a 3-unit high palladium alloy FPD. One hundred and eight measurements were made of 3-unit FPDs, fabricated in a high palladium alloy (2% Au-79% Pd-10% Cu-8% Ga) and constructed on 9 tin dies in vitro. After the castings were fabricated, specimens were measured at 3 specific points per abutment (distal, labial, lingual) before and after soldering. Differences were found in the marginal opening of both copings, before soldering (49.9 microm) and after soldering (48. 3 microm). There was no significant difference in the adaptation of the copings after the soldering procedure (P<.05).

Analysis of stress distribution in a screw-retained implant prosthesis

Four types of implant superstructures were screwed onto implant bodies, and the strains created around the implant bodies were compared and analyzed within the IMZ Implant System. Three IMZ implants were embedded in the center of a polyurethane block (30 x 40 x 30 mm), and a total of 16 superstructures was fabricated by 4 methods: 1-piece cast, 1-piece cast/split soldering, soldering, and passive fit. Six strain gauges were placed on the surface of the block 1 mm apart. Three embedded implants were numbered, and a fixed partial denture was placed on these implants and screwed by a torque wrench using 14.5 Ncm torque. This procedure was repeated 7 times for each fixed partial denture, and each created strain was measured when the last screw was tightened. In all fixed partial dentures, strains were produced around the implant bodies when screws retaining the prosthesis were tightened, and the strain was relieved with unscrewing. The magnitude of strain was greater with the 1-piece cast method or the section/solder method than with the soldering and passive-fit methods. Of the 2 soldering methods, when the screw on the middle implant was tightened before those on the terminal 2 implants, the magnitude of strain was lower with the soldering method than with the 1-piece cast/split soldering method. When the order of screw tightening was changed, there were significant differences in the magnitude of strain at each gauge with the soldering method. With the passive-fit method, no differences in the magnitude of strain attributable to the order of screw tightening could be detected. The magnitude of strain produced around a screw-retained implant prosthesis was significantly lower with the passive-fit method when compared to the other 3 fabricating methods. Furthermore, the implants prepared by the passive-fit method were not affected by the order of screw tightening.

An investigation into the effect of solidification shrinkage on distortion of casting and flexure strength of various solders for base metal alloys. A laboratory study

This study was undertaken to determine the distortion due to solidification shrinkage and to evaluate the flexure strength properties of soldered joints between combination of three commercially available base metal alloys and solders. Ninety rectangular bars were cast using three commercially available alloys for soldering (30 bars of each alloys). Ten bars of each alloy were soldered using electrical soldering unit and solder recommended by respective manufacturers, twenty bars (ten pairs each) were soldered using other solders. Length between the references markings on the specimen were measured using a vernier caliper and travelling microscope to check the possible solidification shrinkage. The specimens were then subjected to flexure strength evaluation using a universal testing machine. Statistical comparison of dimensional changes due to solidification shrinkage and flexure strength evaluation of soldered joints was done using analysis of variance test. Recommended Ni-Cr solders showed less shrinkage compared to that of the Co-Cr solders from the tested specimens and superior flexure strength was found in Co-Cr alloy specimen (P value P > .001) soldered with Co-Cr alloy solders.

Clinical Experiences of Implant-Supported Prostheses with Laser-Welded Titanium Frameworks in the Partially Edentulous Jaw: A 5-Year Follow-up Study

BACKGROUND

Titanium frameworks have been used in the endentulous implant patient for the last 10 years. However, knowledge of titanium frameworks for the partially dentate patient is limited.

PURPOSE

To report the 5-year clinical performance of implant-supported prostheses with laser-welded titanium frameworks in the partially edentulous jaw.

MATERIALS AND METHODS

A consecutive group of 383 partially edentulous patients were, on a routine basis, provided with fixed partial prostheses supported by Brånemark implants in the mandible or maxilla. Besides conventional frameworks in cast gold alloy, 58 patients were provided with titanium frameworks with three different veneering techniques, and clinical and radiographic 5-year data were collected for this group.

RESULTS

The overall cumulative survival rate was 95.6% for titanium-framework prostheses and 93.6% for implants. Average bone loss during the follow-up period was 0.4 mm. The most common complications were minor veneering fractures. Loose and fractured implant screw components were fewer than 2%. An observation was that patients on medications for cardiovascular problems may lose more implants than others (p < .05).

CONCLUSIONS

The clinical performance of prostheses with implant-supported laser-welded titanium frameworks was similar to that reported for conventional cast frames in partially edentulous jaws. Low-fusing porcelain veneers also showed clinical performance comparable to that reported for conventional porcelain-fused-to-metal techniques.

Distortion of three-unit implant frameworks during casting, soldering, and simulated porcelain firings

PURPOSE

The aim of this study was to assess distortion inherent in casting, soldering, and simulated porcelain firings of screw-retained, implant-supported three-unit fixed partial dentures (FPDs).

MATERIALS AND METHODS

Ten wax patterns were fabricated on a die-stone cast containing two implants, 20 mm apart from center to center. Five specimens were cast in a high-palladium alloy, exposed to simulated porcelain firings, sectioned, and then soldered with low-fusing solder. Five specimens were cast, sectioned, soldered with high-fusing solder, and then exposed to simulated porcelain firings. For each specimen, two horizontal and six vertical distances between appropriately scribed reference points were measured with a traveling microscope. Comparisons were made among the various measurements taken after wax-pattern fabrication, casting, high- and low-fusing soldering, and each porcelain firing. Data were analyzed using a repeated-measures factorial ANOVA (alpha = 0.05).

RESULTS

Significant difference was detected in the amount of horizontal distortion during casting (53 +/- 24 microns) and high-fusing soldering (-49 +/- 50 microns), as well as in the amount of horizontal distortion during high-fusing soldering (-49 +/- 50 microns) and low-fusing soldering (17 +/- 26 microns). However, no clinically significant difference was found in the amount of horizontal distortion during casting, low-fusing, and high-fusing soldering. The greatest amount of distortion during the simulated porcelain firings took place during the oxidizing cycle.

CONCLUSIONS

Soldering did not improve the casting misfit of a three-unit implant-retained FPD model. Metal-ceramic implant frameworks should be oxidized before intraoral fit evaluation.

Infrared gold alloy brazing on titanium and Ti-6Al-4V alloy surfaces and its application to removable prosthodontics

PURPOSE

This study investigated the area size of the flow of a gold braze alloy on commercially pure titanium and Ti-6Al-4V alloy plates, and elemental composition at the interface was determined. In the second part of this study, the tensile strengths of titanium plates brazed using a gold alloy were investigated.

MATERIALS AND METHODS

Chips of Type IV gold alloy and silver braze alloys were melted onto commercially pure titanium and Ti-6Al-4V surfaces in a dental infrared radiation unit. Flow area of each braze alloy was measured using a digital image analyzer. Tensile specimens (n = 5) were also prepared by infrared brazing using the braze alloys. Five specimens for each combination of the two titanium plates and the two braze alloys were subjected to tensile loading using a Universal testing machine. Electronprobe microanalysis of x-rays at cross-section of the brazed joints to determine elemental composition across the interface, as well as scanning electron microscopic observation at the fracture surfaces, were also conducted.

RESULTS

The braze alloys flowed well and spread over the Ti and Ti-6Al-4V plates. Braze alloy type significantly influenced flow, and the gold alloy flowed less on the titanium materials. The mean tensile strengths of Ti and Ti-6Al-4V plates brazed using the gold braze alloy were 219 MPa and 417 MPa, respectively. The fracture surfaces of Ti-6Al-4V specimens with the gold braze alloy exhibited typical ductile behavior. Ti with the same braze alloy showed brittle surfaces. A greater concentration of Cu was found at the Ti with gold braze interface.

CONCLUSION

The flow and the tensile strength of the gold alloy coating on titanium surface by means of an infrared brazing is adequate for dental use.

Comparison of interface relationships between implant components for laser-welded titanium frameworks and standard cast frameworks

With the introduction of new techniques for the fabrication of frameworks for implant-supported prostheses comes the need to understand how the components used compare to those used for conventional cast frameworks. The relationship of measured machining tolerances between conventional implant components and those components used for stereo laser-welded implant frameworks was determined using a standardized protocol. Statistically significant differences in the horizontal interface relationship were found between paired implant components, which had a mean range from 23.1 to 51.7 microns. From a laboratory and clinical perspective, machining tolerances of implant components represent a variable associated with their manufacturing, which can ultimately affect the fit of a completed prosthesis.

A soldering index for cement-retained implant restorations

Recent advances in implant abutment interface design purport to offer more repeatable, accurate abutment connection. Improved circumferential indexing of the abutments along with a decrease in micromovement may result in a more predictable fit of cement-retained implant prostheses through a simple variation of soldering technique. A technique for developing a more accurate soldering relationship is described.

Photoelastic stress analysis of load transfer to implants and natural teeth comparing rigid and semirigid connectors

STATEMENT OF PROBLEM

Controversy exists regarding the connection of implants to natural teeth.

PURPOSE

This simulation study measured photoelastically the biologic behavior of implants. Stress transfer patterns with variable implant support and simulated natural teeth through rigid and nonrigid connection were examined under simulated functional loads.

MATERIAL AND METHODS

A photoelastic model of a human left mandible edentulous distal to first premolar was fabricated having 2 screw type implants (3.75x13 mm) embedded within the edentulous area. Two fixed prosthetic restorations were fabricated with either a nonsplinted proximal contact or a soldered proximal contact, and cast precision dowel attachment between implant areas and simulated tooth. Simulated vertical occlusal loads were applied at fixed locations on the restorations. Stresses, which developed in the supporting structure, were monitored photoelastically and recorded photographically.

RESULTS

The rigid connector in the 1 implant situation caused only slightly higher stresses in the supporting structure than the nonrigid connector. The distally loaded 1 and 2 implant-supported restoration produced the highest apical stresses, which occurred at the distal implant. The rigid connector demonstrated the greatest stress transfer in the 2 implant-supported restoration.

CONCLUSIONS

Lower stresses apical to the tooth or implant occurred with forces applied further from the supporting abutment. Although the least stress was observed when using a nonrigid connector, the rigid connector in particular situations caused only slightly higher stresses in the supporting structure. The rigid connector demonstrated more widespread stress transfer in the 2 implant-supported restoration. Recommendations for selection of connector design should be based on sound clinical periodontal health of a tooth and the support provided by implants.

Strategies to achieve fit in implant prosthodontics: a review of the literature

PURPOSE

This paper reviews the literature on advanced strategies that attempt to improve fit in implant prosthodontics with reference to the concept of the "distortion equation."

MATERIALS AND METHODS

The majority of the articles reviewed were either clinical or technique articles that advocated strategies to improve fit in implant prosthodontics. A limited number of retrospective and prospective clinical trial studies were included as they related to the topic. Reviewed articles were limited to those that addressed advanced strategies to improve fit. All of the scientific studies included in this review used an in vitro experimental design. The advanced strategies were categorized into methods that address intraoral indexing and methods that use the implant master cast.

RESULTS

Relatively few methods have been scientifically proven to improve fit in implant prosthodontics. Most of the tested strategies still resulted in a slight misfit of the frameworks to the implant abutments/analogues.

CONCLUSION

Multiple factors preclude that the concept of "passive fit" can be achieved in implant prosthodontics, even with the use of advanced strategies. The use of meticulous, accurate implant prosthodontic procedures and the appropriate use of advanced strategies continue to be the recommended means of achieving precise fit of the implant prosthesis to the intraoral abutments.

Six-year follow-up of titanium and high-gold porcelain-fused-to-metal fixed partial dentures

In a randomized clinical study 47 titanium and gold-alloy fixed partial dentures (FPDs) were placed during a 1-year period. In the titanium group (n = 22) all metal substructures were made of unalloyed titanium. The titanium substructures were fabricated by copy milling, spark erosion and laser welding (Procera, Nobelpharma). Ceramic veneering was carried out with Duceratin titanium ceramics (Ducera, Germany). In the control group (n = 25) the high-gold alloy Degudent U (Degussa, Germany) and Vita VMK 68 ceramics (Vita, Germany) were used. The longest observation time was 6 years. Only one FPD had to be removed due to metal-ceramic failure (titanium group). The clinical performance of all 125 porcelain-fused-to-metal (PFM) veneers with respect to the longevity of the metal-ceramic compound was described by Kaplan-Meyer survivor analyses. Relating survival to a completely intact ceramic veneer, the 5-year survivor rate was 84% for titanium and 98% for the high-gold alloy. PFM titanium restorations exhibited a significantly increased risk of metal-ceramic failure. However, concerning defects requiring removal, no significant differences in titanium versus high-gold alloy occurred. There were no significant differences in the survival distributions between crowns and pontics within the two groups.

Assessment of cold welding properties of the internal conical interface of two commercially available implant systems

STATEMENT OF PROBLEM

The cone-screw abutment has been shown to diminish micromovement by reducing the burden of component loosening and fracture. However, anecdotal concern for cold welding of cone-screw joints in implant design has been identified as a potential source for lack of retrievability.

PURPOSE

This comparative study evaluated the loosening torque, as a percentage of tightening torque, for the ITI Straumann and Astra Tech (3.5 and 4.0 mm diameters) implant systems, which use an 8-degree and 11-degree internal cone, respectively.

MATERIAL AND METHODS

Implants and abutments from each system were mounted in a torque device, and a range of tightening torques was applied. Loosening torques were then measured, and the influence of conus angle, interfacial surface area, saliva contamination, and time delay to loosening were all assessed.

RESULTS

The loosening torque only exceeded tightening torque at the highest levels, just before component failure, when plastic deformation was expected. For all clinically relevant levels of torque, both in a dry environment and with components bathed in artificial saliva at 37 degrees C, loosening torque was always seen to be 80% to 90% of tightening torque, demonstrating that cold welding does not occur. There was a high correlation between loosening and tightening torque for all systems tested, but no statistical difference when comparing wet versus dry or comparing individual data for each system.

CONCLUSIONS

It can be concluded that for clinically relevant levels of tightening torque, no problems are anticipated with respect to retrievability.

Laser-welded titanium frameworks for implant-supported fixed prostheses: a 5-year report

The 5-year results from a multicenter study of implant-supported, laser-welded titanium frameworks are reported here as a complement to earlier reported 2-year results. Implant survival rates were satisfactory for patients with titanium frameworks and for a control group of patients with gold-alloy frameworks. At the 5-year review, more patients with titanium frameworks had lost implants than the patients with gold-alloy frameworks, but this difference was not statistically significant. More fractures of the titanium frameworks occurred, compared to the gold-alloy frameworks; again, this difference was not statistically significant. No significant differences in marginal bone levels were found between the 2 groups of patients. Slightly more fractures of artificial teeth occurred in patients with titanium frameworks than in patients with gold-alloy frameworks. Overall results for the titanium frameworks after 5 years in function are encouraging.

[SEM study of machinable vita cerec mark II porcelain surface morphology after HF acid etching]

After HF acid etching under 5 different HF concentrations and 6 exposing times, the surface morphology of machinable porcelain was examined by scanning electron microscope, and the porsosity percentage and ething depth were determined. The results showed that the etching depth increased with the rising of HF concentration and disposing time until 5%-7.5 min. The porosity percentage of ething surface was positively interrelated with HF concentration and disposing time. It was the first time to classify the etching patterns of machinable porcelain into four groups.

Osseointegration for welded and cast prostheses: presentation of two cases

The author describes two cases of full dental arches with only a few natural teeth in place that were treated with multi-dimensional, osseointegrated implants. The former case utilized the technique of titanium casting of the caps on natural teeth and on implants, with intra-oral welding of the caps to the implants. In the latter case, the techniques of welding and casting differ.

Mechanical properties of laser-welded cast titanium joints under different conditions

STATEMENT OF THE PROBLEM

Data on optimal conditions for laser welding commercial pure cast titanium used in dental restorations are sparse and not well established.

PURPOSE

The mechanical properties of laser-welded cast commercially pure titanium under different laser welding conditions were evaluated to find the optimal parameters in terms of duration and voltage (energy level).

MATERIAL AND METHODS

Fifty-seven cast pure titanium specimens according to ISO specification 6871 were divided into nine experimental groups and one control group. The sectioned titanium bars were laser-welded under different duration (8, 10, and 12 ms) and energy (290, 300, and 310 V) levels and evaluated for ranges of ultimate tensile strength (374 to 562 MPa), 0.2% yield strength (206 to 338 MPa), and percentage elongation (2.49% to 10.58%).

RESULTS

Under a suitable laser-welding setting parameter the ultimate tensile strength and 0.2% yield strength of experimental groups were weaker than the unsectioned control specimens (control group ultimate tensile strength 540 +/- 11 MPa, 0.2% yield strength 258 +/- 49 MPa) or too brittle (control group percentage elongation 12.41% +/- 1.83%). Two-way ANOVA, nonparametric tests showed that voltage was the only significant factor for all parameters studied (p < 0.01). Optimal conditions were determined with three-dimensional response curves (305 V, 12 ms ultimate tensile strength; 310 V, 10 ms 0.2% yield strength; 300 V, 12 ms percentage elongation).

CONCLUSION

Optimal duration and voltage (energy level) used in laser welding for cast CP grade I titanium bar was superior to or comparable with and produced properties that were superior to or comparable with the unsectioned control specimens.

Description and evaluation of a simplified method to achieve passive fit between cast titanium frameworks and implants

Because osseointegrated implants have no resilience in bone, passive fit between dental implants and prosthetic superstructures has been identified, both from biologic and mechanical perspectives, as a potential discriminating prognostic factor. Distortion of the metal framework during the casting procedure has been cited as a main cause of misfit. The objectives of the present article were to describe a recently presented method (Cresco Ti Precision method) intended to correct for distortion in cast titanium frameworks, and to elucidate and evaluate the method by photoelastic and strain gauge techniques. The method appears to be an efficient and accurate procedure for correcting for distortion in cast titanium frameworks.

Thermal modeling of laser welding for titanium dental restorations

STATEMENT OF PROBLEM

Concerns of laser welding for titanium dental prostheses are the limited depth of laser beam penetration and extensive surface damage.

PURPOSE

This study used numerical heat transfer simulation to explain this behavior and offers an alternate multiple-pulsed method.

MATERIAL AND METHODS

A one-dimensional finite difference analysis was used to simulate heat transfer in pure titanium and gold during laser welding with a custom-constructed software program.

RESULTS

The thermal gradient profiles revealed the problem to be inherent in titanium's low thermal conductivity; gold did not have this problem. Time-elapsed multiple pulses on titanium relieved this problem by giving the energy time to diffuse into the depth of the material.

CONCLUSIONS

With single-pulse laser irradiation on titanium, an increase in power could not greatly increase melting depth. The excess energy only vaporized the material surface.

One clinical visit for a multiple implant restoration master cast fabrication

The making of a one-piece, long-span, implant-supported prosthesis with conventional procedures frequently has difficulties associated with the accuracy of fit. This article presents a clinical and laboratory procedure for making an accurate implant working cast that facilitates fabrication of the casting on the master cast. The procedure demonstrates the process of sectioning and rejoining of the resin between the transfer copings and then pouring the impression by first joining the analogs alone with impression plaster, sectioning it, and rejoining it again to stabilize the analogs, and finally, using dental stone to pour the impression. Clinical, radiographic, and laboratory (optical microscope) measurements for one clinical implant restoration confirm the accuracy of fit of this one prosthesis made with this procedure. Its advantage is that it can allow fabrication of the final casting on the cast, thereby eliminating the clinical time necessary to obtain repetitive solder indexes, and thus minimizing inconvenience to the patient.

Fit of implant frameworks fabricated by different techniques

PURPOSE

This study evaluated the precision of fit between an implant framework and a patient simulation model that consisted of five implant abutments located in the mandibular symphysis area. One-piece cast frameworks were compared with Procera machined and laser-welded frameworks with laser videography.

MATERIAL AND METHODS

Five frameworks of each type were measured with a laser digitizer and a graphics computer program to determine a single point represented as the "Centroid" for each framework component and each implant abutment. Differences between the paired centroids for each framework/abutment interface are reported as x- and y-axis displacements, and z-axis gaps. The direction of the x- and y-axis displacements was determined.

RESULTS

There were significant differences (p < 0.05) in the precision of fit between both the one-piece cast frameworks and the Procera frameworks, when compared with the abutments in the patient simulation model. The laser-welded framework exhibited a more precise fit than the one-piece casting, with significant differences at four of the five prosthodontic interfaces, when evaluated by the mean z-axis gap at the centroid points.

Electrical discharge machining

This article describes a laboratory technique of achieving the highest degree of passive fit of an implant-retained restoration using electric discharge machining (EDM). This process can save time by eliminating the need for conventional soldering procedures, increase the longevity of the restoration, and when used along with the clinical technique of fabricating a verification index, eliminate the clinical try-in phase.

Tensile strength of soldered gold alloy joints

STATEMENT OF PROBLEM

Little information is available about the mechanical properties of soldered gold alloys after they have undergone various heat treatments.

PURPOSE

This study investigated the influence of heat treating on the strength properties of soldered joints of two gold alloys (NC Type IV and Sofard), which can be age-hardened at intraoral temperature.

MATERIAL AND METHODS

Dumbbell-shaped specimens were cast with each gold alloy and were cut at the center of the connecting bar for soldering. The two halves of the casting were then soldered with two different gold-based solders (Maingold and Degulor). Three different heat treatments were performed on the soldered gold alloy assemblies: solution heat treatment (ST) at 700 degrees C for 5 minutes (treatment A); aging at 37 degrees C for 7 days after ST (treatment B); and aging at 250 degrees C (Sofard) or 400 degrees C (NC type IV) for 15 minutes after ST (treatment C). The tensile strength, elongation and microhardness were evaluated after each heat treatment.

RESULTS

The hardness values of Sofard significantly (p < 0.05) increased during aging at 37 degrees C (treatment B) and produced adequate strengths of the soldered joints, especially with the harder solder (645.7 MPa: Degulor [222 VHN] vs. 493.3 MPa: Maingold [165 VHN]).

CONCLUSIONS

The results of this study indicated the possibility of strengthening soldered joints in the oral environment, thus eliminating the necessity for any additional hardening heat treatment.