Laser welding of cast titanium and dental alloys using argon shielding

Microscopic and Mechanical Characterization of Co-Cr Dental Alloys Joined by the TIG Welding Process

Due to their good mechanical and other properties, cobalt-chromium alloys (Co-Cr) are often used in prosthetic therapy. The metal structures of prosthetic works can be damaged and break, and depending on the extent of the damage, they can be re-joined. Tungsten inert gas welding (TIG) produces a high-quality weld with a composition very close to that of the base material. Therefore, in this work, six commercially available Co-Cr dental alloys were joined by TIG welding, and their mechanical properties were evaluated to determine the quality of the TIG process as a technology for joining metallic dental materials and the suitability of the Co-Cr alloys used for TIG welding. Microscopic observations were made for this purpose. Microhardness was measured using the Vickers method. The flexural strength was determined on a mechanical testing machine. The dynamic tests were carried out on a universal testing machine. The mechanical properties were determined for welded and non-welded specimens, and the results were statistically evaluated. The results show the correlation between the investigated mechanical properties and the process TIG. Indeed, characteristics of the welds have an effect on the measured properties. Considering all the results obtained, the TIG-welded I-BOND NF and Wisil M alloys showed the cleanest and most uniform weld and, accordingly, satisfactory mechanical properties, highlighting that they withstood the maximum number of cycles under dynamic load.

Applications of Laser Welding in Dentistry: A State-of-the-Art Review

The dental industry without lasers is inconceivable right now. This captivating technology has outlasted other possible alternative technologies applied in dentistry in the past due to its precision, accuracy, minimal invasive effect as well as faster operating time. Other alternatives such as soldering, resistance (spot) welding, plasma (torch) welding, and single pulse tungsten inert gas welding have their pros and cons; nevertheless, laser welding remains the most suitable option so far for dental application. This paper attempts to give an insight into the laser principle and types of lasers used for dental purposes, types of dental alloys used by the dentist, and effect of laser parameters on prosthesis/implants. It is apparent from the literature review that laser assisted dental welding will continue to grow and will become an unparalleled technology for dental arena.

Titanium removable denture based on a one-metal rehabilitation concept

The use of a single metal for all restorations would be necessary because it protects against metal corrosion caused by the contact of different metals. For this "one-metal rehabilitation" concept, non-alloyed commercially pure (CP) titanium should be used for all restorations. Titanium frameworks have been cast and used for the long term without catastrophic failure, whereas they have been fabricated recently using computer-aided design/computer-aided manufacturing (CAD/CAM). However, the milling process for the frameworks of removable partial dentures (RPDs) is not easy because they have very complicated shapes and consist of many components. Currently, the fabrication of RPD frameworks has been challenged by one-process molding using repeated laser sintering and high-speed milling. Laser welding has also been used typically for repairing and rebuilding titanium frameworks. Although laboratory and clinical problems still remain, the one-metal rehabilitation concept using CP titanium as a bioinert metal can be recommended for all restorations.

Characterisation of nanoscale carbide precipitation in as-cast Co–Cr–W-based dental alloys

The results of the present study indicate the importance of local chemistry along with carbide precipitation in the optimization of alloy design for achieving desirable properties such as high strength, ductility, and corrosion resistance in dental Co–Cr-based cast alloys.

The metallurgical characteristics of non-precious alloys using Nd:YAG laser welding

Background

This study aimed to determine the effect of hardness change according to penetration depth in the laser fusing zone and observed the correlation of the microstructure as an Nd:YAG laser was irradiated to Ni-Cr alloy for dental use by setting the spot diameter size with respect to defocusing distances. In all groups, the hardness depth profiles in the laser fusing zone and heat-affecteded zone (HAZ) had larger values than those of the base metal. In addition, the hardness values in places beyond the fusing zone and the HAZ were measured as being quantitatively lower.

Methods

The alloys used in this study were Verabond 2 V, Noritake Super, and Bellabond Plus, which are commercially used non-precious dental alloys. The specimens were cut to have a plate shape with a size of 0.5 × 3.0 × 2.5 mm. This was followed by setting the Nd:YAG laser output, pulse duration, and frequency to 60 W, 10 ms, and 5 Hz, respectively. The laser was then irradiated as the spot diameter condition varied between 0.5 mm and 1.4 mm in accordance with defocusing distance from 0.0 mm to 2.0 mm. After the laser irradiation, a cross-section of the fusing zone in the specimens was observed in terms of laser melted depth, hardness depth profile, and the microstructure of each alloy.

Results

The observation result of the diffusion of the constituent elements and microstructure using field emission scanning electron microscopy, energy dispersive spectroscopy (EDS), and electron probe micro-analyzer showed that the fusing zone revealed a much finer dendritic form than the base metal due to the self-quenching effect after laser melting, while no change in constituent elements was found although some evaporation of the main elements was observed.

Conclusions

These results suggest that each Mo and Si combined inter-metallic compounds were formed on the interdendritic area. Through this study, the laser fusing zone had better hardenability due to the inter-metallic compound and grain refinement effect.

Application of the 2-piece orthodontic C-implant for provisional restoration with laser welded customized coping: a case report

This article presents the application of laser welding technique to fabricate an orthodontic mini-implant provisional restoration in missing area after limited orthodontic treatment. A 15-year-old boy case is presented. Two-piece orthodontic C-implant was placed after regaining space for missing right mandibular central incisor. Due to angular deviation of implant, customized abutment was required. Ready-made head part was milled and lingual part of customized abutment was made with non-precious metal. Two parts then were laser welded (Master 1000, Elettrolaser Italy, Verona, Italy) and indirect lab composite (3 M ESPE Sinfony, St. Paul, MN, USA) was built up. The patient had successful result, confirmed by clinical and radiographic examinations. Before the patient is ready to get a permanent restoration later on, this provisional restoration will be used. This case shows that a two-piece orthodontic C-implant system can be used to maintain small edentulous space after orthodontic treatment.

Mechanical and interfacial characterization of laser welded Co-Cr alloy with different joint configurations

PURPOSE

The mechanical and interfacial characterization of laser welded Co-Cr alloy with two different joint designs.

MATERIALS AND METHODS

Dumbbell cast specimens (n=30) were divided into 3 groups (R, I, K, n=10). Group R consisted of intact specimens, group I of specimens sectioned with a straight cut, and group K of specimens with a 45° bevel made at the one welding edge. The microstructure and the elemental distributions of alloy and welding regions were examined by an SEM/EDX analysis and then specimens were loaded in tension up to fracture. The tensile strength (TS) and elongation (ε) were determined and statistically compared among groups employing 1-way ANOVA, SNK multiple comparison test (α=.05) and Weibull analysis where Weibull modulus m and characteristic strength σ_ο were identified. Fractured surfaces were imaged by a SEM.

RESULTS

SEM/EDX analysis showed that cast alloy consists of two phases with differences in mean atomic number contrast, while no mean atomic number was identified for welded regions. EDX analysis revealed an increased Cr and Mo content at the alloy-joint interface. All mechanical properties of group I (TS, ε, m and σ_ο) were found inferior to R while group K showed intermediated values without significant differences to R and I, apart from elongation with group R. The fractured surfaces of all groups showed extensive dendritic pattern although with a finer structure in the case of welded groups.

CONCLUSION

The K shape joint configuration should be preferred over the I, as it demonstrates improved mechanical strength and survival probability.

Mechanical properties of thin films of laser-welded titanium and their associated welding defects

The aim of this study was to evaluate the mechanical properties of thin films of laser-welded cast titanium using an interference strain/displacement gauge (ISDG) and to analyze factors that affect laser welding. Dog-bone-shaped small specimens of cast titanium were prepared by wire cutting after they were laser-welded. The specimens were divided into three groups according to the gap distance of the laser weld; the control was non-welded titanium. Small specimens without cast defects detected by X-ray screening were measured by a tensile test machine using ISDG, and stress-strain curves were drawn. Finally, the fracture texture was analyzed. The ultimate tensile strengths (UTSs) of specimens with a gap distance of 0.00, 0.25, and 0.50 mm were 492.16 ± 33.19, 488.09 ± 43.18, and 558.45 ± 10.80 MPa, respectively. There were no significant differences in UTS between the test groups and the control group (p > 0.05). However, the plastic deformation and the percent elongation increased as the gap distance increased. Incomplete penetration defects appeared in groups that had small gap distances, which may have affected the properties of the laser-welded titanium. However, the welding material was still pure titanium. These results suggest that an appropriate gap distance should be maintained to improve the application of dental laser welding.

Strength properties of preceramic brazed joints of a gold-palladium alloy with a microwave-assisted oven and gas/oxygen torch technique

STATEMENT OF PROBLEM

The effect of microwave brazing on the strength properties of dental casting alloys is not yet known.

PURPOSE

The purpose of this study was to compare the strength properties of preceramic brazed joints obtained by using a microwave oven and a conventional torch flame for a high noble alloy (Au-Pd).

MATERIAL AND METHODS

A total of 18 tensile bars made of an Au-Pd ceramic alloy were fabricated. Six specimens were cut and joined with a high-fusing preceramic solder in a specially designed microwave oven, and 6 specimens were joined with a conventional natural gas/oxygen torch. The remaining 6 uncut specimens were tested as a control. All the specimens were subjected to testing with a universal testing machine. A 1-way ANOVA was performed for each strength property tested.

RESULTS

The tensile strength of the uncut group was the highest (745 ±19 MPa), followed by the microwave group (420 ±68 MPa) and the conventional torch group (348 ±103 MPa) (P<.001); however, no significant difference in tensile strength was found between the microwave group and gas torch group. The tensile strength of the microwave group exceeded ANSI/ADA Standard No. 88, Dental Brazing Alloys (a joint standard of the American National Standards Institute and the American Dental Association).

CONCLUSIONS

The microwave heating preceramic solder method demonstrated the excellent tensile strength of an Au-Pd alloy and may be an alternative way of joining alloys when a torch flame is contraindicated.

Effect of Nd:YAG laser parameters on the penetration depth of a representative Ni-Cr dental casting alloy

The effects of voltage and laser beam (spot) diameter on the penetration depth during laser beam welding in a representative nickel-chromium (Ni-Cr) dental alloy were the subject of this study. The cast alloy specimens were butted against each other and laser welded at their interface using various voltages (160-390 V) and spot diameters (0.2-1.8 mm) and a constant pulse duration of 10 ms. After welding, the laser beam penetration depths in the alloy were measured. The results were plotted and were statistically analyzed with a two-way ANOVA, employing voltage and spot diameter as the discriminating variables and using Holm-Sidak post hoc method (a = 0.05). The maximum penetration depth was 4.7 mm. The penetration depth increased as the spot diameter decreased at a fixed voltage and increased as the voltage increased at a fixed spot diameter. Varying the parameters of voltage and laser spot diameter significantly affected the depth of penetration of the dental cast Ni-Cr alloy. The penetration depth of laser-welded Ni-Cr dental alloys can be accurately adjusted based on the aforementioned results, leading to successfully joined/repaired dental restorations, saving manufacturing time, reducing final cost, and enhancing the longevity of dental prostheses.

Comparative analysis of different joining techniques to improve the passive fit of cobalt-chromium superstructures

STATEMENT OF PROBLEM

The influence of different joining techniques on passive fit at the interface structure/abutment of cobalt-chromium (Co-Cr) superstructures has not yet been clearly established.

PURPOSE

The purpose of this study was to compare 3 different techniques of joining Co-Cr superstructures by measuring the resulting marginal misfit in a simulated prosthetic assembly.

MATERIAL AND METHODS

A specially designed metal model was used for casting, sectioning, joining, and measuring marginal misfit. Forty-five cast bar-type superstructures were fabricated in a Co-Cr alloy and randomly assigned by drawing lots to 3 groups (n=15) according to the joining method used: conventional gas-torch brazing (G-TB), laser welding (LW), and tungsten inert gas welding (TIG). Joined specimens were assembled onto abutment analogs in the metal model with the 1-screw method. The resulting marginal misfit was measured with scanning electron microscopy (SEM) at 3 different points: distal (D), central (C), and mesial (M) along the buccal aspect of both abutments: A (tightened) and B (without screw). The Levene test was used to evaluate variance homogeneity and then the Welsch ANOVA for heteroscedastic data (α=.05).

RESULTS

Significant differences were found on abutment A between groups G-TB and LW (P=.013) measured mesially and between groups G-TB and TIG (P=.037) measured centrally. On abutment B, significant differences were found between groups G-TB and LW (P<.001) and groups LW and TIG (P<.001) measured mesially; groups G-TB and TIG (P=.007) measured distally; and groups G-TB and TIG (P=.001) and LW and TIG (P=.007) measured centrally.

CONCLUSIONS

The method used for joining Co-Cr prosthetic structures had an influence on the level of resulting passive fit. Structures joined by the tungsten inert gas method produced better mean results than did the brazing or laser method.

Análise metalográfica do titânio puro submetido à soldagem laser Nd: YAG e TIG

INTRODUÇÃO: Os métodos de soldagem mais utilizados em Odontologia não podem ser aplicados ao titânio puro e às suas ligas em função da alta reatividade do titânio com elementos atmosféricos; dessa forma, o mesmo não deve ser soldado por processo comum. OBJETIVO: O objetivo deste trabalho foi avaliar a característica metalúrgica do titânio comercialmente puro sem solda e submetido aos processos de soldagem a laser e TIG. MATERIAL E MÉTODO: Foram confeccionados 15 corpos de prova em titânio comercialmente puro, cinco para cada condição, na forma de hastes cilíndricas, obtidas por fundição odontológica, sob atmosfera de gás argônio e vácuo, com calor produzido por um arco voltaico, com a injeção do titânio sob vácuo-pressão. Três grupos foram formados I: soldagem a laser; II: soldagem TIG, e III: sem solda. Os corpos de prova do grupo I e II foram seccionados ao meio e soldados por TIG e por laser, respectivamente; o grupo III foi mantido sem corte e sem solda, como controle. A análise metalográfica foi realizada sob aumentos de 50×, 100× e 200×, em microscópio. RESULTADO: Pelos resultados obtidos nas micrografias, o titânio comercialmente puro apresentou uma morfologia de grãos equiaxiais da fase α, o cordão de solda a laser apresentou estrutura martensítica e, na TIG, microestrutura Widmanstätten. CONCLUSÃO: A microestrutura martensítica é condizente com a alta taxa de resfriamento proveniente do processo de soldagem a laser. As estruturas martensítica e Widmansttäten são mais refinadas quando comparadas à microestrutura do metal base.

Fatigue performance of joints executed in pure titanium structures with several diameters

This study evaluated fatigue strength of CP-Ti laser-welded joints. Sixty (20/diameter) CP-Ti casted dumbbell rods with diameters of 1.5, 2.0, and 3.5 mm were sectioned and welded using two joint openings (0.0 (00) and 0.6 mm (06)). Six groups were formed, amounting to a total of 9 (n=10) with inclusion of intact groups. Welding was executed using 360 V/8 ms (1.5 and 2.0 mm) and 380 V/9 ms (3.5 mm). Joints were finished, polished, and submitted to radiographic examination to visually analyze presence of porosity (PP). Specimens were submitted to cyclic tests, and the number of cycles until failure (NC) was recorded. Fractured surfaces were examined by SEM. Kruskal-Wallis and Dunn (α=0.05) tests demonstrated that NC was lower for all diameters with 06, and for 3.5 mm/00. NC and PP were found to have a negative correlation (Spearman Coefficient). For CP-Ti frameworks with thin diameters, laser welding is better when structures are juxtaposed.

Deflection load characteristics of laser-welded orthodontic wires

OBJECTIVE

To compare the deflection load characteristics of homogeneous and heterogeneous joints made by laser welding using various types of orthodontic wires.

MATERIALS AND METHODS

Four kinds of straight orthodontic rectangular wires (0.017 inch × 0.025 inch) were used: stainless-steel (SS), cobalt-chromium-nickel (Co-Cr-Ni), beta-titanium alloy (β-Ti), and nickel-titanium (Ni-Ti). Homogeneous and heterogeneous end-to-end joints (12 mm long each) were made by Nd:YAG laser welding. Two types of welding methods were used: two-point welding and four-point welding. Nonwelded wires were also used as a control. Deflection load (N) was measured by conducting the three-point bending test. The data (n  =  5) were statistically analyzed using analysis of variance/Tukey test (P < .05).

RESULTS

The deflection loads for control wires measured were as follows: SS: 21.7 ± 0.8 N; Co-Cr-Ni: 20.0 ± 0.3 N; β-Ti: 13.9 ± 1.3 N; and Ni-Ti: 6.6 ± 0.4 N. All of the homogeneously welded specimens showed lower deflection loads compared to corresponding control wires and exhibited higher deflection loads compared to heterogeneously welded combinations. For homogeneous combinations, Co-Cr-Ni/Co-Cr-Ni showed a significantly (P < .05) higher deflection load than those of the remaining homogeneously welded groups. In heterogeneous combinations, SS/Co-Cr-Ni and β-Ti/Ni-Ti showed higher deflection loads than those of the remaining heterogeneously welded combinations (significantly higher for SS/Co-Cr-Ni). Significance (P < .01) was shown for the interaction between the two factors (materials combination and welding method). However, no significant difference in deflection load was found between four-point and two-point welding in each homogeneous or heterogeneous combination.

CONCLUSION

Heterogeneously laser-welded SS/Co-Cr-Ni and β-Ti/Ni-Ti wires provide a deflection load that is comparable to that of homogeneously welded orthodontic wires.

The effect of disinfecting solutions on bending properties and weight changes of Co-Cr and Ti-6Al-7Nb alloys for dentures

We aimed to evaluate the effect of a denture cleanser and NaOCl 5.25% solution on the modulus of elasticity, the ultimate stress, and the weight changes of Co-Cr and Ti-6Al-7Nb alloys. To evaluate the modulus of elasticity and ultimate stress, 24 Co-Cr and 24 Ti-6Al-7Nb castings were fabricated, immersed in Corega Extradent, NaOCl 5.25%, or water in subgroups of six each. Thirty-six castings were produced to evaluate weight changes, 18 Co-Cr and 18 Ti-6Al-7Nb, and immersed in Corega Extradent, NaOCl 5.25%, or water in subgroups of six each. Changes in the modulus of elasticity, ultimate stress, and weight were calculated before and after immersion in the cleanser or NaOCl 5.25% solution for 45 cycles of 10 min each. Two-way analysis of variance ANOVA and Tukey's test at α = 0.05 were used to analyze the data. The modulus of elasticity and the ultimate stress of Co-Cr decreased after immersion in the solutions. The same properties of Ti-6Al-7Nb decreased in NaOCl 5.25% and increased in the other solutions. Weight loss was observed in Co-Cr alloy in both Corega Extradent and NaOCl 5.25%. A weight increase was observed in Co-Cr in water. An increase was observed in Ti-6Al-7Nb in Corega Extradent, NaOCl 5.25%, and water. The highest decrease (P = 0.05) in the modulus of elasticity and ultimate stress in Co-Cr were observed in NaOCl 5.25%, whereas no statistically significant differences (P > 0.05) were observed in Ti-6Al-7Nb in any of the solutions. A statistically significant weight loss (P < 0.05) was observed when Co-Cr was immersed in NaOCl 5.25%, and a statistically significant weight increase (P > 0.05) was observed when Ti-6Al-7Nb was immersed in Corega Extradent. The Co-Cr alloy bending properties after immersion fulfilled ADA Specification No. 14 for satisfactory clinical performance.

Non-destructive three-dimensional evaluation of pores at different welded joints and their effects on joints strength

OBJECTIVES

The purpose of this study was to measure the porosity in different laser welded cast alloys non-destructively using X-ray micro-focus computerized tomography (micro-CT) and to evaluate the effect of porosity on the tensile strength of the welded joints.

MATERIALS AND METHODS

The welding procedure was conducted in rectangular cast metals, CoCr, Ti and platinum added gold alloy (AuPt). The metal plates were butted CoCr to CoCr (CoCr/CoCr) or Ti to Ti (Ti/Ti) for welding of similar metals and Ti to AuPt (Ti/AuPt) for welding of dissimilar metals. Specimens were welded under several laser-welding conditions; with groove (normal), without groove (no groove), spatter, crack, or no overlapped welding (no overlap) (n=5). Porosity in the welded area was evaluated using a micro-CT. Tensile strength of the welded specimens was measured at a crosshead speed of 1mm/min. Multiple comparisons of the group means were performed using ANOVA and Fisher's multiple comparisons test (α=.05). The relationship between the porosity and the tensile strength was investigated with a regression analysis.

RESULTS

Three-dimensional images of Ti/AuPt could not be obtained due to metal artifacts and the tensile specimens of Ti/AuPt were debonded prior to the tensile test. All other welded specimens had porosity in the welded area and the porosities ranged from 0.01% to 0.17%. The fractures of most of the CoCr/CoCr and Ti/Ti specimens occurred in the parent metals. Joint strength had no relationship with the porosity in the welded area (R(2)=0.148 for CoCr/CoCr, R(2)=0.088 for Ti/Ti, respectively).

SIGNIFICANCE

The small amount of porosity caused by the laser-welding procedures did not affect the joint strength. The joint strength of Ti/AuPt was too weak to be used clinically.

Present status of titanium removable dentures--a review of the literature

Although porcelain and zirconium oxide might be used for fixed partial dental prostheses instead of conventional dental metals in the near future, removable partial denture (RPD) frameworks will probably continue to be cast with biocompatible metals. Commercially pure (CP) titanium has appropriate mechanical properties, it is lightweight (low density) compared with conventional dental alloys, and has outstanding biocompatibility that prevents metal allergic reactions. This literature review describes the laboratory conditions needed for fabricating titanium frameworks and the present status of titanium removable prostheses. The use of titanium for the production of cast RPD frameworks has gradually increased. There are no reports about metallic allergy apparently caused by CP titanium dentures. The laboratory drawbacks still remain, such as the lengthy burn-out, inferior castability and machinability, reaction layer formed on the cast surface, difficulty of polishing, and high initial costs. However, the clinical problems, such as discoloration of the titanium surfaces, unpleasant metal taste, decrease of clasp retention, tendency for plaque to adhere to the surface, detachment of the denture base resin, and severe wear of titanium teeth, have gradually been resolved. Titanium RPD frameworks have never been reported to fail catastrophically. Thus, titanium is recommended as protection against metal allergy, particularly for large-sized prostheses such as RPDs or complete dentures.

Fracture strength of different soldered and welded orthodontic joining configurations with and without filling material

The aim of this study was to compare the mechanical strength of different joints made by conventional brazing, TIG and laser welding with and without filling material. Five standardized joining configurations of orthodontic wire in spring hard quality were used: round, cross, 3 mm length, 9 mm length and 7 mm to orthodontic band. The joints were made by five different methods: brazing, tungsten inert gas (TIG) and laser welding with and without filling material. For the original orthodontic wire and for each kind of joint configuration or connecting method 10 specimens were carefully produced, totalizing 240. The fracture strengths were measured with a universal testing machine (Zwick 005). Data were analyzed by ANOVA (p=0.05) and Bonferroni post hoc test (p=0.05). In all cases, brazing joints were ruptured on a low level of fracture strength (186-407 N). Significant differences between brazing and TIG or laser welding (p<0.05, Bonferroni post hoc test) were found in each joint configuration. The highest fracture strength means were observed for laser welding with filling material and 3 mm joint length (998 N). Using filling materials, there was a clear tendency to higher mean values of fracture strength in TIG and laser welding. However, statistically significant differences were found only in the 9-mm long joints (p<0.05, Bonferroni post hoc test). In conclusion, the fracture strength of welded joints was positively influenced by the additional use of filling material. TIG welding was comparable to laser welding except for the impossibility of joining orthodontic wire with orthodontic band.

Effect of arm design and chemical polishing on retentive force of cast titanium alloy clasps

PURPOSE

Titanium dentures have recently increased in popularity. A decrease in the retentive force of the titanium clasps has frequently been observed. This study investigated the effect of retentive arm design and chemical polishing of titanium clasps.

MATERIALS AND METHODS

Four Akers clasps with 0.25-mm and 0.5-mm undercuts were designed so that the retentive arms were placed at 1/2 and 2/3 of the undercut area. Wax patterns were fabricated and invested in phosphate-bonded Al2O3/LiAl SiO6. They were cast with CP Ti Grade 2, CP Ti Grade 3, and Ti-6Al-7Nb using an argon gas-pressure casting unit (Autocast HC-III). After sandblasting, the castings were chemically treated with an HNO3/HF solution. The retentive force (N) was measured up to 10,000 insertion/removal times. The results (n = 5) were analyzed by ANOVA/Tukey's test (alpha= 0.05).

RESULTS

The retentive forces significantly decreased with increasing immersion time in the HNO3/HF solution (p < 0.05). The retentive force of the 2/3 undercut was significantly greater than that of the 1/2 undercut (p < 0.05). Excluding the initial insertion/removal period of the 2/3 undercut, there were no significant differences among all the titanium metals tested (p > 0.05). As the number of insertion/removal times increased, the retentive force of the 2/3 undercut greatly decreased. The retentive force of the Ti-6Al-7Nb clasps for the 1/2 undercut decreased the least after 10,000 insertion/removal times.

CONCLUSIONS

Chemical treatment for titanium clasps should be performed for 1-5 minutes due to the following factors: accuracy, surface roughness, surface structure, initial retention, and stability of retention. To maintain appropriate long-term retentive force, the retentive arms should be placed in the 1/2 undercut area of the abutment tooth.