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| Year : 2013 | Volume
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| Issue : 6 | Page : 723-729 |
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| A comparison of tensile weld strength and microstructural changes in four arch wires, before and after immersion in 1.23% acidulated phosphate fluoride solution: An in-vitro study |
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Shantanu Tela1, Veera Bhosale2, Ravindranath Sable2, Roopa Abdullah2, Rajshekhar Halli3
1 Department of Orthodontics and Dentofacial Orthopaedics, Jaipur Dental College, Jaipur, Rajasthan, India
2 Department of Orthodontics and Dentofacial Orthopaedics, Bharati Vidyapeeth University Dental College and Hospital, Pune, Maharashtra, India
3 Department of Oral and Maxillofacial Surgery, Bharati Vidyapeeth University Dental College and Hospital, Pune, Maharashtra, India
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| Date of Submission | 15-Feb-2013 |
| Date of Decision | 02-Aug-2013 |
| Date of Acceptance | 09-Nov-2013 |
| Date of Web Publication | 20-Feb-2014 |
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 Abstract | | |
Objectives: The objective of this study is to evaluate and to compare the tensile weld strengths and microstructural changes in four archwires namely beta titanium, stainless steel (SS), blue elgiloy and timolium before and after immersion in 1.23% acidulated phosphate fluoride (APF) solution.
Materials and Methods: The mean tensile weld strength of a weld joint of four arch wires were compared pre-fluoride (Group 1) with post fluoride (Group 2) and the microstructural characteristics of weld joints were evaluated under an optical microscope.
Results: The mean tensile weld strength for beta titanium was 445.64 N/mm 2 , blue elgiloy was 363.26 N/mm 2 , SS was 358.30 N/mm 2 and timolium was 308.62 N/mm 2 . After immersion in fluoride the mean tensile strength for beta titanium was 427.16 N/mm 2 , blue elgiloy was 359.86 N/mm 2 , SS is 349.44 N/mm 2 and timolium is 294.86 N/mm 2 . After immersion in fluoride, the beta titanium and SS had a smooth fusion at the center of weld assembly with characteristic nugget formation. The beta titanium weld assembly had greater tensile weld strength than other welded assemblies. Blue elgiloy ranked second, whereas SS and timolium were third and fourth respectively.
Conclusion: The wires in descending order of their mean tensile weld strength, on evaluation of their weld joints were found to be: Beta titanium > blue elgiloy > SS > timolium. The reduction in tensile weld strength was statistically insignificant for all the archwires after exposure to 1.23% APF at 37°C for 90 min. Keywords: Arch wires, microstructural changes, optical properties, tensile weld strength
How to cite this article:
Tela S, Bhosale V, Sable R, Abdullah R, Halli R. A comparison of tensile weld strength and microstructural changes in four arch wires, before and after immersion in 1.23% acidulated phosphate fluoride solution: An in-vitro study. Indian J Dent Res 2013;24:723-9 |
How to cite this URL:
Tela S, Bhosale V, Sable R, Abdullah R, Halli R. A comparison of tensile weld strength and microstructural changes in four arch wires, before and after immersion in 1.23% acidulated phosphate fluoride solution: An in-vitro study. Indian J Dent Res [serial online] 2013 [cited 2023 Oct 4];24:723-9. Available from: https://www.ijdr.in/text.asp?2013/24/6/723/127621 |
Welding attachments to arch wires can produce changes in their physical as well as mechanical properties. Welding of attachments to the archwire is performed for proper force delivery in orthodontic tooth movement. The welding of wire auxiliaries of the same or different cross section to the arch wire can further increase appliance versatility.
Compromised oral hygiene, a frequent complication with orthodontic treatment, can lead to enamel demineralization and decay. To address this potential treatment complication, Orthodontists commonly prescribe a daily topical fluoride. Although beta titanium and stainless steel (SS) alloys form corrosion-resistant passivation layers, these oxide layers can be disrupted, leading to corrosion susceptibility. It has been reported that beta titanium and SS have exhibited corrosion in the presence of experimental fluoride-based solutions. [1],[2],[3],[4] In addition to corrosive surface changes, it has also been reported that experimental fluoride solutions degrade the tensile strength and microhardness of beta titanium and SS archwires. [2]
Therefore, it is possible that commercially available topical fluoride prophylactic agents may cause a similar corrosive interaction and associated mechanical property degradation of orthodontic wires at the weld joint.
Hence, this study aims to evaluate and to compare alterations in the weld joint strength and microstructural characteristics of four different archwires, namely - beta titanium, SS, blue elgiloy and timolium before and after exposure to 1.23% acidulated phosphate fluoride (APF) at 37°C for 90 min.
| Materials and Methods | |  |
Materials used in the study were:
- Archwires: The following archwires were used in the present study:
- 0.017" × 0.25" SS (Ormco Corporation, Glendora, Calif. - Lot #3105)
- 0.017" × 0.25" beta titanium (Ormco Corporation Glendora, Calif. - Lot #04M6M)
- 0.017" × 0.25" timolium (TP Labs, Indianapolis Lot #2618CN01)
- 0.017" × 0.25" blue elgiloy (Leone, Italy Lot #260997G).
- Orthodontic spot welder: The welding apparatus used was a Rocky Mountain, Model No. 660 welder
- Universal star testing system: Star Testing Systems, Universal Testing Machine Model No. STS248 was used to evaluate the tensile strength of the welded specimen
- Optical microscope: Optical microscope, Richart, Model No. MEF2, with a magnification range of ×50-2000
- Prophylactic agent: The prophylactic rinse used in this study was Phos-Flur mouth rinse 1.23% sodium fluoride acidulated phosphate; 0.04% w/v sodium fluoride pH = 5-6, manufactured by Colgate Oral Pharmaceuticals, Canton Mass; USA.
Method of the study
All the samples were tabulated into two groups in accordance with the two parts of this study. Samples from the four archwires, which were to be evaluated and compared for their tensile weld strength and microstructural properties, were included in Group 1 (pre-fluoride), whereas samples, which were to be tested after exposure to the fluoride prophylactic agent for their tensile weld strength and respective microstructural changes were included in Group 2 [Table 1]. Both these groups were named as the experimental groups.
Experimental groups
Group 1 (pre-fluoride)
A total of 10 welded wire samples from each of the 4 archwires totaling to a size of 40 constituted the Group 1. This group was further divided into Group 1A and Group 1B. Out of the 10 welded wire samples of each group, Group 1A comprised 5 specimen, which were examined for tensile weld strength and the remaining 5 wire samples, which were used for microstructural examination under the optical microscope, constituted Group 1B.
Group 2 (post-fluoride)
10 welded wire samples from each of the 4 archwires totaling to a size of 40 were immersed in Phos-Flur mouth rinse (1.23% sodium fluoride acidulated phosphate; 0.04% w/v sodium fluoride pH = 5-6) in individual plastic containers at 37°C for 90 min and were labeled as Group 2. This group was also further divided into Group 2A and Group 2B. Out of the 10, Group 2A comprised 5 samples, which were examined for tensile test and the remaining 5 wires, which were used for microstructural examination constituted Group 2B.
For measuring the tensile strength, arch wires were overlapped over a length of 5 mm with the wider surface of the rectangular wire segments and welded in straight lengths with the point-to-point electrode configuration at two spots at an equal distance with a single short pulse. The area of the weld joint was approximately 0.026 × 0.032 inches (as measured by the digital caliper) for all the specimens.
Tensile strength test for weld joint is done as follows
The welded assembly of each of the samples was subjected to tensile loading in a universal testing machine. The wire length between the cross-heads of the machine was standardized to 40 mm. The test was conducted at 5 mm/min cross head speed. The full-scale load was set at 1000 N. The load taken to break the weld joint divided by the cross-sectional area of the weld surface gave the value for joint strength in N/mm 2 .
Microstructural examination by optical microscope is done as follows
The straight length of the wires was mounted in a cold setting resin (liquid + powder) and the mounted specimens were polished. After polishing the samples were lapped using levigated alumina powder (i.e. alumina in water suspension) on lapping cloth. Then the samples were etched in reagent (i.e. HCl + HNO 3 in the ratio of 1:3 or HNO 3 + methanol in the ratio of 2%). After 5-10 s the specimens were immediately washed in running water, dried and then the weld area was examined at different magnifications (i.e. ×50, ×100 and ×400) under the optical microscope to evaluate the microstructural changes at the weld area.
Microstructural examination by the optical microscope after exposure to fluoride is done as follows
The welded assembly of each of the welded specimens from the fluoride group (Group 2B) was incubated in fluoride containing agent (10 ml of agent) in individual glass containers at 37°C for 90 min in an incubator machine.
| Results and Observations | | |
It was observed [Graph 1] that beta titanium had the highest weld strength followed by blue elgiloy, SS and timolium respectively.
There is a significant difference between the tensile weld strengths of blue elgiloy and timolium, timolium having a significantly lower strength value (P = 0.005). There is also significant difference of strength between blue elgiloy and beta titanium (titanium molybdenum alloy [TMA]) specimens, blue elgiloy having a significantly lower value (P = 0.001). There is a significant difference between SS and timolium, timolium having a significantly weaker strength than SS (P = 0.010). There is also a significant difference in strengths between SS and beta titanium, SS having a significantly lower strength (P = 0.001). There is a significant difference between beta titanium and timolium, with timolium having a significantly weaker tensile weld strength (P = 0.001). [Table 2] also shows that there is no significant difference between tensile weld strengths of blue elgiloy and SS (P = 0.982). | Table 2: The comparison of pre-fluoride (Group 1) tensile weld strength between four arch wires (within group comparison)
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[Table 3] shows the mean value for five specimens of SS in Group 1A is 358.3 N/mm 2 whereas for the five specimen of Group 2A is calculated to be 349.4 N/mm 2 . Paired t-test was done to compare the mean values for Group 1A and Group 2A for SS. The difference between the mean strengths of SS samples in Group 1A (pre-fluoride) and 2A (post-fluoride) is not statistically significant as indicated by a P value of 0.296. Therefore, [Table 3] indicates that within the SS group, there is no statistically significant decrease in tensile weld strength of the sample after immersion in 1.23% APF at 37°C for 90 min (pH = 5-6). | Table 3: The comparison of pre and post fluoride tensile weld strength for each arch wire (between group comparison)
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The mean value for the five specimen of beta titanium in Group 1A is 445.6 N/mm 2 whereas for the five specimen of Group 2A is 427.2 N/mm [Table 3]. It means that within the beta titanium the mean value of tensile weld strength of Group 1A (pre-fluoride) is greater than that of Group 2A (post-fluoride). Paired t-test [Table 3] was done to compare the mean values for Group 1A and Group 2A for beta titanium. P value of 0.275 indicates that there is no statistically significant decrease in the tensile weld strength of the sample after immersion in 1.23% APF at 37°C for 90 min (pH = 5-6).
The mean value for the five specimens of timolium [Table 3] in Group 1A is 308.6 N/mm 2 whereas for the five specimens of Group 2A is 294.9 N/mm 2 . Paired t-test was done to compare the mean values for Group 1A (pre-fluoride) and Group 2A (post-fluoride) for timolium. P value of 0.442 indicates that within the timolium group, there is no statistically significant decrease in tensile weld strength of the sample after immersion in 1.23% APF at 37°C for 90 min (pH = 5-6).
The mean value for the five specimens of the blue elgiloy in Group 1A is 363.3 N/mm 2 whereas, the value for Group 2A is 359.9 N/mm 2 . Paired t-test [Table 3] was done to compare the mean values for Group 1A (pre-fluoride) and Group 2A (post-fluoride). P value of 0.785 indicates that within the blue elgiloy group, there is no statistically significant decrease in tensile weld strength of the sample after immersion in 1.23% APF at 37°C for 90 min (pH = 5-6). All four wires show that the mean tensile weld strength of Group 1A (pre-fluoride) is greater than that of Group 2A (post-fluoride) put P values indicate that this difference is not statistically significant.
[Table 4] gives the comparison of post-fluoride relative percentage change with respect to the pre-fluoride treatment in tensile weld strength between four archwires. Analysis of variance was done to compare the four archwires for their relative percentage change in tensile weld strength. There is no statistically significant difference in the relative percentage changes among all the four archwires, which means all materials are comparable and show a statistically similar change after exposure to 1.23% APF at 37°C for 90 min (pH = 5-6). | Table 4: The comparison of post-fluoride (Group 2) relative percentage change with respect to the pre-fluoride treatment in tensile weld strength between four arch wires (within group comparison)
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Metallographic examination
Before fluoride exposure (Group 2A)
Micrographs of the weld joint obtained through the optical microscope are shown in color plates 1-8. The common microstructural change observed in the alloy wires was a change in linear/fibrous arrangement of grains in the parent metal to a columnar pattern in the weld area, i.e. there was a change of the alloy from a wrought structure to cast structure.
Beta titanium wire exhibited a small weld area with a characteristic nugget formation. A wide area of demarcation was evident at the margins of the weld nugget indicating a gradual change from wrought to cast structure. There were no observable porosities at the joint surface [Figure 1]. SS wire also exhibited a characteristic nugget formation with no porosities on the weld surface. The demarcating zone from wrought to cast structure was very well evident and the deformation observed on the surface of the wire where the electrode of the spot welder made its contact indicated poor heat tolerance of the wire. The surface of the wire also exhibited scratches and pits on its surface [Figure 2].
The weld joint of timolium exhibited a widely distributed weld area with lack of nugget formation. The melting of the alloy showed inhomogenous wide gaps with intermittent porosities at the center of the joint [Figure 3]. Blue elgiloy wire exhibited a smaller weld area. Its surface showed a poor fusion at the interface. The specimens lacked characteristic weld nugget formation. The melting of the alloy was unable to cover the whole area and showed intermittent gaps and porosities at the center of the joint. Poor heat tolerance at higher voltages was evident with deformation at certain fusion areas [Figure 4].
Metallography after fluoride exposure (Group 2B)
For the fluoride immersed beta titanium, corrosion products were uniformly formed on their surfaces, completely over shadowing the nugget formation [Figure 5]. The overall beta titanium wire surface appeared rougher along the wrought as well as the cast structure. It showed pitting and erosion of the metal surface at the higher magnification, with increased appearance of black spots, which may represent inclusion bodies. In the immersed SS sample, corrosion products were seen distributed all over the wire surface showing little degradation in the structure [Figure 6]. The nugget formation was enveloped by the surface corrosive layer. Chemical segregation within the black areas in the weld zone of the metal matrix was observed at higher magnifications (×1000). For the fluoride immersed timolium, the overall surface roughness was very well evident along with generalized pitting type of corrosion [Figure 7]. The surface corrosion is seen completely covering the wide gaps, which were observed at the center of the weld joint in the Group 2B sample. The microstructural changes for the blue elgiloy samples were not very different from the other wire samples. The weld area was covered by a surface corrosive layer, which lacked any homogeneity [Figure 8].
Collectively, weld surface topography changes following fluoride exposure suggest that the protective oxide layer has been disrupted for all the arch wires, leading to possible corrosive surface changes as would be observed with unwelded portions of the wires.
| Discussion | | |
Orthodontic spot welders use the resistance welding technique by using high-amperage low-voltage electricity. Welding is preferred to soldering to prevent changes in the physical properties of the components being joined. [5] The heat generated in welding is of great magnitude so as to cause melting at the interface. The copper electrodes in spot welders have a low resistance and the alloy wires used for making archwires have a greater resistance. Because of this differential resistance, essentially all the heat generated by the current flow is contained in the welded area. [6] Because sufficient heat is generated at the weld mate interface, the alloy components soften, flow and fuse together under the influence of mechanical pressure, forming a weld nugget. A satisfactory welded joint is the one that is strong, has not undergone oxidation, has little weld flash and set down (the amount the welded wire merges into the main wire) not greater than 25%. [7]
A comparison of tensile weld strength and evaluation of microstructural characteristics of four archwires namely beta titanium, SS, timolium and blue elgiloy was done in the present study.
The other important desirable property for orthodontic wires is corrosion resistance. The most commonly advocated oral hygiene regime usually prescribed during orthodontic treatment, to maintain good oral hygiene and to prevent enamel decalcification, is the use of fluoridated agents, gels and mouthwashes. The second purpose of this study was to determine whether pitting and corrosion on the surface of the weld joint of various orthodontic archwires, following exposure to fluoride prophylactic agents has any effect on its tensile strength, surface and microstructural properties.
In this study, beta titanium (TMA) wire ranked the highest in weld strength with mean tensile weld strength of 445.6 N/mm 2 [Table 2] when directly welded without solder. Nelson et al. [8] first described the welding properties of this wire having a high spring back and weld strength. A superior weld strength of the beta titanium was also reported by Krishnan and Kumar. [9] in their study. Hence, beta titanium offers the potential for many applications during treatment, particularly where welding is required for active tooth movement.
Blue elgiloy ranked second in overall mean tensile weld strength of 363.3 N/mm 2 [Table 2] as heat treatment of elgiloy wires causes precipitation hardening of the alloy, increasing its resistance to deformation. [10]
SS was third in strength with a mean tensile weld strength of 358.3 N/mm 2 [Table 2] but requires soldering to improve strength as has been observed by Burstone in 1981 [11] and Krishnan and Kumar. [9]
Timolium with its smooth surface, reduced friction, low modulus and better strength can be considered an introductory breakthrough in clinical orthodontic practice. [12] However, in the present study timolium shows the least weld strength, mean tensile weld strength of 308.6 N/mm 2 [Table 2] compared with the other three alloys.
Metallography of the welded specimens showed a characteristic weld nugget formation in specimens of beta titanium and SS archwires, but timolium and blue elgiloy lacked this feature [Figure 1], [Figure 2], [Figure 3], [Figure 4]. Timolium exhibited very poor metallographic features with a wider area of melting and porosities or voids at the weld area. This finding is in accordance with the very low-strength values on tensile strength evaluation and can be attributed to the alloying elements such as aluminum and vanadium. [9] Blue elgiloy samples showed a poor fusion at the weld joint because of absence of any characteristic nugget formation, but in spite of this, it was second in overall mean strength. This can be attributed to precipitation hardening of the alloy on heating. | Figure 1: Microstructure of the weld joint of beta titanium archwire at 3 different magnifications under the optical microscope
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 | Figure 2: Microstructure of the weld joint of stainless steel archwire at 3 different magnifications under the optical microscope
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 | Figure 3: Microstructure of the weld joint of timolium archwire at 3 different magnifications under the optical microscope
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 | Figure 4: Microstructure of the weld joint of blue elgiloy archwire at 3 different magnifications under the optical microscope
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Effect of fluoride exposure on weld joints of archwire alloys
The corrosion of these wires in fluoride containing agents may have a clinical implication of an increased frictional resistance during tooth movement which in turn could additionally tax the anchorage units resulting in anchorage loss. Furthermore, the corrosion of the wire surfaces may have an adverse effect on the mechanical properties of the arch wire mainly because of the phenomenon of hydrogen embrittlement. [2]
In this study, it was found that within the SS group there is no statistically significant (P = 0.296) decrease in the tensile weld strength [Table 3] of the sample after exposure to fluoride. Even though, degradation of their mechanical properties, i.e. reduction in tensile strength has been reported due to corrosion and subsequent hydrogen embrittlement by Kaneko et al., [2] in their study on different archwires.
Paired sample t-test within the beta titanium (TMA) specimen in this study did not indicate any statistically significant (P = 0.275) decrease in tensile weld strength of the sample [Table 3] after being exposed to fluoride environment, but percentage wise a maximum reduction of 3.8% [Table 4] was observed within this group, which can be attributed to hydrogen absorption from the cracks developed in the fusion areas. When hydrogen content is larger than several thousand mass ppm, pronounced degradation, such as ductile-to-brittle transition, occurs. The delayed fracture of beta titanium alloys take place because of hydrogen absorption in APF solutions. [13]
Timolium, like the beta titanium (TMA) has a titanium protective film. Elemental analysis of timolium with the help of XRF [12] as shown by Krishnan and Kumar. [9] in 2004 has revealed titanium as the major constituent of the alloy, with both aluminum and vanadium as stabilizing agents. Higher titanium content perhaps explains extensive corrosion of the weld joint.
In APF solutions, the degradation resistance of a wire with a titanium protective film is considered to be lower than that of a wire with a chromium protective film. This explains the minimum percentage decrease of only 0.9% for the blue elgiloy samples. This decrease in tensile weld strength of blue elgiloy is found to be statistically not significant, P = 0.785 [Table 3].
Percentage wise, the beta titanium (TMA) specimen showed a relative change or decrease in tensile weld strength by 3.8%. For the SS, the reduction in strength was about 2.4% and for the blue elgiloy and timolium wires, the relative change in strength was by 0.9% and 3.8% respectively [Table 4]. The overall difference in relative percentage changes among the four archwires was also not found to be statistically significant [Table 4], which means all materials are comparable and show statistically similar change after exposure to APF at 37°C for 90 min (pH = 5-6). The exposure time of 90 min would be equivalent to 3 months of 1 minute daily topical fluoride application or fluoride rinse as stated by Walker et al. in 2005. [14]
The microstructural changes in weld area after immersion in fluoride were observed under an optical microscope at two different magnifications (×400 and ×1000). The corrosive layer is over shadowing the nugget formation in the TMA specimen in Group 2B [Figure 5] and completely covering the wide gaps at the center of the weld joint in the Group 2B timolium wire specimen [Figure 7]. The overall beta titanium wire surface showed pitting and erosion of the metal surface with increased appearance of black spots, which may represent inclusion bodies. Walker et al. [14] had found similar corrosive changes in surface topography of beta titanium wires immersed in topical fluorides (Phos-Flur gel and prevident) for 90 min at 37°C. In the immersed SS sample [Figure 6], chemical segregation within the black areas of the weld joint of the metal matrix was seen along with erosion of the metal surface. Here too, nugget formation was enveloped by the superficial corrosive layer. The weld area becomes susceptible to intergranular corrosion primarily because of chromium carbide precipitation and consequent loss of passivation. The microstructural changes were not much different for the blue elgiloy wire samples when compared with the other archwires after immersion in fluoride. This was evident by the presence of a surface corrosive layer lacking any homogeneity, as shown in [Figure 8]. | Figure 5: Microstructure of the weld joint of beta titanium archwire after immersion in 1.23% acidulated phosphate fluoride at 37°C for 90 min (pH = 5-6), at 2 different magnifications under the optical microscope
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 | Figure 6: Microstructure of the weld joint of stainless steel archwire after immersion in 1.23% acidulated phosphate fluoride at 37°C for 90 min (pH = 5-6), at 2 different magnifications under the optical microscope
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 | Figure 7: Microstructure of the weld joint of timolium archwire after immersion in 1.23% acidulated phosphate fluoride at 37°C for 90 min (pH = 5-6), at 2 different magnifications under the optical microscope
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 | Figure 8: Microstructure of the weld joint of blue elgiloy archwire after immersion in 1.23% acidulated phosphate fluoride at 37°C for 90 min (pH = 5-6), at 2 different magnifications under the optical microscope
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| Conclusion | | |
The wires in descending order of their tensile weld strength, on evaluation of their weld joints were found to be: Beta titanium > blue elgiloy > SS > timolium.
Among the weld surfaces of all the samples studied, only beta titanium and SS had a smooth fusion at the center of the weld assembly, as visible under the optical microscope. A weld nugget formation was evident with SS and beta titanium weld specimens only. Timolium and blue elgiloy showed no nugget formation in any of the samples studied.
The reduction in tensile weld strength was statistically insignificant for all the archwires after exposure to 1.23% APF at 37°C for 90 min. All the tested welded specimens after immersion in fluoride, exhibited corrosive alterations such as pits, tarnish and surface roughness, which might lead to stress corrosion cracking with longer immersion times. However, to confirm the presence of fluoride-related corrosion products, future studies would need to be done using technology such as X-ray diffraction or spectroscopy.
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Correspondence Address:
Veera Bhosale
Department of Orthodontics and Dentofacial Orthopaedics, Bharati Vidyapeeth University Dental College and Hospital, Pune, Maharashtra
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.127621
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3], [Table 4] |
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