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 Table of Contents  
ORIGINAL ARTICLE
Year : 2014  |  Volume : 2  |  Issue : 2  |  Page : 178-183

Evaluation of mechanical properties of recasted dental base metal alloys for considering their reusability in dentistry and engineering field


1 Department of Dental Materials, Yenepoya Dental College, Yenepoya University, Deralakatte, Mangalore, Karnataka, India
2 Department of Prothodontics, Yenepoya Dental College, Yenepoya University, Deralakatte, Mangalore, Karnataka, India
3 Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka, India
4 Department of Dental Materials, Manipal College of Dental Sciences, Manipal, Karnataka, India
5 Department of Oral Pathology, Yenepoya Dental College, Yenepoya University, Deralakatte, Mangalore, Karnataka, India

Date of Web Publication11-Nov-2014

Correspondence Address:
Nandish Bantarahalli Thopegowda
Department of Dental Materials, Yenepoya Dental College, Yenepoya University, Deralakatte, Mangalore - 575 018, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2321-4848.144332

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  Abstract 

Background: Base metal casting alloys are extensively used in dentistry to fabricate many oral appliances and a huge amount is wasted in the form of sprues and buttons during the casting procedure. Recycling and reusing these alloys by clean technologies may save our natural resources from being depleted and as well reduce the cost of the treatment of the patients. Objectives: To study the mechanical properties of recasted dental base metal alloys, and explore possible ways to recycle and reuse in dentistry and other fields of science and technology. Materials and Methods: Two beryllium-free Cobalt-Chromium (Co-Cr) dental casting alloys, Wironit and Wirobond-C, were used for this study. Six groups of specimen (melted once, twice, five, ten, fifteen and twenty times) per each alloy were casted. The tensile strength and hardness of these samples were measured by using universal testing machine and Vickers hardness number (VHN) tester. Results: Tensile strength decreased from 850 MPa to 777 MPa after 5 th recasting and to 674 MPa at the end of 20 th recasting procedure for the Wironit samples. For Wirobond-C samples, tensile strength decreased from 720 MPa to 678 MPa after 5 th recasting and further reduced to 534 MPa at the end of 20 th recasting procedure. Hardness decreased from 380VHN to 335VHN at the end of 20 th recasting for Wironit samples and 328VHN to 247VHN for Wirobond-C samples after 20 th recasting procedure. The slight decrease in their mechanical properties will not have any impact on the clinical performance for dental applications. Conclusion: There is no major degradation in the mechanical properties after recycling, and hence, the left over alloys after casting procedures can be reused in dentistry with a condition to satisfy cytotoxicity tests.

Keywords: Co-Cr alloys, engineering field, tensile strength, vickers hardness number (VHN)


How to cite this article:
Thopegowda NB, Shenoy K, Shankaranarayana RK, Jayaprakash K, Gingipalli K, Vaddya SB, Prabhu S. Evaluation of mechanical properties of recasted dental base metal alloys for considering their reusability in dentistry and engineering field . Arch Med Health Sci 2014;2:178-83

How to cite this URL:
Thopegowda NB, Shenoy K, Shankaranarayana RK, Jayaprakash K, Gingipalli K, Vaddya SB, Prabhu S. Evaluation of mechanical properties of recasted dental base metal alloys for considering their reusability in dentistry and engineering field . Arch Med Health Sci [serial online] 2014 [cited 2019 Mar 19];2:178-83. Available from: http://www.amhsjournal.org/text.asp?2014/2/2/178/144332


  Introduction Top


In dentistry, metals represent one of the four major classes of materials (Ceramics, polymers and composites being other classes) used for the reconstruction of decayed, damaged or missing teeth. An alloy is a mixture of two or more metallic elements and sometimes an important constituent may be metalloid, or even a nonmetal, provided the mixture of elements displays metallic properties. In making an alloy, a parent metal is selected which possesses the most suitable properties and the alloying elements are added to improve the performance of the alloy in the oral cavity for the intended application. For dental applications, alloys usually contain at least four elements, and often six or more elements. [1]

Ever since, Taggart developed the lost wax casting process for the precision fitting castings, several alloy compositions have been made available for use as dental restorative materials. Numerous types of casting alloys have been used for the restoration of teeth. [2] Sixty years ago, only alloys based on gold, or noble metals were used for the fabrication of inlays, onlays, crowns and bridges. In fact, until the early 1950s, gold-based alloys were used widely in the construction of partial denture frames. However, due to the advances in the casting technology and the improvements in the metallurgical aspects, several new base metal alloy compositions have been introduced as an alternative to the existing gold-based alloys. Base metal alloys such as Cobalt-Chromium (Co-Cr), Nickel-Chromium (Ni-Cr) and titanium alloys have been widely accepted by the dental profession for making partial dentures, crowns, bridges, inlays and onlays etc, due to their superior mechanical properties, adequate corrosion resistance and biocompatibility. [3]

Today, majority of all prosthetic restorations used clinically are of base metal alloy compositions. Several reports are available which recommend the use of 50% of previously cast metal alloy buttons or sprues removed from the castings. Some of the manufacturers also recommend the use of such scrap metals for the fabrication of clinically acceptable castings. [4],[5] Although the reasons for not reusing previously cast metal is not well documented, it is presumed that during the casting process certain important minor elements, present in small percentages in the original alloy compositions, may be lost during remelting procedures through volatilization or oxidation. In addition, recasting of alloys has been shown to alter the properties of the alloys with respect to biocompatibility, [6],[7] mechanical properties and corrosion behavior. [8] Several reports are available regarding the effect of using previously cast alloys on their clinical performance. Some of these reports recommend the use of previously cast alloys and some of the report contradicts their use. In addition, little information is available about the number of times an alloy can be recast without losing its properties essential for their clinical performance. This study was performed to explore new ways to recycle and reuse dental base metal casting alloys (Co-Cr alloys) in dentistry and other fields of science and technology (Engineering field) by evaluating the mechanical properties such as tensile strength and hardness.


  Materials and Methods Top


Two commercially available Co-Cr base metal dental casting alloys Wironit (Bego, Wilhelm-Herbst-Strabe, Bremen Germany - LOT 12572) and Wirobond-C (Bego, Wilhelm-Herbst-Strabe, Bremen Germany - LOT 3418) was taken for the study.

Tensile strength measurement

Dumbbell shaped tensile test specimens were prepared in accordance with ISO 6871. Brass die having a gauge length of 15 ± 1 mm and diameter 3 ± 1 mm was first prepared and impression of this brass die was taken using addition polysilicone material by using a cylindrical tray support and this cylindrical impression was cut open and used as split mold to prepare wax/acrylic specimens for lost wax casting technique. Wax specimens were not dimensionally stable and hence 72 acrylic (polymethymethacrylate resin) specimens, (36 samples for Wironit and 36 samples for Wirobond-C) which were stronger and stable were prepared using elastomeric split mold [Figure 1]. Appropriate sprues were fixed for these acrylic samples and invested in phosphate bonded investment material (Bellasun - BegoWilhelm-Herbst-Strabe, Bremen Germany - LOT 0057808). The invested ring containing acrylic samples were placed in furnace at 500°C to 600°C to remove the acrylic samples (acrylic burn out procedure). The alloy samples were prepared by using induction-casting machine (Bego - Fornax-T casting machine - Model 26140) at 1050°C in six stages and allowed to cool by itself overnight to have same rate of cooling for controlling microstructure of the alloy samples. Six samples were prepared at each stage for both Wironot and Wirobond-C alloys [Figure 2]. First group was manufacturer's sample, (1 st casting), second group was melted twice and casted (2 nd casting), third group was melted five times and casted (5 th casting), fourth group was melted ten times and casted (10 th casting), fifth group was melted fifteen times and casted (15 th casting), sixth group was melted twenty times and casted (20 th casting). The sprues were cut off and subjected to sandblasting technique (Aluminum oxide) to remove all the unwanted impurities and electro polished to simulate dental appliances. Tensile strength was measured by Universal Testing machine Instron (Model 3366 UK) using load cells 100 kN at 1mm/min strain rate, until fracture.
Figure 1: (a) Elastomeric Split Mold, (b) Brass Die, (c) Acrylic Samples

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Figure 2: Showing casted alloy samples for tensile strength measurement

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Vickers hardness number measurement

72 Disk shape wax samples of 5 mm in diameter and 3 mm in thickness were prepared by pouring molten inlay wax into the metal mold (36 samples for Wironit and 36 samples for Wirobond-C). Appropriate sprues were fixed for these wax samples and invested in phosphate bonded investment material (Bellasun - BegoWilhelm-Herbst-Strabe, Bremen Germany - LOT 0057808). The invested ring containing wax samples were placed in furnace at 500°C to 600°C to remove the wax samples (wax burn out procedure). The alloy samples were prepared by using induction-casting machine (Bego - Fornax-T casting machine - Model 26140) at 1050°C in six stages and allowed to cool overnight to have the same rate of cooling for controlling microstructure of the alloy samples. Six samples were prepared at each stage for both Wironot and Wirobond-C alloys [Figure 3]. First group was manufacturer's sample, (1 st casting), second group was melted twice and casted (2 nd casting), third group was melted five times and casted (5 th casting), fourth group was melted ten times and casted (10 th casting), fifth group was melted fifteen times and casted (15 th casting), sixth group was melted twenty times and casted (20 th casting). The sprues were cut off and subjected to sandblasting technique (Aluminum oxide) to remove all the unwanted impurities and electro polished to simulate dental appliances. Vickers hardness number (VHN) was measured using 500 g load at 15 sec loading time by using digital micro hardness tester (VHN Tester - Model - MIT- MMTX7A Matsuzawa Co Ltd, Japan).
Figure 3: Showing samples of hardness testing

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  Results Top


The values of tensile strength and VHN of Wironit and Wirobond-C at six different stages is tabulated and given in below tables [Table 1], [Table 2], [Table 3], [Table 4].
Table 1: (a and b) showing tensile test results and statistical analysis of Wironit

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Table 2: (a and b) showing tensile test results and statistical analysis of Wirobond-C

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Table 3: (a and b) showing Vickers hardness number test results and statistical analysis of Wironit

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Table 4: (a and b) showing Vickers hardness number test results and statistical analysis of Wirobond-C

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Tensile strength decreased from 850 to 777 MPa after 5 th recast and to 674MPa at the end of 20 th recasting procedure for Wironit samples. For Wirobond-C samples, tensile strength decreased from 720-678 MPa after 5 th recasting and further reduced to 534 MPa at the end of 20 th recasting procedure, the decrease was statistically significant (P < 0.001). Hardness decreased from 380 to 371 VHN at the end of 5 th recasting and further decreased to 335 VHN at the end of 20 th recasting for Wironit samples and for Wirobond-C samples there was an increase in the hardness from 328 VHN TO 396 VHN at the end of 5 th recasting and later decreased to 247 VHN after 20 th recasting procedure. The decrease in hardness was also statistically significant (P < 0.001). Friedman statistical test was used to assess the effect of recasting on the tensile strength and micro hardness (VHN) of the alloy samples (within the different stages of recasting). The tensile strength and micro hardness (VHN) of alloys at 2 nd recast, 5 th , 10 th , 15th, and 20 th recast were compared with the manufacturer sample using Wilcoxon signed ranks test and statistical significance was fixed at 0.05 levels.

There was a statistically significant decrease (P < 0.0001) in the tensile strength during different stages of recasting (i.e. from 1 st to 20 th recasting). Tensile strength of subsequent recast specimens also showed statistically significant decrease, when compared with the manufacturer's sample (P < 0.05). Similarly, there was a statistically significant decrease (P < 0.0001) in the micro hardness (VHN) during the different stages of recasting (1 st -20 th recast). The micro hardness of subsequent recast specimens also decreased statistically (P < 0.05), when compared with the manufacturer's sample.


  Discussion Top


Recycling has become one of the routine exercises of today's life. Effective and intelligent recycling will help the humanity to get relieved from hazards of dangerous mining process to certain extent, thereby minimizing nonscientific mining. Considering these factors, this study was done to recycle dental base metal casting alloys to reuse in dentistry and later in engineering field. As there was no similar study reported in the literature survey, direct comparison of results was not possible in this study; however, our results support the earlier observation of the following researchers. Pimenta et al. has stated that tensile strength and hardness increased after first casting and later started decreasing after subsequent recasting procedures. [9] Karina et al. studied the micro hardness of different base metal alloys and concluded that there was an increase in their hardness after successive recasting techniques. [10] . Slokar et al. also suggested that hardness remains unchanged after repeated recasting and the hardness can be enhanced by adding titanium to base metal alloys. [11] Peraire et al. observed that there are no drastic changes in the hardness and microstructure even after seven successive remelting and recasting of base metal alloys. [12]

In the present study, two Co-Cr, alloys Wironit and Wirobond-C, were melted and recasted several times (1 st -20 th recast) and analyzed for their tensile strength and micro hardness (VHN). Initially there was very little change in the tensile strength (up to 5 th recast) and then there was slight decrease in the tensile strength, which is negligible in terms of dental applications, since the tensile strength is much more than required for dental appliances. Hardness increased after first recast and later decreased in the subsequent recasting. The microstructural studies may reveal the reason for initial increase in the hardness. The micro hardness values of the tested alloys are in good agreement with the available commercial dental alloys. Considering only the mechanical properties of the above alloys, these alloys can be reused even up to 20 th recast, since the decrease in their mechanical properties is negligible. Many researchers have indirectly showed that there is biocompatible failure due to contamination of alloys and loss of low-fusing components during subsequent recasting procedures, hence a serious thinking was done to reuse the base metal dental casting alloys in other fields and a breakthrough existed by the way of redirecting these used alloys in to the field of engineering. The same composition alloys (Co-Cr alloys) are termed as super alloys in engineering field, because of their high strength and very high corrosion resistance and used as cutting tools, oil well drilling bits, dredging cutters, hot trimming dies, internal combustion engine valves. [13],[14],[15] These instruments do not require biocompatibility, since they are used in industries and hence the left over Co-Cr alloys after casting procedure from dentistry can be safely redirected to engineering field/industries.


  Conclusion Top


Base metal dental casting alloys (Co-Cr alloys) used in the present study showed that there is a slight decrease in the tensile strength and hardness after successive recasting procedures (1 st -20 th recast), but this will not have any effect on the dental applications. Mechanical properties are not only the prime factors to be considered for reusing these alloys, the most important factor is biocompatibility. Many researchers have showed that these types of alloys cannot be directly reused in dentistry because of biocompatibility failures occurring from the impurities and loss of some important low-fusing components during repeated casting procedures. Since huge amount of left over alloys (sprues and buttons) after casting procedure are dumped in dental clinics/dental laboratories, these wasted alloys can be passed on to engineering field/industries to fabricate cutting tools etc. This recycling procedure will bring back fifty percent of the revenue to dental field, resulting in reducing the treatment costs for the patients and environment can be protected, since mining can be reduced to some extent (Green dentistry). Although testing biocompatibility was not within the scope of this study, the authors are continuing the research in this direction to analyze the possible reusability of these recasted alloys in the manufacturing of dental cutting burs and instruments.

 
  References Top

1.Brantley WA. Phillips science of dental materials. Anasuvice KJ. 11 th ed. Amsterdam: Elsevier Publications; 2006. p. 103-5.  Back to cited text no. 1
    
2.Leinfelder KF. An evaluation of casting alloys used for restorative procedures. J Am Dent Assoc 1997;128:37-45.  Back to cited text no. 2
    
3.Asgar K. Casting metals in dentistry: Past - present - future. Adv Dent Res 1988;2:33-43.  Back to cited text no. 3
    
4.Mosleh I, Abdul-Gabbar F, Farghaly A. Castability evaluation and effect of recasting of ceramo-metal alloys. Egypt Dent J 1995;41:1357-62.  Back to cited text no. 4
    
5.Al-Hiyasat AS, Darmani H. The effects of recasting on the cytotoxicity of base metal alloys. J Prosthet Dent 2005;93: 158-63.  Back to cited text no. 5
    
6.Geurtsen W. Biocompatibility of dental casting alloys. Crit Rev Oral Biol Med 2002;13:71-84.  Back to cited text no. 6
    
7.Wataha JC. Biocompatibility of dental casting alloys: A review. J Prosthet Dent 2000;83:224-34.  Back to cited text no. 7
    
8.Ayad MF, Vermilyea SG, Rosenstiel SF. Corrosion behavior of as-received and previously cast high noble alloy. J Prosthet Dent 2008;100:35-40.  Back to cited text no. 8
    
9.Pimenta AR, Diniz MG, Paciornik S. Mechanical and microstructural properties of a nickel-chromium alloy after casting process. Revista Sul-Brasileira de Odontolgia 2012;9:17-24.  Back to cited text no. 9
    
10.Olivieri KA, Neisser MP, De Souza PC, Bottino MA. Mechanical properties and microstructural analysis of a Ni-Cr alloy cast under different temperatures. Braz J Oral Sci 2004;3:414-8.  Back to cited text no. 10
    
11.Slokar LJ, Matkovic T, Matkovic P. Micro structure and hardness of Co-Cr-Ti alloys for dental casting. Metallurgy JA 2004;43:273-7.  Back to cited text no. 11
    
12.Peraire M, Gomis M, Anglada JM, Bizar J, Salsench J, Gil FJ. Effect of recasting on the chemical composition, micro structure, micro hardness, and ion release of 3 dental castings alloys and titanium. Int J Prosthet 2007;20:286-8.  Back to cited text no. 12
    
13.Mishra AK, Hamby MA, Kaiser WB. Metallurgy, microstructure, chemistry and mechanical properties of a new grade of Cobalt-Chromium alloy before and after porouscoating, cobalt-base alloys for biomedical applications, ASTM STP 1365. In: 1 st ed. Disegi JA, Kennedy RL, Pilliar R, editors. American Society for Testing and Materials. West Conshohocken; 1999. p. 71-2.  Back to cited text no. 13
    
14.Khanna OP. Material science and metallurgy. Vol 6. New Delhi: Dhanpatrai Publications; 2004. p. 24-6.  Back to cited text no. 14
    
15.Thopegowda NB, Shenoy K, Shankarnarayana RK, Kukkila J, Vaddya SB, Gingipalli K. Recycling of materials used in dentistry with reference to its economical and environmental aspects. Int J Health Rehabil Sci 2013;2:140-5.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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