The Effect of Argon Inert Gas on the Laser Welding Quality of Co-Cr and Ni-Cr Base Metal Alloys ()
1. INTRODUCTION
The need for improved and functional prosthetic restorations was the primary reason for the development of dental alloys and appropriate welding materials that increase the strength of dental prostheses. This need for durable additives but also the requirement for low-cost alloys, led to the predominance of base metal alloys and in particular Co-Cr and Ni-Cr alloys. These alloys have improved properties such as modulus of elasticity, durability, corrosion resistance and low density. These are a few of the base metal alloy characteristics that lead to the manufacture of thinner, lighter and more durable dental appliances in comparison to gold alloys.
A factor that is required to be taken into consideration by the dental technicians during the manufacture of prosthetic restorations is the moment of casting. If care is not taken, there is a risk of overheating the alloy which can lead to casting failures, to the extent that these restorations cannot be inserted into the oral cavity [1 - 3].
In the event of casting mishandling, casting defects may occur (e.g., microporosity) posing a risk that the prosthetic restoration will break in a weak point, due to the accumulation of stresses and the repeated bending fatigue [4 , 5].
Therefore, key factors that influence the quality of such a prosthetic restoration are:
1) The chemical composition of the alloy
2) The physicochemical composition
3) The metallographic structure
4) The mechanical behaviour of the alloy determined by its mechanical properties
Defects in the structure of cast prostheses and stress concentration areas (porosity, foreign bodies, investment, etc.) are often observed. In addition, loading of the prosthetic restorations is conducted through cyclically repeated loads, causing fatigue and finally leading to their fracture. Fractures can also occur by an accident (e.g., drop and impact). In such instances, proper welding should be conducted, both in terms of strength and accuracy [6 - 11].
Many and important are the researches that have been done from time to time on the subject of the resistance of the welds of the basic dental alloys with laser [12 - 26]. In most researches, the specimens have been cut in the middle, the two pieces have been held in contact with the help of a special base and then the welding was done with penetration of the laser beam, in order to evaluate the quality (strength) of the welding. In a small number of researches, the beveling of the ends of the two pieces with the creation of an X-shaped flap has also been studied [27 - 39].
The purpose of this research was to study the effect of Argon inert gas on the laser welding quality of Co-Cr and Ni-Cr base metal alloys, which are widely used as Fixed Prosthodontics alloys in Dental Laboratories. Furthermore, the scope of the research was to compare the process of laser welding applied in the daily practice (separation of the prosthetic work, immobilization of the two pieces in the oral cavity, transfer on to a new cast, formation of two cones in contact on the side walls of the separation point using the welding material, agglomeration of the two cones with one or more laser attacks), with process used in the previous researches.
2. MATERIALS AND METHODS
2.1. Specimens’ Preparation
Thirty-six specimens were manufactured using a special machinable wax via the subtraction method from a wax disc using a special milling machine (CAD/CAM) (Figure 1). The thickness of each specimen was 3 mm and the total length 42 mm, according to the ISO 6892 specification. Two dental alloys were used to cast the wax specimens. A Co-Cr alloy (Auriloy N.P. Supreme, Aurium Research, San Diego, California, USA) and a Ni-Cr alloy (Auriloy NP Star, Aurium Research, San Diego, California, USA). The composition provided by the company for the Co-Cr alloy was Co 61%, Cr 27%, Mo 6%, Si 1%, C(x), W 5%, Mo 6%, Fe(x), Mn(x) while for the Ni-Cr was Ni 61.2%, Cr 25.8%, Mo 11%, Si% 1.5%, Al(x), Mn(x). The laser-welded group specimens were cut and welded using a special jig. Two laser welding wires were used, one for the Co-Cr alloy (LWNPCo, Aurium Research, San Diego, California, USA) and one for the Ni-Cr alloy (LWNPNi, Aurium Research, San Diego, California, USA), with compositions similar to those of the parent alloys.
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Figure 1. (a) Illustration of the specimen, from https://regbar.com/wp-content/uploads/2019/09/ISO-6892-1-2009.pdf (b) Wax specimen manufactured via CAD/CAM (subtractive manufacturing).
2.2. Radiographic Evaluation
In order to evaluate the degree of porosity of the cast alloys, all specimens were examined using X-rays (General Medical Merate SpA CPI-CMP20, Seriate, Bergamo, Italy). The specimens were placed on a digital X-ray plate (Fuji Film FCR Capsula X Digitizer, Fuji Film, Tokyo, Japan) and irradiated (90 kV, 71 mAs, with head distance from the plate 66 cm).
The shape and size of the specimens did not allow the detection of small pores which were eventually detected and observed in the fractography.
2.3. Welding
The test specimens, after casting, were cut via a 0.5 mm thick cutting disc and welded to a specially designed and fabricated base for this purpose (Laser Star Riverside RI 02915, Orlando, USA). Τhe device settings were: peak power 260 V, pulse duration 3.5 ms pulse repetition 1.5 Hz and spot diameter 0.2 mm. Then, one half of the cut specimen was detached from the jig and a cone was formed in the center of its cross-section by laser welding. Similarly, the same cone was formed on the other part of the cut specimen. When the two pieces were placed back onto the jig, and the two cones formed in the cut area were in light contact, the laser welding of the specimen begun. After the initial welding of the cones and the formation of an X-shaped bevel, the remaining gaps were further filled by laser welding (Figure 2). After the laser welding, the specimens were lightly ground via a special grinding disc (4 mm thick) to smoothen the surface of the weld. During this process, a continuous rotational motion was utilized to maintain the cylindrical shape of the specimen.
2.4. Tensile Strength Testing
The specimens were tested on a Universal Testing machine (Instron6022, Norwood, USA) and the tensile strength and elongation were then calculated. The crosshead speed was set at 1.5 mm/min. The 80% of the specimens fractured in the welded area.
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Figure 2. The cut area and the formation of the cones are displayed. The corresponding laser welding wire was used to fill the gaps created.
2.5. Fractography
After the specimens were fractured, the fractured surface was evaluated to assess the morphology of the welded areas. With the utilisation of the Scanning Electron Microscope (JEOL JSM 6510 QLV, Peabody, USA) a fractographic analysis was performed on the welded areas.
2.6. Statistical Analysis
SPSS 15 for Windows (SPSS Inc., Chicago, USA) was used to statistically evaluate the results.
Due to the low number of specimens (eight), nonparametric tests were applied for statistical analysis, using standard statistical software (SPSS version 15.0 for Windows, SPSS, Chicago, Illinois, USA). The Kruskal-Wallis test was used to assess variations of the means among the 6 different groups. The Wilcoxon test was used to determine the significance of differences in the stress magnitudes between all possible pairs of notch conditions, with a probability cut-off of P = 0.05.
In order to better document this research finding, mainly from a statistical point of view, the statistical evaluation of a multifactorial experiment without repetition of measurements (that means on different specimens) (factorial experiment) was applied. The statistical method simultaneously examined the effect of two factors, the presence or absence of inert gas and the material utilised (type of alloy). These two factors were examined both independently (influence of only the material or only the presence of inert gas) but also in combination (effect of both factors simultaneously.
3. RESULTS
3.1. Descriptive Statistics
Table 1 shows the fracture values of the specimens for each group.
Table 2 lists the tensile strength values of the specimens for each group.
Table 3 shows the complete data of the descriptive statistics for all groups as they were calculated by the statistical program SPSS Statistics.
Table 4 presents the results for both the Kruskall-Wallis statistical test and the Mann-Whitney statistical test in all possible pairwise comparisons. The differences that are statistically significant (P < 0.05) are marked in bold.
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Table 1. Fracture values of all groups (in Newtons).
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Table 2. The calculated tensile strength values for all specimens in MPa.
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Table 3. Data of the descriptive statistics for all groups.
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Table 4. Statistical Evaluation of Results
Evaluation of the variance by Kruskall-Wallis P = 0.002. Evaluation of the difference in mean values between all possible pairs between groups according to Mann-Whitney.
3.2. Fractography
Fragmentation of selected fractured specimens post-welding is shown in Figure 3.
Ni-Cr with Argon: More plastic deformation is observed. A circular layering without homogenization is observed. In particular, the alloy is arranged in circular concentric regions which are not homogenized, which means they have voids which also have circular concentric orbits. In the center of the specimen there is good homogenization of the alloy (no voids present). As the layering develops (moving towards the outer layers), the homogenisation improves, with a reduction of the concentric gaps.
Ni-Cr without Argon: The fracture that occurred over the bonding area shows the characteristic circular layering of the alloy with concentric circles (presenting gaps between them in the same circular arrangement). Better homogenization of the alloy without voids and cracks is observed in the center of the specimen and in the periphery.
Cο-Cr with Argon: In the fracture area that took place in the welded area, there is a circular layering of the welded alloy not presenting good homogenization. In the center, it appears that the homogenization is of higher degree without voids and cracks, while some porosity is also observed. Similarly, there are areas of the circular layers—rings of the welded alloy, where penetration in a radial direction in between the layers is observed.
Cο-Cr without Argon: This specimen’s image shows a more brittle fracture (compared to the Ni-Cr alloy), while better homogenisation can also be seen. Traces of circular layering of the alloy (arrangement in circular concentric rings-layers) are visible, while no clear dendritic structure is observed.
4. DISCUSSION
The longevity and mechanical behaviour of the restorations, apart from the required proper design (avoidance of stress concentration areas), also depend on the structural continuity of the alloy after casting. As laboratory casting may present various defects, it is always possible to fracture due to the concentration of stresses and flexural fatigue that takes place during functional use [4 , 9 , 38]. In cases of fracture, the laser technology used for welding the fractured parts of the restoration is of utmost importance. It is very important to investigate the strength of laser welds to improve the longevity of welded restorations [4 , 9 , 38].
The radiographic examination that was performed showed macroscopic structural continuity of the specimens for all groups. This assessment was conducted because according to other research, several pores were observed in the main mass of the specimens, but also in the welded area [11 - 19]. In this research the specimens were made by the CAD/CAM method, using laminated wax, which were then invested, in order to increase the accuracy of dimensions. On radiographic examination, it appeared that there was no development of large pores in the specimens.
Regarding the tensile strength of the welded specimens in the present study, this was shown to be less than the tensile strength of the non-welded specimens, however this difference was not found to be statistically significant. This finding applies to both Co-Cr and Ni-Cr alloys. A review of the available to us literature shows that most researchers agree that non-welded specimens are more durable than laser welded specimens. The reason for this finding is attributed to the poor penetration of the laser beam, given that the specimens in these studies were cut and repositioned with their cut ends in contact for welding [12 , 15 , 32 , 35 , 36 , 38].
The largest variation between the resistance values was observed in the groups.
All studies support the fact that the final fracture strength decreases after welding. This finding is common to all studies regardless of differences in specimen design and load testing. These findings are attributed to a variety of parameters, e.g., pore development, incomplete welding penetration, melting during laser welding, welding parameters, experience of the technologist, etc. [13 - 21 , 34 - 39].
The presence of argon inert gas was found to cause a decrease in tensile strength when compared to the absence of argon inert gas, however this difference was not found to be statistically significant. In the available to us literature, it seems that opinions on the same subject matter differ, as some researchers consider the presence of inert gas to be beneficial (improving the tensile strength of the weld) while other researchers consider it not presenting any difference [12 , 14 , 38 , 39].
Using the multifactorial experiment without repetition of measurements (that means on different specimens) (factorial experiment) was applied and the simultaneous examination of the effect of two factors, the presence or absence of inert gas and the material utilised (type of alloy), was made. These two factors were examined both independently (influence of only the material or only the presence of inert gas) but also in combination (effect of both factors simultaneously). It has been shown that the factor of the material, as an independent factor, affects the tensile strength to a statistically significant degree, in contrast to the factor of the presence of inert gas which does not affect to a statistically significant degree. This is in agreement to other researchers arguing that the alloying agent (material) determines the bond strength of basic alloys by laser and not the presence of inert gas [12 , 14 , 38 , 39]. In addition, it has been shown that neither the combined effect of both of the above factors has an effect on the strength of the welds to a statistically significant degree.
However, the fractography revealed microporosity and microcracks in almost all specimens, a fact which is attributed to the nature of laser welding taking place in the laboratory, where apart from the quality of the laser welding device and the welding parameters set, the technologist’s skills also affect the final laser welding quality. It is proposed to change the welding parameters during the welding process after the initial welding of the cones to achieve better homogenization (melting) of the welding material towards the periphery.
5. CONCLUSIONS
The following conclusions emerged from this research study:
1) The parameters of the laser welding device used in this study led to satisfactory tensile strength of the laser welded specimens, as although having lower tensile strength than the non-welded specimens, they did not show a statistically significant difference.
2) The presence or absence of argon inert gas during laser welding did not show a statistically significant difference. Thus, the presence of argon inert gas does not affect the quality of the laser weld.
3) The material utilised to manufacture the specimens had a statistically significant effect on the tensile strength, as the Cr-Co alloy specimens had higher tensile strength than the Cr-Ni welded specimens.
4) The morphology of the circular stratification of the laser welding material generated voids in the welded area, but both their arrangement and the intermediate stratified bridges of welded material observed did not significantly affect the tensile strength of the specimens.
5) The process of laser welding applied in daily practice (separation of specimen, formation of two cones in contact, laser welding of the two cones, filling the gap via laser welding), is considered to be satisfactory.