Abstract
Objectives
To describe the influence of pulse shaping on the behavior of a palladium-based dental alloy during laser welding and to show how its choice is effective to promote good weld quality.
Methods
Single spots, weld beads and welds with 80% overlapping were performed on Pd–Ag–Sn cast plates. A pulsed Nd:Yag laser was used with a specific welding procedure using all the possibilities for pulse-shaping: (1) the square pulse shape as the default setting, (2) a rising edge slope for gradual heating, (3) a falling edge slope to slow the cooling and (4) a combination of a rising and falling edges called bridge shape. The optimization of the pulse shape is supposed to enhance weldability and produce defect-free welds (cracks, pores…) Vickers microhardness measurements were made on cross sections of the welds.
Results
A correlation between laser welding parameters and microstructure evolution was found. Hot cracking and internal porosities were systematically detected when using rapid cooling. The presence of these types of defects was significantly reduced with the slow cooling of the molten pool. The best weld quality was obtained with the use of the bridge shape.
Significance
The use of a slow cooling ramp is the only way to significantly reduce the presence of typical defects within the welds for this Pd-based alloy studied.
1
Introduction
The laser welding technique requires the management of many parameters depending on the thermo physical properties of the material, the environment and the need for a shield, the laser and its process parameters (power, pulse duration, pulse repetition rate, focal and pulse shaping), the operator’s skill and the welding procedure. Most of the laser devices dedicated to prosthetic dentistry do not propose any other choice than using a square pulse shape as a default setting for which the maximum power is released within the material for all the set time. The improvement of a few recent pulsed Nd:Yag laser devices has proposed the possibility of using pulse shaping to get a better control on the thermal cycle during welding. With the use of laser pulse shaping, the energy density delivered into the material can be released in different ways during the pulse period: a slow or a fast preheating process, a fast or a slow cooling process or a combination of a slow preheating and a slow cooling, are possible. Those different options are known to be effective to prevent weld defects such as porosities and hot cracking occurring in sensitive materials as shown in the fields of aeronautic and automobile industries for aluminum alloys .
Palladium-based alloys are used in dentistry for metal–ceramic restorations, for implant supported prosthesis and may also be used for removable partial dentures. They show a fine casting accuracy, high ductility, good mechanical properties and good bonding to porcelain as well as good corrosion passivity in the oral environment and a lower cost compared to gold alloys . Conventional brazing methods are frequently preferred to laser welding for the assembly of such alloys . Papers relative to the laser welding of those kinds of alloys are sparse. However, authors all conclude in the poorer effectiveness of pulsed laser welding compared to brazing methods, when a high sensitivity to hot cracking exists as in the case of Palladium-based alloys . The aim of this paper was to investigate the benefit of using temporal pulse shaping to promote a good quality of welds thanks to suitable thermal conditions in a Pd–Ag–Sn dental alloy.
2
Material and methods
2.1
Laser apparatus
A “Mark Uno 65J” pulsed Nd:Yag laser manufactured by Orotig (Verona, Italy) was used. This laser offers the opportunity of choosing between five different pulse shapes summarized in Fig. 1 . The value of fluency, i.e. energy density (J/cm 2 ), given for each pulse shape has been calculated as a proportion of the square pulse shape. The square pulse shape is the default setting for which the maximum power is released for all the pulse duration and is available in all pulsed Nd:Yag lasers devices.
2.2
Material
The Callisto Implant 60 ® tested is a Pd–Ag–Sn dental alloy supplied by Ivoclar/Vivadent © Liechtenstein (composition given in Table 1 ). The small amounts of low melting points elements such as Sn, Zn and Ga are used to improve the molten alloy fluidity. Ir and Re are used to improve the grain refinement of the alloy. Au and Pt promote the corrosion resistance as well as the biocompatibility of the alloy. Considering the thermal properties of the main components, this alloy is supposed to present a good thermal conductivity (Pd = 0.72 W cm −1 deg −1 ; Ag = 3.97 W cm −1 deg −1 ) and thermal diffusivity (Pd = 0.24 cm 2 s −1 ; Ag = 1.74 cm 2 s −1 ) despite a low coefficient of absorption to the Nd:Yag wavelength (Pd = 0.26; Ag = 0.03) . The high reflectivity makes the Pd–Ag–Sn alloy absorb a low fraction of the incident radiation; the high thermal conductivity provokes a rapid heat transfer, avoiding the concentration of energy in the weld pool and finally the lower viscosity of the welding pool limits its expansion before the solidification. As a consequence, this alloy requires a high energy density applied onto the surface of the material.
| Alloy Implant Callisto60 ® | Pd | Ag | Sn | Au | Ir | Zn | Pt | Ga | Re |
|---|---|---|---|---|---|---|---|---|---|
| (in wt.%) | 60 | 25.2 | 7.5 | 2 | 2 | 1.6 | <1 | <1 | <1 |
2.3
Experimental procedure
Plates (20 mm × 10 mm × 1.5 mm) were cast thanks to the lost wax technique. Wax patterns were placed in a dental casting investment (FlexVest ® investment and mixing liquid, Ivoclar-Vivadent Co., Lichtenschtein) inside a ring. The invested ring was put into a preheating vacuum furnace (850 °C for 60 mn) and cast thanks to a blow torch (oxygen 1.4 bar and propane gas 0.7 bar). Castings were then divested, sandblasted with alumina powders (100 μm) and 2 bars pressure, ultrasonically cleaned and cut in halves with a low speed slicing machine under water spray. After sandblasting, the welding edges of the plates were polished using a 1200 grade SiC paper to get an efficient physical joint between them. The surface later degreased with acetone and subjected to the laser treatment. To get a better understanding of what occurs into the material during laser welding, different conditions were tested: single spots on plate, bead on plates and laser welds. For the two last conditions, 80% overlapping of the spots was completed. For each test conditions, three spots, two beads on plate and two welds were performed. Only one cross section was prepared for single spots whereas for the two others, three were polished.
The laser operating conditions were determined after initial preliminary experiments presented elsewhere : 1 kW for output power, 6 ms for pulse duration, 2 Hz for pulse frequency, 0.4 mm for focal spot size with the focal point situated at the specimen surface to obtain the highest power density. The specimens were welded under argon shielding gas. The fluency was estimated to 4800 J/cm 2 for the square pulse shape. The use of a smoothing pass after welding was systematically completed to give the weld an esthetic aspect. Only the focal spot size was increased up to 1.2 mm to smooth the weld on both sides.
2.4
Methods of investigation
The surface aspect of each laser spot and each bead on plates was observed with a 3D profilometric technique (WYKO ® NT1100 de Veeco © ): width, depth and roughness were measured . The X-ray tomography (Nanotom by Phoenix ® ) was used to quantify the volume of potential weld defects such as cavities detected inside single spots . SEM examinations (JEOL JSM-6360A) were performed on cross section weld for single spot, bead on plates and weld assembly. WDS analysis (wavelength-energy dispersive X-ray spectrometric analysis) was used for quantitative element analysis. Vickers microhardness measurements were performed with VMHT AUTO ® (LEICA © ) in the based material, in the heat affected zone and in the nugget area, at the mid-part of the weld, with a 100 g charge.
2
Material and methods
2.1
Laser apparatus
A “Mark Uno 65J” pulsed Nd:Yag laser manufactured by Orotig (Verona, Italy) was used. This laser offers the opportunity of choosing between five different pulse shapes summarized in Fig. 1 . The value of fluency, i.e. energy density (J/cm 2 ), given for each pulse shape has been calculated as a proportion of the square pulse shape. The square pulse shape is the default setting for which the maximum power is released for all the pulse duration and is available in all pulsed Nd:Yag lasers devices.
2.2
Material
The Callisto Implant 60 ® tested is a Pd–Ag–Sn dental alloy supplied by Ivoclar/Vivadent © Liechtenstein (composition given in Table 1 ). The small amounts of low melting points elements such as Sn, Zn and Ga are used to improve the molten alloy fluidity. Ir and Re are used to improve the grain refinement of the alloy. Au and Pt promote the corrosion resistance as well as the biocompatibility of the alloy. Considering the thermal properties of the main components, this alloy is supposed to present a good thermal conductivity (Pd = 0.72 W cm −1 deg −1 ; Ag = 3.97 W cm −1 deg −1 ) and thermal diffusivity (Pd = 0.24 cm 2 s −1 ; Ag = 1.74 cm 2 s −1 ) despite a low coefficient of absorption to the Nd:Yag wavelength (Pd = 0.26; Ag = 0.03) . The high reflectivity makes the Pd–Ag–Sn alloy absorb a low fraction of the incident radiation; the high thermal conductivity provokes a rapid heat transfer, avoiding the concentration of energy in the weld pool and finally the lower viscosity of the welding pool limits its expansion before the solidification. As a consequence, this alloy requires a high energy density applied onto the surface of the material.
| Alloy Implant Callisto60 ® | Pd | Ag | Sn | Au | Ir | Zn | Pt | Ga | Re |
|---|---|---|---|---|---|---|---|---|---|
| (in wt.%) | 60 | 25.2 | 7.5 | 2 | 2 | 1.6 | <1 | <1 | <1 |
2.3
Experimental procedure
Plates (20 mm × 10 mm × 1.5 mm) were cast thanks to the lost wax technique. Wax patterns were placed in a dental casting investment (FlexVest ® investment and mixing liquid, Ivoclar-Vivadent Co., Lichtenschtein) inside a ring. The invested ring was put into a preheating vacuum furnace (850 °C for 60 mn) and cast thanks to a blow torch (oxygen 1.4 bar and propane gas 0.7 bar). Castings were then divested, sandblasted with alumina powders (100 μm) and 2 bars pressure, ultrasonically cleaned and cut in halves with a low speed slicing machine under water spray. After sandblasting, the welding edges of the plates were polished using a 1200 grade SiC paper to get an efficient physical joint between them. The surface later degreased with acetone and subjected to the laser treatment. To get a better understanding of what occurs into the material during laser welding, different conditions were tested: single spots on plate, bead on plates and laser welds. For the two last conditions, 80% overlapping of the spots was completed. For each test conditions, three spots, two beads on plate and two welds were performed. Only one cross section was prepared for single spots whereas for the two others, three were polished.
The laser operating conditions were determined after initial preliminary experiments presented elsewhere : 1 kW for output power, 6 ms for pulse duration, 2 Hz for pulse frequency, 0.4 mm for focal spot size with the focal point situated at the specimen surface to obtain the highest power density. The specimens were welded under argon shielding gas. The fluency was estimated to 4800 J/cm 2 for the square pulse shape. The use of a smoothing pass after welding was systematically completed to give the weld an esthetic aspect. Only the focal spot size was increased up to 1.2 mm to smooth the weld on both sides.
2.4
Methods of investigation
The surface aspect of each laser spot and each bead on plates was observed with a 3D profilometric technique (WYKO ® NT1100 de Veeco © ): width, depth and roughness were measured . The X-ray tomography (Nanotom by Phoenix ® ) was used to quantify the volume of potential weld defects such as cavities detected inside single spots . SEM examinations (JEOL JSM-6360A) were performed on cross section weld for single spot, bead on plates and weld assembly. WDS analysis (wavelength-energy dispersive X-ray spectrometric analysis) was used for quantitative element analysis. Vickers microhardness measurements were performed with VMHT AUTO ® (LEICA © ) in the based material, in the heat affected zone and in the nugget area, at the mid-part of the weld, with a 100 g charge.
3
Results
To determine the influence of pulse shaping on the material behavior, different criteria were selected. The first one was the surface aspect. The second criterion was the welding depth. The last one was the absence of defects inside the welds.
3.1
Effect of pulse shaping on the surface aspect of the welds
It’s important to avoid in each spot, the eventual presence of surface defects such as pit crater, undercut or crack produced by the laser impact. Those defects could be the starting point of corrosion due to saliva infiltration in the joint and could probably give weld brittleness. The morphology of each spot was studied before and after smoothing. The surface of the spot showed an undercut around the outline of the spot with the square pulse shape (A) as a result of the high fluency released into the material ( Fig. 2 a) . This undercut was not observed for the (E) pulse shape due to its low fluency. A curved spot was systematically observed with the choice of the square shape (A) and the ramp up slope shape B ( Fig. 2 b). The smoothest spots were observed with the ramp down slope shape (C) and the bridge shape (D). Specific defects were reliable to the human factor such as a crater resulting from inappropriate settings of the welding parameters ( Fig. 2 c). The R sk value (skewness) provides information about surface roughness measuring the asymmetry of the profile about the mean plane, and is calculated using the following formula:
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