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Joint strength of laser-welded titanium
dental materials Dental Materials 18 (2002) 143±148
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Joint strength of laser-welded titanium J. Liu, I. Watanabe*, K. Yoshida, M. Atsuta Department of Fixed Prosthodontics, School of Dentistry, Nagasaki University, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan. Received 25 October 2000; accepted 6 April 2001
Abstract Objective: The objective of this study was to examine the joint strength of titanium laser-welding using several levels of laser output energy [current (A)]. Methods: Cast titanium plates (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were prepared and perpendicularly cut at the center of the plate. After the cut halves were ®xed in a jig, they were laser-welded using a Nd: YAG laser at several levels of output energy in increments of 30 A from 180 to 300 A. The penetration depths of laser to titanium were measured under various conditions for output energy, pulse duration, and spot diameter to determine the appropriate conditions for these parameters. Based on the correlation between the results obtained for penetration depth and the size of the specimens (thickness: 0.5 and 1.0 mm, width: 3.0 mm), the pulse duration and spot diameter employed in this study were 10 ms and 1.0 mm, respectively. Three laser pulses (spot diameter: 1.0 mm) were applied from one side to weld the entire joint width (3.0 mm) of the specimens. Uncut specimens served as the non-welded control specimens. Tensile testing was conducted at a crosshead speed of 2 mm/min and a gage length of 10 mm. The breaking force (N) was recorded, and the data (n 5) were statistically analyzed. Results: For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents of 240, 270, and 300 A were not statistically (P . 0.05) different from the non-welded control specimens. There were no signi®cant differences in breaking force among the 1.0 mm thick specimens laser-welded at currents of 270 and 300 A, and the non-welded control specimens. Signi®cance: Under appropriate conditions, joint strengths similar to the strength of the non-welded parent metal were achieved. q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Laser welding; Joint strength; Penetration depth
1. Introduction Titanium is one of the most advantageous metals used for dental restorations because of its excellent corrosion resistance and biocompatibility [1±3]. The most common means for joining metal in dentistry is soldering. Conventional methods for dental soldering cannot be used for titanium because of its high melting point and extremely high reactivity with ambient elements at high temperatures. Titanium can be cast or soldered only in vacuum or an argon environment. One of the methods employed for soldering titanium is infrared soldering [4]. This method concentrates infrared rays on the area (focus: approx 10 mm dia.) of soldering; the solder and parent metal are quickly heated, and the process is accomplished in a short time. However, when titanium alloy solders with low fusion points, such as Ti±Ni, Ti±Cu and Ti±Ni±Cu, are employed to solder pure titanium, the * Corresponding author. Tel.: 181-95-849-7689; fax: 181-95-849-7688. E-mail address: [email protected] (I. Watanabe).
corrosion resistance may be reduced because of hazardous elements in the solder and galvanic corrosion between different types of metals. Another method for joining titanium is laser welding. The use of laser welding in dentistry has increased during the past decade [4±14]. Lasers are well known as devices that amplify light by stimulated emission of radiation. To weld dental alloys, crystals of yttrium, aluminum and garnet (YAG) doped with neodymium (Nd) are mainly used to emit laser beams (Nd: YAG laser). Since laser energy can be concentrated on a small area, there are fewer effects of heating and oxidation on the area surrounding the spot to be welded [9]. It is easy to weld dental prostheses using this method since no further materials, such as investment material or a gas torches, are needed, as is the case for conventional dental soldering. The advantages of laser welding are that the parent metals can be welded without solder, and also the parent metal can be used as solder, if necessary. The corrosion resistance will not be reduced because by using the same metal for the solder as for the
0109-5641/02/$22.00 + 0.00 q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(01)00033-1
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J. Liu et al. / Dental Materials 18 (2002) 143±148
parent metal, the mechanical strength of the weld joint will not suffer. The factors affecting the mechanical strength of joints welded by laser welding are the types of metals welded, and the wave length, peak pulse power, pulse energy, output energy, pulse duration, pulse frequency and spot diameter of the laser. In most laser welding machines used for dentistry, the output energy (current or voltage), pulse duration and spot diameter of the laser can be adjusted, and the combinations of these three variables change the penetration depth of the laser into the metal in the area being welded. The penetration depth by laser is different among the parent metals used for dental restorations because the rate of laser beam absorption, thermal conductivity and melting point are different in each metal. In general, it can be said that the greater the rate of laser beam absorption and the lower the thermal conductivity, the greater the penetration depth to each metal. Among dental alloys, base metals (Co, Cr, Ni, Mo and Ti) have a greater rate of laser beam absorption and lower thermal conductivity compared to noble metals (Au, Ag, Pt and Pd) and therefore, are advantageous for laser welding [13]. Titanium in particular has a very low thermal conductivity (0.17 W cm 21´deg 21), which is approximately one twentieth that for Ag (3.97 W cm 21´deg 21) and Au (2.97 W cm 21´deg 21) [13]. Therefore, it is assumed that to weld titanium restorations, laser welding is the most advantageous method. The purpose of this study was to examine the joint strength of titanium laser-welded at several levels of laser output energy. To determine appropriate conditions for pulse duration and spot diameter, the penetration depth to titanium by laser was investigated at different settings for output energy, pulse duration and spot diameter. 2. Materials and methods Two types of plastic plate patterns (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were cast from commercially pure titanium (CP Ti) (JIS Type II, Selec Co., Osaka, Japan). The patterns were invested in mold rings with a magnesia-based investment material (Selevest CB, Selec Co., Osaka, Japan). The molds were allowed to bench-set at room temperature for 60 min and were then placed in a burn-out furnace. The burn-out schedule before casting followed the manufacturer's instructions. The CP Ti was cast into molds using a centrifugal casting machine (Ticast Super R, Selec, Osaka, Japan). After casting, the molds were bench-cooled to room temperature. Each cast specimen was then retrieved from the investment, sandblasted with 50 mm alumina powder and ultrasonically cleaned with acetone for 5 min. All the cast plates were examined through radiography with a dental unit (Coronis 90, Asahi Roentgen Inc., Ltd, Kyoto, Japan) to determine whether there was any noticeable internal porosity. The 70 kvp X-ray source (current: 10 mA) was positioned perpendicular to the ®lm at a distance of 50 cm
from the specimen. If there was any porosity in the cast plate, it was excluded. To determine the conditions for pulse duration and spot diameter, cast titanium blocks (5.0 £ 3.0 £ 30.0 mm 3) were prepared, and their surfaces were polished with No. 600 SiC paper. Two cast blocks were ®xed in a jig with the 5.0 £ 30.0 mm 2 surfaces facing each other. They were then laser-welded at their interface. The laser welding apparatus (TLL7000, Tanaka Laser Co., Tokyo, Japan) used in this study utilizes yttrium, aluminum and garnet (YAG) crystals doped with neodymium (Nd) (Nd: YAG laser). The focus of the laser was set at the interface of the two blocks, and the laser was aimed perpendicular to the line. A laser pulse was applied on only one side of the blocks so that the two blocks could easily be broken apart and separated with pliers. The penetration depth of the laser into the titanium was then measured on the broken surfaces using a close-up photography system (M System, Olympus, Tokyo, Japan) and computer graphics. The conditions used were: current of 160±300 A, pulse duration of 1±13 ms, and spot diameter of 0.4±1.8 mm. The cast plates (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were perpendicularly cut at the center of the plate. After the cut surfaces were polished with No. 600 SiC paper, the cut halves were ®xed in a jig and weld-bonded using the laser welding machine. Based on the correlation between the results obtained for penetration depth and the size (thickness: 0.5 and 1.0 mm, width: 3.0 mm) of the specimens, the pulse duration and the spot diameter employed were 10 ms and 1.0 mm, respectively. The output energy (current: A) used ranges from 180 to 300 A in increments of 30 A. Three laser pulses (spot diam: 1.0 mm for each shot) were applied from one side to cover the joint width (3.0 mm) of the specimens. An uncut cast specimen was also reserved as the nonwelded control specimen. Tensile testing was conducted with a universal testing machine (AGS-10 kNG, Shimadzu, Kyoto, Japan) at a crosshead speed of 2 mm/min and a gage length of 10 mm. The breaking force (Bf: N) was recorded when the specimen fractured, and the means and standard deviations were calculated (n 5) using ANOVA and Tukey's Post Hoc test at the P , 0.05 level of signi®cance. After tensile testing, the fracture surface was observed using a scanning electron microscope (S-3500N, Hitachi, Tokyo, Japan). The area of the fracture was also examined under a magni®cation of 32 £ to correlate breaking force with the type of fracture. The type of fracture was classi®ed into three categories: fracture within the welded spot (A), fracture within the parent metal (C), and fracture between the welded spot and the parent metal (B). 3. Results Fig. 1 presents the penetration depths resulting from a pulse duration of 10 ms at different levels of output energy (current: A). Increasing the current increased the penetration
J. Liu et al. / Dental Materials 18 (2002) 143±148
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Fig. 1. Penetration depth measured at a pulse duration of 10 ms.
Fig. 2. Penetration depth measured at a current of 220 A.
depth of laser to titanium. This was more remarkable when the spot diameter was less than 1.2 mm (0.4±1.0 mm). The penetration depth varied from 0.52 mm (160 A) to 2.27 mm (300 A) at a spot diameter of 1.0 mm. The penetration depths at a current of 220 A are shown in Fig. 2. There were no notable differences in penetration depth among pulse durations except for the pulse duration of 1 ms, which had the lowest value. This was more remarkable when the spot diameter is less than 1.2 mm. Results of tensile testing of the specimens laser-welded at a pulse duration of 10 ms and a spot diameter of 1.0 mm are
summarized in Table 1, and the data are depicted in Fig. 3. The solid lines (Fig. 3) indicate the breaking force for the non-welded control specimens (0.5 and 1.0 mm). For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents of 240, 270, and 300 A was not statistically (P . 0.05) different from the non-welded control specimens. There were no signi®cant differences in breaking force among the 1.0 mm thick specimens laser-welded at currents of 270 and 300 A, and the nonwelded control specimen. Table 2 shows the type of fracture for the welded specimens. The 0.5 mm thick specimens
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Table 1 Breaking force of the laser-welded specimens with various conditions Thickness (mm)
Breaking force (N) Control
Current (A) 300 a
0.5 1.0 a
270 a
757 (64) 1532 (121) a
804 (59) 1617 (60) a
240 a
794 (69) 1504 (83) a
a
748 (73) 780 (182)
210
18
503 (136) 251 (62)
270 (103) 219 (63)
Indicates no statistical difference (P . 0.05) in the same row.
Fig. 3. Breaking force of the specimens.
laser-welded at less than 240 A and the 1.0 mm thick specimens welded at less than 270 A fractured within the welded spot. Several of both the 0.5 or 1.0 mm thick specimens laser-welded at a higher current, fractured between the welded spot and the parent metal or within the parent metal. Representative fracture surfaces observed by SEM are shown in Fig. 4. The upper region of Fig. 4(A) is the fracture surface of the welded titanium, whereas the lower region shows the non-welded surface polished with SiC paper. There are several pores (indicated by arrows) in the welded region of the laser-welded specimens [Fig. 4(B)]. Fig. 4(C) is a magni®cation of the welded region in Fig. 4(B) where ®ne-grained acicular crystals can be seen. Fig. 4(D) shows Table 2 Mode of fracture failure (A: fracture within the welded spot, B: fracture between welded spot and parent metal, C: fracture within the parent metal) Thickness (mm) Current (A)
0.5 1.0
180
210
240
270
300
AAAAA AAAAA
AAAAA AAAAA
AABBB AAAAA
AABBC AAABC
AAABB AABCC
the fracture surface of the control specimen (1.0 mm thick). The reaction layers are indicated by arrows in the outermost edge (cast surface). Dimple fractures are found in the inner region of both the 0.5 mm [Fig. 4(E)] and the 1.0 mm [Fig. 4(F)] thick specimens. The dimple size in the 1.0 mm thick specimen was larger compared to the 0.5 mm thick specimen [Fig. 4(E) and (F)]. 4. Discussion The joint strength of laser-welded restorations is affected by the metals used and the conditions of the laser welding device. In this study, the conditions for output energy (current), pulse duration and spot diameter of the laser welding machine were varied, and the penetration depth of the laser to titanium varied with each combination of the three variables. When the pulse duration was ®xed at 10 ms (Fig. 1), increasing the output energy (current) increased the penetration depth of the laser to titanium. This was particularly remarkable when the spot diameter was less than 1.2 mm (0.4±1.0 mm). This ®nding indicates that decreasing the spot diameter to less than 1.2 mm and
J. Liu et al. / Dental Materials 18 (2002) 143±148
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Fig. 4. SEM photographs after tensile test. (A) Fracture surface of the 0.5 mm specimen laser-welded at a current of 210 A. (B) Fracture surface of the 0.5 mm specimen laser-welded at a current of 180 A. (C) Fracture surface of the 0.5 mm specimen laser-welded at a current of 180 A. (D) Fracture surface of the control specimen (1.0 mm). (E) Fracture surface of the control specimen (0.5 mm). (F) Fracture surface of the control specimen (1.0 mm).
increasing the current causes deeper penetration by the laser (Fig. 1). When the current was ®xed at 220 A (Fig. 2), there were no remarkable differences in penetration depth among pulse durations (4, 7, 10 and 13 ms), except for a pulse duration of 1 ms that showed the lowest value, especially when the spot diameter was less than 1.2 mm. In fact, the penetration depth/spot diameter curves with different currents were similar among the pulse durations of 4, 7, 10 and 13 ms. Watanabe et al. [14] investigated the penetration depth of laser to gold alloy using the same laser welding machine and compared the data with those for titanium. This tendency for no signi®cant differences in penetration depth among the pulse durations (except for 1 ms which showed the lowest values for titanium) was similar to the results for gold alloy, but the penetration depth of the laser to titanium was signi®cantly (P , 0.05) greater compared to that of the gold alloy. This difference in penetration depth occurs because the rate of laser beam absorption and thermal conductivity are different in these metals. Titanum has a lower thermal conductivity value (0.17 W cm 21´deg 21), which is approximately one seventeenth of that for Au (2.97 W cm 21´deg 21). Titanium also has a greater rate (0.4%) of laser beam absorption compared to that of gold alloy (0.03%) [13]. The high rate of laser beam absorption and low thermal conductivity of titanium make it easy for the laser to penetrate into this metal. In this study, the output energy (current) of the laser welding apparatus was varied and the joint strengths of the cast titanium specimens of different thicknesses (0.5 or 1.0 mm) were investigated at a pulse duration of 10 ms and a spot diameter of 1.0 mm. For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents higher than 240 A was not statistically (P . 0.05) different from the non-welded control specimens. The fracture surface of the specimen
laser-welded at a current of 210 A [Fig. 4(A)] indicates that even though there are some non-welded regions polished with No. 600 SiC paper, the laser almost reached the other side of the 0.5 mm thick specimen. This is because the shape of the laser-welded region is a campanulate shape [Fig. 4(A)]. If the entire area of the 0.5 £ 3.0 mm surface were welded by laser, it would be necessary to increase the current to more than 210 A. When the laser is applied to the titanium surface at a pulse duration of 10 ms and a spot diameter of 1.0 mm (Fig. 2), the penetration depth into the titanium laser-welded at a current of 240 A is approximately 1.0 mm. A current of 240 A is thought to be strong enough to weld the whole area of the 0.5 £ 3.0 mm surface because of its high penetration depth. By welding this entire area, the joint strength increased and resulted in a breaking force similar to the non-welded control specimens. As for the 1.0 mm thick specimens, the specimens laser-welded at a current greater than 270 A showed breaking force values similar to the non-welded control specimens. Therefore, it is thought that a current of 270 A penetrates deep [between 1.2 mm (260 A) and 1.8 mm (280 A)] enough to weld all the 1.0 £ 3.0 mm surface. The reason why all the specimens laserwelded at a current less than 210 A (for the 0.5 mm thick specimens) and less than 270 A (for the 1.0 mm thick specimens) fractured within the welded spot is that the specimen surface was not completely welded. Thus, under suitable conditions, joint strengths similar to those for the non-welded parent metal could be achieved by laser welding in this study. This phenomenon was in agreement with previous studies [6,8]. In the SEM micrographs, many small pores were observed in the welded region [Fig. 4(B)]. These pores are attributed to the incorporation of gas such as argon and ambient air in the chamber of the laser welding machine. Titanium has high reactivity with ambient elements at high
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temperature. During the short time it takes to complete the welding processes, gas is incorporated into the molten titanium, making many small pores. The laser produces a hollow on the surface of the welded region in any metal. Many laser shots reduce the cross-sectional area of the welded region because of the hollows. In this study, three laser shots were applied to the surface from one side, creating the hollow areas. Despite the pores and reduced area of the welded region, joint strength comparable to the nonwelded parent metal was achieved under appropriate conditions. This is due to the strengthening of the welded region by the laser [Fig. 4(C)]. The fracture surface of the laserwelded region displayed ®ne-grained acicular crystals, which indicate hard and brittle fracture. Note that the control specimens fractured in a ductile manner (dimple fracture) [Fig. 4(E) and (F)]. In conventional dental soldering, the parent alloys are soldered with different types of alloy solder, which reduces the strength of the soldered joints and accounts for the failure at the joints of soldered restorations. Watanabe et al. [15] investigated the tensile strength of soldered gold alloy joints and found that the tensile strength of the soldered joint depended on the strength of the parent metal and solder. They indicated that the use of hard alloy solder in conjunction with hard parent metal produced soldered joints with great tensile strength. In laser welding, the same metal as parent alloy can be used for the solder, if necessary. Since joint strengths similar to the nonwelded parent metal could be achieved by laser welding, as observed in this study, titanium restorations laser-welded with pure titanium solder will be reliable in the oral environment. Acknowledgements This investigation was supported in part by a Grant-inAid for young scientists, (A)11771229 and (A)11771232, and a Grant-in-Aid for scienti®c research (B)11694293
from the Ministry of Education, Science, Sports and Culture, Tokyo, Japan.
References [1] Williams DF. Titanium and titanium alloy. In: Williams DF, editor. Biocompatibility of clinical implant material, Boca Raton: CRC Press, 1984. p. 44±7. [2] Kasemo B. Biocompatibility of titanium implants: surface science aspects. J Prosthet Dent 1983;49:832±7. [3] Kadoma T. Study on biocompatibility of titanium alloys. Kokubyo Gakkai Zasshi 1989;56:263±88. [4] Wang RR, Welsch GE. Joining titanium materials with tungsten inert gas, laser welding and infrared brazing. J Prosthet Dent 1995;74:521± 30. [5] Wang RR, Chang CT. Thermal modeling of laser welding for titanium dental restorations. J Prosthet Dent 1998;79:335±42. [6] Chai T, Chou CK. Mechanical properties of laser-welded cast titanium joints under different conditions. J Prosthet Dent 1998;79:477±83. [7] Yamagishi T, Ito M, Fujimura Y. Mechanical properties of laser welding of titanium in dentistry by pulsed Nd: YAG laser apparatus. J Prosthet Dent 1993;70:264±72. [8] Neo TK, Chai J, Gibert JL, Wozniak WT, Engelman MJ. Mechanical properties of titanium connectors. Int J Prosthodont 1996;9:379±93. [9] Roggensack M, Walter MH, Boning KW. Studies on laser- and plasma-welded titanium. Dent Mater 1993;9:104±7. [10] SjoÈgren G, Andersson M, Bergman M. Laser welding of titanium in dentistry. Acta Odontol Scand 1988;46:247±53. [11] Berg E, Wangner WC, Davik G, Dootz ER. Mechanical properties of laser-welded cast and wrought titanium. J Prosthet Dent 1995;74: 250±7. [12] Yamagishi T, Ito M, Oshida Y. Tensile strength and elongation of laser-welded titanium. J Dent Res 1993;72:13 (Abstr. No. 224). [13] Togaya T, Shinosaki T. Introduction to laser welding in dentistry. Quint Dent Tech 1999;24(6):42±51. [14] Watanabe I, Liu J, Miura E, Tanaka Y, Shiraishi T, Hisatsune K, Atsuta M. Penetration depth of cast titanium and gold alloy by laser. J Dent Res 2000;79:187 (Abstract No. 347). [15] Watanabe I, Watanabe E, Atsuta M, Okabe T. Tensile strength of soldered gold alloy joints. J Prosthet Dent 1997;78:260±6.
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Joint strength of laser-welded titanium J. Liu, I. Watanabe*, K. Yoshida, M. Atsuta Department of Fixed Prosthodontics, School of Dentistry, Nagasaki University, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan. Received 25 October 2000; accepted 6 April 2001
Abstract Objective: The objective of this study was to examine the joint strength of titanium laser-welding using several levels of laser output energy [current (A)]. Methods: Cast titanium plates (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were prepared and perpendicularly cut at the center of the plate. After the cut halves were ®xed in a jig, they were laser-welded using a Nd: YAG laser at several levels of output energy in increments of 30 A from 180 to 300 A. The penetration depths of laser to titanium were measured under various conditions for output energy, pulse duration, and spot diameter to determine the appropriate conditions for these parameters. Based on the correlation between the results obtained for penetration depth and the size of the specimens (thickness: 0.5 and 1.0 mm, width: 3.0 mm), the pulse duration and spot diameter employed in this study were 10 ms and 1.0 mm, respectively. Three laser pulses (spot diameter: 1.0 mm) were applied from one side to weld the entire joint width (3.0 mm) of the specimens. Uncut specimens served as the non-welded control specimens. Tensile testing was conducted at a crosshead speed of 2 mm/min and a gage length of 10 mm. The breaking force (N) was recorded, and the data (n 5) were statistically analyzed. Results: For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents of 240, 270, and 300 A were not statistically (P . 0.05) different from the non-welded control specimens. There were no signi®cant differences in breaking force among the 1.0 mm thick specimens laser-welded at currents of 270 and 300 A, and the non-welded control specimens. Signi®cance: Under appropriate conditions, joint strengths similar to the strength of the non-welded parent metal were achieved. q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Laser welding; Joint strength; Penetration depth
1. Introduction Titanium is one of the most advantageous metals used for dental restorations because of its excellent corrosion resistance and biocompatibility [1±3]. The most common means for joining metal in dentistry is soldering. Conventional methods for dental soldering cannot be used for titanium because of its high melting point and extremely high reactivity with ambient elements at high temperatures. Titanium can be cast or soldered only in vacuum or an argon environment. One of the methods employed for soldering titanium is infrared soldering [4]. This method concentrates infrared rays on the area (focus: approx 10 mm dia.) of soldering; the solder and parent metal are quickly heated, and the process is accomplished in a short time. However, when titanium alloy solders with low fusion points, such as Ti±Ni, Ti±Cu and Ti±Ni±Cu, are employed to solder pure titanium, the * Corresponding author. Tel.: 181-95-849-7689; fax: 181-95-849-7688. E-mail address: [email protected] (I. Watanabe).
corrosion resistance may be reduced because of hazardous elements in the solder and galvanic corrosion between different types of metals. Another method for joining titanium is laser welding. The use of laser welding in dentistry has increased during the past decade [4±14]. Lasers are well known as devices that amplify light by stimulated emission of radiation. To weld dental alloys, crystals of yttrium, aluminum and garnet (YAG) doped with neodymium (Nd) are mainly used to emit laser beams (Nd: YAG laser). Since laser energy can be concentrated on a small area, there are fewer effects of heating and oxidation on the area surrounding the spot to be welded [9]. It is easy to weld dental prostheses using this method since no further materials, such as investment material or a gas torches, are needed, as is the case for conventional dental soldering. The advantages of laser welding are that the parent metals can be welded without solder, and also the parent metal can be used as solder, if necessary. The corrosion resistance will not be reduced because by using the same metal for the solder as for the
0109-5641/02/$22.00 + 0.00 q 2002 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(01)00033-1
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J. Liu et al. / Dental Materials 18 (2002) 143±148
parent metal, the mechanical strength of the weld joint will not suffer. The factors affecting the mechanical strength of joints welded by laser welding are the types of metals welded, and the wave length, peak pulse power, pulse energy, output energy, pulse duration, pulse frequency and spot diameter of the laser. In most laser welding machines used for dentistry, the output energy (current or voltage), pulse duration and spot diameter of the laser can be adjusted, and the combinations of these three variables change the penetration depth of the laser into the metal in the area being welded. The penetration depth by laser is different among the parent metals used for dental restorations because the rate of laser beam absorption, thermal conductivity and melting point are different in each metal. In general, it can be said that the greater the rate of laser beam absorption and the lower the thermal conductivity, the greater the penetration depth to each metal. Among dental alloys, base metals (Co, Cr, Ni, Mo and Ti) have a greater rate of laser beam absorption and lower thermal conductivity compared to noble metals (Au, Ag, Pt and Pd) and therefore, are advantageous for laser welding [13]. Titanium in particular has a very low thermal conductivity (0.17 W cm 21´deg 21), which is approximately one twentieth that for Ag (3.97 W cm 21´deg 21) and Au (2.97 W cm 21´deg 21) [13]. Therefore, it is assumed that to weld titanium restorations, laser welding is the most advantageous method. The purpose of this study was to examine the joint strength of titanium laser-welded at several levels of laser output energy. To determine appropriate conditions for pulse duration and spot diameter, the penetration depth to titanium by laser was investigated at different settings for output energy, pulse duration and spot diameter. 2. Materials and methods Two types of plastic plate patterns (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were cast from commercially pure titanium (CP Ti) (JIS Type II, Selec Co., Osaka, Japan). The patterns were invested in mold rings with a magnesia-based investment material (Selevest CB, Selec Co., Osaka, Japan). The molds were allowed to bench-set at room temperature for 60 min and were then placed in a burn-out furnace. The burn-out schedule before casting followed the manufacturer's instructions. The CP Ti was cast into molds using a centrifugal casting machine (Ticast Super R, Selec, Osaka, Japan). After casting, the molds were bench-cooled to room temperature. Each cast specimen was then retrieved from the investment, sandblasted with 50 mm alumina powder and ultrasonically cleaned with acetone for 5 min. All the cast plates were examined through radiography with a dental unit (Coronis 90, Asahi Roentgen Inc., Ltd, Kyoto, Japan) to determine whether there was any noticeable internal porosity. The 70 kvp X-ray source (current: 10 mA) was positioned perpendicular to the ®lm at a distance of 50 cm
from the specimen. If there was any porosity in the cast plate, it was excluded. To determine the conditions for pulse duration and spot diameter, cast titanium blocks (5.0 £ 3.0 £ 30.0 mm 3) were prepared, and their surfaces were polished with No. 600 SiC paper. Two cast blocks were ®xed in a jig with the 5.0 £ 30.0 mm 2 surfaces facing each other. They were then laser-welded at their interface. The laser welding apparatus (TLL7000, Tanaka Laser Co., Tokyo, Japan) used in this study utilizes yttrium, aluminum and garnet (YAG) crystals doped with neodymium (Nd) (Nd: YAG laser). The focus of the laser was set at the interface of the two blocks, and the laser was aimed perpendicular to the line. A laser pulse was applied on only one side of the blocks so that the two blocks could easily be broken apart and separated with pliers. The penetration depth of the laser into the titanium was then measured on the broken surfaces using a close-up photography system (M System, Olympus, Tokyo, Japan) and computer graphics. The conditions used were: current of 160±300 A, pulse duration of 1±13 ms, and spot diameter of 0.4±1.8 mm. The cast plates (0.5 £ 3.0 £ 40 and 1.0 £ 3.0 £ 40 mm 3) were perpendicularly cut at the center of the plate. After the cut surfaces were polished with No. 600 SiC paper, the cut halves were ®xed in a jig and weld-bonded using the laser welding machine. Based on the correlation between the results obtained for penetration depth and the size (thickness: 0.5 and 1.0 mm, width: 3.0 mm) of the specimens, the pulse duration and the spot diameter employed were 10 ms and 1.0 mm, respectively. The output energy (current: A) used ranges from 180 to 300 A in increments of 30 A. Three laser pulses (spot diam: 1.0 mm for each shot) were applied from one side to cover the joint width (3.0 mm) of the specimens. An uncut cast specimen was also reserved as the nonwelded control specimen. Tensile testing was conducted with a universal testing machine (AGS-10 kNG, Shimadzu, Kyoto, Japan) at a crosshead speed of 2 mm/min and a gage length of 10 mm. The breaking force (Bf: N) was recorded when the specimen fractured, and the means and standard deviations were calculated (n 5) using ANOVA and Tukey's Post Hoc test at the P , 0.05 level of signi®cance. After tensile testing, the fracture surface was observed using a scanning electron microscope (S-3500N, Hitachi, Tokyo, Japan). The area of the fracture was also examined under a magni®cation of 32 £ to correlate breaking force with the type of fracture. The type of fracture was classi®ed into three categories: fracture within the welded spot (A), fracture within the parent metal (C), and fracture between the welded spot and the parent metal (B). 3. Results Fig. 1 presents the penetration depths resulting from a pulse duration of 10 ms at different levels of output energy (current: A). Increasing the current increased the penetration
J. Liu et al. / Dental Materials 18 (2002) 143±148
145
Fig. 1. Penetration depth measured at a pulse duration of 10 ms.
Fig. 2. Penetration depth measured at a current of 220 A.
depth of laser to titanium. This was more remarkable when the spot diameter was less than 1.2 mm (0.4±1.0 mm). The penetration depth varied from 0.52 mm (160 A) to 2.27 mm (300 A) at a spot diameter of 1.0 mm. The penetration depths at a current of 220 A are shown in Fig. 2. There were no notable differences in penetration depth among pulse durations except for the pulse duration of 1 ms, which had the lowest value. This was more remarkable when the spot diameter is less than 1.2 mm. Results of tensile testing of the specimens laser-welded at a pulse duration of 10 ms and a spot diameter of 1.0 mm are
summarized in Table 1, and the data are depicted in Fig. 3. The solid lines (Fig. 3) indicate the breaking force for the non-welded control specimens (0.5 and 1.0 mm). For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents of 240, 270, and 300 A was not statistically (P . 0.05) different from the non-welded control specimens. There were no signi®cant differences in breaking force among the 1.0 mm thick specimens laser-welded at currents of 270 and 300 A, and the nonwelded control specimen. Table 2 shows the type of fracture for the welded specimens. The 0.5 mm thick specimens
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Table 1 Breaking force of the laser-welded specimens with various conditions Thickness (mm)
Breaking force (N) Control
Current (A) 300 a
0.5 1.0 a
270 a
757 (64) 1532 (121) a
804 (59) 1617 (60) a
240 a
794 (69) 1504 (83) a
a
748 (73) 780 (182)
210
18
503 (136) 251 (62)
270 (103) 219 (63)
Indicates no statistical difference (P . 0.05) in the same row.
Fig. 3. Breaking force of the specimens.
laser-welded at less than 240 A and the 1.0 mm thick specimens welded at less than 270 A fractured within the welded spot. Several of both the 0.5 or 1.0 mm thick specimens laser-welded at a higher current, fractured between the welded spot and the parent metal or within the parent metal. Representative fracture surfaces observed by SEM are shown in Fig. 4. The upper region of Fig. 4(A) is the fracture surface of the welded titanium, whereas the lower region shows the non-welded surface polished with SiC paper. There are several pores (indicated by arrows) in the welded region of the laser-welded specimens [Fig. 4(B)]. Fig. 4(C) is a magni®cation of the welded region in Fig. 4(B) where ®ne-grained acicular crystals can be seen. Fig. 4(D) shows Table 2 Mode of fracture failure (A: fracture within the welded spot, B: fracture between welded spot and parent metal, C: fracture within the parent metal) Thickness (mm) Current (A)
0.5 1.0
180
210
240
270
300
AAAAA AAAAA
AAAAA AAAAA
AABBB AAAAA
AABBC AAABC
AAABB AABCC
the fracture surface of the control specimen (1.0 mm thick). The reaction layers are indicated by arrows in the outermost edge (cast surface). Dimple fractures are found in the inner region of both the 0.5 mm [Fig. 4(E)] and the 1.0 mm [Fig. 4(F)] thick specimens. The dimple size in the 1.0 mm thick specimen was larger compared to the 0.5 mm thick specimen [Fig. 4(E) and (F)]. 4. Discussion The joint strength of laser-welded restorations is affected by the metals used and the conditions of the laser welding device. In this study, the conditions for output energy (current), pulse duration and spot diameter of the laser welding machine were varied, and the penetration depth of the laser to titanium varied with each combination of the three variables. When the pulse duration was ®xed at 10 ms (Fig. 1), increasing the output energy (current) increased the penetration depth of the laser to titanium. This was particularly remarkable when the spot diameter was less than 1.2 mm (0.4±1.0 mm). This ®nding indicates that decreasing the spot diameter to less than 1.2 mm and
J. Liu et al. / Dental Materials 18 (2002) 143±148
147
Fig. 4. SEM photographs after tensile test. (A) Fracture surface of the 0.5 mm specimen laser-welded at a current of 210 A. (B) Fracture surface of the 0.5 mm specimen laser-welded at a current of 180 A. (C) Fracture surface of the 0.5 mm specimen laser-welded at a current of 180 A. (D) Fracture surface of the control specimen (1.0 mm). (E) Fracture surface of the control specimen (0.5 mm). (F) Fracture surface of the control specimen (1.0 mm).
increasing the current causes deeper penetration by the laser (Fig. 1). When the current was ®xed at 220 A (Fig. 2), there were no remarkable differences in penetration depth among pulse durations (4, 7, 10 and 13 ms), except for a pulse duration of 1 ms that showed the lowest value, especially when the spot diameter was less than 1.2 mm. In fact, the penetration depth/spot diameter curves with different currents were similar among the pulse durations of 4, 7, 10 and 13 ms. Watanabe et al. [14] investigated the penetration depth of laser to gold alloy using the same laser welding machine and compared the data with those for titanium. This tendency for no signi®cant differences in penetration depth among the pulse durations (except for 1 ms which showed the lowest values for titanium) was similar to the results for gold alloy, but the penetration depth of the laser to titanium was signi®cantly (P , 0.05) greater compared to that of the gold alloy. This difference in penetration depth occurs because the rate of laser beam absorption and thermal conductivity are different in these metals. Titanum has a lower thermal conductivity value (0.17 W cm 21´deg 21), which is approximately one seventeenth of that for Au (2.97 W cm 21´deg 21). Titanium also has a greater rate (0.4%) of laser beam absorption compared to that of gold alloy (0.03%) [13]. The high rate of laser beam absorption and low thermal conductivity of titanium make it easy for the laser to penetrate into this metal. In this study, the output energy (current) of the laser welding apparatus was varied and the joint strengths of the cast titanium specimens of different thicknesses (0.5 or 1.0 mm) were investigated at a pulse duration of 10 ms and a spot diameter of 1.0 mm. For the 0.5 mm thick specimens, the breaking force of the specimens laser-welded at currents higher than 240 A was not statistically (P . 0.05) different from the non-welded control specimens. The fracture surface of the specimen
laser-welded at a current of 210 A [Fig. 4(A)] indicates that even though there are some non-welded regions polished with No. 600 SiC paper, the laser almost reached the other side of the 0.5 mm thick specimen. This is because the shape of the laser-welded region is a campanulate shape [Fig. 4(A)]. If the entire area of the 0.5 £ 3.0 mm surface were welded by laser, it would be necessary to increase the current to more than 210 A. When the laser is applied to the titanium surface at a pulse duration of 10 ms and a spot diameter of 1.0 mm (Fig. 2), the penetration depth into the titanium laser-welded at a current of 240 A is approximately 1.0 mm. A current of 240 A is thought to be strong enough to weld the whole area of the 0.5 £ 3.0 mm surface because of its high penetration depth. By welding this entire area, the joint strength increased and resulted in a breaking force similar to the non-welded control specimens. As for the 1.0 mm thick specimens, the specimens laser-welded at a current greater than 270 A showed breaking force values similar to the non-welded control specimens. Therefore, it is thought that a current of 270 A penetrates deep [between 1.2 mm (260 A) and 1.8 mm (280 A)] enough to weld all the 1.0 £ 3.0 mm surface. The reason why all the specimens laserwelded at a current less than 210 A (for the 0.5 mm thick specimens) and less than 270 A (for the 1.0 mm thick specimens) fractured within the welded spot is that the specimen surface was not completely welded. Thus, under suitable conditions, joint strengths similar to those for the non-welded parent metal could be achieved by laser welding in this study. This phenomenon was in agreement with previous studies [6,8]. In the SEM micrographs, many small pores were observed in the welded region [Fig. 4(B)]. These pores are attributed to the incorporation of gas such as argon and ambient air in the chamber of the laser welding machine. Titanium has high reactivity with ambient elements at high
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temperature. During the short time it takes to complete the welding processes, gas is incorporated into the molten titanium, making many small pores. The laser produces a hollow on the surface of the welded region in any metal. Many laser shots reduce the cross-sectional area of the welded region because of the hollows. In this study, three laser shots were applied to the surface from one side, creating the hollow areas. Despite the pores and reduced area of the welded region, joint strength comparable to the nonwelded parent metal was achieved under appropriate conditions. This is due to the strengthening of the welded region by the laser [Fig. 4(C)]. The fracture surface of the laserwelded region displayed ®ne-grained acicular crystals, which indicate hard and brittle fracture. Note that the control specimens fractured in a ductile manner (dimple fracture) [Fig. 4(E) and (F)]. In conventional dental soldering, the parent alloys are soldered with different types of alloy solder, which reduces the strength of the soldered joints and accounts for the failure at the joints of soldered restorations. Watanabe et al. [15] investigated the tensile strength of soldered gold alloy joints and found that the tensile strength of the soldered joint depended on the strength of the parent metal and solder. They indicated that the use of hard alloy solder in conjunction with hard parent metal produced soldered joints with great tensile strength. In laser welding, the same metal as parent alloy can be used for the solder, if necessary. Since joint strengths similar to the nonwelded parent metal could be achieved by laser welding, as observed in this study, titanium restorations laser-welded with pure titanium solder will be reliable in the oral environment. Acknowledgements This investigation was supported in part by a Grant-inAid for young scientists, (A)11771229 and (A)11771232, and a Grant-in-Aid for scienti®c research (B)11694293
from the Ministry of Education, Science, Sports and Culture, Tokyo, Japan.
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