- Email: [email protected]
Mechanical properties of laser welds of titanium in dentistry by pulsed Nd:YAG laser apparatus
by pulsed
properties af &laser welds &WAG laser apparatus
Toshio Yamagishi, DDS,a Michio Ito, PhD, Yoshiaburo Fujimura” Matsumtrto Dental College, Nagano, Japan The use of titanium
in dentistry
is increasing,
of titanium
in dentistry
MSC,~ and
and an adequate
method of joining
titanium units of restorations is needed. This study examined the welding of titanium with a normal pulse N&YAG laser. Laser welding of titanium was effective when performed in an argon atmosphere. ANOVA for the three-point bending test showed a correlation between the bending strength of a weld, the atmosphere under which the irradiation is performed, and the intensity of the irradiation. (J PROSTHET DENT 1993;70:264-73.)
T. ltamum and titanium alloys are being used more frequently in dentistry because of their superior mechanical properties, biocompatibility, and safety,l but an adequate method of joining titanium remains to be developed. Various attempts have been made to solder titanium.*, ” Among the many methods of joining metals, dentistry has relied on the investment soldering method, which is complicated and time-consuming. Other methods of soldering t,it,anium include electrical resistance soldering, infrared soldering, and plasma soldering.4-6 For titanium to be soldered without oxidizing, the area to be joined must be isolated from the surrounding air. Therefore, in all of these methods, the procedure must be performed quickly, and heat -releasing procedures must be restricted to a very small area. With laser welding, it is possible to join parts by the selfwelding of the metal parts themselves. Although numerous studies have been made on the use of the laser for welding other dental metals, 7-20few have been made on the welding
of
the input voltage and pulse frequency. The beam could be focused by visual observation because the area being irradiated could be viewed on a monitor. In this experiment, laser irradiation was performed at the focal point of the condensing lens. Laser welding was tested in both air and argon atmospheres; by “air” we refer to the atmosphere in our laboratory, and by “argon atmosphere” we refer to the procedure of using a nozzle to blow argon gas onto the area that is being irradiated. The rate of argon gas supply was 5 Llmin throughout the experiment.
titanium.~i-":'
In this st,udy, pure titanium Nd:YAG laser. The propert,ies with the original titanium by ing test and Vickers hardness
plates were welded with an of the welds were compared use of the three-point bend(Hv) test.
MATERIAL AND METHODS Laser welding apparatus .4 normal pulse, Nd:YAG laser welding apparatus (model ML-2220A, Miyachi Technos, Tokyo, Japan) was used i Fig. 1). Table I lists the technical specifications for the laser. The irradiation power was controlled by regulation of
‘instructor, lnstirute for Dental Science. ,:\:;soclate Professor, Institute for Dental Science. Vlanager. 1,aser Engineering Section, J. Morita Corporation, Ky:to. Japan. ’ pJyright I’ 1993 by t.he Editorial Council of THE JO~JRNALOF ~‘IUWI’HY’I’I(’ DF~I’IS’IX‘,‘. .L, i ,,?.‘2-3913/9:1/$1.110 + .lO.
10/l/47853
Fig.
1. General view of laser welding apparatus.
Table I. Laser welding apparatus specifications Laser medium Irradiation power Pulse range Voltage range Optical fiber Focal length
model ML-2220A Neodymium-YAG Maximum 30 J/P 0.3 mS -9.9 ms DC 250 V -499 V SI-ITOO-SY6 M 70 mm
J/P, Joulesper pulse;MS, milliseconds;SI, Step Index type; SY, trade naw of SI fiber; M, meter.
YAMAGISHI,
ITO,
AND
THE
FUJIMURA
JOURNAL
OF PROSTHETIC
DENTISTRY
Table II. Types of pure titanium used and their chemical composition Rolled titanium plate (RT)
RT
RT
Manufacturer Size
Nippon
stainless steel
5X30Xlmm
Cast titanium plate (CT) Kobe steel 10 g ingot, 5 X 30
Xlmm . ..: ::..,: .,.: ‘.,;::::...:..,. -...::. ,_
* + + + + ..~~:~~~~~~~~~~~~:,, l + + l + :;.,:;_.,.:;..,._.. ~..‘(..... ::.;.::;.:.;:
pJ
Fig.
Fwio”
zone
2. Schematic diagram for Vickers hardness test.
Table II shows the pure titanium used and its chemical composition. Surface observations were made by using pure titanium rolled plates (RT) with Japanese Industrial Standard II equivalence (JIS II) and cast plates of pure titanium. Cast titanium test pieces (CT) were made with pure titanium ingots with JIS II equivalence. Nonintrusive inspection by means of x-ray examination was performed to eliminate any flawed pieces.
Surface
observation
After etching was completed, a metallurgical microscope was used to observe the area of the RT that had been irradiated in a blown argon atmosphere. Table III shows the irradiation parameters. A mixture of 2 % HF, 10% HNOs, and 88% Hz0 was used for etching. Observation of the irradiated RT was then carried out with a scanning electron microscope (SEM) and an x-ray analyzer, (model JCXA733, Nihon Denshi, Tokyo, Japan).
Three-point
bending
test
Test pieces of RT 30 mm long, 5 mm wide, and 1 mm thick were cut in half with a precision cutting device (model Fine Cut, Heiwa Technica, Tokyo, Japan) and the 15 mm pieces were then welded back together. Both the top and bottom surfaces of the RT were welded so that approximately 70% of the irradiated area overlapped. Table III shows the irradiation parameters. After five test pieces were prepared for each set of parameters, a three-point bending test was performed with an autograph (model AG-5000D, Shimadzu, Tokyo, Japan). The distance between fulcrums was 20 mm and the crosshead speed was 0.5 mm/min. The bending strength and 0.2 % proof stress were then measured. Table V shows the results of repeated measurement of ANOVA for the bending strength. The broken edges were also observed with SEM.
Vickers
hardness
test
A hardness test was performed on RT that was 1 mm thick, 5 mm wide, and 30 mm long. Each piece was cut in
SEPTEMBER
1993
-
Code of specimen Element (%) Hydrogen Oxygen Nitrogen Ferrum Titanium
RT
CT
0.0027 0.08 0.01 0.05 Balance
0.01 0.15 0.03 0.15 Balance
half with a precision cutting device and the 15 mm pieces were then welded back together. Both the top and bottom surfaces of the test pieces were irradiated so that approximately 70% of the welded areas overlapped. Table III shows the irradiation conditions. The welded pieces were then cut perpendicular to the weld, that is, lengthwise, and the cut surface was mirror polished. As shown in Fig. 2, Vickers hardness was measured at the center of the welds with a micropenetrometer (model HMV 2000, Shimadzu, Tokyo, Japan) using a force of 300 g for 15 seconds. Measurement was made at approximately 60 pm deep from each surface.
RESULTS Surface observation Fig. 3 is a microscopic illustration of a 30 9%overlapped weld of an RT irradiated in an argon atmosphere, with an energy intensity of approximately 3 joules per pulse (J/P). Granular isometric crystals were observed in the area not irradiated, and a Widmansttitten structure was seen in the irradiated part. SEM observations of the surface of an RT (Fig. 4) and a CT (Fig. 5) were made after a single irradiation. Spots a and b were made in air and spots c and d were made in an argon atmosphere. The irradiation power for spots a and c was 6.5 J/P and for spots b and d 14 J/P. Spots a and c had a diameter of 700 to 800 pm and were almost perfectly circular, whereas spots b and d had a diameter of 1200 to 1300 pm and had an irregular circular shape. The edges of spots a and c on the RT had a regular pattern of radiating lines. The radiating line pattern of spot d was more regular than that for spot b, and concentric circles were visible on the outer edge of the spot. Radiating lines were also noted on the edge of spot a for the CT, although they were not as clear as those for the RT. The melted pattern of spot b was markedly irregular and a crack appeared in its center. Spot d showed a radiating line pattern similar to the one for the RT and concentric circles at the edge. 265
THE
JOURNAL
OF PROSTHETIC
YAMAGISHI,
DENTISTRY
ITO,
AND
FUJIMURA
Fig. 3. Laser-welded section on the surface of a pure rolled titanium plate, etched. Original magnification of part A is ~5 and part B, X20. a, Granular isometric crystal are observed in area not irradiated. b, Widmanstiitten a-structures are observed in irradiated area.
Fig. 4. SEM photo of pure rolled titanium plate surface appearances 6.5 J/P air; b, 15 J/p air; c, 6.5 J/P argon; d, 15 J/P argon.
‘ritble .”
III.
Laser irradiation
and welding conditions
Duration of pulse (mW
.Observation fA4 SEM
Bending test, Hardness test _..-..i __.-.-_- ____-
of fusion zone; a,
3 5
5 2, 5, 8
riosphere by 5 I,/min argon gas blown.
Energy
level
(J/P) (approx) a 6.5, 10 3.5, 6.5, 10, 15, 20 2, 6.5, 10
Overlapping ratio
Atmosphere
0.3
Argon*
0.7 0.7
Air, argon Air, argon Air, argon
Joint 1
Bur.1 But .t
YAMAGISHI,
ITO,
AND
FUJIMURA
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 5. SEM photo of cast pure titanium plate surface appearances of fusion zone; a, 6.5 J/P air; b, 15 J/P air; c, 6.5 J/P argon; d, 15 J/p argon.
Table IV. Results of three-point Irradiation
bending test
intensity
3.5 J/P
6.6 J/P
10 J/P
16 J/P
20 J/P
Air
Bending strength
143.9
Proof stress Argon Bending strength Proof stress
(8.3)
318.5
-*
(27.6)
121.5
-*
c39.1**
133.1 (63.8) 187.0 (16.7)***
219.1 (20.5)***
275.6
(133.3)
521.8
(38.4)
1 *
(49.2)
406.1 (40.1)
491.5
(112.0)
662.9
(30.1)
401.2
(51.3)
426.3
(25.2)
657.0
(44.5)
664.4
(29.8)
419.8 (19.3)
411.0 (10.3)
RT Bending strength: 687.3 (8.8) Proof stress: 409.6 (8.4) J/P, Joules per pulse; ( 1, Standard deviation; RT, unwelded titanium specimen. *It was possible to measure none of five test pieces; these data were not counted as zeros. **It was possible to measure in only one out of five. ***It was possible to measure in two out of five; these data are the average of two. @,Highly significant (p < 0.025). Because of the brittle property these test pieces tended to break within
Three-point
bending
test
Table IV shows the results of welding by irradiating RT. The results of the measurement were calculated to find the average value and standard deviation. Figs. 6 and 7 show the results for the bending strength and the proof stress in both atmospheres. The point marked a shows the results for the original RT before irradiation; the bending strength was 687.3 + 8.8 MPa, and the proof stress was 409.6 f 8.4 MPa. Points b, c, d, e, and f show the results after welding at an intensity of 3.5 J/P, 6.5 J/P, 10 J/P, 15 J/P, and 2OJ/P. In air, it was possible to measure the proof stress for none of five test pieces at an intensity of 3.5 J/P and 6.5 J/P, and for only one of five at an intensity of 10 J/P. In an argon atmosphere, it was possible to measure the proof stress for SEPTEMBER
1993
the proportional
hit
at low intensities.
two of five samples at an intensity of 3.5 J/P and 6.5 J/P; the average values of two measurements are shown in Table IV and Fig. 6. Because of their brittle property, these test pieces tended to break down within the proportional limit at low intensities. Therefore, all data of the proof stress could not be shown clearly in Table IV. Table V shows the repeated measures ANOVA for the three-point bending test. Bending strength was correlated with the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity at the 0.01 level of significance. Scheffe’s multiple-range test was used to determine which combinations were significantly different. Fig. 8 shows the confidence limits (95 %) for the mean bending strength (confidence limit 66.2). In both types of 267
THE
JOURNAL
OF PROSTHETIC
YAMAGISHI,
DENTISTRY
0
b
a
ITO,
AND
FUJIMURA
d
C
6. Results of three-point bending test comparison of the bending strength in air and in an argon atmosphere. a, Unwelded specimen; b, 3.5 J/P; c, 6.5 J/P; d, 10 J/P; e, 15 J/P; f, 20 J/P. Bending strengths were similar to a. Fig.
m
Original
0
Air Argon
a
b
d
C
e
Fig. 7. The results of three-point bending test comparison of the proof stress in air and in an argon atmosphere. a, Unwelded specimen; b, 3.5 J/P; c, 6.5 J/P; d, 10 J/P; e, 15 J/P; f, 20 J/P. *, It was possible to measure none; these data are not counted as zeros. **, It was possible to measure in only one of five. ***, It was possible to measure in two; these data were the average of two. These test pieces broke within the proportional limit, and the proof stress could not be measured.
Table
V.
Analysis of variance for three-point bending strength ss 1.31851.40
1
Irradiation power Atnlosphereiirradiation power
209262I .77
4 4
Error ‘I’otal
214392.26 2538324.97
Atmosphere
26X
-
Source
99459.54
DF
40 49
MS
F
P
131851.40 523155.44
24.600 97.607
24864.89
4.639
5359.81
VOLUME
70
NUMBER
3
THE
JOURNAL
,OF
PROSTHETIC
DENTISTRY
YAMAGISHI,
ITO,
AND
FUJIMURA
800-
Q 6004 5 P 400gul P z 2002 0 0
I 3.5
I 6.5
I 10
I 15
I 20
Irradiation level (J/P) Fig. 8. Effect of confidence limits (95 % ) for the bending strength. Error bar indicates confidence limit (66.2). Bending strength tends to increase at high intensities. At intensity of 20 J/P, the mean values are similar in both atmospheres. However, it is clear that the bending strength tends to become brittle at lower intensities. The difference between the two atmospheres is shown in this figure.
atmospheres, greater irradiation power resulted in greater bending strength. Fig. 9 shows some of the test pieces after the three-point bending test. Piece A is the original titanium before irradiation. Piece B cracked along the weld and C, welded at 20 J/P in an argon atmosphere, did not crack. Fig. 10 shows SEM observations of the broken-surface RT. The RT had been welded at 6.5 J/‘P and was broken by the three-point bending test. Spots a and b were welded in air and c and d in an argon atmosphere. Spot b is a magnification of a and d is a magnification of c. These SEMs revealed no marked differences between irradiation in air and irradiation in an argon atmosphere. Hardness
test
Figs. 11 through 13 show the results of the hardness tests for welded RT test pieces. The vertical axis represents the hardness, the horizontal axis represents the distance measured, and the 0 represents the center of the weld. Distances were determined by considering the minimum possible distance in which the test could be performed without any effect from the pressure marks of a single hardness test. Fig. 11 shows the results of a hardness test for a 2 J/P weld. The hardness of the original RT was 150 to 160 Hv. With the welded area taken as the center, the range of increasing hardness was approximately 800 pm. The maximum hardness in air is 530 Hv, but the maximum hardness was only 350 Hv. Fig. 12 shows the results of a hardness test for a 6.5 J/P weld. With the welded area taken as the center, the range of increasing hardness was approximately 1000 pm. The maximum hardness in air was 560 Hv and the
SEPTEMBER
1993
Fig. 9. Specimens after bending test. A, Unwelded specimen, not fractured; B, fractured; C, not fractured (20 J/P, argon).
range of the area greater than 400 Hv was 800 grn. The maximum hardness in an argon atmosphere was only 300 Hv. Fig. 13 shows the results of a hardness test for a 10 J/P weld. With the welded area as the center, the range of increasing hardness was approximately 1000 pm. The range of hardness increased with the irradiation energy. When welded in air, the maximum hardness was 600 Hv and the range of hardness larger than 450 Hv was 800 pm. Hardness also increased when test pieces were welded in an argon atmosphere with the maximum hardness at 430 Hv. The difference in the increased hardness between welding in air
269
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
YAMAGISHI,
ITO,
AND
FUJIMURA
Fig. 10. SEM photo of fracture surface of welded specimens after bending test of pure rolled titanium plates (6.5 J/p). a and b, Air; c and d, argon. Defects such as gas pores were clearly visible. increase in hardness was greater for pieces welded in air than for pieces welded in an argon atmosphere.
600
DISCUSSION 500 5
ZJ 400 6 G .c
300
2 3 .’
200
Fig. 11. Vickers hardness test results pure rolled titanium plates (2 J/P). -,
and in an argon atmosphere
after welding of Air; ---, argon.
was not as great as with a 6.5
J/P weld.
These findings demonstrated that the hardness increased with the irradiation energy for the pieces welded in air and those welded in an argon atmosphere. However, the
A significant obstacle in successfully welding titanium is control of the argon atmosphere to isolate the titanium from the air and prevent oxidation. Fig. 14 shows various nozzles used to create an argon atmosphere. Type al blows argon directly out of the brass tip, which has an inner diameter of 3 mm. Type a2 has a closed tip with several perforations (see b). Type a3 can blow gas from opposite directions. In the present experiment, type a2 was used to blow argon with a minimum purity of 99.9%. The rate of argon flow was 5 L/min, but this particular flow rate needs to be reexamined. It would be more economical to perform the work inside a chamber filled with argon, but it is extremely difficult to design a chamber that contains argon and makes it possible to perform the necessary procedures. We are now trying to develop a work stand that can be used to blow argon from both above and below. When irradiation was carried out in a blown argon atmosphere at an output power of 3 J/P, a-Type Widmanstatten structures were observed on the irradiated area. These a-type structures developed when there was little cooling from the range of the P-type structures and are considered somewhat mechanically inferior to the martensitic a-type structures, which also occurred when the metal was cooled from the range of the &type structures.“* SEMs showed cracks in the center of areas irradiated in air. However, cracks were not observed when irradiation was carried out in an argon atmosphere. Therefore, it is essential to ir-
VOLUME
70
NUMBER
3
YAMAGISHI,
ITO,
AND
FUJIMURA
THE
6OC
60C
200
200
100 0
100 d
0
500
5oo
pm
Fig. 12. Vickers hardness test results after welding of pure rolled titanium plates (6.5 J/p). -, Air; ---, argon.
Fig.
SEPTEMBER
1993
OF PROSTHETIC
DENTISTRY
I
1
I
500
0
500
J
Pm Fig. 13. Vickers hardness test results after welding of pure rolled titanium plates (10 J/P). -, Air; ---, argon.
14. a and b, Argon gas flow nozzles experimentally
radiate the metal in an argon atmosphere to prevent the oxidation that leads to cracking. When welded in air, the bending strength of test pieces was less than 20% of that of the original material at irra-
JOURNAL
prepared.
diation powers of 3.5 J/P and 6.5 JP; 50% at 10 J/P; and 90% at 20 J/P. In an argon atmosphere, the bending strength was less than 20% of that of the original material at 3.5 J/l?; 50% at 6.5 J/P; 70% at 10 J/P; and 90% at 15
271
THE
JOURNAL
OF PROSTAETIC
DENTISTRY
and 20 J/P. In both atmospheres, greater irradiation power resulted in greater bending strength. When welded in air at powers of 3.5,6.5, and 10 J/P, the test pieces were too brittle to measure proof stress, but at powers of 15 and 20 J/P, the proof stress was similar to that for the original material. Test pieces welded in an argon atmosphere at 3.5 and 6.5 J/P were too brittle to measure proof stress. To attain a three-point bending strength equivalent to the original material, it is necessary to weld at greater than 20 J/P in air. In an argon atmosphere, this can be accomplished at approximately 15 J/P. However, it cannot be concluded that mechanical properties can be improved simply by increasing the irradiation power. SEMs show relatively large cracks in welds made in air at 15 J/p and an extreme roughness for surfaces irradiated at 20 J/P. Also, Sjiigren reported that the tensile strength of titanium rods was adequate when welded at 12 J/P and 18 J/P, but not when the power was increased to 30 J/P.21 As shown by the ANOVA (Table V) and Fig. 8 (confidence limits), there is a significant relationship between bending strength and the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity. In other words, laser welding was effective when performed in an argon atmosphere and, at the same time, the results markedly differed with the intensity of the irradiation. Scheffe’s multiple-range test showed a highly significant difference between “air, 10 J/P” and “argon, 10 J/P” (p < 0.025). However, further research is necessary to determine and identify the best combination of these two factors. SEMs of broken edges revealed that the overlapped spots were S-shaped and that there were numerous minute holes that were apparently caused by the mixture of gases with the molten metal. However, no distinct, morphologic difference was observed between the pieces welded in air and those welded in an argon atmosphere. The effects of various flow rates for the argon and various irradiation powers remain to be studied. Hardness increased toward the center of the welded area. As the powers were increased incrementally from 2 to 6.5 to 10 J/P, the range of the area that was harder than the original material expanded from 800 pm to 1000 pm. The hardness of the test pieces welded in an argon atmosphere never exceeded 500 Hv. The increase in hardness was less for test pieces welded in an argon atmosphere than for those irradiated in air. Therefore, for the same irradiation power, the pieces welded in an argon atmosphere were less oxidized than those welded in air, partly because in the argon atmosphere the test pieces were isolated from oxygen and partly because the irradiated area was cooler and the effect of heat on the area weaker. Laser welding of titanium offers several advantages over the soldering methods. Because the laser can be focused on a relatively small spot, the effect of heat on the titanium is
YAMAGISHI,
ITO,
AND
FUJIMURA
weaker than that of exposure to a flame. The process is less time-consuming because no investment material is needed. Because the joint is formed by the original metal melted together without an intermediary of solder, there should be little problem with reduced corrosion resistance. The future use of lasers to weld titanium and other dental metals will depend on gaining a better understanding of the correct irradiation parameters for the welding of each kind of metal.
CONCLUSIONS There is a significant relationship between bending strength and the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity. Laser welding is effective when performed in an argon atmosphere. At the same time, the results differed markedly with the intensity of the irradiation. Further research is necessary to determine the best combination of these two factors. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Miura I, Ida K. Chitan no shika-riyou. 1st ed. Tokyo: Quintessence Publ, 1988;11-41. Togaya T. Dental application of soldering of titanium and its alloy. J Gifu Dent Sot 1986;13(1):60-93. Yabugami M, Togaya T. The situation of soldering of titanium and titanium-alloy. Quintessence Dent Technol (Japan) 1984;9:927-34. Shimada J. A soldering of titanium and alloy with IR soldering equipment. J J Dent Mater 1991;10:362-75. Hagiwara Y, Osawa H, Shibuya M, et al. A study of dental soldering. Tensile strength of nickel-chromium base metal presoldered by focused energy of infrared. J Jpn Prosthodont Sot 1988;32:814-20. Igarashi T, Noguchi H, Yuda M, et al. Gendai no shika rou-suke tekunikku. Quintessence Dent Technol. Extra issue. Tokyo: Quintessence Publ, 1989:309-15. Earvolino LP, Kennedy JR. Laser welding of aerospace structural alloys. Welding J 1966;March:127S34S. Alwang GW, Cavanaugh AL, Sammartino E. Continuous butt welding using a carbon dioxide laser. Welding J 1969;March:1107S-115s. Gordon TE, Smith DL. Laser welding of prostheses-an initial report. J PROSTHET DENT 1970;24:472-6. Gordon TE, Smith DL. A laser in the dental lab. Laser Focus Magazine 1970;June:37-9. Gordon TE, Smith DL. Laser welding of ceramic fixed prostheses. Dental Digest 197O;July:306-9. Smith DL, Burnett AP, Gordon TE. Laser welding of gold alloys. J Dent Res 1972;51:161-7. Preston DJ, Reisbick MH. Laser fusion of selected dental alloys. J Dent Res 1975;54:232-8. Eshleman JR, Svitser JR, Moon PC. Heat treatment of laser-welded gold alloys. J PROSTHET DENT 1976;36:655-9. Minamizato T. The use of laser welding in prosthodontics. The first report: Basic study on laser welding of dental materials. J Jpn Prosthodont Sot 1974;17:524-9. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering, or casting in one piece. J Dent Res 1977;56:128-34. Benthem H van, Munster JV. Korrosionsversuche an Dentallegierungen vor und nach dem LaserschweiBen. 1. Hochgoldhaltige Dentallegierungen. Dtsch Zahnarstl Z 1985;40:286-9. Dielert E, Kasenbacher A. Lotungen, Mikroplasma-und LaserstrahlschweiBungen an Dentallegierungen. Dtach Zahnarztl Z 1987;42:647-53. Kakimoto K. Basic study on laser welding of Ag-Pd.Cu-Au alloy. J Jpn Prosthodont Sot 1987;31:1143-56.
YAMAGISHI,
ITO, AND FUJIMURA
THE JOURNAL
20. Dobberstein H, Orlick H, Fischer P, Zuhrt R. Experimentelle Untersuchungen sum LaserschweiBen van Co-Cr-Legierungen mit einem gepulsten Nd:YAG-Laser. Zahn Mund Kieferheilkd 1989;77:573-9. 21. Sj6gren G, Anderson M, Bergman M. Laser welding of titanium in dentistry. Acta Odontol Stand 1966;46:247-63. 22. Yamagishi T, Ito M, Masuhara E. Basic study on laser welding of titanium in dentistry by pulsed Nd:YAG laser apparatus Part 1. J Jpn Sot Laser Dent 1991;2:53-4. 23. Yamagishi T, Ito M, Masuhara E. Mechanical properties of laser welds of titanium and other dental alloys by pulsed Nd:YAG laser apparatus Part 1. J J Dent Mater. 1991;10(6):763-72.
Availability
of JOURNAL
OF PROSTHETIC
DENTISTRY
24. Muraksmi Y, Kamei K. Hitetaukiuzoku zairyougaku. Tokyo: Asakura syoten, 1978;106-42. Reprint requests to:
DR.TOSHIO YAMAGEX-II INSTITIJTE FOR DENTAL SCIENCE, MAT~UMOTO DENTAL COLLEGE 1780 GOBARA, HIROOKA, SHIOJIFU NAGANO 399-07 JAPAN
back
issues,
1987-1992
Back issues of THE JOURNAL OF PROSTHETIC DENTISTRY are available for purchase from the publisher, Mosby, at a cost of $7.50 per issue. (Foreign postage is not included.) The following quantity discounts are available: 25 % off on quantities of 12 to 23, and one third off on quantities of 24 or more. Please write to Mosby, Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, or call (600)325-4177, ext. 4351, or (314)453-4351 for information on availability of particular issues for that period from 1987 to 1992. If unavailable from the publisher, photocopies of complete issues are available from University Microforms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (313)7614700.
SEPTEMBER
1993
273
properties af &laser welds &WAG laser apparatus
Toshio Yamagishi, DDS,a Michio Ito, PhD, Yoshiaburo Fujimura” Matsumtrto Dental College, Nagano, Japan The use of titanium
in dentistry
is increasing,
of titanium
in dentistry
MSC,~ and
and an adequate
method of joining
titanium units of restorations is needed. This study examined the welding of titanium with a normal pulse N&YAG laser. Laser welding of titanium was effective when performed in an argon atmosphere. ANOVA for the three-point bending test showed a correlation between the bending strength of a weld, the atmosphere under which the irradiation is performed, and the intensity of the irradiation. (J PROSTHET DENT 1993;70:264-73.)
T. ltamum and titanium alloys are being used more frequently in dentistry because of their superior mechanical properties, biocompatibility, and safety,l but an adequate method of joining titanium remains to be developed. Various attempts have been made to solder titanium.*, ” Among the many methods of joining metals, dentistry has relied on the investment soldering method, which is complicated and time-consuming. Other methods of soldering t,it,anium include electrical resistance soldering, infrared soldering, and plasma soldering.4-6 For titanium to be soldered without oxidizing, the area to be joined must be isolated from the surrounding air. Therefore, in all of these methods, the procedure must be performed quickly, and heat -releasing procedures must be restricted to a very small area. With laser welding, it is possible to join parts by the selfwelding of the metal parts themselves. Although numerous studies have been made on the use of the laser for welding other dental metals, 7-20few have been made on the welding
of
the input voltage and pulse frequency. The beam could be focused by visual observation because the area being irradiated could be viewed on a monitor. In this experiment, laser irradiation was performed at the focal point of the condensing lens. Laser welding was tested in both air and argon atmospheres; by “air” we refer to the atmosphere in our laboratory, and by “argon atmosphere” we refer to the procedure of using a nozzle to blow argon gas onto the area that is being irradiated. The rate of argon gas supply was 5 Llmin throughout the experiment.
titanium.~i-":'
In this st,udy, pure titanium Nd:YAG laser. The propert,ies with the original titanium by ing test and Vickers hardness
plates were welded with an of the welds were compared use of the three-point bend(Hv) test.
MATERIAL AND METHODS Laser welding apparatus .4 normal pulse, Nd:YAG laser welding apparatus (model ML-2220A, Miyachi Technos, Tokyo, Japan) was used i Fig. 1). Table I lists the technical specifications for the laser. The irradiation power was controlled by regulation of
‘instructor, lnstirute for Dental Science. ,:\:;soclate Professor, Institute for Dental Science. Vlanager. 1,aser Engineering Section, J. Morita Corporation, Ky:to. Japan. ’ pJyright I’ 1993 by t.he Editorial Council of THE JO~JRNALOF ~‘IUWI’HY’I’I(’ DF~I’IS’IX‘,‘. .L, i ,,?.‘2-3913/9:1/$1.110 + .lO.
10/l/47853
Fig.
1. General view of laser welding apparatus.
Table I. Laser welding apparatus specifications Laser medium Irradiation power Pulse range Voltage range Optical fiber Focal length
model ML-2220A Neodymium-YAG Maximum 30 J/P 0.3 mS -9.9 ms DC 250 V -499 V SI-ITOO-SY6 M 70 mm
J/P, Joulesper pulse;MS, milliseconds;SI, Step Index type; SY, trade naw of SI fiber; M, meter.
YAMAGISHI,
ITO,
AND
THE
FUJIMURA
JOURNAL
OF PROSTHETIC
DENTISTRY
Table II. Types of pure titanium used and their chemical composition Rolled titanium plate (RT)
RT
RT
Manufacturer Size
Nippon
stainless steel
5X30Xlmm
Cast titanium plate (CT) Kobe steel 10 g ingot, 5 X 30
Xlmm . ..: ::..,: .,.: ‘.,;::::...:..,. -...::. ,_
* + + + + ..~~:~~~~~~~~~~~~:,, l + + l + :;.,:;_.,.:;..,._.. ~..‘(..... ::.;.::;.:.;:
pJ
Fig.
Fwio”
zone
2. Schematic diagram for Vickers hardness test.
Table II shows the pure titanium used and its chemical composition. Surface observations were made by using pure titanium rolled plates (RT) with Japanese Industrial Standard II equivalence (JIS II) and cast plates of pure titanium. Cast titanium test pieces (CT) were made with pure titanium ingots with JIS II equivalence. Nonintrusive inspection by means of x-ray examination was performed to eliminate any flawed pieces.
Surface
observation
After etching was completed, a metallurgical microscope was used to observe the area of the RT that had been irradiated in a blown argon atmosphere. Table III shows the irradiation parameters. A mixture of 2 % HF, 10% HNOs, and 88% Hz0 was used for etching. Observation of the irradiated RT was then carried out with a scanning electron microscope (SEM) and an x-ray analyzer, (model JCXA733, Nihon Denshi, Tokyo, Japan).
Three-point
bending
test
Test pieces of RT 30 mm long, 5 mm wide, and 1 mm thick were cut in half with a precision cutting device (model Fine Cut, Heiwa Technica, Tokyo, Japan) and the 15 mm pieces were then welded back together. Both the top and bottom surfaces of the RT were welded so that approximately 70% of the irradiated area overlapped. Table III shows the irradiation parameters. After five test pieces were prepared for each set of parameters, a three-point bending test was performed with an autograph (model AG-5000D, Shimadzu, Tokyo, Japan). The distance between fulcrums was 20 mm and the crosshead speed was 0.5 mm/min. The bending strength and 0.2 % proof stress were then measured. Table V shows the results of repeated measurement of ANOVA for the bending strength. The broken edges were also observed with SEM.
Vickers
hardness
test
A hardness test was performed on RT that was 1 mm thick, 5 mm wide, and 30 mm long. Each piece was cut in
SEPTEMBER
1993
-
Code of specimen Element (%) Hydrogen Oxygen Nitrogen Ferrum Titanium
RT
CT
0.0027 0.08 0.01 0.05 Balance
0.01 0.15 0.03 0.15 Balance
half with a precision cutting device and the 15 mm pieces were then welded back together. Both the top and bottom surfaces of the test pieces were irradiated so that approximately 70% of the welded areas overlapped. Table III shows the irradiation conditions. The welded pieces were then cut perpendicular to the weld, that is, lengthwise, and the cut surface was mirror polished. As shown in Fig. 2, Vickers hardness was measured at the center of the welds with a micropenetrometer (model HMV 2000, Shimadzu, Tokyo, Japan) using a force of 300 g for 15 seconds. Measurement was made at approximately 60 pm deep from each surface.
RESULTS Surface observation Fig. 3 is a microscopic illustration of a 30 9%overlapped weld of an RT irradiated in an argon atmosphere, with an energy intensity of approximately 3 joules per pulse (J/P). Granular isometric crystals were observed in the area not irradiated, and a Widmansttitten structure was seen in the irradiated part. SEM observations of the surface of an RT (Fig. 4) and a CT (Fig. 5) were made after a single irradiation. Spots a and b were made in air and spots c and d were made in an argon atmosphere. The irradiation power for spots a and c was 6.5 J/P and for spots b and d 14 J/P. Spots a and c had a diameter of 700 to 800 pm and were almost perfectly circular, whereas spots b and d had a diameter of 1200 to 1300 pm and had an irregular circular shape. The edges of spots a and c on the RT had a regular pattern of radiating lines. The radiating line pattern of spot d was more regular than that for spot b, and concentric circles were visible on the outer edge of the spot. Radiating lines were also noted on the edge of spot a for the CT, although they were not as clear as those for the RT. The melted pattern of spot b was markedly irregular and a crack appeared in its center. Spot d showed a radiating line pattern similar to the one for the RT and concentric circles at the edge. 265
THE
JOURNAL
OF PROSTHETIC
YAMAGISHI,
DENTISTRY
ITO,
AND
FUJIMURA
Fig. 3. Laser-welded section on the surface of a pure rolled titanium plate, etched. Original magnification of part A is ~5 and part B, X20. a, Granular isometric crystal are observed in area not irradiated. b, Widmanstiitten a-structures are observed in irradiated area.
Fig. 4. SEM photo of pure rolled titanium plate surface appearances 6.5 J/P air; b, 15 J/p air; c, 6.5 J/P argon; d, 15 J/P argon.
‘ritble .”
III.
Laser irradiation
and welding conditions
Duration of pulse (mW
.Observation fA4 SEM
Bending test, Hardness test _..-..i __.-.-_- ____-
of fusion zone; a,
3 5
5 2, 5, 8
riosphere by 5 I,/min argon gas blown.
Energy
level
(J/P) (approx) a 6.5, 10 3.5, 6.5, 10, 15, 20 2, 6.5, 10
Overlapping ratio
Atmosphere
0.3
Argon*
0.7 0.7
Air, argon Air, argon Air, argon
Joint 1
Bur.1 But .t
YAMAGISHI,
ITO,
AND
FUJIMURA
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 5. SEM photo of cast pure titanium plate surface appearances of fusion zone; a, 6.5 J/P air; b, 15 J/P air; c, 6.5 J/P argon; d, 15 J/p argon.
Table IV. Results of three-point Irradiation
bending test
intensity
3.5 J/P
6.6 J/P
10 J/P
16 J/P
20 J/P
Air
Bending strength
143.9
Proof stress Argon Bending strength Proof stress
(8.3)
318.5
-*
(27.6)
121.5
-*
c39.1**
133.1 (63.8) 187.0 (16.7)***
219.1 (20.5)***
275.6
(133.3)
521.8
(38.4)
1 *
(49.2)
406.1 (40.1)
491.5
(112.0)
662.9
(30.1)
401.2
(51.3)
426.3
(25.2)
657.0
(44.5)
664.4
(29.8)
419.8 (19.3)
411.0 (10.3)
RT Bending strength: 687.3 (8.8) Proof stress: 409.6 (8.4) J/P, Joules per pulse; ( 1, Standard deviation; RT, unwelded titanium specimen. *It was possible to measure none of five test pieces; these data were not counted as zeros. **It was possible to measure in only one out of five. ***It was possible to measure in two out of five; these data are the average of two. @,Highly significant (p < 0.025). Because of the brittle property these test pieces tended to break within
Three-point
bending
test
Table IV shows the results of welding by irradiating RT. The results of the measurement were calculated to find the average value and standard deviation. Figs. 6 and 7 show the results for the bending strength and the proof stress in both atmospheres. The point marked a shows the results for the original RT before irradiation; the bending strength was 687.3 + 8.8 MPa, and the proof stress was 409.6 f 8.4 MPa. Points b, c, d, e, and f show the results after welding at an intensity of 3.5 J/P, 6.5 J/P, 10 J/P, 15 J/P, and 2OJ/P. In air, it was possible to measure the proof stress for none of five test pieces at an intensity of 3.5 J/P and 6.5 J/P, and for only one of five at an intensity of 10 J/P. In an argon atmosphere, it was possible to measure the proof stress for SEPTEMBER
1993
the proportional
hit
at low intensities.
two of five samples at an intensity of 3.5 J/P and 6.5 J/P; the average values of two measurements are shown in Table IV and Fig. 6. Because of their brittle property, these test pieces tended to break down within the proportional limit at low intensities. Therefore, all data of the proof stress could not be shown clearly in Table IV. Table V shows the repeated measures ANOVA for the three-point bending test. Bending strength was correlated with the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity at the 0.01 level of significance. Scheffe’s multiple-range test was used to determine which combinations were significantly different. Fig. 8 shows the confidence limits (95 %) for the mean bending strength (confidence limit 66.2). In both types of 267
THE
JOURNAL
OF PROSTHETIC
YAMAGISHI,
DENTISTRY
0
b
a
ITO,
AND
FUJIMURA
d
C
6. Results of three-point bending test comparison of the bending strength in air and in an argon atmosphere. a, Unwelded specimen; b, 3.5 J/P; c, 6.5 J/P; d, 10 J/P; e, 15 J/P; f, 20 J/P. Bending strengths were similar to a. Fig.
m
Original
0
Air Argon
a
b
d
C
e
Fig. 7. The results of three-point bending test comparison of the proof stress in air and in an argon atmosphere. a, Unwelded specimen; b, 3.5 J/P; c, 6.5 J/P; d, 10 J/P; e, 15 J/P; f, 20 J/P. *, It was possible to measure none; these data are not counted as zeros. **, It was possible to measure in only one of five. ***, It was possible to measure in two; these data were the average of two. These test pieces broke within the proportional limit, and the proof stress could not be measured.
Table
V.
Analysis of variance for three-point bending strength ss 1.31851.40
1
Irradiation power Atnlosphereiirradiation power
209262I .77
4 4
Error ‘I’otal
214392.26 2538324.97
Atmosphere
26X
-
Source
99459.54
DF
40 49
MS
F
P
131851.40 523155.44
24.600 97.607
24864.89
4.639
5359.81
VOLUME
70
NUMBER
3
THE
JOURNAL
,OF
PROSTHETIC
DENTISTRY
YAMAGISHI,
ITO,
AND
FUJIMURA
800-
Q 6004 5 P 400gul P z 2002 0 0
I 3.5
I 6.5
I 10
I 15
I 20
Irradiation level (J/P) Fig. 8. Effect of confidence limits (95 % ) for the bending strength. Error bar indicates confidence limit (66.2). Bending strength tends to increase at high intensities. At intensity of 20 J/P, the mean values are similar in both atmospheres. However, it is clear that the bending strength tends to become brittle at lower intensities. The difference between the two atmospheres is shown in this figure.
atmospheres, greater irradiation power resulted in greater bending strength. Fig. 9 shows some of the test pieces after the three-point bending test. Piece A is the original titanium before irradiation. Piece B cracked along the weld and C, welded at 20 J/P in an argon atmosphere, did not crack. Fig. 10 shows SEM observations of the broken-surface RT. The RT had been welded at 6.5 J/‘P and was broken by the three-point bending test. Spots a and b were welded in air and c and d in an argon atmosphere. Spot b is a magnification of a and d is a magnification of c. These SEMs revealed no marked differences between irradiation in air and irradiation in an argon atmosphere. Hardness
test
Figs. 11 through 13 show the results of the hardness tests for welded RT test pieces. The vertical axis represents the hardness, the horizontal axis represents the distance measured, and the 0 represents the center of the weld. Distances were determined by considering the minimum possible distance in which the test could be performed without any effect from the pressure marks of a single hardness test. Fig. 11 shows the results of a hardness test for a 2 J/P weld. The hardness of the original RT was 150 to 160 Hv. With the welded area taken as the center, the range of increasing hardness was approximately 800 pm. The maximum hardness in air is 530 Hv, but the maximum hardness was only 350 Hv. Fig. 12 shows the results of a hardness test for a 6.5 J/P weld. With the welded area taken as the center, the range of increasing hardness was approximately 1000 pm. The maximum hardness in air was 560 Hv and the
SEPTEMBER
1993
Fig. 9. Specimens after bending test. A, Unwelded specimen, not fractured; B, fractured; C, not fractured (20 J/P, argon).
range of the area greater than 400 Hv was 800 grn. The maximum hardness in an argon atmosphere was only 300 Hv. Fig. 13 shows the results of a hardness test for a 10 J/P weld. With the welded area as the center, the range of increasing hardness was approximately 1000 pm. The range of hardness increased with the irradiation energy. When welded in air, the maximum hardness was 600 Hv and the range of hardness larger than 450 Hv was 800 pm. Hardness also increased when test pieces were welded in an argon atmosphere with the maximum hardness at 430 Hv. The difference in the increased hardness between welding in air
269
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
YAMAGISHI,
ITO,
AND
FUJIMURA
Fig. 10. SEM photo of fracture surface of welded specimens after bending test of pure rolled titanium plates (6.5 J/p). a and b, Air; c and d, argon. Defects such as gas pores were clearly visible. increase in hardness was greater for pieces welded in air than for pieces welded in an argon atmosphere.
600
DISCUSSION 500 5
ZJ 400 6 G .c
300
2 3 .’
200
Fig. 11. Vickers hardness test results pure rolled titanium plates (2 J/P). -,
and in an argon atmosphere
after welding of Air; ---, argon.
was not as great as with a 6.5
J/P weld.
These findings demonstrated that the hardness increased with the irradiation energy for the pieces welded in air and those welded in an argon atmosphere. However, the
A significant obstacle in successfully welding titanium is control of the argon atmosphere to isolate the titanium from the air and prevent oxidation. Fig. 14 shows various nozzles used to create an argon atmosphere. Type al blows argon directly out of the brass tip, which has an inner diameter of 3 mm. Type a2 has a closed tip with several perforations (see b). Type a3 can blow gas from opposite directions. In the present experiment, type a2 was used to blow argon with a minimum purity of 99.9%. The rate of argon flow was 5 L/min, but this particular flow rate needs to be reexamined. It would be more economical to perform the work inside a chamber filled with argon, but it is extremely difficult to design a chamber that contains argon and makes it possible to perform the necessary procedures. We are now trying to develop a work stand that can be used to blow argon from both above and below. When irradiation was carried out in a blown argon atmosphere at an output power of 3 J/P, a-Type Widmanstatten structures were observed on the irradiated area. These a-type structures developed when there was little cooling from the range of the P-type structures and are considered somewhat mechanically inferior to the martensitic a-type structures, which also occurred when the metal was cooled from the range of the &type structures.“* SEMs showed cracks in the center of areas irradiated in air. However, cracks were not observed when irradiation was carried out in an argon atmosphere. Therefore, it is essential to ir-
VOLUME
70
NUMBER
3
YAMAGISHI,
ITO,
AND
FUJIMURA
THE
6OC
60C
200
200
100 0
100 d
0
500
5oo
pm
Fig. 12. Vickers hardness test results after welding of pure rolled titanium plates (6.5 J/p). -, Air; ---, argon.
Fig.
SEPTEMBER
1993
OF PROSTHETIC
DENTISTRY
I
1
I
500
0
500
J
Pm Fig. 13. Vickers hardness test results after welding of pure rolled titanium plates (10 J/P). -, Air; ---, argon.
14. a and b, Argon gas flow nozzles experimentally
radiate the metal in an argon atmosphere to prevent the oxidation that leads to cracking. When welded in air, the bending strength of test pieces was less than 20% of that of the original material at irra-
JOURNAL
prepared.
diation powers of 3.5 J/P and 6.5 JP; 50% at 10 J/P; and 90% at 20 J/P. In an argon atmosphere, the bending strength was less than 20% of that of the original material at 3.5 J/l?; 50% at 6.5 J/P; 70% at 10 J/P; and 90% at 15
271
THE
JOURNAL
OF PROSTAETIC
DENTISTRY
and 20 J/P. In both atmospheres, greater irradiation power resulted in greater bending strength. When welded in air at powers of 3.5,6.5, and 10 J/P, the test pieces were too brittle to measure proof stress, but at powers of 15 and 20 J/P, the proof stress was similar to that for the original material. Test pieces welded in an argon atmosphere at 3.5 and 6.5 J/P were too brittle to measure proof stress. To attain a three-point bending strength equivalent to the original material, it is necessary to weld at greater than 20 J/P in air. In an argon atmosphere, this can be accomplished at approximately 15 J/P. However, it cannot be concluded that mechanical properties can be improved simply by increasing the irradiation power. SEMs show relatively large cracks in welds made in air at 15 J/p and an extreme roughness for surfaces irradiated at 20 J/P. Also, Sjiigren reported that the tensile strength of titanium rods was adequate when welded at 12 J/P and 18 J/P, but not when the power was increased to 30 J/P.21 As shown by the ANOVA (Table V) and Fig. 8 (confidence limits), there is a significant relationship between bending strength and the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity. In other words, laser welding was effective when performed in an argon atmosphere and, at the same time, the results markedly differed with the intensity of the irradiation. Scheffe’s multiple-range test showed a highly significant difference between “air, 10 J/P” and “argon, 10 J/P” (p < 0.025). However, further research is necessary to determine and identify the best combination of these two factors. SEMs of broken edges revealed that the overlapped spots were S-shaped and that there were numerous minute holes that were apparently caused by the mixture of gases with the molten metal. However, no distinct, morphologic difference was observed between the pieces welded in air and those welded in an argon atmosphere. The effects of various flow rates for the argon and various irradiation powers remain to be studied. Hardness increased toward the center of the welded area. As the powers were increased incrementally from 2 to 6.5 to 10 J/P, the range of the area that was harder than the original material expanded from 800 pm to 1000 pm. The hardness of the test pieces welded in an argon atmosphere never exceeded 500 Hv. The increase in hardness was less for test pieces welded in an argon atmosphere than for those irradiated in air. Therefore, for the same irradiation power, the pieces welded in an argon atmosphere were less oxidized than those welded in air, partly because in the argon atmosphere the test pieces were isolated from oxygen and partly because the irradiated area was cooler and the effect of heat on the area weaker. Laser welding of titanium offers several advantages over the soldering methods. Because the laser can be focused on a relatively small spot, the effect of heat on the titanium is
YAMAGISHI,
ITO,
AND
FUJIMURA
weaker than that of exposure to a flame. The process is less time-consuming because no investment material is needed. Because the joint is formed by the original metal melted together without an intermediary of solder, there should be little problem with reduced corrosion resistance. The future use of lasers to weld titanium and other dental metals will depend on gaining a better understanding of the correct irradiation parameters for the welding of each kind of metal.
CONCLUSIONS There is a significant relationship between bending strength and the irradiation atmosphere, the irradiation intensity, and the combination of atmosphere and intensity. Laser welding is effective when performed in an argon atmosphere. At the same time, the results differed markedly with the intensity of the irradiation. Further research is necessary to determine the best combination of these two factors. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Miura I, Ida K. Chitan no shika-riyou. 1st ed. Tokyo: Quintessence Publ, 1988;11-41. Togaya T. Dental application of soldering of titanium and its alloy. J Gifu Dent Sot 1986;13(1):60-93. Yabugami M, Togaya T. The situation of soldering of titanium and titanium-alloy. Quintessence Dent Technol (Japan) 1984;9:927-34. Shimada J. A soldering of titanium and alloy with IR soldering equipment. J J Dent Mater 1991;10:362-75. Hagiwara Y, Osawa H, Shibuya M, et al. A study of dental soldering. Tensile strength of nickel-chromium base metal presoldered by focused energy of infrared. J Jpn Prosthodont Sot 1988;32:814-20. Igarashi T, Noguchi H, Yuda M, et al. Gendai no shika rou-suke tekunikku. Quintessence Dent Technol. Extra issue. Tokyo: Quintessence Publ, 1989:309-15. Earvolino LP, Kennedy JR. Laser welding of aerospace structural alloys. Welding J 1966;March:127S34S. Alwang GW, Cavanaugh AL, Sammartino E. Continuous butt welding using a carbon dioxide laser. Welding J 1969;March:1107S-115s. Gordon TE, Smith DL. Laser welding of prostheses-an initial report. J PROSTHET DENT 1970;24:472-6. Gordon TE, Smith DL. A laser in the dental lab. Laser Focus Magazine 1970;June:37-9. Gordon TE, Smith DL. Laser welding of ceramic fixed prostheses. Dental Digest 197O;July:306-9. Smith DL, Burnett AP, Gordon TE. Laser welding of gold alloys. J Dent Res 1972;51:161-7. Preston DJ, Reisbick MH. Laser fusion of selected dental alloys. J Dent Res 1975;54:232-8. Eshleman JR, Svitser JR, Moon PC. Heat treatment of laser-welded gold alloys. J PROSTHET DENT 1976;36:655-9. Minamizato T. The use of laser welding in prosthodontics. The first report: Basic study on laser welding of dental materials. J Jpn Prosthodont Sot 1974;17:524-9. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering, or casting in one piece. J Dent Res 1977;56:128-34. Benthem H van, Munster JV. Korrosionsversuche an Dentallegierungen vor und nach dem LaserschweiBen. 1. Hochgoldhaltige Dentallegierungen. Dtsch Zahnarstl Z 1985;40:286-9. Dielert E, Kasenbacher A. Lotungen, Mikroplasma-und LaserstrahlschweiBungen an Dentallegierungen. Dtach Zahnarztl Z 1987;42:647-53. Kakimoto K. Basic study on laser welding of Ag-Pd.Cu-Au alloy. J Jpn Prosthodont Sot 1987;31:1143-56.
YAMAGISHI,
ITO, AND FUJIMURA
THE JOURNAL
20. Dobberstein H, Orlick H, Fischer P, Zuhrt R. Experimentelle Untersuchungen sum LaserschweiBen van Co-Cr-Legierungen mit einem gepulsten Nd:YAG-Laser. Zahn Mund Kieferheilkd 1989;77:573-9. 21. Sj6gren G, Anderson M, Bergman M. Laser welding of titanium in dentistry. Acta Odontol Stand 1966;46:247-63. 22. Yamagishi T, Ito M, Masuhara E. Basic study on laser welding of titanium in dentistry by pulsed Nd:YAG laser apparatus Part 1. J Jpn Sot Laser Dent 1991;2:53-4. 23. Yamagishi T, Ito M, Masuhara E. Mechanical properties of laser welds of titanium and other dental alloys by pulsed Nd:YAG laser apparatus Part 1. J J Dent Mater. 1991;10(6):763-72.
Availability
of JOURNAL
OF PROSTHETIC
DENTISTRY
24. Muraksmi Y, Kamei K. Hitetaukiuzoku zairyougaku. Tokyo: Asakura syoten, 1978;106-42. Reprint requests to:
DR.TOSHIO YAMAGEX-II INSTITIJTE FOR DENTAL SCIENCE, MAT~UMOTO DENTAL COLLEGE 1780 GOBARA, HIROOKA, SHIOJIFU NAGANO 399-07 JAPAN
back
issues,
1987-1992
Back issues of THE JOURNAL OF PROSTHETIC DENTISTRY are available for purchase from the publisher, Mosby, at a cost of $7.50 per issue. (Foreign postage is not included.) The following quantity discounts are available: 25 % off on quantities of 12 to 23, and one third off on quantities of 24 or more. Please write to Mosby, Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, or call (600)325-4177, ext. 4351, or (314)453-4351 for information on availability of particular issues for that period from 1987 to 1992. If unavailable from the publisher, photocopies of complete issues are available from University Microforms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (313)7614700.
SEPTEMBER
1993
273