Heat treatment of laser-welded gold alloys

Heat treatment

of laser-welded

gold

alloys

J. R. Eshleman, D.D.S.,* J. R. Svitzer, D.D.S., and P. C. Moon, M.S., Ph.D.** Virginia Commonwealth University, Medical College of Virginia, School of Dentistry, Richmond, Vu., and Vinton, Pa.

R

ecent work by Hulingl has shown that laser-welded connectors are more accurate than soldered or unicast connectors in fixed partial dentures. Gordon and associates2 demonstrated that the strength of laser-welded joints for several dental gold alloys is comparable to the ultimate strength of conventionally soldered alloys. In that study, a preliminary evaluation of the effect of heat treatment on the welded joints was performed by measuring microhardness. To, further investigate the properties of laser-welded joints, the present study was undertaken to evaluate the yield strength in both as-welded and heat-treated conditions for five representative alloys. METHOD Five dental gold alloys were chosen for study, with at least one alloy from each type being used in fixed prosthodontics. Tensile specimens of Ney B-2,? G-3,? SMG-2,t Ceramco-O,$ and Jelenko F$ were cast using the manufacturers’ recommendations. Uniform wax patterns for casting were formed with molten wax under pressure in a brass mold (Fig. 1). Ten tensile specimens were welded for each test. All surfaces to be welded were milled flat for maximal contact between the samples. Gordon and Smith3 have shown that a space of less than 0.001 inch between samples is necessary for the best result. The castings were held in apposition by use of a spring-loaded specimen holder (Fig. 2). The gauge lengths were approximately 0.675 inch. The

Presented at the 51st General Washington, D. C.

Session of the International

This research was supported by A. D. Williams Fellowship College of Virginia. *Associate Professor, Department of Restorative Dentistry. **Assistant

Professor, Department

of Restorative

+The J. M. Ney Company,

Bloomfield,

$J. F. Jelenko

Inc., New Rochelle,

& Company,

Association Grant

for Dental

No. 3558(577),

Research, Medical

Dentistry.

Conn. N. Y.

655

656

Eshleman,

Svitrer,

and

Fig. 1. Brass mold used to construct Fig. 2. Spring-loaded

J. Prosthet. Dent. December. 1976

Moon

wax patterns.

specimen holder used in laser-welded

castings.

The average values and standard deviations of the mechanical properties of as-welded and heat-treated laser-welded joints are compared and the statistical significance is given Table

I.

Alloys Ceramco-0 As-welded Heat-treated (A) Significant Confidence level SMG-2 As-welded Heat-treated (A) Significant Confidence level Ney G-3 As-welded Heat-treated (B) Significant Confidence level Ney B-2 As-welded Heat-treated (C) Significant Confidence level Jelenko F As welded Heat-treated(D) Significant Confidence level

S. D. of yield stress (p.s.i.)

Ultimate tensile strength (psi.)

S. D. of ultimate tensile strength

38,200 19,200 Yes 0.005

2,600 2.200

38,200 37,400 No -

2,600 4,600

35,200 37,500 No -

4,100 2.600

35,200 43,000 Yes 0.005

47,300 77,600 Yes 0.005

6,600 5.200

35,200 38,600 Yes 0.010 34,700 40,700 Yes 0.005

Yield stress (p.s.i.)

Per cent of elongation

S. D. of per cent of elongation

0.0

0.0

10.2 Yes 0.005

2.3

4,100 4,300

0.0 1.0 Yes 0.005

0.0 I.0

47,300 82,400 Yes 0.005

6,600 8,400

0.0 0.6 Yes 0.005

2,900 2.900

48,600 45,900 No -

5,400 2,600

9.6 3.2 Yes 0.005

4.6 2.7

1,800 3.100

48,600 54,400 Yes 0.010

3,400 6,000

12.7 7.5 Yes 0.010

4.5 4.3

0.0 0.6

LeEend: A, fired three times at 1,200” to 1,800” F.-air cooled each time; B, 1,300” F. for lOminutes--water quench-600” -F. for 30 minutes-water quench; C, 1,300” F. for 10 minutes-water quench-450” F. for 30 minutes-water quench; D, 1,300” F. for 10 minuteswater quench-500” F. for 15 minutes-air cool.

Volume 36 Number 6

Heat

treating

laser-welded

gold alloys

657

~ 60,000

cl welded OS

’G 70,000

7.5%

60,000

heat llln!ll treated

s 50,000 2 40,000 b

1.0%

3opoo

Q 20,000 z 2

0%

10,000 .“I

G-3

6-2

F SMG-2 ALLOY

0

Fig. 3. The yield stressesof as-welded and heat-treated alloys are compared. cross section of the tensile specimens measured 0.058 by 0.087 inch. This size approximates the conventional solder joint in fixed prostheses. All specimens in the welded group were welded with an International laser model NWS-J.* The specimens were welded on all four surfaces in the manner developed by Gordon, Burnett, and Smith.2 First, they were welded with a 0.015 inch spot size at 2.5 joules of energy and then overwelded with an 0.030 inch spot size at 5.0 joules of energy. All welding was done for 2 msec. Each weld spot was overlapped by one half to one third to ensure complete welding. The welded specimens were then pulled on the Instron tensile testing machine at a crosshead speed of 0.02 inch per minute. A second group of welded samples were heat treated in a furnace (as listed in Table I) according to the manufacturers’ recommendations before they were tested in the Instron machine. The heat treatment used for the ceramic golds simulated the clinical treatment for a porcelain veneer which is fired three times. The yield stress was determined by the intersection of a 0.2 per cent offset elongation with the stress-strain curve. The per cent of elongation was calculated by dividing the change in specimen length by the gauge length. The change in specimen length is determined from the crosshead speed of the Instron and from the stress-strain curve.

RESULTS Table I reports the average values for the test data obtained from all welded samples including percentage of elongation, yield stress, ultimate strength, and standard deviations. The Student t test was applied to the average values, and the standard deviations were measured for the as-welded and heat-treated conditions for each alloy. From this calculation, the statistical significance and level of confidence of the difference between the as-welded and heat-treated values were determined. As noted from Table I, most heat-treated and as-welded values differ significantly, but small differences may not be clinically significant. The bar graph (Fig. 3) compares the useful or yield stress and per cent of *International

Laser Systems, Inc., Orlando, Fla.

658

Eshleman,

Svitter,

and Moon

J. Prosthet. Dent. December, 1976

elongation of each alloy after welding and again after heat treatment. The percentage of elongation for the as-welded condition is demonstrated in the bars of the graph. The percentages shown outside the bars represent the elongation after heat treatment. In the ductile welded samples, most of the elongation occurred in the weld zone. The ductility reported is, therefore, a very conservative estimate of the elongation within the weld joint since the weld zone is much smaller than the gauge length. When the welded joints were brittle, the yield stress was that stress reported at fracture. DISCUSSION The Ney G-3, a type IV alloy, exhibited no ductility in the as-welded condition and only a little when heat treated in the furnace. However, the alloy does have a high yield stress when heat treated. For Ney B-2, a type III alloy, heat treatment probably degrades the clinical usefulness of the weld. The yield strength is only slightly increased, but the desirable property of ductility is greatly diminished in the heat-treated condition. Heat treatment of welds for this alloy is not recommended. Jelenko F, another type III alloy, responds somewhat differently to heat treatment. As compared to B-2, there is a greater increase in yield strength with retention of adequate ductility. The two ceramic-metal alloys, Ney SMG-2 and Ceramco-0, had no ductility in the as-welded condition and similar yield stresses, But, after passing them through the firing cycle, which simulates the clinical application of porcelain, the SMG-2 had no statistically significant change in yield stress and a minimum amount of ductility. On the other hand, Ceramco-0 demonstrated a drastic decrease in the yield stress and a great increase in ductility. A yield stress value that is much below 30,000 p.s.i. is thought to indicate questionable strength for clinical application. For this reason, laser welding of Ceramco-0 is not recommended. The variable behavior of the alloys observed in this study demonstrated the need to individually evaluate any alloy to be laser welded before clinical application. SUMMARY (1) Three alloys, G-3, SMG-2, and Ceramco-0, showed no ductility in the aswelded condition. (2) The Ney G-3 as-welded samples had no ductility, but heat treating greatly increased their yield stress. (3) The simulated ceramic firing cycle created a small amount of ductility in SMG-2, but the lowering of the yield stress in Ceramco-0 renders the welds dangerously weak even with improved ductility. (4) The Jelenko F as-welded specimens were acceptable. Heat treatment improved the joints slightly, but this is not considered necessary. (5) The Ney B-2 as-welded specimens produced satisfactory joints which did not benefit from heat treatment. The large loss in ductility with heat treatment is considered undesirable. (6) The unpredictable behavior of Ceramco-0 in this study suggests the need to evaluate each alloy individually before clinical application.

Volume 36

Heat

Number 6

treating

laser-welded

gold alloys

659

References 1. Huling, J. S., Jr.: An Evaluation of Distortion of a Three-Unit Fixed Prosthesis Laser Welding, Conventional Soldering, or Cast in One Piece, Thesis, Tufts Boston, Mass., 1975. 2. Gordon, T. E., Jr., Burnett, A. P., and Smith, D. L.: Laser Welding of Gold Dent. Res. 51: 161-167, 1972. 3. Gordon, T. E., and Smith, D. C.: Laser Welding of Prostheses, an Initial PROSTHET. DENT. 24: 472-476, 1970.

Joined by University, Alloys,

J.

Report,

J.

DRS. ESHLEMAN AND MOON MEDICAL COLLEGE OF VIRGINIA SCHOOL OF DENTISTRY RICHMOND, VA. 23298 DR. SVITZER 414 S. POLLARD ST. VINTON, VA. 24179

IADR Clinical

evaluation

PROSTHODONTIC

of porous

rooted alumina

ABSTRACT ceramic

dental

implants

L. J. Peterson, J. J. Klawitter, A. M. Weinstein, B. M. Pennel, and R. M. McKinney. Medical College of Georgia, Augusta, Ga., and Tulane University, New Orleans, La. The purpose of this study was to evaluate porous rooted AlzOJ ceramic implants. A120r has high strength, is chemically stable, is biocompatible, and is able to accept tissue ingrowth when fabricated in a suitable form. Cylindrical implants measuring 5.5 mm in diameter and 16 mm in length, 10 mm of which was the porous root portion, were implanted in dogs. Two pore sizes were evaluated, a small pore size designed for attachment by fibrous tissue ingrowth and a large pore size designed for bone tissue ingrowth. The small pore implant had a mean pore diameter of 25pm and 40% porosity; the large pore mean diameter was 350pm with a 50% porosity. Forty-three implants, 21 large and 22 small pore, were placed and splinted for eight weeks. Clinical evaluation of implant mobility, pocket depth, and radiographic appearance was performed at 1, 2, 4, and 6 months. The early response of the investing tissue was a rather severe inflammatory response. Bone loss varying from slight to severe was seen radiographically. When the Bone resorption continued and adfixation was removed , 27 implants were exfoliated. ditional implants were lost, until, at 6 months, all 43 implants were lost. Re-examination of the dense material of the crown portion of the tooth revealed a porosity approximately 7am in diameter which allowed the migration of bacteria through the implant and thus bypass of the biological seal supplied by peripheral tissue adaptation. The A1203 implants uniformly failed due to bacterial invasion of the bony socket through the microporous structure of the ‘non-porous” crown portion of the implant. Reprinted from the Journal editor, and the American Dental

of Dental Association

Research with permission (copyright holder).

of the author,

the