Strength properties of soldered joints from various ceramic-metal combinations

Strength properties of soldered joints from various ceramic-metal combinations Robert S. Staffanou, D.D.S., M.S.,* Malcolm D. Jendresen, D. D. S. * * * University

of California,

Ryle A. Radke, D.D.S.,**

School of Dentistry,

San Francisco, Calif.

M

ost available information on the strength properties of soldered joints deals primarily with precious metals soldered to precious metals’ or with gap distance in relation to joint strength.’ However, with the ever changing prices of gold, the use of precious metals in the dental practice is diminishing. The ability to join base metal alloys, semiprecious alloys, and precious metal alloys, in varying combinations, will undoubtedly become more desirable in the future. Clinical experience indicates that gold alloys seem to be superior from the standpoint of workability in marginal finishing, and that they are less technique sensitive than the harder base metal alloys. It seems likely, however, that the joining of different structured alloys by means of soldering will be essential. Very little information has been published dealing with the successful joining of base metal, precious, or semiprecious alloys. A recent article states that the joining of precious and nonprecious alloys is not possible,” while an article published in 1974 implies that successful soldering of these alloys is possible.’ Another project also supports the premise that the precious metals can be successfully soldered to semiprecious metals with the statement “. . under softened conditions, the hardness of the joint was greater than that of the mother metal.“” In 1975, Preston” stated that laser fusion of different metals

Supported 50019

in

part by in part

and

NIDR Interagency by NIH Research

Agreement Support

YOl-DEGrant RR-

05305. Presented before the Pacific Coast Society of Prosthodontists, San Francisco, Calif., and the American Academy of Restorative Dentistry, *Associate tistry. **Assistant ***Professor Dean for

Chicago, Clinical Professor,

has resulted in joints superior to conventional soldered joints. This may indeed be the technique of the future, but it is impractical at this time. It is well documented that the soldering of precious metal to another precious metal, when properly executed, results in joints that are close to, and in some cases superior to, the strength of the parent metal.’ Likewise, several articles deal with the need for proper gap space and design to create the strongest possible solder joint.‘. ‘. ” Other investigators have addressed the problem of soldering porcelain units both prior to and after porcelain application.“-” Since the joining of metal alloys in all combinations will undoubtedly become increasingly important, this study was designed to evaluate the strength properties of soldered joints between all combinations of precious, semiprecious, and base metal alloys. Part I was designed to evaluate postsoldering procedures with low-fusing solders (Alboro HF* 1,565 to 1,585” F). This part evaluates the assembly of restorative units after the porcelain has been applied to the metal. Part II was designed to evaluate high-heat (torch) soldering of fixed partial dentures prior to porcelain application.

PART I: POSTSOLDER STUDY Materials and methods Preparation of specimens. Plastic patterns containing four specimens 0.25 by 1.0 cm were sprued and invested in Accu-Cast? ceramic investment. The investment molds were then placed in a cold oven, the temperature raised to 1,300” F in 1 hour, and heat soaked for 2 hours. The alloy specimens were

Ill. Professor,

Department

Department

and Chairperson, Research.

OOZZ-3913/80/010031

and

+ 09$00.90/O

of

Restorative

of Restorative Division

ID 1980

Den-

Dentistry.

‘J.

of Biomaterials;

Associate

The

Co.

C. V. Mosby

F. Jelenko

tAccu-Cast bles Div.,

Co., Ceramic Columbus,

New

Rochelle,

Investment, Ohio.

THE JOURNAL

N. Y. Dent-tal-Ea,

OF PROSTHETIC

Mfg.,

Consuma-

DENTISTRY

31

STAFFANOU,

Fig. 1. Metal soldering.

specimens

apposed

and

invested

RADKE,

AND

JENDRESEN

for

Fig. 3. Photomicrograph of base metal (B) soldered (S] to base metal (B). Dark line between solder (S) and metal (B) is interpreted as an oxide layer.

Fig. 2. Photomicrograph (S) to precious metal (9.

of precious

metal

(P)

soldered

then cast with an induction-type casting machine to standardize the casting procedure. Soldering procedure: Blocks of dental investment were made to align and support the metal specimens for soldering. The metal specimens were visually opposed on the investment supports and with the use of a template invested at a gap distance of approximately 0.3 mm’ (Fig. 1). A piece of solder was then placed over the joint, and the specimen was placed in a porcelain oven which had been preheated to 1,400” F. Immediately upon placing the specimen in the oven, 30 inches of vacuum was obtained. When the invested metal was at 1,400” F, the temperature of the furnace was raised as rapidly as possible until the solder flowed (1,585” to 1,600” F). This method is described by Skinner” who states that one should solder in the shortest possible time at the lowest possible temperature to reduce porosity. As soon as the solder flowed, the temperature was reduced, the vacuum released, and the specimen removed and bench cooled to room temperature. During soldering it was noted that the precious metals and semiprecious metals needed very little flux application, whereas base metal specimens required a very heavy flux application to ensure 32

Fig. 4. Higher magnification, of base metal (B) solder (S) interface showing less concentration of suspected oxide layer than Fig. 3. proper solder flow. After soldering, all joints were finished to as near the original specimen diameter as possible. Two specimens of each metal combination were embedded in Bake-Lite,* metallurgically polished, and inspected at various magnifications on a Metallograph.? Six specimens of each combination were subjected to testing on the Instron Universal Testing MachineS (with a crosshead speed of 0.02 m/min) to *Bake-Lite $Unitron Mass. SInstron Canton,

Ring

Forms,

Metallograph, Universal Mass.

Buehler

Ltd.,

Unitron

Instrument

Testing

JANUARY

Machine,

1980

Evanston,

Instron

VOLUME

111. Co.,

New

Engineering

43

Highland, Corp.,

NUMBER

1

STRENGTH

Table

PROPERTIES

I. Strength

OF SOLDERED

of soldered

JOINTS

alloy

specimens Average Yield

Soldered

alloys*

Precious-precious metal Semiprecious-semiprecious metal Base metal-base metal Precious-base metal Semi-precious-base metal Semiprecious-precious metal “As cast” alloys Jelenko “0” Jelstar Jelbon

strength (Psi)

57,000 30,000

DENTISTRY

0.2)

25,000,OOO 7,600,OOO

63,000 76,000

(1.4 (4.2) (13.7) (0.7)

49,000

(3.6)

70,500 96,000

(0.6)

to 1585”

(5.3)

45,000

(7.6) (5.0)

Modulus elasticity

percentage elongation

(5.9) (17.5)

6.5%

(3.5) (2.6)

0.3% 3.4%

of

2.0%

(16.3)

2.0%

(1.7)

7,500,000

(3.0)

8.4%

60,000

(3.0)

105,000 96,000

(6.7) (5.3)

12,300,OOO 15,600,OOO

(9.4) (0.6)

6.4% 17.0%

29,100,OOO

(0.6)

0.5%

65,000 48,000

12,000,OOO

of

F).

“As cast” specimens of each metal type were subjected to conventional tensile tests, using the Instron Universal Testing Machine to obtain a baseline for comparison with the soldered specimens. The soldered specimens subsequently were subjected to the same tensile tests. The results of this testing are shown in Table I. Precious metal-to-precious metal combination (Fig. 2). This joint showed good surface apposition and contact at the solder-metal interface and ‘was deemed to be a good joint. Tensile stress/strain data supported this with a yield strength of 28,130 psi and an ultimate tensile strength of 43,845 psi (Table I). Fifty percent of these specimens fractured in the joint, and’50% fractured within the metal, indicating that the joint was as strong or stronger than the parent metal. Base metal-to-base metal combination (Fig. 3). This solder joint showed a suspected oxide layer between the solder and the metal specimen, but the tensile results (Table I) again showed that this does not detract from the joint strength. This suggested oxide layer was less apparent at high magnification (Fig. 4). The yield strength and the ultimate tensile strength equaled 76,420 psi. All base metal specimens failed at the joint. Semiprecious metal-to-semiprecious metal combination (Fig. 5). This joint showed a dark line at the interface between the solder and the metal specimen. This area was suspected to be an oxide layer which formed before the solder wet the solid surface. It appeared that this oxide layer was not detrimental to OF PROSTHETIC

10,000,000 13,000,000

44,000

(2.1)

Results

JOURNAL

(3.2) (3.6)

(3.2)

51,000 76,000

evaluate the yield strength, ultimate tensile strength, modulus of elasticity, and percentage of elongation of the soldered specimens.

THE

tensile (psi)

28,000

29,000

Coefficient of variation (%); n = 6. *Solder = Alboro HF, J. F. Jelenko (1565”

Ultimate strength

the joint. The tensile’ testing results (Table I) supported this premise producing a yield strength of 5 1,000 psi and an ultimate tensile strength of 62,800 psi. Fifty percent of these specimens fractured in the joint area, and 50% of the specimens failed in the metal, giving further proof of the joint integrity. Precious metal-to-base metal combination (Fig. 6). This specimen showed good joining between the solder and the precious metal. The suspected oxide layer again appeared on the base metal specimen. Tensile tests (Table I) supported the integrity of this union with an average yield strength of 29,000 psi and an ultimate tensile strength of 45,900 psi. Fifty percent of these specimens failed at .the joint area, and 50% fractured in the precious metal, indicating that the joint was comparable to the strength of the precious metal. Semiprecious metal-to-base metal combination (Fig. 7). Good union of these specimens was seen at the solder joint. An oxide layer was seen on the surface of both specimens where they abut the solder. The tensile test results showed a joint with intermediate strength properties (Table I), a yield strength of 57,000 psi, and an ultimate tensile strength of 65,300 psi. Fifty percent of these specimens failed in the joint area, and 50% of the specimens failed in the semiprecious metal, showing a joint strength comparable with the strength of semiprecious metal. Semiprecious metal-to-precious metal combination (Fig. 8). The photomicrographs of this joint showed close contact of both metals with the solder. The oxide layer on the semiprecious metal specimen was less pronounced in this combination. The tensile tests showed an acceptable joint in relation to the metal strength properties with a yield strength of 30,200 psi and an ultimate tensile strength of 48,000 33

STAFFANOU,

Fig. 5. Photomicrograph of semiprecious (SP) metal soldered (S) to semiprecious (SP). This combination also shows a suspected oxide layer at metal (SP) solder (S) interface. Black voids are areas of porosity.

psi (Table I). All of these specimens fractured within the precious metal, showing a joint comparable with the strength of the ceramic gold alloy.

Discussion and conclusions Soldered joints in all of the metal combinations tested appeared to have satisfactory strength for use in intraoral restorations. Tensile test results of all joints would tend to substantiate this finding. Tensile testing showed all joints to be in an acceptable strength range for clinical use based on clinical success of soldered joints in service. All joints were as strong, or stronger, than the weaker of the two metals used in combination. The base metal, however, is stronger than the solder itself, hence the reason that all base metal-to-base metal tensile failures occurred in the joint area. The precious metal exhibited a softening through the casting and soldering proce: dure. The manufacturer’s specifications for this metal are a yield strength of 65,300 psi and an ultimate tensile strength of 72,500 psi. After casting, the precious metal had a yield strength of 49,000 psi and an ultimate tensile strength of 60,000 psi. After soldering, this dropped to a yield strength of 30,000 psi and an ultimate tensile strength of 43,800 psi. This softening effect reflects either heat treatment of the alloy or atomic diffusion which completely changes the composition of both the solder and the gold alloy with resultant weakening and embrittlement of the joint. The semiprecious metal produced a slight drop in yield strength and ultimate tensile strength when soldered, with the most dramatic property change in

34

RADKE,

AND

JENDRESEN

the percentage of elongation. The manufacturer’s specifications and the “as cast” specimens had a 20% elongation. After soldering, the percentage of elongation dropped to 2%, showing a significant effect of heat hardening. Base metal alloy specimens showed a drop in yield strength and ultimate tensile strength in the soldered specimens. As mentioned before, this is assumed to be an indication of solder strength rather than strength on the cast metal itself. The manufacturer states that the percentage of elongation is 2%, but in this study, 0.5% elongation was the maximum obtained, with most specimens falling to 0.3% for both the “as cast” and the soldered specimens. Semiprecious and base metal alloy specimens showed surface oxidation before the solder flowed over the opposing surfaces. This was more evident in the base metal alloys. Precious metal could be soldered with minimal fluxing, semiprecious metal required a light fluxing, and base metal specimens required heavy fluxing to ensure solder flow and wetting. This heavy flux requirement could lead to staining problems if procelain existed at the areas adjacent to the solder joint. It is possible that on dissimilar metal combinations, the area assumed to be an oxide layer could be an artifact caused from the difference in the polishing rates of the two metals. Further studies are indeed necessary to answer the obvious question concerning the effects of corrosion, the formation of new alloys in the solder joint area, and the effect upon the mechanical properties of these soldered combinations. It should be noted that dissimilar metal combinations increase the risk for corrosion.‘3 It is impossible to predict the degree of the effects of corrosion on the mechanical properties; however, the effects are almost always detrimental.” In all tests, the joint strength obtained in the study was considered to be acceptable in relation to the strength of the cast metal itself.

PART II: PRESOLDER STUDY Methods and materials Preparation of specimens. Same as for Part I. Soldering procedure. Blocks of dental investment were made to support the metal specimens for soldering. The metal specimens were opposed on the investment supports at a gap distance of 0.3 mm and invested as in Part I. The joints were then fluxed, and the specimens were placed in an oven preheated to 900” F for 15 minutes. After removal from the on a soldering block, oven, they were placed

JANUARY

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Fig. 6. A, Precious (PI metal soldered (S) to base (B) metal. Lower magnification shows dark line suspected to be oxide formation. B, This is much less apparent upon higher magnification and could be an artifact.

Table II. Strength

of presoldered

alloy

specimens

Yield Presoldered

alloys*

strength (psi)

Precious-precious Semiprecious-semiprecious Base-base Precious-base Semiprecious-base Semiprecious-precious “As cast” alloys Jelenko “0” Jelstar Jelbon

THE JOURNAL

Thermometer, Corm.

OF PROSTHETIC

Model

DENTISTRY

Modulus of elasticity 8,030,OOO

Average percentage elongation

of

(13%)

30,650

(4.7%)

(20%)

1.2%

(6.5)

46,000

(2.0%)

50,500

(2.8%)

14,000,000

(3.5%)

55,800 37,700 53,000

(2.7%)

(2.7%) (7.6%) (3.4%)

25,160,OOO 13,400,000 13,000,OOO

(15%) (9%) (8.7%)

(2.0) (0.3)

(5%) (0.8%)

55,800 50,800 67,000

1.2% 0.3% .?.5% .3.3%

(3.4) (2.0)

32,600

(3%)

38,300

(12%)

7,400,ooo

(14%)

1.6%

(8.4)

49,000

(3.6%)

60,000

(3.8%)

12,300,OOO

(9.4%)

0.4%

70,500 96,000

(0.6%) (5.3%)

105,000

(6.%) (5.3%)

15,600,000

(0.6%)

17.0%

29,100,OOO

(0.6%)

0.5%

combinations)

refluxed, and soldered with a gas-oxygen torch. The specimens were then allowed to bench cool to room temperature. To observe and record the soldering temperature at the joint, an infrared optical thermometer* was employed (Fig. 9). This instrument reads infrared radiation from heated metal during the soldering procedure and translates this infrared calorimetric reading into degrees Farenheit on a digital readout scale. Prior to testing the soldered joints, “as cast” specimens of each alloy type were subjected to conventional tensile testing, using the Instron Universal Testing Machine (with a crossInfrared Stamford,

tensile (psi)

25,400

Coefficient of variation = (%); n = 6; cross head speed = 0.02 m/set. *Solder = Jelenko “0” (precious and semiprecious

*Optitherm neering,

Ultimate strength

12-8723,

Barnes Engi-

96,000

and Jelbon

(all base metal combinations)

head speed of 0.02 m/min). This was done to obtain a baseline for comparison with the soldered specimens. The soldered specimens were finished back to their original joint diameter and then subjected to the same tensile tests (Table II). Two specimens of each sdldered combination were embedded in BakeLite, metallurgically polished, and viewed on a metallograph.

Results Precious metal-to-precious metal combination (Fig. 10). This combination showed good surface apposition at the solder-metal interface. The solder used (Jelenko “0”) had a fusing range of 2,010” to 2,090” F. The optical thermometer showed a solder-

35

STAFFANOU,

Fig. 7. Base (B) metal soldered metal.

(S) to semiprecious

(SP)

RADKE,

AND

JENDRESEN

Fig. 10. Precious metal (P) soldered to precious (PI metal. Solder exhibits large grain structure.

Fig.

Fig. 8. Precious metal (P) soldered (S) to semiprecibus (SP) metal. Irregular outline of precious (P) specimen is probably a result of polishing.

11. Base metal (B) soldered (S/ to base metal (8). Black voids are areas of porosity. Solder (S) metal (8) interface is indistinct.

Fig. 9. Optical

Fig. 12. Semiprecious (SP) metal soldered (S) to semiprecious metal. Some fuzziness is evident as solder (S) metal (SP) interface.

via infrared 36

thermometer for recording radiation from the specimen.

temperature

JANUARY

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Fig. 13. Precious metal (P) soldered (S/ to semiprecious (SP] metal. Precious IP) solder (S) interface is indistinct, indicating atomic diffusion.

Fig. 14. Semiprecious (SP) metal soldered (S] to base (B) metal. Metal-solder interfaces are very indistinct and diffuse, indicating a high degree of atomic diffusion.

ing temperature range for all specimens from 1,997” to 2,137” F, with an average working temperature of 2,066” F. This measurement gave assurance that the solder was fused in the proper temperature range without grossly overheating or underheating the metal. Tensile stress/strain data showed a yield strength of 25,400 psi and an ultimate tensile strength of 30,650 psi. These values were slightly lower then those of the postsoldered group (Table I). The solder joint showed large-grain structure and porosity consistent with higher heat application. Eighty-three percent of the fractures during the tensile test occurred in the joint area, indicating that these joints were somewhat weaker than the postsoldered joints. However, they were still in the range of the postsoldered joints from a strength standpoint. Base metal-to-base metal combination (Fig. 11). This joint showed large areas of porosity and a rather indistinct interface between the solder and the metal specimen. This indicated atomic diffusion due to the high heat necessary to create solder flow. The optical thermometer indicated a temperature range from 2,076” to 2,188” F with an average working temperature of 2,123” F. These specimens were soldered with Jelenko’s Jelbon solder (2,250” F). The tensile data revealed a yield strength of 55,800 psi and an ultimate strength of 55,800 psi, which was some 20,000 psi lower than the postsolder data from the same combination. This lowering would be consistent with the finding of atomic diffusion which causes weakening and embrittlement of the joint. All fractures of this metal combination were in the joint area, as were the postsolder specimens. Semiprecious metal-to-semiprecious metal com-

Fig. 15. Base (B) metal soldered to precious (P) metal. Metal-solder interfaces are indiscernible and show a high degree of atomic diffusion.

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DENTISTRY

bination (Fig. 12). This joint area showed a definite demarcation but a somewhat fuzzy appearance at the solder-metal interface, showing that some atomic diffusion had taken place. The tensile stress data showed a yield strength of 46,000 psi and an ultimate tensile strength of 50,500 psi, which is 5,000 and 13,000 psi, respectively, lower than the strength of the postsoldered joints. Eighty-eight percent of the fractures in this metal combination were in the joint area. The strength should be sufficient for oral restorations. Again, the optical thermometer showed a fusion range from 1,960” to 2,175” F with an average working temperature of 2,036” F. This combination was soldered with Jelenko “0” solder (2,010” to 2,090” F). 37

STAFFANOU,

Semiprecious metal-to-precious tion (Fig. 13). Photomicrographs

metal combina-

showed close contact of both metals with the solder but with a definite tendency toward atomic diffusion on the precious metal side. This was demonstrated by the lack of a sharp demarcation between the solder and the precious metal itself. The solder also exhibited very large-grain structure and some porosity from the high heat necessary to solder these alloys. This grain growth was due to diffusion occurring in the solder, whereby small solder grains coalesced into larger ones from the high heat application and subsequent cooling. During soldering the optical thermometer showed a soldering temperature of 1,95 1’ to 2,35 1’ F with an average fusion temperature of 2,092” F. Tensile testing showed the yield strength to be 32,600 psi (vs 30,000 psi for the postsoldered joints) and an ultimate strength of 38,300 psi (vs 48,000 psi for the postsoldered joints). All of the specimens fractured in the joint area. This would be compatible with the photomicrograph finding of large-grain structure and atomic diffusion (both of which tend to weaken the joint). This metal combination was soldered with Jelenko “0” solder. Strength properties should be sufficient for clinical restorations.

Semiprecious metal-to-base-metal combination (Fig. 14). Photomicrographs of these joints showed a very diffuse, indefinite joint area. It was difficult to discern any definitive solder-metal interface. There was a high degree of atomic diffusion as a result of the heat application necessary to achieve solder flow. The optical thermometer recorded a soldering temperature range of 2,187” to 2,234” F with an average soldering temperature of 2,210” F. This combination was soldered with Jelbon solder (2,250” F). It was a difficult metal combination to solder, and perhaps the heat application was prolonged to achieve flow. If so, this would account for the greater amount of atomic diffusion. Tensile testing showed a yield strength of 53,000 psi which was not markedly different from the 57,000 psi obtained with the postsoldered specimens. The ultimate strength was 67,000 psi, which was similar to the 65,000 psi obtained in testing of the postsoldered joints. Sixtytwo and one-half percent of the specimens fractured in the joint area and 37.5% fractured in the semiprecious metal, indicating that the joints were as strong as the weaker of the two metals and adequate for clinical restorations.

Precious metal-to-base metal combination (Fig. 15). Photomicrographs of this combination showed

38

RADKE,

AND

JENDRESEN

evidence of atomic diffusion by virtue of the indiscernible demarcation between the solder and the metal specimens. Ninety percent of the fractures during testing of this combination were in the joint, and 10% were in the cast precious metal. The optical thermometer showed a soldering temperature range from 2,020” to 2,116” F with an average soldering temperature of 2,070” F. This combination was soldered with Jelbon solder. Tensile testing showed a yield strength of 37,700 psi, which was considerably higher than the 29,000 psi obtained in the postsoldered joints. The ultimate strength was 50,800 psi, which was approximately 5,000 psi higher than the postsoldered joint strength. The joint strengths were acceptable for clinical restorations. However, this was an impractical combination due to the fact that the fusing temperature of the solder (2,250” F) was very close to the casting temperature of the precious metal (2,300” F), and a great deal of sagging was noted. Undoubtedly, this temperature incompatibility also contributed to the high degree of atomic diffusion at the joint.

Discussion As a general observation through this portion of the study, the photomicrographs of the presolder joints (high heat) showed the lack of a crisp demarcation between the solder and the metal being joined. Postsoldered joints were very definite. This lack of definition indicates atomic diffusion from the amount and duration of the heat necessary to fuse the solder. This is not surprising, since high-fusing solders flow from only 200” to 300” F. below the melting range of the parent metal. Some joints showed large-grain growth, again as a’result of the heat application and subsequent cooling. Bench cooling from soldering temperature to room temperature promotes grain growth which can weaken the joint. Ideally, soldered restorations should be quenched after 5 minutes of bench cooling. To standardize the treatment of the specimens, however, it was decided to allow all specimens to cool to room temperature after soldering. Since all were treated in exactly the same manner, the results can be compared. The base metal alloys seemed to show greater amounts of atomic diffusion than the precious metals. Some joints showed high degrees of porosity, which is caused by the high heat volatizing the base metal components of the solder and creating porosity. Tensile testing did show some differences in joint strengths (Table II). The strengths of precious-

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to-precious, base-to-base, semiprecious-to-semiprecious, and semiprecious-to-base were essentially the same as the postsoldered joint strengths. The precious-to-base combination gave a somewhat higher strength value than that of the postsoldered joint of the same combination. It is felt that none of the joint strengths, whether presoldered or postsoldered, showed dramatic differences. An observation made during the presolder study, however, was the high number of defective joints. Many joints fractured so early in machine loading that no results were recordable. This was markedly different from the postsoldering study in which no failures of this type occurred. Interestingly, these joints visually appeared to be good joints and could not be broken with digital pressure. Since the purpose of this study was to determine ;f acceptable joint strengths could be achieved, joints failing before results were recorded were not included. The intent was to see ifsatisfactory joints were possible, not to evaluate what percentage of the time they could be obtained. The high number of early failures can probably be attributed to the amount and duration of the heat needed to create solder fusion. The optical ‘thermometer assured us that we did not grossly exceed the solder fusion temperature. The high heat requirement, however, still resulted in atomic diffusion of the joints and grain growth, both of which contribute to joint weakening. Distortion, in the form of sagging, was also observed during the soldering of some combinations. The results indicate that satisfactory strengths can be achieved with either presoldered or postsoldered methods. The general observation was that consistently better solder joints can be accomplished, with a higher percentage of success by using the more controllable postsoldering method. Overheating with the oxygen-gas torch is difficult, if not impossible, to prevent. As mentioned in Part I, there is a definite corrosion potential created by the dissimilar metals as well as by the new alloys being formed in the solder joint as a result of atomic diffusion. This potential would seem greater with the presolder joint due to the greater amount of atomic diffusion evidenced.

THE JOURNAL

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DENTISTRY

Further research is needed to evaluate the corrosion .effect upon the mechanical properties of these soldered combinations. We

would

support study.

like

and

to thank

J. F. Jelenko

generosity

in

and

supplying

Company

the

metal

for used

their

in

this

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