DENTAL ALLOY JOINING 1

Dental Alloy Joining Metal-joining operations are usually divided into three categories; Brazing Soldering Welding 2

Dental Alloy Joining The primary difference between soldering and brazing is that brazing requires a heating temperature above 450 C but below the solidus temperature of the metal components being joined. Both soldering and brazing require a filler metal usually called a solder. 3

Dental Alloy Joining Soldering operations are done below 450 C and Brazing operations are done above 450 C 4

Dental Alloy Joining For some unknown reason in dentistry the term soldering is used to describe the joining of metal components using a filler metal. This is technically incorrect as all metal joining in dentistry is generally done above 450 C which is the process of brazing. 5

Dental Alloy Joining So we too will use this term soldering to describe the process of joining metal components together. 6

Dental Alloy Joining The difference between soldering (brazing) and welding is that welding may not require a solder, the metal surfaces to be joined are fused locally. Welding is also generally done at higher temperatures than soldering and generally involves the melting of one or more of the surfaces being joined. If solder is used, its melting point is similar to that of the metal being joined. 7

SOLDERING 8

Soldering Soldering can be used for two purposes: Joining as in the fabrication of a fixed partial denture, where different components are cast individually and then joined together. 9

Soldering Building to build up regions of metal restorations where the dimensions should be increased, such as for missing proximal contact areas. Some casting defects can be corrected with soldering. 10

PROCESS OF SOLDERING 11

Process of Soldering The action of solder on the surface of a metal or alloy is to form a continuous metal structure in which there is metal crystal grain growth between the solder and the alloy. There is the formation of strong intermetallic bonds. 12

Process of Soldering Bonding is contingent on wetting of the joined surfaces by the solder, and not on melting of the metal components. When a solder joint is done properly, there should be no fusion or alteration of the two components being joined. 13

IDEAL PROPERTIES OF A DENTAL SOLDER 14

Ideal Properties of a Dental Solder Lower fusing 50 C-100 C lower than melting point of the components, so as to prevent the distortion and melting Free-flowing Silver tends to help the solder adhere and flow freely to dental gold alloys. Solders with a higher melting temperature have a lower surface tension and flow more easily through narrow gaps 15

Ideal Properties of a Dental Solder Strong should be as strong as the components being melted Corrosion resistant, so as to prevent tarnishing and discolouration Non-pitting more pitting occurs when there is an increased amount of base metal in the solder 16

Ideal Properties of a Dental Solder Same colour the colour of the solder should match that of the alloy Solder must be compatible with the oxide-free parent metal, but it does not necessarily have a similar composition 17

HEATING SOURCE FOR SOLDERING 18

Heating Source for Soldering Torch flame: the same gas/air combination used for casting dental alloys Furnace: this is generally done in a porcelain firing furnace but a casting furnace may also be used. Less heat is lost to other parts of the restoration frame work or the ambient atmosphere when using a furnace. 19

SOLDERING INVESTMENT 20

Soldering Investment Soldering investment is used as a master cast to assemble the parts of a metal frame work and hold them in place. It can also be used to protect very thin areas of the metal restoration, such as fine metal margins. 21

Soldering Investment Soldering investments should not expand as much as casting investments 22

PRINCIPLES OF SOLDERING 23

Principles of Soldering The aim of soldering is to form a good adhesive bond; however this is not possible if oxides are allowed to form on the surface of the alloys prior to soldering. If soldering is carried out correctly it will be the outer layer of the crystal grains, which are changed, and not the total structure. 24

Principles of Soldering For soldering to be successful basic practical steps should be followed: Ensure the surface of the alloy is free from dirt and oxides Place the components as close together as possible without touching 25

Principles of Soldering If using soldering investment use the smallest amount possible Place anti-flux over the alloy where solder is not required Place flux in the joint to be soldered 26

Principles of Soldering Select a suitable solder which melts 50 C-100 C below the melting point of the components Heat the components evenly by either using the reducing zone of the flame or a furnace 27

Principles of Soldering When evenly cherry red apply the solder if not already present Do not remove the heat before the solder is applied or melted, as oxides will form on the surface of the alloy and soldering will fail 28

Principles of Soldering 31

LASER WELDING 33

Laser Welding A laser generates a coherent, high intensity pulse of light that can be focused. By selecting the duration and intensity of the pulse, metals can be melted in a small region without extensive microstructural damage to the surrounding areas. 34

Laser Welding In laser welding, the beam is focused at the joint to melt the opposing surfaces. Due to the expansion from the locally high temperature and the change of state, the two liquid surfaces contact and form a weld on solidification. 35