CAST AND WROUGHT BASEMETAL ALLOYS Dental applications Cast

CAST AND WROUGHT BASE-METAL ALLOYS

Dental applications � Cast Co/Cr alloys � RPD framework � Porcelain-metal restorations � Cast Ni/Cr alloys � RPD framework � Cr and bridges � Porcelain metal restorations � Cast Ti and Ti alloys � Crowns � Bridges � RPD framework � Implants � Wrought Ti and Ti alloys � Implants � Crowns � Bridges � Wrought S/S alloys � Endodontic instruments � Orthodontic wires and brackets � Preformed crowns � Wrought Co/Cr/Ni alloys � Ortho � Wrought Ni/Ti alloys � Ortho � wires and endo files Wrought beta Ti alloys( Ti/Mo) � Ortho wires

General Requirements - Non toxic non allergic. - Resistant to corrosion. - Satisfactory physical and mechanical properties. - Easy to process and handle. - Readily available, relatively inexpensive constituents

Co/Cr alloys - Almost all RPD frameworks are constructed in Co/Cr. Small percentage of frameworks are constructed in Ni/Cr. - Chromium should not be less than 20% and Co, Cr and Ni altogether should not be less than 85% of the alloy by weight. - Physical properties are controlled by the presence of minor alloying elements such as: carbon, molybdenum, beryllium, tungsten, and aluminum.

Function of various alloying elements - Cr is responsible for tarnish and corrosion resistance, but if content is higher than 30% the alloy is difficult to cast and it becomes brittle. So the maximum allowed Cr content should be 28 or 29%. - Co and Ni are interchangeable. Co increases elastic modulus (rigidity), strength and hardness more than Ni.

Functions of various alloying elements - C is essential in the alloy, if quantity is >0. 2%, the alloy becomes too hard and brittle for dental use. If quantity < 0. 2%, the alloy’s yield and ultimate tensile strengths become very low - When the alloy is Ni containing alloy, its carbon content is usually made significantly lower if it was to used for porcelain metal restorations.

- Mo in 3 -6% contributes to the strength of the alloy. - Al in Ni containing alloys forms a compound of Ni and Al (Ni 3 Al) which increases the ultimate tensile and yield strengths of the alloy considerably. - As little as 1% to 2% beryllium to Ni based alloys lowers the fusion range by about 100 C, but this will affect the alloys ductility and corrosion resistance - Si and Manganese increase fluidity and castability of these alloys. - Ni content is difficult to control unless castings are made in vacuum or under argon. Ni more than 0. 1% will reduce the ductility of the casting.

� Elevating temperature 100 degrees above melting temperature will result in a casting with poor surface due to increased reaction with the investment.

Heat treatment of base-metal alloys - Generally: heating cobalt-based alloys reduce yield strength and elongation. However, annealing Co/Cr alloys (by heating the alloy to 1225° C) will lead to increased yield strength and ductility. - When performing soldering and welding procedures, the lowest possible temperature should be used with the shortest possible time of heating.

Melting temperature - Ranges from 1400° C -1500 ° C. - Addition of 1%-2% beryllium lowers the melting temperature by 100 ° C.

Elongation - Because of their toughness, partial denture clasps cast of alloys with a high elongation and tensile strength do not fracture in service as often as do those with low elongation.

Elastic modulus - The higher the elastic modulus, the more rigid a structure - Elastic modulus of base-metal alloys is approximately double the modulus of Type IV cast dental gold alloys

- Hardness is an indication of the ease of finishing Hardness the structure and its resistance to scratching in service. - The higher hardness of the cast base-metal alloys as compared with gold alloys requires the use of special polishing equipment, which may be considered a disadvantage, but the finishing operation can be completed without difficulty by experienced operators. - It is a common practice to use electrolytic polishing for a portion of the finishing process, which reduces the time and effort necessary for mechanical finishing operations.

Fatigue fracture - Clasp arms of RPDs are most prone to fracture. - Comparisons among cobalt-chromium, titanium, and gold alloys shows that cobalt-chromium alloys possess superior fatigue resistance

Toxicity Precautions should be taken to avoid exposure to metallic vapor, dust, or grindings containing beryllium and nickel. The safety standard for beryllium dust is 2 ug/m 3 of air for a time weighted, 8 -hour day. A higher limit of 25 ug/m 3 is allowed for a minimum exposure time of less than 30 minutes. Physiological responses may range from contact dermatitis to severe chemical pneumonitis.

� A cobalt-chromium alloy without nickel or other non-nickel containing alloy should be used on patients with a medical history indicating an allergic response to nickel.

Wrought alloys

-An alloy that is wrought is one that is worked by being forged or hammered. A cast alloy is when the molten alloy is poured into a mold to give it its shape. A wrought alloy is stronger as it has been cold-worked, pounded into shape. It may have been heated and then cooled slowly to anneal it to make it stronger. Between working it and annealing it the molecules are brought closer together giving it more strength.

Dental applications Wrought Ti and Ti alloys �Implants �Crowns �Bridges � Wrought S/S alloys �Endodontic instruments �Orthodontic wires and brackets �Preformed crowns � Wrought Co/Cr/Ni alloys �Ortho wires and endo � files � Wrought Ni/Ti alloys �Ortho wires and endo files � Wrought beta Ti alloys( Ti/Mo) �Ortho wires

Wrought Stainless Steel Alloys - Steel is an iron-carbon alloy. - Stainless steel are alloys of iron and carbon that contain chromium, nickel, manganese, and other metals to improve properties and give the stainless quality to the steel. - Usually, stainless steel alloys are not cast, but instead are used in the wrought form. -Applications include: Orthodontic appliances and fabrication of endodontic instruments, such as files and reamers, temporary space maintainers, prefabricated crowns, and the various clinical and laboratory instruments.

Composition - Ferritic alloys: chromium steel alloys (15 -25%), C, Si, Mo. Used in the construction of instruments and equipment. - Martensitic alloys: Cr steel alloys with lower content of Cr (12 -18%). Can be hardened with heat treatment, used in the construction of instruments and sometimes orthodontic appliances. - Austenitic alloys: chromium 18% and nickel 8%, carbon (0. 08% and 0. 20%), titanium, manganese, silicon, molybdenum, niobium, and tantalum (in minor amounts) to give important modifications to the properties. The balance (= 72%) is, of course, iron. These have the highest corrosion resistance.

Function of alloying elements - Chromium gives corrosion resistance. Approximately 13% chromium is needed to produce corrosion resistance in pure iron, and the necessary proportion is increased with the addition of carbon to form steel. - Chromium resists corrosion well because of the formation of a strongly adherent coating of chromium oxide on the surface, which prevents further reaction with the metal below the surface.

-The formation of such an oxide layer is called passivation. The surface coating is not visible, even at high magnification, but the film adds to the metallic luster of the metal. - Increasing chromium content more than 28% will lead to the formation of a carbide layer at the boundaries of the grains, instead of the oxide, lowering the corrosion resistance and producing a brittle alloy. This process is called sensitization.

- The degree of passivity is influenced by a number of factors, such as alloy composition, heat treatment, surface condition, stress in the appliance, and the environment in which the appliance is placed. - In dental applications, the stainless characteristics of the alloys can therefore be altered or lost by excessive heating during assembly or adaptation, using abrasives or reactive cleaning agents, which can alter the surface conditions of the appliance; and even by poor oral hygiene practices over prolonged periods.

- Molybdenum increases resistance to pitting corrosion. - The elements present in small quantities (stabilizing elements) such as titanium, niobium and tantalum tend to prevent the formation of iron or chromium carbides. - Chemical action is reduced if the surface is smooth and polished. - Soldering with gold or silver produce electrogalvanic action which reduce the stainless properties.

- Heat treatment above 650° C (annealing) produces recrystallization of the micro structure of the alloy and the formation of chromium carbides with lower corrosion resistance. - Heat treatment between 400 -500 °C for 5 -120 seconds removes the internal residual stresses produced by cold working of the alloy.

SS orthodontic wires - Cold worked using pliers. - Once prepared the wires should be heat treated to 450 °C for one minute. - Soldering is difficult and requires skill. Borax fluxes are not satisfactory and the use of fluoride containing fluxes is required. - Gold or silver solders can be performed more easily. - Silver has lower melting point reducing the chance for annealing of steel.

Properties of SS wrought wire alloys

SS endodontic instruments - Numerous endodontic instruments are classified for hand use or as motor-driven instruments. - The most common instruments are the K type of root canal files and reamers. These are manufactured by machining a stainless steel wire into a pyramidal blank, either square or triangular in cross section, and then twisting the blank to form a spiral cutting edge. - A file with a rhombohedral cross section has also been introduced.

- - Mechanical properties are dependant on file geometry, direction of loading and composition. E. g Rhomohedral are less stiff than K-files. Both are weaker in counterclockwise, so twist <¼ turn.

NICKEL-TITANIUM ENDODONTIC INSTRUMENTS - Contain about 56% Ni and 44% Ti by weight, which calculate to be 50% of each by atoms. In some instances, <2% of cobalt may be substituted for nickel - These alloys can change their structure from austenitic (body-centered cubic) to martensitic (close-packed hexagonal) as a function of stress during root canal preparation. - The modulus of Ni-Ti austenite is 120 GPa, and that of martensite is 50 GPa. This effect results in what is termed super-elasticity, and when stress is removed the alloy returns to original shape without permanent deformation and the alloy becomes austenite again.

NICKEL-TITANIUM ENDODONTIC INSTRUMENTS The super-elasticity of Ni-Ti permits deformations of 8% strain in endodontic files with complete recovery (only less than 1% is allowed in SS alloys). - Has higher strength, lower elastic modulus and comparable corrosion resistance to SS. -

Base metal prefabricated crowns - - SS crowns are recommended to restore primary teeth. Have reasonable strength and hardness.

Wrought Co/Cr/Ni alloy - - 40% Co, 20% Cr, 15% Ni, and some other elements. Used for Orthodontic wires Formed and shaped then heat-treated for 7 min at 482 C.

Wrought Ni/Ti alloy - - Nitinol orhtodontic wire was introduced in 1972. Has high resiliency, limited formability, and thermal memory. 55% Ni, 45 Ti. Possesses a temperature transition range (TTR). At temperatures below the TTR, the alloy can be deformed plastically. When the alloy is then heated from below to above the TTR, a temperature-induced crystallographic transformation from a martensitic to an austenitic microstructure occurs and the alloy will return to its original shape. Hence, nickel-titanium is called a shape-memory alloy. Has comparable properties to SS, but has maximum elastic deflection.

Wrought Beta- Ti alloy - - A titanium-molybdenum alloy known as betatitanium was introduced in 1979 as a wrought orthodontic wire. Beta Ti: Body centered cubic crystal lattice. composition: 78% titanium, 11. 5% molybdenum, 6% zirconium, and 4. 5% tin and is supplied as wrought wire. Has lower strength and elastic modulus, max. deflection, lower yield strength, and good ductility, weldability, and corrosion resistance. Has large working range

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