alloy 2 & investment material

Casting Alloys - Part II

Contents

  • Alloys for All-Metal Prostheses:

    • High Noble and Noble Alloys

    • Predominantly Base Metals

    • Titanium and Titanium Alloys

  • Alloys for Metal-Ceramic Prostheses:

    • High Noble and Noble Alloys

    • Predominantly Base Metals

    • Titanium and Titanium Alloys

  • Alloys for Removable Partial Dentures:

    • Base Metal Alloys

    • Titanium-Based Alloys

Alloys for All-Metal Prostheses

I. High Noble and Noble Alloys

II. Predominantly Base Metals

III. Titanium and Titanium Alloys

High Noble and Noble Alloys

Types of Alloys

  • Gold-Based Alloys:

    • Type 1

    • Type 2

    • Type 3

    • Type 4

  • Silver-Palladium Alloys

Gold-Based Alloys

  • For prosthetic dental applications, elements are added to gold:

    • Platinum: Improves hardness, elasticity, and melting temperature.

    • Palladium: Traditionally used but replaced by platinum due to cost.

    • Copper: Makes the alloy heat-treatable.

    • Silver: Neutralizes the reddish color of copper and needed in palladium-based alloys for color.

Properties of Gold-Based Alloys

  • Type 1: Soft; designed for inlays not subjected to significant forces.

  • Type 2: Used widely for inlays; superior mechanical properties but less ductility.

  • Type 3: Crowns and onlays for high-stress areas.

  • Type 4: Used in high-stress areas like bridges and partial dentures.

Composition Summary

  • Changes in alloy color can be balanced by increasing copper, silver, and palladium.

  • Higher silver and copper reduce corrosion resistance.

Heat Treatment

  • Softening: 700 °C for 10 minutes, quenched in water. Increases ductility.

  • Hardening: 200-450 °C for 15-30 minutes, improves strength.

Silver-Palladium Alloys

  • Predominantly silver. Contains at least 25% palladium for nobility and tarnish resistance.

  • Copper-free alloys may mimic properties of Type 3 gold alloys.

  • Potential for tarnish and corrosion increases with copper content.

Predominantly Base Metals

Nickel-Chromium and Cobalt-Chromium

  • Used less for all-metal prostheses, mainly for small castings (crowns, FDPs).

  • Cobalt improves strength and corrosion resistance.

  • Chromium: Ensures alloy corrosion resistance and strength.

Typical Composition (Base Metal Alloys)

  • Nickel-Chromium (Ni-Cr): 58.7% Ni, 25% Cr

  • Cobalt-Chromium (Co-Cr): 62.9% Co, 23% Cr

Important Elements

  • Molybdenum: Increases corrosion resistance, strength, decreases thermal expansion.

  • Beryllium, Carbon, Manganese: Refine grain size, increase strength, but lower ductility.

Mechanical Properties of Base Metal Alloys

  • Generally higher hardness and elastic modulus than noble metal alloys.

  • Yields strength and hardness are crucial but more difficult to manipulate due to their hardness.

  • Use of die stone and expanded investment reduces fitting challenges.

Titanium and Titanium Alloys

  • titanium forms an oxide film for corrosion protection and is highly biocompatible.

  • High melting point and oxidation requires careful handling in casting.

Alloys for Metal-Ceramic Prostheses

I. High Noble and Noble Alloys

II. Predominantly Base Metals

III. Titanium and Titanium Alloys

Joining of Dental Alloys

Soldering and Brazing

  • Soldering: Below 450 °C, brazing: Above 450 °C.

  • Flux: Used to reduce oxidation on the surface, enhancing joint quality.

  • Soldering involves preparation, assembly, and heat application.

Welding

  • Fuses parts by heat, may not require a filler metal.

Investment Materials

  • Used to create molds for casting with different classifications based on binder type.

Properties of Investments

  • Gypsum-Bonded Investments: Used for lower melting alloys (~700 °C).

  • Phosphate-Bonded Investments: Used for higher melting alloys including metal-ceramic prostheses.

  • Ethyl Silicate Bonded Investments: Less common due to complex procedures but used for high-fusing applications.

Setting Time and Expansion

  • Gypsum and phosphate investments exhibit different expansion characteristics based on their binder composition.

  • Hygroscopic expansion compensates for gold alloy shrinkage during cooling.