Metals

💡General

1. Base Metal Alloys (Co-Cr, Ni-Cr): High casting shrinkage of 2.0% - 2.3% due to their high melting temperatures.
2. Gold / Noble Metal Alloys: Lower casting shrinkage of approximately 1.2% - 1.6%
3. Propane = Best gas for melting alloys
4. Vitallium = Alloy of Co-Cr
5. Ammonia = Absorbed on surface of Gold foil for protective film
6. Dental Burs = negative rake angles
7. Jeweler's rouge (iron oxide, \text{Fe}_2\text{O}_3) = extremely fine abrasive compound used as a final polishing agent to impart a high luster and shine to noble metal alloys

💡Terms

• Atomic attraction: The force of mutual pull between atoms. Pure direct filling gold undergoes cold welding when its pristine atomic lattices contact one another, creating strong metallic bonds without needing heat.
• Homogenization: A heat-treatment process (often called "solution heat treatment") used to eliminate chemical segregation or dendritic coring within a cast alloy structure to make it uniform throughout.
• Brazing: A joining process in which filler metal with a melting temperature above 450°C is melted and drawn into a thin joint space by capillary action to bond parent metals.
• Annealing: Heat-treating direct gold prior to placement to drive off volatile surface contaminants (like ammonia) and restore its cohesive capability. Annealing prepares the gold, but atomic attraction is the core principle that causes cohesion.

💡Temperature

• Above the liquidus line: The metal exists purely in a liquid state.
• Between the liquidus and solidus lines: The alloy exists in both a solid and a liquid state (a two-phase mushy zone).
• Below the solidus line: The metal has completely frozen and exists entirely in a solid state.

💡Fluxes

Fluxes used for soldering base metal alloys (such as cobalt-chromium or nickel-chromium) contain fluoride compounds (like potassium fluoride), borate glass, boric acid, and sodium pyroborate to dissolve resistant chromium oxides

💡Eutectic Alloy

A eutectic alloy is a mixture of two or more metals (or a metal and a non-metal) combined in a specific ratio that yields the lowest possible melting point of any mixture of those constituents.

💡Forms of Iron

• Ferrite (α): Soft, ductile, magnetic at room temperature (BCC structure).
• Austenite (γ): Soft, highly ductile, non-magnetic at high temperatures or when stabilized (FCC structure).
• Cementite (Fe3C): Extremely hard and brittle intermetallic iron-carbide compound.
• Martensite: Extremely hard, strong, and brittle needle-like structure formed by rapid quenching.

💡Wrought Metal

A wrought metal is an alloy or metal that has been mechanically shaped and worked—through processes like rolling, drawing, bending, forging, or swaging—after being cast as an ingot.

Key Characteristics

• Strain Hardening: Cold working increases the alloy's yield strength, ultimate tensile strength, and hardness, while decreasing its ductility.
• Microstructure: The cast grain structure is broken down and elongated in the direction of working, resulting in a fibrous or fine-grained structure.
• Flexural Strength & Resilience: Displays higher proportional limit and elasticity compared to its cast counterpart.
• Heat Treatability: Can be heat treated (e.g., stress-relief annealed to restore ductility or age-hardened).