Manufacturing Of Silver Amalgam Alloys
MANUFACTURE OF ALLOY POWDER
Lathe-Cut powder
To make lathe-cut powder, an annealed ingot of alloy is placed in a milling machine or in a lathe and is fed into a cutting tool or bit. The chips removed are often needle like, and some manufacturers reduce the chip size by ball milling.
Homogenizing Anneal
Because of the rapid cooling conditions of from the as-cast state, an ingot of an Ag-Sn alloy has a cored structure and contains non-homogenous grains of varying composition. A homogenizing treatment is performed to re-establish the equilibrium phase relationship. The ingot is placed in an oven and heated at a temperature below the solidus for sufficient time to allow diffusion of atoms to occur and the phases to reach equilibrium. The time of heat treatment may vary, depending on the temperature used and the size of the ingot, but a thermal time of 24 hours at the selected temperature is not unusual.
At the conclusion of the heating cycle, the ingot is brought to room temperature for the succeeding steps in manufacture. The proportion of phases present in the ingot after cooling is affected by the manner in which the ingot is cooled. If the ingot is withdrawn from the heat treatment oven rapidly and quickly quenched, the phase distribution will remain essentially unchanged. On the other hand, if the ingot is permitted to cool very slowly, the proportions of phases will continue to adjust towards the room temperature equilibrium ratio.
For example, in a Ag-Sn alloy, rapid quenching of the ingot results in the maximum amount of Beta phase retained, whereas slow cooling results in the formation of the maximum amount of the Gamma phase.
Particle Treatments
Once the alloy ingot has been reduced to cuttings, it is treated with acids. The exact function of this treatment is not entirely understood, but it is probably related to preferential dissolution of specific components from the alloy. Amalgams made form acid washed powders tend to be more reactive than those made from unwashed powders.
The stress induced into the particle during cutting and ball milling must be relieved or they slowly release over time, causing a change in the alloy, particularly in the amalgamation rate and dimensional change occurring during hardening. The stress relief process involves an annealing cycle at a moderate temperature of 100degrees C for several hours.
Atomized Powder
Melting together the desired elements makes atomized powder. The liquid metal is atomized into fine spherical droplets of metal. If the droplets solidify before hitting a surface, the spherical shape is preserved, and these atomized powders are called “spherical powders”.
Particle Size
The manufacturer controls maximum particle size and the distribution of sizes within an alloy powder. Most significant influence on amalgam properties is the distribution of sizes around the average. For example, very small particles greatly increase the surface area per unit volume of the powder. A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam.
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