the mercury/tin/copper amalgam reduces the melting point of the amalgam. the mercury alloys with the other metals & does not evaporate. on hardening they form an intimately mixed and bound alloy.
think of your mouth. you probably have some 'silver' fillings. these are really a 'bronze' amalgam of tin/copper/silver and some other metals that increase the strength of the resultant alloy. the mercury is mixed with the metals to form a paste that is essentially the other metals dissolved in the mercury, which hardens as the mercury alloys itself with the other metals, becoming intimately and permanently part of them (and raising the meting point as the amalgam becomes an alloy). the mercury in your fillings is bound up & not free to poison you. it is of course subject to chemical attack that may leech out one or more components and eventually cause failure.
(a somewhat simplistic explanation to save space, it ignores the percentages of the components & their effect)
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