Elements are pure substances that cannot be broken down into any other substance. Combining elements creates a new class of substances known as compounds. Dissociating compounds through specific chemical processes can return the compound back into its constituent elements.
Aluminum and copper are, of course, both metallic elements that can be mixed together to make a specific type of compound, known as an alloy. These are considered to be mixtures of metals which have better physical properties than any of the original metals.
According to the link, "A homogeneous mixture is a composition of two or more pure substances in a definite proportion, displaying uniform characteristics," or in your specific case, an alloy of copper and aluminum.
So your aluminum and copper plate would be considered an homogeneous mixture.