Class 1 material is a designation of the Resistance Welding Manufacturers Alliance (RWMA) which describes a group of copper alloys with good strength and high electrical conductivity. Class 1 is often used to weld aluminum and other good conductors like brasses and bronzes which require high weld currents.
Class 1 Bar Stock
In addition to welding aluminum and other highly conductive materials, it is frequently used for welding galvanized steels. Galvanized steels require more current than bare steel and the class 1 material can handle this increase.
The product description and properties of Class 1 can be found in the Resistance Welding Manual section 18-3, available at RWMA/AWS.
There are other Class materials with other properties all shown in the RWMA manual. Their properties allow for welding work pieces with both higher and lower resistances than the aluminum and galvanized steel mentioned here.