The bar is probably made of aluminum. The accepted value for the density of Aluminum is 2.7 g/cm^3, so your calculated density of 2.8 is within 3% of the accepted value.
If you examine a chart of accepted values for the density of various metals, you will find that most of the metals are considerably denser that aluminum, with the next nearest being titanium with a value of 4.5 g/cm^3. Looking at metals that are lighter than aluminum, magnesium is the closest at 1.7 g/cm^3. Since neither of these values is anywhere near 2.8 g/cm^3, it would be reasonable to conclude that your sample has to be made of aluminum.