if density of metal bar is 2.8 g/cm^3, what could be the conclusion?comparing calculated density and the metal bar is probably Aluminum
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.