Steel and copper can be friction stir welded. But, the problem of difference in melting points and high thermal conductivity of copper are to be addressed. The melting point of copper is 10830C and that of steel is around 14000C. Because of the high thermal conductivity of Cu, relatively low tool axis offset should be given towards Cu. If tool axis offset is not given, excessive heat generation may cause thick and highly brittle IMC layer to form at the joint interface resulting in weak joints. Also, keep the high temperature material on the advancing side.

Please check below article and link:

http://www.sciencedirect.com/science/article/pii/S0924013606005048

The tool supplies heat and the pin apart from supplying heat also stirs the material At a given temperature materials with different strength, melting point and thermal conductivity requires different flow stress in order to move and be mixed. At zero offset materials on both side of the faying line are mixed in equal quantity. During dissimilar materials welding Cu-Steel (e.g. Cu- 1083oC temperature relatively soft and highly thermally conductive Vs Steel-  ~1540oC very strong and poor in thermal conductivity in comparison to Cu) when appropriate off is chosen in the Cu side more of the Cu is mixed with less of Steel. Thus, the inequality of flow stress is compensated by the difference in relative mass of materials being mixed.

pardon me, is there any suitable ratio in doing so (offset distance) i.e cu-steel? Thanks.

For such type of dissimilar materials, you would always be better off with a non-rotating shoulder type of tool design (which is patented by three large industrial firms already). This would give you the best dissimilar joint quality (in butt joint) of Cu to Steel. 

Offset is required for multiple reasons. I assume butt joint configuration.

(1) Non uniform heat distribution is required on both sides of the dissimilar metals.

(2) It may prevent partial melting of the material which has very low melting point compared to the other materials to be joined.

(3) Minimize the formation of intermetallic compounds.

(4) Improve tool life.

How tool offset exactly measured from wheather from centre of tool axis or edge of tool?

@Gurunath Shinde  Tool offset is the distance between the center of tool axis and abutting edges. Hence, it should be from center of tool axis. Tool offset is mostly applicable for butt joints.

@Gurunath - Tool offset depends on the max. diameter of your pin/probe (max. because some might use a conical or tri-flute arrangement). After aligning the center of the tool axis to the butt joint line, you need to offset (to the side of the softer material, e.g. Al to steel, move into the Al) by the amount of half the diameter as you measured above. It is crucial that your dissimilar material set up is perfect and that the edges of the workpieces are uniform (i.e. well cut or machined) so that you avoid any chance of tool breakage or overheating while friction stir welding dissimilar materials in a butt joint.