You can use the following parameters:

1. Tool geometry: Left hand threaded tool pin

2. Plunge depth is around 0.2mm

3. Tool pin should be in offset positions i.e. 1 to 2 mm should be in aluminum side.

4. Tool rpm: 800 to 1200

5. Tool traverse speed: 80 mm/min to 120mm/min

Hi Pankaj, thank you for your response.

However my question was more in relation to overlap joints only. I'm not sure I follow why an offset is required for an overlap joint, could you elaborate - for butt welds, it is fully understood.

Have you had lab success in joining Al (any series) to high strength steels (over 800MPa) in an overlap joint? Did they pass bend tests? What was your achieved shear tensile strength?

Ok, I did experiment on butt joint not in overlap joints. But I know that more area of shoulder should be touched with aluminium.

You may refer the following article. 

A preliminary report on the strength and metallography of a bimetallic friction stir weld

joint between AA6061 and MIL-DTL-46100E High Hardness steel 

http://www.focushopeindustries.com/uploads/content/Bimetallic%20Friction%20Stir%20Process%20White%20Paper.pdf

But it is interesting to think for research parameters for joining AA 6061 and High strength steel. If any opportunity to work for such project we may collaborate.