In my opinion your parameters, especially your rotation speed, are high. If too high rotation speed is used, more heat is generated around your tool and your material reach a high level of softening. This is why your material can goes below your shoulder creating a lot of flash defects. Thus, the flash is the material missing in your weld.

Maybe you can also decrease your tilt angle (1.5-2°) and pay attention to the normal force.

Thanks for the answer. What is the minimum shoulder dia required for welding? I am using a 18 mm dia tool.

Your tool geometry depends mainly of your material thickness. Pin profile (conical, cylindrical, thread or not...) is also an important parameter for material flow inside the joint, mostly in case of dissimilar welding. A good approximation of tool diameter is to keep the ratio shoulder diameter /  pin diameter approximately equal to 3.

You will find many papers for discussion of tool geometry.