Aluminum has a tendency to form voids or cracks in the weld nugget as it solidifies. These voids are generally not desirable. In many cases this condition can be improved with good force follow up or application of a secondary forging force near the end of the weld cycle.
Aluminum has higher shrinkage during solidification than steel. The use of low-inertia, low-friction weld head assures good follow up and will help reduce defects during solidification.
Another option is the application of a variable force. The weld is made at one force and then the force is increased as the nugget solidifies to improve the soundness of the final nugget. Timing is critical to make sure the increased force is after the current and before the nugget has solidified.
The goal of these methods is to prevent the voids from forming or welding them closed with forging force as the nugget solidifies.
For more information reference - Recommended Practices for Resistance Welding, AWS Standard, AWS C1.1 Section 4.3.3.2.