A counter-rotating spindle includes a shoulder tool, a pin tool inserted in the shoulder tool, a first motor that is connected with the pin tool and rotates the pin tool, and a second motor that is connected with the shoulder tool and rotates the shoulder tool. The direction and the speed of the rotation for the pin tool may be selected independently from the speed and direction of rotation of the shoulder tool enabling optimization of the friction stir welding process. Counter-rotating the pin tool and the shoulder tool may improve the mixing abilities and efficiency of the pin tool and may prevent the galling effect between the pin tool and the shoulder tool. The counter-rotating spindle may be used, for example, for continuous path friction stir welding and spot welding of aluminum and its alloys, including cast alloys, as well as joining similar and dissimilar alloys.