There are many advantages to joining dissimilar metals and shapes.
Save on material cost by combining different metal types and grades. Use a corrosion-resistant metal that stands up to chemicals and lower cost material for unexposed areas of the part. The same is true for high-stress parts made with a combination of different material strengths depending on load area.
Reduce weight by substituting a solid shaft with a pipe/tube body fused with solid ends.
Save on machining and raw materials by replacing a solid stock blank with weldments conforming more closely to final part size and shape.
Friction welding provides versatility, strength, and savings to manufacturing.