Friction stir lap welded low carbon steel and ductile iron: microstructure and mechanical properties

Abstract

To resolve the poor weldability of ductile irons, this study employed AISI 1008 low carbon steel in the pure ferrite phase as the top plate and ductile iron as the bottom plate and varied the rotational speeds combined with travelling speeds that ranged between 40 and 70 mm min⁻¹ to conduct a friction stir lap welding test. After welding, the weldments underwent microstructure analysis and hardness testing followed by a tensile shear test to evaluate the joint strength. At low rotational speeds <800 rev min⁻¹, the experimental results indicated the following: the interfacial regions of the two materials could not join completely; the matrix structure of the stir zone was primarily composed of pearlite; the original spherical shape of the graphite in the ductile iron matrix changed into a striped conformation; the weldments possessed low average maximum tensile load; and the fracture site was in the vicinity of the weld interface. At the transition parameter of 850 rev min⁻¹, the average maximum tensile load of weldment increased, and the tensile fracture site was in the interface of retreating site. The results under high rotational speeds of 900 and 1000 rev min⁻¹ indicated the following: an excellent joining effect was achieved; the interfacial regions of the carbon steel and ductile iron primarily comprised pearlite, although the vicinity of the retreating side and the stir zone matrix of ductile irons were composed of martensite structures; individual graphite granules were present; the average maximum tensile load of the weldments was the highest; and the fracture site was located in the low carbon steel base metal.