This research paper presents the mathematical model to design a stepped pin profile tool and its durability index. A mathematical model is being developed by considering tools that should generate the same heat as conventional conical tools and stepped tools by varying the shoulder to pin diameter ratio (D/d). Aluminum alloy AA6061-T6 with a thickness of 6 mm fabricated using a butt-type weld configuration and newly developed conical stepped pin profile tools with water as a cooling medium. The result shows that a conical stepped tool shows better mechanical properties of the welded joints compared with a conventional conical tool. When compared to other stepped conical pin profile tools, one with a (D/d) of 3 has superior mechanical and microstructural properties and as the D/d ratio increases, the tool pin durability index falls.