The EU aims for reducing automotive emissions by 30-40% by 2030 which is mainly achieved by lightweight, electrification and fuel efficiency approaches. The steel industry responded by developing advanced-high strength (AHSS), press hardening steels (PHS) and enhanced machinability steels. AHSS and PHS combined with Al cast nodes and Al-extrusions have grown in bodywork to reduce weight, improve performance and safety. The successful joining of enhanced machinability steels in ICE engines promotes fuel efficiency. Stir4Steel will provide efficient, environmental-friendly and cost-effective methodologies to join these materials. It will enable multi-material designs for battery trays, shock towers/front rails, B-pillars and monoblock pistons, leading to weight-optimized components, energy efficiency and lower CO2 footprint. The global project aim described will be reached through the achievement of the following technical objectives: • Objective 1: To develop Friction Stir Welding (FSW) and refill Friction Stir Spot Welding (RFSSW) based joining solutions for Steel/Al joints with AHSS and PHS; • Objective 2: To advance the FSW technology to weld enhanced-machinability steel grades components. • Objective 3: To model the mechanical performance of FS-welded Steel/Al joints and high-machinability steel grades; • Objective 4: To produce and test 4 steel-optimised, multi-material demonstrator car parts; • Objective 5: To simulate the industrialisation process for assembly using such new joining techniques; • Objective 6: To identify cost and environmental elements which influence the vehicle with a systematic LCA. Led by a tip-of-the-spear consortium consisting of steel producers, car manufacturers, FSW joining technologists, and innovation experts, Stir4Steel aims to pave the way for more sustainable applications and higher penetration of advanced steel grades in the automotive as well as aviation, trains and shipbuilding sectors.