What materials are used in modern BIW construction and why?

Modern BIW uses a multi-material approach: Mild steel (cost-effective for less critical areas), High Strength Steel/AHSS (900-1500 MPa for crash-critical members like B-pillar), Ultra High Strength Steel (hot-stamped, >1500 MPa for intrusion prevention), Aluminum (closures, subframes for weight reduction), Magnesium (instrument panel beam), and Carbon Fiber Reinforced Plastic (premium vehicles). Material selection balances strength, weight, cost, formability, joinability, and repairability.